The Research On Fault System In Longmen Shan Foreland Basin

It is well-known that the gas resources are rich in western Sichuan foreland basin.Thus, western Sichuan foreland basin is an important energy succeeding base forSichuan Basin and great potential for exploration is shown here. Petroleumexploration shows that the ancient traps in late Indosinian to Yanshanian are theeffective trap. The discovered gas fields mostly locate in the Yanshanian paleostructure slope or high part. Himalayan tectonic movement is a key factor to naturalgas enrichment and high-yield layer. However, it cannot be ignored that fractureplays an important role in migration, accumulation and destruction of oil and gas: anopen fault can be a channel not only to transport hydrocarbon to traps which forms anew reservoir, but also to carry hydrocarbon out of traps which destroys the formedreservoir. A closed fault will help block the transportation of oil and gas and gatherinto a new reservoir. Once we are clear about when the fracture opens and closes, thesituation of gas enrichment in fault trap can be determined. Therefore, the analysis ofthe fault system serves as an important role to the oil and gas exploration.The data of Longmen Shan foreland basin acquired by the geophysical prospectingteam in SINOPEC is collected and analyzed in the research. Structural interpretationare respectively done with the seismic workstation interpretation softwareLANDMARK, computer interpretation software DISCOVERY, attribute extractionsoftware VVA and balanced cross-section software2D MOVE. The purpose ofbalanced cross-section is to test and verify the faults interpretation’s integrity, andexplain the periods of seismic formation. From the seismic data of same placeacquired in different time (2D data collected before Wenchuan earthquake and a partof3D seismic data collected after Wenchuan earthquake),seismic profiles of differentperiods are contrasted to provide a basis for oil and gas secondary migration analysis. And several conclusions are drawn.Firstly, the fault system of the foreland basin is mainly concentrated in the LongmenShan thrust belt and the Longquan Shan tectonic zone, substantially paralleled toLongmen thrust nappe with the movement from north to east mostly. Three majorfaults exist in the areas: Guankou fault, Pengxian fault and Longquan Shan fault.Both Guankou fault and Pengxian fault are broken from the surface to the basement.Longquan Shan fault developes mainly above the gypsum layers of JialingjiangFormation in Lower Triassic Strata.Secondly, the western Sichuan foreland basin remains a stable sedimentary conditionwith no fracture and the stratigraphic thickness varies little from the late Hercynianto early Indo-China. Until the middle of the Indo-Chinese movement, the forelandbasin had occurred a strong lifting movement so that the Longmen Shan region isbeginning to have a significant structural deformation and layers started to fold.Under the late Indosinian movement, folds in the layers turned obvious. At the sametime Guankou fault began to form. With the increasing of movement in the earlyYanshan, the Longquan Mountains begin to form. Until the early Himalayas,structure layout had been basically formed. Stress of the layers began to shift toextrusion pressure from pulling tension and layer folding continues and normal faulttransforms to thrust fault.Thirdly, the seismic source of Wenchuan earthquake is located in the northwest ofthe work area. Wenchuan earthquake has a dramatic impact on the northwestern areaand the area perpendicular to the trend. Due to severe earthquake movement,thereare seven new small faults detected in the2D seismic profiles, mainly located in thearea in which Line4and Line124intersected with Line261.The new faults mainlypositioned in Mianzhu-Shifang-Guanghan-Deyang area. Under compressive force,layers are folded in various levels or even fractured in most regions. In comparison,remote regions from seismic source or some area paralleling to the strike wereslightly affected.