Formation Mechanics And Exploration Targets Evaluation Of Ordovician Paleokarst Reservoir In Western Ordos Basin

Many geological problems are solved by the theory and technical methods of karst geomorphology,carbonate sedimentary petrology,carbonate reservoir geology,reservoir geochemistry,sequence stratigraphy,logging geology,petroleum geology,petroleum geochemistry,etc.These problems include forming conditions,macro-and-micro-features of paleokarst,karst facies,main controlling factors,distribution rule of karst reservoir,hydrocarbon accumulation conditions and exploration potential evaluation in the Ordovician,western Ordos Basin.The results of study indicate that intra-platform beach,marginal-platform reef-beach are the favorable sedimentary environment of soluble lithology(grainstones)in Zhuozishan and Kelimoli formations.The formation types are mainly continuous dolomite and continuous limestone in Zhuozishan formations.Its karst reservoirs are mainly intergranular dissolved pores and dissolved holes.The formation types are mainly continuous limestone and limestone-dolomite sandwich type in Kelimoli formations.The scale of its karst cave is bigger.The four identification mark of palaeokarst reservoir are established by drilling cores,field outcrop observation,slice appraisal and well logs analysis.Combined with the geochemical study,the palaeokarst of study area is divied into three types of syngenetic karst,supergene karst and burial kars.It established the model of intra-platform beach karst,shallow tidal flat karst and bedding deep phreatic karst.The relationship between sequence and palaeokarst indicates that the first sequence boundary corresponds to karst surface of weathering crust in the top of Ordovician,third sequence obviously controls syngenetic kars and bedding karst.The well segments of karst are belonged to the highstand systems tract and transgressive systems tract.The main factors of palaeokarst are the structure,lithology,paleoclimate conditions and ancient karst landforms.It shows that paleo-reservoirs exist in the studied area by the organic petrology and quantitative analysis of electron probe.They are divided into three types,Kelimoli top pore filling,Zhuozishan top pore filling and Wulalike fissure filling(?).The study of hydrocarbon accumulation conditions indicated that the main source rocks of natural gas are the shale of Pingliang and Kelimoli formation in the fracture-vuggy carbonate reservoir of study area,the reservoir space involves pores,dissolution pores,holes and fissures,the cap rock included the iron-aluminum mudstone,argillaceous rocks of Benxi formation and the shale of Pingliang formation,the source-reservoir-cap forming assemblages characteristics mainly include the self-generating and self-preserving reservoirs of lower-middle Ordovician strata(O2p?O1k)and the upper-lower Paleozoic source-mixed reservoirs of lower Ordovician(O1m).Finally,the accumulation model of paleokarst reservoir is established,the favorable exploration target areas are preferably selected by the results of seismic interpretation.