Mineral composition characteristics and petroleum geological significance of tight sandstone of Longtan Formation in Weixin area, eastern Yunnan

Tight gas is an unconventional resource developed in low-porosity and ultra-low permeability sandstone and has rich potential oil and gas reserves. However, due to its tight and low permeability, fracturing is a necessary means for efficient development of tight gas. The reservoir characteristics and material composition of sandstone are of great significance to the preliminary evaluation of resource potential and production capacity of the dense sandstone reservoirs. By using X-ray diffraction, microscopy, scanning electron microscopy and other technical means, ten outcroppings of tight sandstone from coal measures of Permian Longtan Formation in Weixin area of eastern Yunnan Province were qualitatively and quantitatively analyzed. The results show that the mineral components of tight sandstone in Longtan Formation are mainly clay minerals and quartz, with an average content of 42.5% and 30.1% respectively, followed by feldspar with an average content of 14.7%. In addition, a small amount of calcite and anatase are found in some samples, with an average content of 9.05% and 14.4% respectively. The brittleness index of Longtan Formation tight sandstone is 0.575 obtained by the mineral component method, which is favorable for hydraulic fracturing. The characteristics of clay mineral assemblage show that the Longtan Formation sandstone is a deep and low permeability sandstone reservoir, and the material source is related to the weathering products of magmatic hydrothermal and basalt. The tight sandstone reservoirs of Longtan Formation in the study area are high clay mineral reservoirs, and especially the widespread chlorite and kaolinite have a negative effect on pore development and affect the migration and storage of coal measure gas. Generally, it belongs to a deep low-permeability high clay mineral sandstone reservoir. However, the high content of carbonate minerals in the upper part of Longtan Formation, filled in pores and fissures, has strong anti-compaction effect on sandstone and is also the material basis of later dissolution. It is easy to form secondary pores and promote the formation and development of tight sandstone gas reservoirs. Therefore, The upper part of Longtan Formation can be used as the first choice for the next step of tight sandstone reservoir reconstruction and tight gas development.