Geological Characteristics of Chang 7 Lacustrine Shale, Ordos Basin, China

Xiangzeng Wang, Lixia Zhang, Chengfu Jiang, Jiangbo Sun, Chao Guo, Yongping Wan, Binghua Sun, Bojiang Fan, and Chao Gao
Shaanxi Yanchang Petroleum (Group) CO., LTD., Xian, China

Shale gas has received widespread interest because of their emergence as clean energy. As the source rock for shale gas, shale is widely studied by researchers. The studies on lacustrine shale is extremely poor at present, though the there has long been progress on the studies on marine shale.

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.

The sedimentary environments of Chang 7 shale are deep lake facies and shore-shallow lake facies. The thickness of individual organic-rich shale beds ranges from 15 to 45 m. The matrix types of Chang 7 shale include Type Ⅰ and Type II Kerogens. The Content of Total Organic Carbon content ranges from 1.76 to 5.88%. The Vitrinite reflectance value ranges from 0.52 to 1.25%.

The reservoir of Chang 7 shale includes sandy interlayer, sandy shale and sheer shale. The reservoir spaces of Chang 7 shale include primary intergranular pores, secondary dissolved pores, organic pores, tectonic fractures and lamellation fractures. The porosity and permeability is favorable in the sandy interlayer, with an average porosity of 7%, and average vertical permeability of 0.000923md. The property of porosity and permeability is relatively poor for sandy shale. The property of porosity and permeability is extremely poor in the sheer shale, with the porosity ranging from 1.69% to 3.54%, the vertical permeability ranging from 0.000058md to 0.000091md.

Most of the Chang 7 shale samples contain shale gas. The average gas content is 6.86 cm³/g according to the desorption experiments results. The average adsorbed gas is 1.64 cm³/g under the subsurface pressure according to the isothermal adsorption results. For part of the Chang 7 shale samples, the shale gas occurs in association with liquid oil.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013