Composition of generated and expelled hydrocarbons and phase evolution of shale oil in the 1^st member of Qingshankou Formation, Songliao Basin

Shales in the first member of the Qingshankou Formation (the Qing 1 Member) in the Songliao Basin exhibit a large variation of organic matter maturity and strong heterogeneity of hydrocarbon mobility. How to analyze the shale oil phase and physical properties has posed a major challenge for efficient shale oil exploitation and production. We investigate the characteristics of shale oil composition evolution by carrying out closed and semi-closed organic matter pyrolysis experiments on low-maturity shale samples, in which compensation correction for light hydrocarbon loss of retained hydrocarbons is performed based on the composition of generated and expelled hydrocarbons. By integrating the burial and thermal evolution histories of typical wells in main source kitchens in the Central Depression, we explore the phase evolution pattern of shale oil. Furthermore, we identify play fairways for light shale oil exploration and outline protection conditions for maintaining production pressure. The results reveal that under geological conditions, light components’ proportions and gaseous hydrocarbon content in shale oil increase progressively with the maturity of organic matter. Concurrently, the phase envelopes evolve from high dew point temperature (DPT) and low bubble pointpressure(BPP) toward low DPT and then to high BPP with an increase in the organic matter maturity. In the Qijia-Gulong Sag, the shale oil reservoirs of the Qing 1 Member evolved into light oil reservoirs in the middle stage of the Nenjiang Formation deposition. In the Changling Sag, these reservoirs began to evolve into light oil reservoirs at the end of the Nenjiang Formation deposition. In contrast, these reservoirs in the Sanzhao Sag have been consistently present as black oil reservoirs. Shale oil within both black and volatile oil reservoirs remains in a single liquid phase. In the Central Depression of the Songliao Basin, the light shale oil reservoirs of the Qing 1 Member are primarily distributed at the center of the Qijia-Gulong Sag and in the northern Changling Sag, with maturity of organic matter(R_o) varying from 1.3 % to 1.6 % and formation pressure from 12.2 to 22.4 MPa.