By means of surface and subsurface geological combination research, we use the organic and inorganic geochemistry to describe the geological conditions of lower Paleozoic gas formation and gas reservoir characteristics in the Sichuan basin,reveal mutual coupling relations between the destroyed oil pool and gas reservoir, and establish the model of lower Paleozoic petroleum accumulation. Both lower Cambrian and lower Silurian are two sets of high-quality source rocks in the Sichuan basin, with an average thickness of about 100m. Their distributions show a significant inheritance, with three centers of hydrocarbon-generating intensity like as southern Sichuan,eastern Sichuan and western Hubei. Several gas layers in the lower Paleozoic sequence in the Sichuan basin exist,including the middle-upper Cambrian Xixiangchi Formation, lower and Middle Ordovician Nanjinguan Formations and Baota Formations,and lower Shiniulan Formation (called Xiaoheba Formation in east Sichuan basin). The matrix porosity of reservoir less than 1% accounts for 70%-80% ,but the high quality reservoirs such as karstic,fracture,and reef limestone reservoir with the effective porosity of 4%-ll% are locally developed. The upper Ordovician,lower Silurian mudstones and the middle-lower Cambrian gypsum rocks as two types of regional cap rock,are mainly distributed in the southern and northeast parts of Sichuan basin. The destroyed oil pools and natural gas suggests the potential gas field in lower Paleozoic reservoir would be mainly distributed in the Leshan-longnvshi paleo-uplift areas. The Paleozoic natural gas mostly derived from oil cracking has high dry coefficient, and high content non-hydrocarbon gas. The natural gas in the eastern and southern Sichuan basin is sourceed from the Silurian and Cambrian source rocks,respectively. The process of gas accumulation in lower Paleozoic is as follows: oil generation-yearly oil pool (accumulation)→oil pool adjustment (transformational migration)→ early gas reservoir(oil cracking)→gas reservoir destroyed or kept in present-day (tectonic adjustment). Oil cracking in early oil pools are main source of present lower Paleozoic gas reservoirs, and the early oil reservoir has importantly controlled accumulation and distribution of present-day gas reservoir.
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维普
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