A NEW MODEL FOR CALCULATION OF GAS CONTENT IN TERRESTRIAL SHALES: A CASE STUDY IN THE KUQA DEPRESSION, THE TARIM BASIN, WESTERN CHINA

The shale gas content is an important index for the shale gas potential and reserve evaluation. In this study, the main controlling factors of each parameter in Langmuir model and Ambrose model are identified by various experiments and analytic methods, including indirect prediction method, isothermal adsorption test, XRD analysis, Total Organic Carbon (TOC) measurement, pyrolysis and porosity measurement. The least square method and the Levenberg-Marquardt method are used to optimize the parameters in each model. With analysis above, a new calculation model that is applicable to the shales in the Kuqa Depression, the Tarim Basin, which is the case study area, is established. A further research on influential factors of gas content in Well YN2 is implemented, which suggests the gas content of terrestrial shales to be mainly influenced by TOC content and less effected by the contents of clay minerals and quartz. The main parameters in the new calculation model as optimized are determined to be TOC, depth and gas saturation. The Jurassic shale gas in Well YN2 is speculated to be mainly adsorption gas, with a dominant proportion of 74.92%~90.10% in the total gas content. As the formation depth grows, the free gas content rises continuously, whereas the adsorption gas content first grows and then approaches the equilibrium or even tends to decrease slightly. The target layer, Yengisar Formation, has a great potential in terms of shale gas reserves, with an average total gas content of 6.14 m³/t.