HYDROGEOLOGICAL ORIGIN OF THE ORDOVICIAN STRATIFORM PALEOKARST RESERVOIRS, IN THE SOUTHERN TAHE OILFIELD, TARIM BASIN, CHINA

Paleokarst vuggy porosity is an important reservoir component, which has been proven of significant hydrocarbon potential in Lower-Middle Ordovician in southern Tahe oilfield. An integrated approach was applied in order to better understand and characterize these reservoirs by integration of seismic data with thin sections and cores observation, well logging interpretation, fluid inclusions and geochemistry analysis. Spatial distribution of these paleokarst vug reservoirs was firstly demonstrated. In attempt to define their hydrogeological origin, geochemical evidences and hydrogeological conditions were then illustrated. Three sets of karstified vugs in sizes from a few to dozens of millimeters, occur at 10-60 m, 60-120 m, and 140-200 m below the exposure surface (T₇⁴), respectively. These karstified vugs principally distribute between pinch-out line of Sangtamu Formation and Qiaerbake Formation, with an extension range less than 10 km. These stratiform paleokarst vugs were interpreted to be generated predominantly in early Hercynian, as supported by micropores related to subaerial exposure during 1^st phase of Middle Caledonian have been destructed and calcite samples in south still show similar δ¹³C and δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios with that in north of Tahe area. Semi-confined zones in southern Tahe area are favorable positions for the formation of gravity driven deeply circulating meteoric water, which creates Ordovician stratiform vugs developed along karst boundaries. This study displays that deeply buried paleokarst vug levels altered by hypogenic karstification are effective reservoirs, although they were buried into depth of over 5500 m, and should be considered as critical ultra-deep petroleum-bearing target in future exploration.