[1] Lower Aptian carbonates in the Bab Basin at the southern Neo-Tethys margin record significant environmental changes across Oceanic Anoxic Event 1a (OAE1a). A long-lasting negative shift of carbon-isotope ratios (d 13 C) associated with a distinct decrease in oxygen-isotope ratios (d 18 O) in orbitolinid-rich carbonates characterizes the onset of OAE1a (Livello Selli), supporting a hypothesis that a long-lasting volcanic CO 2 emission is the main cause of OAE1a, inducing global warming. A bloom of microencrusters (Lithocodium and Bacinella) across the proto-Bab Basin occurred synchronously at the beginning of the subsequent positive d 13 C excursion, responding to the global carbon-cycle perturbations. The carbonates, formed during the OAE1a, show higher strontium-isotope ratios ( 87 Sr/ 86 Sr) compared with those of global seawater; this was likely caused by a local influx of isotopically heavier strontium, along with nutrients, into the proto-Bab Basin. These biotic proliferations were triggered by an increased nutrient supply induced by intensified continental weathering due to the global warming suggested by the increase in d second-order transgression and lasted until the early stage of the highstand after the OAE1a. The Livello Selli corresponds to the early stage of this transgression.

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Increasing the recoverable reserves from oil fields by extracting from tar zones is becoming more desirable in the Middle East. One approach for improved definition of tar zones is to understand the factors which affected the deposition and distribution of asphaltenes within the target interval. In this paper we outline how integrated 1-D and 3-D basin modelling was used to identify the timing of hydrocarbon generation and expulsion from the Jurassic source rock to charge a prolific Jurassic carbonate reservoir formation of an oil field, offshore Abu Dhabi, UAE and Qatar. The source rock is modelled to be in the peak oil mature window today, with the onset of oil generation from the Cenomanian to the Turonian, depending on modelled and assumed source rock kinetics. The onset of oil expulsion was from the earliest Paleocene. Measured bulk fluid parameters in the reservoir formation have a significantly higher Gas-Oil Ratio (GOR) and elevated API gravity values when compared with predicted values. A possible mechanism to explain this discrepancy would be to invoke the contribution of higher GOR fluids from more mature source rocks within the fetch area of the field. Thermochemical sulphate reduction of anhydrite layers in the reservoir is predicted to have begun during the Eocene. Major uplift and erosion in the Oligocene and Mio-Pliocene significantly reduced reservoir pressure and temperature. This reduction in pressure and temperature is modelled to have caused precipitation of solids, gravity segregation and flocculation at the then oil-water contact, depositing the main tar zone and patchy tar in the reservoir beneath this zone as charge continued through time. We present a detailed review, interpretation and 3-D basin model; the first study of its kind conducted on this oil field. The 3-D basin model predicts the timing of the deposition and distribution of asphaltenes in the carbonate reservoirs of the studied field and demonstrate that local problems need to be understood in their regional context.

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This paper was prepared for presentation at the 8th Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, U.A.E., 11-14 October 1998.

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