Fluid inclusions in authigenic quartz and hydrocarbon accumulation in the Dengying Formation in Renhuai,northern Guizhou

The study of stages and processes of the Sinian hydrocarbon accumulationis of important significance in the oil and gas exploration and deployment of the "Lower Association" petroliferous system in northern Guizhou. In this paper, the emphasis is placed on systematic analysis of fluid inclusions and oxygen isotope compositions in authigenic quartz and calcite as the pore-space fillings from the hydrocarbon reservoirs in the Sinian Dengying Formation in Renhuai, northern Guizhou. The reconstruction of paleogeothermal fields is based on bitumen reflectances of the Lower Cambrian Niutitang Formation in combination with the homogenization temperatures of fluid inclusions in authigene quartz in the Dengying Formation. The results of research have proved that the fluid inclusions occurring in authigenic quartz can be divided into three stages, with homogenization temperatures of 87.1-98.8℃, 109.4-126.8℃ and 171.3-190.2℃, respectively. The homogenization temperatures of fluid inclusions in calcite filled in the later stages range between 95.3℃ and 116.4℃ . The equivalent vitrinitere flectances of the Lower Cambrian Niutitang Formation vary from 3.11% to 3.43% . Judged from the homogenization temperatures of the fluid inclusions, it is believed that paleogeothermal gradients in the late Caledonian-Yanshanianmay vary from 2.61℃/100 m to 2.76℃/100 m. The δ¹⁸O_PDB values range from -15.0‰ to -7.8‰ for dolomite filled in individual stages, and from -16.1‰ to -14.2‰ for calcite filled in the later stages. It is inferred that the formation temperatures are estimated to be 92.0-212.5℃ and 74.5-86.3℃, respectively. Three stages of oil and gas accumulation and one stage of gas reservoirs destruction are recognized, including the first stage of oil and gas accumulation during the middle to late Caledonian shallow burial stage (442-428 Ma);the second stage of oil and gas accumulation during the late Hercynian to early Indosinian medium burial stages (252-232Ma); the third stage of gas accumulation during the early Yanshanian deep burial stage (170-155Ma), and one stage of gas reservoirs destruction during the early Himalayan (ca. 42-30Ma) at the depths of 2963-3741m in the Dengying Formation in Renhuai, northern Guizhou.