Cong-Sheng Bian
Research Institute of Petroleum Exploration & Development, China
Title: Contribution of overall moderate uplift of basin to Coal-bearing tight sand gas accumulation
Biography
Biography: Cong-Sheng Bian
Abstract
As an important unconventional gas resource, tight sand gas distributes widely in different petroleum basins. In coal-bearing formations of Upper Triassic in the Sichuan Basin and the Carboniferous and Permian in the Ordos Basin, coal measure strata and tight sandstone constitute widely distributed source-reservoir assemblages, and form the basic conditions for the forming of large tight sand gas fields. Similar to most tight gas basins in North America, the Sichuan Basin and Ordos Basin all experienced overall moderate uplift and denudation in Meso-cenozoic after earlier deep burial. Coal seam adsorption principles reveal and actual sample simulation experiment results show that in the course of formation uplift, pressure drop and desorption occur in coal measure strata, resulting in the discharge of substantial free gases, accounting for 28-42% of total gas expulsion from source rocks; simultaneously, the free gases formerly stored in the pores of coal measure source rocks were also discharged in a large scale due to volumetric expansion resulted from formation uplift and pressure drop. Based on experimental data, the gas totally discharged in the uplift period of Upper Paleozoic in the Ordos Basin and Upper Triassic Xujiahe Fm in the Sichuan Basin is calculated as 3-6×108 m3/km2. Geological evidences for gas accumulation in the uplift period are found in the gas reservoir analysis of the above two basins. Firstly, natural gas discharged in the uplift period has lighter carbon isotope and lower maturity than that formed in the burial period, belongs to the one generated at early stage of source rocks, and is absorbed and stored in coal measure strata. Secondly, physical simulation experiment results under high-temperature and high-salinity inclusions and almost actual geologic conditions confirm that substantial gas infusion and accumulation occured in the uplift period of coal measure strata of the two basins. Diffusive flow infusion is the main mode for gas accumulation in the uplift period, which can reach 56×1012m3 in the uplift period of Xujiahe Fm of the Sichuan Basin, compensates the diffusive loss of gas in the gas reservoirs, and has an important contribution to the formation of large gas fields. The above insight has promoted the gas resourc extent and potential of the coal measure tight sand uplift area; therefore, it needs to reassess the formerly believed unfavorable areas where the the uplift scale is larger, so as to get better resource potential and exploration prospect.