Hydrothermal activities of Paleogene Shahejie Formation in Qikou Sag and its effects on lacustrine carbonate reservoirs

Key words:  structural control  Paleogene  lacustrine carbonate  reservoir property  hydrothermal activities  Qikou Sag

Abstract:

Based on observations of cores and thin sections, combined with the analysis of fluid inclusions and geochemical data, the control of hydrothermal activities on lacustrine carbonate reservoir porosity distributions of Paleogene Shahejie Formation in Qikou Sag was studied. The microscope analyses of carbonates show widely distributed saddle dolomites, which are associated with pyrites and quartzs. The geochemical data reveal light REE enrichment and positive Eu anomalies in most samples, coupled with low⁸⁷Sr/⁸⁶Sr ratios and high homogenization temperatures of fluid inclusions, indicating that Shahejie carbonates were deposited in hydrothermal environments. Numerous dissolution pores, corrosion fissures, and intergranular pores formed with CO₂-enriched hydrothermal fluids, and the quality of reservoir property has been significantly improved. The subsequent dolomitization plays an important role, as they make the porosity slightly reduced. All available evidences show a strong structural control by extensional strike-slip faults. Negative flower structures on wrench faults are favored drilling sites for hydrothermal reservoir facies, probably resulting from Neogene thermal events.