Variation in Pore Space and Structure of Organic-rich Oil-prone Shales from a Non-marine Basin: Constraints from Organic Matter and Minerals

Organic matter(OM) and minerals are major particle components of lacustrine shales. The influence of OM and minerals on pore space and structure in organic-rich oil-prone shales containing a large range of total organic carbon(TOC)contents is poorly understood. In this study, we investigated the variation in pore space and structure of low mature lacustrine shales in the Songliao Basin(NE China), based on a study of the mineralogy, petrography, geochemistry, and geophysical properties of shales. Different pore types make markedly different contributions to the mineral surface area(MSA) and pore volume(PV) of the shales. There exists a negative correlation between MSA/PV and TOC in mesopores(r~2 = 0.75/0.65) and macropores(r~2 = 0.74/0.68), and a positive correlation in micropores(r~2 = 0.59/0.64), which are associated with the variation of mineral and TOC contents. A positive relationship between the throat/pore ratio and TOC(r~2 = 0.82) shows an increase in throat radius and decrease in pore radius with increasing TOC content. This relationship is supported by the reduction in mean pore diameter(MPD) for large pores and increase in MPD for small pores. These variations are related to the decreased pores by quartz plus feldspar(Q + F) content, increased throats by clay minerals, and enhanced pore-fill by OM. We propose that the variation of OM and minerals is a key control on the pore space and structure of low mature organic-rich oil-prone shales.