Lacustrine Shale Gas Exploration in Yanchang Exploratory Block
Wang, Xiangzeng; Zhang, Lixia; Jiang, Chengfu; Sun, Binghua; Gao, Chao; Fan, Bojiang; Guo, Chao; Wan, Yongping; Sun, Jianbo; Hu, Haiwen; Yanchang, Shaanxi
Shale gas has received widespread interest because of their emergence as clean energy. However, shale gas exploration is limited within the marine shale; research on terrestrial shale is extremely poor.
Terrestrial dark shale is well developed in Chang-7 Member in Triassic Yanchang Formation in Ordos Basin, China. The research area, Yanchang Exploratory Block, covers the southeast part of Ordos Basin. The successful ignition of Well Liuping 177 indicates the breakthrough in lacustrine shale gas exploration. Comparing with marine shales in North America, the lacustrine shale has its own unique characteristics in the following aspects.
Reservoir Sedimentology: The sedimentary facies is deep lake or shore-shallow lake deposits. Thickness of individual organic-rich shale beds ranges from 15 to 45 m. The organic matter was derived from higher terrestrial plants and is rich in fossil stem, lithophyll, and charcoal debris.
Geochemical Characteristics: Kerogens include sapropel and mixed types, mainly Type II. TOC content ranges from 1.76 to 5.88%. Vitrinite reflectance value ranges from 0.52 to 1.25%.
Reservoir Characteristics: The lacustrine shale is composed of a variety of minerals. The content of clay minerals is relatively high and ranges from 37.4 to 72.8%. The content of brittle minerals is low and ranges from 20 to 30%. Sandy interbeds are present; and the porosity and permeability of sandy shale are larger than those of pure shales.
Gas Bearing Characteristics: The average gas content is 6.86 cm3/g according to results of desorption experiments. The average adsorbed gas is 1.64 cm3/g under the subsurface pressure according to the isothermal adsorption. Shale gas occurs in association with petroleum extensively.
Our results show that the lacustrine shales are characterized by consistent thickness of single beds, high TOC content, abundant gas prone matrixes, multiple sandy interbeds, and high porosity and permeability and high gas content. They are all favorable for the formation shale gas system. These understandings on terrestrial shale provide us guidelines to select favorable exploration targets.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013