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Law of Carboniferous Hydrocarbon Accumulation of Tazhong Area in Tarim Basin

Abstract

The accumulation conditions and models, controlling factors, distribution patterns of Carboniferous hydrocarbon in Tazhong area are documented in this paper based on the theory and methods of sequence stratigraphy, structural Geology, physical and numerical simulation as well as the meshwork-carpet type hydrocarbon accumulation theory. Tarim basin is a superimposed basin, multistage tectonic movements formed multistage regional unconformities and a series of faults. Key elements of accumulation (including structure, faults and sequences) constitute the three-layer structure of the meshwork-carpet type hydrocarbon accumulation system: source layer, unconformity complex, storage layer. According to the configurations of unconformities and system tracts above and below it, two main unconformity complexes are identified in Carboniferous strata. Based on the relationships between faults and unconformity complexes, faults in Carboniferous strata are divided into two categories: faults that cut source layers and faults that cut storage layers. Through physical and numerical simulation of petroleum migration and accumulation, the study discusses the mode and mechanism of petroleum's vertical and lateral migration that is controlled by fault conducting capacity and reservoir heterogeneity. If resultant force of migration is positive, the oil tends to migrate laterally to the positive rhythm reservoirs such as Donghe Sandstone, Gravel-bearing Sandstone and delta sandstone of Sandstone-Mudstone member, and migrates vertically through faults that cut storage layers, then accumulates in lithologic-stratigraphic trap, forming fault block, fault-lithologic or other types of reservoirs. If resultant force of migration is negative, the unconformity complex temporarily store and accumulate petroleum, forming stratigraphic onlap, unconformable screened or low-amplitude structure reservoirs. Based on the research, the paper indicates that the enrichment of hydrocarbon in Carboniferous strata is controlled by structure, faults and sequences. Petroleum diverges along the unconformity complexes and converges through faults. Structure and faults control the migrating direction; structure and sequences control the development of unconformity complex, the distribution of lithologic-stratigraphic trap and the scale of source-reservoir-seal combination; the coupled relation of faults and sequences control the filling ability and effectiveness of lithologic-stratigraphic trap.