The Lower Triassic Sudair Formation represents the deposits of an epeiric carbonate-evaporite-siliciclastic platform. It is widely known as top seal for the underlying Khuff reservoir in some of the largest hydrocarbon accumulations in the world (e.g. the North Dome and the Ghawar field). Towards the seaward edge of the platform however, in northeast Oman for example, the sealing anhydrites and shales pinch out. Dolomite layers turn increasingly grainy, forming up to 8 m-thick individual grainstone bodies. Thus, on a regional scale, a top seal turns laterally into a potential reservoir. This paper outlines facies, depositional environment and stratigraphic architecture of the Middle Mahil Member, the outcrop equivalent of the Sudair Formation in northeast Oman.
The Middle Mahil, some 260 m thick, is made up of just a few facies types: argillaceous, muddy, microbially-laminated, graded, and cross-bedded dolomites. These represent tidal flat, backshoal and shoal environments. Facies are arranged in regular sequences; the smallest units being 2–5 m thick cycles. A clear hierarchical organization of sequences is apparent with bundling of 4–8 cycles into cycle sets, and 2–3 cycle sets forming depositional sequences. The lower third of the Middle Mahil is dominated by rather muddy textures, the middle part is grain-dominated and the upper part again is muddier with a higher percentage of microbial laminites. The maximum grainstone thickness is observed in the middle of the Middle Mahil, at the interpreted zone of maximum flooding.
Overall the Middle Mahil shows “layer-cake” type architecture over distances of 4–8 km. Grainstone layers are laterally continuous at this scale. Pinching and swelling geometries of grainstones are widespread. Grainstones gradually increase in thickness towards the northeast, the direction of the seaward platform edge. Mapping of Middle Mahil grainstones suggest excellent reservoir potential in such platform margin settings. The observations from Oman provide a calibration point to explorationists targeting the Lower Triassic carbonate sequence in the Middle East.
An epeiric platform developed during the Triassic flooding of the Arabian Peninsula along the passive margin of the Neo-Tethys Ocean. This platform is characterized by gradual lateral facies changes and well known for its uniform log character (Osterloff et al., 2004). Stratigraphic surfaces, such as subaerial unconformities or maximum flooding zones can be traced for hundreds of kilometers (Sharland et al., 2001). Vertically the platform is made up of rapidly changing, mostly aggradational sequences that visually resemble the hundreds of thin layers of a layered cake.
The Sudair Formation is part of this Triassic epeiric platform. It is referred to as Middle Mahil Member in outcrops in northeast Oman (Koehrer et al., 2010). The formation is developed across the entire Arabian Peninsula (Figure 1). Lithologically three broad belts can be distinguished on a platform-wide scale (Ziegler, 2001):
Surrounding the Saudi Arabian hinterland, the platform consists of shales with intercalated sandy and muddy dolomitic streaks (Osterloff et al., 2004; Vaslet et al., 2005). In the central part of the platform the Sudair (Middle Mahil) consists of an intercalation of shales, anhydrites and dolomite layers (Alsharhan and Kendall, 1986). Only isolated grainy streaks occur in overall tight dolomites. Towards the seaward margin of the platform, shales and anhydrites decrease in thickness or pinch out while dolomites become increasingly grainy (Figure 1). Hydrocarbons are known to occur in matrix and/or fracture pores. Only matrix properties are described here.
To characterize the grainy platform margin dolomites, an outcrop analog study was set up in cooperation of Shell with the University of Tübingen and Petroleum Development Oman (PDO). Initial results, based on a first reference section at the Saiq Plateau, are outlined below. More extensive work, also covering the regional scale, is currently underway, and will be published in a forthcoming paper. The aim of the study is to investigate grainstone geometries and stratigraphic architecture as analog for the subsurface.
The type section of the Sudair Formation (Middle Mahil) and its stratigraphic equivalents is located in the Al Arid escarpment near the Riyadh-Jeddah road at Khashm Sudayr in Saudi Arabia. Steineke et al. (1958) were the first to subdivide Permian – Triassic sedimentary rocks there into two formations, the Khuff and the overlying Sudair Shale and to systematically investigate the Triassic sedimentary rocks in Saudi Arabia. Powers et al. (1966) established the type section in Saudi Arabia.
The outcrop reference section for northeast Oman proposed by Glennie et al. (1974) is located on the Saiq Plateau of the Al Jabal al-Akhdar (Montenat et al., 1976), the location described in this paper. Other important outcrops are located near Muscat (Weidlich and Bernecker, 2003), close to Ras Al Khaimah, at the Musandam Peninsula (Hudson et al., 1954; Hudson, 1960, Maurer et al., 2008), near Al Ain in the United Arab Emirates (UAE) (Alsharhan, 1989) and in the Shiraz region of Iran (Insalaco et al., 2006). The subsurface reference section in Oman is well Y-85 in the Yibal field (Hughes Clark, 1988; Osterloff et al., 2004).
Economically the Sudair (Middle Mahil) is important as aquitard, seal and reservoir depending on the location. Close to the Saudi Arabian hinterland the Sudair Formation is a clastic wedge that constitutes the most important aquitard in western Saudi Arabia and Syria, covered by numerous hydrogeological studies (e.g. Al-Aswad and Al-Bassam, 1997; Alsharhan et al., 2001).
In the central part of the platform, the Sudair Formation constitutes the top seal for hydrocarbon accumulation in the underlying Khuff Formation, such as the Ghawar field in Saudi Arabia, the Awali field in Bahrain, the North Dome-South Pars field in Qatar and Iran, and the Yibal field in Oman (Alsharhan and Nairn, 1997; Shariff, 1986; C. von Winterfeld oral communication, 2009). Interbedded, mostly muddy dolomites are well-known drilling hazards, in places containing overpressured brines and the occasional hydrocarbon show.
At the seaward margin of the platform, the Sudair Formation turns into a reservoir. It contains gas in Oman and Kuwait (Al-Eidan et al., 2005; Malek et al., 2005; Husain et al., 2008). Hydrocarbons are also known from the time equivalent of the Sudair Formation in Iran (Szabo and Kherapdir, 1978). Massive, partially grainy dolomites of the Lower Dashtak Formation (Khaneh Kat Formation) contain gas in the interior of the Fars Province (Bordenave, 2002).
AREA OF INVESTIGATION
The platform-margin dolomites are analyzed at the Saiq Plateau that stretches in the heart of the Al Jabal al-Akhdar in northeast Sultanate of Oman. Kilometer-scale exposures there give a rare insight into the Sudair (Middle Mahil) Formation in potential reservoir facies.
The study area is located near the town of Nizwa, some 160 km southwest of Oman’s capital Muscat (Figure 2). The Saiq Plateau provides some exceptional exposures. These outcrops are up to several kilometers wide, free of vegetation and construction and affected by comparably mild tectonics only, related to late Cretaceous ophiolites emplacement (Glennie, 2005).
The Triassic Sudair Formation (Middle Mahil Member) is part of the AP6 tectono-stratigraphic megasequence (first-order) of Sharland et al. (2001). AP6 ranges from Mid Permian to Early Jurassic. The lower boundary of this megasequence is base Middle Permian (probably base Wordian) and its upper boundary the lower Jurassic unconformity.
The Sudair Formation (Middle Mahil) and its equivalents are regarded to represent Lower Triassic (Olenekian) deposits (ca. 250–246 Ma) (Figure 3) (Alsharhan, 1993; Sharland et al., 2001) based on diagnostic foraminifera and palynomorphs (Maurer et al., 2008; Forbes et al., 2010).
The Sudair Formation is referred to as Middle Mahil Member (Koehrer et al., 2010) in outcrops of the Al Jabal al-Akhdar. There the Middle Mahil is about 260 m thick (Figure 4). It rests conformably above the Lower Mahil (outcrop equivalent of the Upper Khuff Formation), from which it is separated by a brecciated surface. Its base is marked by a 3-m-thick interval of laminated shale beds, which are purple to olive green in color, intercalated with argillaceous dolomites. These are one of the very rare shaly beds in the entire Permian – Triassic sequence at the Al Jabal al-Akhdar and thus excellent marker beds. They are associated with characteristically high gamma-ray readings (> 60° API). This ‘base Middle Mahil shale’ is regionally developed and can be correlated to the subsurface (Osterloff et al., 2004). Genetically it is the distal tip of a proximal clastic wedge (Murris, 1980) (Figure 1). It is one of several argillaceous units prograding across the platform during regressions.
The upper boundary of the Middle Mahil Member is placed at a karst breccia. This layer separates it from the overlying Upper Mahil Member, regarded as outcrop-equivalent of the Jilh Formation (Koehrer et al., 2010). The Upper Mahil, the youngest Triassic deposit in the area of investigation, is covered by an erosive unconformity. A thin veneer of Lower Jurassic reddish sandstones and shales belonging to the Mafraq Formation (Sahtan Group) covers this unconformity (Baud et al., 2007). Immediately above follow Lower and Middle Jurassic shallow-marine limestones of the Sahtan Group (Glennie et al., 1974) (Figure 4).
Early Triassic times at the Arabian Platform are characterized by relative tectonic quiescence (Dercourt et al., 1993; Konert et al., 2001). Uniform subsidence predominated during a passive continental margin stage. This quiescence on the plate-wide scale is locally superimposed at the Al Jabal al-Akhdar for example, by a more eventful tectonic history. This difference is related to the paleotectonic position of the investigated area, located a few tens of kilometers from the plate margin of the Arabian Shield.
At the margin Cimmerian terranes (Iran, Pakistan, and Afghanistan) detached during the Early Permian and Late Triassic (Angiolini et al., 2004) and again during the Mid and Late Jurassic (Indian subcontinent and Madagascar). These events resulted in breakup unconformities leaving behind substantial stratigraphic gaps in northeast Oman. Thickness and facies patterns during the Mesozoic are known to be more variable in the area of investigation compared to the more uniformly subsiding interior of the Arabian Shield (Pratt and Smewing, 1993; Le Métour, 1995; Gnos et al., 1997).
Transitional-greenhouse conditions during the Triassic led to low-amplitude, high-frequency sea-level oscillations (Markello et al., 2004). These tend to favor development of meter-scale shallowing-upward cycles with short-lived subaerial exposure. Carbonate grains are mostly oolitic-peloidal with microbial lamination ubiquitously developed. The Early Triassic experienced a faunal recovery following the end-Permian extinction. This led to a gradual upwards increase in skeletal grains.
The paleogeographic position of the study area changes from about ca. 25° to 15° south of the equator (Stampfli et al., 2001; Stampfli and Borel, 2002), through a steady northwards drift during the Triassic. Thus Arabia moved towards the southern tropical zone (Konert et al., 2001). Extensive evaporite deposits in Central Arabia, such as the Middle Triassic Khail anhydrite, are testimony for warm arid conditions.
Paleogeographically the study area is part of an extensive Triassic carbonate shelf fringing the Arabian Platform. Carbonates grew at the eastern, seaward margin of the Neo-Tethys for thousands of kilometers. A vast ocean surface, open to the east and located in the predicated hurricane tract, exposed the carbonate shelf to westwards blowing trade winds. Hurricanes might have constantly swept the shallow platform (Markello et al., 2004). Tidal conditions on the contrary are likely to have been in the micro-tidal range only.
Outcrop sections are characterized with both traditional and digital field methods. Two complete and two nearly full sections are logged sedimentologically. These sections are distributed across the Saiq Plateau with a spacing of 4–8 km (Figure 2). Logged properties include depth, lithology, texture, sedimentary structures (biogenic and physical), grain size, components, sequences and rock color (Figure 5). Outcrop photos were generated with a Panasonic Lumix DMC-TZ3. Outcrop data was digitized using the core logging software WellCad 4.2.
The location of section (UTM 572138 East and UTM 2548647 North) and sample points are recorded with a hand held GPS unit (eTrex, Summit, Garmin). The accurate dip of each sample point was recorded together with the accurate dip measured with a field compass, GEKOM by Breithaupt from Germany, to establish the true vertical thickness of sections.
Natural gamma radiation is measured with a portable spectral GR tool every 0.5 m for 15 seconds using the portable spectrometer GS-256 by Geophysika A.S. Brno, Czech Republic. The tool works with a sodium Iodide crystal.
Samples for microfacies, stable isotopes and biostratigraphy are taken every 2–5 m. Altogether 207 samples are collected from four sections and some lateral points in between them. These were analysed with a Leitz Ergolux AMC. White paper was used to enhance the color contrast for component recognition in the dolomitized sections (Dravis, 1991).
Carbon and oxygen stable-isotope analyses were performed on 72 dolomitic samples at the University of Bochum, Germany. Sample material was carefully pulverized with a pestle. About 5 mg of untreated sample powder of each sample was reacted with 100% H3PO4 at 70°C for 2 hours in an off-line vacuum line using a Finnigan Gasbench II. Carbon and oxygen isotope ratios of the generated CO2 were measured on a Finnigan Delta S mass spectrometer at the University of Bochum. For this reaction an acid fractionation factor of 1.00993 was used. Data was reported in the usual δ-notation in permille (‰) relative to the known isotope reference standard Vienna Peedee Belemnite Standard (V-PDB) (Coplen, 1994). The precision for the carbon (δ13C) and oxygen (δ18O) isotopic composition of the dolomite is better than 0.08‰ and 0.14‰, respectively. Data was not corrected for differential fractionation of calcite and dolomite during the dissolution by phosphoric acid as rock samples were only collected from the dolomitized part of the Saiq Plateau section.
Representative grainstone layers were walked-out laterally to investigate lateral facies relationships, changes in grainstone geometries and stratigraphic architecture, i.e. possible presence of shingles or clinoforms.
LITHOFACIES TYPES (LFT), LITHOFACIES ASSOCIATIONS (LFA) AND DEPOSITIONAL MODEL
Lithofacies Types (LFT)
The Middle Mahil Member at Saiq consists of dolomites and (dolomitic) shales. Anhydrites are absent, not even traces of anhydrites are observed.
Components of Middle Mahil dolomites clearly reflect the preceding mass extinction at the closing Permian. While skeletal grains are very scarce, evidence of microbially influenced deposition and cementation is ubiquitous. These are microbial laminite beds, flakestones (flat pebble conglomerates) and microbially-coated intraclasts. Ooids and peloids however dominate the component spectrum. Note, however, dolomitization makes differentiation between both components challenging.
Grain size in Middle Mahil is generally smaller than in the underlying Lower Mahil (Khuff) deposits. Arenitic components predominate, while rudstones and large intraclasts are rare. About 32% of the sequence consists of grain-dominated textures (potential reservoir); the rest is mud-dominated or microbially laminated.
Fourteen facies types are distinguished based on detailed logging and microfacies analysis (Figure 6, shown in red). These are captured in a facies atlas that covers all facies of Permian – Triassic sequences, as they show many similarities. Depositional characteristics such as lithology, carbonate texture (M, W, P, G, R, B, F), rock colors in the outcrop, physical and biogenetic sedimentary structures, grain size, components (biota, bioclasts and non-skeletal allochems), sorting, thickness of different rock units are recorded and used to distinguish lithofacies types (LFT) captured in the facies atlas shown in Figures 7 to 20.
Facies of the Sudair (Middle Mahil equivalent) in the subsurface of Oman (Osterloff et al., 2004) are similarly composed of dolomites with few limestone intercalations. However they are finer grained and frequently anhydrite cemented. The percentage of microbial laminites is higher and there is a significant amount of microbreccia due to dissolution (von Winterfeld, oral communication, 2010).
The Sudair (Middle Mahil equivalent) in the UAE consists of limestones with interbedded dolomites, argillaceous mudstones and anhydritic dolomitic limestones (Alsharhan, 1993). In Qatar the Sudair section (Middle Mahil equivalent) is made up of microcrystalline dolomite with interbedded argillaceous limestones and green or purple shales with sand/silty streaks and anhydrites especially in the northern offshore area (Schlumberger, 1981; Whittle and Alsharhan, 1995).
Event and Marker Beds
Field mapping revealed the presence of some distinct facies that are laterally continuous at outcrop scale and occur in specific stratigraphic positions. They are useful for lateral facies mapping and correlation. Because of its significance they are described separately below.
Disturbed beds (‘Seismites’)
Distinctly unstratified grayish to dark colored layers are 50–70 cm thick. They consist of overturned beds, soft sediment deformation and bed bound microfractures. Sedimentary structures include brittle deformation of unoriented clasts (Figure 21), fluidization structures, soft sediment injections and convolute bedding. Characteristic is the close association of soft sediment deformation and brittle deformation of an intraformational layer. The encountered sedimentary fabrics in the outcrop sections are ascribed to seismic activity. Seismites, a term first proposed by Seilacher (1969; 1984), initially means re-deposited sedimentary layers formed by seismic activity in a tectonically active region. Three main zones build up such a strata package. At the base, a clast-dominated layer, passing gradually into a fine grained, central “soupy layer”. Post event lamination forms the third and smallest unit. Dark, brittle deformed clasts (mudstones, 4–20 cm in length, 2–5 cm in thickness) lie conformably, though unoriented within a light grayish, muddy matrix. This basal unit is interpreted to mirror the initial phase of the seismic event. The high percentage of brittle deformed clasts suggests that most of the sediment was quite solid during deformation.
Liquefaction processes, such as soft-sediment injections indicates initial sediment with higher water content. Fluidization structures, indicating expulsion of pore water from a liquefied bed, are regarded as the cause of convolute bedding and are mainly observed in the soupy layer. Convolute lamination, varying in intensity, gradually passes into an undeformed 3–5 cm thick microbial layer, forming the top of the seismic fabric. Synsedimentary brecciation (Figure 21), observed in few hand specimens right below this layer, might indicate a repetition of a second less intense seismic shock.
One of the very few skeletal beds in the lower Middle Mahil occurs near the base of the member (Figure 22). The bed, only 30 cm thick is a completely dolomitized dark gray skeletal (bivalves, gastropods) pack- to grainstone. Structures include normal grading with erosive bases and scour surfaces. The bed, commonly low angle laminated, occasionally shows decimeter-scale channel features. The layer represents event beds in a grainy backshoal environment. Channels are probably rib channels associated with backwash. The bed is likely identical with bivalve shell beds described by Richoz (2006) and Insalaco et al. (2006).
Facies Associations (LFA)
The fourteen facies are grouped into genetically related lithofacies associations (LFA) based on facies stacking and the interpretation of depositional process and setting. Three LFAs are distinguished in the Middle Mahil at Saiq:
LFA3 (tidal flat),
LFA4 (lower-energy backshoal),
LFA5 (high-energy shoal).
Lithofacies types (LFT) and lithofacies associations (LFA) are captured in the table shown in Figure 23.
Facies composition and stacking pattern in the Middle Mahil are rather simple. Lithofacies variability is limited. Essentially three facies associations (LFA’s) dominate (compare Alsharhan, 1993 and Whittle and Alsharhan, 1995):
Tidal flat (LFA3).
Laterally adjacent facies of the primary depositional environment are reconstructed from the superposition of LFAs and stratigraphic surfaces. The size of the outcrop also permitted to document lateral facies relationships by walking them out.
The most seaward environment (Figure 24) recorded at the Saiq Plateau are storm-and wave-dominated shoal deposits (LFA5). They consist of well-sorted cross-bedded grainstones commonly 0.5–5 m thick, made up of a mixture of ooids and peloids. Trough cross bedding, excellent sorting and stacking of such grainstones suggests these to be in situ shoals, effectively representing the Middle Mahil carbonate factory.
Muddy intercalations within these grainstone layers occur, but they range in thickness only between 2–10 cm. Lateral tracing highlighted the patchy nature of these muddy interbeds. Most of them disappear after a few tens to hundreds of meters due to erosion by an overlying grainstone body. Two grainstone bodies were walked out laterally across four complete and 27 partial sections with an average distance of 2,000 m. All grainstones thicker than 2 m can be traced across the entire area investigated (8 x 8 km).
Frequently these grainstones are covered by 0.05 to 1 m thick microbial laminites. Like muddy interbeds, microbial layers are patchily developed. Laterally they pass into reworked intraclast flakestones and coated intraclasts over distances of hundreds of meters. This suggests a temporal cover of shoal bodies with microbial mats during decreasing accommodation. Field observations suggest sheet-like shoal body architecture. These are intercalated with thin muddy and microbial beds probably reflecting the original grainstone morphology.
Two distinct energy zones are interpreted (Figure 24):
Storm- and wave-dominated shoal and proximal backshoal with local exposure,
Restricted marine, intermittently wave agitated to quiet water distal backshoal and tidal flat.
The regional geological setting, particularly the interpreted lateral persistence of facies associations (Osterloff et al., 2004), suggests the Middle Mahil at Saiq represents the seaward, high-energy (shoal-backshoal) portion of an epeiric platform.
The weathering profile of outcrop walls readily portrays the Middle Mahil as cyclical. Sedimentological logging provides a detailed picture of these cyclic sequences. Sequences of hierarchical orders are defined in terms of changes of accommodation versus supply. Thus sequences are subdivided into a transgressive portion during increasing accommodation (A) versus supply (S), maximum flooding surface (mfs) during maximum A:S, regressive portion during decreasing A:S and maximum regression surface (mrs) during minimum A:S.
Adopting the nomenclature by Kerans and Tinker (1997), sequences are assigned to one of four orders:
Cycle sets (fourth-order),
A reference section was defined at the Saiq Plateau. It is used here to delineate the sequences. The section is easily accessible, located 5 km to the NE of the Al Jabal al-Akhdar hotel, and has the following GPS coordinates: UTM 572138 East, UTM 2548647 North, Elevation: ca. 2,420 m.
The smallest correlative sequences at Saiq are cycles, 2–5 m thick. These are the fundamental stratigraphic building blocks of the Middle Mahil. Cycles consist basically of two alternative facies associations, typically composed of 2 to 5 distinct lithofacies: (1) grainy, higher-energy beds, and (2) muddy, lower energy beds (in places argillaceous).
The Middle Mahil consists of three cycle motifs, made up of inter-bedded grainy and muddy beds:
In total 48 cycles compose the Middle Mahil (Figures 29). These are numbered from top to bottom starting with cycle #1 at top Middle Mahil, reference section meter 0 and ending with cycle #48 at base Middle Mahil, reference section meter 256.
Muddy Backshoal Cycle Motif
Description: The cycles are 2–4 m thick (Figure 25). They record the muddiest textures of the Middle Mahil. Basically these are coarsening up (cleaning up)/fining up (dirtying up) sequences. The lower part of a cycle starts with green, occasionally reddish thinly laminated argillaceous dolomites (LFT1). Upwards they pass into more massive, bioturbated (Thalassinoides) mudstones to wackestones (LFT2b). These are overlain by lightly gray, pinkish (occasionally greenish) decimeter thick, thinly laminated peloidal, intraclast mud- to wackestones (LFT 2a). Components change from silt-size particles to sand-size particles that are mainly peloidal intermixed with rare skeletals, ooids and intraclasts.
Interpretation: The lower part of this cycle type records a transition from argillaceous dolomites deposited during early transgressive phases to clean finely grained bioturbated dolomites, well reflected in the gamma-ray signature. This is interpreted as landward stepping of distal backshoal facies types (LFT 2b) over argillaceous and finely grained proximal backshoal and mudflat deposits (LFT 1). Maximum A:S is interpreted at the grainiest cleanest, coarsest, highest energy dolomites, on top of thick LFT 2b in places with skeletal components. The upper part of this cycle motif shows returning muddy, low-energy sedimentary rocks (LFT 2a) interpreted as decreasing A:S. This cycle type is limited to the lowermost Sudair sequence MS-3 described below.
Grainy Backshoal Cycle Motif
Description: Cycles are 2–4 m thick (Figure 26). They are grainier than the muddy backshoal cycles but show a similar coarsening-up then fining-up motif. The cycle starts, over a sharp erosive base, with graded (LFT 5a1) to bioturbated wackestone. These are 10–40 cm thick. Higher up grayish packstones appear, occasionally grainstones (LFT 5c2), in places containing coated intraclast layers. The grain-dominated strata are 30–80 cm thick. The upper part of this cycle motif consists of light greenish, thinly bedded mud- to wackestones (LFT 2a). These pass into bioturbated wackestones (LFT 2b) that are covered by microbial laminites (LFT 3a), several centimeters to a few decimeters thick.
Interpretation: The lower, coarsening up part records landward stepping of backshoal to distal shoal facies over intertidal/proximal lagoonal strata during increasing A:S. Maximum A:S is recorded at the grainiest, cleanest layer. The upper part experiences a return to lower energy proximal lagoonal to intertidal facies types and reflects decreasing A:S conditions. Minimum A:S is marked on top of thin microbial, tidal flat deposited layers. This cycle motif occurs in the upper part of Sudair sequence MS-3 and MS-1 described below. It might constitute marginal reservoirs.
Grainy Shoal to Backshoal Cycle Motif
Description: Cycles are usually 3–5 m thick (Figure 27). They are dominated by grainy textures, i.e. stacked physically stratified backshoal to shoal beds (LFA 4 – LFA 5). Layers of densely packed, often imbricated rib-up clast packstones are made up of reworked microbial laminites (LFT 5b1). They cover the sharp erosive base of the cycle. These beds are several centimeter to a few decimeters thick. They pass rapidly into graded packstones that turn into high-angle trough-cross bedded peloidal-oolitic grainstones during an overall coarsening up trend. These rather massive grainstone bodies make up the gross of this cycle motive and are several decimeters to a few meters thick. They are covered, often above a sharp boundary, by microbial laminites (LFT 3a) that occasionally show tepee structures. These “microbial-caps”, several centimeters to few decimeters thick, may be replaced by bioturbated “muddy-caps” (LFT 2b) in some cases.
Interpretation: A coarsening up trend from shallow-water microbial mats to thick wave reworked shoal beds in the lower part of the cycle records a clear increase in A:S. It is associated with a rapid landward stepping of shoal bodies. Maximum A:S is interpreted in the upper, coarser and cleaner part of the trough cross bedded grainstones. Due to its massive nature and lack of clear sedimentological indicators it is difficult to determine exactly the position of maximum flooding. Pragmatically the top of the grainstones is picked for correlation purposes. The arrival of laminites or bioturbated wackestones heralds the seaward stepping of tidal flats or low-energy, shallow water backshoal facies deposits during decreasing A:S conditions. As these cycle types are connected with an increase of the A:S ratio, they typically occur around the transgressive peaks of higher order sequences, i.e. cycle sets and sequences. Minimum A:S is recorded at the top of the microbial mats. The cycle constitutes the bulk of Sequence 2 in the central portion of the Middle Mahil Formation. It is the most common cycle motif and records rather clean, reservoir prone units.
An interpretation of the relative geographic position of the three cycle motifs along the depositional gradient is shown in Figure 28.
Cycle Sets (Fourth-order)
Cycles show a regular bundling, with 4–8 cycles (fifth-order) arranged in a cycle set. Cycle sets are defined based on facies stacking pattern, presence and thickness of certain indicator facies (e.g. dissolution breccia) in combination with the outcrop gamma-ray signature. The Middle Mahil reference section (Figure 29) is divided into eight cycle sets. These have average thicknesses of 20–30 m. The numbering of the cycle sets, referred to as Mahil Cycle Sets (MCS) is linked to the three Middle Mahil Sequences described below.
Middle Mahil Sequence 3
Description: The lowermost sequence is built up of cycle set 3.3, 3.2 and 3.1 (Figures 29 and 30) and has an average thickness of 84 m. The base, at section meter 256, consists of red and greenish shales (LFT1), followed by argillaceous mottled dolomites (LFT2). Just above appears a locally, just 5–10 cm thick thrombolite. It is observed in all logged Saiq sections. The gamma-ray signature of Sequence 3 clearly indicates an upward-decrease of the shale content. The boundary between the lower and upper part is placed ca. 47 m above top Khuff, on top of the first prominent peloidal grainstone package (LFT5d1). The part above this grainstone consists of muddy and grainy backshoal cycles, i.e. cycle set 3.2 upper and cycle set 3.1.
Interpretation: Sequence 3 is interpreted as an overall transgression from tidal flat conditions towards deeper lagoonal settings. The first prominent grainstone interval, up to 12 m thick, clearly constitutes shoal deposits of a more seaward environment. An interpreted fall in relative sea-level, and therefore the reduction of accommodation space, is marked by the deposition of muddy, bioturbated lagoonal sedimentary rocks above the thick grainstone. Individual grainy intersections are interpreted as “spill-over lobes” from more distal environments washed into proximal lagoons during high-energy events.
Middle Mahil Sequence 2
Description: The second Middle Mahil Sequence 2 is 76 m thick. It consists of cycle set 2.2 and 2.1 (Figures 29 and 30). The unit is dominated by grainstones. The base of the sequence is placed at section meter 172. The lower part consists of muddy and grainy backshoal deposits that rapidly give way to shoal deposits. The gamma-ray signature shows a rapid cleaning up fully in line with the facies change from muddy to grainy textures. The thickest grainstone occurs between section meter 120 and 132 and is 12 m thick. This interval coincides with the lowest gamma-ray readings. The upper part of the sequence is muddier and grainstones get thinner. This trend is reflected in a subtle increase in gamma-ray readings. The top of Sequence 2 is at section meter 96.
Interpretation: Open-marine shoal bodies step in a landward direction during the lower transgressive part. Mfs (maximum A:S) is interpreted at the top of the thickest grainstone body. Seaward stepping grainstone shoals give way to grainy and muddy backshoal beds during the upper regressive part.
Middle Mahil Sequence 1
Description: The upper Middle Mahil Sequence 1 is some 96 m thick (Figures 29). Base Sequence 1 is at section meter 96. It consists of cycle set 1.3 to 1.1. The lower part, some 50 m thick includes cycle set 1.3 and the transgressive part of cycle set 1.2. The main constituents of this interval are upwards increasing grainstone bodies. This part shows a cleaning up gamma-ray signature. The upper part, 46 m thick, consists of the regressive limb of cycle set 1.2 and the entire cycle set 1.1. This part is dominated by microbial laminites (LFT3a). These facies types pass towards the top into reddish to yellowish dissolution related lithofacies types, such as dissolution breccias interbedded with microbial layers showing birds-eye structures. The light gray-whitish, angular clasts of this monomict breccias range in size from few cm up to 80 cm in length. This part of MS-1 shows a very distinct increase in gamma-ray readings. The top of the upper part is top Middle Mahil, section meter 0, marked by a 20–50 cm thick, reddish to greenish paleo-soil with micro-karst surface.
Interpretation: The transgressive part of Middle Mahil Sequence 1 is seen as a landward stepping of shoal facies, followed by a regressive shift towards backshoal related deposits. Maximum flooding is interpreted at the top of the thickest grainstone body at section meter 45 m, well reflected by a low gamma-ray signature. The regressive part of the sequence reflects the ongoing lithofacies association shift from backshoal over tidal mudflat conditions, to supratidal marsh settings. Exposure related strata mark the sequence boundary. A 50 cm thick paleo-soil is interpreted as the top of the Middle Mahil Formation on the Saiq Plateau. The lithofacies association shift, connected with the change of the deposited facies is also reflected by a dirtying-upward of the gamma-ray signature (Figures 29).
A packaging of the Middle Mahil (Sudair) in three distinct sequences is observed in the subsurface of Oman (Osterloff et al., 2004), the UAE (Alsharhan, 1993) and Qatar (Schlumberger, 1981) (Figure 31).
The Sudair in well Yibal-85 in Oman (Osterloff et al., 2004) is 220 m thick. The section is made up of three distinct units. A lower argillaceous unit is 65 m thick and consists of dolomites, shales and a distinct limestone bed. A 100 m-thick middle unit made up of clean dolomites intercalated with thin anhydrites follows it. An upper unit, some 55 m thick is again more argillaceous made up of dolomites and shales.
Alsharhan (1993), from the offshore UAE, describes a lower more argillaceous unit, some 80 m thick. A cleaner middle Sudair unit succeeds it, some 120 m thick. A more argillaceous upper unit covers this, some 75 m thick.
Similarly the Sudair in Qatar, some 300 m thick, consists of three units. A lower argillaceous unit, some 65 m thick is followed by a rather clean middle carbonate some 125 m thick. A 110 m-thick argillaceous-dolomitic unit covers it. This consists of a middle shale passing into an anhydrite covered by an upper shale (Whittle and Alsharhan; 1995, Schlumberger, 1981).
Hence the three Middle Mahil sequences identified at Saiq might be regionally correlative (Figure 31) and a tool to map out the Sudair (Middle Mahil) as top seal or reservoir prone sequence.
Triassic Supersequence (Second-order Sequence)
The three sequences of the Middle Mahil (Sudair equivalent), each 76–96 m thick, are part of a Triassic supersequence (Figure 3). The lower two sequences show an upward increase in grainy textures. This thickening up, cleaning up trend of grainstones is well expressed in the weathering profile in outcrops and the gamma-ray pattern in the subsurface.
While the lowermost Sequence 3 contains red shales, thick argillaceous-mottled dolomites and thin grainy dolomites only, the overlying Sequence 2 contains significantly thicker grainstone units increasing upwards from 0.3 to 5 m. Likewise grainstones composition change upwards. Grainstones in the Sequence 3 contain more muddy intercalations and signatures of subaerial exposure. Grainstones of the overlying Sequence 2 are free of such features. Instead they show uniformly trough-cross bedded bodies with upwards increasing grainstone thicknesses from 3 m at the bottom to 12 m in the middle. This thickest grainstone layer, in the middle of the Middle Mahil, reflects maximum accommodation and landward stepping of the depositional system. It is interpreted as maximum flooding probably corresponding to MFS Tr30 of Sharland et al. (2001).
Grainstones gradually decrease in thickness in the uppermost part of Sequence 2 and in Sequence 1. Grainstones are replaced by mottled mudstones and most notably microbial laminites. These herald a return of lower accommodation conditions and seaward stepping of the carbonate platform. The shallowest point of the supersequence is recorded in the overlying Upper Mahil Member (Jilh equivalent) where dominant karst surfaces are developed.
The stratigraphic subdivision and correlation with regional and global reference scales is challenging due to pervasive dolomitization and poor faunal content. Micropaleontological investigations of the Lower Triassic Mahil Formation at Al Jabal al-Akhdar have several limitations, namely the pervasive dolomitization of the sedimentary rocks and a depauperate fauna, probably related to the persistence of adverse environmental conditions throughout the Early Triassic. Biotas, especially in shallow marine environments, reveal a protracted recovery phase following the end-Permian mass extinction (e.g. Hallam, 1991; Wignall and Hallam, 1993; Erwin, 1994; Schubert and Bottjer, 1995).
The sedimentary rocks are predominantly composed of non-skeletal components with abundant traces of microbial activity. Low-diversity, high-abundance opportunistic forms, which were able to thrive in stressed environments, characterize the fauna.
Few specimens of Cornuspira mahajeri (see also “Cyclogyra sp.” det. A. Baud in Richoz et al., 2005) occur in fine peloidal dolomicrites with intercalated shale horizons at the base of the Middle Mahil Member (lower part of Sequence 3) (Figure 32-1).
Groves and Altiner (2005) confined a “disaster fauna” with the smaller foraminifera Earlandia sp., Rectocornuspira kalhori, and Cornuspira mahajeri, commonly associated with the widespread microbialite facies, to the lowermost Triassic. However, these morphologic simple forms may persist into younger (Olenekian) deposits (e.g. Galfetti et al., 2008).
At the Saiq Plateau, Cornuspira mahajeri occurs adjacent to the only dolomite bed with shell fragments in Sequence 3, the “Claraia beds” (Figure 22) together with a prominent positive δ13CCarb isotope shift (discussed below), which points to a Late Induan/Early Olenekian (Late Dienerian/Early Smithian) age.
No foraminifera have been recovered in the upper part of Sequence 3 and lower part of Sequence 2. This is probably also due to the pervasive dolomitization particularly of grainstone layers. Foraminifera reappear in the upper regressive part of Sequence 2, but become more frequent in Sequence 1. Dolomitized litho-/bioclastic, and peloidal grainstones yield mostly monospecific assemblages with numerous Hoyenella sinensis (Figure 32-4. 6, 7). Rare co-occurring specimens at the base of Sequence 1 have been provisionally assigned to cf. Gandinella silensis (Figure 32-2), but its scarcity and poor preservation currently prevents an exact determination. One strongly dolomitized sample at the nearby Wadi Sahtan in the lower part of Sequence 1 yielded two random sections of cf. Meandrospira? sp. (Figures 32-3). This, however, needs confirmation by analyzing additional samples.
The fauna provides only loose age constraints, because Hoyenella sinensis has been found from the Late Induan up to the Middle Triassic (Rettori, 1994; Maurer et al., 2008). The species is likewise an opportunistic form and typically occurs in stressed environments, probably with increased salinity. The rare and poorly preserved associated forms may hint to a Late Olenekian (Spathian) age, which is also indicated by the isotope data below. Such interpretation is further supported by similar foraminiferal assemblages from the upper Bih and Hagil formations at Musandam (northern UAE) (Maurer et al., 2008).
Numerous studies (e.g. Richoz, 2006; Koehrer et al., 2010) highlighted the value of stable-isotope analysis at the Al Jabal al-Akhdar. Vahrenkamp (1996), Coy (1997), Le Bec (2004), and Immenhauser et al. (2004) describe isotope-based stratigraphic correlations of Al Jabal al-Akhdar outcrops to Omani subsurface sections.
Recent works on δ13CCarb isotopes in the Early Triassic reveal several pronounced positive excursions. These are traceable throughout the Tethys from Italy to China (e.g. Payne et al., 2004; Korte et al., 2005; Horacek et al., 2007a,b; Galfetti et al., 2007). It is supposed that the δ13CCarb curve is representative of the global Early Triassic Ocean water geochemistry and has high potential for accurately dating sedimentary successions via chemostratigraphy. It has also been shown that dolomitization at the Al Jabal al-Akhdar did not alter the trends in δ13CCarb isotope composition by comparison with subsurface sections (Coy, 1997; Le Bec, 2004; Vahrenkamp, Atudorei, 1999).
Positive excursions have been explained with the burial of isotopically light organic carbon due to ocean anoxia/stratification in combination with enhanced nutrient levels due to increasing siliciclastic input (Horacek et al., 2007b).
According to Payne et al. (2004) and Horacek et al. (2007a, b) the δ13CCarb isotope curve displays three positive excursions (Induan/Olenekian, Early/Late Olenekian, Olenekian/Anisian) in the Early/Middle Triassic with the following characteristics (Figure 33):
P/T boundary: 2–4 per mil (‰) negative excursion (from 4‰ to 0‰)
Values slowly increase (to 2-4‰) with minor negative excursion(s) in the early Induan (Griesbachian).
Induan (Dienerian)/Olenekian (Smithian): ca. 8‰ positive excursion (values sharply rise from 2–4‰ to 8‰ and back to 0‰)
Values remain low in the Early Olenekian (Smithian) (-2‰ to 0‰)
Early/Late Olenekian (Smithian/Spathian): ~5‰ positive excursion (values from -2‰ to 3‰ and back to 0‰)
Late Olenekian (Spathian)/Anisian (Early/Middle Triassic): ~6‰ positive excursion (values from -1‰ to 5‰).
Stable values (~2‰) throughout the Middle Triassic.
According to Galfetti et al. (2007) (data derived from outer platform deposits in China) the shift during the Late Induan/Early Olenekian (Dienerian/Smithian) is less pronounced (~2‰), instead they showed a more pronounced shift at the Early/Late Olenekian (Smithian/Spathian) boundary. The positive excursion during the Late Olenekian (Spathian)/Anisian is more protracted than in other illustrations.
Interpretation of the Isotope Curve from the Saiq Plateau
The δ13CCarb isotope curve shows a protracted decline from 3–4‰ in the uppermost Saiq Formation to 0–1‰ close to the base of the Mahil Formation (Koehrer et al., 2010). Values increase in the Lower Mahil Member to +2–3‰ from 20 m above P/T boundary onward (Figure 33). The uppermost part of the Lower Mahil displays two short negative excursions at Saiq and Wadi Sahtan, before the values rise to around +3–4‰. Positive values persist into the lower part of Middle Mahil Member around the “Claraia” beds and sharply drop to low values of 0–1‰. At Wadi Sahtan another positive excursion to +3‰ is recorded at 190 m above P/T boundary, which is less well represented on the Saiq Plateau between section meter 240 and 220. The remaining Middle Mahil shows values around +1–2‰ with a slight trend to higher values towards the upper part of section.
The isotope data from the Saiq Plateau show close similarities to those from Wadi Sahtan, and we follow herein the stratigraphic interpretation of Richoz et al. (2005). There are no indications for major stratigraphic gaps in the succession. Thus the isotope curve may be interpreted as follows:
The δ13CCarb values in the Late Permian (Changhsingian) Saiq Formation are above +3‰. Towards the inferred Permian/Triassic (P/T) boundary close to the Saiq/Mahil boundary values are decreasing with a minimum around 0–1‰. A recovery to more positive values of +3‰ took place in the Early Induan (Griesbachian). In the Late Induan (Dienerian), the δ13CCarb values are around +2–3‰ similar to the values in section Zal (Iran) (Figure 33). The two negative excursions in the uppermost part of the Lower Mahil may hint to similar negative shifts in Italy. But negative excursions are more suspicious, as meteoric diagenesis may produce similar effects. The Induan/Olenekian (Dienerian/Smithian) boundary is marked by the pronounced positive trend close to the base of the Middle Mahil Member. with values rising to a maximum of almost +4‰, followed by a steep and undisturbed decrease to low values around 0.1‰ in the Early Olenekian (Smithian). The Early/Late Olenekian (Smithian/Spathian) boundary may be reflected by the positive excursion at section meter 190 in Wadi Sahtan, respectively 220 m above P/T boundary on the Saiq Plateau (Figures 29 and 33).
In summary the stratigraphic analysis provides indications for an Lower Triassic (Olenekian) age of the Middle Mahil deposits, based on bio– and isotope stratigraphy.
The extensive Saiq outcrop permits lateral tracing of certain beds. Four sections each some 4–8 km apart where logged sedimentologically and with outcrop gamma-ray. Cycles and sequences can be recognized at all sections. The thickness of grainy versus muddy facies changes laterally at every stratigraphic interval from cycle to sequence level. Overall the section shows a “layer-cake” type architecture (Figure 34) in the area of observations. Shingling or clinoforms were explicitly searched for. Two grainstone bodies were laterally traced in a stratigraphic framework along distinct marker beds. This gave a first impression of the lateral continuity of potential reservoir bodies on a scale of several kilometers.
Bivalve content was walked out in a 30 cm thick dolomite body, the ‘Claraia bed’ (Figure 22) of cycle set 3.3 near section meter 230 (Figure 29). Thickness of this grainy dolomite changes up to 33%. The percentage of mud versus grain varies up to 50%. Other grainstone bodies show up to 25% thickness variations over a distance of a few kilometers.
Shingling or clinoforms were not observed in the area of investigation. Correlation of small-scale cycles revealed gradual increase in grainstone thickness towards the NE, towards the perceived platform margin.
In summary sequence-stratigraphic analysis shows that the Sudair platform can be subdivided into three sequences, similar to the Khuff sequences below, which probably extend regionally as comparison with sections in the UAE and Qatar suggests. Mapping these can be useful for identifying the most reservoir-prone, middle grain-dominated sequence on an exploration scale. The three small-scale cycle motifs are useful at a production scale to identify reservoir trends by changing cycle composition. Grainstones show a pinching and swelling architecture and are fairly continuous on the outcrop scale.
The Middle Mahil Formation (Sudair equivalent) is interpreted as Early Triassic, Olenekian in age. On the Saiq Plateau it represents a seaward platform margin setting. It consists of clean dolomites. Anhydrites are absent and shales are very thin. The succession is about 260 m thick and made up of 14 distinct lithofacies types. About 30% of the succession consists of grain-dominated facies that represent high-energy shoal deposits.
Three basic cycle motifs are distinguished: muddy backshoal, grainy backshoal and shoal to backshoal cycles. Correlation of these cycles highlights changes in texture that might reveal areas of thicker grainstone possibly corresponding to areas of higher net-to-gross (NtG) in the subsurface for that matter.
The proposed sequence-stratigraphic framework subdivides the section into
3 sequences (third-order sequences),
8 cycle sets (fourth-order),
48 cycles (fifth-order).
The best reservoir is located in the middle sequence around overall (second-order) maximum flooding. Grainy facies thicken towards the NE, the perceived platform margin. Stratigraphic cross-sections show an overall “layer-cake-type” architecture with pinching and swelling of grainstones. Grainstones thicker than 2 m show lateral persistence across 8 x 8 km at the Saiq Plateau. The outcrop analysis suggests excellent reservoir quality of comparable subsurface sections at the Lower Triassic seaward margin of the Neo-Tethys Ocean.
Our study of Middle Mahil outcrop analogs at Saiq show the seaward margin of the Sudair platform as potential reservoir. This observation adds to the commonly held connotation of the Sudair as ‘Sudair shale’ or top seal. An additional potential reservoir can be expected at similar platform margin positions that however go along with an increasing seal risk.
The authors gratefully acknowledge the contributions of Gordon Forbes (PDO), Daniel Vachard (University of Lille), Gordon Coy (PDO), Claus von Winterfeld (PDO), Michele Claps (PDO), Suleiman Al-Kindy (PDO), Aly Brandenburg (PDO), Jan Schreurs (PDO) and Alan Heward (Petrogas). Ulrike Schulte (University of Bochum) is thanked for stable-isotope analyses. Shuram Oil & Gas is thanked for fieldwork logistics support. An anonymous reviewer and GeoArabia’s Editor-in-Chief are thanked for their comments, and GeoArabia’s designer Arnold Egdane is thanked for preparing the final graphics and layout. We thank ALT (Luxemburg) for access to WellCAD software.
ABOUT THE AUTHORS
Michael C. Pöppelreiter studied at the Mining University of Freiberg, Germany, the Postgraduate Research Institute of Sedimentology, United Kingdom, and the University of Tubingen, Germany, where he earned a PhD in 1998. Since then, Michael has worked as sedimentologist/3-D modeller with Shell in Holland, as carbonate geologist/3-D modeller at Shell’s Bellaire Technology Center in Houston, USA and at present, he is Team Leader at the Qatar Shell Research and Technology Centre in Doha, Qatar where he is coordinating the Khuff/Sudair outcrop analogue study. Michael published numerous papers on carbonate reservoirs, reservoir modelling and borehole image log technology. He is Honorary Professor at the University of Tuebingen, Germany. His research interests include structural control on reservoir distribution in carbonate reservoirs.
Christoph J. Schneider studied Geosciences at the University of Tuebingen (Germany), focusing on Sedimentary and Petroleum Geology. His diploma thesis (2009) was on facies analysis, sequence stratigraphy and static modelling of a Sudair Formation equivalent (Al Jabal al-Akhdar, Sultanate of Oman). His study has been sponsored by Petroleum Development Oman and Shell QSRTC. In June 2009 Christoph joined Wintershall Holding GmbH as a geologist, working on static geological modelling and time-depth conversion. Currently he is working on carbonate reservoir characterization in the Wintershall Qatar Branch in Doha, with specific interest in porosity/permeability prediction for carbonate reservoirs.
Michael Obermaier studied Geosciences at the Universities of Tuebingen (Germany), and Miami (Florida, USA). His diploma thesis (2009) at the University of Tuebingen was on facies characterization and regional 3D-modeling of a Sudair Formation outcrop-equivalent (Sultanate of Oman). Michael is currently working as research associate and PhD student at the Center for Applied Geosciences (University of Tuebingen). His PhD thesis, funded by Petroleum Development Oman (PDO), focuses on sequence stratigraphy, reservoir and seal geobodies of the Sudair and Jilh Formations in outcrops and subsurface of Oman.
Holger C. Forke studied Geology and Paleontology at the University of Erlangen. 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 and at the Institute of Geology (University of Erlangen) within the DFG Priority Programme 1054 ‘Late Paleozoic sedimentary geochemistry’. In recent years, he 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 of Natural History, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt University Berlin, Germany.
Bastian Koehrer studied Geosciences at the Universities of Tuebingen (Germany) and Bristol (United Kingdom) and obtained a M.Sc. in Sedimentary Geology in 2007. He then worked at the Center for Applied Geosciences at the University of Tuebingen as research and teaching associate. For his PhD dissertation on the Khuff Formation, Bastian spent 18 months of outcrop mapping in the Sultanate of Oman. The main objective of his thesis, funded by Shell and Petroleum Development Oman, was to establish a regionally valid sequence-stratigraphic framework and geological model of the Khuff Formation that highlights nature and dimensions of potential reservoirs from near well- to subregional-scale. Meanwhile, Bastian works as a geologist at Wintershall in Kassel (Germany). His research interests include carbonate reservoir characterisation and 3D-modelling.
Thomas Aigner studied Geology and Paleontology at the Universities of Stuttgart, Tuebingen and Reading/England. His diploma thesis was on the Geology and Geoarcheology of the Egyptian pyramides plateau in Giza (1982). 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 focussing on basin analysis and modelling (1985-1990). He worked as adjunct lecturer for applied sedimentology at the University of Wuerzburg (1988-1990). Since 1991 Tom is a professor and head of the sedimentary geology group at the University of Tuebingen. In 1996 he was a ‘European Distinguished Lecturer’ for American Association of Petroleum Geologists. In 2007/8 he spent a sabbatical with PDO and Shell Qatar. His current projects focus is on sequence stratigraphy and reservoir characterisation/modelling in outcrop and subsurface.