The Research On The Formation Process And Mechanism Of Gas Pools In Proterozoic To Low Paleozoic Erathem, Sichuan Basin

The thesis summarizes the exploration conditions and results of Proterozoic-Low Paleozoic Erathem in Sichuan basin. Petroleum exploration in Sichuan basin has lasted forty years and drilled more than fifty wild cat wells. These wells distributed at Leshan-Longnvsi paleouplift and orogenic belts around Sichuan basin. Till now, there is a gas field (Weyuan gas field) with proved reserves of 408.6×10~8m~3(2000), a gas-brone area (Ziyang gas-brone area) with probable reserves 102×10~8m~3 in upper Sinian system, and five gas wells(Wei42, Wei52, Wei78, Wei92 and Shui 2) in Cambrian sytem in Weiyuan gas field. In the other area of the basin, there are six gas show wells. Except Weiyuan gas field there are only five industrial wells (Zi 3, Dongshen 1, Heshen 1, Wuke 1 and Tai 13). The success rate for the petroleum exploration in Sichuan basin is no more than 10%.Therefore the present thesis has firstly studied the evolution of energy field and change process of hydrocarbon phase state in the Sinian to Low Paleozoic natural gas pools in Sichuan basin. Before the whole uplift of Sichuan basin by Himalayan movement, the Sinian to Low Paleozoic Erathem in Sichuan basin had suffered deep bury and high temperature stage. The paper has quantitatively simulated the buried and geothermal histories of the Sinian to Low Paleozoic Erathem by apatite fission track and fluid inclusion data.There was strong uplift at Himalaya movement period in Sichuan basin and its peripheral orogenic zones. The main uplift stage was Neogene. The rise rates exceeded 100m/ma, and the rise height exceeded 4200m. The uplift stage can be divided into three stages: the first stage, differential uplift stage, late Cretaceous to Paleogene; the second stage, whole basin uplift stage with the rise rates of more than 40m/Ma and the rise height of more than 1000m; the third stage, fast uplift stage with the rise rate of more than 100m/Ma and the rise height of more than 1500m exceped the west Sichuan. The uplift history in the peripheral orogenic zones of Sichuan basin can also be divided into three stages, the first stage(120Ma-65Ma) with the rise rate of 15-93m/Ma and the rise height of 1000m; the second stage(65-25Ma) with the rise rate of 5-47m/Ma and the rise height of 250-1800m; the third stage (25Ma-) with the rise rate of 60-300m/Ma and the rise height of 1300 to more than 3000m.The paleo-geopressure was overpressure by deep buried. But it is normal pressure now. It showed that the geopressure field suffered the process of overpressure to normal pressure. The gas in Sinian-Low Paleozoic Erathem are thermal cracked gas by high temperature. Hydrocarbon pools suffered evolutional process from paleo-oil pool to paleo-gas pool. Because of the overpressure by thermal cracked gas, some gas (CH4) dissolved in water by overpressure and high temperature.Based on the studying above, the present thesis has analyzed the gas reservoir characteristics and formation mechanism in the Sinian to Low Paleozoic Erathem, taking Weiyuan-Ziyang and Dingshan as examples.The formation process of Ziyang Sinian gas-borne area is: (Ziyang—Weiyuan) palaeo-oil pool→oil thermal crack→oil-cracked gas pool→gas pool modified by uplift→present gas borne area.The formation process of Weiyuan Sinian gas field is: (Ziyang--Weiyuan) palaeo-oil pool→oil thermal crack→the gas largely dissolved in water→the water with dissolved gas migrated to Weiyuan area by uplift and escaped from water in Weiyuan→present gas field.The destroy process of Dingshan structure is: palaeo-oil pool→plaeo-oil pool destroyed→palaeo-oil pool→oil thermal cracked and formed paleo-gas pool→gas escape by uplift and fault actionThe thesis has summarized the characteristics of petroleum pools in the Sinian to Low Paleozoic Erathem in Sichuan basin:(1) The process, generating and exhausting hydrocarbon from mature organic matter, happened before Triassic Period and made the petroleum pool multi-formation in early time. The main characters of this process are difference of hydrocarbon generation and exhaustion, multi-migration and accumulation;(2) the process of oil cracking to gas before middle Jurassic Epoch produced gas and bitumen and made the gas pool formation in middle period. The main characters of the process are deep buried and oil cracking gas;(3) the process of structural trap formation and uplifting denuation led to the gas pool formation in late period. The main characters of the process are uplifting denuation and late period gas pool formation.