Organic matter enrichment patterns and sedimentary environments of shale oil: a case study on the second member of the Funing Formation in the Gaoyou Sag, Subei Basin, China

Located within the Subei Basin, the Gaoyou Sag is abundant in hydrocarbon resources and exhibits significant potential for shale oil exploration and development. However, the organic matter enrichment patterns and palaeoenvironmental evolution of the second member of the Funing Formation (E 1 f 2 ) in the Huazhuang area remain poorly understood, particularly regarding the complex origins of organic matter and their coupling with sedimentary environments. To address these issues, representative shale and crude oil samples from the E 1 f 2 interval were systematically analysed using gas chromatography-mass spectrometry, X-ray diffraction and inductively coupled plasma mass spectrometry. The results show that the average pristane/phytane (Pr/Ph) ratio is 0.58, the V/Cr ratio is close to 1 and the average V/(V + Ni) ratio is approximately 0.7. The Sr/Cu ratio is significantly greater than 10 in the lower section but markedly less than 10 in the upper section. The Sr/Ba ratio ranges from 0.06 to 1.23, progressively decreasing from bottom to top. A comprehensive analysis indicates that the lower E 1 f 2 was deposited in a saline lacustrine anoxic environment, while the upper E 1 f 2 transitioned to brackish-water conditions under a warm and humid climate. Based on the distribution of the C 27 –C 29 steranes, the organic matter is determined to have been mainly derived from a mixture of lower aquatic organisms and terrestrial higher plants. The warm–humid climate, decreasing water salinity and bottom-water anoxia jointly promoted the enrichment and preservation of organic matter. This study establishes an organic matter enrichment model co-controlled by palaeoclimate and palaeosalinity, providing a theoretical basis for shale oil exploration in similar lacustrine basins.