Petroleum Geological Conditions And Exploration Prospects Of Lower Paleozoic In Sichuan Basin

For oil and gas exploration of Lower Paleozoic in Sichuan Basin, there is no significant progress except the gas reservoir of Ordovician and Cambrian in Weiyuan and Silurian gas reservoir of Wuke 1# in East Sichuan basin. Research in pass 50 years reveals the process of hydrocarbon accumulation and controlling factors is not clear is one of the most important reasons for poor efficiency exporation. This paper has aimed on the Lower Paleozoic in Central, Westeastern, Southeastern, Northern Sichuan basin, West Hubei province-East Chongqing city. Particularity of Lower Paleozoic petroleum geology conditions has been deep characterized. By application latest theory of conventional and unconventional oil and gas accumulation, characteristics and distribution of Paleo oil pools and reservoir bitumens, Paleo oil pools acummulation and destruction process of Lower Paleozoic, underlying Sinian and overlying Permian, main contronling factors of conventional gas reservoir, coupling relationship between paleo oil pool and gas reservoir, the differences of paleo oil pools between Lower Paleozoic and Sinian have been detaily analysed. In addition, micra & micro reservor charateristics and storage space (naro-scale pores) of source shale reservoir have been deep studied, so is the types and formation mechanism of shale reservoir. Finally, hydrocarbon potential of Lower Paleozoic has been evaluated based on the the basic conditions and integrated results about hydrocarbon accumulation from the disscusion of conventional oil and gas accumulation coupling with unconventional oil and gas accumulation.Lower Cambrian Niutitang Fornation and the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation are two sets of high quality source rocks, especially at their bottom. The two sets source rocks comprised of shallow water shelf black or gray-black shale, which was characterized by good organic matter type (Ⅰ1 andⅡ1), high maturate (high maturity-over-mature), high quality (of the Lower Cambrian TOCmax=8.0%, Lower Silurian TOCmax=5.0%), complicated hydrocarbon generation and evolution (two staged hydrocarbon generation in Cambrian). The hydrocarbon generation centers of two set source rocks have been with inheritance, and also developed three centers:Southern, Northeastern Sichuan and Western Hubei province.Different areas in Sichuan basin, such as uplift and slope, have developed obviously different layers of reservoir, and storage space types are very related to the layers in different area. The main carbonate reservoir rock types of lower Paleozoic are crystalline dolomite, oolitic limestone, oolitic dolomite, dolarenite and reef- shoal limestone. The main storage spaces are intercrystalline (dissolution) pores, dissolved pores of gypsum, caves. Matrix porosity is generally less than 2%, and is IV class reservoir charactered by low porosity and lower permeability. That of Cambrian physical property is better than that of Ordovican and Silurian vertically. Micang mountain and paleo-uplift is the best, followed by the Southeastern Sichuan basin, that of Hubei Province is in third.There are three sets of regional cap rocks in Sichuan basin, whose are Lower Cambrian mudstone, Upper Ordovician and Lower Silurian mudstone, Middle-Upper Cambrian gypsum. They mainly located in Southern, Northeastern Sichuan basin. The fluid characteristics in Central, Northeastern Sichuan basin and Western Hubei province-East Chongqing revealvs that the three regional caps have lost its ability in some time, which caused the preservation condition had been destroyed and fluid cross-layer flowed in vertical.Lower Paleozoic high quality reservoirs have been controlled by the depositional environment, dolomitization, gypsum dissolution, karst, dissolution caused by external erosional fluid, fracturing. The destruction and preservation mechanism of caives have been contronlled by internal fluid filling-cementation, external erosional fluid filling- dissolution-precipitation, hydrocarbon emplacement and thermal cracking process, decompression and cooling effect in uplifting process. Formation mechanism of high-quality reservoirs in different times co-exsits the common and personality. In short, the formation and preservation mechanisms of high-quality reservoirs are very closely related to a series of water-hydrocarbons (rock) effects caused by the Caledonian paleo-uplift and Lower Paleozoic oil& gas accumulation in Sichuan basin.Sinian reservoir bitumens are distributed in the whole basin, and Sinian paleo oil pool is a super large scale lithological-strutural paleo oil pool. Lower Paleozoic reservoir bitumens are mainly distributed in Cambrian and Ordovician karst reservoirs in the core-slope zone of Leshan-longnvshi paleo-uplift. Lower Paleozoic paleo oil pool is smaller and scattered, and is mostly lithological -structural paleo oil rerservoirs.The distribution of reservoir bitumens are controlled by the paleo-uplift and reservoir rock characteristics. The paleo oil pool has significant effect on current gas reservoir accumulation and distribution.The paleo oil pools and natural gas shows in Lower Paleozoic reservoir are mainly distributed in Leshan-longnvshi paleo-uplift areas. The natural gas in Paleozoic is very dry, with high content non-hydrocarbon, which is mostly from cracked paleo oil reservoirs. The main gas is from the Silurian source rocks in eastern Sichuan, and from Cambrian source rocks in southern Sichuan. Oil and gas accumulation process of lower Paleozoic is as follows:oil generation→paleo oil pool (accumulation)→paleo oil pool adjustment (transformational migration)→paleo gas reservoir(oil cracking)→paleo gas reservoir destroyed or present-day gas reservoir formation(tectonic adjustment).For the conventional oil and gas exploration, good exploration prospects can not be ignored in southern area near the the southeastern edge of Sichuan basin for Cambrian. Southern-Eastern Sichuan basin is the most favorable target area for Ordovician further exploration. Eastern Sichuan-Eastern Chongqing will be the most favorable exploration area for Silurian.For the unconventional oil and gas exploration, Lower Cambrian shale, which is characterized by less oil and gas shows in drilling, high maturity, is not conducive to the shale gas accumulation in Sichuan basin. Upper Ordovician-Lower Silurian black shale, which is characterized high-quality, moderate maturity, micro-porosity well development in muddy siltstone and very fine sandstone, is with good geological conditions for the formation of shale reservoir.