Tectonic Evolution And Fracture Stage In Hexingchang, Dongtai And Luojiang Area Of The Western Sichuan Depression

The Western Sichuan Depression(WSD) is located between Micang nappe in the north and Leshan-Yaan in the south. Its northwest boundary is Longmenshan nappe, and Its east boundary is the Longquan fault fold belt. The WSD was developped the strong tectonic movement under the impact of the Longmenshan orogenic belt in the Indo-Chinese Epoch, Yanshan-Himalayan Epoch, and the Sichuan Basin was also controlled by the Longmen Shan. The formation of WSD was began in the Indo-Chinese Epoch, and continued to develop into the foreland basin in the Yanshan Epoch. But it entered the shrinking of phase in the Sichuan Epoch, and it also formed a foreland basin.in the f Himalayan Epoch.The Triassic and later 10 seismic reflection layers and the corresponding geological layers were identified by the contrast of the wave drag and the well in The synthetic seismogram project.On the base of careful seismic profiles, we found faults were developed fully in the study area with multi-direction, many Epochs. Combing with the regional geological setting and the principle of balanced cross section, we provred that the faults can be devided into 4 Epochs as followings:firstly, EW trending structural zone in the Indo-Chinese Epoch;secondly,Yanshan NNE trending structural zone in the Yanshan Epoch;thirdly, NNW trending structural zone in the Sichuan Epoch;fourthly, NS trending structural zone in the Himalayan Epoch.By analyzing the geophysical interpretation sections, we had identified two structural traps in the Luo River and Hexingchang. They had different characteristics in the different depth: the deeper structure was very complex, the EW trending folds which were formed in the Indo-Chinese Epoch were involved the later Jure type of folding in the Yanshan Epoch,resulting into development of multiple types of traps; the middle mainly were the NNE trending structural traps;the shallow were damaged strong ,leading to the small structural traps. destructive. By the use of structure superposition analysts, we founded that: in the Indo-Chinese Epoch it developed the nearly EW direction and vertical fold;in the Yanshan Epoch it developed the nearly NNE direction and vertical fold, resulting that the dome-shaped folds were formed.; in the Sichuan Epoch it developed the nearly NNW direction fold, which enhanced or rehabilitated the early structural traps. superimposed on the basis of the ancient structure, strengthen or modify the trap;in the Himalayan Epoch NS trending structural zone mainly influenced the epilimmion,and the deep layers were less affected.In summary, our study suggests that: the Indo-Chinese Epoch and the early Yanshan Epoch created the earlier traps, and provided migration channels for oil and gas; hydrocarbon generation began in the late Indo-Chinese Epoc, and continued to achieve the peak in the mid-Yanshan Epoc; oil and gas accumulation was saved in the ancient structural traps. Therefore, the Paleocene tectonic events before the Sichuan tectonic event or superposition structural traps controlled the formation of accumulation of gas, whinch were the best traps .Hovever, after the Sichuan tectonic event, the traps are ineffective,Which showed less contributement in the storage and transport of gas,or isolated and even destroy the original structural traps in some areas.