Research On Reservior Forming Condition Of Upper Majiagou Formation In Yichuan-Fuxian Area

The Ordos Basin is a sedimentary basin on the second continent of China and has abundant oil and gas resources.Yichuan-Fuxian area is located in the southeastern part of Ordos Basin.Yican 1 and Yi15 wells in this area have been tested for gas and obtained industrial gas flow,which shows good exploration potential.Taking the upper Majiagou Formation in Yichuan-Fuxian area as the research object,on the basis of summing up the previous research results,this paper analyzed the hydrocarbon source rock characteristics,sedimentary facies characteristics,reservoir characteristics and source-reservoir-cap combination and other reservoir-forming conditions in the study area.Through dissecting typical gas producing wells in the study area,the main controlling factors of natural gas reservoir formation were summarized,the reservoir-forming model was established,and the favorable reservoir-forming areas were optimized according to the main controlling factors of reservoir formation in the study area.The main findings are as follows:?1?The source rocks of the upper Paleozoic coal measures are widely covered with hydrocarbon generation in Yichuan-Fuxian area.The maturity of organic matter is high and is in the over-mature stage.The thickness of source rocks is 50⁸0m,and the gas generation intensity is?25⁴0?×10⁸m³/km².The upper Paleozoic Marine carbonate hydrocarbon source rock development,organic matter abundance TOC between 0.15%⁰.25%,the overall low abundance,and the types of organic matter are mainly type I and II₁,organic matter maturity in a mature stage,the intensity of angry for?5¹0?×10⁸m³/km².?2?The sedimentary period of Mawu Member of Ordovician Majiagou Formation is a limited carbonate platform under the condition of epicontinental sea in Yichuan-Fuxian area.The upper assemblage of Mawu Member mainly develops gypsum dolomite,gypsum dolomite,mud dolomite,karst breccia dolomite,and karst pore,gypsum moulding pore and fissure as the main reservoir space,but the pore is mainly composed of mud microcrystalline dolomite,powder fine crystal dolomite and karst breccia dolomite.The porosity and permeability are relatively low,which are very low porosity and low permeability reservoirs.?3?The upper assemblage of Majiagou Formation in Yichuan-Fuxian area has a good upper and lower Paleozoic source-reservoir transport system,favorable sedimentary facies and karst palaeogeomorphology.The type of source-reservoir-cap assemblage is the weathering crust palaeokarst source-reservoir-cap assemblage of upper and lower reservoirs.The main types of traps are unconformable traps and lithologic traps.?4?The gas source correlation analysis shows that the upper gas source of the majiagou formation is dominated by the upper gas source of the Paleozoic coal measures,and the lower Paleozoic carbonate source rocks are auxiliary.The upper assemblage of Majiagou Formation in Yichuan-Fuxian area has good source-reservoir transport system,favorable sedimentary facies and karst palaeogeomorphology of upper and lower Paleozoic.Combining with the main factors of reservoir formation in the study area,three favorable areas for reservoir formation were selected,namely yichuan area,fuxian area,and the southern area of yichuan-fuxian county.