Caledonian karstification of Ordovician carbonates in the Tahe oilfield, northern Tarim basin, Northwest China, and its petroleum geological significance

Intensive geological, geophysical and geochemical studies indicate that polyphase Caledonian karsfification occurred inside and at the top of the Ordovician in the southern part of the Tahe oilfield. The karstification is mainly manifested by the absence of the conodont zone, caving of wells and mud leaking in wells, onlap of the base of the upper Ordovician reflection wave group in the seismic section and erosional truncation of its top and Sr isotope features of calcites filling in core caves. The Caledonian karst reservoirs were mainly controlled by such factors as unconformity, faults, lithology and paleogeomorphology, showing distinct fault-controlled and stratabound features. Thus it is predicted that the areas of favorable reservoir rocks would be located in stacked sites of erosional areas of paleoweathering crust, areas of Caledonian faulting and areas of shoal reservoir rocks, e.g. the plunging crown of the axial zone of the Akekule anticline and the NE- and nearly N-Strending fault zones in the southwest and southeast parts of the Tahe oilfield.