The genesis of bitumen and its effect on hydrocarbon accumulation

In recent years, large amounts of bitumen have been discovered in the Permian Lower Wuerhe Formation (P(2)w) reservoir exploration of the Mahu Sag on the northwestern margin of the Junggar Basin. However, bitumen genesis and its influence on oil accumulation remain unclear, affecting the subsequent exploration and development in this area. Therefore, this study analyzed the genesis and source of bitumen and its influence on hydrocarbon accumulation through chromatography, microscopic observation, inclusion temperature tests, and reservoir physical property experiments. Studies have shown that the crude oil produced in the early period of the Fengcheng Formation (P(1)f) was destroyed due to tectonic movement. Crude oil undergoes biodegradation, where the lighter components are dispersed and lost. The remaining heavy components undergo high temperature conditions during long-term burial to form bitumen. Microscopic observation showed that the overlying reservoir has a higher bitumen content and the underlying reservoir has a lower bitumen content. After the bitumen was dissolved, the porosity and permeability of the overlying reservoir increased by 6.77% and 34.67 mD from the original 3.53% and 3.41 mD, respectively. The porosity and permeability of the underlying reservoir increased by 3.21% and 27.68 mD from the original 5.86% and 1.51 mD, respectively. This shows that the initial physical properties of the overlying reservoir were better than those of the underlying reservoir. Therefore, early crude oil is more likely to enter the overlying reservoir, resulting in a higher bitumen content. The difference in the homogenization temperature of inclusions and the maturity of bitumen indicates that the oil and gas accumulation in this area has undergone two stages. Early filling with oil occurred in the late Permian, but the reservoir was destroyed because of tectonic movements. The residual components formed bitumen during the long burial, which occupied most of the reservoir space and made it difficult for late crude oil to enter the pores.