A Forward And Inversion Study On The Main Density Interfaces In Sichuan Basin

Sichuan basin, being located in the northwest of Yangtze block in South China, is the complicated superimposed basin because different basin types inherited or overlaid in different periods. Every tectonic interface is the residue of prototype basin. Reliable evidence is provided to study geometry structure of middle and upper Yangtze region in South China and establish marine oil and gas theory by analyzing spatial distribution of tectonic interfaces which are Indo-Chinese, Caledonian and basement interfaces.Based on geological, gravity and magnetic, seismic, drilling, logging, the physical properties of rocks and testing data, the experiments show that the result of forward and inversion is more reasonable for the gravity method resolution and the accuracy of data by using 3-D forward gravity parabolic model, matching filter and Parker-Oldenburg methods. The spatial variation pattern of the three main interfaces in the Sichuan basin nowadays is initially established. So the hybrid method based on forward and inversion is put forwardThe result indicates that gravity anomalies pattern among the three main density interface is the approximate consistency overall, which is basement controls caprock, deep structure dominates shallow structure. However, local region it is inconsistent as a result of structural deformation and tectonic stress field. The depth of each density interface nowadays in this area is relatively simple, which is similar to the shape of the diamond in short. The maps of depth show that the dominant uplift area with NE strike in the middle of the Sichuan basin intersects in the basin southern margin with the eastern edge and western edge depth gradient zone. The relationship between lithological and depth changes of the basement is analyzed by the sedimentary basement residual gravity anomalies. The result indicates that the basement with the characteristics of weakly magnetic and low-density mainly distributes in the northwestern and southeastern parts of the Sichuan basin and the basement with the characteristics of strong magnetic and high-density mainly distributes in the middle of the basin. The conclusion is obtained by studying fault. The first is that in the north and the middle part of Huayingshan fault zone cuts caprock, disappears in the basement and in the south part of it may cut deeper depth. Secondly, Qiyaoshan fault zone already existed in Jinningian,and re-activities from southeast to northwest pressure stress. Finally, the gravity anomalies among the three major density interface are inconsistent because of the basement of Yangtze block subduction northward beneath the southern Daba Mountains.