The Tectonization and Petroleum Accumulation in Southeast Sichuan Basin

Abstract: Sichuan basin is one of Chinese largest petroliferous basins, as well as a significant natural gas producing region in southwest China. As a new gas exploring field, the lower paleozoic of southeast Sichuan basin has almost not been studied, because its arming strata were deeply buried, and it is characterized by the multiphase deformation, the multiphase bed forming, the perplexing tectonic, the high-over evolution of petroleum, and the gas distribution and migration controlled by tectonic background, basin structure trellis and fault zones. In recent years, a lot of exploring work indicates that the studying area has such a capacious gas foreground that it can be chosen as an exploration-tactic superseding area of Sichuan basin in future.This paper deals with the study of the rules of accumulation and distribution of petroleum, which is based on the analysis of the tectonic background, the features of structural evolution, the types of the tectonization and their significant influences for the formations of gas pools. Then the promising areas were pointed out and some advices for next exploration work were given.The study includes: (1) the study of the strata order and the sediment environment; (2) the study of the tectonic background and the evolution stage; (3) the study of the tectonic deformation of the basin; (4) the study of the typical gas pool and the bed forming; (5) the study of the petroleum system; (6) the selection of the prospective areasAccording to sufficient practical data and the combination of open field work with indoor work, the article has achieved much, such as bellow:The lower paleozoic of southeast Sichuan basin is made of marine deposits, whichcould be divided into four grade- II sequences and thirty three grade-III sequences.The elongational environment from Caledonian to Hercynian caused the foundation of deep faults. It was the compresso-shear stress field towards NW in south China that caused ejective structures in southeast Sichuan, the transformation began in Indosinian movement, and their main tectonization happened in the late period of Himalayan orogeny.Local structures in southeast Sichuan are formed in the same tectonization mechanism. The differences of folding scale, the folding ships and the folding degrees are related closely to their tectonic locations, directions and patterns of the stress, and their vertical organizations of layers, and the conditions of basements and boundaries. Most of the founded anticlines can be regarded as superimposed bodies of two-level flower-structure borne in Mesozoic and lower Paleozoic, Their evolutionary history could be divided into three stages as pre-folding tension, primary folding and final forming.There are plenty of source rocks, reservoirs, and caprocks in lower Paleozoic, and the source-reservoir-caprock assemblages are abundant too, and different types of traps have been found in the studying area, most of which are structural traps.Almost all of the lower Paleozoic source rocks in south ease Sichuan basin are high-over evolved, which mainly produce gas. The complexity of petroleum system are indicated by the source providing, the bearing system, the pool-forming process, the system composition, the location of accumulation and other aspects. The petroleum system has characteristics of late accumulation and bulk sealing. The scale and location of gas pool are determined by geological structures, and the middle deformed areas are promising ones which should be regarded as targets of next exploration…
Key words: Southeast Sichuan; tectonization; folding; petroleum system; accumulation condition

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