A Study On Shale Gas Of Lower Silurian Longmaxi Formation In Sichuan Basin And Its Peripheral Areas

Sichuan basin, located at the western part of China, is an important oil & gas accumulation area. There are two suites of Paleozoic regional marine source rocks (Lower Cambrian Niutitang Formation Shale and Lower Silurian Longmaxi Formation Shale). In this thesis, we are mainly focusing on the Longmaxi formation shale gas reservoir-forming conditions and its exploration prospects.The Longmaxi strata were were mainly deposited in shallow - deep water environments, included deep-water shelf, neritic deep-water basin and marginal slope facies.esis mainly consists of deep-water shelf, neritic deep-water basin and marginal slope depositions. The Longmaxi interval is dominated by fine-grained sediments, comprised of black carbonaceous shale, radiolarian siliceous mudstone, silt mudstone, argillaceous siltstone.Dark/Grey shale is aboundant of graptolite. Its Source areas are Hannan (paleo-)land , Sichuan northern uplift , Sichuan central uplift to the west.The last is not the dominated source areas.Zhenba-Guanyin to the north,Yunyang-Shizhu-Guanyinqiao to the east and Yibing-luzhou to the sourth are difined .High quality source rock occurred on its bottom, between 20m and 120m in thick ,dominated by I-II types of kerogen, rich aboundant of organic contents(TOC range from 1.0% to 5.0%).There exsits four high thermal maturity zones in study areas, such as northern areas,northeastern areas, western Hubei-eastern Chongqiong areas,sourthern areas,with Ro range of 1.2%～4.3%. Especially, high thermal evolution of source rock in the Longmaxi strata is closely related to 259Ma thermal activities in sourthern parts of Sichuan basin; while deeper burial depth during post-deposition is the key factor to its high thermal activities in northeastern parts of Sichuan basin.The studied areas experienced multi-tectonic events superimposition and reworking.Especially the folds and uplifts during the Yanshan– Himalayan periods, which caused high dip-angle fractures development. Fractures could improve mudstone/shale porosity and permeability and facilitate the adsorbed gas desorption.. Varieties of pores are found in the Longmaxi formation shale, such as intragranular organic micropores, illite sheetlike micropores, microfractures; intergranular calite/dolomite/quartz micropores; intergranular plagioclase dissolution miropores; flakelike mica pores; pyrite-framboid nano - to microintercrystalline pores. The occurrence mechanism of nano- and micropores includes mineral alteration during the diagenesis, unstable mineral dissolution, organic matter generation and thermal evolution.Based on the comparison of Barnett shale(Fort Worth basin) and Longmaxi formation(Sichuan basin), they have similar depositional setting, korogen types (I～II) and hydrocarbon generation and thermal evolution processes. But the latter is relatively deeper burial depth (from 1600m to 4200m), richer clay minerals and thermal maturity (generally Ro>2%), quick rising rate (44m/Ma) and lower gas content (ranging from 1.73m~3/t to 3.28m~3/t).The bitumen in Carboniferous system palaeo pools(140×108t) and the natural gas in conventional gas field groups(2575.3×108m~3) are source from Longmaxi formation.Longmaxi formation shale once expulsed a lot of hydrocarbon during the organic thermal evolution processes.However,its present gas content is more than America's shale gas recovery/exploitation limits(1.1m~3/t). According to the above analysis, Longmaxi formation shale has better exploration prospect in southeastern part of Sichuan basin and western Hubei - eastern Chongqing areas with moderate burial depth and good sealing properties.