AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop

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Implications of Fault Geometry on Sealing Capacity in Paleozoic Unayzah Reservoirs - South Ghawar


Sandstone members of the Paleozoic Unayzah Group have a huge hydrocarbon potential in the Kingdom of Saudi Arabia. The South Ghawar trend proved to be gas bearing by discovering considerable accumulations in Unayzah Group at different locations. In the study area, the NW-SE trending master-fault to the west of the main fields is assumed to provide the lateral seals to the hydrocarbon traps. To reduce the uncertainty associated with such faulted traps, it is important to study the geometry and the sealing capacity of the bounding fault. The top of Khuff Formation, Khuff-C reservoir, Unayzah-Asand, Unayzah-B sand horizons, and the master-fault west of the main fields were structurally mapped using 3D seismic data. The geometric relationships between horizons and the fault were modeled in order to build the most geologically realistic 3D fault model. Fault displacement attribute calculation has indicated three major elliptical displacement patterns along the fault. This distribution of the fault throw over the fault surface suggests that the master-fault has evolved as a group of NW-SE aligned faults that linked into one major fault. A three dimensional fault and horizons model was populated with lithological and petrophysical data (Volume of shale) of the wells around the bounding fault. Fault displacement profiles and lithologies around the fault have been incorporated into a simple fault seal analysis workflow. Across fault juxtaposition diagrams of reservoir and non-reservoir units were used as a predictive tool of whether the fault plane is sealing or leaking. The sealing capacity of the studied fault was found to vary vertically and along the fault strike due to variations in layer thickness, lithology and fault displacement. Information on pressure distribution plots and hydrocarbon water contacts have been used to calibrate and cross-check our findings, which appear to be compatible with the sealing and non-sealing regions along the studied fault. The cross-fault juxtaposition approach, like the one used in this study, can be routinely performed to address the risks and uncertainties associated with fault related prospects.