Petrology of Siltstone Laminae in Zhangjiatan Shale of the 7th Member of Yanchang Formation and Their Significance for Shale Gas, Ordos Basin, China

Lei, Yuhong; Wang, Xiangzeng; Luo, Xiaorong; Zhang, Lixia; Zhang, Likuan; Jiang, Chengfu; Cheng, Ming; Yu, Yuxi

Zhangjiatan shale in the 7th Member of Yanchang Formation, Ordos Basin, China, is a potential reservoir of continental shale gas. It appears that laminated siltstone laminae affect gas accumulation in and production from the shale. The role of siltstone laminae in shale gas accumulation is assessed on the basis of their petrologic features and spatial distribution using outcrop, core, well-log, and microscopic and SEM data. The siltstone laminae occur mainly in the upper and lower parts of Zhangjiatan shale and are 0.2-2 mm thick and 5-30 laminae/m in distribution (i.e. density). Siltstone/shale ratio ranges from 5 to 25%. Framework grains are quartz and feldspars. Clay minerals are mainly illite and parallel to bedding plane. Minor fossil and organic debris and pyrite are present. Pore types include primary inter-granular among framework grains, dissolutional in feldspar, crystalline in cements, inter-granular around clay minerals, intraparticle in organic matter, and fracture. Low-angle cracks occur in shale where siltstone laminae are present. Some cracks are open, while others cemented by calcite. The number of cracks apparently increases with the density of siltstone laminae. Some cracks grew along bedding plane. Such phenomenon results in anisotropic hydraulic property: the horizontal permeability of the shale is 2-5 times larger than the vertical permeability. Results of water emersion test of shale cores indicate release of natural gas generally along siltstone lamination surfaces. Collected gas volume correlates positively with the density of siltstone laminae. Hence, siltstone laminae not only provide pore space for gas accumulation, but also serve as carriers for shale gas flow into boreholes. Understanding the depositional environments and diagenetic processes of siltstone laminae is important to predict sweet pots and assess shale gas resources.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013