The pores evolution of lacustrine shale induced by smectite-to-illite conversion and hydrocarbon generation: upper Triassic Yanchang Formation, Ordos Basin, China

The lacustrine shales (e.g. shales deposited in the lake) are important commercial production targets for the oil and gas industrial. The pore network of shales has a significant effect on the occurrence state of natural gas&oil and reserve calculation. The exploration practice has indicated that the pore network of lacustrine shale is very different from that of marine shale. Although many researches have proposed that the pore network of lacustrine shales is influenced by clay minerals, however, the origin, evolution and preservation of pores in the lacustrine shales are still obscure. This study aims to reveal the effect of synchronous hydrocarbon generation and smectiteillite conversion on the formation and evolution of pores in lacustrine shales from the Chang #7 (C7) Member of the Upper Triassic Yanchang Formation in Ordos Basin, China. The middle and lower part of C7 Member is known as the ?Zhangjiatan? shale, which is focused by oil shale exploration recently in China. The combination of analytical techniques was used to investigate the pore evolution related to clay minerals. The clay minerals within shales from the C7 Member consist of illite-smectite mixed layer minerals, illite, kaolinite and chlorite, accounting for 25.9?59.1% of the whole rock. There is a positive relationship between density functional theory (DFT) pore volumes and the content of clay minerals. DFT method is appropriate for all types of the pore, especially micropores. The mesopores (2?50 nm) dominated in the shale samples, while the organic matter only hosted a small amount of micropores ( 2 nm). In the study, it?s shown that the development of mesopores is controlled by the smectite-illite conversion. The mesopores volume and surface areas will decrease with the smectite layers converting into illite. However, the marcopores ( 50 nm) can increase with the formation of illite layers, caused by the minerals volume shrinks and overpressure. The smectite-to-illite conversion is favorable for the pore network under the moderate thermal maturity. The synchronous smectite-to-illite conversion and hydrocarbon generation is a critical mechanism for the formation and preservation of pores in the lacustrine shale.