The Research Of Geological Conditions For Marine Oil And Gas In Western Sichuan

The sturdy area located transition zone between Songpan-Ganzi geosyncline and northwest continent margin of Yangtze platform. Platform base formed during Jinning activity,belonged to passive continental margin from Paleozoic to Mesozoic. In the transition zone of western Sichuan, The geosyncline evolved cratonic marginal rift basin From Caledonian to early Indosinian, platform evolved cratonic around basins and intracratonic sag basins. These prototype basins of different property had favorable tectonic-deposition setting of the source rock. This research defined dark gray limestone, shale as source rock, mapped the map of plane distribution of source rock. Source rock in geosyncline contained: Devonian, Silurian, Upper Triassic, Lower Cambrian and Middle Permian, which is different from Longmenshan area, While Middle Permian and Middle-Lower Triassic in depression of western Sichuan. By evaluation of source rock, indicated that TypeⅠsource rock area located northwest of the line between Mianyang and Mianzhu. Density of producting hydrocarbon is 30～130×108m3/km2. TypeⅡsource rock area located Mianyang, Deyang, Zhongjiang, Jintang. Density of producting hydrocarbon is 10～30×108 m3/km2. TypeⅢsource rock area located westof the line between Shifanf and Chengdu. Density of producting hydrocarbon is less than 10×108m3/km2. Density of producting hydrocarbon of TypeⅠsource rock of Middle Permian is 30～55×108m3/km2 in east of the line between Hanwang, Pengzhou and Wu1 well. West of the line is TypeⅡsource rock of Upper Permian, Density of producting hydrocarbon is 10～30×108m3/km2. East of the line is TypeⅢsource rock, Density of producting hydrocarbon is 8～10×108m3/km2.By the analysis of source rock, oil seepage, GC, GC-SQ and carbon isotope, we deem that source of oil seepage and asphalt in continental reservoir came from both continental and marine. Marine reservoir had characteristics of multi-source. For example, the characteristic of GC-SQ in continental sandstone (J2s1) of Houba is similar to Zhong 46 well (T3x2 reservoir), and similar to the map of source rock(D2j), GC-SQ of asphalt sample of Shawozi formation in Hanwang, Jiangyou, Feixiangguan formation in Wudu, and Yutongliang are highly similar. From the analysis of evolution of source rock indicated that source rock was matured before Anxian activity. Oil and gas migrated to slope and interplatform(From front Longmenshan to depression of western Sichuan). So predicted that the area from front Longmenshan to depression of western Sichuan was not destroyed by later tectonic movement, may find petroleum reservoir of marine source rock– marine reservoir and marine source rock– continental reservoir.By the petroleum geological survey in Longmenshan area and the analysis of related samples, we had acquaintanceship for regional source-reservoir-seal combinations as follow:①Black shale, dark gray microcrystalline limestone (Z1ds), black shale (∈1) in basin-lower slope as source rock. Fracture dolomite (Z2dn) in exposed shoal-reef as reservoir. Shale(∈1, S) as seal.②Gray-black shale (Z2dn2,∈1, S1) in basin and black shale, dark gray microcrystalline limestone (D1-2) in basin-shelf as source rock. Sandstone, limestone, dolomite (S1-2,D2-3) in shoal-reef. as reservoir. Shale (D2-3, C1z) in lagoon-tidal flat as seal.③Dark muddy microcrystalline limestone, black shale (P2-3) in outer ramp and swamp as source rock. Limestone, dolomite (P2-3,T1-2) in shoal as reservoir. Mudstone, gypsum and salt rocks (T1-2) and mudstone (T3) as seal. These combinations had preserved the oil and gas in western Sichuan. The burial depth of first combination is more than 8,000 meters in depression of western Sichuan. The second combination only developed in Longmenshan arera, which supply the third combination to some extent. The third combination had exploration significance.Comprehensive evaluation: By the influence of late Indosinian - Himalayan tectonic movement, Paleozoic-Mesozoic strata was uprifted and eroded, thrusted, deformed in western Sichuan. The condition for oil and gas preservation was destroyed. From seismic profiles, indicated that Paleozoic-Mesozoic strata had complete"autochthon"tectonic zone, which was covered by marine-continental seal in middle-late Indosinian-Yanshan epoch. And be seal by imbricate low-angle thrust fault in west, preservation condition was better. Oil and gas from order destroyed reservoir migrated into new reservoir. From the angle of comprehensive evaluation, east of Jiangyou-Dujiangyan fracture and west of depression in western Sichuan may find medium and large reserves of natural gas. Western Sichuan, Longquanshan tectonic zone, Zhongjiang-Huilong nose tectonic zone had favorable preservation condition, May developed shoal facies reservoir, which are key area for exploration in western Sichuan.