Skip to Main Content

Middle East Models of Jurassic/Cretaceous Carbonate Systems

SEPM Special Publication No. 69, Copyright ©2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-075-1, p.315–334.

Abstract

Two major factors, in addition to the global eustaric sea-level changes, controlled the deposition of the Cretaceous stratigraphic sequences in the Mesopotamian Basin in central and southern Iraq: (1) the culmination of the Neo-Tethys rifting resulting from the movement of the Arabian Plate into the tropical and subtropical climatic zones during the Cretaceous, and (2) the contemporaneous growth of giant structures due to salt diapirism or relative basement-block displacement, particularly during middle and Late Cretaceous.

The Early Cretaceous carbonate ramp of the Yamama Formation underwent a gradual clastic invasion that started during the late Early Cretaceous. This was followed by tectonic activity beginning in the late middle Cretaceous that converted a ramp system into a block-faulted shelf/platform system by the end of Cretaceous.

Within the three groups of the Lower, middle, and Upper Cretaceous, referred to as the Thamama, Wasia, and Aruma Groups in the Arabian Gulf region, seven major sedimentary cycles are recognized. Three of these cycles or major sequences are attributed to the Lower Cretaceous Thamama Group, and each of the Wasia and Aruma Groups of the middle and Upper Cretaceous, respectively, are divided into two cycles. These major sequences are defined within their depositional time intervals as early Berriasian-Valanginian, Valanginian-Barremian, Barremian-middle Aptian, middle Aptian-Albian, Cenomainian-early Turonian, Turonian-Campanian, and Campanian-Maastrichtian. Each of the seven cycles can be subdivided into large- and medium-sized accommodation cycles on the basis of detailed sedimentological and paleontological data.

The Cretaceous sediments of the Mesopotamian Basin host one of the richest petroleum systems in the world. Each major sequence contains weLl-recognized stacked carbonate and sandstone reservoirs. These are overlain by multiple, tight carbonate and shale seals. The late Barremian-middle Aptian dolomitic sequence (i.e., the Shuaiba Formation) may be the only exception. Potential source rocks charging these reservoirs are either associated within the same major sequence or group, particularly in the Lower and middle Cretaceous, or are older, such as the Upper Jurassic Naokelekan and Sargelu Formations.

The Cretaceous might contain more plays either as new fields or at deeper levels in the present fields. More reserves are likely in the stratigraphic traps of the Mishrif, Zubair, and Nahr Umr formations. Formations like the Hartha, Ahmadi, Khasib, and Mauddud may prove to be productive outside the presently prospective areas.

Introduction

The Cretaceous of Iraq, particularly in the Mesopotamian Basin of central and southern Iraq, has been subjected to intensive study since the middle of this century because it hosts around 80% of Iraq’s proven oil reserves together with more than 70% of its probable unproven reserves (Fig. 1 A). However, only a limited amount of this work has been published (e.g., Dunnington, 1967; Gaddo, 1971; Al-Siddiki, 1978; Ibrahim, 1981,1983,1984; Ali and Aziz, 1993; Sadooni, 1993,1995b, 1996; Aqrawi, 1996; Aqrawi et al, 1998). These rocks form the main carbonate and sandstone reservoirs in most of the super-giant fields of the Mesopotamian Basin.

Fig. 1A.

—A simplified basement tectonic map of Iraq (modified after Buday and Jassim, 1987) showing area of study and location of the Mesopotamian Basin among the longitudinal and transverse blocks separated by faults (dashed lines). Note the main fields as black areas.

Fig. 1A.

—A simplified basement tectonic map of Iraq (modified after Buday and Jassim, 1987) showing area of study and location of the Mesopotamian Basin among the longitudinal and transverse blocks separated by faults (dashed lines). Note the main fields as black areas.

After more than a half-century of exploration in Iraq, the Cretaceous succession still produces more pleasant surprises than any other geological system. The Cretaceous comprises more than 3000 m of sediments distributed among 44 formations and includes by all standards the thickest, the most varied, and the most widespread sedimentary successions in the Midd le East.

The Cretaceous Period encompassed the maximum rifting of the Neo-Tethys. The limits of the Cretaceous flooding extended beyond most of the previous Jurassic shorelines, and it evolved later into small isolated basins and playas during the Tertiary. Cretaceous deposits in the Mesopotamian Basin show an extensive horizontal and vertical facies differentiation, including the development of different reef types (sponge-stromatoporoid, algal, and rudist) as well as regional gradational spatial changes from carbonates to evaporites, or even to elastics. Carbonates are the major Cretaceous rocks in the Mesopotamian Basin, but some thick clastic units are also present. This was due in part to the low stand of sea level and the prevalence of humid conditions during certain periods of the Cretaceous (Murris, 1980; Beydoun, 1991). The stratigraphic relationships in the Cretaceous of Iraq, correlated to the Cretaceous of some other Arabian Gulf countries, are shown in Figure 2. In addition, a lithostratigraphic correlation of the Cretaceous succession in three selected wells, Majnoon-1, Ratawi-1, and Ghlaisan-1 (wells Mj-1, Rt-1, and Gh-1 in Fig.IB), is shown in Figure 3.

Fig. IB.

—Map of the study area showing the location of the studied wells mentioned in the text.

Fig. IB.

—Map of the study area showing the location of the studied wells mentioned in the text.

Fig. 2.

—Stratigraphic relationships of the Cretaceous formations of Iraq and their equivalents in some Arabian Gulf countries, updated and amalgamated from various sources.

Fig. 2.

—Stratigraphic relationships of the Cretaceous formations of Iraq and their equivalents in some Arabian Gulf countries, updated and amalgamated from various sources.

Fig. 3.

—Stratigraphic correlation of the Cretaceous succession in three selected wells (Ghlaisan-1, Ratawi-3, and Majnoon-1, referred to as Mj-1, Rt-1, and Gh-1 in Fig. 1). For horizontal scale see Fig. 1. (Modified after Al-Siddiki, 1978a).

Fig. 3.

—Stratigraphic correlation of the Cretaceous succession in three selected wells (Ghlaisan-1, Ratawi-3, and Majnoon-1, referred to as Mj-1, Rt-1, and Gh-1 in Fig. 1). For horizontal scale see Fig. 1. (Modified after Al-Siddiki, 1978a).

This study is an attempt to develop a preliminary sequence stratigraphic framework for the Cretaceous by distinguishing the major stratigraphic sequences in each of the three Cretaceous groups (i.e., the Thamama, Wasia, and Aruma) in the Mesopotamian Basin. This approach is based on integration of the available sedimentologic and chronostratigraphic data, and integrating these with establ ished global eustatic sea-level changes (Haq et al., 1987), taking into consideration the basic concepts of sequence stratigraphy (e.g., Vail et al, 1977; Van Wagoner et al., 1990), particularly those applying to carbonate rocks (Loucks and Sarg, 1993). Some of these major sequences are also subdivided into large- and medium-sized accommodation cycles whenever more detailed sedimentologic and paleontologic data are available. We also discuss the petroleum prospects of each major sequence together with the future hydrocarbon potential of Cretaceous petroleum plays.

Formation Nomenclature: A Historical Background

The stratigraphic framework of the Cretaceous in southern Iraq was originally formulated by Owen and Nasr (1958) (Fig. 2). This work was based on the extensive early fieldwork by geologists of the Iraq Petroleum Company (IPC) and the Basra Petroleum Company (BPC) during the Forties and Fifties. There were two stratigraphic nomenclatures for the northern (including central parts) and southern Iraq. This double nomenclature was the result of concession boundaries rather than real geological variations. The IPC was working in northern Iraq and the BPC was operating in southern Iraq, and each company relied on its own stratigraphic nomenclature.

Chatton and Hart (1961) attempted a regional correlation between the two provinces, but their findings remained unpublished. Since that time more data have been accumulated from two main sources: the exploration program conducted by the Iraqi National Oil Company (INOC) during the Seventies and the Eighties, and from the Regional Geological Survey (Geosurv), carried out by the State Establishment for Geological Survey and Minerals Investigation.

Some earlier results of Geosurv and INOC work appear in Buday (1980) and Buday and Jassim (1987), who also compiled much earlier data dating back to Bellen et al. (1959). The main findings of Geosurv survey were summarized in Jassim et al. (1984). The modifications proposed by Hart and Chatton (1960, 1961, 1962a, 1962b, 1962c), and the new findings of the INOC exploration program and the Geosurv survey, together with many individual studies, particularly those in the form of M.Sc. and Ph.D. theses in various universities across Iraq and elsewhere, brought some profound changes to the understanding of the Cretaceous stratigraphy of Iraq, particularly in the Mesopotamian Basin. Because most of these nomenclatural modifications have never been published before, a short summary of them is given in Table 1. These modifications (as in the current study) will red uce the confusion arising from the large number of formation names and will facilitate regional correlation for sequence stratigraphic purposes.

Table 1.

—Some later unpublished modifications to the Cretaceous stratigraphic framework of the Mesopotamian Basin, after being formulated principally by Owen and Nasr (1958), and revised in more detail in the Iraq stratigraphic lexicon (Bellen et al., 1959).

Author(s)Modifications
Chatton and Hart (1961) The Gir Bir (Cenomanian-?Turonian), Mergi (Turonian) and M’sad (?upper Cenomanian) are unnecessary synonyms of the Mishrif Formation. 
Chatton and Hart (1962b), Aqrawi (1983), Sherwani (1983) Fahad (Turonian), Maotsi (Turonian), and Mahliban (Upper Cenomanian) Formations are local variants of the intertonguing between the Rumaila and Mishrif Formations. 
Chatton and Hart (1960), Sadooni (1978) Upper Qamchuqa Formation is equivalent to the Mauddud Formation and the Lower Qamchuqa Formation is equivalent to the Shu’aiba Formation. The terms Mauddud and Shu’aiba Formations should be used in northern Iraq also. 
Chatton and Hart (1960) Upper Sarmord and Rim (?Albian) Formations are considered as parts of the Nahr Umr Formation, while Middle Sarmord should be attached to the Ratawi Formation and only the Lower Sarmord is considered as the real Sarmord facies as originally defined. The Karimia (?Tithonian-?Berriasian) and Makhul (?Tithonian) Formations are considered as local variants of the Sarmord Formation 
Chatton and Hart (1960) Garagu Formation (Valanginian-?Hauterivian) is considered as an equivalent to the Yamama Formation. Jib’ab (Upper Senonian), Digma (Maastrichtian), and Qurna (Maastrichtian) Formations are synonyms of the Shiranish Formation. 
Chatton and Hart (1961) Unification of the Aqra (?Maastrichtian), and Bekhme (Upper Campanian-?Lower Maastrichtian) Formations into a single Formation (i.e., Aqra Formation). 
Chatton and Hart (1961) The Pilsener Formation (Upper Campanian-Lower Maastrichtian), to be replaced by the Hartha Formation 
Gaddo(1961) Announcement of the Batiwah Formation, which consists of greenish gray calcareous shale interbedded with grayish, dense limestone. The formation is proposed for the neritic shale and limestone of Albian age, and represents a transition between the sandstone and shale of the Nahr Umr Formation and the limestone of the Albian part of the Qamchuqa Formation. 
Chatton and Hart (1961) Announcement of the Kifel Formation, which consists of anhydrite and pelletal limestone of lower Turonian, and represents the lagoonal equivalent of the Mishrif Formation. 
Jassim et al. (1984) Announcement of the Jid Formation as the phosphate-bearing equivalent of the Hartha-Tayarat Formation (Campanian-Maastrichtian) in the Western Desert 
Al-Naqib(1967) Upgrading the Dibs Member of the obsolete Pilsener Formation into the Safawi Formation. 
Aqrawi (1983) and Sherwani (1983) Replaced the names of Maotsi, Fahad, and Mahliban formations by Mishrif and Rumaila formations in central Iraq, as they are in southern Iraq. 
Author(s)Modifications
Chatton and Hart (1961) The Gir Bir (Cenomanian-?Turonian), Mergi (Turonian) and M’sad (?upper Cenomanian) are unnecessary synonyms of the Mishrif Formation. 
Chatton and Hart (1962b), Aqrawi (1983), Sherwani (1983) Fahad (Turonian), Maotsi (Turonian), and Mahliban (Upper Cenomanian) Formations are local variants of the intertonguing between the Rumaila and Mishrif Formations. 
Chatton and Hart (1960), Sadooni (1978) Upper Qamchuqa Formation is equivalent to the Mauddud Formation and the Lower Qamchuqa Formation is equivalent to the Shu’aiba Formation. The terms Mauddud and Shu’aiba Formations should be used in northern Iraq also. 
Chatton and Hart (1960) Upper Sarmord and Rim (?Albian) Formations are considered as parts of the Nahr Umr Formation, while Middle Sarmord should be attached to the Ratawi Formation and only the Lower Sarmord is considered as the real Sarmord facies as originally defined. The Karimia (?Tithonian-?Berriasian) and Makhul (?Tithonian) Formations are considered as local variants of the Sarmord Formation 
Chatton and Hart (1960) Garagu Formation (Valanginian-?Hauterivian) is considered as an equivalent to the Yamama Formation. Jib’ab (Upper Senonian), Digma (Maastrichtian), and Qurna (Maastrichtian) Formations are synonyms of the Shiranish Formation. 
Chatton and Hart (1961) Unification of the Aqra (?Maastrichtian), and Bekhme (Upper Campanian-?Lower Maastrichtian) Formations into a single Formation (i.e., Aqra Formation). 
Chatton and Hart (1961) The Pilsener Formation (Upper Campanian-Lower Maastrichtian), to be replaced by the Hartha Formation 
Gaddo(1961) Announcement of the Batiwah Formation, which consists of greenish gray calcareous shale interbedded with grayish, dense limestone. The formation is proposed for the neritic shale and limestone of Albian age, and represents a transition between the sandstone and shale of the Nahr Umr Formation and the limestone of the Albian part of the Qamchuqa Formation. 
Chatton and Hart (1961) Announcement of the Kifel Formation, which consists of anhydrite and pelletal limestone of lower Turonian, and represents the lagoonal equivalent of the Mishrif Formation. 
Jassim et al. (1984) Announcement of the Jid Formation as the phosphate-bearing equivalent of the Hartha-Tayarat Formation (Campanian-Maastrichtian) in the Western Desert 
Al-Naqib(1967) Upgrading the Dibs Member of the obsolete Pilsener Formation into the Safawi Formation. 
Aqrawi (1983) and Sherwani (1983) Replaced the names of Maotsi, Fahad, and Mahliban formations by Mishrif and Rumaila formations in central Iraq, as they are in southern Iraq. 

Cretaceous Sequence Stratigraphy: The Controlling Factors

Cretaceous sedimentation in the Mesopotamian Basin was controlled by a combination of both regional and local factors in addition to global eustatic sea-level changes. Some regional factors such as the culmination of the Tethys rifting is evident from the widespread distribution of the Cretaceous sediments in most parts of the Arabian Plate. Also, the movement of the Arabian Plate to the tropical or the subtropical zone occurred at this time (Murris, 1980; Beydoun, 1991,1993). Local factors are the effect of either salt diapirism or movement of basement blocks. As a result, three main categories of factors contributed to the final nature and distribution of the Cretaceous sediments in the Mesopotamian basin: tectonic, climatic, and eustatic.

Tectonic Influence

A major local factor affecting sediment type in the Mesopotamian Basin of Iraq may have been the continuous growth of some structures since Early Jurassic time. By comparing the Mishrif Formation’s thicknesses in two wells (one on the :rest and the other on one flank) of the Rumaila Field, it is evident that the structure has grown since Early Cretaceous to the present, although the growth rate has declined with time (Fig. 4). This growth has created a difference of around 128 m between the two wells (Al-Sakini, 1992). Ala (1974) and Sadooni (1993) suggested that movement of the Infracambrian Hormuz Salt might have influenced sedimentation during most of the Cretaceous. Fuloria (1976)noticed a correspondence between reservoir-quality Mishrif facies and Yamama fades, which may reflect such tectonic control on both the Yamama and Mishrif Formations, of Early and middle Cretaceous ages, respectively. Patch reefs and oolite shoals are the two main reservoir facies that developed on the crestal parts of the field structures, whereas the flanks and low areas between these were filled with muddier facies (Sadooni, 1993; Aqrawi et al., 1998).

Fig. 4.

—Growth of the Rumaila North oilfield during geologic time. (Modified after Al-Sakini, 1992.)

Fig. 4.

—Growth of the Rumaila North oilfield during geologic time. (Modified after Al-Sakini, 1992.)

This halokinetic tectonism played a minor role in controlling sedimentation during most of the Early Cretaceous, but during the late Albian (i.e., at the beginning of the middle Cretaceous), tectonism became more conspicuous, with possible activation of the basement faults and relative displacement of blocks (Fig.lA) in addition to salt diapirism (e.g., Aqrawi, 1996; Sadooni, 1993, 1995b, 1996; Aqrawi et al., 1998). These local tectonic effects led to minor variations in facies, particularly in central Iraq, at the western edges of the Mesopotamian Basin, and some areas adjacent to the Iranian borders near Amarah. These contrasts resulted in the establishment of different formation names.

Climatic Factors

It is generally agreed now that during most of the Cretaceous many parts of the Arabian Plate were in the tropical to subtropical zones (Beydoun, 1991). Murris (1980) notes the association of clastic materials with humid tropical periods. Aqrawi (1996) suggests the dominance of a humid tropical climate during the Late Cretaceous (particularly Turonian-Campanian) by study of the clay mineralogy of the Khasib, Tanuma, and Sa’di formations in the Mesopotamian Basin. Detrital illite and kaolinite are the main clay minerals detected in the clay fractions of these three formations. However, periods of arid climate might have occurred occasionally, as during the late Cenomanian, as reflected by the deposition of Kifel Formation evaporites ending the regression of the Cenomanian-Early Turonian cycle.

Wind exerted a considerable influence on the nature, location, and distribution of sediments in the Mesopotamian Basin. With the present level of available information, however, it is not possible to comment on the wind direction and its intensity during the Cretaceous. What is evident from the careful examination of the nature of the sediments, however, is that major lateral differences in sediment nature take place across the different flanks of the same structure. As a result, the vertical differentiation in sediment types is usually accompanied by differences between the two flanks of the structures, as in the West Qurna and Ratawi fields, of southern Iraq (Figure 5 A, B). A consistent pattern of fades distribution may indicate a windward/leeward effect. Alternatively this differentiation may be partially due to the difference in the gradient of the slope of the growing structures.

Fig. 5.

—A) Vertical and horizontal facies differentiation of the Yamama Formation in the West Qurna Field, Mesopotamian Basin. These variations are probably the result of a combination of structural and climatic conditions. B) Vertical stacking of the Yamama Formation facies in the West Qurna Field may indicate a low-energy reef.

Fig. 5.

—A) Vertical and horizontal facies differentiation of the Yamama Formation in the West Qurna Field, Mesopotamian Basin. These variations are probably the result of a combination of structural and climatic conditions. B) Vertical stacking of the Yamama Formation facies in the West Qurna Field may indicate a low-energy reef.

Furthermore, it seems that a gradual increase in the intensity of the energy level with time is reflected by the change from the low-energy patchy stromatoporoid-algal reef of the Yamama Formation (Lower Cretaceous) to the rudist reef of the Mishrif Formation (middle Cretaceous). It is difficult, however, to attribute this shift to a change in the wind intensity only, because it may be the result of several other factors, such as a profound shift in the whole climate (Sadooni, 1998), gradual evolution of the major structures, evolution of the carbonate platform to a more exposed setting, or even a change to shallow-water-dominated parasequences.

Eustatic Sea-Level Changes

The stratigraphy of the Mesopotamian Basin is still far from being fully studied, and so this proposed sequence stratigraphic framework (Fig. 6) is a baseline step towards establishing detailed future studies on the high-resolution sequence stratigraphy.

Although regional and local factors together played an important role in shaping the Cretaceous depositional successions in the Mesopotamian Basin, it is possible to recognize the eustatic control on the major sequences or cycles. In some few cases, it is possible also to relate smaller, higher-frequency cycles to these events (e.g., Aqrawi et al., 1998). A full analysis of the role of eustatic factors requires a detailed chronostratigraphic analysis of the rock units, which is not yet possible.

Most previous paleontological work in Iraq has been concerned with the determination of broad formation age rather than the recognition of specific biozones. As a result, many single, homogeneous lithologic units may contain significant time breaks. Although the major sequences, which usually emcompass more than one formation and up to four formations, can be correlated with Haq et al. (1987) curve with reasonable confidence, other higher-frequency cycles might be the result of tectonism, particularly during middle and Late Cretaceous.

However, some more detailed analysis was carried out in order to subdivide some of the major sequences into large-scale and medium-scale shallowing-up cycles on the basis of the available lithological and wireline log information. The major cycles (or sequences) can be correlated regionally across the Middle East, particularly into neighboring countries (Alsharhan and Kendall, 1991). Lack of accurate time lines and identified boundary surfaces makes it more difficult to correlate the other smaller sequences, even within the Mesopotamian Basin itself (see Aqrawi et al., 1998, for a detailed case of the middle Cretaceous).

Major Stratigraphic Sequences And Their Petroleum Prospects

The Cretaceous succession in the Mesopotamian Basin can be subdivided into three groups following the terminology used in the Arabian Gulf region (e.g., Powers et al., 1966; Harris et al., 1984; Kendall et ah, 1991; Alsharhan and Nairn, 1986,1988,1990). These are the Thamama, Wasia, and Aruma Groups, which are Lower, middle, and Upper Cretaceous, respectively (Fig. 2). These Groups are easily recognized in the Cretaceous successions of the Mesopotamian Basin, and are introduced to the Iraqi geological column in order to unify the general stratigraphic terminology of the Middle East. The bases for such division are the major unconformity surfaces, which are initially used as separating time lines between the main Cretaceous sedimentary cycles for regional studies (Bellen et al., 1959; Buday, 1980). However, such unconformities are usually prominent in the shallower stable shelf areas but are not easily traced into the deeper parts of the basin, such as the eastern Mesopotamian Foredeep region near the Iranian border. The recognizable sequences are named according to their time interval of deposition, as follows (Fig. 6).

Fig. 6.

—The seven major sequences distinguished throughout the Cretaceous sediments of the Mesopotamian Basin as compared to Haq et al. (1987) cycles.

Fig. 6.

—The seven major sequences distinguished throughout the Cretaceous sediments of the Mesopotamian Basin as compared to Haq et al. (1987) cycles.

Lower Berriasian-Valanginian Cycle

The lower contact of this sequence is marked by the sharp facies change from the evaporites of the Upper Jurassic Gotnia Formation to the argillaceous limestones of the Lower Cretaceous Sulaiy Formation (Sadooni, 1997). However, this contact between the Jurassic and Cretaceous is gradational in the southern parts of the basin, and Sulaiy deposition might have began in the Late Jurassic. Redmond (1964) and Al-Siddiki (1978a) place the Jurassic-Cretaceous boundary in the lower part of the Sulaiy Formation. Accordingly, the Bramkampella sp. zone is characteristic of the lowermost parts of the Sulaiy, which is either lowermost Cretaceous or uppermost Jurassic, whereas the Everticyclammina sp. zone is characteristic of the lower parts of the Yamama Formation. The upper boundary of this sequence is placed at the base of the Ratawi Formation.

In addition to the Sulaiy and Yamama Formations of the Mesopotamian Basin, the sequence also includes a bathyal fades of organic-rich shales and marly limestones of the Chia Gara Formation, the lower part of the Balambo Formation, an outer-ramp, marly limestone of the Sarmord Formation, and inner-ramp, highstand oolitic limestone and patchy sponge-stromatoporoid reefs of the Sulaiy and Yamama formations (Fig. 5A, B). Although it was reported (Al-Siddiki, 1978a) that the Sulaiy, Yamama, and Ratawi formations form a single, homogeneous succession that is difficult to differentiate, detailed petrographic examination of materials from these formations and regional well-log correlation indicate the presence of numerous short breaks. These breaks (up to 14 of them just in the Yamama Formation) are characterized by concentrations of petrified plant remains and are probably associated with lowstand settings. Major breaks were used to subdivide the Yamama Formation into three reservoir units separated by two permeability barrier units formed of organic-rich, argillaceous materials. These breaks were correlated over most of the studied area and may be due to eustatic factors and, hence, could be correlated with the Haq et al. (1987) cycles. The vertical distribution of the Yamama facies indicates that the formation consists of three stacks of inner-ramp sediments separated by two outer-ramp facies. Each inner-ramp setting shallows up into either an oolitic shoal or a patch reef. The outer-ramp facies are formed of mudstones or shales (Sadooni, 1993). In the deeper parts of the basin to the east, it is very difficult to establish any break in what appears to be a record of continuous sedimentation.

According to Al-Sakini (1992), the Sulaiy Formation is a possible source rock in the major Cretaceous reservoirs of the northern Arabian region and extends northward to Hemreen Mountains, which separate the Mesopotamian tectonic zone from the Foothill zone in Iraq. He indicated that the formation enters the thermal maturation zone at a depth below 3000 m. Al-Haba and Abdulla (1989) reported that the Chia Gara and Balambo Formations are among the most important Lower Cretaceous source rocks of Iraqand suggested that the Balambo Formation is the main source for the Yamama Formation.

The Yamama Formation is the main Lower Cretaceous carbonate reservoir in southern Iraq. The formation produces from a combination of leached oolitic facies and slightly dolomitized horizons with porosity values of up to 22% and permeability of about 300 md. Although the formation is still under exploration, major finds of light oil were reported from West Qurna, North Rumaila, Rafedain, and many other oilfields (Al-Siddiki, 1978b; Sadooni, 1993).

Valanginian-Barremian Cycle

This cycle starts with the first clastic pulse that invaded all the Cretaceous basins of Iraq, including the Mesopotamian Basin. This clastic front resulted in both the Ratawi (mainly argillaceous limestone and shale) and the Zubair (sandstone and shale) formations. It extended to Jambur Field in northern Iraq, where it was sandwiched between the carbonates of the Garagu Formation. In the deeper eastern parts of the basin, the Balambo Formation was still being deposited.

The Ratawi Formation was first introduced by Nasr (1950, unpublished report). Owen and Nasr (1958) described the formation from the Ratawi-1 well in southern Iraq, where it consists of massive, pyritic, greenish to dark shales in terbedded with pseudo-oolitic, detrital, pyritic limestones. The main fossils are pseudochrysalinids, cyclamminids, trocholinids, and oysters. The age of the formation was considered to be Neocomian, but Bellen et al. (1959) and Buday (1980) suggested a Hauterivian-Valanginian age. Chatton and Hart (1962a) amended the definition of the formation by emphasizing the lithological differences between the Ratawi and Yamama formations. They defined the Yamama Formation as a calcarenite, and the Ratawi included a more argillaceous suite. Sadooni (1993) documented these distinctions with petrographic evidence.

The Ratawi Formation represents the beginning of the clastic front that culminated during deposition of the Zubair Formation. In its type locality at the Zubair-3 well, the Zubair Formation has been divided into five distinctive divisions, from top to bottom (Bellen et al., 1959):

  1. shale with two zones of sandstones and a minor amount of siltstones;

  2. sandstones with subordinate shales and siltstones;

  3. black or greenish black, fissile, hard shale with local sandstone streaks;

  4. sandstones with subsidiary amounts of siltstones;

  5. greenish black shales with a sandstone-siltstone zone.

Further drilling in the Mesopotamian Basin indicated that these subdivisions are of only local importance, and the final reports of other wells drilled in Rumaila, Nahr Umr, and other fields are filled with alternative schemes of subdivisions. Farther to the north, Ali and Aziz (1993) described the Zubair Formation in East Baghdad Field as consisting of deltaic lobe cycles, in which the uppermost parts are marked by facies indicating wave and tidal action and overlain by shale that ends the cycle. Each cycle contains four different facies of channels, marsh, chenier, and shelf facies. They suggested that the cyclic nature of the formation was a result of regional fluctuations of the sea level due to the Alpine Orogeny.

The Zubair Formation covers most parts of the Mesopotamian Basin and is considered identical in both age and lithology variation to the Biyadh Formation of Saudi Arabia (Powers et al., 1966; Buday, 1980). The formation shows wide vertical and horizontal variation. These are reflected in the number of the cycles, in their facies changes, and in the sandstone/shale ratio. The formation even grades partially into carbonates in some of the eastern fields, such as Halfayia, and into marly limestones of the Sarmord Formation in northern Iraq.

The depositional regime of the Zubair Formation is not fuJly understood. Although the formation may have been deposited in a fluviAl-deltaic complex, a marine influence in some parts of the basin is indicated by the presence of thick shale units and the regional distribution of sandstone bodies of relatively regular thickness. The dominance of the clastic sediments could be attributed to the humid climatic conditions, but other tectonic or local factors may also have contributed to the final configuration of the Zubair basin. The Zubair Formation is the main reservoir in Rumaila North, Rumaila South, Zubair, and many other giant oilfields in southern Iraq. The reservoir consists of three main sand units AB, DI, and LN, with an average porosity of 20% for each unit and average permeabilities of 600,1000, and 850 md, respectively. The reservoir units are separated by two shale permeability barriers. The thickness of the net pay is around 100 m, mostly of well-sorted sand containing oil of 34° API (Al-Sakini, 1992).

The Ratawi Formation also is a reservoir rock in other Iraqi oilfields, such as Rumaila North and Zubair. The reservoir units are formed of partially leached oolitic and skeletal limestones. According to Al-Sakini (1992), the formation has porosity of 16-18%, which improves significantly in the crestal parts of these fields.

Barremian-Middle Aptian Cycle

The carbonate system recovered from the clastic invasion with the deposition of carbonates of the Lower Qamchuqa Formation in northern Iraq. These deposits consist of neritic shelf shoals with large benthonic foraminifera. In the Mesopotamian Basin, the Shuaiba Formation consists of such shelf carbonates. This formation name is widely used elsewhere in the Arabian Gulf (Alsharhan, 1995).

The Shuaiba Formation was introduced by Rabanit (1951, unpublished report). Owen and Nasr (1958) described it as dolo-mitic limestone, coarsely crystalline with some neomorphosed rudists and Orbitolina spp. and Choffatella decipiens Schlumberger. Bellen et al. (1959) amended this and redescribed the formation from the Zubair-3 well to be composed of the following major units, in upward order:

  1. pseudo-oolitic limestone with angular sand grains;

  2. argillaceous, aphanitic limestone;

  3. chalky limestone;

  4. recrystallized globigerinid limestone with glauconite;

  5. shaly limestone and shale.

The sediments of the Shuaiba Formation represent a trans-gressive depositional system that was abruptly terminated by an unconformity. In this study, the Shuaiba Formation is considered to be a complete cycle, because it is bounded by a transgressive surface at the base and an unconformity at the top.

The upper contact of the Shuaiba Formation is unconformable with the overlying Nahr Umr Formation, and signals the termination of the Lower Cretaceous major sedimentary cycle (i.e., the Thamama Group), and the beginning of a new major sedimentary cycle (Fig. 6).

Although it is one of the main rudist-bearing reservoirs in onshore and offshore regions of the northern Arabian Gulf (Hamdan and Alsharhan, 1991; Alsharhan, 1995), the Shuaiba is not a major reservoir in the Mesopotamian Basin.

Middle Aptian-Albian Cycle

The base of this cycle includes the “second clastic front”. It contains less sand but extends over nearly the same regions of the first clastic front represented by the Lower Cretaceous Ratawi and Zubair Formations. Stratigraphically the clastic sediments of this cycle belong to the Nahr Umr Formation. The deep-water Balambo Formation was still being deposited in the eastern basinal parts of the basin, while in the north a carbonate system was reestablished with the deposition of the Mauddud Formation (previously called the Upper Qamchuqa Formation). Basin-margin lagoons and sabkhas of the Jawan Formation occupied the northwestern parts of the country (Sadooni, 1978, 1995a).

The Nahr Umr Formation was described from the Nahr Umr-2 well in southern Iraq by Owen and Nasr (1958) as consisting of black shale interbedded with medium- to fine-grained sandstones with lignite, amber, and pyrite. For correlation and production purposes, the formation was divided into two members, the Third and the Fourth Sands. These two major sand members are separated by a major shale unit. Hart and Chatton (1960) divided the Nahr Umr Formation into two units, the Nahr Umr Shale and the Nahr Umr Sand. This is because the sand component of the formation practically disappears in northern Iraq. In the upper third of the formation, a limestone unit is designated as the Dair Limestone Member. The fossil content of this unit is very similar to that of the overlying Mauddud Formation. This is one of the justifications to include the Nahr Umr and Mauddud formations in a single cycle.

Fig. 7.

—The Upper Albian-Coniacian formations and main lithologies of the Mesopotamian Basin as compared to Haq et al. (1987) cycles.

Fig. 7.

—The Upper Albian-Coniacian formations and main lithologies of the Mesopotamian Basin as compared to Haq et al. (1987) cycles.

Ibrahim (1981, 1983) investigated the Albian depositional regime, and in particular the Nahr Umr Formation, in some detail in southern Iraq. He suggested that the formation was deposited in a continental setting that ranged from flood plain to delta with probable marine and eolian influence. Alsharhan (1991) studied the stratigraphic status and sedimentological setting of the Nahr Umr Formation in the United Arab Emirates and noted that the deposition of this formation follows the sea-level curves of Haq et al. (1987). Reviewing the sedimentological settings and petroleum potential of the Albian elastics in the western parts of the Arabian Gulf region, including southern Iraq, Alsharhan (1994) noted that the Albian elastics (with different local names) were deposited in a complex setting of alluvial plain to lower coastal plain and shallow marine shelf.

The contact between the Nahr Umr and the Mauddud formations is gradational, as the clastic component decreases upward. The Mauddud is among the few unit names that have been imported to the stratigraphy of Iraq. The formation was first described by Henson (1940, unpublished report) from the Dukhan-1 well in Qatar, and later was amended by Owen and Nasr (1958). The formation has been found also in Burgan-10 well in Kuwait. Al-Shamlan (1975) studied the formation in detail from some of the Kuwaiti oilfields.

According to Owen and Nasr (1958), the formation consists of 110-168 m of “organic, detrital, locally pseudo-oolitic limestones broken by local green or bluish shale”. In most of the southern Iraqi oilfields, the Mauddud Formation consists of alternating orbitolinid-bearing limestone with dolomitic limestone and dolomite. Extensive dolomitization of the formation has significantly obliterated its original depositional textures and makes interpretation of the depositional setting difficult (Al-Shididi et al, 1995).

Because of its high shale content the Nahr Umr Formation is not a major oil reservoir in the Mesopotamian basin, as its equivalent Burgan Formation is in Kuwait. Oil, however, was tested in some of the giant fields such as Nahr Umr, Ratawi, Rachi, and Majnoon (Alsharhan, 1994). This oil may be generated from indigenous source beds from the relatively thick shale units or from the underlying shales of the Zubair Formation. The full potential of this formation is not fully investigated, because most of the exploration in the Mesopotamian Basin is still directed toward major structures, whereas the probability of stratigraphic entrapment in the sandstone units of the formation and/or fractured reservoirs in the shales is high.

Although the Mauddud Formation is a good reservoir in northern Iraq (e.g., Kirkuk and Kabbaz fields), its oil potential in the Mesopotamian Basin is relatively low and restricted at present to the Ratawi Field and possibly to the small field of Badra on the Iraq-Iran border. In Ratawi Field, the Mauddud consists of foraminiferAl-alga I limestones and dolomites. Two reservoir units are separated by massive limestone. The average porosity of each reservoir is around 18%, and some of the pores are filled with heavy oil (Al-Sakini, 1992).

Cenomanian-Lozver Turonian Cycle

Activation of the basement faults and differential movement of fault blocks led to prominent horizontal variation in sedimentary deposits in the Mesopotamian Basin (Fig. 2). The upper boundary of this sequence is a well-developed unconformity in the Lower Turonian, and the lower contact shows a visible change in facies from the Mauddud carbonates to the Ahmadi black fissile shales. During the same time, the “Third Clastic Front” was represented by the Rutbah Sandstone Formation in western Iraq and the Ahmadi Shale in the Mesopotamian Basin. This front is, however, of a smaller scale than the previous ones represented by the Zubair and Nahr Umr Formations. A sequence stratigraphic scheme for the Upper Albian-Lower Turonian is here proposed (Fig. 8).

Fig. 8.

—The stratigraphic relations of the Cenomartian-Lower Turanian cycle in the Mesopotamian Basin (after Aqrawi and Khaiwka, 1986, modified after Chatton and Hart, 1961).

Fig. 8.

—The stratigraphic relations of the Cenomartian-Lower Turanian cycle in the Mesopotamian Basin (after Aqrawi and Khaiwka, 1986, modified after Chatton and Hart, 1961).

The Mishrif Formation rudist-bearing carbonates were deposited together with the equivalent Gir Bir Formation in the north, following deposition of the sub-basinal, transgressive Rumaila Formation chalky and marly limestones in the Mesopotamian Basin and the basinal carbonates of the Dokan Formation in the north. The latter were deposited in the deeper eastern and intrabasinal parts of the same basin. The Balambo Formation was still being deposited in the bathyal parts of the two basins. The intertonguing of the Mishrif and Rumaila facies in the unstable central parts of the Mesopotamian Basin led some earlier workers to rename their equivalents in these areas the Mahilban, Fahad, and Maotsi formations (Aqrawi, 1983; Sherwani, 1983; Aqrawi and Khaiwka, 1986, 1989; Sherwani and Aqrawi, 1987; Aqrawi et al., 1998). Chatton and Hart (1961) proposed a preliminary model for the deposition of the whole Cenomanian-Lower Turonian cycle (Fig. 8), which was later modified by Aqrawi and Khaiwka (1986).

The Mishrif Formation consists of two large-scale regressive cycles separated by an intraformational unconformity (Reulet, 1982; Aqrawi et al., 1998), particularly on the eastern side of the Mesopotamian Basin near the Iranian border, such as in Amarah Field (Fig. 9). This unconformity can be correlated with the intraformational shale beds of the Mishrif in the southwestern parts of the basin, such as in the Nasiriyya (Kareem et al., 1988) (Fig. 10) and Gharraf (Aqrawi et al., 1998) fields. Each of the two main regressive cycles of the Mishrif Formation can be subdivided into three medium-scale cycles or sequences, particularly in the eastern side of the basin, such as in the Amarah, Halfaya, and Rafidain fields. Almost all the later coarsening-up cycles consist of one reservoir pay zone represented by rudstone and/ or rudistid grainstone/packstone facies.

Fig. 9.

—Two different-scale sedimentary cycles of the Mishrif Formation in the well Amarah-1 (well Am-1 in Fig. 1).

Fig. 9.

—Two different-scale sedimentary cycles of the Mishrif Formation in the well Amarah-1 (well Am-1 in Fig. 1).

Fig. 10.

—Depositional environments of the Mishrif Formation in the Nasiriyya area, referred as well Ns-1 in Fig. 1 (modified after Kareem et al., 1988).

Fig. 10.

—Depositional environments of the Mishrif Formation in the Nasiriyya area, referred as well Ns-1 in Fig. 1 (modified after Kareem et al., 1988).

The rudist-bearing carbonates of the Mishrif Formation are the principal reservoir facies in the Mesopotamian Basin (Al-Khersan, 1975; Aqrawi et al., 1998). Dominated by these coarser-grained facies, the Mishrif Formation provides better reservoir zones in the eastern fields, particularly those located in the shallow, uplifted areas, which are also characterized by high subsidence rates along the Amarah paleo-high (Aqrawi et al., 1998). The hydrocarbons in the Mishrif could be sourced partly from the underlying and basinal equivalent Rumaila Formation marly limestones, as well as the deeper carbonates of the Dokan and Balambo Formations. Tight muddy layers in the Mishrif Formation provide local seals for these reservoir units and the overlying Kifel Formation evaporites where present. The sub-basinal Khasib Formation provides the regional seal. The Mishrif petroleum system is one of the unique systems in the whole Arabian Gulf region, but particularly in the Mesopotamian Basin from southern Iraq to the Hemreen Mountains (Fig. 1A). Reservoirs are characterized by high porosities (20% average) and permeabilities (average of 200 md, but may reach up to > 1 d). They contain about 40% of the Cretaceous and up to 30% of the total Iraq oil reserves, which are characterized by 26-28° API (Al-Sakini, 1992).

Upper Turonian-Lower Campanian Cycle

The three formations constituting this cycle, the Khasib, Tanuma, and Sa’di formations, are bounded by two recognizable unconformities in the Mesopotamian Basin, and they may represent a single sequence in the Upper Cretaceous Aruma Group (Aqrawi, 1996). The sub-basinal Oligostegina facies of the Khasib Formation carbonates formed in the central and southern parts ofIraq during the Turonian after a considerable phase of nondeposition or erosion in early Turonian. A thin calcareous shaly unit of the Tanuma Formation was followed by oolitic grainstone reservoirs (Aqrawi, 1996). A thick Sa’di Formation dominated by wackestone and packstone facies overlies the shaly Tanuma throughout the basin.

The major cycle consists of a mixed siliciclastic-carbonate depositional system in a tectonically unstable middle-shelf environment. The basin configuration was subjected to various changes during deposition of this sequence, which are reflected in the isopach maps of the three formations (Aqrawi, 1996). At least five medium-scale and three large-scale coarsening-up accommodation cycles can be recognized in this major cycle (Fig. 11). Some of the smaller cycles are continuous through adjacent formations. The presence of only detrital illite and kaolinite in the three formations may indicate a tropical or subtropical climate during their deposition (Aqrawi, 1996). This major sequence consists of several carbonate reservoir zones represented by porous oolitic grainstone and skeletal packstone facies, particularly in the Tanuma and Khasib formations, and mainly occurs in the oilfields of Central Iraq, such as the East Baghdad Field. The Tanuma also provides reservoirs in a few southern oilfields. The Khasib and Tanuma carbonate reservoirs account for up to 14% of the Cretaceous (and about 10% of the whole Iraq) oil reserves, and have porosities of up to 30% (Al-Sakini, 1992). The interbedded shale units provide the local seals, but they are not proved as source rocks. The extensive shaly Tanuma provides a regional seal for the Khasib reservoirs, and the tight basinal carbonate facies of the lower parts of the Sa’di Formation may provide a regional seal for the Tanuma reservoirs in addition to the Tanuma shaly beds.

Fig. 11.

—Sedimentary cycles of the Turonian-lower Campanian sequence in the Mesopotamian Basin.

Fig. 11.

—Sedimentary cycles of the Turonian-lower Campanian sequence in the Mesopotamian Basin.

Campanian-Maastrichtian Cycle

The Campanian-Maastrichtian sequence is bounded above by the upper Maastrichtian unconformity, which separates the Cretaceous from the Tertiary in most parts of Iraq. Block faulting of the shelf, which started during the deposition of the previous two cycles (i.e., cycles 5 and 6), was well advanced during this cycle. Rudist patch reefs of the Hartha Formation in the Mesopotamian Basin and the Aqra and Bekhme formations in the northern parts of Iraq developed on the horsts, while deep water carbonates represented by the Shiranish Formation developed in between (Figs. 12 and 13).

Fig. 12.

—Structurally controlled facies development during the Campanian-Maastrichtian. The rudist reefs of the Hartha, Aqra, and Bekhme formations grow on the horsts, whereas the muddy carbonate facies of the Shiranish Formation accumulated in deep grabens, after an intensive period of block-faulting movements started in the Cenomanian.

Fig. 12.

—Structurally controlled facies development during the Campanian-Maastrichtian. The rudist reefs of the Hartha, Aqra, and Bekhme formations grow on the horsts, whereas the muddy carbonate facies of the Shiranish Formation accumulated in deep grabens, after an intensive period of block-faulting movements started in the Cenomanian.

Fig. 13.

—Stratigraphic relationships of the main upper Campanian-Maastrichtian carbonate facies in the Mesopotamian Basin (modified from Chatton and Hart, 1961).

Fig. 13.

—Stratigraphic relationships of the main upper Campanian-Maastrichtian carbonate facies in the Mesopotamian Basin (modified from Chatton and Hart, 1961).

The Hartha Formation consists of two major informal sequences, the upper and lower Hartha. Sadooni (1996) divided the Hartha in central Iraq into eight petrographic and petrophysical units. The formation consists of shallow-water foraminiferal shoal and rudist-biostromal deposits, separated by deeper-water muddy carbonates. The Hartha is underlain in some areas by the Safawi Formation evaporites (Al-Naqib, 1967) and overlain by the deep-water sediments of the Shiranish Formation. This, in turn, is overlain by the shelf carbonates of the Tayarat Formation. The sequence terminated after the deposition of the Tayarat, which represents the last phase of Cretaceous deposition before the unconformity separating the Upper Cretaceous Aruma Group from the Tertiary Umm er Radhuma Formation.

According to Al-Sakini (1992), the sediments of this sequence contain less than 2% of the Cretaceous oil reserves in Iraq. The Hartha Formation produced around 700 barrels of oil from the Merjan-1 well in northwest Iraq, with encouraging oil shows in the northern extensions of the East Baghdad and Ahdab fields. This oil may be generated from the deep-water marly limestone of the Shiranish Formation, particularly from the deeper parts of the basin eastward, where these sediments matured enough to generate oil.

Potential New Cretaceous Plays

Iraq’s largest hydrocarbon reserves are hosted in the Cretaceous sediments, particularly in the Mesopotamian Basin (Al-Sakini, 1992; Ibrahim, 1996). The Cretaceous formations in the Mesopotamian Basin are a unique petroleum system, and are among the richest in the world. Although clastic and carbonate reservoirs, together with their regional and local seals, are found throughout the Cretaceous, the source rocks in many cases are either Lower Cretaceous or Upper Jurassic shale and marly formations (Al-Haba and Abdulla, 1989).

Although the Cretaceous hydrocarbon reserves of Iraq could be increased dramatically by updating the usage of development techniques (Al-Gailani, 1996), exploration of new plays in the Cretaceous system remains high, particularly if attention is diverted from the major structures and structural traps and focused on new geographic areas and stratigraphic traps.

The Yamama Formation is perhaps one of the most promising carbonate reservoirs, because of its wide geographic distribution over most parts of southern Iraq and the presence of a relatively thick section of porous oolitic and skeletal limestones. During the last two decades, the Yamama has provided substantial additions to Iraqi reserves in some southern fields where production was formerly from the younger Mishrif Formation, such as West Qurna, or from new fields, such as Nasiriyya.

Both of the major clastic reservoir formations, the Zubair and Nahr Umr Formations, were deposited in complex continental systems with marine influence. A better understanding of these systems requires high-resolution seismic surveys to help define their geometry and architecture. Such surveys will probably reveal the presence of potential reservoirs of stratigraphic entrapment that contain large quantities of oil away from the normal large structures already producing or those explored earlier.

Possible stratigraphic traps can be found within the rudist buildups and grainstones of the Mishrif Formation in areas outside the usual structures. Delineation of such porous units requires high-resolution seismic surveys accompanied by detailed sedi-mentological work on some of the exploration wells to determine the extension of the reefal bodies extending beyond the structures.

During the last few years, the Sa’di, Tanuma, and Khasib formations have produced oil in central parts of Iraq such as in the East Baghdad Field. The Hartha Formation produced oil from West Iraq that is located outside the usual oil province in the central and the southern parts of the country. The Mauddud Formation also has a high oil potential in some of the southeastern structures on the Iraq-Iran border. More studies are needed in these areas to explore such potential fully.

Finally, some non-reservoir formations may become reservoirs in other unexplored areas, such as the Ahmadi limestone member in the restricted Iraqi offshore territory, as is the case in Kuwaiti-Saudi Arabian offshore. Extension of the exploration efforts to such areas may succeed in finding new plays.

Summary And Conclusions

Cretaceous sedimentation in the Mesopotamian Basin of Central and Southern Iraq was controlled by local tectonic activity (particularly basement-block displacement and diapirism), and other regional tectonics such as the culmination of Neo-Tethys rifting, in addition to global eustatic sea-level changes. Although tropical and subtropical climate dominated most of the Cretaceous, some recognizable dry periods are well reflected by the deposition of evaporites such as the Kifel and Safawi formations.

This preliminary Cretaceous lithosrratigraphic study shows that the Cretaceous formations in the Mesopotamian Basin can be divided into the same three main groups (Thamama, Wasia, and Aruma of Lower, middle and Upper Cretaceous, respectively) used in the whole Arabian Gulf region. In addition, the formations can be further grouped into seven major cycles named according to their time interval of deposition. Some of these major cycles are further subdivided into large- and medium-scale accommodation cycles where additional detailed sedimentological and paleontological data are available.

The Sulaiy and Yamama Formations were deposited on a carbonate ramp and formed the first Cretaceous major cycle. From the middle Valanginian, the ramp underwent flooding of elastics from the uplifted areas in the southwest. Initial stages of this are represented by the thin siltstone and sandstone units of the Ratawi Formation. Clastic deposition reached its peak, creating a major clastic-dominated cycle, with the deposition of the Zubair Formation during the Hautervian-Barremian. These elastics extended as far as the Kirkuk Field in northeastern Iraq.

The regional clastic influx, which covered most parts of the Arabian Plate, was terminated with the establishment of a new carbonate shelf in the Aptian, when the Shuaiba Formation was deposited, creating the third and final major sequence of the Lower Cretaceous.

After a period of a nondeposition or erosion represented by a well recognized, regional unconformity over large parts of the Middle East, the shelf was once again subjected to a significant clastic influx represented by the Albian Nahr Umr Formation, followed by deposition of Mauddud carbonates. During the Cenomanian-early Turonian after deposition of the transgres-sive shales and chalky and marly limestones of the Ahmadi and Rumaila Formations, respectively, another major sequence characterized by rudist reefs and related buildups of the Mishrif Formation dominated the shelf areas. Deposition of this major cycle ended in arid climatic conditions, as seen by the Kifel evaporites, followed by a period of erosion or nondeposition during early Turonian. Local tectonic influence is clearly reflected in the large- and medium-size accommodation cycles.

Deposition of the Khasib, Tanuma, and Sa’di formations, as a complete carbonate-clastic sequence, began during the Turonian and lasted until the Campanian. The climate changed again to tropical. Strong tectonic activity was initiated during late middle Cretaceous (i.e., during deposition of the Mishrif Formation) and led to the development of a block-faulted shelf during the Campanian-Maastrichtian. Rudist patch reefs of the Hartha Formation and its equivalents flourished on highs, while deep-water marly limestones of the Shiranish Formation occupied the low regions of the basin. Cretaceous deposition was terminated during the Maastrichtian by regional erosion after the deposition of the Tayarat Formation carbonates. This uppermost unconformity separates the Cretaceous from the Tertiary sequences.

The Cretaceous petroleum system in the Mesopotamian Basin is world ranking in terms of size. Stacked reservoirs are common, created by both local and regional seals, and sourced mainly by hydrocarbons generated in the Lower Cretaceous or Upper Jurassic. Although the Cretaceous system is relatively well explored by traditional methods in the Mesopotamian Basin, it still provides new hydrocarbon prospect potential, both outside the basin and in nonstructural plays that have not yet been proven.

New plays may be found in the Yamama Formation in other old fields that are producing now from the middle and Upper Cretaceous or from new fields especially in the Basrah-Amarah-Naseriyya triangle. Stratigraphic traps are expected to be found in the carbonate system of Mishrif as well as in the clastic systems of Zubair and Nahr Umr. Formations such as the Ahmadi, Khasib, Mauddud, and Hartha may prove to be good reservoirs in areas outside the present “conventional” exploration zones, as demonstrated by the Hartha Formation in central and western Iraq or the Mauddud Formation along the Iraq-Iran border.

References

Ala
,
M.A.
,
1974
,
Salt diapirism In Southern Iran
:
American Association of Petroleum Geologists, Bulletin
 , v.
58
, p.
1758
1770
.
Al-Gailani
,
M.
1996
,
Iraq’s significant hydrocarbon potential remains relatively undeveloped
:
Oil and Gas Journal
 ,
July
29
1996
, p.
108
111
.
Al-Haba
,
Y.Q.
Abdullah
,
M.
1989
,
A geochemical study of hydrocarbon source rocks in northern Iraq
:
Oil and Arabic Co-operation Journal
 , v.
15
, p.
11
15
(in Arabic).
Ali
,
A.J.
,
Aziz
,
Z.R.
1993
,
The Zubair Formation, East Baghdad Oilfield, central Iraq
:
Journal of Petroleum Geology
 , v.
16
, p.
353
364
.
Al-Khersan
,
H.
1975
, Depositional environments and geological history of the Mishrif Formation in southern Iraq:
Ninth Arab Petroleum Congress
 ,
Dubai
,
United Arab Emirates
, paper no. 121 (B-3), p.
1
18
.
Al-Naqib
,
K.M.
1967
,
Southwest Iraq
:
U.S. Geological Survey
 , Professional Paper 560-G,
54
p.
Al-Sakini
,
J.
1992
, Summary of the petroleum geology of Iraq and the Middle East:
Naft-Al-shamal Press
 ,
Kirkuk
,
180
p. (in Arabic).
Al-Shamlan
,
A.A.
1975
, Petrographic and microfacies analysis of the Mauddud Formation in Kuwait:
Ninth Arab Petroleum Congress
 ,
Dubai
,
United Arab Emirates
, paper 126 (B-3),
34
p.
Alsharhan
,
A.S.
1991
,
Sedimentological interpretation of the Albian Nahr Umr Formation in the United Arab Emirates
:
Sedimentary Geology
 , v.
73
, p.
317
327
.
Alsharhan
,
A.S.
1994
,
Albian elastics in the western Arabian Gulf region: a sedimentological and petroleum-geological interpretation
:
Journal of Petroleum Geology
 , v.
17
, p.
279
300
.
Alsharhan
,
A.S.
1995
,
Facies variation, diagenesis, and exploration potential of the Cretaceous rudist-bearing carbonates of the Arabian Gulf
:
American Association of Petroleum Geologists, Bulletin
 , v.
79
, p.
531
550
.
Alsharhan
,
A.S.
,
Kendall
,
C.G.St.C
1991
,
Cretaceous chrono-stratigraphy, unconformities and eustatic sea-level changes in the sediments of Abu Dhabi, United Arab Emirates
:
Cretaceous Research
 , v.
12
, p.
379
401
.
Alsharhan
,
A.S.
,
Kendall
,
C.G.St.C
1995
,
Facies variation, depositional setting and hydrocarbon potential of the Upper Cretaceous rocks in the United Arab Emirates
:
Cretaceous Research
 , v.
16
, p.
435
449
.
Alsharhan
,
A.S.
Nairn
,
A.E.M.
1986
,
A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part I. Lower Cretaceous (Thamama Group), stratigraphy and paleogeography
:
Journal of Petroleum Geology
 , v.
9
, p.
365
392
.
Alsharhan
,
A.S.
,
Nairn
,
A.E.M.
1988
,
A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part II. Mid-Cretaceous (Wasia Group), stratigraphy and paleogeography
:
Journal of Petroleum Geology
 , v.
11
, p.
89
112
.
Alsharhan
,
A.S.
,
Nairn
,
A.E.M.
1990
,
A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part III. Upper Cretaceous (Aruma Group) stratigraphy and paleogeography
:
Journal of Petroleum Geology
 , v.
13
, p.
247
266
.
Alsharhan
,
A.S.
,
Nairn
,
A.E.M.
1993
, Carbonate platform models of Arabian Cretaceous reservoirs, in
Simo
,
J.A.T.
Scott
,
R.W.
Masse
,
J.-P.
eds, Cretaceous Carbonate Platforms:
American Association of Petroleum Geologists
 ,
Memoir
56, p.
173
184
.
Al-Shididi
,
S.
Thomas
,
G.
Delfaud
,
J.
1995
,
Sedimentology, diagenesis, and oil habitat of Lower Cretaceous Qamchuqa Group, Northern Iraq
:
American Association of Petroleum Geologists, Bulletin
 , v.
79
, p.
763
779
.
Al-Siddiki
,
A.A.
1978a
, Subsurface geology of southeastern Iraq:
Tenth Arab Petroleum Congress
 ,
Tripoli
,
Libya
,
47
p.
Al-Siddiki
,
A.A.
,
1978b
, Yamama’s oil: a big exploration project executed by national staffs:
Iraqi National Oil Company, First Science Conference
 ,
Baghdad
, Iraq,
23
p. (in Arabic).
Aqrawi
,
A.A.M.
1983
,
Depositional environment and stratigraphy of the Rumaila Formation and equivalents in selected boreholes, middle and southern Iraq
:
unpublished M. Sc. thesis
 ,
University of Baghdad
, 115 p. (in Arabic).
Aqrawi
,
A.A.M.
1996
,
Carbonate-siliciclastic sediments of the Upper Cretaceous (Khasib, Tanuma and Sa’di Formations) of the Mesopotamian Basin
:
Marine and Petroleum Geology
 , v.
13
, p.
781
790
.
Aqrawi
,
A.A.M.
Kareem
,
B.A.
Al-Rekabi
,
Y.R.
1988
, Diagenetic environments of a shelf carbonate sequence (Mishrif Formation) of middle Cretaceous, southern Iraq:
General Management For Reservoir Development, Ministry of Oil
 ,
Baghdad
, Iraq (unpublished report).
Aqrawi
,
A.A.M.
Khajwka
,
M.H.
1986
,
Depositional environment ol Rumaila Formation (Cenomanian) in selected boreholes, central and southern Iraq
:
Geological Society of Iraq, Journal
 , v.
19
, p.
77
95
.
Aqrawi
,
A. A.M.
Khaiwka
,
M.H.
1989
,
Microfacies analysis of Rumaila Formation and equivalents (Cenomanian) in Mesopotamian basin, a statistical approach
:
Journal of the University of Kuwait (Science)
 , v.
16
, p.
143
153
.
Aqrawi
,
A. A.M.
Thehni
,
G.A.
Sherwani
,
G.H.
Karhem
,
B.M.A.
1998
,
Mid-Cretaceous rudist-bearing carbonates of the Mishrif Formation: an important reservoir sequence in the Mesopotamian Basin, Iraq
:
Journal of Petroleum Geology
 , v.
21
, p.
57
82
.
Beydoun
,
Z.R.
1991
,
Arabian plate hydrocarbon geology and potential— a plate tectonic approach
:
American Association of Petroleum Geologists, Studies in Geology
  no.
33
,
77
p.
Beydoun
,
Z.R.
1993
,
Evolution of the Northeastern Arabian Plate margir and shelf: Hydrocarbon habitat and conceptual future potential
:
Institue Français du Pétrole
 , Revue, v.
48
, p.
311
345
.
Buday
,
T.
1980
,
The Regional Geology of Iraq; Vol. 1, Stratigraphy and Palaeogeography
:
Mosul, Iraq, Dar Al-Kutub Publication House
 ,
445
p.
Buday
,
T.
Jassim
,
S.Z.
1987
, The Regional Geology of Iraq. Vol. 2, Tectonism, Magmatism and Metamorphism:
Geological Survey and Mineral Investigation
 ,
Baghdad
,
Iraq
,
351
p.
Burchette
,
T.P.
1993
, Mishrif Formation (Cenomanian-Turonian), southern Arabian Gulf:
carbonate platform growth along a cratonic basir margin
 , in
Simo
,
J.A.
Scott
,
R.W.
Masse
,
J.-P.
eds, Cretaceous Carbonate Platforms:
American Association of Petroleum Geologists Memoir 56
 , p.
185
199
.
Chatton
,
M.
Hart
,
E.
1960
, Revision of the Tithonian-to-Albiar stratigraphy of Iraq:
Iraq Petroleum Company unpublished report Oil Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1961
, Review of the Cenomanian tc Maastrichtian stratigraphy in Iraq, The Cenomanian Cycle:
Irac Petroleum Company unpublished report, Oil Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1962a
, Announcement of a rock unit redefined Ratawi Formation:
Iraq Petroleum Company unpublished report, Oi Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1962b
, Review of the Cenomanian tc Maastrichtian stratigraphy in Iraq, the Upper Campanian-Maastrichtian cycle:
Iraq Petroleum Company unpublished report Oil Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1962c
, Announcement of a new rock unit, Kife Formation:
Iraq Petroleum Company unpublished report, Oil Explo ration Company archive
 ,
Baghdad
,
Iraq
.
Dunnington
,
H. V.
1967
,
Stratigraphic distribution of oilfields in the Iraq-Iran-Arabia Basin
:
Institute of Petroleum, Journal
 , v.
53
, p.
129
161
Fuloria
,
R.C.
1976
, Petroleum prospects analysis of southern Iraq, with particular reference to Yamama Formation:
Iraq National Oil Com pany unpublished report, Southern Petroleum Organization, Geo logical Laboratories
 ,
Basrah
,
Iraq
.
Gaddo
,
J.
1961
, Announcement of a new rock unit, Batiwah Formation:
Iraq Petroleum Company unpublished report, Oil Exploration Com pany archive
 ,
Baghdad
,
Iraq
.
Gaddo
,
J.
1971
,
The Mishrif Forma tionpaleoenvironment in the Rumaila/ Tuba/Zubair region of south Iraq
:
Geological Society of Iraq, Journal
 , v.
4
, p.
1
12
.
Hamdan
,
A.R.A.
Alsharhan
,
A.S.
1991
,
Palaeoenvironments and palaeoecology of the rudists in the Shuaiba Formation (Aptian) United Arab Emirates
:
Journal of African Earth Sciences
 , v.
12
, p.
569
581
.
Haq
,
B.U.
,
Hakdenbol
,
J.
Vail
,
P.R.
1987
,
Chronology of fluctuating sea levels since the Triassic (250 million years to present)
:
Science
 , v.
235
, p.
1156
1167
.
Harris
,
P.M.
,
Frost
,
S.H.
Seiglie
,
G.A.
Schneidermann
,
N.
1984
, Regional unconformities and depositional cycles, Cretaceous of the Arabian Peninsula, in
Schlee
,
J.S.
ed., Interregional Unconformities and Hydrocarbon Accumulation:
American Association of Petroleum Geologists, Memoir 36
 , p.
67
80
.
Ibrahim
,
M.W.
1981
,
Lithostratigraphy and subsurface geology of the Albian rocks of southern Iraq
:
Journal of Petroleum Geology
 , v.
4
, p.
147
162
.
Ibrahim
,
M.W.
1983
,
Petroleum geology of Iraq
:
American Association of Petroleum Geologists, Bulletin
 , v.
67
, p.
97
130
.
Ibrahim
,
M.W.
1984
,
Geothermal gradients and geothermal oil generation in southern Iraq
:
A preliminary investigation: Journal of Petroleum Geology
 , v.
7
, p.
77
86
.
Ibrahim
,
M.W.
1996
,
Study sizes up Iraq’s reserves, exploration status, production potential
:
Oil and Gas Journal
 ,
June
24
1996
, p.
53
55
.
Jassim
,
S.Z.
Karim
,
S.
Basi
,
M.
Al-Mubarak
,
M.
Al-Munir
,
M.
1984
, The final report on the regional geological survey of Iraq, 3, stratigraphy:
Geological Survey and Mineral Investigation, unpublished report, Oil Exploration Company archive
 ,
Iraq.
Kareem
,
B.M. A.
Aqrawi
,
A. A.M.
Shubbar
,
B.A.
1988
,
Sedimentologi-cal and environmental study of the Mishrif Formation in Well Ns-2, Nasiriyya area, southern Iraq
:
General Management For Reservoir Development, Ministry of Oil, Baghdad, Iraq (unpublished report in Arabic).
 
Kendall
,
C.G.St.C
Bowen
,
B.
Alsharhan
,
A.
Cheong
,
D.K.
Stoudt
,
D.
1991
,
Eustatic controls on carbonate facies in reservoirs, and seals associated with Mesozoic hydrocarbon fields of the Arabian Gulf and the Gulf of Mexico
:
Marine Geology
 , v.
102
, p.
215
238
.
Loucks
,
R.G.
,
Sarg
,
J.F.
eds.,
1993
,
Carbonate Sequence Stratigraphy: Recent Developments and Applications
:
American Association of Petroleum Geologists, Memoir 57
 ,
545
p.
Murris
,
R.J.
1980
,
Middle East: Stratigraphic evolution and oil habitat
:
American Association of Petroleum Geologists, Bulletin
 , v.
64
, p.
597
618
.
Owen
,
R.M.S.
Nasr
,
S.N.
1958
, Stratigraphy of the Kuwait-Basrah areas,in
Weeks
,
L.G.
ed., Habitat of Oil, A Symposium:
American Association of Petroleum Geologists
 , p.
1252
1278
.
Powers
,
R.W.
1968
,
Saudi Arabia
:
Lexique Stratigraphique International, HI, Asie, Fasc. 10a., Centre National de la Recherché Scientifique
 ,
177
p.
Powers
,
R.W.
Ramirez
,
L.F.
Redmond
,
CD.
Elberg
,
EX.
, Jr.,
1966
,
Sedimentary geology of the Saudi Arabia
:
U.S. Geological Survey, Professional Paper 560-D
 ,
147
p.
Redmond
,
CD.
1964
,
Lituolid foraminifera from the Jurassic and Cretaceous of Saudi Arabia
:
Micropaleontology
 , v.
10
, p.
405
414
.
Reulet
,
J.
1982
, Carbonate reservoir in a marine shelf sequence, Mishrif Formation, Cretaceous of the Middle East, in
Reeckman
,
A.
Friedman
,
G.M.
eds., Exploration for Carbonate Platform Reservoirs:
New York
,
John Wiley & Sons
, p.
165
173
.
Sadooni
,
F.N.
1978
,
Sedimentology and petroleum prospects of the Lower Cretaceous Qamchuqa Group, northern Iraq
: unpublished Ph.D. thesis,
University of Bristol
,
UK
,
376
p.
Sadooni
,
F.N.
1993
,
Stratigraphic sequence, microfacies and petroleum prospects of the Yamama Formation, Lower Cretaceous, southern Iraq
:
American Association of Petroleum Geologists, Bulletin
 , v.
77
, p.
1971
1988
.
Sadooni
,
F.N.
1995a
,
Diagenetic features of some subsurface Tertiary-Cretaceous evaporites from northern Iraq
:
Carbonates and Evapor-ites
 , v.
10
, p.
45
53
.
Sadooni
,
F.N.
1995b
, Applications of borehole logs in depositional and diagenetic facies characterization of carbonate reservoirs:
First Indian Conference on Petroleum Geophysics, Dehradun
 ,
India
, Proceedings,
17
p.
Sadooni
,
F.N.
1996
,
Stratigraphic and lithological characteristics of Upper Cretaceous carbonates in central Iraq
:
Journal of Petroleum Geology
 , v.
19
, p.
271
288
.
Sadooni
,
F.N.
1997
,
Stratigraphy and petroleum prospect of Upper Jurassic carbonates in Iraq
:
Petroleum Geoscience
 , v.
3
, p.
233
243
.
Sadooni
,
F.N.
1998
,
Cretaceous rudist-bearing carbonate belt of the Arabian Platform: a probable indicator of a regional climatic front
:
(abstract) ENERGEX’98, Bahrain.
 
Sherwani
,
G.H.
1983
,
Depositional environments and stratigraphic relationships of the Mishrif Formation in selected boreholes, middle and southern Iraq
: unpublished MSc thesis,
University of Baghdad
,
Baghdad
(in Arabic).
Sherwani
,
G.H.
,
Aqrawi
,
A.A.M.
1987
,
Lithostratigraphy and environmental considerations of Cenomanian-early Turonian shelf carbonates (Rumaila and Mishrif Formations) of Mesopotamian basin, central and southern Iraq (abstract)
:
American Association of Petroleum Geologists, Bulletin
 , v.
71
, p.
614
.
Stocklik
,
J.
1972
,
Iran
:
Lexique Stratigraphique International, F. 9b, Asie, Centre National Recherche Scientifique, Paris
 ,
376
p.
Sugden
,
W.
Standring
,
J.A.
1975
, Qatar Peninsula:
Lexique Stratigraphique International, F 10b3, Asie, Centre National Recherche Scientifique
 ,
Paris
,
127
p.
Vail
,
P.R.
,
Mitchum
,
R.M.
, Jr
Todd
,
R.G.
Widmier
,
J.M.
Thompson
,
S., III
Sangree
,
J.B.
Bubb
,
J.N.
Hatlelid
,
W.G.
1977
, Seismic stratigraphy and global changes of sea level, in
Mitchum
,
R.M.
Vail
,
P.R.
Sangree
,
J.B.
eds, Seismic Stratigraphy—Applications to Hydrocarbon Exploration:
American Association of Petroleum Geologists, Memoir 26
 , p.
49
212
.
Bellen
,
R.C. Van
Dunnington
,
H.V.
Wetzel
,
R.
Morton
,
D.M.
1959
, Iraq:
Lexique Stratigraphique International, F 10a, Asie, Centre National Recherche Scientifique
 ,
Paris
,
333
p.
Wagoner
,
J.C. Van
Mitchum
,
R.M.
Campion
,
K.M.
Rahmanian
,
V.D.
1990
,
Siliciclastic sequence stratigraphy in well logs, cores, and outcrops: concepts for high resolution correlation of time and facies
:
American Association of Petroleum Geologists, Methods in Exploration Series 7
 ,
55
p.

Acknowledgments

The authors would like to acknowledge the two reviewers, Drs. A.J. Lomando (Chevron Overseas) and CM. Walkden (University of Aberdeen), for their valuable suggestions and notes. Thanks are also due to Anne Keth Nyvoll (of Smedvig Technologies, Stavanger) for redrafting the figures.

Figures & Tables

Contents

GeoRef

References

References

Ala
,
M.A.
,
1974
,
Salt diapirism In Southern Iran
:
American Association of Petroleum Geologists, Bulletin
 , v.
58
, p.
1758
1770
.
Al-Gailani
,
M.
1996
,
Iraq’s significant hydrocarbon potential remains relatively undeveloped
:
Oil and Gas Journal
 ,
July
29
1996
, p.
108
111
.
Al-Haba
,
Y.Q.
Abdullah
,
M.
1989
,
A geochemical study of hydrocarbon source rocks in northern Iraq
:
Oil and Arabic Co-operation Journal
 , v.
15
, p.
11
15
(in Arabic).
Ali
,
A.J.
,
Aziz
,
Z.R.
1993
,
The Zubair Formation, East Baghdad Oilfield, central Iraq
:
Journal of Petroleum Geology
 , v.
16
, p.
353
364
.
Al-Khersan
,
H.
1975
, Depositional environments and geological history of the Mishrif Formation in southern Iraq:
Ninth Arab Petroleum Congress
 ,
Dubai
,
United Arab Emirates
, paper no. 121 (B-3), p.
1
18
.
Al-Naqib
,
K.M.
1967
,
Southwest Iraq
:
U.S. Geological Survey
 , Professional Paper 560-G,
54
p.
Al-Sakini
,
J.
1992
, Summary of the petroleum geology of Iraq and the Middle East:
Naft-Al-shamal Press
 ,
Kirkuk
,
180
p. (in Arabic).
Al-Shamlan
,
A.A.
1975
, Petrographic and microfacies analysis of the Mauddud Formation in Kuwait:
Ninth Arab Petroleum Congress
 ,
Dubai
,
United Arab Emirates
, paper 126 (B-3),
34
p.
Alsharhan
,
A.S.
1991
,
Sedimentological interpretation of the Albian Nahr Umr Formation in the United Arab Emirates
:
Sedimentary Geology
 , v.
73
, p.
317
327
.
Alsharhan
,
A.S.
1994
,
Albian elastics in the western Arabian Gulf region: a sedimentological and petroleum-geological interpretation
:
Journal of Petroleum Geology
 , v.
17
, p.
279
300
.
Alsharhan
,
A.S.
1995
,
Facies variation, diagenesis, and exploration potential of the Cretaceous rudist-bearing carbonates of the Arabian Gulf
:
American Association of Petroleum Geologists, Bulletin
 , v.
79
, p.
531
550
.
Alsharhan
,
A.S.
,
Kendall
,
C.G.St.C
1991
,
Cretaceous chrono-stratigraphy, unconformities and eustatic sea-level changes in the sediments of Abu Dhabi, United Arab Emirates
:
Cretaceous Research
 , v.
12
, p.
379
401
.
Alsharhan
,
A.S.
,
Kendall
,
C.G.St.C
1995
,
Facies variation, depositional setting and hydrocarbon potential of the Upper Cretaceous rocks in the United Arab Emirates
:
Cretaceous Research
 , v.
16
, p.
435
449
.
Alsharhan
,
A.S.
Nairn
,
A.E.M.
1986
,
A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part I. Lower Cretaceous (Thamama Group), stratigraphy and paleogeography
:
Journal of Petroleum Geology
 , v.
9
, p.
365
392
.
Alsharhan
,
A.S.
,
Nairn
,
A.E.M.
1988
,
A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part II. Mid-Cretaceous (Wasia Group), stratigraphy and paleogeography
:
Journal of Petroleum Geology
 , v.
11
, p.
89
112
.
Alsharhan
,
A.S.
,
Nairn
,
A.E.M.
1990
,
A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part III. Upper Cretaceous (Aruma Group) stratigraphy and paleogeography
:
Journal of Petroleum Geology
 , v.
13
, p.
247
266
.
Alsharhan
,
A.S.
,
Nairn
,
A.E.M.
1993
, Carbonate platform models of Arabian Cretaceous reservoirs, in
Simo
,
J.A.T.
Scott
,
R.W.
Masse
,
J.-P.
eds, Cretaceous Carbonate Platforms:
American Association of Petroleum Geologists
 ,
Memoir
56, p.
173
184
.
Al-Shididi
,
S.
Thomas
,
G.
Delfaud
,
J.
1995
,
Sedimentology, diagenesis, and oil habitat of Lower Cretaceous Qamchuqa Group, Northern Iraq
:
American Association of Petroleum Geologists, Bulletin
 , v.
79
, p.
763
779
.
Al-Siddiki
,
A.A.
1978a
, Subsurface geology of southeastern Iraq:
Tenth Arab Petroleum Congress
 ,
Tripoli
,
Libya
,
47
p.
Al-Siddiki
,
A.A.
,
1978b
, Yamama’s oil: a big exploration project executed by national staffs:
Iraqi National Oil Company, First Science Conference
 ,
Baghdad
, Iraq,
23
p. (in Arabic).
Aqrawi
,
A.A.M.
1983
,
Depositional environment and stratigraphy of the Rumaila Formation and equivalents in selected boreholes, middle and southern Iraq
:
unpublished M. Sc. thesis
 ,
University of Baghdad
, 115 p. (in Arabic).
Aqrawi
,
A.A.M.
1996
,
Carbonate-siliciclastic sediments of the Upper Cretaceous (Khasib, Tanuma and Sa’di Formations) of the Mesopotamian Basin
:
Marine and Petroleum Geology
 , v.
13
, p.
781
790
.
Aqrawi
,
A.A.M.
Kareem
,
B.A.
Al-Rekabi
,
Y.R.
1988
, Diagenetic environments of a shelf carbonate sequence (Mishrif Formation) of middle Cretaceous, southern Iraq:
General Management For Reservoir Development, Ministry of Oil
 ,
Baghdad
, Iraq (unpublished report).
Aqrawi
,
A.A.M.
Khajwka
,
M.H.
1986
,
Depositional environment ol Rumaila Formation (Cenomanian) in selected boreholes, central and southern Iraq
:
Geological Society of Iraq, Journal
 , v.
19
, p.
77
95
.
Aqrawi
,
A. A.M.
Khaiwka
,
M.H.
1989
,
Microfacies analysis of Rumaila Formation and equivalents (Cenomanian) in Mesopotamian basin, a statistical approach
:
Journal of the University of Kuwait (Science)
 , v.
16
, p.
143
153
.
Aqrawi
,
A. A.M.
Thehni
,
G.A.
Sherwani
,
G.H.
Karhem
,
B.M.A.
1998
,
Mid-Cretaceous rudist-bearing carbonates of the Mishrif Formation: an important reservoir sequence in the Mesopotamian Basin, Iraq
:
Journal of Petroleum Geology
 , v.
21
, p.
57
82
.
Beydoun
,
Z.R.
1991
,
Arabian plate hydrocarbon geology and potential— a plate tectonic approach
:
American Association of Petroleum Geologists, Studies in Geology
  no.
33
,
77
p.
Beydoun
,
Z.R.
1993
,
Evolution of the Northeastern Arabian Plate margir and shelf: Hydrocarbon habitat and conceptual future potential
:
Institue Français du Pétrole
 , Revue, v.
48
, p.
311
345
.
Buday
,
T.
1980
,
The Regional Geology of Iraq; Vol. 1, Stratigraphy and Palaeogeography
:
Mosul, Iraq, Dar Al-Kutub Publication House
 ,
445
p.
Buday
,
T.
Jassim
,
S.Z.
1987
, The Regional Geology of Iraq. Vol. 2, Tectonism, Magmatism and Metamorphism:
Geological Survey and Mineral Investigation
 ,
Baghdad
,
Iraq
,
351
p.
Burchette
,
T.P.
1993
, Mishrif Formation (Cenomanian-Turonian), southern Arabian Gulf:
carbonate platform growth along a cratonic basir margin
 , in
Simo
,
J.A.
Scott
,
R.W.
Masse
,
J.-P.
eds, Cretaceous Carbonate Platforms:
American Association of Petroleum Geologists Memoir 56
 , p.
185
199
.
Chatton
,
M.
Hart
,
E.
1960
, Revision of the Tithonian-to-Albiar stratigraphy of Iraq:
Iraq Petroleum Company unpublished report Oil Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1961
, Review of the Cenomanian tc Maastrichtian stratigraphy in Iraq, The Cenomanian Cycle:
Irac Petroleum Company unpublished report, Oil Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1962a
, Announcement of a rock unit redefined Ratawi Formation:
Iraq Petroleum Company unpublished report, Oi Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1962b
, Review of the Cenomanian tc Maastrichtian stratigraphy in Iraq, the Upper Campanian-Maastrichtian cycle:
Iraq Petroleum Company unpublished report Oil Exploration Company archive
 ,
Baghdad
,
Iraq
.
Chatton
,
M.
Hart
,
E.
1962c
, Announcement of a new rock unit, Kife Formation:
Iraq Petroleum Company unpublished report, Oil Explo ration Company archive
 ,
Baghdad
,
Iraq
.
Dunnington
,
H. V.
1967
,
Stratigraphic distribution of oilfields in the Iraq-Iran-Arabia Basin
:
Institute of Petroleum, Journal
 , v.
53
, p.
129
161
Fuloria
,
R.C.
1976
, Petroleum prospects analysis of southern Iraq, with particular reference to Yamama Formation:
Iraq National Oil Com pany unpublished report, Southern Petroleum Organization, Geo logical Laboratories
 ,
Basrah
,
Iraq
.
Gaddo
,
J.
1961
, Announcement of a new rock unit, Batiwah Formation:
Iraq Petroleum Company unpublished report, Oil Exploration Com pany archive
 ,
Baghdad
,
Iraq
.
Gaddo
,
J.
1971
,
The Mishrif Forma tionpaleoenvironment in the Rumaila/ Tuba/Zubair region of south Iraq
:
Geological Society of Iraq, Journal
 , v.
4
, p.
1
12
.
Hamdan
,
A.R.A.
Alsharhan
,
A.S.
1991
,
Palaeoenvironments and palaeoecology of the rudists in the Shuaiba Formation (Aptian) United Arab Emirates
:
Journal of African Earth Sciences
 , v.
12
, p.
569
581
.
Haq
,
B.U.
,
Hakdenbol
,
J.
Vail
,
P.R.
1987
,
Chronology of fluctuating sea levels since the Triassic (250 million years to present)
:
Science
 , v.
235
, p.
1156
1167
.
Harris
,
P.M.
,
Frost
,
S.H.
Seiglie
,
G.A.
Schneidermann
,
N.
1984
, Regional unconformities and depositional cycles, Cretaceous of the Arabian Peninsula, in
Schlee
,
J.S.
ed., Interregional Unconformities and Hydrocarbon Accumulation:
American Association of Petroleum Geologists, Memoir 36
 , p.
67
80
.
Ibrahim
,
M.W.
1981
,
Lithostratigraphy and subsurface geology of the Albian rocks of southern Iraq
:
Journal of Petroleum Geology
 , v.
4
, p.
147
162
.
Ibrahim
,
M.W.
1983
,
Petroleum geology of Iraq
:
American Association of Petroleum Geologists, Bulletin
 , v.
67
, p.
97
130
.
Ibrahim
,
M.W.
1984
,
Geothermal gradients and geothermal oil generation in southern Iraq
:
A preliminary investigation: Journal of Petroleum Geology
 , v.
7
, p.
77
86
.
Ibrahim
,
M.W.
1996
,
Study sizes up Iraq’s reserves, exploration status, production potential
:
Oil and Gas Journal
 ,
June
24
1996
, p.
53
55
.
Jassim
,
S.Z.
Karim
,
S.
Basi
,
M.
Al-Mubarak
,
M.
Al-Munir
,
M.
1984
, The final report on the regional geological survey of Iraq, 3, stratigraphy:
Geological Survey and Mineral Investigation, unpublished report, Oil Exploration Company archive
 ,
Iraq.
Kareem
,
B.M. A.
Aqrawi
,
A. A.M.
Shubbar
,
B.A.
1988
,
Sedimentologi-cal and environmental study of the Mishrif Formation in Well Ns-2, Nasiriyya area, southern Iraq
:
General Management For Reservoir Development, Ministry of Oil, Baghdad, Iraq (unpublished report in Arabic).
 
Kendall
,
C.G.St.C
Bowen
,
B.
Alsharhan
,
A.
Cheong
,
D.K.
Stoudt
,
D.
1991
,
Eustatic controls on carbonate facies in reservoirs, and seals associated with Mesozoic hydrocarbon fields of the Arabian Gulf and the Gulf of Mexico
:
Marine Geology
 , v.
102
, p.
215
238
.
Loucks
,
R.G.
,
Sarg
,
J.F.
eds.,
1993
,
Carbonate Sequence Stratigraphy: Recent Developments and Applications
:
American Association of Petroleum Geologists, Memoir 57
 ,
545
p.
Murris
,
R.J.
1980
,
Middle East: Stratigraphic evolution and oil habitat
:
American Association of Petroleum Geologists, Bulletin
 , v.
64
, p.
597
618
.
Owen
,
R.M.S.
Nasr
,
S.N.
1958
, Stratigraphy of the Kuwait-Basrah areas,in
Weeks
,
L.G.
ed., Habitat of Oil, A Symposium:
American Association of Petroleum Geologists
 , p.
1252
1278
.
Powers
,
R.W.
1968
,
Saudi Arabia
:
Lexique Stratigraphique International, HI, Asie, Fasc. 10a., Centre National de la Recherché Scientifique
 ,
177
p.
Powers
,
R.W.
Ramirez
,
L.F.
Redmond
,
CD.
Elberg
,
EX.
, Jr.,
1966
,
Sedimentary geology of the Saudi Arabia
:
U.S. Geological Survey, Professional Paper 560-D
 ,
147
p.
Redmond
,
CD.
1964
,
Lituolid foraminifera from the Jurassic and Cretaceous of Saudi Arabia
:
Micropaleontology
 , v.
10
, p.
405
414
.
Reulet
,
J.
1982
, Carbonate reservoir in a marine shelf sequence, Mishrif Formation, Cretaceous of the Middle East, in
Reeckman
,
A.
Friedman
,
G.M.
eds., Exploration for Carbonate Platform Reservoirs:
New York
,
John Wiley & Sons
, p.
165
173
.
Sadooni
,
F.N.
1978
,
Sedimentology and petroleum prospects of the Lower Cretaceous Qamchuqa Group, northern Iraq
: unpublished Ph.D. thesis,
University of Bristol
,
UK
,
376
p.
Sadooni
,
F.N.
1993
,
Stratigraphic sequence, microfacies and petroleum prospects of the Yamama Formation, Lower Cretaceous, southern Iraq
:
American Association of Petroleum Geologists, Bulletin
 , v.
77
, p.
1971
1988
.
Sadooni
,
F.N.
1995a
,
Diagenetic features of some subsurface Tertiary-Cretaceous evaporites from northern Iraq
:
Carbonates and Evapor-ites
 , v.
10
, p.
45
53
.
Sadooni
,
F.N.
1995b
, Applications of borehole logs in depositional and diagenetic facies characterization of carbonate reservoirs:
First Indian Conference on Petroleum Geophysics, Dehradun
 ,
India
, Proceedings,
17
p.
Sadooni
,
F.N.
1996
,
Stratigraphic and lithological characteristics of Upper Cretaceous carbonates in central Iraq
:
Journal of Petroleum Geology
 , v.
19
, p.
271
288
.
Sadooni
,
F.N.
1997
,
Stratigraphy and petroleum prospect of Upper Jurassic carbonates in Iraq
:
Petroleum Geoscience
 , v.
3
, p.
233
243
.
Sadooni
,
F.N.
1998
,
Cretaceous rudist-bearing carbonate belt of the Arabian Platform: a probable indicator of a regional climatic front
:
(abstract) ENERGEX’98, Bahrain.
 
Sherwani
,
G.H.
1983
,
Depositional environments and stratigraphic relationships of the Mishrif Formation in selected boreholes, middle and southern Iraq
: unpublished MSc thesis,
University of Baghdad
,
Baghdad
(in Arabic).
Sherwani
,
G.H.
,
Aqrawi
,
A.A.M.
1987
,
Lithostratigraphy and environmental considerations of Cenomanian-early Turonian shelf carbonates (Rumaila and Mishrif Formations) of Mesopotamian basin, central and southern Iraq (abstract)
:
American Association of Petroleum Geologists, Bulletin
 , v.
71
, p.
614
.
Stocklik
,
J.
1972
,
Iran
:
Lexique Stratigraphique International, F. 9b, Asie, Centre National Recherche Scientifique, Paris
 ,
376
p.
Sugden
,
W.
Standring
,
J.A.
1975
, Qatar Peninsula:
Lexique Stratigraphique International, F 10b3, Asie, Centre National Recherche Scientifique
 ,
Paris
,
127
p.
Vail
,
P.R.
,
Mitchum
,
R.M.
, Jr
Todd
,
R.G.
Widmier
,
J.M.
Thompson
,
S., III
Sangree
,
J.B.
Bubb
,
J.N.
Hatlelid
,
W.G.
1977
, Seismic stratigraphy and global changes of sea level, in
Mitchum
,
R.M.
Vail
,
P.R.
Sangree
,
J.B.
eds, Seismic Stratigraphy—Applications to Hydrocarbon Exploration:
American Association of Petroleum Geologists, Memoir 26
 , p.
49
212
.
Bellen
,
R.C. Van
Dunnington
,
H.V.
Wetzel
,
R.
Morton
,
D.M.
1959
, Iraq:
Lexique Stratigraphique International, F 10a, Asie, Centre National Recherche Scientifique
 ,
Paris
,
333
p.
Wagoner
,
J.C. Van
Mitchum
,
R.M.
Campion
,
K.M.
Rahmanian
,
V.D.
1990
,
Siliciclastic sequence stratigraphy in well logs, cores, and outcrops: concepts for high resolution correlation of time and facies
:
American Association of Petroleum Geologists, Methods in Exploration Series 7
 ,
55
p.

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal