ABSTRACT

Saudi Aramco is conducting an exploration program to discover additional non-associated gas reserves in the Ghawar Area. The program has successfully discovered significant sweet gas and condensate reserves in the pre-Khuff siliciclastics and has further increased our understanding of the Paleozoic petroleum system.

The Lower Permian Unayzah Formation is the principal pre-Khuff hydrocarbon reservoir in the Southern Ghawar Area, where it contains both oil and gas. The Unayzah consists of fluvial to marginal marine sandstones. The Devonian Jauf Formation is the principal pre-Khuff reservoir in the Northern Ghawar Area, where it hosts the recently discovered giant Hawiyah gas-condensate field. The Jauf consists of shallow marine sandstones that exhibit unusually high porosities considering the burial depths. The primary source rock for pre-Khuff hydrocarbons is the basal “hot shale” of the Lower Silurian Qalibah Formation. Maturation modeling of these shales indicates hydrocarbon generation began in the Middle Triassic (oil) and continues to the present (dry gas).

Pre-Khuff hydrocarbon traps are found in simple four-way closures as well as more complex structural-stratigraphic traps on the flanks of Hercynian structures. Trap formation and modification occurred in four main phases: (1) Carboniferous (Hercynian Orogeny); (2) Early Triassic (Zagros Rifting); (3) Late Cretaceous (First or Early Alpine Orogeny); and (4) Tertiary (Second or Late Alpine Orogeny). Structures in the Ghawar Area show differences in growth histories, which have impacted the amount and type of hydrocarbons contained.

INTRODUCTION

As a result of the increasing demand for natural gas in the Kingdom of Saudi Arabia, Saudi Aramco has carried out an aggressive exploration program to discover additional non-associated gas reserves near existing facilities. The first phase of this program began in early 1994 and focused on the pre-Khuff of the Ghawar Area (Figures 1 and 2), where non-associated sweet gas accumulations were known to exist.

The initial exploration well of this program, located on the eastern flank of Central Ghawar (Hawiyah Well-200), was completed in August of 1994 as a deeper pool gas-condensate discovery. Since the Hawiyah discovery, several pre-Khuff penetrations have been drilled, resulting in significant additions to Saudi Aramco’s non-associated gas and condensate reserves base.

In addition to the new wells, several thousand line kilometers of 240-fold split spread 2-D seismic data using longer offsets of 5 to 6 kilometers (km) have been acquired since the program was initiated. The new information gained from the drilling, testing, and seismic operations has substantially increased our knowledge of the pre-Khuff geology of the Ghawar Area.

The purpose of this paper is to synthesize this data to provide an insight into the Paleozoic hydrocarbon geology of the Ghawar Area. Furthermore, this paper addresses the growth history of Ghawar and surrounding structures to determine the relationship of structural growth to the hydrocarbon occurrences in the Ghawar Area. The generalized geologic map of the southern part of the Arabian Peninsula with the Ghawar Study Area is shown in Figure 1.

STRATIGRAPHIC FRAMEWORK AND BASIN EVOLUTION

The pre-Khuff sediments in the Ghawar Area are composed of a massive sequence of siliciclastic sediments with a thickness range from approximately 8,000 feet (ft) in the basinal areas to 2,000 ft on the crest of Ghawar, where significant erosion has occurred. This erosion is the result of uplift associated with the Hercynian Orogeny, which we consider to be of Carboniferous age in the Ghawar Area. These clastics were deposited in terrestrial to shallow marine environments on the stable passive margin of Gondwana (Beydoun, 1991).

With the exception of the Hercynian Unconformity, which is overlain by the Unayzah or Khuff formations, the stratigraphic succession of the pre-Khuff in the Ghawar Area is generally conformable. Pre-Hercynian thickness variations observed are thought to be due to broad epierogenic uplifts of the Arabian Plate, creating intracratonic sag basins with intervening gentle arches. Figure 3 shows the generalized Paleozoic stratigraphic column of the Ghawar Area. A composite log showing the wireline log characteristics and major reservoirs of the Paleozoic is shown in Figure 4. Figure 5 shows the subcrop of the pre-Unayzah and pre-Khuff (where the Unayzah Formation is absent) unconformities.

Precambrian-Early Cambrian

The Precambrian to Lower Cambrian “basement rocks” of the Ghawar Area are composed of steeply dipping, fractured metasediments. The basement complex of the Arabian Shield was subjected to compressional movements associated with the Idsas Orogeny (McGillivray and Husseini, 1992). This event created the dominant present-day north-south structural grain in the Ghawar Area. Subsequent rejuvenation of the north-south structural elements has controlled the structural development of the sedimentary cover rock and has had a major influence on the hydrocarbon distribution of the area.

Khuff penetrations (in black).

The north-south orientation of major basement-involved structures such as Ghawar can readily be seen on gravity maps, where structural highs are generally associated with positive gravity anomalies. Northwest-southeast structural elements which run parallel to the Najd-fault system (Stern, 1985; Husseini and Husseini, 1990) are also present in the Ghawar Area, but are of secondary importance.

The basement complex has been penetrated by two wells in the Ghawar Area. In both wells, the basement is composed of highly-fractured metasediments, which have a distinctive high resistivity, high velocity (high sonic travel times), and moderate gamma-ray wireline log character. These metasediments have been radiometrically dated at 671 and 604 million years (Ma).

Cambrian-Ordovician

Following peneplanation of eastern Arabia and the formation of the pre-Saq Unconformity (PSU), terrestrial to marginal marine sediments of the Saq Formation were deposited. The Saq in the Ghawar Area consists of arkosic sandstones and red micaceous siltstones in the lower part with a generally cleaner, skolithus-bearing sandstone unit at the top. This upper, shallow-marine sandstone is well developed in the Haradh Area (Figure 2), and is informally referred to as the “Saq-A sandstone”. The upper Saq at Ghawar has sporadic hydrocarbon shows, but is a poor reservoir in the Ghawar Area due to silica cementation.

Following Saq deposition, a sequence of micaceous sandstones with subordinate shales was deposited, which is assigned to the Qasim Formation (Williams et al., 1986). The Qasim was deposited in a shallow marine environment, and is made up of stacked sequences reflecting eustatic sea level changes. The Qasim embraces two distinct shale units in Northern Ghawar, the Ra’an and Hanadir, which grade into siltstone and sandstones to the south. This southward increase in sand content, which is observed in several pre-Khuff units, is thought to be due to epierogenic uplift (ancestral Central Arabian Arch?) in the southern Ghawar Area.

Following deposition of the Qasim Formation in the Late Ordovician, glaciation affected most of present-day western Arabia. Glacial and periglacial sediments have been recognized in the outcrop belt of Saudi Arabia, and consist of tillites, sandstones, and siltstone units of variable thickness (McClure, 1978; Vaslet, 1987, 1989). Recent field work in the Qasim Area has demonstrated the presence of striated glacial pavements, faceted and striated basement boulders, and tillites of Late Ordovician age (Senalp and Laboun, 1996). These deposits have been collectively referred to as the “Sarah-Zarqa Formation”, though recently it has been argued whether this terminology should be maintained (Senalp and Laboun, 1996).

These glaciogenic sediments do not appear to extend to the Ghawar Area, where this interval is represented by a relatively clean, fine- to medium-grained sandstone referred to as the “Sarah Formation”. The Sarah Formation occurs immediately beneath the Qusaiba Member and represents a lowstand deposit associated with Gondwana glaciation. The base of the formation represents an erosional unconformity (Type 1 sequence boundary), and is known as the pre-Zarqa/Sarah Unconformity (PZSU).

Numerous shows have been reported from the Qasim and Sarah formations in the Ghawar Area, but, as yet, no measurable hydrocarbon flow rates have been established. The best shows have been observed in the Sarah, which flowed gas to surface. However, no sustainable flow rates have been obtained, which is due to low permeability.

Silurian

In the Early Silurian (Llandoverian), sea level rose due to deglaciation and resulted in the widespread deposition of the Qalibah Formation. The upward-coarsening, progradational Qalibah Formation consists of a lower Qusaiba Member and an upper Sharawra Member (Mahmoud et al., 1992).

At the base of the Qusaiba Member, an organic-rich shale persists and is regionally correlatable. This shale, with its distinctive high gamma-ray signature on logs, is referred to as the “hot shale”, and is considered the principal source rock for Paleozoic hydrocarbons in Saudi Arabia (McGillivray and Husseini, 1992; Mahmoud et al., 1992; Cole et al., 1994; Halpern and Carrigan, 1995 unpublished report). The “hot shale” was deposited during the rapid Early Silurian transgression over Gondwana following glaciation, and is thought to represent a condensed section with the top of the “hot shale” corresponding to the maximum flooding surface.

The thickness of the “hot shale” in the Ghawar Area varies considerably, and ranges from 229 ft in Southern Ghawar to 9 ft in Northern Ghawar. A significant acoustical impedance contrast occurs at the base of the Qusaiba, resulting in a persistent seismic reflection. This reflector is the most reliable seismic event in the pre-Khuff section.

A sandstone/siltstone unit, informally referred to as the “Mid-Qusaiba Sand”, is present as a key stratigraphic marker within the Qusaiba throughout most of the Ghawar Area. The unit is composed of a crudely-thickening and coarsening-upward sequence and most probably represents a progradational basin floor fan system. These sandstones have had significant gas shows in the Ghawar Area, and have flowed gas at commercial rates in one well in Southern Ghawar (Haradh).

The overlying Sharawra Member consists of micaceous sandstones, siltstones, and shales representing a continuation of the progradational, coarsening-upward sequence. Sporadic hydrocarbon shows have been encountered in the Sharawra, but formation analyses indicate poor reservoir development.

A Late Silurian hiatus is observed in the Ghawar Area, as part of the uppermost Silurian (Wenlockian-Ludlovian) appears to be missing. This missing section is thought to reflect non-deposition rather than tectonic uplift and erosion (Mahmoud et al., 1992).

Devonian

The Tawil Formation was deposited after the “Silurian Hiatus”, and consists of coarser clastics. The Tawil is primarily composed of fine- to medium-grained gray to reddish sandstones with subordinate micaceous siltstones and shales. These thin, discontinuous micaceous siltstones and shales yield a series of sharp “gamma spikes” that give the Tawil a characteristic log signature. The Tawil was deposited in a fluvial to marginal marine environment.

The age of the Tawil is generally considered to be Early Devonian, though recent biostratigraphic analyses indicate the Tawil to include the latest Silurian (Pridolian). The Tawil varies considerably in thickness, ranging from 1,010 ft to 225 ft in the Ghawar Area. This thickness variation is thought to be due to depositional control, though an erosional unconformity at the base of the Jauf Formation cannot be ruled out. The Tawil is commonly tightly cemented with silica and kaolinite, and is considered a poor reservoir.

After deposition of the Tawil, the shallow marine sands of the Jauf Formation were deposited over a broad shelf. The Jauf sandstones have the most favorable pre-Khuff reservoir development in the Ghawar Area, and are the host sands for the recently discovered giant gas-condensate field at Hawiyah. The Jauf consists of fine- to medium-grained sandstones that lack the silica cement that is so prevalent in other pre-Khuff sandstones of the Ghawar Area. Instead, the reservoir sandstones are weakly-cemented with authigenic illite clay. The Jauf Reservoir is further discussed in the “Reservoir Rock” section in this paper.

The reservoir sandstones of the Jauf are capped by a very distinctive shaley/silty unit informally called the “D-3B” stratigraphic marker. On wireline logs, it is characterized by a shaley/silty zone with relatively high resistivities and velocities which rests directly on porous sands of the Jauf Reservoir (Figure 8). This zone is progressively better developed to the northeast, where it forms the seal for the Jauf gas-condensate accumulation at the offshore Abu Safah field, which is located approximately 70 km north of the island of Bahrain. The unit is characterized by an abundance of marine sphaeromorph acritarchs of Saudi Aramco’s “D-3B” palynozone.

The D-3B marker unit is considered to have been deposited during a sea level rise and may represent a condensed section with a maximum flooding surface. Regional log and biostratigraphic correlations indicate this unit to be correlative to the Hammamiyat Limestone Member of Northwest Arabia (Al-Hajri et al., 1996). The age of the Jauf Formation is Early to Middle Devonian (Emsian-Early Givetian).

Overlying the Jauf Formation is the Middle to Upper Devonian Jubah Formation. The Jubah is characterized by a rather monotonous sequence of very fine- to medium-grained sandstones with subordinate siltstones and shales. The thickest occurrence of the Jubah is at Abqaiq, where 1,451 ft were penetrated. Sporadic shows in the Jubah have been observed, but log and test analyses indicate the sands to be of poor reservoir quality.

Carboniferous-Early Permian

As a result of the collision of Gondwana and Laurentia in Early Carboniferous(?) time, the northeast margin of Gondwana was transformed from a passive to an active margin (Beydoun, 1991). This tectonic event is termed the Hercynian Orogeny, and has had a dramatic effect on the geology of Arabia. Major north-south trending, basement-involved horst blocks formed in Central and Eastern Saudi Arabia during this event, and are the sites of significant amounts of erosion. On the crestal portion of Ghawar, for example, approximately 3,500 ft of section was removed. The fault-bounded Hercynian structures, such as Khurais (located 100 km west of Ghawar) and Ghawar, contain significant oil and gas reserves, and are generally well-expressed on gravity maps as positive anomalies.

Sediments of Early Carboniferous age are unknown in the Ghawar Area. During this time an extensive erosional surface developed, which is referred to as the “pre-Unayzah Unconformity” (PUU) or Hercynian Unconformity. The generalized pre-Permian geology of the area is shown on the PUU Subcrop Map (Figure 5). This map illustrates the significant amount of uplift and erosion at Ghawar during the Hercynian Orogeny.

Deposition of the Middle Carboniferous(?) to Lower Permian Unayzah Formation followed the Hercynian Orogeny. The basal Unayzah Formation consists of fine- to coarse-grained fluvial/alluvial sands that filled the relict topography due to differential erosion of the Hercynian structures (Al-Laboun, 1987). In Southern Saudi Arabia and Oman, glacial sediments of the Lower Unayzah(?), and the Omani equivalent Al-Khlata Formation, were deposited (McClure, 1980; McClure et al., 1988; Hughes-Clarke, 1988; Levell et al., 1988). These glaciogenic sediments may be represented in the Ghawar Area by a southeasterly-thickening wedge of undated, uncorrelatable siliciclastics in the Haradh-Tinat Area.

In the Southern Ghawar Area, the Unayzah forms the principal pre-Khuff hydrocarbon reservoir, and is divided into a lower Unayzah-B Reservoir, a middle Siltstone Member, and an upper Unayzah-A Reservoir. The Unayzah is missing in Northern Ghawar, where the base of the Khuff Formation rests unconformably on pre-Hercynian sediments.

Late Permian-Triassic

Following deposition of the Unayzah Formation, a major transgression occurred throughout Arabia, during which the predominantly carbonate Khuff Formation was deposited. This transgression coincided with Late Permian rifting along the Zagros suture and the opening of the Neo-Tethys ocean (Sengor et al., 1988; Beydoun, 1991).

The Khuff consists predominantly of cyclic shallow water carbonates and evaporites that generally thicken to the east. In the Ghawar Area, the Khuff ranges in thickness from approximately 1,400 ft at Haradh to 1,750 ft at Abqaiq. The Khuff carbonates contain major non-associated gas reserves with varying amounts of hydrogen sulfide. The hydrogen sulfide content, in general, increases to the north in the Ghawar Area, concurrent with increasing reservoir temperatures.

At the base of the Khuff Formation, transgressive sands and shales occur which are commonly referred to as the “Basal Khuff Clastics”. The regionally extensive shales, along with associated tight carbonates, form the major regional seal for pre-Khuff hydrocarbon accumulations in Central and Eastern Saudi Arabia.

At the end of Khuff deposition in the Early Triassic, shales and subordinate carbonates of the Sudair Formation were deposited. This was followed by deposition of a mixed assemblage of shales, carbonates, and sands of the Jilh Formation during continued opening of the Neo-Tethys. This extensional phase resulted in the reactivation of earlier Hercynian faults and therefore formed an important growth event for pre-Khuff hydrocarbon-bearing structures in the Ghawar Area (see discussion on “Trap Formation”).

Jurassic-Early Cretaceous

Following the extensional structural adjustments in the Triassic, the Arabian Plate returned to a major period of tectonic stability. A thick sedimentary sequence dominated by shallow water carbonates accumulated on a broad, stable platform (Alsharhan and Kendall, 1986). These rocks contain remarkably uniform and aerially extensive source, seal, and reservoir units that host the world’s largest oil reserves (Murris, 1980).

In the Ghawar Area, the platform was west-dipping into an intrashelf basin located to the west of Ghawar. This westerly dip was reversed in Late Cretaceous time, when the Arabian Plate started to tilt eastward due to differential loading of the plate margin.

Late Cretaceous-Tertiary

In Late Cretaceous time, tectonic activity again interrupted passive margin depositional conditions on the Arabian Plate (Grabowski and Norton, 1995). At this time, the subduction complex of the Neo-Tethys collided with Oman, resulting in uplift and emplacement of the Semail Ophiolite Complex in Oman (Glennie, 1995). This tectonic episode has been termed the First Alpine Event (Loosveld et al., 1996). In Eastern Saudi Arabia, this tectonic event led to significant structural growth and is represented stratigraphically by the pre-Aruma Unconformity (PAU). Most structures in the Ghawar Area show growth during this period. Additionally, in Late Cretaceous time, the dominant westward tilt of the Ghawar Area was reversed, with the regional dip changing to the east.

Late Alpine tectonic movements affected Arabia from the Middle to Late Tertiary. At this time, Arabia separated from Africa along the Red Sea Rift and collided with Eurasia, resulting in compression of the Arabian Plate (Beydoun, 1991). In Oman, this collision resulted in the main uplift of the Oman Mountains and has been termed the Second Alpine Event (Loosveld et al., 1996). In Eastern Saudi Arabia, this event led to major reactivation of pre-existing structures and is represented stratigraphically by the pre-Neogene Unconformity (PNU). Additionally, the regional eastward tilting of the Arabian Plate, initiated in the Late Cretaceous, continued.

The significance of these tectonic events, and the effects on the growth histories of structures in the Ghawar Area, is further discussed in the section entitled “Trap Formation” later in this paper.

HYDROCARBON GEOLOGY

Exploration History

Saudi Aramco has drilled approximately 45 pre-Khuff wells in the Ghawar Area as of mid-1997. These pre-Khuff penetrations are shown in Figure 2. Depths to the base of the Khuff range from approximately 12,000 to 15,000 ft.

Pre-Khuff hydrocarbons in the Ghawar Area were first discovered at Shedgum in 1980. The Shedgum discovery well flowed sweet gas and condensate from the Devonian Jauf Reservoir. The next major pre-Khuff gas discovery was at Haradh in 1982. The discovery well flowed significant quantities of sweet gas from both the Permian Unayzah-A and Unayzah-B Reservoirs. Also in 1982, light, sweet oil was discovered at Tinat in the Unayzah-A Reservoir. In 1984, non-associated sweet gas was discovered at Sahba in the Unayzah-A Reservoir (Figure 2).

The next major discovery in the pre-Khuff in the Ghawar Area did not occur until 1994, when the Hawiyah-200 well discovered gas-condensate in the Jauf Reservoir on the east flank of Ghawar. Hawiyah-200 flowed 20.2 million cubic feet gas per day (mmcfgpd) of sweet gas with 3,286 barrels per day (bpd) of condensate. Hawiyah-200 was the first test of the recently defined Jauf flank play at Ghawar, and has provided a major impetus to explore for similar structural/stratigraphic traps along the flanks of Ghawar, as well as other deeply-eroded Hercynian structures.

In early 1997 another major gas-condensate discovery was made at the Waqr structure, located southwest of Ghawar. The discovery well flowed in excess of 40 mmcfgpd from the Unayzah Formation. The Waqr structure is located along a separate north-south structural trend which, like Ghawar, originated in the Hercynian (Figure 2).

The pre-Khuff petroleum system is graphically depicted in Figure 6, and shows the essential elements of a prospective hydrocarbon system: source, reservoir, seal, and timing of trap formation and hydrocarbon generation.

Source Rock

Based on carbon isotope and chromatographic fingerprinting techniques, the primary source rock for Paleozoic hydrocarbons in the Ghawar Area is the basal “hot shale” of the Lower Silurian Qusaiba Member of the Qalibah Formation (Mahmoud et al., 1992; Cole et al., 1994; Halpern and Carrigan, 1995, unpublished report). These basal Qusaiba shales have an average total organic carbon (TOC) richness of 3%, with a maximum observed TOC of 6.15% (Monnier et al., 1990, unpublished report; Cole and Colling, 1994, unpublished report). The organic matter in the Qusaiba has been identified as predominantly amorphous Type II kerogen.

Other potential Paleozoic source rocks have been evaluated, but were determined to be poor source rock candidates due to low organic richness, insufficient thickness, and/or poor lateral continuity. These analyses include shales from the basal Khuff, Unayzah, Jauf, and Qasim formations.

Reservoir Rock

The major pre-Khuff hydrocarbon reserves discovered to date in the Ghawar Area are from the Lower Permian Unayzah Formation and the Lower to Middle Devonian Jauf Formation. The Unayzah Formation is the principal pre-Khuff hydrocarbon reservoir in the Southern Ghawar Area (Haradh, Sahba, Waqr and Tinat), while the Jauf Formation is the primary reservoir to the north (Hawiyah, ‘Uthmaniyah and Shedgum).

The Unayzah Reservoir is generally divided into two units: the upper Unayzah-A Reservoir and the lower Unayzah-B Reservoir (Figure 7). These reservoir units are separated by a siltstone member, which is reddish to gray in color in the Southern Ghawar Area. This siltstone separator is well-developed in Southern Haradh, Sahba, and Tinat, but is poorly-developed to the north. The Unayzah is missing in Central and Northern Ghawar and thickens to the south. Thickness ranges from 0 ft in Southern Hawiyah to nearly 1,600 ft at Tinat, where a lower Unayzah unit (or its equivalent) is present beneath the Unayzah-B Reservoir.

The Unayzah-A Reservoir has tested gas at Haradh, Sahba, and Harmaliyah. Premium crude oil occurs in the Unayzah-A at Tinat, and is the only oil occurrence thus far found in the pre-Khuff of the Ghawar Area. Porosities range from 5-25%, and average around 12% in the pay zones. Gas flow rates from the Unayzah-A Reservoir are highly variable, ranging from less than 5 mmcfgd to more than 40 mmcfgd.

The Unayzah-B Reservoir has only flowed measurable gas in southern Haradh. Porosities in this well, and surrounding Unayzah-B penetrations, are generally low and average approximately 6%. Higher flow rates in the Unayzah-B are attributable to fracture-enhanced permeability.

The Jauf Reservoir in the Ghawar Area is very well developed and displays unusually high porosities considering the depth of burial. The Jauf Reservoir is the productive pre-Khuff unit at Shedgum, Hawiyah, ‘Uthmaniyah, and Abu Safah fields. Log correlations of Jauf penetrations in the Eastern Province of Saudi Arabia show excellent reservoir development and continuity.

The Jauf Reservoir ranges in thickness from 292 to 474 ft in the Ghawar Area. The type log for the Jauf Formation is shown in Figure 8. The Jauf can be crudely divided into two units based on log character. The lower unit is characterized by “cleaner” looking sands on the gamma-ray, with few shale-siltstone intercalations. The upper unit is characterized by “dirtier” looking sands (higher gamma-ray), with several shale intercalations. Both fining- and coarsening-upward sequences are evident in the upper unit. Reservoir quality is best developed in the upper unit at Ghawar.

As noted previously, the Jauf Reservoir lacks the silica cementation that is so prevalent in other pre-Khuff siliciclastic units. The reservoir sands are weakly-cemented with authigenic illite clay, which occurs as grain coatings, as well as pore lining and bridging filaments (Cocker and Al-Shahab, 1995, unpublished report). The illite grain coatings have inhibited quartz cementation, and is primarily responsible for the preservation of high porosity in these deeply-buried sandstones.

The abundance of dispersed illite in the reservoir sands has a significant effect on wireline log responses, and, if not accounted for, can lead to pessimistic formation evaluations (Figure 8). Among these potential “pitfalls’” is the lowering of resistivity values due to the excess bound water and high cation exchange capacity of the illites. This can lead to pessimistic water saturation calculations and potentially bypassed low-resistivity pay zones. Sands with resistivities as low as 0.35 ohm-meters and Archie-derived water saturations as high as 65% have produced water-free hydrocarbons in the Jauf Reservoir.

Additionally, the presence of illite in the sands increases the gamma-ray response due to the potassium, which makes the sands look “dirtier”. An additional log evaluation “pitfall” caused by the presence of illite is the suppression of the “gas effect” noted on neutron-density logs. This suppression of the neutron-density curve crossover is caused by the higher apparent neutron porosity due to the abundant bound water and correspondingly high hydrogen index associated with the illite.

The Jauf Reservoir is host to the recently discovered giant gas-condensate field at Hawiyah, and is also the productive pre-Khuff gas reservoir at Shedgum, ‘Uthmaniyah, and Abu Safah. Porosities up to 30% are observed in the Jauf Reservoir, which is extremely high considering the present-day depths of approximately 14,000 ft. As noted earlier, the preservation of these high porosities is due to the presence of illite grain coatings.

Seal Rock

The major regional seal for the pre-Khuff petroleum system corresponds to the transgressive shales and carbonates of the basal Khuff Formation. These shales and tight carbonates were deposited over a widespread area during the Late Permian sea level rise. The basal Khuff forms the top seal to the Unayzah Reservoir in four-way structural closures in the Southern Ghawar Area. The basal Khuff also forms the top seal to the Jauf Reservoir in the combination structural-unconformity type traps at Shedgum, Hawiyah, and ‘Uthmaniyah. In the unconformity-related traps, the basal Khuff unconformably overlies a truncated Jauf Reservoir section, which was eroded during the Hercynian Orogeny.

The Jauf Reservoir may also be sealed by the informally named “D-3D Zone” (Figure 3). This shaley interval is a widely correlatable transgressive unit directly overlying the Jauf Reservoir sandstones. As noted previously, this unit is progressively better-developed to the northeast, where it forms the seal for the Jauf gas accumulation at the offshore Abu Safah field. To the southwest, this unit gets sandier, resulting in a progressive loss of sealing capacity.

Lateral seals associated with Hercynian faults are also important in controlling hydrocarbon accumulations in the Ghawar Area. These pre-Khuff faults have juxtaposed the down-dropped Jauf Reservoir sands against shales, siltstones, and tight sandstones of the pre-Jauf section on the upthrown fault block. These faults, in turn, are overlain and sealed by the basal Khuff Formation; though reactivated faults cutting the Khuff are known to exist at Ghawar. It is also considered possible that the fault planes, with low calculated smear-gouge ratios, could act as effective seals.

Trap Formation

Structural development and trap formation in the Ghawar Area took place in four distinct stages: the Carboniferous stage associated with the Hercynian Orogeny; the Early Triassic stage associated with the rifting along the Zagros suture and the opening of the Neo-Tethys sea; the Late Cretaceous stage associated with the collisional event that led to the emplacement of the Semail Ophiolite Complex in Oman (First Alpine Event); and the Middle to Late Tertiary stage associated with the opening of the Red Sea and collision of Arabia with Eurasia (Second Alpine Event). Three structural cross-sections illustrating the trapping configurations for pre-Khuff hydrocarbons in the Ghawar Area are shown in Figures 9 to 11. The locations of these cross-sections are shown on the PUU subcrop map (Figure 5).

Interestingly, the eight structures studied in the Ghawar Area (Figure 2) show different growth histories. Figure 12 is a summary chart of the individual structures in the Ghawar Area showing the differing growth histories as well as the presence of associated gravity anomalies and hydrocarbons.

To evaluate these growth histories, a series of isochron, isopach and time structure maps of critical horizons were generated using most of the available higher fold seismic data. The maps selected to illustrate the four main growth periods include: Base Khuff to Base Qusaiba Isopach (Carboniferous Hercynian Event); Jilh Dolomite/Mid-Jilh Reflector to Top Khuff/Base Khuff Isochron (Triassic Zagros Rifting Event); pre-Aruma Unconformity to Shu’aiba Isochron (Late Cretaceous First Alpine Event); and Top Aruma Time Structure (Tertiary Second Alpine Event). The later time structure map was selected because a consistent regional seismic pick on the pre-Neogene Unconformity (PNU) could not be made. It is reasoned that the top of the Aruma Formation would have been deposited as a relatively flat surface, and that significant relief of that horizon is attributable to the Middle-Late Tertiary growth event.

Carboniferous Growth Event (Hercynian)

The Hercynian Growth Event is evident on the Base Khuff to Base Qusaiba Isopach, which is shown in Figure 13. The isopach thinning indicated on this map is due to erosion associated with the Hercynian Orogeny of probable Carboniferous age. Flattening of seismic sections on the Base Khuff also illustrates this growth event, as shown in Figure 14. Note that on the isopach map significant Hercynian growth is evident at Ghawar, Tinat, Sahba, and Waqr. Only minor thinning occurs at Harmaliyah. No thinning occurs at Abqaiq and Udaynan, indicating these structures to be non-Hercynian. Note that the Abqaiq structure, as well as several other “late” structures in the area such as the Dammam and Awali Domes, most probably originated by post-Paleozoic movements of Infracambrian salt. These structures were not affected by Hercynian movements, and have preserved Upper Paleozoic sections on the crest.

Hercynian structures in the Ghawar Area are more economically attractive, in that the Paleozoic reservoirs contain higher BTU gases with significant condensate yields. These structures also tend to be more fully-filled to structural spill point.

Early Triassic Event (Zagros Rifting)

The next major growth event occurred in the Early Triassic in response to rifting along the Zagros Suture and the opening of the Neo-Tethys Sea. During this extensional event, earlier Hercynian structures were reactivated, further enhancing these structures. This growth event is illustrated by the Jilh Dolomite (or Mid-Jilh Reflector) to Top Khuff (or Base Khuff) Isochron Map (Figure 15). Note that Early Triassic growth is only associated with Hercynian structures (i.e., Ghawar, Sahba, Tinat, and Waqr). No structure in the Ghawar Area can be attributable to Triassic growth only.

Late Cretaceous Event (First Alpine Orogeny)

A major growth phase in the development of structures in the Eastern Province occurred in the Late Cretaceous. Uplift and erosion associated with Late Cretaceous tectonic activity created the widespread erosion surface of the pre-Aruma Unconformity (PAU). All structures studied in the Ghawar Area, with the exception of Tinat, have been affected by this growth period. The Late Cretaceous growth event is represented by the PAU-Shu’aiba Isochron Map (Figure 16). A flattened seismic section on the PAU is shown in Figure 17.

Note the significant growth that took place at Ghawar and Abqaiq in the Late Cretaceous. The younger Abqaiq structure originated at this time and continued to grow in the Tertiary. At Ghawar, which originated in the Hercynian, the variable thinning noted indicates that the component structural blocks making up the giant structure, grew at differing rates.

Weak to moderate structural growth occurred at Sahba, Waqr, Niban, and Harmaliyah during this time, while Tinat experienced very little to no Late Cretaceous growth. The Udaynan structure also formed at this time, but had an unusual development that is unique among the Ghawar Area structures. The Udaynan structure originated as a result of the reversal of regional dip from the west to the east in Late Cretaceous time. Prior to the Late Cretaceous, the Udaynan Area was a “hinge zone” where the sediments thickened westward into the basin. In Late Cretaceous time, the basin shifted to the east, reversing the dip at Udaynan and creating the structure. The eastward tilting continued in the Tertiary, further enhancing the east limb of the Udaynan structure.

Mid-Late Tertiary Event (Second Alpine Orogeny)

The final episode of structural development in the Ghawar Area occurred in the Middle-Late Tertiary. This growth event is shown by the Aruma Time Structure Map (Figure 18). Note that there is considerable relief at the top Aruma level at Ghawar and Abqaiq, indicating significant post-Aruma growth of these structures. This relief is also shown on the seismic section of the ‘Ain Dar, Shedgum, and Abqaiq structures (Figure 19). It appears the Tertiary growth has simply enhanced the previously existing structural configurations at Ghawar and Abqaiq, with no new structural trends developing.

Other structures in the Ghawar Area show weak to moderate growth during this episode, and were primarily affected by the continued northeast tilting of the entire region during this time. This tilting has had a negative effect on lower relief structures in the area, effectively “opening up” the previously existing west closure of several horizons.

Generation and Migration

As previously discussed, the primary source rock for the Paleozoic hydrocarbons in the Ghawar Area is the “hot shale” of the Lower Silurian Qusaiba Member. Figure 20 is a burial history diagram of the Udaynan well (Figure 5), which shows the maturation history of the Base Qusaiba. The Udaynan burial history closely approximates that of the basinal areas immediately east and west of Ghawar, which are the most proximal “kitchens” for hydrocarbon generation in the Ghawar Area.

The burial history diagram indicates that the Base Qusaiba entered the oil window at approximately 230 Ma, in the Middle to Late Triassic. The Base Qusaiba progressed through the oil window and entered the wet gas/condensate window at approximately 125 Ma, in the Early Cretaceous. The Base Qusaiba then proceeded through the wet gas window and entered the dry gas window at approximately 60 Ma, in the Early Tertiary. The Base Qusaiba remains in the dry gas window at present, though the generative potential may be spent in the deepest portions of the basins.

The relationship of the source rock maturation and migration to trap development was previously shown in Figure 6. The exploration ramifications of this petroleum system model are significant. The model indicates that oil generation and migration occurred after Carboniferous (Hercynian) and Early Triassic growth, but before the development of Late Cretaceous and Tertiary structures. Therefore, only earlier-formed Hercynian and Early Triassic structures in the area can be considered prospective for oil. Late structures, such as Abqaiq and Udaynan, would only be prospective for gas, with later structural development favoring drier gases. The model further indicates that earlier-formed oil accumulations would be displaced by later-arriving gas, providing adequate migration pathways existed.

The above scenario could explain two anomalies that occur in the pre-Khuff of the Ghawar Area: the Unayzah oil accumulation at Tinat, and the prominent oil staining of the Jauf gas-condensate reservoir at Shedgum and Hawiyah. The Tinat oil occurrence is anomalous because, at 15,000 ft, it is deeper than the gas/condensate accumulations at Hawiyah and Shedgum (14,000-14,500 ft). The source rock is considered the same (Qusaiba), and the present-day geothermal gradients are very similar (about 1.40°F/100 ft). However, the growth histories of the two structures are different, with Tinat experiencing very little of the late growth that affected Ghawar (Figure 10).

Both Ghawar and Tinat originated in the Hercynian, which would have allowed oil generated in the Mesozoic to migrate to these structures. The late growth that occurred at Ghawar is thought to have reactivated the Hercynian faults, allowing gas that was being generated at the time to migrate to the structure and displace the previously existing oil accumulation. This would explain the oil staining that is observed in the Jauf Reservoir at Ghawar. The Tinat structure, with minor late growth and fault reactivation, may not have received the late-arriving gas to displace the oil. The high Gas/Oil Ratio noted at Tinat (2,100) may be due to partial migration of gas into the structure, or in-situ cracking of the oil to gas.

CONCLUSIONS

Current exploration efforts to increase the non-associated gas reserves in the Kingdom are focusing on the pre-Khuff siliciclastics. The main exploration objectives are sandstones of the Middle Carboniferous (?) to Lower Permian Unayzah Formation and Lower to Middle Devonian Jauf Formation.

Pre-Khuff hydrocarbon traps are found in simple anticlinal closures as well as more complex combination structural-stratigraphic traps. The structural-stratigraphic traps involve faulting and/or truncation of the Jauf Reservoir on the flanks of Hercynian structures. The recent Hawiyah gas-condensate discovery on the eastern flank of Ghawar is an example of this type of trap. The major top seal for the pre-Khuff accumulations in the Ghawar Area are shales and tight carbonates of the transgressive basal Khuff Formation.

Trap formation and modification are attributed to four distinct growth periods: the Carboniferous (Hercynian Orogeny), Early Triassic (Zagros Rifting), Late Cretaceous (First Alpine Orogeny), and Tertiary (Second Alpine Orogeny). Structures in the Ghawar Area show differences in growth histories, which have impacted the amount and type of hydrocarbon occurrences found in the structures.

The primary source rock for the pre-Khuff hydrocarbons is the basal “hot shale” of the Lower Silurian Qusaiba Member of the Qalibah Formation. The basal Qusaiba shales began oil generation in the Triassic and proceeded through the wet gas-condensate window in the Cretaceous. The Qusaiba next entered the dry gas generation phase in the Early Tertiary, and remains in the dry gas window at present.

ACKNOWLEDGMENTS

The authors wish to thank the Ministry of Petroleum and Mineral Resources and the management of Saudi Aramco for permission to publish this paper. We also thank Dr. M.I. Al-Husseini and two anonymous referees for their reviews of this manuscript, which resulted in an improved version.

ABOUT THE AUTHORS

Lawrence E. Wender was a Geological Consultant with the Area Exploration Department of Saudi Aramco, having been involved in the study and exploration of the Paleozoic of Saudi Arabia. He recently rejoined Mobil Oil Corporation and has nearly 20 years of oil industry experience. Lawrence holds a MSc degree in Geology from the University of Utah.

Jeffrey W. Bryant joined Saudi Aramco in 1990 as a Geologist working frontier exploration of the northwestern region of Saudi Arabia. Since 1993 he had been exploring for Paleozoic deep gas reserves in the Eastern Province of the Kingdom. Jeffrey has nearly 20 years of oil industry experience, including Alaska exploration and US Gulf Coast development with Exxon, and Gulf Coast of Mexico exploration with Agip. Jeffrey holds a MSc in Geology from the University of Arizona. He is currently with CMS Energy.

Martin F. Dickens is a Geophysical Specialist with Saudi Aramco, having joined the company in 1991. He has 20 years of international experience. Since 1993 he has been exploring for hydrocarbons in the Eastern province of Saudi Arabia. Martin holds a BSc in Geophysics from the University of Southampton.

Allen S. Neville is a Geophysical Specialist with Saudi Aramco. He was previously with Gulf Oil Corporation and has over 20 years of oil industry experience. Since joining Saudi Aramco in 1987, he has been involved in the study and exploration of the Paleozoic in Saudi Arabia. Allen holds a MSc in Geology from Wright State University.

Abdulrahman M. Al-Moqbel is a Geophysicist with Saudi Aramco. He graduated in 1995 from the University of Pacific (California) with a BSc in Geophysics. He has worked in the Geophysical Data Processing Division and the Area Exploration Division. Abdulrahman has been involved in the study and exploration of the Paleozoic hydrocarbons in the Eastern Province of Saudi Arabia. He is a member of the AAPG.

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