Dolomitization and its impact on porosity development and preservation in the deeply burial Lower Ordovician carbonate rocks of Tarim Basin, NW China

Abstract

The deeply buried dolostones (>6000 m) of the Lower Ordovician Penglaiba Formation are currently the key targets of hydrocarbon exploration in the Tarim Basin. Petrographic, isotopic, and fluid inclusion microthermometric data were obtained in an attempt to determine the origin of dolostones and its effect on porosity development. Based on the petrography examination, three matrix dolomites and two dolomite cements were identified: very finely to finely crystalline, nonplanar-a to planar-s (D1) dolomite; finely to medium crystalline, planar-s to planar-e (D2) dolomite; medium to coarse crystalline, nonplanar-a (D3) dolomite; medium to coarse crystalline dolomite cements (Cd and Sd). D1 dolomite, which commonly occurs widely distributed throughout the study area, is characterized with δ13CVPDB values of −2.4 to −0.1‰, δ18OVPDB values of −8.1 to −4.1‰, and 87Sr/86Sr ratios of 0.70899–0.70979, inferring the seawater origin. D2 dolomite, merely confined to one side of the stylolite, has δ13CVPDB values of −1.8 to −0.4‰, δ18OVPDB values of −8.3 to −3.7‰, and 87Sr/86Sr ratios of 0.70871–0.70955, suggesting that D2 is precipitated in shallow-middle burial conditions and the dolomitizing fluids is derived from the residual primitive seawater in the formation. D3 dolomite is considered to be formed at higher temperatures under deep burial conditions considering its crystal textures and the δ18OVPDB values of −9.0 to −6.8‰. The two dolomite cements, Cd and Sd dolomites, are characterized with the inclusion Th values of 119–163 °C and 87Sr/86Sr ratios of 0.70941–0.70993, inferring the hydrothermal origin. Based on the distribution, petrography and geochemistry data of D1, the mainly hydrological system for dolomitization in the Penglaiba Formation is interpreted to be the seepage-reflux model. Intercrystalline pores and vugs are two types of pores associated with dolomitization, the latter being the most dominant type that is related to hydrothermal dolomitization.