ABSTRACT

The upper part of the Saiq and lower part of the Mahil formations in the Oman Mountains represent outcrop time-equivalents to the highly prolific, hydrocarbon-bearing subsurface Middle and Upper members of the Khuff Formation (K4-K1 reservoir intervals). In this study, four outcrops sections on the northern flank of the Oman Mountains (Al Jabal al-Akhdar region) are sedimentologically documented and integrated with the sequence-stratigraphic scheme initially developed at the Saiq Plateau reference section.

The focus of this study is the description of the distribution and textural variation of grainstones as potential reservoir facies on a subregional (ca. 60 x 40 km) scale. Stratigraphic cross-sections are constructed based on two sequence-stratigraphic orders: (1) one second-order supersequence (DS2 18) that provides a well-constrained general framework and (2) four third-order depositional sequences (KS1-KS4), within which subtle temporal and spatial variations of grainstones occur. From these correlations predictive rules and correlation lengths of shoal bodies are extracted.

The presence of Khuff grainstones is strongly governed by stratigraphic position. Thicker and more abundant grainstones are present during the early transgressive (KS4) and late regressive (KS1) portions of the supersequence. Thinner and less abundant grainstones are present during the late transgression (KS3 and lower KS2) and early regression (upper KS2). They are absent around the second-order zone of maximum flooding (middle KS2). High lateral continuity of correlated cycle sets is observed, suggesting the absence of significant tectonic activity of the area during the Late Permian and Early Triassic.

Integrated litho-, bio-and sequence stratigraphy provides a robust framework for correlation on a regional scale (ca. 700 km). The Oman Mountains area shows a more distal facies pattern on the Khuff platform compared with other Khuff reservoir sections in the region. This is especially evident around KS2 maximum flooding with muddy foreshoal and offshoal deposits in contrast to mainly oolitic shoal deposits in the Musandam (UAE) and offshore Fars (Iran) area.

INTRODUCTION

Significant volumes of natural gas in the Middle East are produced from the middle and upper part of the Middle Permian to Lower Triassic Khuff Formation (reservoir zones K4-K1) (Figure 1) (e.g. Al-Jallal, 1987; Sharland et al., 2001).

The Khuff Formation and its time-equivalents on the Arabian Platform are examples of carbonate ramp deposits, covering a plate-wide epeiric carbonate shelf composed of subtidal to supratidal limestones, dolomites and evaporites with prominent carbonate shoal complexes (e.g. Aigner and Dott, 1990; Al-Jallal, 1995). As a consequence of the extremely gentle topography of the platform, a “layer-cake”-type stratigraphic architecture is commonly attributed to the Khuff Formation on a 10s of km-scale. Major changes in gross depositional environment are documented to only occur over larger distances on the scale of 100s of km (e.g. Sharland et al., 2001, 2004; Osterloff et al., 2004; Insalaco et al., 2006; Maurer et al., 2009).

This study is part of a research project on Khuff grainstone bodies carried out in the Al Jabal al-Akhdar area (Oman Mountains) in the Sultanate of Oman (Figure 2). Its aim is to unravel the geometries and distribution of Khuff grainstone as potential reservoir bodies. Initially a one-dimensional facies and sequence-stratigraphic framework was proposed by Koehrer et al. (2010) for the Khuff time-equivalent strata of the Saiq Plateau. Subsequently, Khuff grainstone geometries were documented on the Saiq Plateau from near well (< 2 km)-scale (Zeller et al., 2011) to field (< 10 km)-scale (Koehrer et al., 2011). The present paper focuses on grainstone architecture on a subregional (60 x 40 km)-scale. It presents new sedimentological and biostratigraphic data from outcrop sections of Middle to Upper Khuff time-equivalent strata in (1) Wadi Sahtan, (2) Wadi Bani Awf, (3) Wadi Mistal and (4) Wadi Hedek that complement the analyses from the Saiq Plateau. 2-D correlations and stratigraphic crosssections across the Oman Mountains are constructed. Correlations are analyzed to develop predictive rules and reveal correlation lengths for Khuff grainstone bodies.

METHODS AND DATABASE

The outcrops investigated in this study are situated in the northern part of the Oman Mountains and on the Saiq Plateau (Figure 2). Free of vegetation and construction they allow detailed field investigation. Each outcrop was sedimentologically described in detail using both outcrop observations and thin sections. The Wadi Sahtan, Wadi Bani Awf, Wadi Mistal and Saiq Plateau sections were also studied biostratigraphically.

Previous work in the Oman Mountains was carried out during extensive mapping by the BRGM (Rabu et al., 1986, 1993; Le Métour et al., 1990; Bechennec et al., 1993). Coy (1997), Richoz (2006) and Richoz et al. (2010) studied the diagenesis and geochemistry of the Saiq and Mahil formations.

Facies characterization of the Khuff Formation and its outcrop time-equivalents in the Gulf region was carried out mainly by Alsharhan (1993), Alsharhan and Kendall (1986), Alsharhan and Nairn (1994a, b), Al-Jallal (1987, 1994, 1995), Al-Aswad (1997), Schlumberger (1981), Insalaco et al. (2006), Weidlich and Bernecker (2007), Maurer et al. (2008, 2009), Koehrer et al. (2010) and Kolodka et al. (2012), leading to the interpretation of the major Khuff facies belts (Figure 1).

In this study, facies analysis is based on 215 rock samples collected from the outcrops. Thin sections were prepared and analyzed with a standard transmitted light microscope for microfacies characterization and biostratigraphic analysis. Microfacies analysis focuses on faunal and algal assemblages, diversity patterns and environment-specific, abiotically or microbially induced components. For each thin section certain sedimentary features (e.g. sorting, gradation, fenestral fabrics, bioturbation) and diagenetic observations (e.g. dolomitization, recrystallization and cementation) were recorded.

Main constituents were grouped into abiotic components (e.g. ooids, peloids and cortoids), metazoans, algae and benthic foraminifera, followed by a semi-quantitative analysis (very rare, rare, common and abundant). Depending on preservation, foraminifera and algae were determined on species-or genus-level for biostratigraphy and paleoenvironmental interpretation. Results from the microfacies analysis were integrated into the scheme of facies (Table 1) and facies associations (Table 2) adopted from Koehrer et al. (2010).

Microfacies analysis of thin sections from the Saiq and Mahil formations is hampered partly by dolomitization. Rock samples are affected to various degrees by mimetic replacement (fabric preserving) to complete destruction into finely crystalline dolomite (non-fabric preserving). The “white paper technique” (Delgado, 1977; Dravis, 1991) was applied to enhance contrasts in thin-sections and allow observation of depositional fabrics and identification of microfossils in dolomites with poor preservation. Nearly complete fabric destruction is widespread in the Lower Mahil Formation of the wadi sections. There, textures and facies associations were mainly derived from dolomite crystal-sizes, components and sedimentary structures as well as from direct comparison with the much better preserved samples from the Saiq Plateau outcrop (Koehrer et al., 2010).

Outcrop gamma-ray measurements were carried out during field work using a portable GR spectrometer (model GS-512, manufactured by Geofyzika, Czech Republic). The spectrometer is equipped with a 3 x 3 inch NaI (TI) scintillation detector collecting natural gamma-radiation at the rock surface. Total counts were measured for 15 seconds with a sample point spacing of 1 m. Around specific stratigraphic boundaries sample point spacing was increased to 0.5 m. An interval of 15 seconds was chosen after test measurements of 180, 90, 30 and 15 seconds resulted in very similar GR trends. These GR measurements proved to be very useful for stratigraphic correlations and sequence interpretation.

Vertical stacking patterns were analyzed and interpreted within a sequence hierarchy (second-to fifth-order) according to the terminology proposed by Kerans and Tinker (1997). The accommodation versus sediment supply concept outlined in Cross et al. (1993), Cross and Lessenger (1998), Homewood et al. (2000) was applied for cycle, cycle set and sequence definition.

Logged sections were digitized with the software WellCAD and correlated based on the biostratigraphic framework, lithostratigraphic marker beds, outcrop GR logs and cycle set boundaries.

GEOLOGIC FRAMEWORK

Tectonic and Paleogeographic Setting

The Khuff Formation was deposited during the fragmentation of the supercontinent Pangaea. It formed during the syn-to post-rift phase of the Neo-Tethys Ocean in the Middle Permian to Early Triassic (Pillevuit, 1993; Stampfli and Borel, 2002). Throughout the Permian, the study area was probably located on a structural basement high (Searle, 2007). Early Permian continental clastics of the Gharif Formation present in the SW-interior of Oman pinch-out towards the Oman Mountains in the northeast (Blendinger et al., 1990). Due to seafloor-spreading of the newly formed Neo-Tethys Ocean and associated drowning of the Arabian Platform in Mid-Permian times, a thin sliver of initial continental clastics was transgressively onlapped by marine deposits (Glennie et al., 1974; Rabu et al., 1993; Searle, 2007). These carbonate platform deposits covered the rifted continental margin across parts of the Sultanate of Oman. They are dated as Middle Permian (Lower Khuff) in the Al Jabal al-Akhdar area (e.g. Rabu et al., 1993). During the Late Permian and Early Triassic (Middle to Upper Khuff), the study area was located relatively close to the outer Arabian Platform margin, some tens of kilometers landward (NE) of the present Omani coastline (Figure 1) (Ziegler, 2001; Konert et al., 2001; Searle, 2007). During the Late Cretaceous First Alpine Orogeny, the Permian to Cretaceous carbonate platform deposits of the Oman Mountains were faulted and folded, which transformed the region of Al Jabal al-Akhdar into a westward dipping anticline (Searle, 2007). Allochthonous deposits that include younger deep ocean sediments from the Neo-Tethys and the overlying ophiolites were thrusted over the older sedimentary rocks of the Proterozoic to Cretaceous (autochthonous unit) (Glennie, 2006). In the studied outcrop sections, main evidence of the structural history is shown by brittle faulting and fracturing, strike-slip faulting and bedding-parallel low-angle faulting (e.g. oral comm. J. Mattner, 2007 and P. Richard, 2009).

Outcrop Stratigraphy

Permian-Triassic strata in the Oman Mountains (Al Jabal al-Akhdar) is subdivided into two formations by Glennie et al. (1973, 1974) (Figure 3):

  • Saiq Formation (Permian), time-equivalent to the Lower and Middle Khuff Formation (K7-K3 reservoir units)

  • Mahil Formation (Triassic), time-equivalent to the Upper Khuff (K2-K1 reservoir units), Sudair and Jilh formations

The boundary between the Saiq and the Mahil formations, the “Saiq/Mahil Formation Boundary”, is well marked by a whitish-colored recessive step in the slope profile on the Saiq Plateau (Rabu et al., 1986; Coy, 1997). In the wadis, this color change is less prominent, but can be seen on satellite images (e.g. Glennie, 2006). In this paper, we follow the delineation of the “Saiq/Mahil Formation Boundary” from the Saiq Plateau (Koehrer et al., 2010) and extrapolate it to the adjacent wadi sections. It occurs some 100 m below the occurrence of a clastic interval with thinly interbedded red to olive-green shale beds, regarded as the formation boundary in the wadis by some other workers (e.g. Baud and Bernecker, 2010).

The sections studied in this paper cover a stratigraphic interval that is time-equivalent to the Middle to Upper Khuff Formation (reservoir zones K4-K1) (Figure 3). Detailed outcrop investigations on Lower Khuff (K5-K7) time-equivalents in the Oman Mountains are currently underway (Bendias et al., in prep. and Walz et al., in prep).

FACIES AND SEQUENCE STRATIGRAPHY Facies and Depositional Model

Facies were grouped into grain-dominated textures (i.e. potential reservoir facies) and mud-dominated textures (i.e. non-reservoir facies) (Figure 4, Tables 1 and 2, Plates 1 and 2):

Grain-dominated facies

Grain-dominated facies consist of well-sorted oolitic/peloidal grainstones (shoal deposits), coarsegrained intraclastic rudstones (proximal shoal margin) and poorly sorted cortoidal and bioclast/peloid-rich pack-to grainstones (moderate-energy backshoal) (Figure 4).

Oolitic/peloidal grainstones (Figure 5d) are often pervasively dolomitized and display a non-mimetic replacement of calcite into finely crystalline dolomite (Plate 2, picture 8). Biotic assemblages are often poorly preserved. Associated with these shoal deposits are poorly sorted cortoidal and bioclast/peloid-rich pack-to grainstones (Plate 2, picture 7) having a moderate to poor preservation, but show the highest biodiversity in smaller foraminifers and algae. Some of the pack-/grainstones display abundant microbial activity with micritic envelopes and oncoidal encrustations around grains.

Cortoids may be lumped together to form complex aggregate grains (Plate 2, picture 6), or merge into a microbial bindstone fabric. These beds are interpreted as protected areas behind the high-energy shoals with periodic reworking during storm events (Figure 4).

Mud-dominated facies

Interbedded between grain-dominated facies are bioclast-rich wacke-to packstones (Figure 5c). These beds were interpreted as foreshoal deposits initially (Figure 4). Detailed microfacies analysis revealed that a number of samples are characterized by dominant dasycladacean algae (Mizzia, Velebitella) (Plate 2, picture 11), gastropods, bivalves and common staffellid/miliolid assemblages. The dominance of green algae and absence of stenohaline metazoans indicate a backshoal environment for this facies (see also biofacies scheme in Insalaco et al., 2006 and facies interpretation in Maurer et al., 2009).

In contrast, mud-dominated foreshoal deposits are characterized by a higher diversity of metazoans (echinoderms, brachiopods, bryozoans and rugose corals) and dominance of gymnocodiacean algae (Gymnocodium) (Plate 2, pictures 9-10). Normally graded tempestite sheets and bioturbated mudstones of the distal foreshoal and offshoal environment are mainly present in the basal part of the Mahil Formation (Figures 5e and 5f). Burrowed mud-to wackestones (Figure 5b) and microbial laminites (Figure 5a) represent more restricted lagoonal and tidal flat environments. Biotic assemblages display a low diversity, mostly represented by common Earlandia and Hemigordiellina, small Globivalvulina and rare small lagenids (Plate 2, pictures 1, 2 and 4). Microbial laminites exhibit typical sedimentary features of the intertidal environment like fenestral fabrics and circumgranular cracks. Mudstone layers in-between microbial sheets yield tiny sparitic globules, surrounded by a thin dark micritic wall interpreted as “Calcispheres” (Plate 2, picture 3).

Sequence Stratigraphy

A high-resolution one-dimensional sequence-stratigraphic framework was proposed by Koehrer et al. (2010) for Khuff time-equivalent strata on the Saiq Plateau as a reference section (Figure 3). A fourfold stratigraphic hierarchy was defined and consequently used hereafter:

Cycles

Cycles (possibly fifth-order), 1 to 5 m in thickness, are the fundamental stratigraphic building blocks of the studied sections. They were found to be very useful for high-resolution correlation over distances of a few kilometers as shown on the well-exposed 8 x 8 km large Saiq Plateau outcrop (Koehrer et al., 2011). Reservoir quality trends might be revealed by tracing distinct changes in cycle composition on a field-scale. Since this study is aimed on correlations on a several 10s of km-scale, we focus on larger-scale cycle sets and sequences in this paper.

Cycle Sets

Systematic vertical stacking of Khuff facies types is most evident in transgressive-regressive, usually tens of m-thick cycle sets (most likely fourth-order). In the studied interval 20 cycle sets (KCS1.1-KCS4.11) were recognized. Depending on their stratigraphic position, they are between 10-30 m thick. Cycle sets are defined mainly based on facies stacking pattern in combination with the outcrop gamma-ray signature. Four principal cycle set motifs, previously described in Koehrer et al. (2010) were defined (Figure 5):

(a) Foreshoal Motif: This cycle set motif is made up of thinly-interbedded mud-to packstones of the offshoal and foreshoal facies association (Figure 5). Grainstones are absent. The vertical stacking pattern of this motif is interpreted as a systematic shallowing-upward from the offshoal below storm wave base (SWB) to distal to proximal foreshoal environment below fair weather wave base (FWWB) during regression.

(b) Shoal Motif: This motif is composed of mud-dominated foreshoal facies types with wacke-to packs tone texture and well-sorted oolitic/peloidal grainstones of the shoal facies association (Figure 5). The thickening-upward of high-energy facies types in the upper part of the cycle set is interpreted as upward-shallowing/shoaling from a proximal foreshoal setting into the shoal depositional environment during regression.

(c) Backshoal Motif: Cycle sets of this motif consist of grainstones of the high-energy shoal and mud-dominated wacke-to packstones of the moderate and low-energy lagoonal backshoal facies associations (Figure 5). The sparse occurrence of mud-dominated foreshoal facies types is limited to the zone of maximum accommodation in the middle part of the motif. The vertical facies stacking pattern of the cycle set motif suggest an upward shallowing into the low-energy backshoal facies association from the high-energy shoal and moderate-energy foreshoal facies association during regression.

(d) Tidal Flat Motif: The tidal flat cycle set motif is generally made-up of wacke-to packstones belonging to the backshoal facies association and microbial laminites with mud-to wackestone texture (Figure 5). These laminites represent the most proximal deposits of the tidal flat environment. Interbedded are several dm-thick grainstones beds that may represent spill over lobes and backwash deposits within a lagoonal setting. The development of muddy cycle set caps in the upper part of the cycle set motif is interpreted as the systematic shallowing-upward from the high-energy shoal and low-energy backshoal (lagoonal) into the tidal flat facies association during regression.

Sequences

The stratigraphic interval covered in this paper was subdivided into four third-order sequences (KS1-KS4) (Figure 3). Sequence-stratigraphic characteristics of the Saiq Plateau section are described in detail by Koehrer et al. (2010). Four additional outcrop sections were investigated in wadis on the northern flank of the Oman Mountains: Wadi Sahtan (Figures 6 and 7), Wadi Bani Awf (Figures 8 and 9), Wadi Mistal (Figure 10) and Wadi Hedek (Figures 11 and 12). Sequence-stratigraphic characteristics of all sections are summarised in Table 3.

(a) Khuff Sequence KS4: The KS4 is the thickest of the four investigated sequences ranging from 152 m in Wadi Mistal to 171 m in Wadi Hedek (Table 3). It can be subdivided into 11 cycle sets (KCS4.1-KCS4.11) (Figures 6, 8, 10 and 11) and directly overlies a microbial laminite unit up to 2 m thick (“Microbial Marker 2”). This prominent bed is found in four out of five sections, only faulted-out in the Wadi Sahtan section.

The transgressive part (KCS4.11-KCS4.7) is mainly built of mud-dominated cycle sets of the tidal flat motif.

The maximum flooding surface (MFS) of KS4 (MFS P30) is tentatively placed in the middle of cycle set KCS4.6 at maximum thickness of grainstones interbedded with muddy foreshoal textures, indicating the most open-marine conditions.

In the regressive part (KCS4.5-KCS4.1), sections are dominated by the shoal cycle set motif. The upper sequence boundary of KS4 (SB KS4/KS3) is placed at a broadly correlatable bed of restricted facies types with microbial laminations (“Microbial Marker 3”). It is further marked by a prominent positive excursion of the outcrop GR-curve (Figures 6, 8, 10 and 11).

(b) Khuff Sequence KS3: The KS3 varies in thickness between 62 m in Wadi Mistal and 69 m in Wadi Hedek (Table 3). It consists of four cycle sets (KCS3.1-KCS3.4) (Figures 7, 9, 10 and 12).

The transgressive part of the KS3 (KCS3.4-KCS3.3) is dominated by cycle sets of the shoal motif and muddy cycle sets of the backshoal motif.

The MFS of the KS3 is placed within an up to 1 m thick coral floatstone bed (“Coral Marker”) in KCS3.2, containing abundant rugose fasciculate corals, partially in in-situ life position. This bed is found in Wadi Hedek, Wadi Mistal and on the Saiq Plateau. In Wadi Sahtan and Wadi Bani Awf, the “Coral Marker” bed is not present. There, the MFS (MFS P40) is placed within bioclast-rich packstones with some coral fragments and other open-marine fauna.

During the regressive part (KCS3.1), cycle sets are entirely composed of cycle sets of the shoal motif. The upper sequence boundary of KS3 (SB KS3/KS2) is placed on top of thickly amalgamated grainstone beds present in all sections, reaching a thickness of up to 7 m. No evidence for any missing sections was found in the upper part of the KS3, as suspected by Al-Husseini and Matthews (2010). This is confirmed by the presence of a diverse foraminiferal fauna, as outlined in the following section on biostratigraphy.

(c) Khuff Sequence KS2: The sequence ranges in thickness from 55 m (Saiq Plateau) to 79 m in Wadi Hedek (Table 3). The KS2 is composed of three cycle sets (KCS2.1-KCS2.3) (Figures 7, 9 and 12). As the upper part of the KS2 in Wadi Mistal was not accessible for sedimentological logging, only the lowermost 32 m were incorporated into the study.

The transgressive part of the KS2 (KCS2.3) consists of a single half-cycle set of the foreshoal motif.

The KS2 MFS (Tr10) is picked within thinly-bedded mud-to wackestone beds within KCS2.2. These beds are interpreted to represent open-marine, distal tempestite sheets deposited near storm wave base (SWB).

The regressive part of the KS2 (KCS2.1) is mainly made-up of mud-dominated foreshoal cycle set motifs and grain-dominated shoal cycle set motifs towards the top. The upper sequence boundary of KS2 (SB KS2/KS1) is placed at the top of the thickest grainstone body in all of the sections, further constrained by a negative excursion in the outcrop GR-log in Wadi Sahtan and Wadi Bani Awf (Figures 7 and 9).

(d) Khuff Sequence KS1: The KS1 varies in thickness from 46 m in Wadi Bani Awf to 84 m in Wadi Hedek (Table 3) and usually consists of two cycle sets (KCS1.1-KCS1.2) (Figures 7, 9 and 12). Due to the inaccessibility of the outcrop, the KS1 was not logged in Wadi Mistal. A significant increase in thickness of the KS1 up to 35 m is recorded in Wadi Hedek. There, a third cycle set (KCS1.0) is interpreted which is not present in the other sections.

The transgressive part of the KS1 (lower KCS1.2) is thin and reaches only up to 10 m on the Saiq Plateau.

The KS1 MFS is placed within muddy graded foreshoal deposits of the KCS1.2. It is further marked by a subtle increase in the outcrop GR-readings.

The regressive part of the KS1 (upper KCS1.2-KCS1.0) consists of cycle sets of the shoal motif. Ooid grainstones and microbially-coated intraclast-rich rudstones are arranged in a very distinct coarsening-upward trend. The KS1 sequence boundary is marked by an up to 5 m thick polymict breccia (“Top Breccia”). The breccia diminishes in thickness from south to north across the sections investigated. In Wadi Hedek, it is absent. Breccia formation is not fully understood and might be influenced by post-depositional tectonics during the Late Cretaceous (oral comm. J. Mattner, 2007).

Supersequence

The entire Middle to Upper Khuff succession is interpreted as a single asymmetrical, transgressive-regressive second-order supersequence. It most likely corresponds to the DS2 18 defined by Al-Husseini and Matthews (2005) (Figure 3).

The transgressive portion (KS4-lower KS2, approx. Saiq Formation) is between 240 m (Wadi Mistal) and 268 m (Wadi Hedek) thick. Its base is marked by the “Microbial Marker 2” marker bed.

The MFS of the supersequence corresponds to the KS2 MFS (Figures 7, 9 and 12).

The regressive portion (upper KS2-KS1, approx. Lower Mahil Formation) varies in thickness from 78 m on the Saiq Plateau to 112 m in Wadi Hedek. The supersequence boundary is marked by the “Top Breccia” marker bed. It is interpreted to be equivalent to the second-order supersequence boundary SB2 17 of Al-Husseini and Matthews (2005) (Figure 3). It is transgressively capped by a prominent shale interval with individual brick red to olive green shale layers up to 0.2 m in thickness, defined as Lower Mahil-Middle Mahil (Khuff-Sudair) formation boundary (Poppelreiter et al., 2011).

BIOSTRATIGRAPHY AND REGIONAL CORRELATION

A general outline and preliminary correlation of the Saiq Formation with standard Tethyan and global scales has already been proposed based on biostratigraphic analysis from the Saiq Plateau (Koehrer et al., 2010). Additional samples from the sections Wadi Sahtan, Wadi Bani Awf and Wadi Mistal now provide an improved database with several species previously unrecognized (Figures 13-16). Especially noteworthy is the presence of several globivalvulinid (Paradagmarita, Paremiratella) species in the KS3 (Figure 17), recorded for the first time in the Al Jabal al-Akhdar area.

Biostratigraphic database

Deposits of the upper part of the Saiq Formation (KS4-basal KS2) are sandwiched in between two major biotic events (Figure 17):

  • The end-Guadalupian (Capitanian) crisis (Isozaki, 2009; Bond et al., 2010) in the Al Jabal al-Akhdar sections shows the gradual demise of large, internally complex fusulinids (schwagerinids, verbeekinids), alatoconchid bivalves and cerioid corals during the upper, regressive part of the KS5 (oral comm. L. Walz, 2010).

  • The Permian Faunal Extinction (PFE) records a very sharp transition in the lower KS2 from bioclast-rich pack-/grainstones to azoic mudstones of the Lower Mahil Formation with a nearly complete loss of biota. Fossils remain very scarce during the upper KS2 to KS1, with bioturbation traces present in all sections.

Carbonate production in the KS4-KS1 is dominated by non-skeletal grains with a high proportion of microbially-mediated precipitation. The associated benthic fauna is characterized by the dominance of species with inferred high tolerance to fluctuations in salinity, trophic conditions, oxygenation and hydrodynamic regimes. The absence of classical Late Permian biostratigraphic markers (e.g. palaeofusulinids, colaniellids) is probably attributed to adverse environmental conditions as well as biogeographic isolation along the southern, passive margin of Gondwana (Altiner et al., 2000; Ueno, 2003).

Biostratigraphy in the studied sections relies on the occurrence of smaller benthic foraminifers, which might serve as useful biostratigraphic markers on a subregional to regional-scale. The biostratigraphic subdivision acknowledges the first occurrence datum (FOD) of certain marker species. The stratigraphic position of stage boundaries of the base Wuchiapingian (Dzhulfian) and Changhsingian (Dorashamian) are derived from tentative correlations of smaller foraminiferal occurrences with Late Permian index fossils from other Tethyan sections (Nestell and Pronina, 1997; Pronina-Nestell and Nestell, 2001; Gaillot et al., 2009).

In each sequence, the biostratigraphic content was evaluated:

(a) Khuff Sequence KS4: The transgressive part of KS4 (KCS4.11-KCS4.7) begins right above the last occurrence (LOD) of a regionally important biostratigraphic marker, the distinctive miliolid foraminifer Shanita amosi (Insalaco et al., 2006). Sections in Wadi Sahtan, Wadi Bani Awf and Wadi Mistal show moderately diverse faunal assemblages mostly present in pack-/grainstones with common miliolids (Neodiscopsis cf. ambiguus) (Figure 17), rare lagenids (Frondina permica, Rectostipulina spp.) and biseriamminids (Retroseptellina decrouezae, Globivalvulina cf. vonderschmitti, Paraglobivalvulina sp., Dagmarita chanakchiensis). Staffellids (Sphaerulina sp., Nankinella minor) occur sporadically and are predominantly preserved as indeterminable molds.

Around the interpreted maximum flooding interval of the KS4 (KCS4.6), the majority of samples yield a poorly preserved foraminiferal fauna, mainly due to the presence of strongly dolomitized oolitic/peloidal grainstones. Besides miliolids, lagenids, and staffellids, the FOD of the staffellid Neomillerella mirabilis is noteworthy, which is rarely found in the sections Wadi Bani Awf and on the Saiq Plateau (Figures 13, 15, 17 and Plate 1).

During the regressive part of KS4 (KCS4.5–KCS4.1) the foraminiferal fauna becomes more diversified and the preservation displays a better mimetic replacement. The entry of several new species is observed, related to an overall, second-order transgressive trend. This is also indicated by the presence of rare corals and echinoderms in KCS4.3. The most important biotic event is the FOD of large miliolids (Glomomidiellopsis uenoi), Paremiratella robusta (Wadi Bani Awf) and Paradagmarita cf. monodi (Wadi Mistal) (Figure 17, Plate 1) during KCS4.2 and KCS4.1.

(b) Khuff Sequence KS3: Biotic assemblages show moderate to high diversity in the KS3. Besides the already reported species from the uppermost KS4, lagenids (Ichthyofrondina latilimbata) (Plate 1, picture 15) and Paradagmarita spp. (Plate 1, pictures 3–5) were found in the sections Wadi Bani Awf (Figure 15) and Wadi Mistal (Figure 16). Unfortunately, wall composition, especially in the more minute forms, are poorly discernible and fossil determination is often only possible using the “white paper method” (Plate 2, pictures 12–13) (Coy, 1997).

(c) Khuff Sequence KS2: The basal KS2 shows an abrupt facies change from fossiliferous pack-/grainstones to dolomitized mudstones, almost barren of fossil remains. Neither a thrombolitic horizon (e.g. Baud et al., 1997; Masaferro et al., 2004; Insalaco et al., 2006; Ehrenberg et al., 2008; Maurer et al., 2009), nor the characteristic “disaster fauna” (Rectocornuspira kalhori, Microconchus (“Spirorbis”) phlyctaena), present in many other boundary sections (Groves and Altiner, 2005; Hughes, 2009), were found in the Al Jabal al-Akhdar area. These “spirorbiform” tubes are often assigned to annelids, but belong to the microconchids (Weedon, 1994; Taylor and Vinn, 2006). Furthermore, the whole Lower Mahil Formation has poor fossil recovery, which might also partly be accountable to the pervasive dolomitization. Age constraints (latest Permian to late Induan) were inferred from isotopic data (Richoz, 2006) and the sparse fossil fauna in the basal Middle Mahil Formation (Pöppelreiter et al., 2011).

(d) Khuff Sequence KS1: The KS1 is almost entirely made up of strongly dolomitized oolitic/peloidal grainstones and intraclastic rudstones, nearly barren of fossil remains. No biostratigraphic age constraints were made in this sequence.

Revised Regional Correlation

Biostratigraphic correlation with subsurface from offshore Fars

A detailed outcrop to subsurface study from the Zagros Mountains in the north to offshore Fars area in the south has been performed by Insalaco et al. (2006). Six paleoecological systems (PS 1–PS 6) are documented in the Upper Dalan Member and lower Kangan Formation (time-equivalent to the Middle and Upper Khuff) in Iran (Figure 18). This approach links the established sequence-stratigraphic framework with biostratigraphic data and paleoecological interpretation and gives the opportunity to compare the data with the sections from the Al Jabal al-Akhdar area.

(a) Paleoecological Systems PS 1 to PS 3: Inner shelf deposits of PS 1 (i.e. KS4–KS4b2) yield common miliolids, including the FOD of Neodiscopsis ambiguous, globivalvulinids, staffellids and dasycladacean algae (Figure 18). More open-marine shoal deposits with gymnocodiacean algae are present in PS 2 (i.e. KS4b3–KS4c2) and witness the FOD of Rectostipulina quadrata, followed by a maximum flooding interval with a lagenid-rich facies. The PS 3 (i.e. KS4c2 to KS4c4) is made up of oolitic shoals turning into hypersaline lagoonal deposits. Neomillerella mirabilis first occurs in this part together with Paradagmarita zaninettiae.

Similar faunal assemblages were found in the KS4 (KCS4.11–KCS4.5) of the Al Jabal al-Akhdar. However, stratigraphic appearance of species slightly differs. The first occurrences of Neodiscopsis cf. ambiguus, Paraglobivalvulina mira and Rectostipulina quadrata were already recorded in the upper KS5 (Shanita amosi zone), which is probably closer to the true FOD in the Late Capitanian (Bond et al., 2010). In the work of Insalaco et al. (2006), the upper KS5 was not studied. Neomillerella mirabilis, reported from offshore Fars (KS4c; Gaillot and Vachard, 2007) and Saudi Arabia (Khuff C; “Staffella hupehensis”, Hughes, 2009), was also found in the sections from Al Jabal al-Akhdar. Species of Paradagmarita have so far not been detected in the lower part of KS4.

(b) Paleoecological System PS 4: PS 4 embraces most of the KS3a sequence of Insalaco et al. (2006) and marks the appearance of large lagenids (Aulacophloia martiniae) in a lagenid-rich muddy lagoonal facies with common bryozoa and brachiopods (Figure 18). Insalaco et al. (2006) and Hughes (2009) considered this facies to represent maximum flooding intervals. This facies and biotic assemblages were not found in the sections from Al Jabal al-Akhdar, where lagenid foraminifera are generally rarely observed. The absence of this characteristic facies generates difficulties for a precise correlation with our sections, where maximum flooding intervals are recorded by bioclastic pack- to grainstone facies with open-marine metazoans (rugose corals, echinoderms) and common gymnocodiacean algae.

Sequence KS3a of Insalaco et al. (2006) was initially correlated with the lower part of KS3 in Koehrer et al. (2010). This correlation however causes the problem of a considerably reduced sediment thickness of the entire KS3 (60–65m) compared to KS3a/3b in offshore Fars (102 m) and Musandam (126 m). The KS3 in the Al Jabal al-Akhdar sections also lacks distinct biostratigraphic evidence for a separation into two individual sequences as in offshore Fars and Musandam. Therefore it is more appropriate to correlate sequence KS3a of Insalaco et al. (2006) with the upper KS4 (KCS4.4–KCS4.1) of our sections in the Oman Mountains (Figure 18).

(c) Paleoecological System PS 5: The faunal assemblage of PS 5 (KS3a4 to KS3b) in offshore Fars displays the FOD of several Changhsingian marker species (Glomomidiellopsis uenoi, Paremiratella robusta) and diverse paradagmaritin species (Figure 18). Biostratigraphic data from Wadi Sahtan, Wadi Bani Awf and Wadi Mistal similarily show the occurrence of Glomomidiellopsis uenoi, Paremiratella robusta and common Paradagmarita species in the uppermost KS4 and KS3. It is therefore supposed that only the KS3b of Insalaco et al. (2006) correlates with the KS3 in the Al Jabal al-Akhdar sections (Figure 18).

(d) Paleoecological System PS 6: The last Paleoecologic System (PS 6) is represented by the KS2a/b interval of Insalaco et al. (2006). It includes the PFE (Permian Faunal Extinction) followed by an azoic interval, thrombolitic facies and presence of Rectocornuspira kalhori and Microconchus (“Spirorbis”) phlyctaena in the earliest Triassic (Figure 18). The PFE is present also in the Al Jabal al-Akhdar area, but lacks the thrombolites and earliest Triassic disaster fauna. Compared to the described Middle/Upper Khuff deposits from offshore Fars, sections from the Al Jabal al-Akhdar show more similarities to the outcrops from the Zagros Mountains with coral-rich deposits in the Changhsingian and muddy offshoal deposits in the lower KS2.

Biostratigraphic correlation with outcrops in Musandam

The Bih Formation in the western part of the Musandam Peninsula (UAE), ranging from Permian to Lower Triassic, has been studied by Maurer et al. (2008, 2009) (Figure 18) and embedded into the sequence-stratigraphic scheme proposed by Insalaco et al. (2006). Accordingly, the investigated part of the Bih Formation range from KS4c to KS1. Maurer et al. (2009) note a diversity increase of large miliolids (Glomomidiellopsis uenoi) and biseriamminids (Paremiratella robusta, Paradagmarita monody) during the KS3b. The highest diversity of benthic foraminifera occurs in lagoonal deposits and leeward shoals. Lagenids (Pachyphloia, Nodosinelloides, Rectostipulina), typically present in the muddy intervals of maximum flooding intervals of offshore Fars, are rarely present. The described diversity increase and the disappearance of Permian fauna and algal flora at the Permian Faunal Extinction serve as tie points to correlate the sequences with the sections from the Al Jabal al-Akhdar area. Comparison of facies types indicate that the Oman Mountains area generally shows a more distal facies pattern on the Khuff platform, which is especially evident during the lower KS2 with muddy foreshoal and offshoal deposits compared to backshoal to oolitic shoal deposits in the Musandam area (Figure 18).

GRAINSTONE ARCHITECTURE ON A SUBREGIONAL-SCALE

Sequence-Stratigraphic Correlation

Outcrop sections were correlated across the study area (60 x 40 km) (Figure 2). Two correlation lines were constructed slightly oblique to the assumed north-easterly paleo-dip direction of the Khuff carbonate ramp (Ziegler, 2001):

  • (1) W-E transect that runs from Wadi Sahtan to Wadi Hedek (Figure 19), and

  • (2) S-N transect from the Saiq Plateau to Wadi Hedek (Figure 20).

The “Saiq/Mahil Formation Boundary”, well-marked in all five outcrop sections, was chosen as the datum for correlation. It occurs in the transgressive part of the KS2 and is easily recognized by a major textural change from dark gray grain-dominated deposits of the Saiq Formation to pale gray mud-dominated deposits of the Lower Mahil Formation. All additional marker beds outlined above further back the correlation (Figure 3):

As much as practicable cycle set boundaries (Figure 5) were used to increase resolution within the sequences and add time lines to the stratigraphic correlation (Figures 19 and 20). Cycle set boundaries were interpreted by:

  • tidal flat caps of microbially laminated boundstones (tidal flat motif), marked by prominent positive excursions of the outcrop GR-log,

  • muddy lagoonal caps of burrowed to vertically rooted mud-to wackestones (backshoal motif), accompanied by higher outcrop GR values as well,

  • facies offset from higher-energy shoal-associated grainstones (shoal motif) to open-marine mud-dominated foreshoal deposits.

In most parts of the section such as KS4 and KS3, cycle sets can be traced with confidence across the study area. In intervals of thickly developed grainstones (e.g. KS1) without muddy cycle set caps, well-traceable marker beds and biostratigraphic information, the definition of cycle set boundaries was however problematic. Whereas sequence and cycle set boundaries are correlative, thinner individual cycles could not be correlated with confidence. The number of cycles per cycle set was found to vary between the sections in certain intervals, making correlation uncertain. Koehrer et al. (2011) showed that 1 to 5 m thick cycles can be successfully used for stratigraphic correlations on a field-scale (< 10 km), but possibly not beyond.

Lateral and Vertical Grainstone Architecture

Two cross-sections across the Oman Mountains show the interpreted distribution of grainstones within each of the 10 to 30 m thick cycle sets (Figures 21 and 22). From these correlations, proportions and dimensions of grainstone bodies are extracted.

Sequence-scale (KS4–KS1)

The stratigraphic architecture of grainstones was characterized on a third-order sequence-scale (Figures 21 and 22):

(a) Khuff Sequence KS4

Grainstone percentages range between 51% in Wadi Hedek to 87% in Wadi Bani Awf (Table 3). On average, grainstones (mainly composed of peloids) cover 66% of the sequence gross thickness.

Transgressive part (KCS4.11–KCS4.7): On the W-E transect, an aggradational facies pattern is observed between the sections (Figure 21). Grainstones, variably composed of ooids and peloids, make up 62% of the interval in Wadi Bani Awf where they are the thickest. They are continuous on the scale of correlation (60 km), with average correlation lengths of 48 km and a maximum thickness of up to 5 m. On the S-N transect, facies changes are more pronounced and occur on a scale of up to 40 km (Figure 22). Several m-thick muddy textures of the backshoal and tidal flat facies association present on the Saiq Plateau laterally turn into thinner, grainy shoal deposits towards the Wadi Hedek section. This change is accompanied by an increase of grainstone percentage from 35% to 55%.

Zone of maximum flooding (KCS4.6): On the N-S transect, up to 7 m thick grainstone bodies are observed in this interval on the Saiq Plateau. Grainstones thickness decreases signifcantly down to a few dm or they pinch-out completely towards Wadi Hedek, where muddy foreshoal deposits dominate (Figure 22). On the W-E transect, a drop in grainstone percentages to a minimum of 55% is recorded in this interval (Figure 21). Massive grainstones up to 8 m thick are present in the proximal sections of Wadi Sahtan and Wadi Bani Awf. They turn laterally into thinner grainstone streaks that are separated from each other by m-thick mud-dominated foreshoal deposits in Wadi Mistal and Wadi Hedek. Average grainstone correlation lengths drop to a minimum of 39 km.

Regressive part (KCS4.5–KCS4.1): An aggradational to progradational facies pattern is also observed during the regressive part of the KS4 on both transects. Grainstones up to 6 m thick, mainly made up of peloids, constitute 67% of this interval. On the W-E transect, grainstones reach a maximum thickness of 10 m and form continuous reservoir sheets extending over the whole studied area (Figure 21). Average correlation lengths are 47 km. In the proximal part of the study area, thickest grainstone bodies are present in Wadi Sahtan and Wadi Bani Awf. In distal sections of Wadi Mistal and Wadi Hedek grainstone percentages drops down to 54%. Grainstones are less amalgamated with a higher percentage of several m-thick intercalated muddy open-marine foreshoal deposits. On the S-N transect, subtle progradational patterns and more apparent lateral facies variations occur on the scale of 10–20 km (Figure 22). Thicker grainstones up to 5 m present in Wadi Hedek laterally turn into thinner grainstone bodies towards the Saiq Plateau.

(b) Khuff Sequence 3

Grainstones are mainly composed of peloids with minor skeletal components. Grainstone percentages range from 33% at the Saiq Plateau to 70% in Wadi Hedek (Table 3) with average values of 59%.

Transgressive part (KCS3.4–KCS3.3): An aggradational facies pattern is apparent in the W-E cross-section, similar to the regressive part of the KS4 (Figure 21). Peloid-dominated grainstone bodies up to 4 m thick show average correlation lengths of up to 52 km and sheet-like geometries across the entire study area. Intercalated are 1–2 m thick muddy foreshoal or locally backshoal textures. In the S-N cross-section, up to 2 m thick, muddy backshoal deposits are present on the Saiq Plateau in the early transgressive part. They turn laterally into some 3 m-thick grainstones present in Wadi Hedek on a scale of 38 km. Grainstone percentage laterally changes from 35% on the Saiq Plateau to 60% in Wadi Hedek.

Zone of maximum flooding (KCS3.2): Around KS3 maximum flooding, grainstone percentages drop to values of around 50%. These are up to 3 m thick and mainly composed of peloids. On the W-E transect, grainstones bodies decrease in maximum thickness to 3 m in Wadi Bani Awf and Wadi Mistal (Figure 21). Laterally grainstone more commonly pass into mud-dominated foreshoal deposits up to 5 m in thickness on the scale of 10–20 km. Within this interval, grainstones are patchier developed. On the S-N transect, 1–3 m thick grainstones constitute 65% of the section in Wadi Hedek, but are completely absent on the Saiq Plateau (Figure 22).

Regressive part (KCS3.1): Investigated sections turn grainier again in both cross-sections (Figures 21 and 22). Grainstone volumes slightly increase to 54%. An upward-thickening pattern of peloid-dominated grainstone bodies with maximum thicknesses from 5 to 7 m is observed. Average correlation lengths are 52 km. At the very top of the KS3, the thickest grainstone body up to 7 m thick on the Saiq Plateau exhibits textural continuity across the entire study area in all sections.

(c) Khuff Sequence KS2

The lowest grainstone percentage of all sequences (average of 29%) is recorded in the KS2. Grainstones are dominated by peloids and intraclasts. Their percentages range between 13% (Saiq Plateau) to 43% (Wadi Sahtan) (Table 3).

Transgressive part (KCS2.3): In the transgressive part of the KS2, grainstone volumes drop to 21% of the bulk rock volume (Figures 21 and 22). Peloid-dominated grainstone bodies reach a maximum thickness of 4 m. Average lateral correlation lengths drop down to 48 km. Thinner, several dm-thick grainstones are not persistent on the scale of correlation and pinch-out after 5–20 km.

Zone of maximum flooding (KCS2.2): A major facies change occurs across the “Saiq/Mahil Formation Boundary” from the shoal (KCS2.3) into the foreshoal environment (KCS2.2) over an interval of a few meters (Figures 21 and 22). No grainstones are present in any of the sections investigated.

Regressive part (KCS2.1): Peloid- and intraclast-dominated grainstones re-appear and increase in abundance to values of 26% and in thickness from virtually zero up to 8 m just below the sequence boundary. Grainstone body correlation lengths slightly increase to 49 km on average. In W-E direction, pronounced facies changes are apparent (Figure 21). 1–8 m thick, partly amalgamated peloidal-intraclastic grainstones present in Wadi Sahtan and Wadi Bani Awf turn into mud-dominated foreshoal textures towards the E (Wadi Mistal, Saiq Plateau and Wadi Hedek). In these more distal sections, grainstone beds are separated from each other by muddy foreshoal deposits. In S-N direction, a high facies similarity is observed in Wadi Hedek and on the Saiq Plateau (Figure 22). Open-marine tempestite sheets and bioturbated mudstones several meters in thickness are successively overlain by grainy shoal deposits during regression.

(d) Khuff Sequence KS1

Ooid and intraclast-dominated grainstone percentages range between 61% on the Saiq Plateau to 88% in Wadi Hedek (Table 3). KS1 contains the highest average grainstone percentages of 80%.

Transgressive part (KCS1.2): On the W-E transect, this interval is very thin (up to 5 m) to virtually absent and entirely made up of muddy foreshoal facies. On the S-N transect, several dm-thick grainstone streaks are present on the Saiq Plateau and in Wadi Hedek. They laterally pass into mud-dominated foreshoal deposits already after a few km.

Zone of maximum flooding (KCS1.2): Decimeter-thick mud-dominated foreshoal facies types constitute this zone. No grainstones are present.

Regressive part (KCS1.1–KCS1.0): A very similar distribution of facies associations is observed in the both transects (Figures 21 and 22). A thickening-upward trend of ooid-/intraclast-dominated grainstones combined with an increase in grainstone percentage from 66% at the base to 83% at the top is observed. Grainstones in the basal part are between 1–2 m thick and extend for 5–10 km. They increase in thickness up to 15 m and up to 53 km in average lateral extent. Intercalated between the massive grainstone piles are dm-thin muddy foreshoal deposits.

Supersequence-scale (DS218)

The grainstone distribution on a second-order supersequence-scale reflects an overall transgressive (KS4 to lower KS2) to regressive (upper KS2 to KS1) trend.

Transgressive part: In the early transgressive part (KS4), grainstones are predominantly composed of peloids and cortoids. Grainstone bodies with a maximum thickness of up to 10 m are present in the sections to the west (Wadi Bani Awf and Wadi Sahtan) (Figures 21 and 22). Thinner grainstones, only up to 5 m thick, have more muddy dm-thick intercalations towards the east (Saiq Plateau, Wadi Mistal and Wadi Hedek). Grainstone volumes reach values up to 67% of the bulk rock volume. Most of the logged grainstone bodies can be correlated between the outcrop sections. The late transgressive part includes KS3 and lower KS2. Grainstones are peloid-dominated. Grainstone abundance decreases down to 38%. A decrease in maximum thickness from 10 m to 4 m is observed. Average lateral correlation lengths of grainstone bodies decrease from 52 km in the early transgressive part down to 48 km in the late transgressive portion of the supersequence.

Zone of maximum flooding: The zone of second-order maximum flooding is interpreted just above the “Saiq/Mahil Formation Boundary” within the KS2. It is associated with a drastic facies change from shallow-water grainy to deeper-water muddy facies associations. In this zone, shoal-associated grainstones completely disappear. The percentage of muddy foreshoal deposits reaches 100% in places. Muddy beds in this interval have a cumulative thickness of up to 35 m.

Regressive part: In the regressive part (upper KS2 and KS1), grainstones are predominantly composed of ooids and coarse intraclasts. Grainstones increase again in thickness from virtually zero in the interval around the “Saiq/Mahil Formation Boundary” to 15 m towards the top of the section (Figures 21 and 22). The average correlation lengths of grainstone bodies increase to 53 km. The observed facies patterns indicate the transition from the muddy offshoal environment into the grainy shoal environment during second-order regression. The decrease of accommodation space towards the top is also reflected by strong amalgamation and an aggradational stacking pattern of shoal-associated grainstones.

Predictive Rules of Grainstone Geometries

This study reveals the distinct character of grainstones in Middle to Upper Khuff time-equivalent strata (KS4 to KS1) on a 60 x 40 km-scale. Observations in outcrops can be synthesized as follows:

Grainstone volume

Thicker (up to 10–15 m) grainstones with thin muddy intercalations are present during the early transgressive (KS4) and late regressive (KS1) portions of the second-order supersequence. Thinner (up to 7–8 m) grainstones with thicker muddy intercalations are present during the late transgressive (KS3 to lower KS2) and early regressive (upper KS2) portions of the second-order supersequence. Shoal-associated grainstone bodies are completely absent around the second-order zone of maximum flooding (middle KS2).

Grainstone composition

Shoal-associated grainstones deposited in the transgressive part of the second-order supersequence (KS4 to lower KS2) mainly consist of peloids. Bioclasts are most commonly present in grain-dominated packstones, indicative of the moderate-energy backshoal environment. Grainstones deposited during second-order regression (upper KS2 and KS1) are dominated by ooids and intraclasts. Compositional changes within the correlated grainstones and their leaching potential may be directly translated into petrophysical properties of analogous subsurface reservoirs. Early diagenetic leaching of the ooids may create highly porous reservoirs with oo-moldic porosity and rather small pore throats and low permeabilities. In contrast, peloids are less commonly dissolved. In the absence of dolomitization, peloidal grainstones most likely have the potential to form reservoir bodies with moderate to high interparticle porosity and potentially higher permeabilities.

Grainstone correlation

Sequences (51–170 m thick) and cycle sets (1–30 m thick) are well-correlatable on the tens of km-scale. They provide robust stratigraphic time lines on the scale of investigation. Individual cycles (1–5 m thick) could not be correlated on this scale. By tracing distinct changes in cycle composition, they may yield changing reservoir quality trends on a field-scale (< 10 km). Grainstones show mainly sheet-like geometries across the entire study area. Average correlation lengths range from 47 to 53 km.

Grainstone architecture

Vertical and lateral grainstone dimensions are controlled by stratigraphic position. Grainstone architecture is mainly aggradational. Minor landward and basinward shifts of the gross depositional environment can be interpreted on the sequence-scale in the area of investigation. This facies similarity and overall aggradational stratigraphic architecture give the system a “layer-cake” type appearance, suggesting almost flat platform topography and wide facies belts on the tens of kilometer subregional scale.

CONCLUSIONS

  • (1) Four outcrop sections of Middle and Upper Khuff time-equivalent strata (Saiq and Mahil formations) were investigated in wadis on the northern flank of the Oman Mountains. It is possible to link these additional sections in the stratigraphic framework developed previously for the Saiq Plateau outcrop.

  • (2) Micropaleontological studies on the faunal and algal content provide refinement of the paleoenvironmental interpretation and on facies stacking patterns. The presence of several important regional biostratigraphic markers of smaller benthic foraminifera are reported for the first time in the Al Jabal al-Akhdar area. Integrated sequence-biostratigraphy and paleoenvironmental interpretation provides a robust framework for the correlation of Khuff time-equivalent deposits on a regional scale (ca. 700 km).

  • (3) Grainstone architecture was analyzed on a 60 x 40 km-scale. Individual cycle set boundaries together with bio- and lithostratigraphic markers were used as time lines for stratigraphic correlation.

  • (4) Constructed cross-sections show a very similar vertical and lateral facies pattern and high lateral continuity of the depositional sequences (KS1–KS4) and of their component cycle sets, indicating the absence of significant tectonic activity of the area during the Late Permian and Early Triassic.

  • (5) Presence, architecture and thickness of grainstone facies is strongly governed by stratigraphic position. Thicker and more abundant grainstones are present during the early transgressive (KS4) and late regressive (KS1) portions of the supersequence. Thinner and less abundant grainstones are present during the late transgression (KS3 and lower KS2) and early regression (upper KS2). Shoal-associated grainstone bodies are completely absent around the second-order zone of maximum flooding (middle KS2).

  • (6) Second-order cyclicity also dictates the composition of grainstone facies. Grainstones are peloid-dominated during transgression (KS4 to lower KS2) and ooid/intraclast-dominated during regression (upper KS2 and KS1). Compositional changes within the correlated grainstones and their diagenetic potential may be directly translated into petrophysical properties of analogous subsurface reservoirs (oo-moldic versus interparticle porosity).

  • (7) Regional comparison of facies and sequence-stratigraphic stacking patterns indicates that the Oman Mountains area represents a more distal facies position on the Khuff platform. This is especially evident around KS2 maximum flooding with muddy foreshoal and offshoal deposits in contrast to mainly oolitic shoal deposits in the Musandam (UAE) and offshore Fars (Iran) area. Hence care should be taken by directly comparing facies and reservoir proportions in the specific depositional sequences of the Oman Mountains with other Khuff reservoir sections in the region. However the basic patterns of grainstone reservoir distribution within a sequence-stratigraphic framework are expected to represent “rules of thumb” that may well have predictive value elsewhere.

ACKNOWLEDGEMENTS

This study is part of an extra-mural research project of the University of Tuebingen, sponsored by Shell and Petroleum Development Oman. We would like to thank Shell and PDO and their focal points J. Amthor, A. Brandenburg, J.-M. Dawans, G. Forbes and J. Schreurs for assistance and financial support. The Omani Ministry of Oil and Gas is thanked for permission to publish the paper. We are grateful to our Khuff team members M. Zeller (now University of Miami), C. Schneider (now Wintershall), L. Walz (now Shell), M. Haase (now ExxonMobil), D. Bendias, M. Obermaier (both University of Tuebingen), in addition to S. Richoz (University of Graz) for help and useful ideas. The authors would also like to thank D. Vachard (University of Lille) for the initial determination of the microfossils from the Saiq Plateau section. P. Jeisecke (University of Tuebingen) is thanked for the preparation of the thin sections. Shuram Oil and Gas (Muscat) is acknowledged for fieldwork logistics. We are also very grateful to ALT for providing access to WellCAD software. The final version of this manuscript benefited from the comments by two anonymous reviewers and M. Al-Husseini, and the proof-reading by GeoArabia’s Assistant Editor Kathy Breining. GeoArabia’s Nestor ‘Nino’ Buhay IV is thanked for designing the final version of the figures.

ABOUT THE AUTHORS

Bastian Koehrer studied Geosciences at the Universities of Tuebingen/Germany and Bristol/UK (MSc in 2007). He earned a PhD in carbonate sedimentology from the University of Tuebingen in 2011 with an outcrop study on the Khuff Formation in Oman, funded by Shell and Petroleum Development Oman. He joined Wintershall Holding GmbH, where he is presently enrolled in a company-internal graduate programme, alternating between different job assignments in Germany and Qatar. Amongst others, Bastian is a member of the EAGE, AAPG and DGMK and has published several papers on carbonate sequence stratigraphy and outcrop analogs. His research and working interests include production-scale reservoir characterization and reservoir quality prediction as well as geostatistical 3-D modeling.

bastian.koehrer@wintershall.com

Thomas Aigner studied Geology and Paleontology at the Universities of Stuttgart, Tuebingen/Germany and Reading/UK. For his PhD dissertation on storm depositional systems (1985) he worked at the Senckenberg-Institute of Marine Geology in Wilhelmshaven and spent one year at the University of Miami in Florida. He then became an Exploration Geologist at Shell Research in Rijswijk/Holland and Houston/Texas focusing on basin analysis and modeling (1985–1990). Since 1991 Tom has been a professor and Head of the Sedimentary Geology Group at the University of Tuebingen. 1996 he was a ‘European Distinguished Lecturer’ for the AAPG. His current projects focus is on sequence stratigraphy and reservoir characterization/modeling in outcrop and subsurface.

t.aigner@uni-tuebingen.de

Holger Forke studied Geology and Paleontology at the University of Erlangen/Germany. His diploma thesis and PhD dissertation (2001) focused on the biostratigraphic correlation of Carboniferous–Permian deposits from the Southern Alps (Austria) and Urals (Russia). He has then worked at the Senckenberg Research Institute in Frankfurt, Main, at the Institute of Geology (University of Erlangen; DFG Priority Program 1054 ‘Late Paleozoic Sedimentary Geochemistry’) and participated in expeditions and mapping campaigns to Svalbard and the Canadian Arctic in cooperation with the Norwegian Polar Institute, University of Bremen, and BGR Hannover. His work mainly deals with Late Paleozoic foraminifera and conodonts with emphasis on the application for sequence biostratigraphy. He is currently a Guest Researcher at the Museum für Naturkunde –Leibniz Institute for Research on Evolution and Biodiversity (Humboldt, University Berlin) and involved as a consultant in several academic and industry-related projects of the Khuff Formation (Oman, Saudi Arabia).

holger.forke@gmx.de

Michael Pöppelreiter studied at the Mining University of Freiberg/Germany, the Postgraduate Research Institute of Sedimentology/UK and the University of Tuebingen/Germany, where he earned a PhD in 1998. Since then, Michael has worked as Sedimentologist/3-D Modeler with Shell/Holland, as Carbonate Geologist/3-D Modeler at Shell’s Bellaire Technology Center in Houston/Texas and as Senior Carbonate Geologist at the Qatar Shell Research and Technology Centre in Doha/Qatar. At present, Michael works as Principal Geologist and carbonate subject matter expert with Shell in Kuwait. He has published numerous papers on carbonate reservoirs, reservoir modeling and borehole image log technology. He is guest lecturer at the University of Tuebingen. His research interests include structural control on reservoir distribution in carbonate reservoirs.

m.poppelreiter@shell.com