Main Structural Interfaces Of Sichuan Basin Obtained From Inversions Of Gravity Anomalies Through Modified Very Fast Simulated Annealing Method

Sichuan basin, lying in the northwest of Yangtze block in South China, is a typical Paleozoic and Meso-cenozoic era marine and continental superimposed basin. Since the basement consolidated, Sichuan basin experiences frequent tectonic movements, sedimentary formations are intensely reformed as well as geological condition, multiple cyclic tectonic movements make petroleum systems more complex. In order to construct the distribution of marine strata of Sichuan basin, this paper tried to make profile inversions combined with drilling and seismic data using aimulated annealing method,then we obtain inverse undulations of the Indo-china interface, Caledonian interface and top of the basement, which exist density contrasts caused by the three major tectonic movements.Based on geological, gravity and magnetic, seismic, drilling, logging data, this paper inverses interface undulation and density parameter using 2 dimensional rectangle gridded model as forward model combined with modified very fast simulated annealing method. The forward model is flexible, which can construct arbitrary interfaces and adapt to especially for the complicated geological models in practical situations. According to the fitting precision, we set different size of grids. Simulated annealing method is easy to add constraint conditions with high precision. Thus three main structural interface undulations of the Sichuan basin could be solved efficiently.Undulations of three structural interfaces indicate that basal undulation dominates distribution of caprock, deep structure dominates shallow structure. The fluctuation of formation is tectonic response and inheritance. Basal distribution pattern is mainly resulted from Indo-china movement and Caledonian movement. Therefore the basin presented uplift in the west and depression in the east during Paleozoic era, while uplift in the east and depression in the west during Meso-Cenozoic era. Overall, tectonic movement is the basic reason of current interface undulation. Detachment layers generally exist in basin, mainly include the Lower-Middle Triassic and gypsum-salt rock, Silurian shale bed, Lower-Middle Cambrian mud shale or gypsum-salt rock and Lower Sinian mud shale, which not only separate uncoordinated deformation between structural layers but also protect underlying structure. The detachment layer of top of the basement control deformation of the whole basin, inducing different deformation intensity and style in different area. In addition, unconformity also compartmentalizes the lower and upper structural layers, displayed different deformation styles.The results suggest that gravity anomalies are mainly induced by interface undulation and density contrast, especially the Property and buried depth of the basement, while internal uneven density in layers just induces shortwave anomalies. The relative high gravity anomaly of middle uplift induces by the uplift of the basement joint with crystalline basement lithology, relative low density, deep buried depth of basin basement in the north, west and east of the basin produce low gravity anomaly. While in southern areas buried depth of basement is shallow with low gravity anomaly, thus gravity anomaly induced by the nature of the basement is more remarkable.