Distribution and characteristics of paleo-oil reservoirs in Cambrian Longwangmiao Formation in Anyue area of Sichuan Basin, China

The Anyue gas field is a giant marine gas field with the largest reserves of a single reservoir and the oldest strata in China. In this area, the proved gas reserves in the Cambrian Longwangmiao Formation are 4, 404×10⁸ m³, which are believed to originate from crude oil cracking according to substantial evidences. The paleo-oil reservoirs in the area have been destroyed, and their fluid properties, scale and other characteristics are unclear, despite of the distribution preliminarily determined. In this paper, the paleo-oil reservoirs in the Cambrian Longwangmiao Formation in the Moxi–Gaoshiti area were analyzed thoroughly through conventional microscopic observation, together with quantitative grain fluorescence (QGF), total scanning fluorescence (TSF), and micro-laser Raman analysis. The study results indicate that the Longwangmiao Formation contains bitumen, which is mainly distributed in four types of pore space, i.e. vein-like fractures, dissolution pores, intergranular pores, and intercrystalline pores. The bitumen in vein-like fractures was formed mostly in relation to hydrocarbon migration, while the bitumen in dissolution pores, intergranular pores and intercrystalline pores was formed by in-situ cracking of petroleum, mostly as a result of thermal alteration. The QGF index suggests the existence of paleo-oil reservoirs in certain scale in the study area, and the high QGF-E intensity indicates the property of residual bitumen. The laser Raman spectroscopy reveals the maturity of the extensive solid bitumen in reservoir rocks. The well-cross section illustrates that the paleo-oil reservoirs are mainly layered, have experienced mostly in-situ cracking, and match with the present gas reservoirs. (paper)