Evaluation method of hydrocarbon yield of source rocks in open, semiopen, and closed systems

Evaluation method of hydrocarbon yield of source rocks in open, semiopen, and closed systems: A case study on the K1qn Formation, northern Songliao Basin, China

Hydrocarbon yield of source rocks is an important parameter in the evaluation of oil and gas resources, and its value determines the potential of conventional and unconventional oil and gas resources. There is no oil cracking into gas in the thermal pyrolysis experiment data of open system; whereas kerogen cracking into oil in closed system is involved in the calculation of oil cracking into gas. However, most source rocks in sedimentary basins are a process of hydrocarbon generation and hydrocarbon expulsion, which could lead to insufficient understanding of hydrocarbon yield of source rocks. Based on multiple thermal pyrolysis experiment data, three hydrocarbon generation kinetic models, and actual geological data (burial history, thermal history, and hydrocarbon generation threshold), we established the evaluation method and chart of hydrocarbon yield of source rocks under open, semiopen, and closed systems by using hydrocarbon generation kinetics method. The concept of degree of openness was proposed. From a closed system to an open system, the degree of openness increases gradually, and its value changes from 0 to 1. Taking the K1qn Formation in the northern Songliao Basin as an example, the hydrocarbon expulsion efficiency of the K1qn Formation source rock is approximately 70%, and it can be approximated as the degree of openness. Based on the proposed method for evaluating hydrocarbon yield of source rocks, the charts of hydrocarbon yield of source rocks under open system, semiopen system with a degree of openness of 0.7, and closed system of the K1qn Formation in the northern Songliao Basin were established. The oil and gas yields of the K1qn Formation source rocks in semiopen system with a degree of openness of 0.7 are approximately 540 mgHC/gTOC and 105 mgHC/gTOC at a burial depth of 2000 m, respectively. The results show that the hydrocarbon yield of source rocks in semiopen system is closer to the hydrocarbon yield of source rocks under geological condition. We consider the thermal pyrolysis experiment data, hydrocarbon generation kinetics model, and geological data to propose a very valuable evaluation method of hydrocarbon yield of source rock, which has a solid theoretical basis and strong applicability.