Longmenshan The Northern Section Of Groundwater Dynamic Characteristics And Hydrocarbon Preservation Conditions

Longmenshan divided by three major NE-trending faults, i. e, from west to east the Maoxian-Wenchuan fault, Beichuan-Yinxiu fault, Anxian-Guanxian fault into"back-mountain belt","front-mountain belt"and"piedmont belt". They are Songpan-Ganzi fold-belt, Maoxian-Wenchuan ductile shear belt, Longmenshan over- thrust nappy belt and foreland basin of western Sichuan. On the vertical direction of structural-line, Longmenshan divided by HanWang, DuJiangyan (Guanxian), TianQuan.YaAn into"North section","Middle section"and"South section"from north to south. Longmenshan thrust belt has four major slip horizons on the longitudinal of it:①coal measure stratum of XuJiahe formation;②a nhydrock and salt beds of Upper LeiKoupo;③mud rock and muddy limestone of FeiXianguan formation and Lower Permian;④mud rock and muddy limestone of Cambrian and Lower Devonian. North section of Longmenshan has a good condition of hydrocarbon source rocks, rocks-reservoir-cap combinations and traps. A lot of oil and gas seepages have been discovered wildly. The hydrocarbon seepage distributed in the sedimentary rocks from Silurian (Paleozoic) to Jurassic (Mesozoic).The main hydrocarbon source rocks are Sinian, Lower Cambrian, Lower Permian, Upper Permian and Upper Triassic. The unsuccessful explorations: KuangShanliang, TianJingshan, HeWanchang and the successful explorations such as ZhongBa disclosed the complexities of stucture in the Longmenshan thrust belts. The most important reservoirs are bioclastic-limestone of MaoKou formation,ChangXing formation in Permian; dolostone of JiaLingjiang formation, LeiKoupo formation; lithic quartz sandstone of XuJiahe formation in Middle and Upper Triassic. The type of the reservoir is fracture-pore.On the basis of Remote Sensing, Structural Geological, Hydrogeological, in combination with the wild-field surveys, the structural characteristics of the north section of Longmenshan were studied systematically, the studied suggested that the hydrocarbon preservation condition is decided by the structure condition:(1):By the analysis of oil field water composition, this paper elected Na+, Mg2+,Ca2+ and Cl- ,S O42-,HCO3- as the essential index to researching the source of the oil field water in HeWanchang and ZhongBa. Results show that the water of HeWangchang gas field have a low salinity but high ratio of Na+/Cl- , it was typical infiltration water. Because of high opening degree of hydrogeology, underground water in HeWanchang was in a well-oxygenated environment that was a bad factor of petroleum. As the explorations has indicated that: there is no oil accumulation in HeWanchang but some geothermal water which produced from JiaLingjiang formation and LeiKoupo formation (Triassic).(2) In the west of Longmenshan thrust belt,HeWanchang, the beds are monocline which have trend of NE-SW, the dip angle of rock is very steep, and it becomes gentle near the basin rapidly. The Remote-Sensing and wild-field surveys indicated that rocks of Triassic are exposed as a result of strenuous tectonic movements, on the other hand, the crack of rocks is well-developed; the corrosion of carbonate rocks of Jialingjiang formation and Leikoupo formation is very intensive. This special geology character will make the ground water seep into ground directly. There is lack of water-tight faults and another important factor: the upper and lower rocks on the beds of Triassic are impermeable, so water can only fluid-though the layers of Triassic which have high permeability, by the huge difference of land height, water of infiltration could take the place of sedimentary-water and destroy the hydrocarbon pool in the traps which on the lower land.(3): In contrast, there are some NE-trending, NW-direction water-tight faults in northwest of ZhongaBa, this type of fault could prevent ground water infiltrating to the depth of the basin. The Triassic layers can not accept the atmospheric water or ground water as these stratum are buried in depth of basin, as a result of that, sedimentary-water system lost connection with infiltration water, these two difference type of hydrodynamic force system are separated by the huge water-tight faults located in the piedmont belt of Longmenshan. Finally, petroleum is saved from destruction that comes from the infiltration water like HeWanchang anticline. Moreover, ZhangMing fault, which located in the southwest of ZhongBa oil field also have fluid-closing ability, this will provide a important condition of accumulating of oil and gas.(4): Oil field water of ZhongBa has a wild difference to the water in HeWanchang: its salinity is high and the water type is sulfate-sodium type or chloride-calcium type. This type of oil field water is a typical sedimentary-water, it represent a reducing and closed environment. Underground water fluid slowly, even ceased in an environment like that, it was favorable for accumulating of oil and gas. The prospect of petroleum in this area will be the trap where far from the"piedmont belt",the aim reservoirs must be carbonate rocks of Triassic and Upper Permian.