--> The Large but Unrecognized Potential in Ordovician Carbonate Plays in North Texas and South Oklahoma

AAPG Southwest Section Annual Convention

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The Large but Unrecognized Potential in Ordovician Carbonate Plays in North Texas and South Oklahoma


Carbonate E&P in Asia has provided much new information on carbonate heterogeneity that has led to significant challenges and new approaches to current theories and practices in carbonate reservoirs; especially facts related to Palaeozoic carbonates, such as: • Cavernous reservoirs could be detectable as double- point bright spots in seismic data • Large cavities (vugs to caverns) are extensively distributed in carbonates down to more than 7000 m as the main producing reservoirs in tight carbonates. Many of them are far from unconformity and faults • Extremely high carbonate heterogeneity led to complex reservoir compartments and different fluid contacts, resulting in highly productive wells (up to 5.3 MMBO or 300 BCF) very close to dry holes within producing fields • A large potential of hydrocarbons (up to 90%) have been left in many mature fields and areas Re-examining a 3D seismic survey in North Texas with the above knowledge, we observed large potential in Lower Ordovician Ellenberger/Arbuckle carbonates. Cavernous reservoirs are evident and extensive, unevenly distributed at thrust-faulted uplift within an area of 60,000 acres and a thickness of about 3000 ft. With few penetrations, these thick and highly- heterogeneous carbonates become a deeper frontier play that lies under shallow mature fields. Like cases in Asia, the potential could be >20MMBO recoverable in this area with well flow rates of >1MBO/D and cumulative >1MMBO. The cave features are relatively subtle in other areas due to poor seismic quality and possibly less cave distribution. In addition, the overlaying mature play of Upper Ordovician Viola carbonates still has moderate potential due to limited seismic quality, complex thrust faulting, highly variable thickness (0 to 1000 ft) and strong reservoir heterogeneity (tight to cave). These two plays provide highly economic targets for drilling in North Texas and South Oklahoma which are basically unrecognized due to limited knowledge and approaches on carbonate heterogeneity. Carbonate- targeted seismic and drilling strategies developed in Asia focus on revealing and characterizing the heterogeneity, reducing drilling and completion cost, and optimizing production and recovery efficiency, which is the effective solution to efficiently monetize the large, but complex, potential.