Main factors controlling the formation of giant marine carbonate gas fields in the Sichuan Basin and exploration ideas

So far, 12 giant marine carbonate gas fields have been discovered in the Sinian-Middle Triassic of the Sichuan Basin and marine carbonate gas reservoirs have become an important natural gas exploration domain and direction in the Sichuan Basin. In order to further expand the exploration domain of marine carbonate gas reservoirs in the Sichuan Basin, this paper firstly analyzes the basic characteristics of the discovered giant marine carbonate gas fields. Based on this, the main factors controlling the formation of giant marine carbonate gas fields were summarized systematically and the corresponding exploration ideas and evaluation methods were put forward. And the following research results are obtained. First, the giant marine carbonate gas fields in the Sichuan Basin have high quality source-reservoir-cap assemblages of lower source rock and upper reservoir with the Lower Cambrian, the Lower Silurian and the Upper Permian as the major source rocks, the mound-shoal complex of the Upper Sinian Dengying Formation, the grain shoal of the Lower Cambrian Longwangmiao Formation, the Carboniferous dolomite, the reef-shoal complex of the Upper Permian Changxing Formation-Lower Triassic Feixianguan Formation and the grain shoal of the Lower Triassic Jialingjiang Formation-Middle Triassic Leikoupo Formation as the main reservoirs, and the Triassic evaporite, the Lower Cambrian shale and the Middle Permian mudstone as the regional cap rocks. Second, the formation of giant marine carbonate gas fields in the Sichuan Basin is mainly controlled by the following three factors: Palaeotectonic environment and tectonic evolution control the formation and distribution of uplift-depression pattern in the basin, and thus control the formation and distribution of major hydrocarbon source rocks and large-scale reservoirs; Palaeogeomorphic high belts of palaeo-uplifts and palaeo-slopes control the distribution of high-energy depositional facies belts (such as reefs and shoals), large-area karst reservoirs and ancient oil/gas reservoirs; Regional and direct cap rocks control the spatial distribution and enrichment of giant gas fields. In conclusion, the exploration ideas for the giant marine carbonate gas field in the Sichuan Basin are proposed as: determining favorable exploration plays according to the uplift-depression pattern, searching for high-energy depositional facies belts in the palaeogeomorphic high belts, searching for reservoirs based on seismic prediction, and determining targets based on ancient and modern trap conditions. In addition, the evaluation methods are to determine the lithofacies and palaeogeographic pattern of the basin by virtue of well-seismic-outcrop comprehensive analysis, precisely characterize high-energy depositional facies belts of reef (mound)-shoal complex by means of impression method, residual thickness method and seismic facies, and comprehensively evaluate favorable exploration plays and targets based on the conditions of source rocks, reservoirs and cap rocks. © 2021, Natural Gas Industry Journal Agency. All right reserved.