The Study Of Gas Geochemical Characteristics Of Accumulationin The Upper Paleozoic Of Southwest Ordos Basin

The Southwest Ordos Basin is low porosity low permeability lithologic gas reservoir, and its main gas reservoirs are Permian He8 of Shihezi formation and Shan1 of shanxi formation. Based on the previous research results and the exiting exploration data, using both the petroleum system theory and pool-forming dynamic, guided by the theory and method of Petroleum Geology, Sedimentary Petrology, Hydrocarbon Accumulation Geology and Oil & Gas Geochemistry, this article detailed studies the gas geochemical characteristics, the natural gas origin, the gas source rock characteristics and the gas accumulation period. The main conclusions and innovative achievements that obtained by this article are as follows: In the Southwest Ordos Basin, the upper palaeozoic gas of this area is mainly coal gas generated hydrocarbon source rocks, and a small amount of oil type gas mixes in. The dark mudstone as a whole presents a trend of gradually becoming thinner from north to south. The organic matter of source rock in relatively high abundance, kerogen belongs to type Ⅲ. And the thermal evolution is during the high mature-post mature phases. The averageof the intensity of gas generation is 15.5×l08m3/km2. It presents a trend of gradually becoming smaller from north to south. And the formation and distribution of the upper palaeozoic gas reservoir is controlled obviously by gas source condition. The research of upper palaeozoic natural gas in the Southwest Ordos Basin shows that the averageof CH4 is 88.60%, the average of aridity coefficient is 98.41%. The δ13C1 of gas is mainly distributed in-26‰~-32‰. The δ13C2 is mainly distributed in-26‰~-33‰. The δ2D1 is mainly distributed in-160‰~-190‰. The δ2D2 is mainly distributed in-140‰~-180‰. The gas maturity level is between 1.2%~1.8%, and is in the stage of mature to high mature.The main driving force for gas migration of Benxi formation, Shan2 in the Southwest Ordos Basin is abnormal pressure formed by hydrocarbon generation. The main driving force for gas migration of Shan1 and He8 is molecular diffusion reaction force, and the upper palaeozoic gas has the characteristics of proximal deposit.migration accumulation.The analysis of the upper palaeozoic inclusion and source rock simulation shows that at late Triassic, the upper palaeozoic hydrocarbon source rock enters hydrocarbon generation threshold. It is in low mature stage, and begins to produce hydrocarbon. Basically no gas discharge. At early-middle Jurassic, the source rock is in mature stage, and the hydrocarbon generation quantity increases gradually. Begin to have a gas discharge. At late Jurassic to early Cretaceous, the source rock is in high mature-over mature stage and the generation and expulsion of hydrocarbons is abundant. At early Cretaceous, the hydrocarbon-generating peak is reached. After early Cretaceous, the organic matter maturity basically has no change, and the generation and expulsion of hydrocarbons trends to stop.In the Upper Paleozoic of southwest basin,Predicted 5 favorable abundance zone and 5320km2 of Predicted area.