AAPG Middle East Geoscience Technology Workshop, Integrated Emerging Exploration Concepts

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Integrated Structural Modeling to Generate a New Play Type Offshore Bahrai


One of the main exploration targets in the Gulf region is the Jurassic Arab section. Four carbonate reservoir cycles (Arab D to Arab A limestones and dolomites) have been deposited under shallow shelf conditions, with evaporitic precipitates as caprocks for each individual subsequence (Arab D - Hith anhydrites). The carbonate reservoirs are laterally continuous over wide distances. The reservoir properties may vary, but for them to become a lateral seal for a stratigraphic trap is rather unlikely. Therefore, the traditional trap type is a four-way dip closure. These traps have a very low amplitude, the actual closure area is small and difficult to locate on seismic data. To extract more information out of the seismic data, a thorough reprocessing has been carried out. Application of several cycles of static correction, deghosting of the seismic data and a well driven PSDM resulted in an enhanced vertical kinematic and dynamic range of the data set. The detailed velocity model still results in a depth conversion error of up to 30 feet, which is in the range of the expected trap height. To cover the range of possible velocity variations, a stochastic depth modeling was applied which yielded in a number of small closures. As a result of the reprocessing, a series of subtle compressional faults have been identified. The introduction of these subtle faults creates a new trap type with potentially larger closures. But more importantly, the lateral trap distribution is no longer random, it is now underpinned by a tectonic concept which makes it possible to search for these traps specifically. The faults also provide a migration pathway for the hydrocarbons from the underlying source rocks into the trap. One of the parameters to evaluate this migration pathway is the brittleness around the fault. This is done by transforming the results of a simultaneous inversion into rock mechanical properties (Youngs modulus and Poisson’s ratio). In crossplot domain the volumes with the highest brittleness are highlighted and compared to a coherency-derived fault- and fracture attribute. The ultimate seal at trap level is then provided by the anhydrite layers. A highly detailed facies model derived from seismic and well data was used for juxtaposition analysis to evaluate the sealing capacity of the faults.