Development characteristics of Lower Cretaceous reservoir bitumen and its relationship with oil and gas genesis in the Lishu Rift, Songliao Basin

Abstract

Different types of bitumen are developed in the Lower Cretaceous reservoirs of the Lishu Rift, Songliao Basin. However, its formation time and genesis are not clear, and the relationship between its development characteristics and the genesis of natural gas is close. To strengthen the research on the bitumen and hydrocarbon genesis will help to define the next direction of natural gas exploration. Therefore, by using various geochemical analysis techniques, including fluid inclusion observation and temperature measurement, laser Raman spectroscopy and natural gas composition, the occurrence state, maturity, formation time, and genesis of different types of bitumen are studied, then different types of oil and gas reservoirs in the study area are identified, and the connection between the development degree of bitumen and the hydrocarbon genesis is clarified. The following research results are obtained. First, colloid-asphaltene bitumen and carbonaceous bitumen are developed in the Lower Cretaceous reservoirs of the Lishu Rift. Carbonaceous bitumen exhibits clear D and G peaks in Raman spectra, and is regular and half-filled or fully filled in the intergranular pores, and lacks fluorescence. Colloid-asphaltene bitumen has various occurrence states, such as vein filling in the fractures or irregular filling in the pores, with brown fluorescence, and weak or undeveloped D peak in the Raman spectrum. Second, carbonaceous bitumen was formed by the cracking of crude oil during the deposition of the Upper Cretaceous Nenjiang Formation and has the characteristics of higher maturity. Compared with carbonaceous bitumen, the reflectivity of colloid-asphaltene bitumen is lower, which is mainly formed by oil and gas migration and differentiation during the hydrocarbon accumulation period. Third, there are various types of oil and gas reservoirs in the study area. The Houwujiahu, Gujiazi and Jinshan areas are dominated by gas reservoirs; the Pijia, Shiwu, Siwujia and Bawu areas are dominated by gas reservoirs and condensate gas reservoirs without oil rings; the Sujiatun and Qinjiatun areas are dominated by condensate gas reservoirs with small oil rings and without oil rings. In conclusion, the development degree of bitumen is closely related to the genesis of natural gas. The gas reservoirs of the Houwujiahu, Gujiazi, and Jinshan areas have a low content of carbonaceous bitumen, which is coal-type gas formed by humic kerogen cracking. The gas reservoirs and condensate gas reservoirs without oil rings in the Pijia, Shiwu, Siwujia, and Bawu areas have higher content of carbonaceous bitumen, which is caused by cracking of kerogen or oil. The content of carbonaceous bitumen of condensate gas reservoirs with small oil rings and without oil rings in the Sujiatun and Qinjiatun areas is higher than 0.25%, which is mainly caused by crude oil cracking.