Enrichment mechanisms and accumulation models of deep tight sandstone reservoirs in Junggar Basin: A case study of Moxizhuang and Yongjin areas

Commercial oil and gas flow was discovered in the Jurassic sandstones of Moxizhuang and Yongjin areas in the central Junggar Basin. The reservoir porosity and permeability are very low in some areas where the buried depth of the Jurassic reaches 6 000 m， belonging to ultra-low porosity and ultra-low permeability reservoirs. In such tight reservoirs， the charging power of crude oil， the source of oil and gas， and the migration process need to be systematically discussed. Biomarkers were used to determine the source of oil and the main source rocks of crude oil in tight sandstone reservoirs to reveal the enrichment process and main controlling factors of crude oil in tight sandstone reservoirs. The slip distances of the faults in horizontal and vertical directions and the sealing properties of the faults were studied by quantitative and semi-quantitative methods， and the relationship between fault size and oil test production was clarified by combining statistics and geological analysis. Moreover， the influence of fault size and fault distance on oil and gas accumulation was explored. Micro-CT scanning， constant-rate mercury injection， nuclear magnetic resonance （NMR）， and physical simulation were applied to systematically describe and characterize the pore structure parameters of sandstones and fluid accumulation models in these tight reservoirs. The results show that the crude oil is mainly from the Permian source rocks. The faults with large sizes and high activity intensity are vertical migration channels of oil and gas， and the faults with horizontal and vertical movement distances of more than 700 m and a size of more than 20 km at geological time have better oil and gas transport capacity. However， the faults with small sizes and short movement distances are mostly closed. The pore sizes of the tight reservoirs are small， mainly in the range of 163.8-207.7 μm， and the throats are 0.5-8.1 μm. The heterogeneity of the micro-pore structures leads to the differential charging and enrichment of crude oil. The oil content of reservoirs with medium porosity and coarse throat is the highest， followed by the reservoirs with large porosity and small throat， and the oil content of reservoirs with small porosity and micro throat is the lowest. The accumulation of Jurassic tight sandstone reservoirs is mainly influenced and controlled by high activity intensity， large faults， and pore structures of tight sandstone.