Occurrence state and oil content evaluation of Permian Fengcheng Formation in the Hashan area as constrained by NMR and multistage Rock-Eval

Abstract

The Hashan area, neighboring the Mahu Sag that is rich in the shale oil resources, showed commercial oil flow in the corresponding lacustrine shales of the Lower Permian Fengcheng Formation (P1f) with reserve scale approximately 789 million tons, presenting great potential for oil exploration. Despite their geographical proximity, the hydrocarbon occurrence and oil-bearing capacity of shale in the Hashan area and Mahu Sag greatly differ owing to the complex tectonic evolution. Therefore, understanding the occurrence state and oil content of the P1f in the Hashan area is crucial for ongoing shale oil exploration activities and the development of the northwestern margin of the Junggar Basin. In this study, an integrated investigation, including petrological observations, scanning electron microscopy (SEM) observation, analysis of nuclear magnetic resonance (NMR) T1–T2 spectra, and conventional and multistage Rock-Eval pyrolysis methods were conducted to evaluate the occurrence state and oil content of the P1f shale in the Hashan area. The results indicate that plagioclase (average 30.7%) and quartz (24.1%) dominate the mineral compositions of the P1f shale samples. A method involving quartz–plagioclase–carbonate minerals is proposed to conduct lithofacies classification. In the Hashan area, the organic matter abundance in the P1f shale is scaled in fair to good range, the thermal maturity ranges from immature to early mature stage, and the primary organic matter types are Types I and II1. Intergranular and dissolution pores are the two most common pore types. The free oil is mostly found in the pores and microfractures of the mineral matrix, whereas the adsorbed oil is mostly adsorbed on the surfaces of kerogen and clay minerals. The high organic matter abundance, quartz content, and porosity account for substantial increase in the oil content, the area rich in shale oil resources coincides with that rich in free oil. The most favorable lithofacies in the Hashan area is the calcareous mudstone/shale, which hosts the highest free oil content (average 2.49 mg), total oil content (15.02 mg/g), organic matter abundance (TOC = 1.88% and S1+S2 = 20.54 mg/g) and porosity (5.97%).