THE ORGANIC GEOCHEMICAL CHARACTERISTICS AND GAS-BEARING PROPERTIES OF MUD SHALE IN THE SHANXI FORMATION IN YAN'AN AREA

Previous studies have shown that the shale gas potential of the Yanchang Formation inYan’an area is large, while the Shanxi Formation has been a few researched.In this paper, the author analyzes the mud shale of Shanxi Formation in the Yan'an area through on-site coring and experimental analysis.Based on the analysis of the organic matter abundance, organic matter type and organic matter maturity and the researching of gas composition and carbon isotope characteristics ,the paper discusses the gas-bearing properties and genesis of the shale gas to provide a basis for the future exploration and development.

In this paper, The rock pyrolysis parameters, organic matter abundance and vitrinite reflectance of more than 100 samples from three wells were measured by OGE-II oil and gas evaluation instrument,and the gas content was measured with field canister gas desorption.

The results show that the TOC distribution of Shan 1 member is between 0.05% and 7.46%,with an average of 1.36%, and the hydrocarbon generation potential is between 0.12 and 11.69mg/g.The TOC distribution of Shan 2 member is between 0.04% and 11.5%, with an average of 2.81%,and hydrocarbon generation data distributed in 0.1～8.56mg/g. According to the analysis,Shanxi formation has the material basis of shale gas generation.The organic microscopic components of mud shale are mainly composed of vitrinite and saprolite,followed by inertinite and exinite .The organic matter types are mainly II₂ and III. The Organic matter maturity research shows that the Ro of 66 shale samples was between 0.55% and 3.34%, with an average of 2.01%.Most of the organic matter thermal evolution has reached the stage of high maturity to over maturity.The mud shale in Shanxi Formation of three wells have different degrees gas content.According to 81 samples, the gas content of Shan 2 member is 0.46~5.54cm³/g, with an average of 1.47cm³/g, and the Shan 1 member is 0.37~1.84cm³/g,with an average of 0.82 cm³/g.The gas is mainly hydrocarbons, with the highest methane content, followed by ethane and propane, and non-hydrocarbon gases mainly N₂ and CO₂.The gas belongs to dry gas with dry coefficient being often more than 0.95.The carbon isotope composition has characteristics of δ¹³C₁＜δ¹³C₂positive order distribution.The δ¹³C₂ value is higher than -27.5‰, which belongs to coal type gas.