Occurrence characteristics and factors that influence shale oil in the Jurassic Lianggaoshan Formation, northeastern Sichuan Basin

Abstract

Well Ping'an 1, located in the northeastern Sichuan Basin, tested 112.8 m³/d of oil from the Lianggaoshan Formation, marking an important breakthrough in shale oil production. However, due to the influence of complex lithofacies, high thermal maturity, and limited exploration, the occurrence and factors that influence shale oil across the different lithofacies of the Lianggaoshan Formation remain unclear, thereby severely impeding exploration and development. This study systematically examines the occurrence and factors that influence shale oil (adsorbed oil and free oil) in the Lianggaoshan Formation through an integrated approach involving petrology, geochemistry, scanning electron microscopy, nuclear magnetic resonance spectroscopy, multi-temperature pyrolysis, and gas chromatography. Results demonstrate the following: (1) Residual shale oil (adsorbed oil) in the study area predominantly exists as oil films, precipitated oil, and residues (asphalt). Oil films appear on the edges of organic matter (OM), clay minerals, pyrite, and felsic minerals. Crude oil adheres as thin layers to mineral edges, with part of the oil resulting from the lipophilic properties of OM. Precipitated oil tends to accumulate around organic pores, and at higher maturity stages, heavy oil components adhere to mineral surfaces as residues. The hydrocarbon loss of the sample is serious, and free oil is difficult to observe due to experimental processing. (2) Based on the experimental results of preserved cored samples, the shale oil in the study area is mainly free oil (0.86–4.46 mg/g, average 1.99 mg/g), followed by adsorbed oil (0.0007–1.36 mg/g, average 0.43 mg/g). The presence of higher concentrations of clay minerals and total organic carbon (TOC) correlates with increased quantities of free and adsorbed oil in the Lianggaoshan Formation. (3) The two-dimensional nuclear magnetic resonance method combined with the “e” index method was employed to quantify the free oil volumes in shale reservoirs of varying maturity within the study area. The recovery coefficient initially increases rapidly and then gradually with increasing R_o (vitrinite reflectance) values. Compared with other lithofacies types, OM-rich laminated argillaceous shale and OM-poor laminated siliceous shale have higher free oil content, which makes them easier to produce, in other words, these two lithofacies should be preferred as favorable exploration and development targets.