Reservoir Characteristics of the Upper Paleozoic Shale in Xiangzhong and Xiangdongnan Depression, Hunan Province, China

Jiaju Liang¹, Fanghao Xu², Shaofeng Bu¹, Qing Yu¹, and Jing Qian¹
1Chengdu University of Technology, Chengdu, Sichuan, China
2University of Regina, Regina, SK, Canada

The capacity of a shale reservoir is primarily related to its mineral components. Quartz is kind of compact-grained in brittle minerals, which is one of the causes that decreases the original pores and thin fractures in the shale reservoirs. The cementation of the calcite, on one hand, would decrease the reservoir volume of the shale reservoirs. On the other hand, the increasing of the content of calcite would enhance the brittleness of the shale reservoirs. Normally, shale with high content of clay minerals accompanied with strong plasticity, which mainly developed planar-cracks, and that has a negative effect on the post-transformation of the shale. However, there are more micro-pores and higher specific surface area than other minerals in same volume, which enables it to adsorb a large amount of shale gas. Therefore, the determination of the ideal mineral components of the shale reservoir should take the contents of both the clay minerals and brittle minerals into consideration in an attempt to find the most favorable balance.

The shale layers of the upper Paleozoic stratas in the study area contain the Tiaomajian Formation (D1t), the Qiziqiao Formation (D1q), the Shetianqiao Formation (D1s) and the Xishankuang Formation (D1x) of Devonian; and the Ceshui Member of Carboniferous(C1d); the Longtan Formation (P1l) and Dalong Formation in Permian(P1d). The shale consists predominantly of dark gray mudstone, gray-black shale and dark gray lime mudstone. The mineral composition of shale is complicated. The content of the clay mineral is ranging from 18% to 75%, with an average of 40.7%. The contents of the detrital minerals, like quartz, feldspar, and authigenic minerals, like calcite and dolomite, fluctuate greatly. The content of quartz is between 7% and 51%, and 31% in average. The average content of calcite is 21.7%. Some of the samples also contain pyrite, which indicates a reducing environment. Although the average content of quartz only reaches 31%, the total contents of the brittle minerals can reach up to 57%, which will greatly benefit the fracturing exploitation of the shale reservoir.

The methods of Field emission scanning electron microscopy, specific surface testing, isotherms adsorption and desorption have been applied for the capacity evaluation of the shale reservoir. The main reservoir space of the shale in the study area can be divided into the following types: frame pores, dissolution pores, organic pores, organism cavities and micro-fractures, etc. The shale from the Longtan Formation and Dalong formation in Permian has a higher specific surface area, while the shale from the Ceshui Member in Carboniferous and the Shetianqiao Formation of Devonian possess larger pores. The shale with higher specific surface area is more capable of adsorbing gas while the shale with larger pores shows an advantage in storing free gas. The adsorption and desorption curves of the majority shale samples from the study area are comparatively flattened and take the form of anti-S, which indicates that their pore throats are well sorted and a favorable environment for the seepage flow is created , thus benefiting the future exploitation of shale gas.

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