Reasons For Formation Of Typical Palaeo-Reservoirs Of Paleozoic In Southern China And Revelation To Exploration

Oil & gas exploration in Paleozoic in southern China have undergone for more than half a century. Huge natural gas fields such as Puguang and Yuanba have been found in Sichuan Basin and its margin and great breaks have been gotten in oil & gas exploration, but except for these, vast areas in southern China can not get commercial exploration results. Many oil & gas shows in Paleozoic in south China prove that Paleozoic in south China possesses favorable hydrocarbon generating conditions and it has undergone great reservoir formation process in geological history. Therefore, to analyze the factors for forming palaeo-reservoirs in Paleozoic in south China and probe the rules of forming palaeo-reservoirs in south China will provide some revelations and references for finding new areas and direction for oil & gas exploration.The study on palaeo-reservoir system in southern China starts from the end of the 1970s. With the discovery of the huge Majiang Palaeo-Reservoir, a great deal of petroleum geologists then began to study the origin of palaeo-reservoirs. Palaeo-reservoirs are the reservoirs that had been destructed but can be resumed, the original/primary reservoirs were formed in relatively early times and many palaeo-reservoirs in southern China were formed in Paleozoic. The palaeo-reservoirs in southern China are distributed widely from western part of Jiangnan-Xuefeng Uplift to vast area of Sichuan Basin, the bitumen, oil shows and well natural gas remained are discovered in pores, fractures, cavities, leach holes and corroded hollows, specially in the fractures. Oil and gas shows in Lower Paleozoic are dominated by bitumen and Ordovician by a spot of oil shows, which are mainly showed in the western side of Xuefeng Uplift and margins of Qianzhong Uplifts. Oil & Gas shows in Upper Paleozoic are dominated by oil shows and downhole gas shows, which are mainly showed in margins of palaeo-uplifts and Central Hunan Depression. The planar distribution of palaeo-reservoirs is controlled by location of uplifts and trend of great deep faults, its main part is distributed in great deep fault in east-northern direction.Great Paleozoic sediments in southern China have set great material base for formation of Palaeo-reservoirs. In early Paleozoic there deposited passive continental margin—wide gentle slope sedimentary layers and late Paleozoic are consisted of platform basin-intracratonic marine sedimentary layers. There widely develop black carbonic shale of lower Cambrian in slope and deep sea basin of lower Paleozoic, which is dominated by ascending oceanic current and provide abundant hydrocarbon source base for formation of palaeo-reservoirs. The dolomites and sandstones in platform and upper slope provide favorable storage space for oil & gas, the mudstones of lower Cambrian and lower Silurian provides preservation conditions for palaeo-reservoirs. All these form the petroliferous combination (z-s Lower Combine) of lower Paleozoic in southern China. Duyun Structural Movement and Guangxi Structural Movement not only provide accumulating places in large scale for formation of palaeo-reserviors of lower Paleozoic in southern China, but also provide motivate force to accumulate oil & gas for palaeo-reservoirs. The palaeo-reservoirs of lower Paleozoic were initially formed in great positive structures. After Caledonian Structure Movement, Yangtze Plate and Cathaysian Plate jointed to form an uniform plate named Southern China Plate. From mid Devonian to mid Triassic, southern China accepted marine deposits with great thickness, on one side it made the oil reservoirs of lower combination(Z-S) transform to gas reservoirs, on the other side new petroliferous combinations came into being in Devonian, Carboniferous and mid Triassic(D-T Combinations).Analyses on typical palaeo-reservoirs of Paleozoic in southern China show: palaeo-reservoirs are comprehensive products from multiple structural movements and their superimpositions and generally the palaeo-reservoirs underwent the evolution course of oil reservoir—gas reservoir-residual in which Caledonian movement and indo-China—Yanshanian movement play key role in paleo-reservoirs farmation and their evolution. The basic reasons for reform and destruction of palaeo-reservoirs are the great elevation and erosion of strata in the later period.According to the analyses above, Paleozoic in south China is still an important field for oil & gas exploration in China. Our exploration works in future should attach more attention to the difference between paleao-reservoirs and present reservoirs. The paleao-reservoirs were nearly destructed finally while the present reservoirs need to be preserved. The forming mechanism of paleao-reservoirs are essentially different. Hydrocarbon resources of paleao-reservoirs come from source rocks. However, the lower Paleozoic source rocks can never be effective source rocks due to their overly thermal maturities. Therefore, hydrocarbons that generated in early times must be preserved in special way and then become effective secondary sources as the substantial supply to the latter reservoir forming. The water-solution gas reservoir forming model of Weiyuan field has set a guidence for discovery of paleo-reservoirs in south China. As for south China area, two conditions must be satisfied for the paleo-reservoirs formation, one is favorable later preserving conditions and the other is that the deeply buried reservoirs in early stage must be considerably uplifted later. Exploration breakthrough will never attained in areas with on preserving conditions or overly buried depth. According to these above analyses, Wuling Depression, Qiannan Depression in south-western edge of Xuefeng Uplift, Zigui Synclinorium, Guizhong Depression in the areas of Western part of Hunan and Huabei Province are considered to be the major exploration area in future.