Mesozoic Geological Characteristics, Accumulation Patterns And Resource Prospect Of Oil Sands In Chuxiong Basin

Chuxiong basin located at the middle part of Yunnan provinceand the west part of Yungui Plateau with the altitude of 1700 to 2200m a.s.l and covered 36500 km2.It is wedge-shaped for the north part iswider than the south part. Oil sands belonging to unconventionaloil-gas resource,is an important alternative resource for conventionaloil-gas resource. Chuxiong basin is a lower-degree explored basin.The distribution regulations,accumulation patterns, resource amount,resource potentials of oil sands have never been systematically studied.In the South of China, the Yangtze platform experiencedCenozoic-Mesozoic tectonic movements, especially the Himalayanfault-tecnotic reconstructions. The organic matters of most areasexperienced several thermal alterations, result in high degree ofmineralization and low oil-bearing. The researching areas are Sazhi,Longjie,Zhonghe,Xiangyun,Nanhua and Yongsheng in Chuxiongbasin in this paper. The oil-sands-forming Mechanism is that ancient oil reservoirsare deformed by tectonic movements and uplifted to surface,simultaneously, a set of processes such as oxidation, biodegradation,leaching make light ingredients escape so that hydrocarbon convertinto oil sands. During the formation of oil reservoir all the processessuch as hydrocarbon generating, migrating and accumulating arecontrolled by the history of tectonic and sediment. Chuxiong basin isa rifting basin belonging to Panxi rift, with a foreland basin beingsuperposed on the south-west part. From Paleozoic when basin isn'tformed to Mesozoic and Cenozoic when basin developed andshrinked,it can be divided into four tectonic evolution stages: riftingstage, depression stage, shrinking stage, uplifting stage. Chuxiongbasin is a large-middle Msozoic -Cenozoic reformed basin, the strataof marine facies, marine-terrigenous facies, terrigenous faciessuccessively deposited, since Triassic when it formed. Theupper-Triassic strata are a set of dark sandstones, claystones and coalmeasures of marine facies, terrigenous facies and marine-terrigenousfacies, being exposed around the margin of the basin with greatthickness. Jurassic formation developed widely, covered on theupper-Triassic conformably, and mainly consist of a set of redinland-river-lake imposed-oxidated sandstones and claystones withcoal measures. Cretaceous sediment is red inland-river-lake facieswhich located at the north part of Chuxiong basin.There had been formed large-scale ancient oil reservoirs in thisbasin. Most of the oil reservoirs was raising to surface and wasdestroyed by multiple tectonic movements,resulting in bitumenoutcrops scattered everywhere. The oil sands mainly distributed in theformation from upper-triassic Shezi to Ganhaizi and Jurassic Zhanghe,and croped out at the slope zone and the top of tectonic. Bitumenfilled in the pores or cracks of sandstone and claystone, arecharacterized by extensive outcrops,thin caps and high maturity.Effective area percent of pore ranges from 8% to 15%, and haveindustrial thickness.Through thin section identification, we analyze sedimentaryfacies, sedimentary environments and reservoir conditions ofbitumen-bearing sandstones. The Reservoir rocks is mainly lithicquartzarenite, as well as a part of lithic arkose, with high maturity ofcomposition and texture. According to causes of formation, thereservoir pores can be classified as primary pore and secondary pore,and the later is primary which formed by clast dissolution,authigenic-mineral dissolution and replacement. Bitumen filled inintergranular pore of fine sands and silt stones. According to thecharacteristics of pores, five types can be classified which areintergranular solution pore, intragranular solution pore, cast solutionpore , micro-pore and crack. Based on the analysis of sedimentaryfacies, we conclude that delta front sub-facies and delta plainsub-facies are favorable facies, and coarst-shallow lake facies,aqueous turbidity, olistostrome are unfavorable for worse physicalquality of container rock.By the analysis of tectonic backgrounds and sedimentaryenvironments, as well as the study on source beds,reservoirs and caps,formation conditions of oil sands can be summarized as follows : (1)There is favorable geological backgrounds;Plenty of hydrocarbonsource and large-scale reservoir is essential for oil sands formation. (2)Carrier-system units such as pore, crack and plane of unconformitycollocated appropriately, and forming an effective migration network.(3) Oil sands experienced several strong tectonic movements. (4)Slope zone and tectonic top are the most advantaged areas.Stong tectonic movements and multiple tectonic deformationresult in the complicated models and processes of oil-sands formation.Both pore bitumen developed in Yanshanian and crack bitumenformed in Himalayan can coexist in those areas, which indicate thatoil sands formed in Himalayan and Yanshanian period. The basicmineralization model of oil sands can be summarized as follows:ancient oil reservoirs were uplifted to surface twice, and oil and gaswere oxidated twice. During Middle-Yanshanian movements, thebasin was uplifted and folded, and in this uplifting period, oil-gasreservoir undergo oxidation and leaching, firstly forming oxidated oilsands. From late-Yanshanian movements to early-Himalayanmovements, the K2-E gypsum-bearing regional cap depositedsuccessively, which make the remaining reservoir to be re-imbededand reformed, then form a new encirclement. Duringmiddle-Himalayan,flaw faults destroy the reservoirs of the north partof the basin. Oil and gas was re-oxidated , resulted in moreworse-quality of oil sands. Late Himalayan movement bringed stronguplift and denudation to this area, ancient oil reservoirs were upliftedto surface and oxidated to the oil sands nowaday.We measure area percent of pore by thin sections and ascertainexploitable quotiety by floatation process, caculate the thickness of oilsand beds based on the basic geological data and profiles. Accordingto different oil-sands-body controlled factors and features of beds, wedelineate the oil sands body limit. Through the study above,we getthe assessment parameters, and then, built up a scientific andreasonable foundation for later work. We assess high-degree studyingareas by oil saturation method. we adopt analog method to assesslow-degree studying areas on the score of factors includingsedimentary facies, lithology, diagenesis and structural features.Firstly, we adopt Monte-Carlo method to assess probabilistic resourceamount of each sub-unit, and get resource probability curve of eachassessment sub-unit, and make probabilistic plus to get higher unitprobabilistic resource amount. In the end, the oil sands resourceamount of Chuxiong basin at different probability is gotten. Theresults show that the geological resource amount of oil sands reach0.21×108 and the exploitable resources amount of oil sands is0.18×108 at the possibilities of 50% respectively. By the syntheticalanalysis of evaluation parameters and geological features,we draw aconclusion that the oil sands of Zhonghe,Xiangyun,Nanhua,Sazhihave greater resource amount and better qualities.