Hydrogeomorphologic architecture of epikarst reservoirs in the Middle-Lower Ordovician, Tazhong Uplift, Tarim Basin, China

Paleokarst vugs and fissures in the Middle-Lower Ordovician from the Tazhong Uplift have been a productive unit in the Tarim Basin for more than two decades. A new approach was applied to delineate the characteristics of these reservoirs by integration of 3D seismic data with thin sections, cores, and imaging logs, as well as related geochemical analysis. Reconstructed hydrogeomorphologic architecture was defined to illustrate the origin of the paleokarst reservoirs in this study. Fissures and fissure-vugs of epikarst reservoirs are the predominant pore type in the Central Uplift in the Tazhong area, which pervasively distributed as planes and steps in shallow depth of 0-30 m below the regional unconformity (T74). The favorable position for reservoir development was intimately related to fissures and faults in NE and NWW directions. Surface topography of the Middle-Lower Ordovician was flat initially, with karren as the most widespread landform category. Hence, surface water tended to be sheet flow and concentrated in places where karrens and cracks developed well, and thus the infiltration strength and exhumation capacity of surface water suffered from extensive constrains due to limited hydraulic gradient, which resulted in immature drainage system and incomplete karst sequence. Therefore, the hydrogeomorphologic architecture in the Middle-Lower Ordovician was likely lack of surface drainages, and a fissure-seepage karst reservoir development model was established to interpret the influence of hydrogeomorpholoic architecture on paleokarst reservoirs distribution. This study confirms that paleokarst fissure-vug system developed in karst initiation period can still be potential petroleum-bearing target in deeply buried subsurface.