Natural gas accumulation models and favorable exploration areas of the Cambrian-Ordovician in the Ordos Basin
After over 30 years of exploration and development,the Cambrian-Ordovician marine strata of Lower Paleozoic in the Ordos Basin have submitted over one trillion cubic meters of 3P reserves in total,and a series of significant theoretical achievements have been obtained.With increasing oil and gas demands in China,the contradiction between sustaining rapid production growth and insufficient exploration reserve replacement is intensified,so it is necessary to search for substantial reserves to solidify resource foundations.To identify new potential natural gas exploration areas,this paper systematically studies the source rock conditions,reservoir characteristics,and natural gas accumulation models of the Cambrian-Ordovician in the Ordos Basin and predicts the favorable exploration areas by analyzing seismic,field profiles,drilling cores,microscopic thin sections and test data,on the basis of existing natural gas exploration achievements.The following research results are obtained.First,the Cambrian in the Ordos Basin exhibits the natural gas accumulation models of"new source rock,old reservoir,lateral hydrocarbon supply"and"lower source rock,upper reservoir,fault-conducted hydrocarbon supply",with the Upper Paleozoic coal-measure source rocks and the Lower Cambrian source rocks serving as the hydrocarbon source rocks of Cambrian gas reservoirs and the extensive fractured-porous(vuggy)dolomite reservoirs facilitating natural gas accumulation.Second,the Ordovician sub-salt layers in the east of the Ordos Basin exhibits a natural gas accumulation model of"self-source rock,self-reservoir,fault-conducted hydrocarbon supply",with the internal argillaceous source rocks and the dispersed soluble organic matters capable of hydrocarbon supply and the dolomite reservoirs along paleo-uplifts providing substantial reservoir space for natural gas enrichment.Third,there are two natural gas accumulation models in the west of the Ordos Basin,i.e.the"self-source rock and self-reservoir within the source rock"of marine shale,and the"upper-source rock,lower-reservoir,fault lateral hydrocarbon supply"of carbonate rock,with the Kelimoli Formation and the Wulalike Formation mudstone(shale)capable of substantial hydrocarbon supply and the dolomite reservoirs along uplifts and the shale reservoirs along depressions offering significant reservoir capacities.In conclusion,the Cambrian in Wuqi-Huanxian and Zhengning-Huangling areas of southern Ordos Basin,the 2nd and 3rd Members of Ordovician Majiagou Formation in Yulin-Jingbian area of eastern Ordos Basin,and the Ordovician dolomite in Taole and Majiatan areas of western Ordos Basin are strategic domains for future substantial petroleum discoveries.The research results are of great guidance to the selection and favorable target evaluation of Cambrian-Ordovician oil and gas exploration areas in the Ordos Basin.
Ordos BasinCambrian-OrdovicianDolomiteMarine shaleNatural gas accumulation model
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