Characteristics of collapsed subsurface paleokarst systems and controlling factors of subsurface paleokarst development in the Lianglitage Formation, Halahatang oilfield, Tarim Basin, NW China

The deeply buried Ordovician Lianglitage Formation (>6500 m) in the Halahatang area, Tarim Basin, is characterized by a paleokarst topography and subsurface, collapsed paleokarstsystems. Based on core and thin-section observation and wireline-log interpretation, this study analyzed basic features of collapsed karst systems and mapped the pattern of collapsed and original karst systems through root mean square (RMS) amplitude and variance attribute fusion. Employment of production data revealed the general rule of these ultra-deep, buried, collapsed systems as reservoirs for oil and gas. This study showed that epigenic subsurface karst systems in the Lianglitage Formation experienced full collapse, filling, and subsequent strong compaction. Passages of subsurface karst systems have a dendritic pattern These ultra-deep and strongly compacted collapsed systems are not favorable for oil and gas production owing to poor reservoir qualities. Factors controlling original subsurface karst development in this area are extensive subaerial exposure during the Falling-Stage Systems Tract (FSST) and Lowstand Systems Tract (LST) periods following deposition of the Lianglitage Formation, faults and fractures formed before or during karstification, and surface drainage systems on top of the Lianglitage unconformity. This study provides a method to comprehensively characterize the features, distribution and mechanism of the ultra-deep buried epigenic karst reservoirs and clarified their low potential as reservoirs for oil/gas exploration and development. It also helps clarify that the true oil pay reservoirs in this reservoir resulted from hypogene dissolution rather than epigenic karst.