Properties of the Lower Cambrian Shale in the Western Sichuan Basin (South China) and Their Effect on Shale Gas Potential

Abstract

The Lower Cambrian Shale (LCS) in the Sichuan Basin in Southwest China has been considered as a potential shale gas play. Integrated characterization on the properties of LCS of Qiongzhusi Formation from Well JY1 located in the western Sichuan Basin is conducted to evaluate its shale gas potential. Trace element ratios V/Cr and Ni/Co reveal that LCS was deposited in an anoxic condition and the abundance of biogenic Ba implies high organic matter productivity in the extensional area inherited from the Late Ediacaran Xingkai rifting event in the western Sichuan Basin during early Cambrian. Organic matter is characterized by 19 % sapropelic and 81 % secondary maceral and average total organic content (TOC) is 2.1 %. The LCS is in dry gas window suggested by Ro greater than 1.3%. The LCS is composed of quartz (averaging at 41.6 %), clay minerals (averaging at 38.7 %) and minor other minerals (e.g. pyrite and carbonate). Porosity ranges from 0.16 % to 6.21 % (averaging at 2.39 %) and low permeability ranges from 0.00305 md to 0.0134 md (averaging at 0.00741 md). 3 types of pores developed: (1) organic pore, (2) inorganic pore including intergranular pore, intragranular pore, interlayer pore of clay minerals and intergranular pore of pyrite, and (3) microfracture including structural microfracture and diagenetic microfracture. Nano-CT images suggest that the connectivity of organic pores increases with the increase of TOC. Positive relationship does not exist between porosity and TOC, which is different from the currently producing Lower Silurian Longmaxi shale with elevated organic porosity for higher TOC shale. The porosity in LCS is positively correlated to Ro for shale with Ro between 1.3 % and 2.0 %, however, it becomes negative with Ro ＞ 2.0 %. Combined measurement of high pressure mercury test and nitrogen adsorption test shows that about 50 % to 70 % pores are 2-10 nm in pore diameter. There exist positive correlations between sorption gas capacity, BJH pore volume, and BET surface area, which indicates the microporous shale could provide massive surface and volume at the microscale for gas to be absorbed. These results clearly indicate that LCS in areas with high TOC, higher porosity and maturity around 2% in the broad synclines and anticlines in the western Sichuan Basin has huge shale gas potential. The unconformity and karst caused by Tongwan movement below the LCS in the central and eastern Sichuan Basin could breach the shale gas accumulation.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018