Hypogenic origin of paleocaves in the Ordovician carbonates of the southern Tahe oilfield, Tarim basin, northwest China

Abstract

The Lower-Middle Ordovician paleocaves at the depths of over 6000 m in the southern Tahe Oilfield have been important objectives of petroleum production in the Tarim Basin. 3D seismic data, drilling well data, image logs, and cores were integrated to delineate the development characteristics, spatial distribution, and morphology of paleocave reservoirs in the Tahe Oilfield. The petrographic, geochemical features and fluid inclusion measurements of the cave and fracture-filling calcite in the Tahe Oilfield were subsequently performed. The formation mechanism of paleocaves in the southern Tahe Oilfield was revealed through the comparative analysis of paleocaves in the north and south of the Tahe Oilfield. The paleocaves in the northern and southern Tahe Oilfield are extensively distributed 0–150 m and 160–350 m away from the regional unconformity, respectively. Different from the diverse paleocave morphologies including single cave passages, anastomotic mazes, and branchwork passages in the northern Tahe Oilfield, the paleocaves in the southern Tahe Oilfield are dominated by isolated chambers and rising shafts. Additionally, almost all the cave and fracture-filling calcite cements in the northern Tahe Oilfield coexist with gray-green sand and clay. While, the cave-filling calcite cements are relatively clean, without any terrigenous minerals. Furthermore, the calcite samples in the northern Tahe Oilfield show δ13C and δ18O values varying from −5.90‰ to −0.39‰, and from −13.47‰ to −7.23‰, respectively. The calcite cements in the southern Tahe Oilfield present similar δ13C values with the Ordovician limestone samples and relatively more negative δ18O values in a range between −15.02‰ and −12.29‰. Compared with the calcite samples in the norther Tahe Oilfield, the cave-filling calcite in the southern Tahe Oilfield shows a narrower range of 87Sr/86Sr ratios varying from 0.709342 to 0.709689. In particular, large amount of two-phase aqueous inclusions observed in the calcite from the southern Tahe Oilfield display relatively higher homogenization temperatures ranging from 90.8 °C to 205.9 °C. These significant differences on the paleocaves between the north and south of the Tahe Oilfield suggest that the paleocaves in the south are overprinted by both gravity-driven deep circulating meteoric water during the early Hercynian and the ascending thermal water possibly occurring from the second episode of the middle Caledonian to the early Hercynian. The paleocaves in the southern Tahe Oilfield are interpreted to be generated by hypogene karst under the constraints of strike-slip fault system and lithostratigraphic sequence in the Ordovician in confined settings. This study is an important supplement to the previous Ordovician meteoric karst model in unconfined zones of the Tahe Oilfield. It indicates that the stratabound isolated paleocaves developed in the lower submember of the Upper Yingshan Formation in the southern Tahe Oilfield have significant exploration potentials.