ABSTRACT

The six surface-piercing salt domes of interior North Oman form prominent topographic and geological features in an otherwise flat, rocky desert environment. These domes in the central part of the Ghaba Salt Basin have been known since the 1950s but very little data has been published on them. Our geological survey in 2001 provided significant new lithological, stratigraphic, and sedimentological information on the rocks exposed in the domes. This paper provides a comprehensive overview of the morphology, geometry, structural geology and geological evolution of the salt domes. Furthermore, it incorporates relevant information from unpublished subsurface studies to place the new geological field data in the context of ongoing exploration for deep hydrocarbon plays in Oman.

A wide variety of rocks is exposed in the salt domes: carbonates, clastics (conglomerates, sandstones, siltstones and clays), volcanics, evaporites and ‘caprocks’. Constituent rocks and structural style vary considerably from one dome to another, but at the surface the main lithological elements of the diapirs are carbonates and evaporites of the ‘Infracambrian’ (Late Precambrian to Early Cambrian) Ara Group, the uppermost unit of the Huqf Supergroup. Very large exotic blocks of bedded Ara carbonates––commonly hundreds of meters long––are well-exposed and form distinctive hills and ridges, thus allowing detailed field observations on intra-salt carbonate ‘stringers’ that have been carried up by rising diapiric salt. A close correlation exists between the facies of the carbonate exotics in the salt domes and Ara ‘stringer’ carbonates penetrated and extensively cored in recent deep exploration wells in the South Oman Salt Basin. This demonstrates the regional significance of the salt domes for the intra-salt ‘stringer’ hydrocarbon play in Oman. Our work has implications for the prospectivity of other ‘Infracambrian’ evaporite basins in Oman, and possibly also for time-equivalent (‘Hormuz’) salt basins elsewhere in the Middle East.

INTRODUCTION

Six surface-piercing salt domes that crop out in the desert of interior North Oman (Figures 1 and 2) have attracted the attention of geologists since the early 1950s but, until now, very little data have been published on these prominent topographic and geological features. By incorporating additional detailed geological field observations made in late 2001 on the salt domes, this paper synthesizes the surface and subsurface geological data collected over many years by geoscientists of Petroleum Development Oman (PDO). In particular, it provides new data on the facies of the remarkable ‘Infracambrian’ carbonate rocks that form large exotic rafts or ‘floaters’ in the exposed evaporitic rocks of the Late Precambrian to Early Cambrian Ara Group (Figure 3). The main objectives of this paper are as follows:

  • (a) Provide a comprehensive overview of the location, morphology, geometry, composition, structural geology and evolution of the six salt domes;

  • (b) Highlight the significance of the exposed Ara Group exotic carbonate blocks in the context of the successful intra-salt carbonate ‘stringer’ play in the South Oman Salt Basin, and the implications for the hydrocarbon prospectivity of the Ghaba Salt Basin in interior North Oman; and

  • (c) Provide a basis for further oil company and academic investigations on the Infracambrian carbonate rocks exposed in these salt domes. Subjects of particular interest are Ara Group sequence stratigraphy and facies analysis, source rock distribution, analog models for reservoir characterization, and static/dynamic modeling (including fracture studies) in support of the development of the South Oman ‘stringer’ hydrocarbon discoveries, together with seismic and structural geological studies.

Since 1997, PDO has made several new oil and gas discoveries in the Harweel area (Figure 2) in the southern part of the South Oman Salt Basin through a revival of the intra-salt Ara carbonate ‘stringer’ hydrocarbon play (Reinhardt et al., 2000). This play was proven in 1977 by the discovery of the deep Birba oil field situated close to the large, much shallower Marmul field, also in the South Oman Salt Basin. Subsequent geological studies (Mattes and Conway Morris, 1990) highlighted the distinctive characteristics of this unusual hydrocarbon play, one of the oldest petroleum occurrences known in the world (see Figure 21). Carbonate ‘stringer’ reservoirs are frequently dolomitized, with an average porosity of 8 to 12 percent at depths of 4 to 5 km. The main ‘stringer’ reservoirs in the South Oman Salt Basin are 80 to 120 m thick. Since they commonly contain intervals rich in organic matter, ‘stringers’ may form a self-charging petroleum system encased in salt, and such reservoirs can yield more than 6,000 b/d of oil in production tests, particularly if the reservoir is highly overpressured.

In 1989, a significant intra-salt oil discovery was made at a depth of about 4 km in the South Oman Salt Basin in exploration well Al Noor-1 (see Al Noor field, Figure 2). The seismically well-defined, intra-salt exploration objective turned out not to be a carbonate ‘stringer’ but a highly overpressured slab of silicilyte up to 400 m thick composed of microcrystalline silica (0.3–1 μm), possibly of organic origin. High porosity (average 23%) and very low permeability (<<1 mD) characterize this unique reservoir in the Athel Formation. It is rich in organic matter (up to 5% total organic carbon) and has high hydrocarbon saturations (>80%). The Athel Formation is thought to have developed as an organic-rich ooze in an anoxic deep-marine setting, coeval with the deposition of carbonate ‘stringers’ in shallower marine areas (Figure 3). Subsequent studies have revealed similar, but generally thinner, silicilyte rocks in other parts of the South Oman Salt Basin. Whilst Athel-equivalent rocks (black cherts of the Fara Formation; Rabu, 1988) appear to be present in the Oman Mountains, no silicilyte has been identified so far in the salt domes of the Ghaba Salt Basin. Notwithstanding the sizeable intra-salt oil discovery at Al Noor, exploration of the carbonate ‘stringer’ play was halted in the early 1990s after PDO had disappointing results from several deep wells drilled in the South Oman Salt Basin. The poor results were mainly due to difficult subsurface imaging and unpredictable (tight) reservoir facies in the ‘stringer’ carbonates.

The recent revival of the ‘stringer’ play in the Harweel area was not accidental. A large investment was made in pre-stack migrated high-quality 3-D seismic surveys, quantitative geophysical studies, and a wide variety of geological investigations (including extensive suites of log data, calibrated by substantial core coverage). Furthermore, an integrated team of petroleum geoscientists, academics, and postgraduate students was established in Oman that investigated several of the unusual aspects of this Late Precambrian to Early Cambrian hydrocarbon play (Grotzinger, 2000; Schröder, 2000; Grotzinger and Amthor, 2002).

So far, no dedicated exploration wells have been drilled to test the ‘stringer’ hydrocarbon play in the large Ghaba and Fahud salt basins in interior North Oman (Figures 2 and 4). However, modern 2-D and some 3-D seismic data from these areas reveal well-defined intra-salt sedimentary bodies at depth (see Figure 22). Moreover, the new field data presented in this paper on the surface-piercing salt diapirs in the Ghaba Salt Basin, unequivocally demonstrate the presence of similar ‘stringer’ carbonates in interior North Oman.

The multiple and complex petroleum systems of the Ghaba and Fahud salt basins have been studied recently by Pollastro (1999) and Terken et al. (2001). In addition, a detailed study of the ‘Dhahaban’ petroleum system in North and Central Oman by Terken and Frewin (2000) traced the geochemically distinct ‘Q’ oil family to a postulated prolific Infracambrian to Cambrian evaporitic source rock at the top of the Ara Group in the Ghaba and Fahud salt basins. Consequently, the carbonate-evaporite rocks exposed in the surface-piercing salt domes of interior North Oman are of particular interest in further unraveling both the deeper stratigraphy and the petroleum prospectivity of these basins (Figures 3 and 4). So far, exploration drilling in a possible fourth salt basin (Figure 2), postulated in the Dhofar area to the west of the South Oman Salt Basin (Blood, 2001), has not proved conclusively the presence of evaporites and/or carbonate stringers in this part of Oman.

BACKGROUND

Salt Domes of the Arabian Gulf Region

Evaporites of late Proterozoic to earliest Cambrian age (often referred to informally as ‘Infracambrian’) occur in several early subsiding rift basins in Oman, Saudi Arabia, the United Arab Emirates, Kuwait, and Iran, with extensions into Pakistan and northwest India. Paleogeographically, with Pakistan and India restored to their likely positions in northeast Gondwana, the original salt basin(s) would have extended for thousands of kilometers along strike, thus constituting the world’s oldest preserved saline giant, the ‘Hormuz Complex’ (Stöcklin, 1968, 1986). The distribution of Infracambrian salt basins in the Middle East (Husseini and Husseini, 1990) is of particular importance since halokinesis is often a controlling factor in creating structural traps in this very rich hydrocarbon province. Recurrent deep-seated basement faulting and related halokinesis is responsible for the formation of major structures over large areas, reinforced or overprinted along, for example, the Zagros fold-thrust belt and the Oman Mountain deformation belt, by Mesozoic and younger compressional tectonics (McQuillan, 1991).

The Hormuz salt is present in more than 200 salt domes distributed throughout the southeastern part of the Cretaceous-Tertiary Zagros fold-thrust belt (Kent, 1970; Jackson, 1991), and also forms the diapiric cores of several Arabian Gulf islands and topographic features along the coast of the United Arab Emirates (Salah, 1996). In Oman, six surface-piercing salt domes were discovered in the early 1950s by aerial and ground reconnaissance surveys in what is now known as the Ghaba Salt Basin (Figure 2). Subsequently, these prominent surface features were investigated in the field by Iraq Petroleum Company (IPC) geologists as part of their early hydrocarbon exploration efforts in interior Oman (for an historic account of these first field surveys see Sheridan, 2000). These early workers (Morton, 1959; Tschopp, 1967) correctly correlated the exposed Infracambrian Ara Group evaporites with the Hormuz succession.

The salt domes in the Arabian Gulf region penetrate thick sequences of Cambrian to Tertiary sedimentary rocks that are locally as much as 10 km thick. In southeastern Iran, the overburden was first deformed in the Zagros fold-thrust belt and then pierced by salt diapirs (Jackson, 1991). In the United Arab Emirates and elsewhere in the Arabian Gulf region, such diapirs create numerous large traps for oil and gas in late Paleozoic to Mesozoic carbonate and clastic reservoirs. Salt domes in the Zagros-Arabian Gulf-United Arab Emirates region expose a diverse assemblage of lithologies of inferred Infracambrian and younger age that occur as exotic blocks ‘floating’ in a mass of deformed halite, gypsum, and anhydrite (Kent, 1979; Salah, 1996). These ‘floaters’ include a variety of sedimentary rocks, rhyolitic and basaltic volcanics, and intrusive diabase sills and dikes (Stöcklin, 1986). Sedimentary rocks include light and dark dolostones (often cherty and with stromatolites); dark-gray, fetid, laminated limestones; buff-pink-brown sandstones; and variegated shales and hematitic siltstones. Less common are conglomerates, and marine limestones and shales that contain possible pre-Middle Cambrian trilobites, brachiopods, and stromatoporoids. The larger rafts and blocks (tens to hundreds of meters in size) generally display successions of several of these rock types, and commonly contain a cyclic arrangement of facies, suggestive of primary deposition within a predominantly carbonate-evaporite environment (Kent, 1979).

Volcanic rocks that have been reported from individual blocks consist of an assemblage of pillow basalts, rhyolitic ignimbrites, and tuffs. Basalts were possibly fed by diabase dikes; such rocks have been locally reworked to form conglomerates deposited above internal surfaces of erosion (Kent, 1970). The interbedded nature of sedimentary and volcanic rocks indicates syndepositional magmatism, supporting the postulated intra-rift setting of the Hormuz salt basins.

With the exception of the limestones and shales, the ‘exotic’ blocks are devoid of fossils and so their age remains uncertain. However, the rocks exposed as exotics in the salt domes (including the volcanics), do not correlate with the Paleozoic and Mesozoic successions exposed and/or drilled in the region. Thus, it is assumed that most of the blocks in the Hormuz sequence must be older, and are probably Late Precambrian to Early Cambrian (‘Infracambrian’).

Salt Domes of Oman: Previous Work

Reconnaissance trips by petroleum geologists to the surface-piercing salt domes of Oman (Figure 1) were made in the early 1950s during the quest for oil that followed important oil discoveries in Iran and Saudi Arabia. The first of these field surveys, undertaken by IPC geologists, identified the major lithological components of the large exotic blocks exposed in the domes (Dunham, 1955). Dark, finely laminated limestones and vuggy dolostones were seen, in addition to halite, anhydrite, red siltstone, and diagenetic nodules (possibly danburite, a calcium boron silicate). Accumulations of ‘slaggy’ carbonate probably represented insoluble cap rock facies. The assemblage was interpreted as a suite of primary evaporite and early diagenetic facies and mineral assemblages. Inclusions of bitumen were identified in calcite crystals in the limestones.

In the early 1960s, the large oil fields of Yibal, Fahud, and Al Huwaisah were discovered in the Fahud Salt Basin in northern Oman by using a combination of surface geological mapping and basic geophysical surveys. As a result, field studies by Nicol and Magnee (1964) focused on assessing the hydrocarbon prospectivity of the area of the salt domes situated farther to the southeast in the Ghaba Salt Basin (Figure 2). It was recognized early that individual faulted flank traps might be small, and that discoveries involving a single dome might be uneconomic. The field work provided few constraints on the stratigraphic position of the exposed exotic blocks. Nicol and Magnee (1964) concluded that the exotic blocks were pre-Mesozoic, and that some lithologies showed similarities with the outcrop stratigraphy in the NNE-trending Haushi-Huqf High that forms the eastern flank of the Ghaba Salt Basin (Figures 2 and 4).

A more important observation by Nicol and Magnee (1964) was that the dark, organic-rich, finely laminated carbonates first reported by Dunham (1955), were present in all salt dome outcrops, whereas other lithologies (such as clastics) were far more limited in volume and distribution. It was inferred that the laminated carbonates had been deposited in tidal-flat settings where they would have been susceptible to early dolomitization. Finally, their reconnaissance work indicated that two of the northerly domes, Jebel Majayiz and Qarat Al Milh, were situated on the long-lived NNW-trending Maradi strike-slip fault zone (Figures 2 and 4).

A substantial effort was subsequently made by Parker and Wessels Boer (1968) to understand the stratigraphic position of the exotic blocks in the salt domes. Their work suggested that the finely laminated carbonates, so commonly present in the salt domes, were correlative with the thin-bedded carbonates present in the lower part of the late Precambrian Buah Formation (Huqf Supergroup) exposed in the Haushi-Huqf High on the eastern margin of the Ghaba Salt Basin. Furthermore, it was suggested that the minor occurrences of non-fossiliferous reddish siltstones could be correlated with the Shuram Formation that stratigraphically underlies the Buah Formation (Figure 3). Authors such as Gorin et al. (1982) emphasized the similarities between the Buah Formation and the carbonates exposed in the Ghaba salt domes. However, since this early work, numerous exploration wells in the Oman salt basins have shown that the thick evaporite-carbonate sequence at the top of the Huqf Supergroup, originally known as the Ara Formation (Hughes Clarke, 1988) but subsequently redefined as the Ara Group (Figure 3), post-dates the Buah Formation even in the center of the salt basins. It cannot be excluded that some of the stromatolitic carbonates observed in the Ghaba salt domes might have been rafted from the upper (possibly evaporitic) part of the Buah Formation. Nevertheless, the combination of recent exploration drilling results, detailed seismic interpretation studies, outcrop studies in the Haushi–Huqf High, and the new geological field data presented here indicate that most, if not all, ‘exotic’ carbonate blocks exposed in the Ghaba salt domes represent intra-salt ‘stringer’ carbonates of the Ara Group.

Hydrocarbon exploration and development activities by PDO in the South Oman Salt Basin were accelerated following the discovery of several new oil fields in this basin in the early 1980s. The work demonstrated the intimate relationship between repeated phases of halokinetic deformation and sedimentation from the early Paleozoic onward (Al-Marjeby and Nash, 1986; Heward, 1990). A wide array of salt tectonic features is evident from 2-D and 3-D seismic surveys calibrated by numerous exploration wells (Al-Barwani et al., 2002). Spatial and temporal links have been established between salt movements in the South Oman Salt Basin and regional basement tectonics in other parts of Arabia (for a description and references see Boserio et al., 1995).

Several relatively shallow (less than 1,200 m deep) exploration wells were drilled in the early 1970s in the vicinity of the Qarn Alam and Qarat Kibrit salt domes in the Ghaba Salt Basin. Only minor oil stains were found in Tertiary rocks and in clastics of the Haushi Group, and some immovable oil was logged in carbonates of the Cretaceous Shuaiba Formation (for stratigraphy see Figure 3).

In the course of the 1990s, the hydrocarbon potential of the area of the salt domes of North Oman received renewed attention as part of the drive to mature additional frontier exploration plays. PDO geologists made several reconnaissance trips to the surface-piercing salt domes to gather additional stratigraphic and structural data (Al-Harthy et al., 1991; Bell and Loosveld, 1994). In addition, vertical and oblique aerial photos were acquired and a significant number of radial 2-D seismic lines were shot over several salt domes with the aim of delineating possible flank traps. Detailed studies were made to rank the various prospects (Faulkner, 1998; Al-Lawatia, 2001). Furthermore, high-resolution gravity and aeromagnetic studies were carried out to constrain geometric models and potential trapping geometries. Finally, based on these studies, a dedicated exploration well was drilled in 1997 close to the Qarn Nihayda salt dome (see next section).

GEOLOGICAL SETTING OF THE GHABA SALT DOMES

The six surface-piercing salt domes in North Oman are located in the Ghaba Salt Basin. The Basin trends in a southwesterly direction from the jebels of the Salakh Arch on the southern edge of the Late Cretaceous-Tertiary deformation front of the Oman Mountains, to the Central Oman High (Figure 2). It is bounded to the east by the west-dipping flank of the similarly trending Haushi–Huqf High, and to the west by the Musallim Slope on the margin of a significant Precambrian structural high (Figure 4). Figure 3 shows the stratigraphy of subsurface interior Oman.

A total of 29 salt structures have been identified in the Ghaba Salt Basin, ranging in type from 8 relatively low relief, deeply buried salt pillows (e.g. Figure 22) to 21 narrow, high-relief salt diapirs (e.g. Figure 5). The diapirs are limited to the deepest part of the Basin (Figure 4) in a zone trending roughly northeast (Figure 6). Based on seismic and gravity data, the top of the relatively undeformed pre-salt Precambrian Huqf sedimentary sequence is estimated to be about l0 km deep in the center of the Basin. The salt deposits belong to the Ara Group, which forms part of the Huqf Supergroup (Figure 3), and is thought to be of latest Proterozoic to earliest Cambrian age (Amthor et al., 2002). Multiple phases of salt movements played an important role in the Cambrian and later history of the Ghaba Salt Basin. Nucleation and growth of salt structures is known to have occurred from at least the early Paleozoic (Droste, 1997) and has continued to the present day in some areas, as evidenced by late Quaternary sedimentation patterns around the salt domes.

Structurally, the salt diapirs are extremely high-relief features (as much as 9 km deep) that pierce the entire stratigraphic post-Ara Group sedimentary succession in the Ghaba Salt Basin (Figure 5). Nicol and Magnee (1964) concluded that the two most northerly salt diapirs, Jebel Majayiz and Qarat Al Milh, are located on the Maradi strike-slip fault zone. This was confirmed by additional field work along this major structural feature (Hanna and Nolan, 1989), and by extensive 2-D and 3-D seismic surveys carried out by PDO in this part of its concession.

The detailed structural evolution of the six surface-piercing diapirs is unlikely to have been uniform, especially as two of them are situated along the repeatedly reactivated deep-seated Maradi strike-slip fault zone, but several general observations can be made as to their complex geological evolution. Following an initial phase of diapir growth through passive downbuilding, the salt pillows or incipient diapirs were progressively buried under a thick sedimentary cover of lower Paleozoic clastics of the Haima Group. Downbuilding is consistent with the development of wide early rim synclines around several of the diapirs, as seen on 2-D and 3-D seismic data. This was followed by a period of quiescence that ended when the diapirs were remobilized during one or more periods of basement reactivation and faulting in the late Paleozoic. Further reactivation occurred during the Late Cretaceous at the time of thrusting of the Oman ophiolite over the carbonate foreland basin (Figure 2). In the Campanian, deformation took place farther south in the foreland domain, by left-lateral movement along basement-involved faults such as the Maradi Fault Zone that cuts across the Ghaba Salt Basin (Figures 2 and 4). Additional growth of the salt structures took place during the Tertiary, probably in several stages that were related to distinct tectonic pulses, both compressional and extensional, which have been well documented by studies of deformed Tertiary sequences in the Oman Mountains and its foreland areas (Rabu et al., 1993). Some of the diapirs in interior North Oman (e.g. Jebel Majayiz and Qarat Kibrit) are still active, as is shown by late Quaternary drainage patterns and the tilting of younger strata. Qarat Kibrit is also subject to active salt dissolution.

The Ghaba Salt Basin salt domes feature prominently on a depth map at top Ara salt level (Figure 6) as most of the diapirs are associated with negative residual gravity anomalies. Seismic imaging revealed a complex network of reverse, strike-slip, and normal faults close to each diapir, as demonstrated by subsurface imaging of the Qarn Alam and Qarat Kibrit salt domes (Figure 5). The number of reverse faults observed points at least locally or temporally to a regionally compressive stress field. Seismic sections across two of the outcropping diapirs (Figure 7a,b) and a buried dome (Qarn Alam oil field, Figure 7c) illustrate several key subsurface features, including salt-piercement geometry, gentle deep-seated rim synclines, and the relationship with basement fault zones.

Based on several integrated geological and geophysical evaluation studies, an exploration well (Qarn Nihayda-1) was drilled close to the Qarn Nihayda salt dome in late 1997 to test the hydrocarbon prospectivity of its eastern flank. It bottomed in the Al Khlata Formation at a total depth of 3,890 m (Figure 8). The well targeted multiple potential Mesozoic and Paleozoic reservoir-seal pairs but only found minor shows in the Natih reservoir. High-resolution 3-D gravity data had been acquired pre-drilling in order to constrain the seismic interpretation over the Qarn Nihayda salt dome, and the drilling results showed that there was good general agreement between the modeled and encountered salt diapir flank geometry.

Subsequent studies of possible traps in the steeply dipping and often faulted strata abutting the flanks of the complex diapirs in North Oman suggest that they may be relatively small in size and have a relatively low probability of exploration success because of fault and top seal risks. Exploration drilling results (oil staining, immovable oil) suggest that later movements had breached earlier traps, at least locally. Despite the recent seismic imaging of the steep salt-sediment interfaces through radial 2-D surveys, it is obvious that structural imaging is still aliased and that, for example, salt overhang and/or pinch-out traps are difficult to define. Future exploration will require high-resolution 3-D seismic surveys to adequately delineate potential oil and gas traps. Additional exploration challenges in this salt-diapir flank hydrocarbon play are drilling problems related to real-time steering of deviated wells, overpressures, and borehole instability (Al-Lawatia, 2001).

Salt-diapir flank traps are well known from hydrocarbon provinces such as the North Sea, Gulf of Mexico, and the Arabian Gulf. However, in the Ghaba Salt Basin substantial reserves have so far only been discovered in structural culminations overlying the crest of buried salt swells and diapirs (for example the Qarn Alam oil field in Figure 7c and the Saih Nihayda field in Figure 22). Although bitumen has been reported from the salt dome outcrops (see above) and minor shows have been observed in exploration wells drilled on their flanks, so far no live hydrocarbon seeps or tarry deposits have been found at the surface. This is despite the diapirs occurring in close proximity to the producing oil fields of Ghaba North, Qarn Alam, and Qarat Al Milh (see Figures 5 and 6).

GHABA SALT DOMES: DESCRIPTION

General

Summary descriptions are given for each of the six surface-piercing salt domes (from south to north) in the Ghaba Salt Basin (Table 1 and Figure 9). However, before describing the individual salt domes it is important to note that, whilst all six diapirs are different in detail, they have several features in common as follows:

  • The diapirs have elevations of only 100 m or less above the surrounding areas (Table 1) but they are prominent features in an otherwise flat and strongly deflated desert setting (Figure 10). They stand out owing to their rugged geomorphic expression and darker color (Figures 13a and 14). All six diapirs are visible and accessible by four-wheel drive from the Muscat-Salalah highway that crosses interior Oman from north to south.

  • The diapirs are roughly circular to irregularly oval in shape, with the largest (Qarn Sahmah) being over 8 km in circumference at the surface (Figure 11).

  • Their irregular topography reflects the strong contrast in rock hardness between the very resistant exotic carbonate blocks and the enclosing evaporite matrix.

  • The nature and volume of the exotic blocks varies from dome to dome, even though Ara Group ‘stringer’ carbonates predominate in all diapirs. No distinct trends have been observed in terms of stratigraphy and/or tectonic juxtaposition of the blocks at surface. This is not surprising considering that these rafts were dragged upward by buoyant salt from depths of at least 3 km (e.g. Al Khlata blocks in the Qarn Sahmah dome) to well in excess of 8 km for many of the Ara carbonate ‘stringers’. Isoclinal folds and flow banding can be seen in the more evaporitic sequences (Figure 12b), whereas more open-folding, dense fracturing, and evaporite veining is common in the thicker rafts of competent carbonates.

The establishment of a detailed ‘stringer’ stratigraphy in the salt domes is hampered by the often chaotic juxtaposition of Ara carbonates within the domes, the locally strong brecciation, and dolomitization, and the frequent lack of suitable sedimentary way-up criteria. Nevertheless, detailed field observations in the salt domes have provided the following information:

  • Locally the ‘stringers’ are inverted.

  • With the likely exception of part of the carbonate sequences exposed in Jebel Majayiz, most ‘stringers’ are probably less than 50 m thick in outcrop, compared to typical thicknesses of 80 to 120 m in the subsurface of the South Oman Salt Basin. It is not known whether this reflects a primary difference in ‘stringer’ development between the two salt basins or if it is due to subsequent salt diapirism, for example through the disintegration of the layered rafts along less-competent (evaporite) intervals during upward halokinetic transport.

  • In the Qarn Sahmah, Qarn Nihayda, Qarn Alam, and Jebel Majayiz salt domes, at least two distinct carbonate lithofacies occur that are tentatively interpreted as representing different Ara carbonate cycles or ‘stringers’ (see field photos in sections below). The predominant lithofacies consists mainly of light- to very dark-gray, finely laminated to thin-bedded, fetid dolomitic limestones and ‘crinkly’ laminated dolomites, with minor intervals of well-developed thrombolitic framestones, fine-grained homogenous carbonate muds, and thin interbeds of cross-bedded carbonate silt and grainstones. Common diagenetic features are randomly distributed evaporite crystals and mottled (‘leopard-skin’) recrystallized beds. The second lithofacies consists mainly of thicker bedded, light- to medium-gray dolomites characterized by bed-parallel cherts, and locally well-developed stromatolites. Thin (centimeter-scale) interbeds of laminated carbonates are common in these chert-rich rocks. Deformation tends to be more intense in this lithofacies, which may be related to the original presence of very thin evaporitic beds. Locally, it may be difficult to distinguish between primary sedimentary structures, early to late diagenetic features, and the effects of salt deformation. Strong dolomitization also helps to mask original features.

  • Because of the extreme aridity of the region, halite dissolution is slow. Halite is exposed in the Qarn Sahmah, Qarat Kibrit, and Qarat Al Milh diapirs (salt is mined from the latter two) and is also present just below the surface in shallow boreholes at Qarn Nihayda. Irregular, whitish anhydritic and gypsiferous breccias and spongy residues and veins occur widely throughout the salt domes, usually in close association with weathered evaporite bodies. Dissolution and reprecipitation of evaporites and the dehydration of gypsum to anhydrite is interpreted as being common in and around the salt diapirs. This is reflected in the presence of light-colored insoluble residues (‘caprocks’) at surface and elevated salt concentrations in shallow water wells drilled in the vicinity of the Qarn Alam salt dome. In order to explain the locally very large number of juxtaposed exotic blocks occurring at the surface, for example at Jebel Majayiz (Figure 11f), it is clear that, over time, large volumes of evaporites must have been dissolved.

  • At or very near the surface, the diapirs pierce whitish, thick-bedded, partially recrystallized late Tertiary micritic carbonates. These rocks locally form a relatively narrow halo of radially dipping, upward curved beds (e.g. Qarn Nihayda, see Figure 11c). No salt-dome induced stratigraphic and/or sedimentary facies variations have been recognized in the Tertiary rocks surrounding the diapirs. The width of the zone of deformation or tilting that affected the Tertiary rocks is relatively narrow (of the order of hundreds of meters) and the contact between the domes and overburden appears to be near vertical, both in outcrop and from seismic data.

  • Field evidence points to recent salt diapir activity in at least the Qarat Kibrit and Jebel Majayiz domes. At the four northernmost diapirs, the pierced Tertiary carbonates are overlain by a thin (less than 1 m thick) veneer of coarse, polymict conglomerates of late Quaternary age. These limestone-rich conglomerates represent the southward, distal extensions of a wide belt of channelized alluvial fans (see upper part of Figure 9a). The fans are related to the intermittent denudation of the rising Oman Mountains, situated more than 100 km to the north, during wetter climatic intervals (see Maizels and McBean, 1990). The Qarn Nihayda diapir is located close to the southern limit of these coalesced alluvial fans (Figure 9a). The southernmost diapir, Qarn Sahmah, lies well beyond the southern extent of the Quaternary gravels and, instead, is encroached on by recent, partially active sand dune ridges (Figure 9b).

  • The diapirs are affected to a variable extent by recent erosion and weathering, including dissolution by meteoric waters (sporadic rainfall and wintertime fogs) and by locally strong eolian deflation. Wind erosion has produced the many occurrences of ventifacts and wind-sculptured, sharp-edged limestone crests and ridges.

  • The outlines of the domes are well defined in the field (Figure 11), with the exception of the northernmost diapir, Qarat al Milh. This one is invaded and partially eroded by a N-S Quaternary wadi, suggesting recent evaporite dissolution by surface run-off and/or shallow underground flow. Several of the diapirs have distinct, small- to medium-scale radial drainage systems superimposed on the earlier N-S trending Quaternary alluvial systems, indicating that at least some of the diapirs are still active (e.g. Qarn Kibrit and Jebel Majayiz, Figures 11e,f and 16). In the case of the relatively large Jebel Majayiz diapir, a marked deflection of the large N-S oriented Quaternary fluvial channel around the eastern flank of this structure shows this dome was active during the Quaternary (Figure 9a).

Qarn Sahmah

The Qarn Sahmah salt dome (Qarn is Arabic for ‘horn’) is located about 18 km east-northeast of the Anzauz oil field. The nearest subsurface control points are exploration wells Qarn Sahmah-1 (1979) and Qarn Sahmah North-1 (1983) 7 km and 16 km respectively from the diapir. This salt dome is the southernmost (Figure 9a) and largest of the six, and forms a roughly circular outcrop with numerous prominent ridges (Figures 9b, 10a, 11a,b). In terms of composition, size, and structural complexity, the Qarn Sahmah salt dome is most similar to the surface-piercing domes of southern Iran and the Arabian Gulf, given its relatively diverse assemblage of both sedimentary and igneous rocks.

The eastern part of this large outcrop exposes recumbent, commonly dismembered folds defined by well-exposed white anhydrite breccia (Figure 12a); dark, laminated or stromatolitic Ara dolomites with associated bedded anhydrite (Figure 12b); and poorly exposed red-and-white banded halite (Figure 12c). Individual stromatolite domes are generally unoriented (Figure 12d), but locally show possible current-influenced alignment. Stromatolitic layers are very finely laminated and laterally continuous, suggesting growth through in situ precipitation (see Pope et al., 2000). Dark carbonates appear to form a high-relief, outward-dipping ‘rim’ that forms the eastern and the west-central flank of the dome. Here the carbonates overlie an approximately 20 m-thick sequence of white, brecciated, gypsum/anhydrite, which in turn overlies red, gray-green mottled, silty shales (Figure 12h). The layered anhydrites are stratigraphically interbedded with the stromatolitic dolostones and halite, which suggests a primary evaporitic origin. In contrast, the irregular anhydritic breccias are regarded as an insoluble residue, or ‘cap rock’, which formed during rise of the salt diapir and dissolution of halite in the presence of percolating groundwater (cf. Harrison, 1996). Many of the carbonates have a strong fetid (sulfurous) smell when freshly broken.

In the center of the Qarn Sahmah dome, finely laminated and fetid dark limestones and dolostones are well exposed in large, coherent exotic blocks, up to several hundred meters long along strike. The blocks consist of successions at least 15 to 20 m thick, in contrast to the thinner carbonate units mentioned above. The finely laminated carbonates (Figure 12e,f) have irregular textures comparable to the ‘crinkly laminites’ described recently from a stringer carbonate (unit A4) in the Ara Group of the South Oman Salt Basin (Schröder, 2000; see Figure 20a). The upper parts are more obviously microbially laminated, showing some evidence for development of local small stromatolites. The presence of bedded anhydrite at the base of the succession, together with interbeds of thin anhydrite commonly overlain by laminated carbonate facies, suggests a cyclic pattern at a scale of 10 to 30 m.

The finely laminated, fetid limestones were analyzed for source-rock potential by Parker and Wessels Boer (1968). The results indicated very low pyrolysis-fluorescence values and only low to marginal maturity (in the order of 0.5–0.7 VRE). This relatively low maturity suggests that these rocks were never deeply buried and that the growth of the diapirs may have started at an early age, consistent with the ‘downbuilding’ mechanism suggested above.

The center of the dome also exposes boulder conglomerates, containing clasts of rhyolite, foliated granodiorite, and sedimentary rocks (Figure 12g), as well as beds of highly immature, red to gray sandstones, and deeply weathered reddish-brown siltstones and clays with angular clay clasts. The sedimentary rocks represent typical diamictites of the Permo-Carboniferous Al Khlata Formation, a rock unit well-known from numerous wells in Oman, as well as from spectacular outcrops in the Haushi-Huqf High along the eastern margin of the Ghaba Salt Basin (Hughes Clarke, 1988; Al-Belushi et al., 1996). A deformed sequence of much better sorted, mostly red sandstone, grayish-brown siltstone, and immature clays (possibly belonging to the lower Paleozoic Haima succession described by Millson et al., 1996 and Droste, 1997), is present in the north-central part of the salt dome. These siliciclastic rocks are faulted against the Ara carbonates.

In addition, relatively small outcrops of red-gray volcaniclastics and dark-gray weathering olivine basalt occur in the north-central part of the Qarn Sahmah dome. The basalt contains large, white centimeter-scale euhedral feldspar phenocrysts that were originally crystals of plagioclase (Fitch and Hudson, 1955). Other minerals are chlorite, biotite, amphibole, apatite, and hematite-replaced magnetite. Several attempts at radiometric dating of the volcanics have not yielded interpretable results, leaving their age unresolved. The weight of field evidence favors an interpretation as a shallow intrusive, possibly a dike or sill (Fitch and Hudson, 1955). Because of the poor outcrop conditions it is unclear what kind of rock it originally intruded (Bell and Loosveld, 1994).

Qarn Nihayda

The Qarn Nihayda salt dome (Figure 9a) is located approximately 20 km southeast of the Saih Rawl oil field and 23 km west-southwest of the Qarn Alam field. It is a markedly NNW-elongated dome with an outer rim of gently deformed Tertiary carbonates (Figure 11c). It forms a sharply defined topographic feature in an otherwise flat plain (Figures 10b and 13ac). An unsuccessful multi-legged exploration well (Qarn Nihayda-1) was drilled in 1997 just to the east of the diapir to test the hydrocarbon potential of possible salt flank traps (Figure 8).

A low-relief area in the southern part of the dome is characterized by small irregular hills of pink, muddy soil encrusted with anhydrite and gypsum, plus dolomite breccias with a white, weathered gypsum patina. Although no halite has been found at the surface, halite was intersected at a depth of only 26 m in one of three shallow bore holes drilled by PDO in the central part of the dome.

The central area is characterized by numerous elongated ridges and irregular hills formed by Ara carbonates (Figures 13b,c). The thinly laminated fetid lithofacies, locally with mottled recrystallized intervals, predominates in the outcropping ‘stringers’ (Figures 13d,e); coherent successions at least 30 m thick can be measured in several individual blocks. Such laminites contain conspicuous intercalated beds, up to a few decimeters thick, of massive lighter gray carbonate mudstone (Figure 13f). These beds were probably deposited during storms, which would have swept muds from shallow-water environments into sub-wavebase settings (Schröder et al., in review). In addition, cherty dolomites and subordinate monomict carbonate breccias occur in this dome, with the former reaching a thickness of at least 25 m. The ‘stringers’ are strongly deformed and, as in the Qarn Sahmah dome, folds are dismembered by faults, creating a rather chaotic appearance in most of the outcrop. Nevertheless, some of the individual ‘stringer’ carbonates can be traced for more than 250 m along strike. Deformation appears to be most intense along the eastern margin of the dome. The topography in the northwestern part of this salt dome is subdued (Figure 11c); the low hills in this area are formed by anhydritic residues covered by Quaternary scree deposits.

Qarn Alam

The Qarn Alam salt dome (Figure 14) is located about 11 km northeast of the Qarn Alam oil field, and 7 km southwest of the Ghaba Resthouse on the Muscat–Salalah highway. Two unsuccessful exploration wells were drilled by PDO in 1973 to test the flanks of the diapir (Qarn Alam Northeast-1, -2). The dome is situated on a NW-trending fault zone (Figure 5). It has very gentle, almost symmetrical western and eastern flanks (Figure 10c) and a distinct radial drainage pattern.

Notwithstanding its relatively small size, the Qarn Alam dome is a significant outcrop because it is dominated by several large, protruding carbonate blocks that have well-developed internal Ara stratigraphy, including ‘crinkly’ laminated carbonates with laterally continuous thrombolite facies (Figures 15ad). This ‘crinkly’ marker allows for straightforward block-to-block correlations. The arrows in Figures 15a,b point to the same sequence boundary marked by a thin, light-colored anhydrite bed within laminated carbonates exposed in different blocks in the central part of the dome. This sequence boundary caps a 2 to 3 m-thick thrombolite framestone shown in Figures 15c,d. Dark components (mesoclots of Kennard and James, 1986; Grotzinger, 2001) form bush-like structures locally, creating significant primary pore space (initally about 50%), now filled with cement (light component). The cement infill was in two phases: an initial isopachous rim cement, probably marine in origin, and later pore-occluding blocky spar. The arrow in Figure 15d points to a void with well developed rim cement. The presence of these rim cements is significant and indicates that this highly porous microbial framework was cemented early, probably during marine lithification, which prevented it from collapsing by compaction during shallow burial. This was a critical factor in contributing to porosity retention within microbialite ‘stringer’ reservoir facies in South Oman Salt Basin (Grotzinger and Amthor, 2002).

In Figure 15a, the thickness of the thrombolitic interval below the sequence boundary varies laterally, suggesting a mounded feature at a scale of tens of meters. Overall, the thrombolitic carbonate facies exposed in the Qarn Alam salt dome has a primary lateral continuity of at least hundreds of meters––the minimum distance summed across the exposed correlatable exotic blocks––suggesting deposition on a relatively flat and stable substrate. This is corroborated by the presence of a very similar, if not stratigraphically identical, thrombolitic framestone overlain by thin anhydritic beds and light-colored limestone in the northern part of Jebel Majayiz, about 25 km to the northeast.

The northern end of the salt dome is marked by a prominent hill formed by a large, northwest-dipping carbonate block. The block consists exclusively of finely laminated, fetid limestone and dolostone (Figures 15e,f). A few thin to medium-thick beds of massive mudstone are intercalated within the succession, as indicated by the arrow in Figure 15e. The finely laminated carbonates are evenly laminated, suggesting the absence of benthic microbial mats in influencing sedimentation. The northwestern part of the dome consists of irregular low-relief outcrops of whitish gypsum and anhydrite, partly covered by gently tilted thin Quaternary gravels. Light-colored anhydritic residues occur locally between the exotic blocks. Silty sediments of unknown affinity form some very small outcrops in the southern half of the dome.

Qarat Kibrit

The Qarat Kibrit salt dome (Qarat is Arabic for plate; Kibrit is sulfur) is located about 11 km west-northwest of the Ghaba North oil field (Figures 5 and 9a). Its surface expression is a well-defined N-S elongated depression with gentle flanks, and a ridge of dark-gray, thinly bedded and laminated, fetid Ara Group carbonates on its western side (Figures 10d, 11e, 16).

Two relatively shallow wells (less than 1,000 m deep) were drilled by PDO in 1973 on the northern flank of the salt dome (Qarat Kibrit-1, -2) and seismic data provide information on the dominant structural style, and the prolonged history of salt movements. Tilting of the Paleozoic-Mesozoic sedimentary rocks took place in Late Cretaceous times due to an active phase of diapirism. Light-colored early Tertiary carbonates of the Umm er Radhuma Formation (Hughes Clarke, 1988) were deposited on top of truncated Mesozoic beds following a major phase of erosion. A subsequent mid-Tertiary phase of diapirism resulted in tilting and erosion of the Umm er Radhuma sediments in the crestal area, followed by deposition of shallow-marine carbonates of the late Tertiary Fars Group.

Seismic and well data indicate that upturned Fars Group carbonates and older sedimentary rocks are present around the dome. Thin tilted Quaternary alluvial fan deposits on the flanks of the outcrop demonstrate recent movements of the diapir. At the same time, dissolution of the salt mass in the center of the diapir appears to be progressively deepening the central depression (see topographic profile in Figure 10d). This process is enhanced by the inward-directed drainage that has developed locally at the edge of the dome (Figure 16), thus increasing the volume of meteoric waters captured during sporadic rainstorms.

With the exception of two other large carbonate rafts (see below), most of the salt dome is a shallow depression relative to the surrounding Quaternary gravel plain (Figure 16). The low-lying area is characterized by a highly irregular topography that is formed by small, laterally discontinuous outcrops of salt, gypsum, anhydrite, minor amounts of clastics (including red siltstones), and relatively small blocks of bedded cherty and/or recrystallized carbonates.

A prominent outcrop in the west-central part of the dome (Figure 17a) consists of a westward-dipping carbonate block, at least 25 m in stratigraphic thickness, with faults and/or shear planes bounded by thick shear-banded halite (which was mined in the past) that contains small rafted carbonate blocks. Bedding planes at the stratigraphically highest level in this outcrop show well-developed fracture and joint systems (Figure 17b). The outcrop is particularly significant in that it contains most of the attributes associated with the Ara A4 ‘stringer’ in the South Oman Salt Basin carbonates (Schröder, 2000).

Most of this block consists of finely laminated, fetid limestones (Figure 17c). Although the limestones are for the most part evenly laminated, they may also be irregularly laminated and analogous to the ‘crinkly’ laminites of the A4 carbonates (Schröder, 2000), which suggests the influence of benthic microbial mats. Beds of massive mudstone, 10 to 20 cm thick, are interbedded with the thinly laminated carbonates (Figure 17d). Similar intercalated massive mudstones are characteristic of the A4 carbonate ‘stringer’ where they occur as event beds, probably emplaced through off-shelf transport of muds during storms. The irregularly laminated facies becomes more abundant at the top of the interval, which is capped by a meter-thick interval of stratiform to digitate stromatolites. In plan view, the stromatolites form clusters of curvilinear crests (Figure 17e), suggesting colonization of wave ripples by benthic microbial mats in a shallow-water environment. The stromatolitic facies is directly overlain by 20 cm of black, organic-rich shale. The occurrence of such black shales in this succession supports the notion of ‘Dhahaban’ carbonate-evaporite source rocks in the Ghaba Salt Basin (Terken and Frewin, 2000).

The black shale is in turn overlain by 30 cm of crudely laminated dolostone and then by white anhydrite, about 80 cm thick (Figure 17f). The anhydrite layer is overlain by several meters of banded pink, white, and brick-red halite (Figure 17g) with thin intercalated layers of rippled red siltstone (Figure 17h). Thus, red siltstones, which have been observed elsewhere in the Ghaba Basin diapirs (Qarn Sahmah, Figure 12h), are present in the Ara Group sedimentary succession. Furthermore, deep exploration drilling in the central (Al Noor) and southern part (Harweel area) of the South Oman Salt Basin intersected thick sequences of coarse, immature red clastics within the Ara salt deposits (Figure 21). The clastics are considered to represent relatively proximal erosion products of the uplifted western margin of the South Oman Salt Basin that underwent compressional and/or strike-slip deformation during the later stages of basin development (see tectonic boundary with Ghudun-Kasfah High on Figure 2).

Jebel Majayiz

The large, N-trending Jebel Majayiz salt dome (Figure 10e) is located about 13 km east-northeast of the Ghaba North field, and approximately 8 km east of the Muscat–Salalah highway. It is situated on the Maradi Fault zone (Figure 6) and forms a prominent topographic high consisting of many large blocks of strongly brecciated gray Ara carbonates with highly variable dip and strike (Figure 11f). The dome is bounded to the east by a faulted contact with gently dipping Tertiary limestones, and by a wide Quaternary wadi whose course was markedly deflected by the growing salt diapir, as shown in Figure 9a. The western flank of the dome is very gentle, with a thin cover of Quaternary gravels.

The large blocks of rafted ‘stringer’ carbonates consist mainly of cherty, stromatolitic lithofacies. Except for locally sandy carbonates and recrystallized gypsum, no other lithologies have been seen during several field visits, but the dome is large and not yet fully explored. In the northern part of the dome, evaporites and anhydritic residues are common, mixed with relatively small, highly irregular blocks of lighter colored limestones. These may represent minor carbonate intervals in a thick evaporite succession. There is an unconfirmed report of salt having been mined in the past from the southern part of the diapir.

The southwestern margin of the dome exposes large and lithologically varied blocks consisting of the two main lithofacies of finely laminated, fetid, dark carbonates and cherty stromatolitic dolomites. Field relations are complex, but it is possible that both lithofacies occur in a single thick ‘stringer’. The thickness of individual sequences may be as much as 100 m. Outcrops are locally fairly continuous. Dips increase from 45° near the outer rim to up to 85° (and locally overturned) closer to the center of the dome. In the central area are exposures of pink mudstones with gypsum, together with some reddish sandy dolomite.

Similar lithologies are exposed in the eastern part of the dome, although stratigraphic thicknesses of complete, intact sections measured in individual blocks do not generally exceed 25 to 50 m. Stromatolitic dolostones are well-developed, including small, isolated columnar structures (Figure 18a) that pass stratigraphically upward into larger domal stromatolites, up to 50 to 100 cm wide (Figure 18b), that had original synoptic relief as much as 60 cm. In other exotic blocks, the stromatolites form small buildups a few meters high, with individual columns having more conoform geometry (Figure 18c). The various stromatolite facies are associated with fine clastic carbonates, in some cases possibly peloidal grainstones (Figure 18d) and packstones. Plan-view cross-sections of the larger stromatolitic domes show that they are strongly elongate, suggesting that sustained, high-velocity tidal or wave current action occurred during their growth.

Laterally continuous beds of thrombolitic framestone are present within the laminated lithofacies, exposed both in cross-section (Figure 18e) and in plan view (Figure 18f). A sequence identical to that shown on Figure 15b was found in the northern half of Jebel Majayiz and indicates that the unit had a significant lateral extent. The morphology and scale of the thrombolitic mesoclots are similar to the thrombolites exposed in the Qarn Alam salt dome.

Qarat Al Milh

The Qarat Al Milh salt dome (Milh is Arabic for salt), is situated about 4 km west of the Muscat to Salalah highway and about 5 km northwest of the Qarat al Milh oil field. The diapir and oil field are located on the major Maradi strike-slip zone (Figures 2, 3, 6). It is the northernmost of the six domes (Figure 9a) and is the most poorly exposed. It is small and has only a minor topographic expression (Figure 10f) that does not show up well on aerial photographs as the dome is mostly confined to a broad N-S trending wadi. Although small and relatively poorly exposed, the Qarat Al Milh dome provides further evidence on the characteristics of the Ara Group carbonates.

Carbonate blocks up to a few tens of meters across occur throughout the dome and are commonly associated with layers of anhydrite, and minor outcrops of halite occur amongst weathered blocks of Ara carbonates. The carbonates are light- to dark-gray. Bedded limestones show variability in outcrop-weathering patterns due to subtle internal variation in the thickness of laminae (Figure 19a). Such thinly laminated limestones may pass upward into more thickly laminated to massive limestones, sometimes forming low-relief domal structures suggestive of stromatolites (Figure 19b). In detail, the thinly laminated limestones have a highly irregular lamination texture, involving the deposition of successive irregular laminae that provided a high initial porosity, now filled with calcite (Figure 19c). Close to the presumed stratigraphic tops of carbonate successions, represented by the occurrence of anhydrite, the carbonate facies are more thickly laminated and form discrete, laterally linked small stromatolites with a pustular texture (Figure 19d). Other carbonate facies include early diagenetic mottled structures shown in Figure 19e. The mottled facies is often developed in finely laminated limestones with ‘crinkly’ texture and the mottles represent neomorphic recrystallization of primary lime muds to form aggregates of larger, fabric-obliterating crystals.

Thinly laminated light-gray limestones with an even lamination texture are shown in Figure 19f. These facies are identical to the thinly laminated carbonates in the other salt domes (see Figures 13e, 17c) suggesting that they are distributed regionally throughout the Ghaba Salt Basin. In addition, they occur in ‘stringers’ in the South Oman Salt Basin (see below and Figure 20b). The dome also contains thinly laminated carbonates with a typical ‘crinkly’ texture that are analogous to the crinkly laminites in the ‘stringer’ carbonates of the South Oman Salt Basin.

IMPLICATIONS FOR THE ‘STRINGER’ HYDROCARBON PLAY AND PROSPECTIVITY

Since 1997, several hundred million barrels of recoverable oil and several trillion cubic feet of recoverable gas have been discovered in the Harweel cluster of ‘stringer’ hydrocarbon discoveries in the South Oman Salt Basin (Figures 2 and 21). Recent additional oil and gas discoveries have confirmed the economic significance of this prolific and unusual deep intra-salt hydrocarbon play. Building on PDO’s success in the intra-salt ‘stringer’ hydrocarbon play in the South Oman Salt Basin, one of the aims of our work was to establish which of the critical success factors might be applied to the Ghaba and Fahud salt basins using the geological data from the Ghaba Basin salt domes. Exploration successes result from a combination of the following criteria:

  • Ubiquitous hydrocarbon charge from mature high-quality (Type I/II) source rocks within the carbonate ‘stringers’;

  • Better ability to predict dolomite reservoir occurrences and reservoir quality, based on detailed studies of depositional facies relationships;

  • Excellent sealing properties of the thick Ara salts that encase the ‘stringer’ carbonates, with hydrocarbon bearing reservoirs being frequently highly overpressured; and

  • Acquisition and interpretation of large, high-quality 3-D seismic data sets for use in detailed prospect delineation, mapping, ranking, and risking.

With regard to hydrocarbon charge, the presence of black shale in the Qarat Kibrit salt dome (see Figure 17f) shows that such organic-rich beds associated with ‘stringer’ carbonates are not limited to the South Oman Salt Basin. Similar source rocks in the carbonate-evaporite sequences in North Oman confirms earlier studies by Terken and Frewin (2000) who had postulated the occurrence of rich, near-top Ara source rocks to explain the nature and distribution of hydrocarbon fields in the Ghaba Salt Basin.

Direct comparison of the proven ‘stringer’ play in the South Oman Salt Basin with its postulated equivalent in the Ghaba Salt Basin is hampered by the different nature of the available data sets. A large amount of high-quality subsurface data (well log, core, and 3-D seismic) from more than 40 ‘stringer’ penetrations in the South Oman Salt Basin is available, but the main source of information for the equivalent ‘stringer’ play in the Ghaba Salt Basin is outcrop data from the salt domes, and the mostly 2-D regional seismic data sets of various vintages. Because of this, detailed and predictive play comparisons are not yet possible. For example, key questions remain regarding spatial ‘stringer’ development and carbonate reservoir facies and distribution, source rock distribution and quality, the timing of hydrocarbon generation and, importantly, the diagenetic history and hence the prediction of reservoir quality.

Based on the information presently available, the greatest risk of failure in the ‘stringer’ play is reservoir quality. Shallow-water carbonate platform facies characteristically have high porosity and permeability, whereas deeper water slope and basin facies are tight despite some oil saturations of more than 80 percent. Consequently, as new intra-salt prospects are explored, success will depend on the early prediction of favorable reservoir facies trends in the various stratigraphically distinct ‘stringer’ levels in the Ara Group (Figure 3). The surface-piercing salt domes of North Oman provide the only known outcrop equivalent of the ‘stringer’ carbonates. A thick sequence of post-Buah Formation platform carbonates with stromatolites has recently been mapped in the Huqf–Haushi High to the southeast of the Ghaba Salt Basin (Figures 2 and 4) and interpreted to be at least partly age-equivalent to the Ara Group. They may represent a condensed, basin-margin lateral equivalent of the carbonate-evaporite sequence developed in the salt basins (Nicholas and Brasier, 2000).

Even though individual ‘stringers’ may be more thinly developed in North Oman (and this assumption needs further investigation), the facies that characterize the large exotic Ara carbonate blocks in the outcropping salt domes are very similar to those of the extensively cored intra-salt carbonate stringers of the South Oman Salt Basin. The broader implication of this is that such stringers are indeed present within the Ghaba Salt Basin and therefore constitute deeper exploration targets in addition to the Mesozoic and Paleozoic plays historically pursued in this area (see Figure 4). The carbonate stringers in the South Oman Salt Basin represent former isolated platforms of late Proterozoic to earliest Cambrian age (Amthor et al., 2001; Grotzinger and Amthor, 2002), that can be prolific reservoirs (Reinhardt, 2001). In the South Oman Salt Basin the Ara Group represents at least six third-order cycles of carbonate/evaporite sedimentation in a tectonically active basin (Figure 3). Each cycle consists of several spatially isolated platforms, with reservoirs developed according to primary facies distributions. Enclosing salts provide base and top seals (Figure 21). Ara carbonate platforms formed during transgressive to highstand accommodation conditions superimposed upon a progressive, long-term accommodation increase. Older platforms are thinner and laterally more extensive whereas intermediate platforms are thicker and more differentiated with respect to shelf margin and slope-to-basin facies. Younger platforms are thinner again, often dominated by transgressive systems tracts (TST) deeper water facies, and the youngest consist of numerous small pinnacle reefs.

South Oman ‘stringer’ platform facies include microbial boundstone and framestone, intraclast-peloid-ooid grainstone-packstone, and mudstone. Microbial facies dominate, and have a variety of textures that conform to systematic variations in water depth and the inferred accommodation regime (Grotzinger and Amthor, 2002). Platform interior facies consist of peritidal stratiform stromatolites with pustular, smooth and tufted textures. These pass laterally into thrombolite (Figure 20c) sheet and mound facies, which pass downslope into turbiditic mudstones that interfinger with crinkly laminites (Figure 20a) in the most distal settings. ‘Crinkly laminites’ are widespread in all Ara stringer basinal settings, regardless of relative ‘stringer’ age, and result from accumulation of both pelagic and benthic microbial organisms. These basinal microbialites form one reservoir type whose performance deteriorates in proportion to the influx of turbiditic shelf-derived muds. Other reservoirs are developed principally in shelf-interior to shelf-margin microbialites and associated grainstones. Reservoir quality in microbialites reflects the primary (inefficient) growth fabric of thrombolites, and of the early diagenetic decay of microbial mats in more stromatolitic facies.

Exotic carbonate blocks within the Ghaba salt domes contain facies that are in all respects identical to the main reservoir-forming microbialites of the South Oman Salt Basin, thus providing a valuable additional source of information to the cored rocks from exploration wells in the Harweel field cluster. For example, the cored ‘crinkly laminites’ in the South Oman Salt Basin (Figure 20a) compare well with the thinly laminated carbonates with irregular, crinkly textures from the Ghaba salt diapir rafts shown in Figure 19c,d. In addition, the thrombolites cored in the Harweel area (Figure 20c) are very similar to the thrombolites observed at surface in the Ghaba salt domes (Figures 15c,d and 18e,f).

Other similar, but non-reservoir, facies are the finely laminated carbonates with even lamination textures, (see Figure 20b from the South Oman Salt basin compared with Figures 13e, 15f, 17c, and 19f from the Ghaba salt domes). In South Oman, these finely laminated carbonates with even lamination texture are known to occur as transitional carbonate-evaporite facies that formed during the early stages of marine flooding of the evaporite basin. The first carbonates deposited in most stringers are generally finely laminated with even textures (Figure 20b) and represent subtidal, low-energy conditions. They are strikingly similar to the ‘rhythmites’ formed in other evaporite basins of differing age, and hence may well reflect a strong seasonal or other climatic imprint on deposition. The absence of ‘crinkly’ textures implies low primary organic accumulation rates and the absence of benthic microbial mats.

We consider that continued exploration drilling, combined with further geological investigations of the surface-piercing salt domes in interior North Oman, and the interpretation studies of large new 3-D seismic data sets, will yield substantial additional information on the unconventional but potentially high-reward Ara intra-salt ‘stringer’ hydrocarbon play. This play has been successfully pursued in the South Oman Salt Basin (Figure 21). Based on the new data presented in this paper, it is likely that the Ara ‘stringer’ play is not limited only to this basin but extends into the Ghaba and Fahud salt basins. It is possible that other Infracambrian evaporite basins in the Middle East may have similar carbonate rocks and hydrocarbon potential.

ACKNOWLEDGMENTS

The authors thank the Ministry of Oil and Gas and Petroleum Development Oman for permission to publish this paper. We acknowledge the work of other IPC, PDO, and Shell geoscientists on the salt domes and related topics. In particular, important surface and subsurface studies were carried out by Tom Faulkner, Radha Al Lawatia, Alistair Milne, Pieter Spaak, and Mark Partington (hydrocarbon play analysis), Pascal Richard (structural geology and kinematic modeling), Mohamed Al-Harthy and Eilard Hoogerduin-Strating (field geological data), Salim Al Rawahy and Suzanne Witte (high-resolution gravity and aeromagnetic data). Our colleagues in PDO’s Geomatics department are thanked for the high-quality aerial photography, ground surveys, mapping, and drafting. Text editing and the design and drafting of the final figures was by Gulf PetroLink.

ABOUT THE AUTHORS

Jeroen M. Peters is Oil Director for South Oman in Petroleum Development Oman (PDO). He obtained a BSc in Geology, an MSc in Structural Geology and a PhD in Marine Geology from the University of Amsterdam in The Netherlands. After joining Shell in 1985, he worked in various aspects of Exploration and New Business Development in Shell International EP in The Netherlands, Shell Expro in the United Kingdom, PDO, Shell China Petroleum Development, and Brunei Shell. Jeroen returned to Oman in 1999 as Exploration Director of PDO. He was appointed to his present position in April 2002.

Joeroen.M.Peters@pdo.co.om

Jacek (Jack) B. Filbrandt joined Petroleum Development Oman (PDO) in 1999 and is now head of the Structure and Traps team. He has a BSc in Earth Sciences from Leeds University and a PhD in Structural Geology and Geophysics from University College, Cardiff. Jack joined Shell Expro in 1985 working mainly on 3-D seismic surveys. He subsequently moved to Shell International EP in The Netherlands and worked in New Ventures with responsibility for developing greenfield opportunities, mainly in East Africa and the Middle East. Jack spent three years with Shell Oil in New Orleans, exploring the deepwater Sub-Salt Play. He was leader of the Structural Geology Team at Shell Research in the mid-90s.

Jacek.B.Filbrandt@pdo.co.om

John P. Grotzinger is a Professor at the Massachusetts Institute of Technology (MIT). He has a BS (high honours) from Hobart College (1979), an MS from the University of Montana (1981), and a PhD from Virginia Polytechnic Institute and State University (1985). Between 1985 and 1988 John was a Field Party Chief with the Geological Society of Canada. He was a post-Doctoral Research Fellow and Associate Research Scientist at the Lamont-Doherty Geological Observatory from 1985 to 1992. John joined MIT as an Assistant Professor in 1988 and became Professor in 1995. He was awarded the Presidential Young Investigator Medal of the National Science Foundation in 1990, and the Donath Medal of the Geological Society of America (GSA) in 1992. He was the AAPG Distinguished Lecturer for 1997/98. He is a Fellow of the AAPG and a Senior Fellow of GSA. His current interests involve the development of digital methods for mapping reservoir-scale heterogeneity in outcrop analogs, and the development of geologic models for intra-salt reservoirs in South Oman. During 2001, he spent a sabbatical year in PDO Exploration. On his return to MIT, John was elected to the National Academy of Sciences.

grotz@mit.edu

Mark J. Newall is a Senior Exploration Geologist in Frontier Exploration in Petroleum Development Oman (PDO). He has a BSc from Durham University (1987) and a PhD in Numerical Modelling of Sediment Transport Systems from Liverpool University (1990). He joined Shell in 1990 and has worked at KSEPL and Shell International EP in The Netherlands on the sequence stratigraphic evaluation of sedimentary basin and numerical models for reservoir prediction. In 1996, Mark moved to Sarawak Shell Berhad (Malaysia) to work in the New Business Development Team dealing with acreage evaluation and acquisition. He joined PDO in 2001 and is currently working in the Government Gas Exploration Team with a specific interest in Gas Exploration in South Oman.

Mark.J.Newall@pdo.co.om

Mark Shuster joined Petroleum Development Oman in 2001 and is the Frontier Exploration Asset Manager. He has worked in the petroleum industry since 1986. Prior to his PDO assignment he worked in Australia, as an Exploration Manager for Woodside Energy Ltd and Geological Services Manager and New Opportunities Coordinator for Shell Development Australia. Before Australia, he worked for Shell International (Netherlands) and Shell Oil (Houston) on a variety of applied basin studies and new opportunity evaluations from around the world. Mark has a PhD in Geology from the University of Wyoming and a BSc (Geology) from the University of the Pacific. He is a member of the AAPG, GSA and Geological Society of Oman.

Mark.W.Shuster@pdo.co.om

Hisham A. Al-Siyabi is an Exploration Geologist with Petroleum Development Oman (PDO). He received a BSc in Geology from the University of South Carolina in 1992 and joined PDO the same year. He has an MSc (1994) and a PhD (1998) from The Colorado School of Mines. His PhD dissertation was on the investigation of the sedimentology and stratigraphy of the Pennsylvanian (Carboniferous) deep-water deposits of the Jackfork Group in Arkansas. Hisham rejoined PDO in February 1999 and is working in the Deep Oil Exploration Team.

Hisham.Siyabi@pdo.co.om