Exploration status of the deep Sinian strata in the Sichuan Basin: Formation conditions of old giant carbonate oil/gas fields

Abstract

To construct a giant Sichuan gas province, it is in an urgent need of exploring new replacement areas with abundant natural gas resources and great exploration potential. In this paper, the formation conditions (e.g. source rock, reservoir and play) and the distribution laws of the deep-seated old giant marine carbonate oil/gas fields in China's Craton Basins were investigated systematically. Then, the accumulation and enrichment conditions of deep Sinian natural gas in the Sichuan Basin were analyzed. Finally, potential and favorable target zones of natural gas exploration in the Dengying Formation of Upper Sinian were evaluated. And the following research results were obtained. Firstly, the effectiveness and scale of source kitchens, reservoirs and reservoir–caprock assemblages and the proximity are the necessary conditions for the formation of deep-seated giant carbonate oil/gas fields, and paleo-uplifts, paleo-slopes and paleo-fault zones are the favorable areas of searching deep-seated giant carbonate oil/gas fields. Secondly, in the Sichuan Basin, three sets of high-quality source rocks are developed in the Neoproterozoic–Cambrian System, and their maturity of organic matter is still in the optimal window for cracking gas, so the gas generation scale is large. Thirdly, after the microbial carbonate rocks of Dengying Formation in the Sichuan Basin were reconstructed by constructive diagenesis, effective reservoirs are formed and distributed widely. Fourthly, owing to the integrated source rocks and cap rocks of Dengying Formation, the marginal platform and the intra platform have favorable conditions for proximal hydrocarbon accumulation. In conclusion, the deep Sinian in the Sichuan Basin has good conditions of hydrocarbon accumulation and it is an important replacement area of natural gas exploration. Paleo-uplifts and slopes in the central Sichuan Basin have been the favorable locations of natural gas accumulation for a long period. Large-scale exploration shall focus on four favorable areas, including the marginal platform zone in the fourth member of Dengying Formation, the marginal platform zone in the second member of Dengying Formation, the bioherm shoalbody of Dengying Formation intra platform in the paleo-uplifts and slopes of the central Sichuan Basin, and the bioherm shoalbody of Dengying Formation in the eastern Sichuan Basin.