Reservoir characteristics and controlling factors of the middle–high maturity multiple lithofacies reservoirs of the Lianggaoshan Formation shale strata in the northeastern Sichuan basin, China

Abstract

The Lianggaoshan Formation, which is located in the northeastern part of the Sichuan Basin, is an important shale oil producer. However, due to the effects of complex petrography, a wide range of maturity, and lower organic matter (OM) content in comparison to other areas, the pore characteristics of different lithofacies are unclear, which severely limits exploration and development. Therefore, by using geochemical, petrological and pore structure characterization experiments, this study thoroughly examines the pore properties of several lithofacies in the Lianggaoshan Formation in Northeast Sichuan. The deposits in the study area are OM-rich laminated argillaceous shale (ORLAS), silty mudstone (SM), OM-low laminated siliceous shale (OLLSS), organic-laminated argillaceous shale (OLLAS), OM-low bedded shale (OLBMS), and (fine) siltstone (FS). With higher porosity and macropore + microfracture volume ratio, ORLAS and OLLSS have better reservoir qualities than the other lithologies, while the reservoir properties of SM and FS are the worst. Hydrocarbon generation and diagenesis affecting pore development include thermal maturation of OM, mechanical compaction, dissolution, and cementation. The porosity and medium–large scale pore volume ratio show a tendency to increase and then decrease with maturity, reaching their highest values at an Eq range of (Ro) = ∼1.6%. Dissolution greatly contributes to pore development. The pore space is primarily sealed by cementation and compaction, which is most evident in SM and FS. In addition to thermal maturity, factors affecting the reservoir properties include clay minerals, siliceous minerals, and total organic carbon (TOC). Clay minerals and OM often form OM–clay mineral complexes, which can promote hydrocarbon generation and pore enlargement through interaction. The properties of the reservoir significantly improved when TOC and clay contents increased. However, the lithology of the study area, which contains a significant concentration of siliceous minerals, is unfavorable to reservoir properties.