DIAGENESIS AND PORE CHARACTERISTICS OF HIGH-MATURITY MARINE SHALES: A COMPARATIVE STUDY OF LONGMAXI AND NIUTITANG SHALES FROM SICHUAN BASIN EDGE IN CHINA

Lower Silurian Longmaxi Shale and Lower Cambrian Niutitang Shale are two significant shale horizons being studied for unconventional development in China. In this work, we focused on 50 core samples of seven wells from southeastern and southern margin around Sichuan Basin, to better understand the effects of diagenesis on pore characteristics of the high-maturity marine shales. Field emission scanning electron microscopy (FE-SEM), petrology microscopy, low-pressure gas (N₂ and CO₂) adsorption, X-ray diffraction and geochemical analyses were performed on these samples in the two formation shale reservoirs. Results indicate that the Niutitang Shale has higher TOC contents and maturity than the Longmaxi Shale. Meanwhile, the organic matter (OM) pores in the Niutitang Shale are less developed than those in the Longmaxi Shale, due to the higher maturity and stronger compaction. The Niutitang Shale has a stronger clay mineral transformation and compaction than the Longmaxi Shale. Therefore, the inorganic pores of the Niutitang Shale are also less developed than those in the Longmaxi Shale. Moreover, TOC-normalized OM pore volume is dramatically increased with an increase of maturity in the Longmaxi Shale, and then declined rapidly in the Niutitang Shale. The Longmaxi Shale has a higher OM porosity and larger proportion of OM porosity than the Niutitang Shale. Our results show that Longmaxi Shale has a stronger shale gas sorption and storage capacity than Niutitang Shale in the study area, with a greater development potential.