Second EAGE Workshop on Arabian Plate Geology

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Six maximum flooding surfaces (K90 – K140) of Albian – Early Turonian age were recognised across the Arabian Plate by Sharland et al. (2001). These MFS together with intervening sequence boundaries/correlative conformities and maximum regression surfaces can be reasonably well age calibrated by biostratigraphy (especially ammonites, planktonic foraminifera and nannofossils) and isotope data. This allows us to correlate these surfaces outside of the Arabian Plate and to test their eustatic origin by demonstrating their occurrence on a global scale. We have found evidence for the K90 – K140 sequences in locations ranging from Europe, Arctic North America, through the western interior of the US to offshore Brazil, the west coast of Africa, India and Australia. At all these location the expression of these surfaces is biostratigraphically calibrated and hence we can be confident in a eustatic driver for the K90 – K140 sequences. It is interesting to speculate on the causes of eustacy in the mid-Cretaceous. It seems likely that a growing body of direct and proxy evidence points to a coincidence of climatic fluctuation and eustacy, suggesting that melting and creation of ephemeral polar ice may be a causal mechanism, even in what is commonly regarded as a "greenhouse" time.

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Albian-Turonian carbonates in NE Arabia show a progressive retreat of their shelf margins towards palaeohighs. This retreat is controlled by localised tectonic collapse of the pre-existing platforms and re-location of platform margins onto structurally high areas.

Unnamed clastics at the base of Burgan offer a hitherto un-explored hydrocarbon play in onshore Kuwait. It is a sub-unconformity play and comprise a composite sequence with three systems tracts upwards, namely; transgressive, highstand and followed by another transgressive package. Thickness of whole package varies between 10 and 145’ (generally between 37’to 60’, net pay 40’). Sand bodies in the transgressive system tracts are discontinuous laterally and vertically and that of the highstand system tracts are fairly continuous sheets and shale-out in the distal settings. Thin channel sand bodies of lower transgressive and highstand units entrap oil whenever upper transgressive unit provides effective top seal. The effectiveness of the top seal depends on the vertical limits of incision by overlying lowstand Burgan clastics. The trapping is controlled by both stratigraphic and structural components.

New 3D seismic acquired in the Rub 'Al Khali basin of Saudi Arabia, enables the application of a sequence stratigraphic framework as a basis for seismic interpretation. Emphasis is focused on the Wasia formation depositional environments, and implications on petroleum systems.

A high resolution sequence stratigraphic model has been constructed for the mid-Cretaceous Sarvak Formation in the High Zagros (Iran). The exceptional outcrop quality, displaying the detailed geometrical and facies patterns in the transition zone from carbonate platform to intrashelf basin, offered the rare opportunity to distinguish between the relative control of carbonate sediment supply and accommodation on the depositional geometries of 3rd order and 4th order depositional sequences. A significant distinction has been made in the 3rd order sequences between the early transgression (e-TST) when the system is still flat, and corresponds to a ramp setting, and the late transgression (l-TST) when a carbonate platform to intrashelf basin topography is created. The rate of accommodation creation is identified as the dominant factor controlling the morphology of the depositional profile, and, as such, the driving motor behind the dynamics of this type of carbonate system. The dip angle of the depositional profile has a major influence on: (1) the hydrodynamics of the system, (2) the type of carbonate sediment, and (3) the volume of carbonate sediment produced. This outcrop analogue can be considered as a good reference model for Cenomanian-Turonian carbonate platform margins of the Arabian Plate.

The Natih Formation (Late Albian –Early Turonian) corresponds to a very broad, extensive inner carbonate platform that developed in Oman over more than 1000 km. Because it is overall an aggrading carbonate platform system, the third-order stratigraphic sequences appear very tabular and isopach, which often leads to "layer-cake" correlations, even at the scale of the reservoirs. However, within these third-order sequences, a high level of sedimentary and stratigraphic heterogeneity can develop, and they are linked with: (1)the development and fill of intrashelf basins (different types of clinoforms), (2) phases of subaerial exposure (incisions and forced regressive wedges), and (3) phases of flooding of the platform (tidal channels and bioclastic shoals). These stratigraphic intervals have been studied on several outcrops of the Oman Mountains. The regional stratigraphic framework has been reviewed and extended by regional 3D seismic interpretation. A particular focus was made on specific reservoir-scale sediment bodies, on both outcrops and seismic data, and the technique of synthetic seismic modeling has been used to improve seismic interpretation. The results of this study have been used to refine the static model of the Natih Formation in a giant oil field in northern Oman.

Diagenetic modification of the Albian-Turonian in the Middle East is complex and multiphase, and has provided a critical control on reservoir quality. This paper presents a summary of this diagenetic overprint, gathered from published case studies from across the region. The most important diagenetic events comprise marine cementation, along discontinuities and overprinting bioturbation, incipient karst formation at intraformational sequence boundaries and deep cutting karst at the late Cenomanian/early Turonian unconformity. Finally, leaching of micrite and early diagenetic cements has often created large volumes of secondary micro- and macroporosity. This leaching could have taken place from meteoric pore waters or deep burial brines. This paper will assess the relative importance of these key in order to provide the basis of a predictive regional diagenetic model for the Albian-Turonian of the Middle East.

Rock properties of the Natih Formation in Field F were strongly modified as a result of diagenesis. The paragenetic sequence is well defined, and typical for a carbonate field from this geographic location and time period. It exhibits an interplay between early diagenetic processes, faulting and the tectonically controlled circulation of deep burial fluids. Conclusions are supported by detailed petrographic and geochemical data including isotopes, fluid inclusions and elemental compositions.