The seismic reflection boundary T₇² and the Upper Ordovician carbonate karst reservoirs on the Tazhong uplift, Xinjiang

The seismic reflection boundary T₇²,i.e.,the boundary of the Upper Ordovician Sangtamu Formation and its underlying Lianglitag Formation is interpreted as an important structural-sequence transform boundary in the Upper Ordovician strata on the Tazhong and Tabei uplifts,Xinjiang,and represent the trasition from the Early Palaeozoic carbonate platform-type deposits to terrigenous clastic deposits.The deposits in the two formations cited above were previously assigned to the continuous deposits,and the boundary between them considered as a transgressive onlap and "drowned platform".The studies in recent years show that the T₇² boundary is a regional fossil exposue surface and depositional break.The truncation is well defined at the top of the Upper Ordovician Lianglitag Formation below the boundary,and the residual thickness ranges nearly from 20 m to 600 m,respectively in the Tabei and Tazhong Basins.The fossil exposure and karstification of the weathering crust permit the development of the karsts in the carbonate rocks from the Lianglitag Formation.These karsts occur as major hydrocarbon reservoir horizons along the F1 fault on the northern margin of the Tazhong uplift.A succession of about 300 m-thick palaeokarst cave systems may be traced across the Lianglitag Formation and its underlying Qarbag Formation,Yijianfang Formation and Yingshan Formation on the southern bank of the Tarim River.These palaeokarst cave systems constitute the magnificent hydrocarbon reservoir rocks composed mostly of the limestone palaeokarst systems from the Upper Ordovician Lianglitag Formation on the southern slope of the Tahe Oil Field.