ABSTRACT

In the South Oman Salt Basin (SOSB), the Ara carbonates form an extensively cored, deeply buried intra-salt hydrocarbon play. Six surface-piercing salt domes in the Ghaba Salt Basin (northern Oman) provide the only outcrop equivalents for carbonates and evaporites of the Ediacaran – Early Cambrian Ara Group (upper Huqf Supergroup). Based on fieldwork, satellite images and isotope analysis it is concluded that most of the carbonate bodies (so-called stringers) in the Ghaba salt domes are time-equivalent to the stratigraphically uppermost stringer intervals in the SOSB (A5–A6). Maturity analyses demonstrate that the carbonate stringers in the salt domes were transported with the rising Ara Salt from burial depths of c. 6 to 10 km to the surface. Petrographic and stable-isotope data show that their diagenetic evolution during shallow and deep burial was very similar to the Ara carbonate stringer play in the SOSB. However, during the retrograde pathway of salt diapir evolution, the carbonate stringers were exposed to strong deformation in the diapir stem and diagenetic alterations related to dedolomitisation. As the salt domes contain facies that are in all aspects identical to the deeply buried Ara play in the SOSB, this study provides substantial additional information for hydrocarbon exploration in southern Oman. Moreover, our work has implications for the hydrocarbon prospectivity of the Ghaba Salt Basin and possibly of other Ediacaran – Lower Cambrian evaporite basins in the Middle East, such as for the time-equivalent Hormuz salt basins.

INTRODUCTION

The study of large rock inclusions (so-called rafts, floaters or stringers) in salt diapirs is of broad economic interest as they constitute commercial hydrocarbon reservoirs, although they may also represent potential drilling risks due to their high fluid overpressures. The uplift mechanisms and associated controls on the internal deformation of a salt diapir have been subject of controversial debates in the past 20 years (Talbot and Jackson, 1987; Kupfer, 1989; Talbot and Jackson, 1989; Gansser, 1992; Talbot and Weinberg, 1992; Gansser, 1993; Weinberg, 1993; Koyi, 2001; Callot et al., 2006).

The most extensively studied stringers are the ones exposed in the Iranian salt domes. They are up to 6 km2 in area and c. 80 m thick, comprising volcanic and igneous rocks or sediments such as anhydrite and carbonates. These stringers are commonly interpreted as interlayered with the primary rock salt and lifted with the rising salt diapir from up to 10 km at depth to the surface (Kent, 1979; Gansser, 1992; Talbot and Jackson, 1987). Talbot and Weinberg (1992) argued that the concentric shapes of many salt domes observable in the Kavir area (central Iran) in air photographs (Jackson et al., 1990) are a result of circulation within the diapir. By this process, the initial internal stratigraphy develops into a mushroom-shape, resulting in gentle dips of the intra-salt inclusions for most portions of the upper level of a diapir. Results from analogue modelling have shown that brittle layers passively follow the vertical ascent of ductile layers from the earliest stages until cessation of diapir evolution (Escher and Kuenen, 1929; Koyi, 2001; Callot et al., 2006). During this evolution, the embedded inclusions undergo stretching, leading to boudinage and rotation against the vertical boundaries of the diapir. As diapir growth and salt supply stop, the inclusions start to descend within the diapir, which initiates an internal (secondary) flow within the salt diapirs. During this process, the inclusions undergo folding and create shear zones at the immediate contact with the salt.

From these insights, one could expect that the structural configuration of inclusions at the surface is strongly influenced by the internal kinematics (flow patterns) of a salt diapir. However, detailed mapping of mining galleries in structural shallow levels of various salt diapirs revealed highly complex isoclinal and overturned folding with all geometrically possible shapes and orientations (see Talbot and Jackson, 1987, and references therein). Besides the complexity of the internal structural geology developing during the rise of (natural) diapirs, strong dissolution by groundwaters leads to a structural reconfiguration of the inclusions (Talbot and Jackson, 1987; Weinberg, 1993), which masks interpretations on the style of salt tectonics. Most of the insights on the structural evolution of salt-encased inclusions during diapir rise emerged from the interplay between satellite imaging, air photographs and fieldwork (primarily from the Iranian salt domes) by the workers cited above. These observations give valuable insights into the processes occurring during final diapir evolution. However, none of these works aimed to investigate the internal processes of intra-salt inclusions during diapir rise.

Six surface-piercing salt domes in the Ghaba Salt Basin (GSB) of northern Interior Oman provide the unique opportunity to compare data on structural geology, diagenesis and geochemistry of carbonate stringers, which are stratigraphic and lithologic outcrop equivalents to deeply buried intra-salt carbonates in the South Oman Salt Basin (SOSB, Figure 1). Several field surveys of the salt domes of the GSB were made by petroleum geologists since the early 1950s with the aim to constrain the stratigraphic position of the exposed exotic blocks. Currently, the only work published on the surface-piercing salt domes clearly documented that the stringers are predominantly carbonates and evaporites of the Late Neoproterozoic to Early Cambrian Ara Group in the SOSB, which constitutes one of the most important hydrocarbon plays in Oman (Figure 1; Peters et al., 2003). The Ara Group in the SOSB was deposited as a six-cycles sequence of carbonates and evaporites, which developed into large salt diapirs due to passive downbuilding. The carbonates are buried to a depth of 3 to 6 km and are fully enclosed within the salt that forms a perfect seal for hydrocarbons in the carbonate stringers.

The understanding of the sedimentary facies and platform geometries of this unusual, deeply buried play was considerably improved through the application of an outcrop analogue from the terminal Neoproterozoic carbonate ramps of the Nama Group in southern Namibia (Grotzinger, 2000; Grotzinger et al., 2000; Grotzinger and Amthor, 2002; Adams et al., 2004). However, important aspects of the Ara Group reservoir architecture can not be sufficiently explained by this facies analogue. In contrast to the Namibian outcrops, the sedimentary geometries in the Ara Group were strongly influenced by synsedimentary salt tectonics, causing the considerable thickness increase of the basinal carbonate facies. Based on seismic character alone these thick basinal facies can easily be misinterpreted as reefal build-ups (Al-Siyabi, 2005). Moreover, the distribution of diagenetic phases, which governs the reservoir properties of the Ara carbonates, can not be studied in the Namibian outcrop equivalents. Hence, an analogue model based on the surface-piercing salt domes of the GSB potentially completes the existing reservoir model for the SOSB with respect to salt tectonics and diagenesis. The aim of this paper is therefore twofold: first, to better constrain the dynamics of salt tectonics in northern Oman. Second, to compare and contrast data on lithology, organic matter maturity and diagenesis with existing data from the deeply buried Ara Group in the SOSB. This work contributes to a better general understanding of reservoir quality evolution and diagenetic processes occurring during the uplift of salt diapirs.

GEOLOGICAL SETTING

Ghaba Salt Basin (GSB)

The six surface-piercing salt domes are located in the GSB, which is one of three evaporitic basins constituting the deep subsurface of Interior Oman (Figure 1). The Ara Salt in Oman is thought to be time-equivalent to the Hormuz Salt, which forms a number of basins from the Arabian Gulf region to the Salt Range of Pakistan (Gorin et al., 1982; Edgell, 1991, Allen, 2007). The salt basins of Interior Oman follow a NE-SW alignment, which acted as left-lateral strike-slip faults during Late Neoproterozoic to Early Cambrian times (Loosveld et al., 1996). In the SOSB and the GSB the basement is overlain by the Upper Neoproterozoic to Lower Cambrian Huqf Supergroup, which comprises the Abu Mahara, Nafun, Ara and Nimr groups (Figure 2). The carbonates and evaporites outcropping in the six surface-piercing salt domes of the GSB belong to the Ediacaran – Early Cambrian Ara Group, which also forms the hydrocarbon plays of the SOSB (Peters et al., 2003; Al-Siyabi, 2005; Al-Balushi, 2005).

In the SOSB and partly in the GSB, the development of salt diapirs (Figure 3) is a product of post-depositional salt movement triggered by differential loading of thick continental clastics onto the mobile substrate of the Ara Salt (Loosveld et al., 1996). In the SOSB, the Ara Salt forms the top, side and bottom seal for the Ara hydrocarbon carbonate stringer play. While the Ara Salt diapirs in the SOSB remained unchanged after the phase of passive downbuilding, the Ara Salt diapirs in the GSB underwent reactivation followed by active piercing of the strata overlying the Ara Salt and ultimately the surface (Loosveld et al., 1996; Peters et al., 2003; Filbrandt et al., 2006). Active piercing was initiated by the development of deep-rooted, sinistral strike-slip faults, such as the far-reaching Maradi Zone and Burhaan faults (Figures 1, 2 and 3a). The salt domes Jabal Majayiz and Qarat Al Milh are situated within the basement-involved Maradi Fault Zone (Figure 1; Peters et al., 2003). Movements along the Maradi Fault Zone started during the Late Cretaceous, at the time of thrusting of the Oman ophiolites (Filbrandt et al., 2006), and continued until the Late Neogene as shown by folded Pliocene – Pleistocene wadi gravels along the fault (Hanna and Nolan, 1989). In addition, the diapirs are surrounded by a complex network of normal, strike-slip and reverse faults, pointing to a temporarily compressive stress field.

South Oman Salt Basin (SOSB)

The lithologies of the six surface-piercing salt domes in the GSB are mainly composed by Ara Group evaporites and carbonates, which are clearly correlated to the Ara intra-salt hydrocarbon plays of the SOSB (Peters et al., 2003). From work on drill cores from the SOSB, it is known that the Ara Group spans the Precambrian – Cambrian (Ediacaran – Cambrian) boundary (Amthor et al., 2003) and consists of marine carbonate platform sediments, representing at least six third-order cycles of carbonate and evaporite sedimentation. Each cycle is characterised by the sedimentation of Ara Salt at very shallow water depths, followed by the deposition of 20 to 220 m thick isolated carbonate platforms (so-called stringers) in deeper basins during trangressive periods (Mattes and Conway Morris, 1990). Bromine geochemistry of the Ara Salt (Schröder et al., 2003; Schoenherr et al., 2008) and marine fossils (Amthor et al., 2003) clearly indicate a seawater source for the Ara evaporites. In the SOSB, the Ara Group cycles are termed A1 to A6 from bottom to top (Figure 2). The carbonate and evaporite parts of these Ara Group cycles are termed A1C – A6C and A1E – A6E, respectively (Al-Siyabi, 2005).

Cores from the Ara carbonate cycles A1C to A4C contain up to 5 sequences and display very repetitive facies patterns. The A1C shows one sequence of finely laminated dark carbonates interpreted as basinal facies to peritidal grainstones and thrombolites in the top section. The laterally very extensive A2C interval is the most prolific hydrocarbon reservoirs in the SOSB. The A2C stringers are formed by a breccia unit at the base, followed by 4–5 shallowing upward sequences. The A3C interval contains 2–3 sequences and shows very similar facies to the A2C but is always thicker than 100 m (personal communication, J. Grotzinger, 2007). The main reservoir facies of the A4C in the Greater Birba area (see Figure 1) comprises ‘crinkly’ laminites of the outer ramp and stromatolites and thrombolites of the inner ramp (Schröder et al., 2005). The A5C and A6C intervals are generally composed of similar facies to the A1C to A4C and partially show isolated ‘pinnacle’-like thrombolite reefs. Both intervals have been shown to be mostly non-producing reservoirs. Seismic sections show that the A5C consists of several structurally segmented slabs. The A6 carbonates form the top of the Ara Group and are directly overlain by the Nimr Group clastics and therefore do not form salt-encased stringers. However, in some part of the basin, the A6E evaporites are interbedded with carbonate and siliciclastic units, which form so-called floaters, i.e. relatively thin stringer-like sediment bodies (Al-Siyabi, 2005, Figure 2).

Generally, the shallow-water carbonate platform facies of the Ara Group consists of grainstones and laminated stromatolites, while the platform margin is formed by stromatolites and thrombolites (Figure 4). The slope facies includes organic-rich laminated dolostones and the basinal facies is dominated by sapropelic laminites (Al-Siyabi, 2005). During seawater highstands, prolific oil source rocks formed in the deeper (some hundreds of metres), periodically anaerobic to dysaerobic parts of the basin (mainly slope and basinal mudstones in Figure 4). Density stratification of seawater allowed preservation of a sufficient amount of organic material in the bottom layers as high organic productivity of algal material in the upper water layers was present (Mattes and Conway Morris, 1990).

The Ara evaporites include halite and anhydrite, which replaced primary gypsum (Mattes and Conway Morris, 1990; Schröder et al., 2003). Increasing seawater salinity due to basin desiccation led to the depositional succession carbonate-sulphate-halite, which in turn proceeded into the succession halite-sulphate-carbonate during the following sea-level rise. Anhydrite overlying the carbonate is termed “roof anhydrite”, while anhydrite above the halite is referred to as the “floor anhydrite”. Both anhydrite horizons can be up to 20 m thick. The thickness of the Ara Salt is 10–150 m in the A1 to A4 cycles and can exceed 1,000 m in the A5 and A6 sequences (Schröder et al., 2003).

METHODOLOGY

Samples

In total, we investigated 107 carbonate samples from the six surface-piercing salt domes. Out of the 107 samples, 40 thin sections were prepared and stained with Alizarin Red S to differentiate dolomite from calcite. Bulk rock powders of 36 samples were analysed with X-ray diffraction (XRD) using a D 5000 diffractometer. Total organic carbon was analysed for 60 samples. Solid bitumen reflectance (BRr) measurements for maturity assessment were performed on 29 samples. Stable-isotope analysis was performed on 101 carbonate samples.

Stable Isotopes

Oxygen and carbon-isotope analyses were performed at the laboratory of IFM-GEOMAR, Kiel (Germany). Carbonate powder was dissolved with 100% H3PO4 at 75 °C in an online, automated carbonate reaction device (Kiel Device) connected to a Finnigan Mat 252 mass spectrometer. Isotope ratios are calibrated to the Vienna Pee Dee Belemnite (V-PDB) standard using the NBS-19 carbonate standard. Average standard deviation based on analyses of a reference standard is < 0.07‰ for δ18O and < 0.03‰ for δ13C.

Solid Bitumen Reflectance (BRr)

Maturity analysis requires a sufficient number (c. 50) of reflectance measurements on vitrinite or solid bitumen particles to provide reliable data. Reflected light microscopy under immersion oil revealed that all of the Ghaba Salt Basin samples contain solid bitumen particles, which are too fine-grained (c. 1 µm in diameter) to apply the conventional photometer method. Therefore, we measured the grey values (value of brightness) of the solid bitumen. For this method, a Zeiss Axioplan microscope was interfaced with a Zeiss Axio digital camera and a desktop computer using a “Hilgers” instrument and the relevant software FOSSIL and DISKUS. The programs allow the direct conversion of grey values into mean random reflectance values (Rr in %). The calibration was applied using an Yttrium-Aluminium-Garnet (0.889% Rr) and a Gadolinium-Gallium-Garnet (1.714% Rr) at 10 V for a 40x/0.85 n.a. lens under immersion oil (ne = 1.518).

Total Organic Carbon

The assessment of the carbon content was performed with a LECO multiphase C/H/H2O analyser (RC-412). This instrument operates in a non-isothermal mode with continuous recording of CO2 release during oxidation, thus permitting the determination of inorganic and organic carbon in a single analytical run. All analyses were performed in duplicate and the results were averaged.

FACIES AND STRUCTURAL EVOLUTION

This section provides detailed observations on the sedimentological and structural architecture of constituent lithologies in the Qarn Nihayda and Jabal Majayiz salt domes from satellite images and fieldwork.

Jabal Majayiz Facies

The Jabal Majayiz salt dome (Figure 1) is the second largest in size and is characterised by a high density of carbonate stringers showing an overall chaotic juxtaposition (Figure 5). Generally the lithofacies is dominated by stromatolites and thrombolites. The stringers in the southernmost third and the central eastern part of the dome are almost completely composed of various types of crinkly and pustular laminites, cherty stromatolites and thrombolites. This lithofacies association is exemplarily illustrated in two stratigraphic sections (Figure 6). Both sections show various transitions between crinkly laminites, pustular laminites and massive thrombolites, which can gradually change into stromatolite facies. The mesoscopic appearance of pustular laminites can be very similar to thrombolites, but with an overall laminar structure. In contrast to other parts of the dome, most stringers in the central northern parts (around sample JM 42 in satellite image) are mainly composed of finely laminated carbonates, which can gradually pass into crinkly laminites and stromatolitic layers.

Stromatolites and pustular laminites are usually interpreted as peritidal facies of the inner ramp (Figure 4). The depositional environment of thrombolites ranges from shallow-subtidal to the platform margin and slope (Schröder, 2000). Finely and crinkly laminated carbonates are attributed to slope and outer ramp settings (Schröder et al., 2003). Sandstones in the GSB are often associated with stromatolites and therefore are assumed to be intertidal. Alternations between pustular laminites, thrombolites and crinkly laminites (Figure 6) were also observed in the Birba area of the SOSB (Schröder, 2000). However, the facies in Jabal Majayiz differs from the other salt domes and the SOSB in the high proportion of siliciclastic material, chert and stromatolites. In the SOSB siliciclastics are most commonly found in the A6 interval.

Qarn Nihayda Facies

In Qarn Nihayda (Figure 1) a number of profiles record gradual transitions from regressive conditions with elevated salinity to transgressive open-marine conditions (Figures 7 and 8). At the base of the succession shown in Figure 8, crinkly laminites are overlain by pustular laminites, indicating a shallowing-upwards trend (Figure 4). Displacive anhydrite nodules in the pustular laminites probably indicate a supratidal environment. An overlying residual breccia consisting of crinkly-laminite clasts in an anhydrite-bearing, laminated, micritic matrix, is followed by a residual anhydrite layer. These layers represent restricted marine conditions that possibly led up to rock salt formation. A subsequent flooding event likely led to the dissolution of evaporites, formation of the residual layers and the subsequent deposition of a c. 15-m-thick interval of light-grey, finely-laminated carbonates. Towards the top, these carbonates show an increasing amount of anhydrite rosettes and nodules, which partially display chicken wire structures. These textures most likely indicate a gradual relative sea-level fall that culminated in supratidal conditions. Upsection, a change to more open-marine conditions is indicated by a decreasing content of lath anhydrites in finely laminated to massive carbonates. A further deepening is indicated towards the top of the succession by alternations of finely laminated, dark-grey carbonates with massive, light-grey mudstones. The massive beds, which are interpreted as turbidites (Peters et al., 2003), show slump folds, dewatering channels and load casts; the latter two structures clearly indicate that this stringer is structurally inverted.

Deformation-related Structures at Jabal Majayiz

The structural configuration of stringers, as observed from the satellite image, does not indicate an overall trend (Figure 5). However, the outer stringers tend to strike parallel to the dome axis (N-S). Within the dome, clusters of stringers show consistent strike. One cluster, which covers an area from the centre towards the northeast, is characterised by dominantly NE-striking stringers. Another cluster in the central-western part contains stringers with N-S orientation. This may suggests that some clusters of rock salt and stringers behaved coherently during rise and rotated against other adjacent clusters. The interior of the dome is characterised by a very high density of stringers, which are chaotically juxtaposed and superimposed against each other.

Deformation-related structures are very abundant in Jabal Majayiz. Brecciation is common in all lithofacies (Figure 9a). Cataclasites form along normal faults (up to 2 m wide), that dissect the stringers perpendicular to bedding. Large-scale open folding of the stringers is very common. Isoclinal folding accompanied by strong brecciation and thrusting is also present (Figures 9b–d; for location see JM 2 in Figure 5). Deformation, i.e. folding and thrusting, was most likely accompanied by hydrofracturing as for example indicated by irregular-shaped fractures containing highly fragmented material from adjacent layers (Figure 9d). The occurrence of anhydrite caprock in Jabal Majayiz is limited to the northern and southern parts, and forms an outward dipping periclinal rim around the salt dome (Figures 5 and 9e). Rarely, anhydrite caprock occurs in-between stringers in the dome interior, where it shows a pronounced foliation with a flow-like texture around embedded carbonate clasts (Figure 9f).

Deformation-related Structures at Qarn Nihayda

The satellite image of Qarn Nihayda shows that the dome axis trends NW-SE. Detailed mapping revealed that the strike of most stringers is oriented parallel to the dome axis. This is most apparent along laterally continuous stringer ridges along the western and eastern flank, which are subdivided into several individual blocks (Figure 7). In most parts of the dome centre this general trend is still discernible but in the northern part of the dome centre the stringers show a more chaotic juxtaposition. The dip of most stringers ranges from 45° to 90° with the steepest dipping stringers at the dome margins and flat dipping stringers in the centre (cross section A-A′, Figure 7b). The simplified cross section B-B′ (Figure 7b) shows open folded stringers in the dome centre with the fold axis oriented parallel to the dome axis. In this cross section, stringers in the eastern part dip to the east, while stringers in the western part dip to the west. Despite of this overall configuration (common strike), the individual stringers in the western part can not either be correlated by the use of sedimentological or structural features with the stringers in the eastern part.

Two fold generations can be observed in Qarn Nihayda. For example, the most northwestern stringer dips with 70° to the east and is characterised by gentle to tight asymmetric folds with east and west dipping limbs and a N-S trending fold axis (Figure 10a). The second fold generation represents the gentle bending of the whole stringer with a steeply dipping and E-W trending fold axis, which is common in all of the six salt domes. This fold style is difficult to observe in the field, since interlimb angles are in many cases up to 170°. It becomes more apparent in the satellite image by the slightly curved shape of some stringers (Figure 7). The deformation-related structures are in large parts very similar to Jabal Majayiz. The most obvious deformation-related structures in the field are breccias associated with numerous whitish, mostly calcite cemented veins (Figures 10b–c). These breccias are very common and not tied to a specific lithology. The associated veins are often oriented parallel and perpendicular to bedding, displaying ‘dike-and-sill’ structures (Figure 10c), which generally form by hydrofracturing due to high fluid overpressures (Mandl, 2005).

In some cases, the breccias define up to 2-m-wide damage zones, which are oriented perpendicular to the strike of the stringers. These fault zones likely promote the tectonic dissection of stringers in separated blocks. As the overall fold style of the stringers is open, it is difficult to observe crosscutting relationship between the tectonic breccias and folding. However, in some cases it appears that the breccias are clearly incorporated into folding. A few stringers show bedding-parallel and oblique (< 15°) thrust faults, which crosscut tectonic breccias (Figure 10d). Because marker horizons are lacking, the displacements are not assessable. In addition, a number of faults oriented perpendicular to the bedding are defined by up to 7 cm thick cataclasites (Figure 10e and f). A slaty and fine-grained fabric typically characterises these cataclasites. The occurrence of massive blocks of anhydrite caprock is common in Qarn Nihayda and covers about 90% of the northern and the southern part (see white areas in Figure 7). As observed in the Qarn Sahmah salt dome, whitish to yellowish anhydrite often forms in-between single stringer ridges in the centre of the dome. The mesoscopic fabric is characterised by thin laminae of pink to whitish anhydrite alternating with dark ochre-coloured calcite laminae (Figure 10g). This fabric is often highly fractured and cemented by gypsum or anhydrite (Figure 10h).

Summary: Facies and Deformation

In both salt domes, vertical facies changes in relation to relative sea-level change can be observed (Figures 6 and 8). On the other hand, the relatively chaotic arrangement and strong tectonic modification of stringers in the dome centre of Jabal Majayiz make it difficult to trace lateral facies variations over more than c. 200 m (Figure 5). This is especially true for the northern part of the dome, which is affected by locally strong brecciation. The stringers with the largest lateral continuity can be found at the dome flanks. The stringers in the interior of the Qarn Nihayda dome are also more intensively deformed than at the dome flanks, where lateral facies change can be mapped over several hundred metres. In contrast to Jabal Majayiz, the Qarn Nihayda salt dome comprises carbonate facies that are very similar to the Ara carbonate facies described from the SOSB (e.g. Mattes and Conway Morris, 1990; Schröder et al., 2005). Hence, a detailed facies map of stringers along the flanks of the Qarn Nihayda salt dome will likely provide the best analogue for the reservoirs in the SOSB.

DIAGENESIS

Paragenetic Sequence

Based on 40 thin-sections, a common paragenetic sequence was developed for the six surface-piercing salt domes (Figure 11). Adjacent stringers within the individual salt domes were exposed to different maximum burial temperatures, as indicated by solid bitumen reflectance (see below), and hence might have experienced slightly different diagenetic histories. On the other hand, the paragenetic sequence of all salt domes encompasses the same succession of diagenetic processes from shallow to deep burial and subsequent uplift. The term shallow burial refers to diagenetic processes that occur until the carbonate stringers were completely sealed by the Ara Salt, which can be assumed to occur at a burial depth of around 30 m for rock salt (Casas and Lowenstein, 1989). Consequently, the term deep burial denotes the broad field of diagenetic alterations after the carbonate stringers were fully encased by the Ara Salt. Uplift related processes become active when the diagenetic system becomes at least partially open to external fluids. The general paragenetic sequence developed for the salt domes can be directly compared to the general paragenetic sequence from the SOSB (Schoenherr et al., 2008), which is also based on a basin-wide analysis including the whole range of burial depth. The paragenetic sequence is presented in Figure 11 and supported by photographs in Figure 12 and 13.

Shallow Burial

The earliest diagenetic phase is botryoidal cement that occludes some of the growth framework porosity in the thrombolite facies. Today these cements are either dolomitic or, if calcitic, contain small dolomite inclusions. This suggests multiple phases of replacement. However, their botryoidal form and blunt crystal terminations suggest an aragonitic or high-Mg calcite mineralogy prior to dolomitisation. The precipitation of both mineral phases could have been favoured by terminal Neoproterozoic seawater with an Mg/Ca ratio only slightly lower during the Quaternary (Brennan et al., 2004). The drape of laminae around calcite cemented or open moulds is interpreted as displacive growth of anhydrite nodules in soft sediment before compaction and dolomitisation (Figure 11, Phase 2; Figure 12a). Dolomitisation seems to have occurred simultaneously with, or slightly earlier than, the first phase of carbonate leaching. The vuggy porosity created by this first phase of carbonate leaching is often filled by anhydrite cement (Figure 11, Phase 5). Anhydrite cement also occluded most of the remaining growth framework porosity in the thrombolite facies (Figure 12b). The timing of the development of clusters and single anhydrite laths replacing matrix dolomite, predominantly in the thrombolite and crinkly laminite facies, is equivocal. In many cases the anhydrite lath formed prior to significant stylolitisation. They are therefore attributed to the same phase of anhydrite growth as the shallow burial anhydrite cements (Figure 11, Phase 5).

Deep Burial

In its earliest form silica was only observed in the stromatolite facies of Jabal Majayiz, where it occurs as angular fine siliciclastic grains and as clear to light-brown silica cement (Figure 11, Phase 6). In stromatolite, microcrystalline quartz-cement occludes the intercrystalline porosity of laminae consisting of fine crystalline sub- to euhedral dolomite. Silica cement is also present as equant micro- to megaquartz crystals and length-slow chalcedony cementing fenestral pores in stromatolites. The presence of length-slow chalcedony and the textural relationship with dolomite indicates that the bulk of this silica phase precipitated from solutions high in Mg2+ and SO42- (Folk and Pittmann, 1971) shortly after dolomitisation. A second carbonate leaching phase (Figure 11, Phase 7), which is only of local importance, affected the coarser grained organic-poor parts of crinkly laminites (Figure 12c). In contrast, the darker, organic-rich laminae are preferentially preserved (Figure 12c). This leaching phase seems to be associated with hydrocarbon migration, since solid bitumen is relatively abundant in this porosity type. In some rare examples, solid bitumen is enriched along stylolites but is absent from the matrix. This indicates that some stylolites have acted as pathways for hydrocarbon migration. The onset of stylolite formation hence predates oil migration. Where a textural relationship between stylolites and fractures can be observed, the fractures crosscut the stylolites. The main phase of fracture formation hence seems to postdate stylolite growth. This generation of fractures is normally cemented by dolomite or calcitic cement with small dolomite inclusions, indicating a former dolomitic mineralogy (Figure 11, Phase 9). Microfracture-lining saddle dolomite, which is characterised by curved-shaped crystal facettes and undulose extinction, occurs in few samples. Solid bitumen is relatively rare. For the bulk of the solid bitumen the solid bitumen reflectance data indicate a formation in the deep burial realm (Figure 11, Phase 10).

Uplift Related Processes

In the surface-piercing salt domes of the GSB, the carbonate stringers are intercalated within diapiric caprock, which forms through the dissolution of salt and the passive enrichment of insoluble impurities at the dissolution front (Posey and Kyle, 1988). The caprock consists predominantly of anhydrite, calcite, quartz, feldspar and gypsum. As long as the carbonate stringers are sealed by salt, their diagenetic system is completely closed. When the salt around the carbonate stringers dissolves in structurally shallow levels, the diagenetic system in turn becomes open to external fluids. In one sample, solid bitumen impregnated pores were observed in caprock (Figure 12d), indicating that liquid hydrocarbons were present during or after caprock formation. The burial temperature recorded by the solid bitumen reflectance indicates that caprock formation started at temperatures close to or higher than 110°C, equivalent to burial depth of about 4 km (Figure 11, Phase 12). Caprock formation can continue over long time periods whereby progressive dissolution along the salt-caprock interface leads to underplating of the newly formed residue. Hence the caprock becomes younger towards its base.

The age relationship between diagenetic phases related to caprock formation and dedolomitisation is difficult to evaluate. Most of the uplift related processes seem to occur nearly simultaneously. One of the most important processes is dissolution of evaporites (Figure 11, Phase 14). Partially open growth-framework porosity in thrombolites often contains corroded remnants of former anhydrite cements. Moulds after anhydrite nodules and lath also point to extensive leaching of evaporites. Rehydration of anhydrite to gypsum (Figure 12b) is seldom observed. Some former anhydrite nodules are now filled by calcite. In most cases the calcite is free of inclusions and is interpreted as cement filling the moulds of dissolved anhydrite nodules (Figure 12a). In a few cases small anhydrite inclusions in the calcite point to a direct replacement of anhydrite by calcite (Figure 11, Phase 16) without a void phase.

Calcitisation of dolomite is probably the most important uplift-related process (Figure 11, Phase 17). Small dolomite inclusions in the coarse blocky calcite crystals indicate that calcitisation proceeded through a thin solution-film and not through a solution cavity-fill process (Figure 12 e). Independent of facies, calcitisation is most pronounced adjacent to strongly fractured and brecciated rocks. Outside such zones, calcitisation is facies and fabric selective. In mid-ramp successions of interbedded laminated and massive dolostones, the laminated dolostones are much more pervasively calcitised than the massive ones. On the microscale, the organic-poor, light laminae of crinkly laminites are replaced by calcite, while the organic-rich, dark laminae are predominantly preserved as dolomite (Figure 12e). The geopetal fill in former microclots of thrombolites (Figure 12f) indicates that microcrystalline dolomite was preferentially dissolved (Figure 11, Phase 18) and the void subsequently cemented by sparry calcite (Figure 11, Phase 19). In contrast, coarse to medium crystalline, subhedral dolomite surrounding the former microclot is well preserved. This is one of the rare examples where calcitisation proceeded through a solution cavity-fill process instead of a replacement by thin-film solution without a void phase. Drusy and blocky calcite cement is abundant in fractures (Figure 12e) and breccias (Figure 12g). The absence of any dolomite inclusions indicates that these cements were directly precipitated as calcite and did not form through a replacement of dolomite cement. The lack of any dolomite cement suggests that these fractures were formed during the uplift phase (Figure 11, Phase 13; Figures 13e–f).

Authigenic euhedral quartz crystals of up to several millimetre in length contain calcite and dolomite inclusions in the same crystal (Figure 12h). Often the dolomite inclusions are concentrated in the core and the calcite inclusions in the rim of the quartz crystal. This pattern is interpreted as replacive growth during progressive dedolomitisation.

Dedolomitisation often is accompanied by calcite dissolution (Ayora et al., 1998). In contrast to many other studies on dedolomitisation, there is no evidence for a late uplift-related phase of calcite dissolution, such as open rhombohedral pores (Evamy, 1967) or solution-enlarged fractures. In general, the sedimentary fabric is well preserved (Figures 13a–d) and the different primary facies, such as laminites and thrombolites can be distinguished despite pervasive replacement during dolomitisation and dedolomitisation.

Petrographic Differences between the GSB and SOSB

Shallow Burial

Some of the earliest diagenetic phases observed in some samples from the SOSB, such as micritic and drusy dolomite cement, were not observed in the GSB samples. In contrast to the SOSB, no indication of a very early, shallow burial phase of fracturing was observed in the GSB. However, a much more important difference between the two basins is the complete absence of halite cements and the reduced abundance of anhydrite cements in the stringers of the GSB. Apart from these differences, the shallow burial diagenetic processes are very similar for both basins and some of the early diagenetic phases, like the botryoidal cement, are surprisingly well preserved.

Deep-Burial

The low abundance of solid bitumen in the GSB is one of the most striking differences in comparison to the SOSB. Coke-like solid bitumen indicates a phase of hydrothermal alteration in the SOSB (Schoenherr et al., 2007). In the GSB, coke-like solid bitumen is absent with the exception of one sample taken close to a basalt dike in Qarn Sahmah.

Uplift-related

All processes related to an open diagenetic system during uplift are absent from the SOSB. Fractures are common in the subsurface of the SOSB but are even more abundant in the surface outcrops of the GSB. Most fractures in the SOSB were formed in the deep-burial environment. In the GSB the majority of fractures likely formed during the uplift-phase. In the SOSB the replacement of dolomite by calcite is of local importance, likely caused by thermochemical sulphate reduction in the deep-burial environment (Schoenherr et al., 2008). Petrographically this dedolomite might be difficult to distinguish from dedolomite in the GSB. It therefore can not be completely ruled out that some of the dedolomite attributed to uplift related processes was already formed during deep burial. However, the bulk of dedolomite in the GSB can clearly be attributed to uplift related processes.

Porosity

Growth-framework, fenestral and vuggy pores are more abundant in the GSB than in the SOSB. In total, this porosity increase outbalances the concomitant decrease in intercrystalline porosity in the GSB.

Stable Isotope Data

Stable oxygen and carbon isotopes of 101 carbonate stringer samples, mainly from Qarn Nihayda (n = 52) and Jabal Majayiz (n = 31), were analysed (Figure 14). The samples have δ18O values that range between -10.3 and -0.5‰Vienna Peedee belemnite (VPDB), and δ13C values between -7.7 and 3.9‰ VPDB. The carbonate-matrix samples and primary dolomite cemented veins (Figure 11, Phase 13) display mean δ18O values of -4.0‰ VPDB (σ = 1.58) and mean δ13C values of 1.8‰ VPDB (σ = 1.74). The lowest δ18O values (-10.3 to -6.6‰ VPDB) derive from uplift related calcite vein cements (Figure 11, Phase 13), which also show the most depleted δ13C values in the range of -7.7 to -1.3‰ VPDB. The matrix of the same samples shows isotope values, which invariably plot in the average field of most samples.

The δ18O values from the carbonate matrix samples of the surface-piercing carbonate stringers of the GSB are depleted by about 1.6‰ compared to the subsurface carbonate stringers of the SOSB (Figure 14, Schoenherr et al., 2008). In contrast, the mean value of δ13C, excluding the calcite vein cements, is nearly identical for both settings, although some GSB samples are depleted compared to the SOSB samples. With respect to δ13C most of the matrix samples from the SOSB and many from the GSB plot close to the field of isotopic composition of the Neoproterozoic seawater (Derry et al., 1992; Jacobsen and Kaufman, 1999). Furthermore, two samples from the same stringer in Qarn Nihayda (QN 14, Figures 7 and 14) and two samples from two different stringers in Jabal Majayiz (JM 2 and JM 16, Figures 5 and 14) plot in the field of the short-lived negative δ13C excursion, which marks the Precambrian-Cambrian boundary in the A4C interval of the SOSB (Amthor et al., 2003).

Interpretation and Comparison to the SOSB

With respect to early and deep burial diagenesis the development in the GSB seems to be nearly identical to the SOSB (Schoenherr et al., 2008). The lower abundance of solid bitumen is explained most easily by a lower volume of hydrocarbons in the GSB compared to the SOSB. An alternative explanation could be that the most prolific stringer intervals, such as the A2C, are not exposed in the salt domes. All other major differences are a consequence of diagenetic alterations in association with dedolomitisation in the GSB. Dedolomitisation, the replacement of dolomite by calcite, is caused by Ca-rich solutions. Meteoric water, responsible for halite dissolution during caprock formation, likely is also undersaturated with respect to anhydrite (Posey and Kyle, 1988). When anhydrite is dissolved by meteoric waters, Ca2+ is expelled to the porewater and promotes calcite cementation and replacement of anhydrite by calcite. Calcite precipitation, in turn, decreases the pH and removes carbonate ions from the solution causing dissolution of dolomite (Back et al., 1983). The concurrent calcite precipitation and dolomite dissolution leads to dedolomitisation. This process could be enhanced by bacterial or thermochemical sulphate reduction, which removes sulphate from the porewater and increases alkalinity (Ben-Yaakov, 1973; Reuning et al., 2006), favouring anhydrite dissolution and calcite precipitation. The reduction of dissolved sulphate ions is accompanied by organic matter oxidation. The presence of pore-filling solid bitumen in caprock demonstrates that liquid hydrocarbons were available for sulphate reduction during or after caprock formation. The solid bitumen reflectance indicates a burial temperature of about 110°C. At temperatures above 80°C thermochemical sulphate reduction is more likely to occur than bacterial sulphate reduction. Subsequently, bacterial sulphate reduction could have occurred during later uplift. The presence of saddle dolomite and solid bitumen, which are typical by-products of sulphate reduction (Machel et al., 2001), might indicate that this process was active in the GSB.

The influence of meteoric waters in the GSB is also supported by stable-isotope values. The oxygen isotopic composition of carbonates is directly controlled by temperature and the isotopic composition of the precipitating fluid. The greater burial depth in the GSB, deduced from bitumen reflectance data, could have contributed to the shift towards lighter δ18O values compared to the SOSB. However, the fact that the most negative δ18O values occur in uplift-related, cemented fractures that formed late in the diagenetic history and the positive correlation to δ13C indicate that temperature is perhaps not the main controlling factor. A similar positive correlation between δ13C and δ18O was observed in Pleistocene phreatic cave deposits from the diapiric Jabal Madar dome in northern Oman (Figure 14), which is underlain by Ara Group evaporites (Immenhauser et al., 2007). The authors attributed the trend towards lighter oxygen isotopes to the progressive mixing of saline, deeply circulating meteoric fluids that rose along the diapir stem with descending 18O depleted meteoric freshwaters. The accompanying shift towards lighter δ13C values was interpreted as the incorporation of variable amounts of 12C derived from the oxidation of soil-zone organic carbon derived from land plants (Meyers, 1997; Reuning et al., 2005). Descending meteoric freshwaters would be characterised by lighter δ13C values. The ascending basinal fluids, in comparison, would have obtained a less depleted δ13C signature through dissolution of marine carbonates during their circulation in the basin.

Extensive fluid convection cells, where basinal waters are channelled upward along escape structures bounding the diapir stems, are known from the Gulf of Mexico (Posey and Kyle, 1988). A similar mechanism as proposed by Immenhauser et al. (2007), hence, could explain the observed isotope trend in the surface-piercing salt domes of the GSB. However, an additional source for depleted δ13C values in the surface-piercing salt domes might be the oxidized organic matter from liquid hydrocarbons (with δ13C commonly between -25 and -30 permil) consumed during sulphate reduction (Machel, 2001; Reuning et al., 2002). The phreatic calcites in Jabal Madar (Immenhauser et al., 2007) have more depleted δ18O values than the carbonate stringers of the surface-piercing salt domes. Likely the influence of deeply circulating basinal fluids, with higher δ18O values, was more important at the surface-piercing salt domes than in the cave deposits of Jabal Madar. Alternatively, the meteoric waters, which caused the dedolomitisation of the carbonate stringers at the surface-piercing salt domes might have had a less depleted δ18O signature. Carbonate cements of quaternary conglomerates from south of the Oman Mountains vary between -8 and -1 permil δ18O PDB, indicating a less depleted isotopic composition of shallow groundwater (Burns and Matter, 1995).

The fact that vein filling calcite cements (Figure 11, Phase 13 + 19) show the most negative oxygen and carbon isotope values suggests that faults acted as fluid pathways during dedolomitisation. In contrast to the calcite-cemented veins, the bulk of samples from the carbonate matrix and primary dolomite cemented fractures (Figure 11, Phase 9) show a large overlap with the δ13C values of the subsurface samples from the SOSB. This likely is due to a lower fluid-to-rock ratio outside of the uplift related fractures, which buffers the carbon-isotope values towards the positive marine δ13C values of the host carbonate. Four samples from the GSB plot close to the A4C samples from the SOSB that record the negative isotope excursion of the Cambrian – Precambrian boundary (Amthor et al., 2003). Since these samples from three different stringers in Jabal Majayiz and Qarn Nihayda are only weakly dolomitised, they could have recorded an unmodified secular isotope signal and hence would be time-equivalent to A4C stringers in the SOSB. However, this hypothesis would need to be confirmed by additional carbon-isotope analysis of these stringers.

Implication for Reservoir Properties

Dedolomites are often described as highly porous with a good hydrocarbon reservoir potential. Dedolomitisation can proceed through a direct replacement, where dissolution of dolomite and precipitation of calcite occur simultaneously (Evamy, 1967; Ayora et al., 1998). Alternatively, the dissolving dolomite leaves a void, which is subsequently filled by calcite cement either during the same overall process (Garcia Garmilla and Elorza, 1996), or from a different solution at a different time (Jones et al., 1989; James et al., 1993). Dedolomitisation has been described as (1) porosity increasing due to the predominance of dolomite dissolution over calcite precipitation (Canaveras et al., 1996), (2) as porosity reducing because of cementation of open porosity by calcite (Munn and Jackson, 1980); or as (3) porosity preserving, where dolomite dissolution and calcite precipitation take place pseudomorphically (Evamy, 1967).

Ayora et al. (1998), based on reactive transport modelling and petrographic observation, argue that dedolomitisation can either take place as pseudomorphic replacement on a volume-to-volume basis or non-pseudomorphically on a mole-to-mole basis following the equation:

 

CaMg(CO3)2 + Ca2+ = 2CaCO3 + Mg2+.

They state that pseudomorphic replacement will preserve the porosity, whereas non-pseudomorphic replacement will reduce the porosity since the molar volume of dolomite is less than two molar volumes of calcite. In the case of the GSB, the petrographic evidence clearly points to direct replacement as the dominant process. Evidence for solution-cavity fill, such as geopetal fill, has only been observed sporadically. The preservation of many primary or early diagenetic structures, such as the botryoidal cements, points to pseudomorphic replacement as the dominant process, dedolomitisation itself being porosity preserving (Ayora et al., 1998). Direct calcite precipitation, independent of dolomite replacement, leads to the occlusion of fracture and intercrystalline porosity. A later phase of calcite dissolution, like it has been observed in many other dedolomites (Evamy, 1967; Ayora et al., 1998), is not present in the GSB. Hence, the overall increase in porosity does not seem to be due to the dedolomitisation itself but can be attributed to the dissolution of anhydrite and probably also halite mainly from growth-framework, fenestral and vuggy pores.

In summary, the study of the diagenesis of surface-piercing Ara carbonates can in some respect be helpful for the interpretation of diagenetic relationships in the subsurface of the SOSB since many of the early to deep burial diagenetic phases were preserved by pseudomorphic replacement. A direct comparison of petrophysical (porosity and permeability) and geochemical (e.g. δ18O) properties, on the other hand, is hampered by the strong uplift-related diagenesis in the GSB. To obtain an unaltered carbon-isotope signal, e.g. to identify the A4C interval, it is recommended to use facies, which are less affected by dedolomitisation, such as the massive dolostones of the mid-ramp facies.

SOLID BITUMEN REFLECTANCE AND PALAEO-TEMPERATURES

Samples for both, total organic carbon (TOC) and mean random solid bitumen reflectance (BRr) were selected in order to compare data between several stringers within each salt dome and between the salt domes.

Total Organic Carbon (TOC)

The majority of the samples selected for TOC determination derive from dark and fetid finely laminated carbonates (n = 55), most likely of the basinal facies, which macroscopically appeared to represent potential source rocks. In addition, some samples originate from the crinkly laminite and thrombolite facies. Only 3 out of 60 samples have TOC values higher than 0.5% and that about 90% of all samples have TOC values < 0.1%.

Solid Bitumen Reflectance (BR)

The majority of the samples from which the TOC was determined, and some additional crinkly laminites, thrombolites and one diagenetic cap rock sample were selected for BRr measurements (n = 43 samples). The very low TOC values correlate very well with a very low content of solid bitumen in all samples. Reflected light microscopy under immersion oil has shown that c. 95% of the 43 samples contain solid bitumen, however, besides a few exceptions, most samples contain only trace amounts of very fine-grained solid bitumen, mostly accumulated along laminae. The applied method allows for the measurement of BRr of solid bitumen particles with a rectangular area of at least 0.42 µm2. As most of the solid bitumen particles in all samples studied are smaller than this area, only 29 out of the 43 samples could be used to calculate palaeo-temperatures from the BRr data (Table 1). In order to obtain palaeo-temperature data, the BR r data were converted to VRr data (Schoenherr et al., 2007), which were converted to palaeo-temperatures using the equation of Barker and Pawlewicz (1994) for burial temperatures. The palaeo-temperatures of two samples, situated in close proximity to a basalt dike in Qarn Sahmah, were calculated using the equation of Barker and Pawlewicz (1994) for hydrothermal temperatures. In a few cases, the comparability of maturity data of samples from different locations within one salt dome is hampered by the presence of two generations of solid bitumen. In Qarat Kibrit, the BRr data for 5 samples from a 20-m-thick stringer described in Peters et al. (2003) consistently show two generations, indicating burial temperatures of 200°C and 150°C, respectively. BRr data from a stringer c. 200 m in the north of the first outcrop indicate a burial temperature of 170°C.

In order to test if the symmetric configuration of the stringer ridges observed in Qarn Nihayda matches with a possible symmetry trend of thermal maturation isogrades, we measured the BRr of samples located along the WSW-trending profiles (see Figures 7 and 7b). The dataset indicates highest burial temperatures of 200–250°C of stringers located in the dome centre and of 280–300°C of stringers outlining the eastern flank of the dome (see Figure 7). In one sample, solid bitumen in diagenetic caprock (Figure 12d) records a burial temperature around 110°C indicating that liquid hydrocarbons were present during or after caprock formation. However, it is not clear if the hydrocarbons were sourced from within the carbonate stringers or migrated into the caprock from an external source after dissolution of the surrounding Ara Salt. A contribution from possible salt flank traps seems possible, even though an exploration well just to the east of the diapir proved to be unsuccessful.

In the Qarn Alam salt dome, two samples from outcrops about 300 m apart show nearly identical BRr values, which correspond to burial temperatures of 240°C. Palaeo-temperatures from three stringers of the Qarn Sahmah salt dome vary between 150–200°C. Two thrombolite samples, which have been taken 2 m apart from a c. 400 m long and 1.5 m thick basalt dike indicate palaeo-temperatures of 330–370°C. Interestingly, microstructures of the basalt show cracks, which are cemented by calcite, quartz and subordinate solid bitumen. Palaeo-temperatures from four stringers of salt dome Jabal Majayiz vary between 170–210°C. In summary, this dataset shows that the surface-piercing stringers in the GSB experienced burial temperatures between 140–300°C with the majority of burial temperatures at c. 200°C.

Interpretation and Comparison to the SOSB

The extremely low TOC values (< 0.1%) of most samples from the basinal facies indicate poor source rock potential in the salt domes (Table 2). This is in contrast to the laminites of the A1C-A4C reservoir intervals of the SOSB, which have TOC values in the range of 0.5–5.0% (Schoenherr et al., 2007). The BRr values and the calculated burial temperatures can not be directly compared to the SOSB, since the BRr values in the SOSB are strongly overprinted by hydrothermal alteration (Schoenherr et al., 2007). In the GSB hydrothermal alteration seems to be restricted to the vicinity of the basalt intrusion in Qarn Sahmah. Other evidence for hydrothermal alteration, such as coke-like solid reservoir bitumen, was not observed in the GSB. Palaeo-burial temperatures between 140° and 300°C (average c. 200°C) in the GSB indicate burial depths between 5 and 11 km (average 7 km), assuming the present geothermal gradient of 28°C/km along the Maradi Fault Zone (Pollastro, 1999). Hence most of the stringers were buried deeper than the deepest stringers in the SOSB (c. 6 km).

CONCEPTUAL MODEL FOR SALT DIAPIR EVOLUTION

This section discusses the depositional, diagenetic and structural evolution of the surface-piercing salt domes in four stages and integrates data from the subsurface of the SOSB to highlight the differences caused by the uplift of intra-salt carbonate stringers (Figure 15). The structural evolution is in large parts compiled from the work of Escher and Kuenen (1929), Loosveld et al. (1996), Callot et al. (2006) and Filbrandt et al. (2006). A comparison between the carbonate stringers in the salt dome outcrops of the Ghaba Salt Basin and the stringer intervals in the subsurface of the SOSB is given in Table 2.

Ara Group Deposition (Late Neoproterozoic to Early Cambrian)

The fieldwork of Peters et al. (2003) clearly revealed that the carbonates and evaporites exposed in the six surface-piercing salt domes belong to the latest Ediacaran to Early Cambrian Ara Group, well known from the SOSB (Amthor et al., 2003; Al-Siyabi, 2005). They described an outcrop in Qarat Kibrit that displays the cyclic Ara Group succession of rock salt – anhydrite – carbonate – anhydrite – rock salt. Bromine contents of rock salt from this succession vary between 49–68 ppm (Schoenherr et al., submitted), indicating a normal marine feed for rock salt (Valyashko, 1956). This is consistent with bromine data in the range of 45–109 ppm for the Ara Salt from the SOSB (Schröder et al., 2003; Schoenherr et al., 2008). Although the affinity to the Ara Group is well established for the surface-piercing carbonate stringers, their exact correlation to the Ara cycles (A0C to A6C) as recorded in the SOSB is not straightforward. In the SOSB the stratigraphy is based on geochronology, bio- and chemostratigraphy. Radiometric dating of ash beds brackets the age of the Ara Group in the SOSB between ca. 547 Ma and 540 Ma (Amthor et al., 2003; Bowring et al., 2007). Absolute dating of the stringers is not possible for the GSB, since no ash bed was identified in the surface-piercing salt domes. In the SOSB the fossils Cloudina and Namacalatus are very abundant in the A1–A3 carbonates, especially in the thrombolite facies, but are absent from the A4–A5 level (Amthor et al., 2003). Thrombolites are present in all salt domes except Qarat Al Milh and Qarat Kibrit. Despite this widespread occurrence of thrombolites in the GSB, no Cloudina and Namacalatus fossils were found (Table 2). Their absence suggests that the stringers in the surface-piercing salt domes are younger than A3C.

The A4C stringer interval in the Birba area of the SOSB (Figure 1) is characterised by a negative carbon isotope excursion (Amthor et al., 2003) coincident with a pronounced enrichment in redox-sensitive trace elements (Schröder and Grotzinger, 2007) and a strong gamma-ray signal (Mattes and Conway Morris, 1990). The negative carbon isotope excursion can be correlated world-wide to the Precambrian – Cambrian boundary (Amthor et al., 2003). The presence of a gamma-ray response and trace-element enrichment were not tested for the GSB, but two stringers in Jabal Majayiz (Figure 5) and one stringer in Qarn Nihayda (Figure 7) show the negative isotope signal typical for the A4C interval. Since these samples are only weakly dedolomitised, they could have recorded an unmodified secular isotope signal and hence would be time-equivalent to the A4C stingers in the SOSB (see below). The fact that only three from c. 100 isotope samples show a possible A4C signature leads to the conclusion that the bulk of the stringers in the GSB likely are time-equivalent to the A5 and A6 interval in the SOSB. The A6C carbonates themselves likely do not form stringers but are lateral equivalents to the lowermost Nimr clastics (Figure 15). However, the A5 and A6 cycles in the SOSB contain different salt-encased units of anhydrite, carbonate or siliciclastics. Many of the stringers in the salt domes could represent such units from the A5 and A6 sequences. This interpretation is supported by the abundance of sandstone layers and siliciclastic-rich carbonates in the Jabal Majayiz salt dome (Figure 6), which are similar to the clastic intervals interbedded in the evaporites of the A5 and the A6 sequences in the SOSB (Figure 2). The relatively low source rock potential of the A5C and A6 intervals in the SOSB coincides with the low content of TOC and solid bitumen observed in the surface-piercing stringers.

Only Qarn Sahmah contains also magmatic rocks (granodiorites) and pre-Ara volcanics from much deeper levels (basement and pre-Ara strata) than the uppermost stratigraphic intervals (A5C and A6C) of the Ara Group exposed in the five more northern located salt domes (see Figure 1).

Initial Stage of Passive Salt Diapirism (Middle Cambrian to Late Ordovician)

The differential loading of the thick Nimr and Haima clastics onto the mobile substrate of the Ara Salt caused passive diapirism (downbuilding) until the Nimr grounded on the subsalt strata in the SOSB as well as in the GSB (Loosveld et al., 1996; Peters et al., 2003). Seismic sections presented by Peters et al. (2003, their figure 22) indicate the possible presence of stringers in the Saih Nihayda field in the GSB in a structurally similar position as the intra-salt stringer reservoirs in the SOSB. So far, the intra-salt stringers in the GSB are not a target of hydrocarbon exploration, probably due to difficulties in seismic detection and the deep burial of the stringers. The presence of solid bitumen generally indicates that oil was generated in the surface-piercing stringers, probably during the last periods of Haima deposition (as in the SOSB, Terken et al., 2001). The stringers exposed in the salt domes show very low amounts of solid bitumen and very low TOC values. On the other hand, the presence of buried hydrocarbon (gas?) bearing stringers, equivalent to the A1C to A3C in the SOSB, can not be excluded. As indicated before, the presence of liquid hydrocarbons during or after caprock formation is indicated by solid bitumen (Figure 12d). However, the hydrocarbons source is ambiguous. They could have been derived from within the carbonate stringers or migrated into the caprock from an external source.

Reactive/Active Diapir Growth and Surface Piercement (Late Cretaceous – Recent)

The Late Cretaceous sinistral strike-slip movement along the Maradi Fault Zone and the Burhaan Fault caused reactive diapir growth in transtensive relay settings, e.g. into the space created by pull-apart graben. The salt domes Jabal Majayiz, Qarat Al Milh, Qarat Kibrit and Qarn Alam are directly associated with such strike-slip zones (Peters et al., 2003; Filbrandt et al., 2006). From the Mid-Miocene to the Pliocene – Pleistocene the Zagros Orogeny led to a reversal from a sinistral to a dextral strike-slip movement along, for example, the Maradi Fault Zone (Filbrandt et al., 2006). The displacement in this contractional regime is believed to be limited to some 100 m (Hanna and Nolan (1989), compared to Late Cretaceous sinistral motion of some 10 km (Filbrandt et al., 2006). The squeezing of pre-existing salt domes by the Zagros collision therefore is thought to be a minor influence, but likely contributed to diapir growth and the final piercement of the pre-existing salt dome through the surface. The association of deeply rooted faults with the position of the salt domes Qarn Nihayda and Qarn Sahmah is so far not confirmed by seismic interpretations.

The widespread occurrence of cataclasites and tectonic breccias in the surface-piercing stringers was not observed in the extensively cored Ara carbonate stringers of the SOSB. The most likely structural position of this deformation is the narrow and shallow diapir stem, where differential (σ1 – σ3) stress (up to 5 MPa) and thus deformation intensity is highest within a salt diapir (Schoenherr et al., submitted). This uplift-related structural deformation probably led to the second phase of calcite cemented fracture formation (Figure 11, Phase 13 and 19; Figure 13), and is coincident with the major phase in dedolomitisation.

The highly heterogeneous distribution of palaeo-temperatures within the salt domes shows that the stringers derive from different depths and thus different stratigraphic intervals. It seems unlikely that all of the stringers were at the same structural level before piercement of the surface. The recent structural configuration of stringers and the widespread occurrence of anhydrite caprock in most salt domes (e.g. Figures 9g–h) rather suggest strong dissolution of rock salt at structurally shallow levels and at the surface, which led to rotation and a chaotic juxtaposition of the stringers. These processes strongly masked the original structural configuration of stringers during diapir rise and thus insights into the internal kinematics of the northern Oman salt diapirs. An exception is the near symmetric orientation and periclinal strike of most stringers in Qarn Nihayda (Figure 7), which points to the preservation of the original stratigraphy after piercement of the surface. However, the palaeo-temperatures from the Qarn Nihayda stringers do not show the symmetrical stringer configuration, i.e. increasing palaeo-temperatures towards the dome centre.

Insights from seismic sections and fieldwork suggest a relatively simple geometry of the six surface-piercing salt diapirs with far-reaching diapir stems and cylindrical terminations (Figures 2, 3 and 15). The lack of structures indicating lateral spreading (injection into adjacent strata) of the salt suggests constant confinement by country rock walls during emplacement. The steep dip of the salt (Qarat Kibrit) and of numerous carbonate stringers (e.g. Qarn Nihayda) contradicts a cut through the uppermost structural levels of a mushroom-shaped diapir as suggested by Talbot and Weinberg (1992) for some of the salt plugs in Iran. In contrast to many salt domes in Iran (Jackson et al., 1990) or in the subsurface of northern Germany (Mohr et al., 2007), there is no evidence for the presence of present or past salt glaciers.

CONCLUSIONS

  • The surface-piercing stringers generally comprise the same suite of facies as the deeply buried stringer play in the SOSB and thus represent an important outcrop analogue to study lateral facies development on a scale of up to several hundred metres.

  • Poor source rock development, the near absence of the A4C negative δ13C isotope excursion and the lack of the late Ediacaran fossil cloudina, as well as the abundance of clastics, presumably point to the exposure of the stratigraphically uppermost stringer intervals (A5C and A6 compared to the SOSB) in the salt domes.

  • Palaeo-temperatures point to very deep burial depths of on average 7 km due to passive downbuilding of the Ara Group.

  • The stringers are internally much more deformed than in the SOSB and depleted δ18O isotopes of syn-tectonic veins and the carbonate matrix suggest deformation and dedolomitisation in a diagenetically open system due to the contact with meteoric waters within the diapir stem.

  • Many of the early to deep burial diagenetic phases were preserved by pseudomorphic replacement. A direct comparison of petrophysical (porosity and permeability) and geochemical (e.g. δ18O) properties with the SOSB is hampered by the strong uplift related diagenesis in the GSB. Weakly dedolomitised carbonates, such as the massive dolostones of the mid-ramp facies, should be used to identify the light carbon isotope shift associated with the A4C interval.

ACKNOWLEDGEMENTS

We thank the Ministry of Oil and Gas of the Sultanate of Oman and Petroleum Development Oman (PDO) for granting permission to publish the results of this study. Furthermore, we are grateful to PDO for sample shipment and the many supporting data. Two anonymous reviewers and GeoArabia’s editorial staff are thanked for their helpful suggestions and professional treatment of this manuscript. The manuscript also benefited from a thorough revision by Beke K.A. Rosleff-Sörensen. Uwe Wollenberg is acknowledged for carrying out scanning electron microscopy and XRD measurements. Werner Kraus, Rolf Mildenberger and Phillip Binger are thanked for thin section preparation and Jens Köster for TOC measurements. The final design and drafting by GeoArabia’s Graphic Designer Nestor Niño Buhay is appreciated.

ABOUT THE AUTHORS

Lars Reuning is a Lecturer for carbonate sedimentology at RWTH Aachen University (Germany). He graduated from Marburg University and holds a PhD from the University of Kiel (Germany). Following a short period as Lecturer at the University of Kiel and as Visiting Lecturer at the State University of St. Petersburg (Russia), he joined RWTH Aachen University in 2005. He worked on sedimentology, seismic stratigraphy and diagenesis of carbonates, as well as on the impact of diagenesis on reservoir properties, mainly at the North West Shelf of Australia and in Oman. His main research interests are carbonate-evaporate interactions, carbonate diagenesis and 3-D seismic geometries at carbonate platform slopes.

reuning@geol.rwth-aachen.de.

Johannes Schoenherr is an Exploration Geologist with ExxonMobil Production Germany GmbH. He holds a German Diplom in Geology from Technical University of Darmstadt, Germany (2004), with a main emphasis on structural geology and a PhD from RWTH Aachen University, Germany (2008). His PhD research was focused on the analysis of reservoir and seal quality of the infra-Cambrian Ara play (carbonates and rock salt) in northern and southern Oman. He joined ExxonMobil in 2008 and is currently working in the Technical Subsurface Geoscience East Team with a specific interest in salt tectonics of the Zechstein in North Germany.

johannes.schoenherr@exxonmobil.com

Ansgar Heimann recently graduated from RWTH Aachen University, Germany. In his MSc thesis, he focused on the sedimentology and diagenesis of the surface-piercing salt domes of interior northern Oman.

ahei@gmx.net

Janos L. Urai is currently a Professor of structural geology, tectonics, and geomechanics at RWTH Aachen University and Inaugural Dean, Department of Applied Geoscience, German University of Technology in Oman (GUtech) in Muscat. He is interested in basic and applied aspects of rock deformation in the presence of fluids at a wide range of scales in hydrocarbon reservoirs.

j.urai@ged.rwth-aachen.de

Ralf Littke is Professor of geology and geochemistry of petroleum and coal at RWTH Aachen University, Germany. Ralf’s current research topics include dynamics of sedimentary basins, with special emphasis on temperature and pressure history; generation of hydrocarbon gases and nonhydrocarbon gases as well as petroleum; transport and accumulation of methane and carbon dioxide; and development of new tools in petroleum system modeling.

littke@lek.rwth-aachen.de

Peter A. Kukla graduated in geology from Wuerzburg University, Germany, and Witwatersrand University, South Africa (PhD). His professional career included positions at Witwatersrand University (1986 – 1990), Shell International E&P (1991 – 2000), and at RWTH Aachen University (since 2000) as Full Professor of Geology and Head of the department and Director of the Geological Institute, with research focus on applied sedimentology, reservoir geology, and quantitative geodynamics.

kukla@geol.rwth-aachen.de

Zuwena Rawahi is a Senior Carbonate Geologist in Petroleum Development Oman (PDO) and has been working on the Precambrian stringer play on the South Oman exploration team for the last three years. Prior to that, she worked for seven years on the Shu’aiba Formation. Her main interest is related to carbonate sedimentology and diagenesis.

zuwena.rawahi@pdo.co.com