ABSTRACT

Based on studies of petrographic thin sections from core and cutting samples, the pre-Permian siliciclastics in four deep wells in southern Kuwait were found to be tight. Three of these wells are located on the crestal region of the Burgan Arch, and one on the Umm Gudair anticline. These clastics were encountered beneath a thin brick-red shale of unknown thickness, immediately below the pre-Khuff unconformity at the base of the Permian-Triassic Khuff Formation. The pre-Khuff clastics range in thickness from a few tens of feet to more than 4,000 ft, and overlie a Proterozoic argillite (Economic Basement). Based on Illite Age Analysis (IAA) of samples from cores, the depositional K-Ar age of the pre-Khuff clastics is estimated to be younger than 509 Ma (90 percent confidence interval: 544–481 Ma, i.e. Cambrian-Early Ordovician). The argillite was uplifted through the 300°C isotherm at about 611 Ma (90% confidence interval: 635-588 Ma, i.e Late Proterozoic); its deposition and metamorphism preceded this date. During the Paleozoic, the pre-Khuff clastics were buried to depths of 10,000–15,000 ft, but were subsequently uplifted in the Late Paleozoic. IAA diagenetic K-Ar ages of the Economic Basement (421 Ma; 90 percent confidence interval: 442-397 Ma; Late Ordovician-Early Devonian) and pre-Khuff clastics (369 Ma; 90 percent confidence interval: 404–337 Ma, i.e. Devonian-Early Carboniferous) indicate that by these times the pre-Khuff section was already deposited and undergoing burial diagenesis. The interpretation of gravity data indicates that in Paleozoic basinal regions (e.g. between the Burgan Arch and Umm Gudair Anticline), the Paleozoic sedimentary section is likely to be more complete and may exceed 10,000 ft in thickness.

INTRODUCTION

The Permian-Lower Triassic carbonates of the Khuff Formation and pre-Khuff clastic section constitute important hydrocarbon reservoirs in many parts of the Arabian Peninsula (e.g. Al-Laboun, 1986, 1987; McGillivray and Husseini, 1992; Al-Jallal, 1995; Alsharhan and Nairn 1997; Wender et al., 1998; Konert et al., 2001). These reservoirs are generally believed to be sourced by the Lower Silurian ‘hot shale’ (Mahmoud et al., 1992; Abu-Ali et al., 1999). In Kuwait, the Khuff and pre-Khuff sections are very deep and remain an exploratory target. Specifically, the Khuff Formation is more than 15,000 ft deep, and was penetrated in eight wells. A few of these wells traversed the entire Khuff Formation (about 1,800 ft thick), to encounter undated and tight pre-Khuff clastic sections above a Proterozoic argillite (Economic Basement). One deep well in particular (Well BG-A in Figures 1 and 2), encountered an exceptionally thick pre-Khuff section consisting of 4,290 ft of clastics (17,850–22,140 ft).

Khan (1989), assuming that the Khuff and pre-Khuff succession in well BG-A is conformable, incorrectly correlated the pre-Khuff section to the Upper Paleozoic formations of Saudi Arabia. The structural style and geometry of the pre-Khuff section in Kuwait differs profoundly from the conformable nature of the Khuff and younger formations (Figures 3 and 4). This relationship is evident in Figure 3 that shows a regional Bouger gravity map that highlights uplifted basement-cored structures as positive anomalies (e.g. Burgan Arch). Most of this uplift occurred during the Late Paleozoic (Figure 4). As a result, the succession below the Khuff Formation in basement-cored structures may be as old as Cambrian or infra-Cambrian. Biostratigraphic analysis of these pre-Khuff rocks, however, has not yielded definitive results.

This paper presents new data including Illite Age Analysis (IAA) in well BG-A, suggesting the pre-Khuff section in the Burgan Arch is mostly Lower Paleozoic and possibly infra-Cambrian in age. The paper starts by clarifying the nomenclature that is used in the four deep wells in the study area. It then reviews the results of petrographic and reservoir analysis, as well the IAA and K-Ar age dating. The results are then interpreted in terms of burial and thermal history, and the regional evolution of the Arabian Plate.

NOMENCLATURE

Pre-Khuff and pre-Unayzah unconformities

Several regional Upper Paleozoic unconformities are defined in the Arabian Peninsula. Over major basement-cored structures like the Burgan Arch and Ghawar Anticline (Wender et al., 1998), these unconformities generally converge into the pre-Khuff unconformity (PKU). The PKU defines a sequence boundary at the base of Khuff transgressive deposits (e.g. Strohmenger et al., 2000). In many wells in Saudi Arabia, the Khuff Formation overlies the Permian siliciclastic Unayzah Formation; for example: the reference section of the Unayzah Formation in Hawtah-1 well (Ferguson and Chambers, 1991); Hawtah–6 (Senalp and Al-Duaiji, 1995); or the southern flank of Ghawar field (Wender et al., 1998). The Unayzah Formation is older and distinct from the overlying basal part of the Khuff Formation that generally consists of intercalated siliciclastic and carbonate beds. The Khuff basal section is referred to as the ‘Khuff E member’ by Al-Jallal (1995), the ‘Basal Khuff Clastics’ of the Khuff-D Member (e.g. Senalp and Al-Duaiji, 1995; Al-Hajri and Owens, 2000), or the Ash-Shiqqah Formation (Senalp and Al-Duaiji, 2001).

The pre-Unayzah unconformity is sometimes referred to as the ‘Hercynian unconformity’ (e.g. Wender et al., 1998; Konert et al., 2001). In the deep wells of southern Kuwait, the Unayzah Formation is generally absent and the pre-Khuff and pre-Unayzah unconformities converge. This surface is generally characterized by an abrupt change from low gamma ray (GR) values in the Khuff Formation, to high GR values in the older Paleozoic section (Figure 2). In fact where the GR transition is abrupt, the lower section may generally be inferred to be older than the Unayzah Formation. Immediately below this unconformity surface, mud log reports indicate a thin brick-red shale of unknown thickness in Burgan wells BG-A, BG-B, BG-C, and BG-Cs. In Umm Gudair well UG-X, detailed mud log reports are not available for the sediments that directly underlie the Khuff carbonates. The pre-Khuff unconformity represents a major time break in sedimentation that allowed the induration of the exposed clastics and oxidation of the upper shale deposits (i.e. brick-red shale underlying the unconformity (Figures 2, 9 and 10).

Proterozoic Economic Basement

To the west and south of Kuwait, several deep wells drilled over major structures in eastern Saudi Arabia encountered a Proterozoic section of steeply-dipping metasediments. Similar steeply-dipping metasediments were encountered in several pre-Khuff wells in Kuwait and are here defined as the ‘Proterozoic Economic Basement’ or the equivalent ‘Rayn Basement’ (Al-Husseini, 2000). In Saudi Arabia, the Economic Basement has been radiometrically dated in a few wells. In the Summan Platform, the El-Haba-2 and Khabb-1 wells intersected steeply dipping (up to 70°) metamorphosed shales directly below the Permian Khuff Formation (Al-Husseini, 2000). The metasediments in Khabb-1 were dated at 636–605 Ma (M.N. Bass 1981, in Husseini 1989). In ‘Ain Dar-196 in the Ghawar Anticline, folded sandstones and shales dated at 671–604 Ma were encountered below the Upper Cambrian-Lower Ordovician Saq Formation (T.H. Kiilsgaard and W.R. Greenwood 1976, in Husseini 1989).

BURGAN DEEP PRE-KHUFF WELLS

Well Burgan BG-A

Well BG-A penetrated approximately 4,290 ft of pre-Khuff clastics and recovered six cores (Figure 2). Sixteen thin sections were selected for detailed analysis of rock texture, mineralogy and grain fabric (Figures 5, 6 and 7). Point-count modal analysis and grain size measurements were made on 15 of these thin sections to identify diagenetic alterations that effect reservoir potential and to document pore system properties (Figure 7). In well BG-A, a total of 19 cutting samples were studied petrographically (Figure 2). One sample was taken from each of the cores 1, 2, 5 and 6 and dated using K-Ar and Illite Age Analysis (IAA) as indicated in Figure 2 and Table 1.

Although the lithology of the un-cored pre-Khuff section cannot be uniquely determined from well logs, it is tentatively interpreted (from top to base) as five lithological units (Figure 2):

  • Dominantly red shaly sandstone, and sandstone with minor shale interbeds, about 1,070 ft thick (log depths 17,850–18,920 ft);

  • Dominantly red claystone and silty claystone about 1,130 ft thick (log depths 18,920–20,050 ft);

  • Dominantly red sandstone with minor shale interbeds about 1,270 ft thick (log depths 20,050–21,320 ft);

  • Dominantly red fining-upward sandy conglomerate about 320 ft thick (log depths 21,320-21,640 ft)

  • Dominantly red fining-upward conglomerate about 500 ft thick (log depths 21,640–22,140 ft).

Below the base of the pre-Khuff clastic section, well BG-A encountered nearly 100 ft of Economic Basement (22,140-22,239 ft; Khan, 1989) and reached a total depth of 22,239 ft.

The pre-Khuff section showed an overall fining-upward trend that ranges from poorly-sorted conglomerates and sandy conglomerates; to moderately well-sorted, fine-grained sandstones (Figure 6), containing little depositional clay matrix; to claystones (Figure 2). Grain rearrangement and compaction appear to have begun soon after burial and were ultimately extensive. The cored pre-Khuff clastic section is characterized by well-developed red-bed facies. The cutting samples from the Upper pre-Khuff interval (log depths 17,850–18,920 ft) exhibit the same volcanic fragments, grain textures and mineral constituents as the lower cored sandstone interval (log depths 20,050–21,320 ft). It is likely that the upper interval is part of the same depositional cycle as the lower interval.

Using Folk’s 1974 classification, the pre-Khuff clastics are mostly immature feldspathic litharenites (Figure 5). Quartz and rock fragments are more abundant than feldspars. The rock fragments are mostly volcanic, metamorphic and sedimentary, with a few plutonic and undifferentiated rock fragments. The volcanic material is more mafic than felsic and the metamorphic rock fragments are typically mica poor.

The total cement in these rocks ranges from 0-32.4%. The predominant cement type is dolomite (as much as 25.2%), which precipitated both early and late in the diagenetic history (Figure 8). Dolomite occurs as both a pore- and fracture-filling cement (Figures 7a and 7c). The majority of the dolomite cement is baroque, characterized by curved crystal faces and cleavages and undulose extinction. Baroque dolomite, also called ‘saddle dolomite’, precipitates at high temperatures (possibly > 150°C) and is commonly associated with sulfate-bearing carbonates and hydrocarbons (Radke and Mathis, 1980). In some of the pre-Khuff rocks, dolomite appears to have replaced earlier carbonate cement. Subordinate amounts of quartz cement (0-4.8%), Fe-oxide cement (0-12.4%), pyrite cement (0-2.8%) and Fe-stained clay (0-2.8%) are also present.

Well Burgan BG-C

Well BG-C (Figure 9) penetrated approximately 550 ft of pre-Khuff section (log depths 17,050-17,600 ft), and about 1,880 ft of Economic Basement (log depths 17,600-19,480 ft). There are no cored intervals in this well. Eleven cutting samples were studied petrographically and compared to core samples from wells BG-A and BG-Cs. Well BG-C encountered a thin pre-Khuff brick-red shale of unknown thickness, above approximately 280 ft of dolomite and anhydritic dolomite (questionable lithological interpretation from well logs and cuttings), and 250 ft of shaly sandstone (more reliable interpretation).

Volcanic rock fragments found in cuttings from the interval between 17,050-17,600 ft are similar to those observed in the cored intervals of well BG-A (cores 2, 3 and 4). Lithic fragments, present in cuttings from the interval between 17,600-19,480 ft, are similar to the metamorphic argillites observed in core samples of well BG-A (cores 5 and 6) and well BG-Cs (cores 1 and 2).

The upper part of the pre-Khuff section in well BG-C most probably corresponds to the cored sandstones of well BG-A (log depths 20,050–21,320 ft). The lower part of well BG-C most probably corresponds to the cored argillites of wells BG-A and BG-Cs (Economic Basement).

Well Burgan BG-Cs

Well BG-Cs (Figure 10) is a sidetrack of well BG-C that only penetrated about 70 ft of shale, including a thin brick-red shale (log depth 17,140-17,210 ft), before reaching the Economic Basement (log depth 17,210-18,680 ft). Three samples from well BG-Cs cores 1 and 2 of the Economic Basement were dated using K-Ar (Table 2). Thin sections from nine studied cutting samples showed the same metamorphic rock types as the Economic Basement rocks cored in this well.

Well Umm Gudair UG-X

Well UG-X (Figure 11) drilled approximately 145 ft into the pre-Khuff section (log depth 18,538-18,683 ft), consisting of sandstone and shale alternations. Four cutting samples were studied petrographically and compared to core samples from wells BG-A and BG-Cs. The cutting samples are heavily contaminated by drilling mud solids and halite. Although there are no sandstone fragments observed in the cuttings, quartz grains and volcanic rock fragments are similar to those found in pre-Khuff samples from wells BG-A and BG-Cs.

Porosity and Reservoir Potential

The primary depositional characteristics of the pre-Khuff clastics (coarse-grained, fairly good sorting, low matrix content) suggest they initially had potentially good reservoir quality. The reservoir quality, at any position in the basin, would have been determined by variations in provenance (e.g. more feldspar-rich, fewer ductile grains), burial conditions (less compaction, lower temperatures) and proximity to sources of carbonate cement. However, reservoir potential of the pre-Khuff section in the Burgan and Umm Gudair wells is poor.

Very little macroporosity exists in the pre-Khuff clastics due to a complex diagenetic history (Figure 8). There is no visible intergranular porosity, but a small amount (< 1%) of leached-grain, intragranular, secondary porosity is present. Core porosity measurements (< 0.05–1.2%) do not suggest the presence of significant microporosity.

Although the high cement contents of several samples indicate early (pre-compaction) precipitation of pore-filling dolomite cement (Figure 7a), the remainder of the pre-Khuff rocks experienced major ductile-grain compaction, which severely reduced primary porosity (Figure 7b). The remaining porosity was further reduced by late-stage precipitation of baroque dolomite in both pores and fractures (Figure 7c). The presence of baroque dolomite, micaceous reaction rims around grains (Figure 7d), and extreme deformation of ductile grains indicate that these rocks experienced high temperatures. The illite data (next section) suggest temperatures above 150° C. Fractures are present in three samples, but are completely filled with late-stage authigenic dolomite and silica (Figure 7c).

AGE DATING ANALYSIS

Age dating of the pre-Khuff section is crucial for any future reservoir assessment. Because the drilled pre-Khuff section of all wells is very tight, it is important to determine whether these clastics correspond to potential reservoir rocks, such as the Devonian Jauf, or the Permian Unayzah formations (Wender et al., 1998; Konert et al., 2001), or to older non-reservoir strata. Although IAA dating does not give an unequivocal answer, it does show that the drilled pre-Khuff sections most probably represent rocks of infra-Cambrian to Early Paleozoic age.

Methodology

ExxonMobil Upstream Research Company patented the Illite Age Analysis (IAA) K-Ar isotope dating method (Pevear, 1992, 1994, 1999; Ylagan et al., 2000, 2002). Illite is a general term for the dioctahedral mica-like, potassium-aluminum silicate clay mineral that is common in sedimentary rocks, especially shales. IAA uses K-Ar isotope dating of illite (including mica). This technique is based on the assumption that illite (in sedimentary rocks) is a mixture of detrital (older-than-depositional-age) and diagenetic (younger-than-depositional-age) phases. Illite in shales is thus a mixture of detrital mica and its weathering products, with diagenetic illite precipitated from pore fluids during burial. Two important lines of evidence support this conclusion.

  1. Polytypes are a variety of crystal structure polymorph that are distinguished by various repeated stacking arrangements of identical atomic layers (see Pevear, 1999). Illite mineralogy shows a mixture of 2M1 and 1M polytypes (including 1Md), with 1M increasingly abundant in the finer-size fractions. The 2M1 polytype is expected for the large detrital micas eroded from slates, schists and phyllites. Diagenetic illite that grows in sandstones is exclusively 1M, which suggests that similar material mixed with 2M1 muscovite in shales is also diagenetic.

  2. Typically, in a single shale sample separated into several grain size fractions, measured K-Ar age of a fraction decreases with decreasing grain size. Illite in shales is a mixture of detrital (older) and diagenetic (younger) components, with the latter more abundant in the fine fractions. Thus the age of bulk mixtures of detrital and diagenetic end members is not useful. Instead, the separate ages of the end members are required.

The IAA method estimates the K-Ar ages of the end members, and brackets the detrital and diagenetic ages of sedimentary rocks. Ideally, the detrital age is the mean age of the coarse micas, which may themselves be a mixture. It corresponds to the mean time of uplift and cooling of the source terrain (provenance) below the so-called blocking temperature for muscovite (260-300°C), below which Ar no longer diffuses out of the crystal structure. Note that this “detrital” age is not necessarily the time of crystallization of the detrital grains, nor is it the age of source terrain sedimentation; it may be younger than either one.

The diagenetic age estimates when illite formed in the sediment, typically during heating to about 100-200° C by burial. This estimate could represent a nearly instantaneous event in a case where illite formed in response to an igneous intrusion; or an interval of burial, possibly 10’s of millions of years in duration, in a sedimentary basin. The diagenetic age is always younger than depositional age.

IAA requires K-Ar ages from each of three progressively finer size fractions of a sample. This is accomplished by using centrifugation to divide the sample into three clay-size fractions: (1) coarse (2.0–0.2 μm); (2) medium (0.2–0.02 μm); and (3) fine (< 0.02 μm). Using the clay (<2 μm) fraction generally excludes feldspar, so that the only K-bearing phases are illite and micas. The three K-Ar ages, with appropriate mineral analysis, are used to make the IAA plot that defines the extrapolated end member ages and their uncertainties (Figure 12a).

Illite Age Analysis Results of Kuwait Samples

The IAA technique was applied to seven core samples from wells in Kuwait: two from the pre-Khuff section and five from the Proterozoic Economic Basement ( Tables 1 and 2). In addition, two core samples of Mesozoic age were analyzed to confine the stratigraphic ages and the timing of pre-Khuff tectonic events (red triangles in Figures 12b, 14a, and b).

To estimate the ages of the detrital and diagenetic end members, the proportions of the mineral end members (2M1 and 1M polytypes) in each of the three size fractions were quantitatively determined by XRD (Tables 1 and 2), and the K-Ar dates of the fractions were determined by a commercial laboratory. To obtain the end member ages, the points (normalized to 100% illite) plotted as apparent K-Ar age versus percent (%) detrital illite, were linearly extrapolated to 0 and 100% detrital illite (Figure 12a). The quantification of polytypes in the Kuwait samples was done using a genetic algorithm to match modeled to experimental XRD patterns (Ylagan et al., 2002).

Using both the experimental and modeled XRD patterns for a single size fraction of a sample from well BG-A allowed the quantitative analysis of 1M (diagenetic) and 2M1 (detrital) polytypes. These data, with the measured K-Ar isotope age for the fraction, were used to plot one data point on the IAA plot (Figure 12a). Data points for the other fractions were obtained similarly. The well BG-A samples are clearly mixtures of high temperature (detrital) 2M1 (2-layer, monoclinic crystal form) mica with low temperature (diagenetic) 1M (1-layer, monoclinic) illite (Figure 13a). In the well BG-Cs, all illite is the high-temperature 2M1 form (Figure 13b). For details of the methodology for obtaining the IAA plot and estimating uncertainties, see Ylagan et al. (2002).

Three results stand out in the mineralogy of the pre-Khuff clastics in the wells BG-A and BG-Cs core samples.

  1. The cores contain no smectite or mixed-layer illite/smectite. This suggests a hotter thermal history than in most basins of Mesozoic age or younger. At about 100-150° C, smectite is completely transformed to illite (Huang, et al., 1993).

  2. In well BG-A, the diagenetic illite is a well-ordered 1M structure (compared to the disordered 1Md), suggesting temperatures of about 200°C.

  3. 1M illite is completely absent from well BG-Cs; there is only 2M1, suggesting temperatures near 300°C. The separated illite fractions were examined by atomic force microscopy to check for purity. The well BG-A samples showed thin, elongated illite crystals (Figure 13a) typical of the 1M structure (Pevear, 1999); whereas the well BG-Cs samples showed thicker, blocky or platy crystals typical of 2M1 illite (Figure 13b).

Well BG-A

The deeper two samples from the Economic Basement (core 5/A3; core 6/A4) are argillites with incipient slatey cleavage (tentatively interpreted as greenschist facies). They fall on a separate trend in the IAA plot (Figure 12a: dark blue and green symbols). Their detrital age is 635-588 Ma (90% confidence interval) with a mean of 611 Ma. These metasediments were uplifted and passed through the 300°C isotherm in the Late Precambrian (635-588 Ma). This corresponds to the age of uplift, but not the initial deposition or metamorphism, of the basement.

The diagentic age of the two basement samples is between 442–397 Ma (90% confidence interval), with a mean of 421 Ma (Table 1 and Figure 12a). This Early Silurian to Early Devonian window was the time when the metasediments, essentially an erosion surface on basement, were reburied under the pre-Khuff and underwent retrograde burial metamorphism forming 1M illite. The required temperature to produce diagenetic illite in well BG-A implies a burial depth of between 10,000 and 15,000 ft., the likely thickness of Early Paleozoic pre-Khuff sediments.

The detrital age of the two samples from the pre-Khuff clastics (core 1/A1: core 2/A2) is 544–481 Ma (90% confidence interval) with a mean age of 509 Ma (Table 1 and Figure 12a). The diagenetic age of the same two samples is 404–337 Ma (90% confidence interval) with a mean age of 369 Ma. Both core samples are interpreted to be derived from the erosion of Economic Basement that was uplifted before about 509 Ma (Early Cambrian and Early Ordovician). The detrital age for the pre-Khuff is about 100 Ma younger than that of the basement argillite, suggesting admixed detrital material from younger uplifts is also present.

The pre-Khuff sediments were subsequently buried to a sufficient depth to form diagenetic (1M) illite during 404–337 Ma (Early Devonian to Early Carboniferous). Figure 12a and Table 1 show a slight overlap in the 90% confidence intervals for the diagenetic end member ages for basement and pre-Khuff, indicating these could be the same age. Indeed, they should be, because the diagenetic illite (1M, retrograde) in the basement would have formed during the same burial diagenetic event that formed illite in the pre-Khuff section, which buried it.

Well BG-Cs

The three core samples of well BG-Cs contain only 2M1 mica; therefore an IAA plot could not be drawn. Also the illite crystallinity is much higher (narrower XRD peaks) for these samples (Figure 14a). The ages of the samples span a similar range as those analyzed from the argillite (Economic Basement) of well BG-A (Tables 1 and 2). These results suggest that the Economic Basement was uplifted at about 600 Ma (as in well BG-A) but buried to a much greater depth during the Devonian (Figure 14b). The burial depth was great enough to reach 300 °C (so that 2M1 rather than 1M micas formed or reset the K-Ar clock in previously formed 2M1 illite). In other words, the samples contain Devonian age 2M1 illite in their finest size fractions.

In Figure 14a, illite crystallinity is the Full-Width-at-Half-Maximum (FWHM) of the main illite XRD peak versus core depth plotted. The line on Figure 14a shows a typical trend of decreasing FWHM with depth and temperature (peaks become narrower and sharper with depth). The well BG-Cs sample is off the trend. The FWHM is too low for present depth, suggesting it was uplifted by about 5,000 ft compared with the other samples (red arrow). It seems likely that wells BG-A and BG-Cs are separated by a fault with about 5,000 ft of offset that is pre-Khuff and post-Devonian in age. This hypothesised fault, which offsets the burial diagenesis zones between the two wells (Figure 14a), may be related to the “Hercynian” tectonic event depicted in Phase 5 of Figure 15a.

PROTERZOIC-PALEOZOIC GEOLOGICAL MODEL

Figure 15a and b presents a simplified geological model for the evolution of the Burgan Arch in terms of six phases. The figure also shows the interpreted regional tectono-stratigraphic events of the Arabian Plate (e.g. Al-Husseini, 2000; Sharland et al., 2001; Konert et al., 2001).

Phase 1 represents Late Proterozoic time when the Kuwait region presumably formed part of the Rayn Terrane. The Rayn Terrane extended westwards to the Amar Island Arc at the eastern edge of the Arabian Shield (e.g. Al-Husseini, 2000). In this model, a relatively thick sedimentary section would have been deposited in the region of Kuwait above the Crystalline Basement.

Phase 2 is attributed to the Late Proterozoic Amar or Suturing Orogeny that occurred between about 640–610 Ma (e.g. Al-Husseini, 2000; Nehlig et al., 2002). This orogeny resulted from the closure of a seaway along the western edge of the Amar Arc as evidenced by ophiolite belts along the Amar-Idsas Suture. The suture zone of the collision front was located approximately 300 km west of the Burgan Arch. These regional considerations suggest that the Economic Basement sediments in Kuwait were deformed at about 640–610 Ma. The K-Ar age of the detrital illite end member from the Economic Basement indicates that these metasediments were uplifted through the ~300° isotherm at 611 Ma (635–588 Ma).

The Amar Orogeny was followed by Phase 3 (about 610–540 Ma), a period dominated by erosion over Proterozoic highlands. During the latter part of this post-collisional phase (about 560–540 Ma), the salt associated with the Hormuz Series and Ara Group of Oman was deposited in an extensional horst and graben regime (Al-Husseini, 2000). The Ara salt is dated at 544.5 Ma by U-Pb radiometric techniques (Brasier et al., 2000). The Burgan and Umm Gudair structures probably formed horsts during the deposition of the Hormuz Series (Phase 3 in Figure 15a). The detrital age of the pre-Khuff clastics (544-481; mean 509 Ma) indicates that they are younger than the Hormuz Series (and Ara Group). These clastics are probably equivalent to the Late Cambrian-Early Ordovician Saq Formation.

During the Paleozoic, the Arabian Plate was essentially an east-dipping platform. Regions like the Burgan Arch accumulated an estimated 10,000-15,000 ft of Paleozoic sediments (Phase 4). In the Late Paleozoic, however, the platform was affected by one or more events that uplifted many basement-cored structures, including the Burgan Arch (Phase 5). This tectonic phase (‘Hercynian’ event) appears to have exploited the existing NS-trending weak basement fabric to relift the same Proterozoic NS-trending blocks (Figure 15a). The hypothesized fault (Figure 14b), which offsets the burial diagenesis zones of Phase 4 between the two wells, may be related to the “Hercynian” tectonic event depicted in Phase 5 of Figure 15a.

Burial diagenesis during Phase 4 is reflected in the diagenetic ages of the Economic Basement (442-397 Ma) and the pre-Khuff clastics (404-337 Ma). The older interval (442-397 Ma) includes the Late Silurian regional hiatus seen in many parts of the Arabian Plate (Mahmoud et al., 1992). The younger interval (404-337 Ma) includes the time period generally attributed to the ‘Hercynian’ event (Late Devonian and Early Carboniferous; Sharland et al., 2001). However, we have shown above that the diagenetic ages from these two units may be the same within experimental error, and probably older than the Late Paleozoic ‘Hercynian’ event. Following the Late Paleozoic tectonic event, Phase 6 represents renewed Permian Khuff deposition over the Burgan Arch. This depositional phase continued into the Mesozoic with only minor structural movements (Figure 15a).

CONCLUSIONS

Gravity modeling and age dating of pre-Khuff siliciclastics in four deep wells in southern Kuwait show profound structural deformation took place, primarily in the Late Proterozoic and Late Paleozoic. The Burgan Arch and Umm Gudair structure are regions where the pre-Khuff unconformity cuts deep into Lower Paleozoic or older rocks. These clastics are probably Cambrian-Ordovician or older and have poor reservoir potential. Between the pre-Mesozoic structures and along their flanks, however, synclinal regions appear to contain more complete Paleozoic strata. These may include Silurian source rocks, as well as Devonian and Permian reservoirs regionally sealed by the Lower Khuff Formation.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the management of ExxonMobil Exploration Company, Houston (EMEC); ExxonMobil Upstream Research Company, Houston (URC); and Kuwait Oil Company, Kuwait (KOC) for permission to publish this paper. For helpful discussions we wish to thank our colleagues from EMEC: Christopher A. Johnson, Leonard V. Moore, Steven R. Webb, Larry Wender; KOC: Salah A. AbdulMalek, Abdullatif Y.M. Al-Kandari, Khaled A. Al-Sumaiti, and Hassan Bunain. Appreciation is extended to John Mariano (EMEC) for performing the gravity and magnetics models, and to Dolores A. Claxton (EMEC) for drafting the figures. We wish to thank Ron J. Kleist (EMEC), Leonard V. Moore (EMEC), J. Frederick (Rick) Sarg (EMEC), and Larry Wender (EMEC) for their constructive reviews of this paper. GeoArabia editors Moujahed I. Al-Husseini and Joerg Mattner, and two anonymous reviewers are also thanked for their helpful comments. Design and drafting of the final figures and text editing was by Gulf Petrolink.

ABOUT THE AUTHORS

Christian J. Strohmenger received a Diploma in Geology from the University of Giessen (1983) and a PhD in Sedimentology from the University of Heidelberg, Germany (1988). From 1989 to 1990 he worked as a Research Assistant in carbonate sedimentology and sequence stratigraphy at the University of Geneva, Switzerland. He joined BEB Erdgas und Erdoel GmbH, Hanover, Germany (now ExxonMobil Production Germany, EMPG) in 1990 working as a Carbonate Sedimentologist and Seismic Interpreter. From 1996 to 2002 he was with ExxonMobil Exploration Company in Houston, Texas where he worked on Mesozoic and Paleozoic carbonate and sandstone reservoirs of Kuwait and Saudi Arabia. Christian was seconded to Abu Dhabi Company for Onshore Oil Operations (ADCO), UAE in 2002 working as a Carbonate Stratigraphy Specialist. His current interests are carbonate sequence stratigraphy, sedimentology, and reservoir quality prediction. Christian is a member of AAPG, SEPM, GSA, IAS, and ESG.

cstrohmenger@adco.co.ae

christian.j.strohmenger@exxonmobil.com

Menahi S. Al-Anzi is presently leading Geological Exploration Team of the Exploration group at Kuwait Oil Company(KOC).He received a BSc in Geology from Kuwait University in 1987 and joined KOC in 1988. He has extensive experiences in Exploration and Development geology and has been a member of KOC/Shell offshore Joint study. Menahi has lead the ‘Prospect Evaluation Team’ since 1996, which lead into a number of significant Cretaceous and Jurassic discoveries.His main area of interest includes, petroleum systems, play assessment and exploration strategy. Menahi has attend numerous technical and professional courses, seminars and forums. He is a member of AAPG and SPE.

msanezi@kockw.com

Dave R. Pevear recieved his PhD from the University of Montana in 1968, was professor of geology at Western Washington University for 15 years and worked also for the US Geological Survey during this time. In 1981 he joined Exxon Production Research Company, from which he retired in 2000. He is author or coauthor of 35 refereed articles, four book chapters, one book, one US patent and many technical reports. He is co-editor of book on quantitative mineral analysis. He was a Clay Mineral Society (CMS) Council Member; Continuing Education Committee Chair for 8 years, CMS president (1993-94); CMS Treasurer (1996-2002); Chairman of CMS 1991 Annual Meeting. Co-Organizer and book author of SEPM course on “Clays for Petroleum Geologists & Engineers”. Lecturer in “Clays and Clay Minerals for Petrophysicists and Log Analysists” Short Course at June, 2000 SPWLA meeting, Dallas. He is presently a consultant.

p00bare@pdq.net

Robert F. Ylagan received his AB in Geology from Colgate University and a PhD from the University of Illinois in clay mineralogy and low-temperature geochemistry. He joined Exxon Production Research in 1996 and used mineralogic and geochemical tools to better understand thermal histories of basins and timing of basin forming events. He recently earned an MBA from the University of Rochester and will rejoin Exxon Mobil Upstream Research Company in the upcoming year.

ylagan@frontiernet.net

Tobi H. Kosanke earned her PhD in Geology from Rensselaer Polytechnic Institute and both her Master and Bachelor of Science degrees from Florida International University. As a student she earned the Mineralogical Society of America American Mineralogist Undergraduate Award and the Association of Women Geoscientists Chrysalis Scholarship. She has been working as a reservoir quality specialist at ExxonMobil since 1998. Tobi has experience modeling porosity and permeability of both clastic and carbonate rocks in geologically diverse environments and has applied her skills in the ExxonMobil research, exploration, development and production companies. She is currently working on a regional studies team that identifies new business opportunities for ExxonMobil.

tobi.h.kosanke@exxonmobil.com

G. Scott Ferguson is currently working as a Senior Staff Geologist for Nexen Petroleum International in Calgary, where he is the exploration team leader for Nexen’s Masila Block inYemen. Since 1984 he has held several technical and managerial positions, including Exploration Geologist at Texaco Canada, Senior Geologist secondee to Saudi Aramco, Senior Geologist for Wascana de Venezuela Ltd., Technical Manager for Wascana Energy Ltd., and Senior Geologist with Kuwait Santa Fe. Scott received a BS in Geology from Iowa State University, USA (1981) and a MSc in Geology (Clastic Sedimentology) from McMaster University, Canada (1984).

scott_ferguson@nexeninc.com

Daniel H. Cassiani is a Supervisor of Regional Studies in the Middle East for ExxonMobil. He received his BS in Geology from Boston College in 1979. In 25 years with ExxonMobil, his assignments have ranged from frontier exploration to new and mature field development. His fields of specialization include seismic interpretation, seismic attribute analysis and seismic stratigraphy. He has worked in many of the producing basins of the USA, SE Asia and the West Coast of Africa. He has been working the geology of the Middle East the past seven years. Daniel and his team are currently working a regional study in the Caspian/Middle East Business Unit of ExxonMobil.

dan.h.cassiani@exxonmobil.com

Adel F. Douban is a Senior Geologist with Sipetrol International SA, Egypt branch. He received his BSc in Geology (1980) and MSc in Petroleum Geology (1991) from Alexandria University, Egypt and his PhD (2001) from Cairo University, Egypt. Since 1981 he has worked for Gulf of Suez Petroleum Company (GUPCO) and Western Desert Petroleum Company (WEPCO) as Exploration Geologist, for Esso Egypt as Geological Supervisor and for Kuwait Oil Company (KOC) as Senior Geologist. His main interests include regional geology, sequence stratigraphy, depositional systems and basin modeling. He is an active member of the AAPG and EPEX.

adouban@sipetrol-eg.com