Lithomechanical Properties and Regional Stress Field Analysis of the Lower Silurian Longmaxi Formation Shale in the Central Part of Guizhou Province, China

Di Yang¹, Shugen Liu¹, Yuming Shan¹, Bo Ran¹, Chao Luo¹, Yuehao Ye¹, Shiyu Wang¹, Hongcheng Wan², and Jinming Song¹
¹State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China;
²SINOPEC East China Branch, Nanjing, China

The Lower Silurian Longmaxi Formation shale is one of the most important petroleum source rocks in the Lower Paleozoic strata, southeast of Sichuan basin, China. The shale series is also one of the main targets for shale gas exploration in China. The mudstones, argillaceous siltstones, fine sandstones, gritstones and even thin interbeded fine-grained conglomerates are all included in the term of ‘shale series’ in our studies. As we all known, shale gas production is more effective by hydraulic fracturing based on opening natural fractures. Therefore, it is important to know about the 3-D stress field and the distribution of nature fractures in the studied shale.

The research samples are collected from Qilong village (the Lower Silurian Longmaxi Formation), Xishui, Guizhou province, China. Through the analysis of the joints in the field outcrop, 3-D lithomechanical properties under simulated formation conditions and the characteristics of microscopic fracture development in the laboratory, the following results have been arrived.

(1) From bottom to top of the outcrop section, the TOC contents of the shale are 0.31%~6.69%, with the average of 2.48%; the quartz contents are decreased from 65% in Wufeng Formation to 22% in Longmaxi Formation; and the carbonate contents are increased;

(2) 3-D lithomechanical properties display that the elastic deformation is the leading deformation. Rock rupture patterns are mainly for tensile failure and monoclinic shear failure. With confining pressure increasing, rock compressive strength are increased in different directions, but the Poisson's ratio, Young's modulus and axial strain are increasing only in the direction perpendicular to strata. Under the same confining pressure conditions, the horizontal minimum principal stress is down-dip (Y) in Xishui area, which is coincident with the developing directions of fractures in these areas;

(3) With the increasing of confining pressure, the micro fractures and rupture degree are increased and the effectively connecting pores are also increased.

(4)The dominant fracture direction is NW270-300° and NW350-360° in Xishui area through outcrop measurements. So, it is favorable and potential for shale reservoirs because there would form abundant fractures and micro-fractures during multistage tectonic movements in Sichuan basin.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013