Geological Characteristics And Accumulation Model Of Neogene Oil Sands In Western Qaidam Basin

With the rapid development of economy, the world has a rising demand forenergy. Increasing consumption of people’s daily life and shortage of conventional oilhave made it urgent to explore and develop unconventional oil resources. Oil sands,as an important kind of unconventional oil and gas, its close relationship with theconventional oil and great potential make it been paid more and more attention. Theoil sands resources in Qaidam Basin are rich, mainly distributed in the west and northarea. There are abundant outcrops and oil seepage on the ground. According to thenew round of evaluation of oil and gas resources, the geological resources of oil sandsin Qaidam basin amounted to4.94×108t, revealing much better developing prospect.But the exploration of oil sands is lower, and its geological features and distributionregularity and accumulation model are not well understood. In this paper, we analyzethe Neogene oil sands in western basin, and we hope to provide reference for theexploration and development of regional oil sands, or even the whole basin.Field geological survey reveals that the Neogene oil sands in western QaidamBasin is mainly distributed in Youshashan, Gancaigou, Nanyishan and other high partsof the structure or regions with faults developing. Oil sands bearing layers areXiayoushashan formation, Shangyoushashan formation. Locally, Shizigou formationalso contains. Petrological analysis of Xiayoushashan formation shows that, lithologictypes of the oil sands reservoir are mainly debris-feldspar sandstone, feldsparsandstone and feldspar-debris sandstone, content of clastic particle is77%~97%,mainly comprised of quartz(20%~60%), feldspar (10%~55%) and debris (5%~40%).Mica is rich locally. The compositions of interstitial material are mainly calcareouscement and clay, with the content is about5%~18%. The rock has lowercompositional and structural maturity, reflecting the original sedimentary material wascarried a short distance. Diagenesis analysis shows that it may experience a diagenetic evolution from middle diagenetic phase A to B. Regional sedimentary evolution andgrain size analysis reveal that Xiayoushashan formation mainly deposited Delta plain.With the average porosity of16.18%, and the average air permeability of60.81mD, itbelongs to medium-low porosity and medium-low permeability reservoirs.We operate physical property analysis, group separation and GC-MS analysis onthe extractions of oil sands. The results show that it is much rich in saturatedhydrocarbon (47.48%~58.08%, average52.78%), and it has higher density and lowerviscosity with its density1.01g/cm3and viscosity1976~2010mPa.s when theenvironment temperature is20℃. The characteristics of biomarkers show that, thesource rocks of the oil sands of Xiayoushashan formation are Paleogene and Neogene,which deposited in a reducing and salty environment. The organic matter is mainlyderived from lower aquatic organisms, and the maturity level is low mature to maturestage. The Oil sands endured lower biodegradation or oxidation.Based on the analysis of forming conditions and controlling factors of theNeogene oil sand, we conclude below5aspects. Thick and wide distribution ofeffective source rocks offer material basis for oil sands. The distribution range andsize of oil sands are controlled by sedimentary facies and porous conditions. Variouscombinations of source and reservoir beds provide favorable conditions for oilaccumulation. Multiple periods of tectonic activities is an important factor in theformation of oil sands. Dry climate conditions and extensively distributed salt crustcontribute to the formation of high-quality oil sands. On the late Himalayan age,strong activities result in destroys on the preservation conditions of previous reservoir,some even were directly uplifted to the ground surface. Crude oil had beenexperienced oxidation or biodegradable or other secondary alterations, and oil sandsformed. Dry climate and extensively distributed salt crust have provided goodpreservation conditions, and contributed to the formation of high quality oil sands.