ABSTRACT

The δ13C values of Albian to Cenomanian shallow-marine carbonate sequences of the Natih Formation have been collected from subsurface cores of a key location in Oman. The 450-m-thick stack of shelf carbonates is without significant gaps in deposition. The δ13C data range between 1‰ and 6‰, more-or-less tracking the evolution over time of δ13C in seawater established elsewhere in time-equivalent pelagic carbonate sequences. Anchored by biostratigraphy the isotope profile suggests several additional time correlations. It thus provides significantly enhanced stratigraphic resolution and a key section for regional correlations. In particular, the onset of Natih deposition (Natih F and G members) coincides with the Albian/Cenomanian boundary event, thus placing the base Natih into the Albian. The Natih C and D members were deposited mainly during the Mid-Cenomanian oceanic anoxic event, while the carbon-isotopes signature of the Natih A Member, which is at this locality incomplete due to erosion, documents the onset of the Cenomanian/Turonian boundary event (OAE2). This indicates a latest Cenomanian, possibly Early Turonian age for the top Natih at this subsurface location and suggests an Early Turonian age for the more complete Natih section exposed in the nearby Oman Mountains sections.

Both organic-rich intervals of the Natih Formation (Natih E4b and B2) do not correlate with global Oceanic Anoxic Events indicating a rather local setting for source-rock deposition. This is further supported by an isotopic anomaly associated with the organic-rich Natih B. The anomaly is likely related to near-seabed diagenesis or a temporary limited water exchange of the intra-shelf basin with the open ocean and the incorporation of recycled carbon from oxidized organic matter into the water column and the inorganic carbon pool. The subsurface carbon-isotope profile correlates well with those from nearby outcrop and other subsurface sections adding further confidence that primary signatures are preserved and can be used for correlations. As in other Early Cretaceous shelf sequences of the Arabian Plate oxygen isotopes are lighter than expected for calcite deposited in equilibrium with Cretaceous seawater indicating most likely whole-scale recrystallization and stabilization during shallow burial at slightly elevated temperatures.

INTRODUCTION

Thick sequences of shallow-water carbonates have been deposited during the Early and mid Cretaceous in the Arabian Gulf region. On a coarse scale the sequences are well dated and can be correlated over extensive areas (Murris, 1980; Sharland et al., 2001). However, more detailed subdivisions using sequence stratigraphy (flooding surfaces and sequence boundaries), in combination with other dating techniques, has revealed that correlations are not quite as straight forward (Sharland et al., 2001, p. 264-267; van Buchem et al., 2002, 2011; Homewood et al., 2008). Furthermore, seismic reflection patterns clearly indicate significant heterogeneity in platform growth and hence a complex diachronous infill of accommodation space resulting from repeated flooding of a vast continental shelf at this more detailed scale (Droste and Van Steenwinkel, 2004).

In the shallow-water carbonates of the Arabian platform sequences biostratigraphy has traditionally been the most important tool to constrain ages and to provide control points for correlation of time lines (Simmons, 1994; Witt and Goekdag, 1994; van Buchem et al., 1996, 2002, 2011; Sharland et al., 2001; Schroeder et al., 2010). However, biostratigraphy in shallow-water carbonates remains relatively coarse and controversial giving rise to considerable uncertainty in ages and correlations (Sharland et al., 2001, p. 28; Homewood et al., 2008, p. 48).

Exotic minerals (e.g. zircon in ash beds), which can directly be dated using radioactive isotope decay ratios, are usually rare and have not been found for the mid Cretaceous sequences of the region.

Chemostratigraphy using carbon (13C/12C) and strontium (87Sr/86Sr) stable-isotopes ratios is another tool to increase stratigraphic resolution in marine carbonates (Scholle and Arthur, 1980; Jenkyns, 1980; Burke et al., 1982; Hess et al., 1986; Wagner, 1990; Weissert and Lini, 1991; Vahrenkamp, 1996, 2010; Veizer et al., 1999; McArthur et al., 2001; Jarvis et al., 2006). The technique is based on systematic variations in the isotopic composition of seawater over time. It has been developed from measuring the isotope ratios in well-dated carbonate sections assuming that diagenesis has not significantly altered a primary isotopic composition derived from seawater at the time of deposition. As a result of the abundance of primary carbon in the carbonate system the δ13C signature is less susceptible to alteration during diagenesis and hence often reflects primary marine values. A large number of studies from pelagic and shallow-water carbonate sequences from around the world have confirmed this approach with excellent global isotope curves emerging from these studies for many periods of the earth history (Jarvis et al., 2006).

This paper documents δ13C and δ18O values of the Albian–Cenomanian Natih Formation from a reservoir location in the subsurface of northern Oman (Field “F”; Figures 1 to 3) and compares them with values from pelagic and other shallow-water carbonate sequences of similar age. It is the aim of this study to provide a reference record for one of the most important reservoir-prone, shallow-water carbonate units in the Middle East. A detailed record of changes in carbonate δ13C values will enhance stratigraphic resolution and aid regional correlation for carbonate sequences covering a significant part of the mid Cretaceous time period. Furthermore, the shape and detail of the curves combined with lithology data from the Arabian carbonate platform may provide some further insight into global and local carbon fluxes and the occurrence of oceanic anoxic events.

REGIONAL STRATIGRAPHY

The Wasia Group in Oman is composed of shales and argillaceous limestones of the Albian Nahr Umr Formation (Immenhauser et al., 1999) and the overlying limestones, marls and shales of the Albian–Turonian Natih Formation (Murris, 1980; Simmons and Hart, 1987; van Buchem et al., 1996, 2002, 2011; Droste and Van Steenwinkel, 2004). The Natih stratigraphy is complex as a result of repeated infill of intra-shelf basins and the associated vertical and lateral facies changes (Figures 2 and 3; van Buchem et al., 1996, 2002, 2011; Droste and Van Steenwinkel, 2004; Grélaud et al., 2006; Homewood et al., 2008). Based on outcrop and reservoir sections, seven members have been defined for the Natih Formation (A to G from top to bottom) with a further (local) division into sub-member units (e.g. A1–A7, Figure 3; Hughes Clarke, 1988). The most recent state of regional stratigraphy and biostratigraphy has been summarized in van Buchem et al. (2011).

The time-stratigraphic subdivision of the Natih Formation based on biostratigraphy is not without significant differences depending on fossil groups investigated (see figure 5inHomewood et al., 2008, and references therein). For this paper a subdivision is adopted based on benthic foraminifera (Simmons and Hart, 1987; Smith et al., 1990), rudists (Philip et al., 1995) and ammonites (van Buchem et al., 2005; Bulot, as summarized inHomewood et al., 2008; van Buchem et al., 2011). The entire biostratigraphic data set has been summarized in van Buchem et al. (2011) as follows:

According to this summary the Cenomanian/Turonian boundary is found “near” the Natih B/A boundary (van Buchem et al., 2011).

At the location of Field “F” the almost complete Natih section is represented (Figure 3). Only the top of the Natih A is not found. Instead the top of the Natih A is defined by an unconformity with evidence of significant erosion (mid-ramp facies directly below unconformity with no preserved shallow-water carbonates). The erosion is most likely related to the onset of Late Cretaceous tectonism in Oman prior to the deposition of the overlying Upper Cretaceous shales of the Fiqa Formation. This contrasts with outcrops in the Oman Mountains where a complete section is exposed including shallow-water shoals (van Buchem et al., 2002; Homewood et al., 2008).

DATA AND METHODS

In total, approximately 470 bulk powder samples were collected for this study from four cored sections in Field “F”. Samples were drilled from the matrix of cores composed of micrite and fine grains (undifferentiated abraded fossil fragments, foraminifera and peloids) with a 6 mm bit on a common household drill. Large recrystallized fossils or cements were avoided to minimize the potential impact of diagenesis. On average one sample was taken per meter of core. However, extra samples were taken near depositional surfaces or near any other features that may indicate either breaks in deposition, an unusual diagenetic overprint or other features of interest.

Samples were analyzed for δ13C and δ18O by the Stable Isotope Laboratory of the University of Miami, USA, using standard methods as detailed in Vahrenkamp and Swart (1994) and Swart et al. (2005). Data are reported using the conventional notation as parts per thousand relative to Vienna Pee Dee belemnite (V-PDB). The precision for this method based on repeat analysis is ± 0.08‰.

In addition published outcrop data from the Albian Nahr Umr Formation were integrated (Immenhauser et al., 1999; van Buchem et al., 1996, 2002). Gamma-ray logs are used to provide further stratigraphic control and aid correlation between the cores and with other local and regional sections.

The stratigraphic architecture in these fields is essentially a flat layer cake. Hence, gamma-ray traces are nearly identical and excellent correlations can be achieved. This was used to cross-correlate the data from the four investigated wells to create a combined depth profile covering the entire Natih Formation (Figure 4).

The lithology of the shallow-water carbonate sequences of the Wasia Group including lateral heterogeneity has been described in detail by several authors (van Buchem et al., 1996, 2002; Immenhauser et al., 1999; Grélaud et al., 2006; Homewood et al., 2008). Detailed information from the investigated wells on mineralogy, allochems, fossils, facies, depositional environments, diagenetic features, etc. available from internal company reports and regional studies (Homewood et al., 2008) have been incorporated.

A standard cross-plot is used for δ13C and δ18O profiles versus thickness with the data range fixed from 0‰ to +6‰ δ13C and -8‰ to 0‰ δ18O and thickness in meters. Gamma-ray logs are depth-matched with cored intervals based on core gamma rays and displayed over a very low range of 0–60 API units (American Petroleum Institute) in order to provide significant resolution for these overall relatively pure limestone sections.

After correlation with the well-dated European curve of Jarvis et al. (2006) the Natih section is transferred into a time frame to allow an assessment and a discussion of the duration of sequence-stratigraphic cycles proposed by others (van Buchem et al., 2002, 2011; Grélaud, 2008).

RESULTS

Measured δ13C values vary between 0‰ to +6‰. Profiles of δ13C show significant variation with depth, yet consistent trends in most of the intervals can be observed (Figure 4). While the data vary systematically throughout the core the scatter about the average at any depth is usually less than ± 0.5‰ rarely reaching ± 1‰ (e.g. lower Natih D section, Figure 4). Negative values conclusively indicative of diagenetic overprint are not present in these matrix samples. The range is similar to marine carbon-isotope values reported for this time period from both pelagic and shallow-water sections (Voigt, 2000; Wissler et al., 2003; Jarvis et al., 2006). However, some diagenetic overprint cannot be entirely excluded and may actually be indicated in the relatively light carbon-isotope excursion of the organic-rich interval in the Natih B unit (see discussion below).

The δ18O values vary between -6‰ and -1‰ with most data confined to a narrow range between -5‰ and -3‰ significantly departing from a proposed average isotopic composition of carbonates in equilibrium with Cretaceous seawater of -2‰ δ18O (Figure 4; Lohmann, 1988).

Based on log analysis, supported by energy dispersive x-ray analysis, the mineralogy of the sections is almost exclusively calcite albeit minor admixtures of other minerals do occur. In some intervals dolomite is present in minor quantities (E1 and E2 units); others are argillaceous (D5, C4) and/or organic-rich (B unit) as indicated by increased gamma-ray values. Minerals associated with burial diagenesis are present but only in minor quantity and usually clearly associated with fracture cements (Taberner et al., 2009)

DISCUSSION: CARBON ISOTOPE TRENDS

Marine Signatures

A comparison of the δ13C profiles from Field “F” with time-equivalent pelagic profiles (Voigt, 2000; Jarvis et al., 2006) shows a plausible correspondence in character over most of the section albeit the base line for the carbon-isotope profile is at significantly higher values (Figure 5): 3.5‰ δ13C versus 1.5‰ δ13C in the European pelagic section. Only the Natih B section falls below the marine base line and exhibits trends that do not correspond to any of the pelagic curve (see discussion below). The overall correspondence is taken as evidence that isotopic variations in seawater through time have been preserved in the shallow-water carbonates of Oman for most of the section.

Using the subdivision of the Natih Formation of Hughes Clarke (1988) as defined in Field “F” and the overall biostratigraphic constraints detailed above the following major time lines can be drawn when correlating with the very well-constrained Northern European chalk sections of Jarvis et al. (2006) (Figure 5):

  1. The Albian/Cenomanian boundary event can clearly be identified over the section of the Natih G and F members. This places the top of the Nahr Umr and the lower Natih G and F members into the Late Albian and the overlying Natih E Member into the Early Cenomanian.

  2. A distinct double peak is associated with the Natih D and C members. Using the Jarvis composite curve the Mid-Cenomanian Event I likely corresponds to the first peak near the top of the Natih D Member while the second peak near the top of the C Member may align with the peak on the Jarvis curve at the Middle/Late Cenomanian boundary.

  3. The clearly positive trend towards heavier isotope ratios near the top of the Natih A section in Field “F” is correlated with the rising limb of the OAE 2 anomaly at the Cenomanian/Turonian boundary. At this location the top of the Natih A is defined by an unconformity with evidence of significant erosion (mid-ramp facies directly below unconformity with no preserved shallow-water carbonates), hence it is unclear whether peak values and the Cenomanian/Turonian boundary are preserved. However, the broad plateau with several peaks near the top of the profile may correspond to a Cenomanian/Turonian boundary signature recognized in other profiles (Voigt et al., 2004; Jarvis et al., 2006; Tsikos et al., 2007; Parente et al., 2007) suggesting that the top Natih A at Field “F” may just reach into the Turonian. Consequently, younger units of the Natih A Member exposed in the Oman Mountains almost certainly are of Turonian age, which is supported by ammonite biostratigraphy (Bulot as reported inHomewood et al., 2008) and the occurrence of Hippurites rudists (Philip et al., 1995).

Overall the proposed alignment of the carbon-isotope curves corresponds well with the chronostratigraphy based on biostratigraphy proposed by van Buchem et al. (2011) and summarized above with one exception. The Cenomanian/Turonian boundary is placed higher into the Natih A section, probably near the top of the section found in Field “F”. However, since erosion is evident at Field “F” this is not in contradiction to established biostratigraphic anchor points established in the more complete sections of the Oman Mountains, which place the Natih A into the Early Turonian (rudists, Philip et al., 1995; and ammonites by Bulot as summarized in van Buchem et al., 2011). The most important implication of this finding is that the Natih B including its source-rock intervals is placed entirely into the Late Cenomanian (see discussion below).

In addition a number of additional possible correlation lines have been drawn (see dotted lines in Figure 5). However, due to the low amplitude of the variations these correlations are less certain. Using these additional correlations the following assignments may be made:

  1. The lower Natih E (E4b to top E3) is early Early Cenomanian (Zone Mm in Jarvis et al., 2006).

  2. The E2 and E1 make up the rest of the Early Cenomanian (Zone M. dioxini) and the top E1 likely coincides with the Early/Mid Cenomanian boundary.

  3. The Natih D and C members are Mid Cenomanian, with the top of the C1 unit and perhaps even the lower B sub units (B4 and B3) defining the Mid/Late Cenomanian boundary.

  4. The organic-rich sediments of the Natih B2 unit, with its characteristic low-carbon isotope interval, are Late Cenomanian. Hence the regionally extensive source-rock interval (B2) does not coincide with the global anoxic event OAE2 at the Cenomanian/Turonian boundary, but rather pre-dates it.

Natih B Excursion

The negative excursion over most of the Natih B Member is an anomaly that cannot be correlated to the global isotope signature (Figures 4 and 5). Based on biostratigraphy, the source rocks of the Natih B are Late Cenomanian (van Buchem et al., 2011) but predate, based on the carbon-isotope profiles, the Cenomanian/Turonian boundary and the global OAE2 time period. At the location the source rock is of marine Type I/II origin, immature and with total organic content of up to 15% (Terken, 1999; van Buchem et al., 2002; Al Balushi and Macquaker, 2011). It is composed of numerous small-scale cycles with rhythmic alteration in the organic content (van Buchem et al., 2002; Al Balushi and Macquaker, 2011).

Two explanations for the isotope anomaly come to mind. Al Balushi and Macquaker (2011) proposed deposition of the Natih B2 unit in a basinal setting with frequently alternating periods of anoxia and at least partial oxygenation. Possibly, the near seafloor periodic oxygenation of organic matter may have released enough light carbon dioxide into the pore water to derive a lower bulk rock composition as a result of early diagenesis. Alternatively, oxygenated waters reacting with the organic material previously accumulated at the seafloor during times of anoxia may have released carbon with low isotopic ratios into the basinal waters thereby lowering locally the overall inorganic carbon-isotope signature of the water. Thus a local depositional environment may have caused the departure of the isotope trend from global values. Either way, a clear departure from oceanic signatures resulted pointing towards an at least partially isolated local environment in these small intra-shelf basins. The gradual and smooth trends leading into and out of the anomaly are interpreted to further support that this is either a vestige of very early diagenesis or a local water phenomenon rather than a late diagenetic overprint.

Oxygen-isotope Trends and Diagenesis

While the δ13C values seem to essentially track the character of the evolution of marine values the δ18O data depart from an original marine signature (Figure 4; Lohmann, 1988; Kuhnt et al., 2009) indicating whole-scale recrystallization of the rock in either non-marine waters and/or at elevated temperatures (Budd, 1989; Moshier, 1989; Raven and Dickson, 2007). Overall there is little character in the δ18O profile despite several major third-order sequences boundaries and clear evidence for subaerial exposure (Figures 3 and 4). In addition, the rocks experienced a complex history of burial, tectonic disturbance, and uplift (Terken, 1999; Filbrandt et al., 2006) with clear evidence for burial fluids circulating through the sequence (Taberner et al., 2009). In the cores from Field “F” conclusive evidence has been collected from minor quantities of late stage cements for methanogenesis and sulphate reduction (Taberner et al., 2006, 2009).

However, a late diagenetic overprint significantly altering a bulk signature is considered unlikely because late diagenetic cements are rare and usually associated only with fractures. Sampling of such cements has been avoided for this study. Instead porosities reaching in places 40% suggest that leaching was the dominant diagenetic process that has affected the Natih sequence at this subsurface location. In the most likely scenario carbonates overall experienced complete stabilization to low magnesium calcite, soon after deposition in shallow-burial environments but prior to the major circulation of burial brines triggered by ensuing tectonic events of the Late Cretaceous.

Regional Correlations

Carbon-isotope profiles of the Natih Formation are available from nearby outcrop sections exposed in the Oman Foothills (P. Razin and F. van Buchem, personal communication, 2011) and from other subsurface locations cored in northern Oman (Wagner, 1990) (Figures 1, 6 and 7). The geology of these sections and their subdivision in Natih sub-units is well understood, making a correlation with the subsurface straight forward (van Buchem et al., 2002; Homewood et al., 2008). Carbon-isotope profiles clearly show similar trends as those seen in the subsurface at Field “F”, further supporting that the observed signatures are primary and not influenced by late-stage diagenetic overprints. In the pioneering work on isotope profiling by Wagner in 1990 the value of carbon-isotope profiles for correlation purposes was already recognized (Figure 7) albeit at that time mainly related to diagenetic overprints. The similarity in signatures between the shallow-shelf sections with those from deep-marine sections from Europe clearly points to a primary seawater signature. The lateral change of the negative excursion of the Natih B, both in the outcrop and the subsurface profiles, is related to a change of depositional environment from organic-rich basinal deposits to platform slope deposits (van Buchem et al., 2002; Homewood et al., 2008). This observation further supports that the basinal deposits systematically deviate from normal marine signatures due to a local sub-environment (see discussion above).

Similar Shape but Different Absolute Values

While the δ13C profiles of the Natih shallow-water sequences show a plausible correlation with curves from pelagic sequences, differences do exist in absolute values. In some parts of the section these may be up to 2‰ (Figure 4, Lower Natih E unit). Similar differences in δ13C values between shallow-water shelf and pelagic sequences have been noted previously in the Aptian Shu’aiba Formation (Vahrenkamp, 1996, 2010). A primary facies-related or diagenetic cause for shifts cannot entirely be excluded (Swart and Eberli, 2005; Raven and Dickson, 2007). However, shifts in absolute values have also been documented between pelagic profiles from different localities (Voigt, 2000), indicating some whole-scale differences between water masses or some effect of fractionation. The origin of these variations still remains enigmatic and warrants further attention.

Oceanic Anoxic Events and the Intra-shelf Source Rock Units of the Natih Formation

The Natih Formation contains two regionally significant source-rock intervals with total organic carbon (TOC) reaching up to 15% (Terken, 1999; Al-Balushi and Macquaker, 2011): the Natih E4b and the Natih B, which are equivalent to the Shilaif Formation in the United Arab Emirates (UAE) with its basinal source-rock intervals, and respectively the Safaniya and Mishrif members of the Wasia Formation in Saudi Arabia (Newell and Hennington, 1983; van Buchem et al., 2011). In Oman the Natih B is the better-developed source-rock interval (thicker and more organic rich), while regionally, the Natih E4b equivalent source rocks are at least equally as important (Newell and Hennington, 1983; Alsharhan and Nairn, 1988). However, neither of the source-rock intervals correlates to any of the Cenomanian global Oceanic Anoxic Events. The Albian/Cenomanian boundary event predates the Natih E4b, the Mid-Cenomanian anoxic event predates and the Cenomanian/Turonian boundary event post-dates the Natih B2 source-rock interval. Clearly, the source rocks of the Natih intra-shelf basins are local phenomena apparently not related to global anoxic events.

Instead in these shelf carbonate platforms anoxic events post-date transgressions and are associated with highstand deposits. The Albian/Cenomanian boundary and the Mid-Cenomanian Anoxic Events both followed the lowstand/transgressional input of significant amounts of siliciclastics onto the Natih Shelf (marls and shales of the upper Nahr Umr, G2, F2, D5 and C4 Natih sub-units). The Cenomanian/Turonian event follows the transgression above the Natih C and basinal deposition of the Natih B. The elevated δ13C signatures correlating to global anoxic events coincide on the Natih platform with sea level highstand shallow-water carbonate deposition (G1, F1, E3/2, D1, C1, A3 and likely the eroded shallow water A units above the A1 subunits of the Natih Formation at Field “F”).

CONCLUSIONS

The δ13C values of Albian–Cenomanian, shallow-marine carbonate sequences of the Natih Formation have been collected from subsurface cores of a key location in Oman. The 450 m-thick stack of shelf carbonates is without significant gaps in deposition. The δ13C data range between 1‰ and 6‰ and show plausible similarities with the evolution over time of δ13C in seawater established elsewhere in time-equivalent pelagic carbonate sequences.

Anchored by biostratigraphy, the isotope profile provides significantly enhanced stratigraphic resolution and key tie points for regional correlations. In particular, the onset of Natih deposition (Natih F and G members) coincides with the Albian/Cenomanian boundary event, thus placing the base Natih into the Albian. The Natih C and D members were deposited during the Mid-Cenomanian oceanic anoxic event, while the carbon-isotope signature of the Natih A Member, which is at this locality incomplete due to erosion, documents the onset of the Cenomanian/Turonian boundary event (OAE2). This indicates a latest Late Cenomanian, possibly Early Turonian age for the top Natih at the subsurface location of Field “F” and suggests an Early Turonian age for the more complete Natih section exposed in the nearby Oman Mountains sections.

An isotopic anomaly associated with the organic-rich Natih B intra-shelf basin sequences indicates a temporary limited water exchange with the open ocean and the incorporation of recycled carbon from oxidized organic matter into the isotopic composition of the sediments.

The subsurface carbon-isotope profiles correlate well with those from nearby outcrop sections adding additional confidence that primary signatures are preserved and can be used for correlations. Similarly, as in other Early Cretaceous shelf sequences of the Arabian Plate, oxygen isotopes are lighter than contemporaneous seawater indicating whole-scale recrystallization during shallow burial at slightly elevated temperatures

ACKNOWLEDGEMENTS

I would like to acknowledge fruitful discussions with many colleagues on different topics of the Natih Formation, in particular Henk Droste, Carine Grélaud, Cathy Hollis, Peter W. Homewood, Adrian Immenhauser, Joerg Mattner, Philippe Razin, Conxita Taberner and Frans van Buchem. The second Arabian Plate Workshop was a key event to deepen insights in the Albian–Turonian depositonal system and I thank the many contributors and participants of this event. Special thanks to Philippe Razin for generously sharing outcrop and isotope data from Jabal Salakh. The Stable Isotope Laboratory at the University of Miami provided the very stable and reliable input of thousands of isotope measurements over the past 20 years. I am grateful to Petroleum Development of Oman and the Ministry of Oil and Gas of the Sultanate of Oman for supporting this work and allowing its publication. The author thanks GeoArabia’s Editor-in-Chief Moujahed Al-Husseini and Executive Editor Joerg Mattner for valuable input on content and figures, Assistant Editor Kathy Breining for proofreading the manuscript and Heather Paul-Pattison for an excellent job designing the paper.

ABOUT THE AUTHORS

Volker C. Vahrenkamp is a Carbonate Geology Specialist for Shell International E&P, currently seconded as Exploration Manager to Abu Dhabi Company for Onshore Oil Operations (ADCO). He obtained his BSc in Geology from Universitat Freiburg, Germany, in 1980, his MSc in Engineering Geology from the University of Michigan, USA, in 1983, and his PhD in Marine Geology and Geophysics from the Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, USA, in 1988. Volker’s main interests include chemostratigraphy, the origin of the carbonate pore network and its impact on reservoir fluid movement and seismic response; carbonate diagenesis; modern reservoir analogs, especially the dimensionality of carbonate depositional facies; 3-D reservoir modeling; paleoclimate and its impact on the rock record. Before moving to Abu Dhabi in 2007, he worked as Principal Reservoir Geologist with Petroleum Development of Oman, as a Senior Reservoir Geologist and Seismic Interpreter with Sarawak Shell Berhad, Malaysia, and as a Production Research Geologist for Shell in The Netherlands. Volker has published many articles on various geological topics, has organized a number of core workshops for Arabian platform carbonate sequences and has led numerous field training courses in the Middle East. In 2008, he was an AAPG Distinguished Lecturer. In 1988, Volker received the F.G. Walton Smith Prize for Excellence in Marine Sciences from the University of Miami. In 1982, he received the Scott Turner Earth Science Award from the University of Michigan. Volker is a member of several professional societies including AAPG, EAGE, SPWLA, the Emirates Society of Geoscientists (ESG) and the Geological Society of Oman (GSO).