Influencing factors of micropores in the graptolite shale of Ordovician Pingliang Formation in Ordos Basin, NW China

Abstract

Multiple tests were conducted on graptolite shale samples from Middle Ordovician Pingliang Formation in three typical areas in Ordos Basin to investigate the relationship between micro-nano pore structure, graptolite content, rock composition, TOC, maturity, main and trace elements and gas content. The graptolite of Pingliang Formation concentrates in the black shale at the lower section of this layer. Pores in the shale are diverse in types, including pores made by extracellular polymeric substance (EPS), in bio-graptolite body, as inter-granular void of clay minerals, intra-granular, interstitial space between mineral crystals, micro-fractures, and intra-granular dissolution structures etc. Tests show that graptolite affected the sedimentary environment and shale gas accumulation; graptolites content is positively correlated to TOC, in a certain range, specific surface area is positively correlated to TOC and maturity, etc., as overall a lithologic inorganic and organic ratios. The rare earth elements (REE) patterns of the three areas are similar, indicating the same provenance. REE, Fe, Al and Ti content are negatively correlated to TOC, indicating the high REE content is not caused by organic matter enrichment, but related to the absorption of minerals particles. Gas content is positively correlated to specific surface area, TOC and maturity. The above factors contribute to space and size distribution of micro-nano pores in shale. Key words: graptolite shale, pore type, geochemical parameters, Ordovician Pingliang Formation, Ordos Basin