Hydrocarbon Accumulation Implications of Mesozoic Progressive Evolution of Southern Dabashan Arc-Shaped Thrust Belt in Northeastern Sichuan Basin, China
The southern Dabashan thrust belt is a 50km wide and 200km long, arc-shaped, southwestward one between Qinling-Dabie orogenic belt and northeastern Sichuan foreland basin, China. The sedimentary cover of the belt consists mainly of folded Paleozoic-Lower Mesozoic strata of shallow marine origin and Upper Mesozoic strata of continental facies sequences. The structural style in the belt varies from thick-skinned to thin-skinned thrust structures from NE to SW. The fold and thrust belt has been established as a thrusting system migrated towards the basin by forward propagation during Mesozoic from 175 Ma to 90 Ma. Based on FT(fission track) and a series of balanced two-dimensional cross-section analytic results, we have suggested that the Mesozoic intra-continental evolution of the belt is progressive from NE to SW, and the deformation process is younger and younger and shortening is less and less, respectively. A series of oil, gas and bitumen shows have been found in the thrust belt, and many gas fields in northeastern Sichuan basin and southwestwards near the belt. For example, The Puguang gas field is the biggest gas one recently found in the northeastern Sichuan basin, and the gas and bitumen coexist in the oolitic dolomite reservoirs of the Feixianguan Formation of the Lower Triassic. The Baiguo oil show is located in the northeastern part in the belt, and the oil was charged during mostly late Jurassic-early Cretaceous in the fractured reservoirs of the Feixianguan Formation of the Lower Triassic. After later Cretaceous, with the uplift of the belt and the erosion of the caprock of the reservoirs, the Baiguo oil seepage began. The Miaoba bitumen show is located in the middle part in the belt, and the early oil was charged during mostly middle-late Jurassic in the fractured pore reservoirs of the Wujiaping Formation of the Upper Permian. The bitumen is interpreted to be asphaltene by reason of deasphalting action, and was exposed with the uplift and erosion after later Cretaceous. Because of the Mesozoic progressive deformation of the southern Dabashan arc-shaped thrust belt from NE to SW, the sedimentary thickness of the strata is increasing from the belt to foreland basin. The thrust front may follow the hydrocarbon maturation rear, in other words, the hydrocarbon maturation degree is the lower near the orogenic belt, contrarily, the degree is the higher near the foreland basin. The hydrocarbon maturation is in response to the progressive deformation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009