Differences of marine and transitional shales in the case of dominant pore types and exploration strategies, in China

Abstract

Whether exploration practices of marine shale can be applied to transitional shale remain unclear. To address this issue, it is first necessary to clarify similarities and differences in dominant pore types between the two shales. In this paper, samples were selected from Lower Silurian marine shale and Upper Permian transitional shale, in China. Methods utilized include organic geochemistry, X-ray diffraction, physical property tests and scanning electron microscopy. Results showed that marine shale and transitional shale were similar in OM abundances and thermal maturities, but were different in OM types, mineral components and pore properties. Further, facilitated by image statistics, OM-hosted pores (OMPs) and mineral-hosted pores (MPs) were separated and quantified between the two shales. In particular, more than 90% of pores were OMPs in OM-rich/medium lithofacies of marine shale, whereas above 90% of pores were supplied by MPs in transitional shale. Initially, differences in OM origins and OMP genetic characteristics may trigger the porosity differences. Thermal-genetic OMP, mainly developing in marine shale, features wide distributions, complex surfaces, various sizes, and extensive contributions to pore volume (PV)/pore surface area (PSA). Inheritance OMP, mainly developing in transitional shales, features isolated distributions, smooth surfaces, large diameters, and limited contributions to PV/PSA. Likewise, differences in mineral compositions and MP preservation mechanisms exert influence on the porosity differences. In marine shale, MP is rare and contributes little to PV/PSA due to fierce compaction, cementation and OM occupation. In transitional shale, MP (almost associated with clay flakes) can be protected from cementation and OM occupation, and contributes considerable PV but minimal PSA. It implies that different pore types may impact the reservoir characteristics. Consequently, the exploration targets in marine shales should be OM-rich siliceous lithofacies, whereas targets in transitional shale should be mainly silt-clay lithofacies.