Shale Gas Geological Conditions in the Southeast of the Ordos Basin, Central China
Liu, Chenglin; Xu, Zui; Zheng, Shijing; Liu, Jun
The study area of 2619 square kilometers is located in the south of the Jinxi fold belt of the Ordos basin, where coalbed methane exploration began in the early 1980s. Based on geological field survey, seismic data, data of drilling wells and well logs, geochemical and reservoir tests, and basin modeling, we have investigated shale gas geological conditions and calculated shale gas resources at the study area.
The sedimentary environment evolves from the ocean-land transition of the Taiyuan Group of Lower Permian to the land of the Shanxi Group of Lower Permian. For the mudstones of the Taiyuan Group, the total thickness is 20-45m with the mean of 27.2m, total organic content is 0.9-5.4% with the mean of 2.6%, main Kerogen type is II2, and vitrinite reflectance is 1.6-2.1% with the mean of 1.8%. For the mudstones of the Shanxi Group, the total thickness is 40-100m with the mean of 55.2m, total organic content is 1.7-4.7% with the mean of 2.3%, main Kerogen type is II2, and vitrinite reflectance is 1.6-2.3% with the mean of 1.8%.
Main minerals of lower Permian mudstones contain 53.91% of clay minerals, 37.19% of quartz, and 1.63% of calcite. These mudstones' porosity is less than 2% and permeability is 0.001-0.01mD. Their reserving space is mainly made up of matrix pores (including residual primary pores, organic pores, and solution pores) and fissures.
Lower Permian mudstones and coals begin to generate hydrocarbon in Early Triassic and attain the peak of oil generation in Middle Jurassic and the peak of gas generation in Middle Cretaceous. During the process of gas generation and migration, shale gas, coalbed methane and tight gas accumulate in shales, coalbeds, tight sandtones and limestones at the same time.
Gas log recognitions show that the first member of the Taiyuan Group and the first member of the Shanxi Group are rich in shale gas, the third member of the Taiyuan Group and the second member of the Shanxi Group are rich in coalbed methane, and the second member of the Taiyuan Group is rich in tight gas.
Based on analogs with the Barnet shale of American Fort Worth basin and the Longmaxi shale of Chinese Sichuan basin, Lower Permian shale gas resources are calculated to be 260 billion cubic meters for this study area.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013