Hydrocarbon occurrence characteristics and charging differences in Chengbei Fault Zone based on fluid inclusions

Abstract Hydrocarbon accumulates in multiple layers of the high, middle and low parts of Chengbei fault zone, in vicinity of Qikou Sag and Qinan Subsag, which represents a breakthrough for large-scale hydrocarbon exploration in recent years. Based on the size, abundance, and attitute of hydrocarbon inclusions, as well as their microscopic polarization and fluorescence characteristics, the hydrocarbon occurrence and the charging difference in the high, middle and low faulted terraces were analyzed systematically in this paper. There are two types of occurrence in the intergranular pores and fractures in the reservoirs, namely, the medium to light oil that shows green fluorescent light, and the solid bitumen charged with a certain amount of hydrocarbon that does not show any fluorescent light. This indicates that the zone has experienced at least two stages of hydrocarbon migration and accumulation, and that diversified forms of hydrocarbon exist in the reservoirs of the zone. The characteristics of hydrocarbon inclusions show that two stages of inclusions were found in the tectonic positions of different faulted terrace. The first-stage inclusions were developed in the early secondary enlargement of quartz grains, with a low degree of development abundance; while the second-stage inclusions were developed after the secondary enlargement of quartz grains, with a high degree of development abundance. The second-stage hydrocarbon inclusions of the reservoir samples obtained at tectonic positions of the high, middle and low fault terrace represent the main filling stage of the present-day reservoirs. Their fluorescence characteristics demonstrate the oil charging difference, which is the maturity of the hydrocarbon decreases as the terraces lift up along the slope. Identifying the hydrocarbon occurrence and charging differences from the perspective of inclusions is conducive to recognizing the regularity of hydrocarbon charging and migration and the distribution patterns in fault terrace zones, and lay the foundation for hydrocarbon exploration of slopes in the future.