| Xihu Sag has an enormous scale, a deep sedimentation and is rich in oil and gas resources. Its tectonic characteristics and petroleum geological conditions are complicated because Xihu Sag had experienced multiple periods of rifting and extrusion-inversion effects. Thus making an in-depth study of the fault system features to clarify the geological evolution and tectonic modes of the basin is of vital importance in exploring oil and gas in the depression. Pinghu structural belt is located in the middle of west slope belt which has great potential in oil and gas exploration. The accumulation of oil and gas is strongly connected with features and evolution of faults system. It needs further research of the patterns of faults evolution and its control on sedimentary and oil accumulation. Central reverse structural belt has a great number of inversion structures whose research areas are mainly located in the south of sag. In recent years, it is indicated to have a large potential of tight sandstone gas in the north of central reverse belt. Therefore, the relevant research demands to be improved urgently due to the lack of tectonic study.This research is based on several 2D lines of Xihu sag, two 3D seismic areas of Pinghu structural belt and Central reverse belt, and other geological and drilling materials. The faults were identified and meticulously depicted by using seismic attributes. It gave a systematical and quantitative analysis and research of structural style of faults, stages of structural evolution and formation mechanism from three aspects:geometry, kinematics and dynamics. The different kinds and patterns of fault controlling effects on hydrocarbon accumulation was also analyzed briefly. Main research content and conclusion obtained are listed below:(1) Choose multi-seismic attributes to test and analyze different parameters’ influence on the effects of recognition. Compare the recognition effects of different levels of faults under different attributes. In the end, the best instantaneous attribute, variance cube and C3 coherence attribute for this region are chosen to be the most effective methods of recognizing and depicting fault features.(2) By multi-attribute analysis, different fault systems were recognized in 3D seismic area. Fault boundaries of four seismic reflection horizons (Tg, T30, T21,T20) are pictured which indicates that the extension orientation of faults was converted from NE to NNE. On the vertical profile, faults are divided into two systems. Eight fault assemble patterns are identified:"stepladder", "half graben rift", "broom", "horst", "simple back break inversion", "’y’or anti-’y’shape", "compound’y’shape" and "flower-like shape". Faults in this area are divided into 3 levels and features of main faults in the region are described.(3) Multiple geological analysis methods were used to study the kinematics of faults, such as growth index, fault activity rate, reversal rate analysis and section extension-compression ratio. Result of growth index indicates that Xihu Sag was under extension where the north extension was stronger before T30 and then experienced at least two compressional inverses such as T20 and T12. The inverse of T12 is the most intensive and the inverse strength of north part was stronger than the south. Both the fault activity rate and inversion rate indicate that the inversion intensity of Longjing movement weakened slowly from the north to south. Typical section extension-compression ratio proves that it experienced three extrusions (T30, T20, T12) of which the T12 was the most strongest in this region.(4) Through the study of balanced profiles and analysis of regional stress field, it is concluded that Xihu Sag is mainly affected by the relative motion and collision of India plate, Pacific plate and Eurasian plate. Before the late Eocene, the crowding-in effect of Indian plate played a dominant role, and Xihu Sag was under the dextral transtensional stress field with large-scale chasmic-fault depression. In late Eocene, with the increase of subduction speed of the Pacific plate, the East China Sea Shelf Basin had the first tectonic inversional movement called the Yuquan movement; In Oligocene, vertical subduction led to the basin east-west extrusion, thus basin entered the depression-inversional stage; the second inversion-Huagang movement occurred in late Oligocene (T20); At the end of the Miocene, the East China Sea was in the left-lateral extrusion stress field, and the expansion of the Okinawa trough caused the strongest inversion-Longjing movement (T12).(5)According to the previous research result of oil and gas reservoir in this region, the control function of faults characteristics and combination style over oil and gas reservoir are analyzed:level one faults control the distribution of oil and gas field. Faults of rift period provide the main passages for the movement of oil and gas. Faults of inversion can develop effective traps. Faults of sink can improve native oil and gas reservoirs. Five kinds of structure traps’ styles are recognized in the central inversion region:inversion lower plate fault nose traps, inversion fault bench traps, opposite cross fault bench traps, forward fault anticlinal combination traps, backward fault anticlinal combination traps. They can provide evidence for exploration of this region. |
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