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Origin of the Ordovician stratabound paleocaves in the southern Tahe oilfield, Tarim Basin, China: constrains from cave morphology and speleothem geochemistry

Abstract

The Ordovician stratabound paleocaves at depth of >6100 m in confined karst zones have been proven of significant hydrocarbon potential in the southern Tahe oilfield. An integrated approach was applied to better characterize these reservoirs by integration of seismic data with well drilling data, cores observation, thin sections, geochemistry analysis and fluid inclusions. Paleocaves morphology and infilling features, and speleothem geochemistry both in the northern and southern Tahe area were demonstrated and a detailed comparison study was subsequently performed to constrain the formation time of the stratabound paleocaves in the south. In attempt to define the origin of these paleocaves, hydrogeological conditions were illustrated as well. These karstified paleocaves developed 160-351 m under the regional unconformity (T74) and were predominantly concentrated in the Lower subsegment of the Upper Yingshan Formation (T75-1~T76). The distinctions of paleocaves between the north and the south of the Tahe area revealed that the Lower-Middle Ordovician isolated caves in the southern Tahe area were more likely created by ascending thermal fluids after the 1st phase of the middle Caledonian. In addition, they were induced by the discordance of permeability structure of a hydrostratigraphic framwork, consisting of a confining bed, a upper aquifer, a cave unit and a lower aquifer, in which ascending transverse speleogenesis was driven by a rising flow system. The distribution of the stratabound paleocaves was principally controlled by stratiform flower-like structure between T75-1 and T76 interfaces. This study indicates that it is still possible to form large-scale paleocave systems in confined karst settings, in which the caves far away from the regional unconformity should be considered as critical ultra-deep petroleum-bearing target in future exploration.