Accumulation Model And Resource Potential Evaluation Of Oil Sand From Mesozoic And Cenozoic In Northern Qaidam Basin

Oil sand of the Lower Ganchaigou Formation,Upper Ganchaigou Formation and Caishiling Formation in the Northern Qaidam Basin was analyzed using petrological and geochemical methods to assess the petrophysical properties,relationship between physical properties and lithological properties,organic matter origin/type and thermal maturity,paleoenvironmental conditions,and to correlation between this oil and its potential source rocks.The formation mechanism of light oil sand,reservoir formation model and resource evaluation are also comprehensively analyzed.Petrographic and petrophysical analysis indicates some differences in the reservoir quality between Lenghu and Yuqia oil sand.The oil sand in Lenghu area is characteristic by feldspar sandstone,poor structural maturity and compositional maturity,angular to subangular particles and medium sorting,with mainly litho-feldspathic sandstone and feldspatho-lithicsandstone.Its particle sizes are range from fine to coarse,with angular to sub-angular,poor to well sorted.The connection between rock composition and structure was estimated from the relationships between porosity and permeability with oil content,RQI(reservoir quality index),NPI(normalized porosity index)and FZI(flow zone indicator).The petrophysical results show that the Lenghu reservoir possesses medium porosity and low permeability(poor reservoir quality)due to some clay minerals that occupied pores spaces,resulted in poor connectivity.On the other hand,the reservoir quality of oil sand in Yuqia area is considered to be good with a presence of many large pore spaces within the rocks which enhance frequent and easy flow of fluids in the interconnected pore spaces.The organic matter in the oil sand is of high quality as indicated by the relatively high hydrocarbon fractions and low NSO(nitrogen-sulfur-oxygen or resin)compound.The extracted oils of the studied samples have entered the early maturity stage as suggested by the biomarker maturity parameters.Geochemical analysis indicates that source rock of oil sand in Lenghu area forms in suboxic,low salinity lacustrine environment with algae and higher plant as major source.Paleoenvironmental conditions of the source rock formed the oil of Yuqia oil sand are believed to be under lacustrine influence with relatively high salinity,of suboxic conditions that received mixed aquatic source of predominantly algal input.Oil-source correlation results suggest that source rocks for oil sand in Lenghu are the Middle Jurassic carbonaceous mudstones and organic-rich mudstones,whereas the organic matter of oil sand in Yuqia comes from the Middle Jurassic oil shales.The formation mechanism of the light oil from oil sand in the Northern Qaidam Basin has been studied and solved.It is considered that the Paleogene oil sand in the Northern Qaidam Basin was formed under a continuous filling with some biodegradation.The filling rate is higher than the degradation rate,which results in the degradation being concealed.This is suggested by the oil seepage on the surface of the field,the difference in composition of the oil from surface and subsurface,the existence of 25-norhopane and tectonic thermal evolution analysis.The formation of oil sand has a good material basis.Jurassic source rocks that are widely distributed in the Northern Qaidam Basin are known to be good quality.Most of them entered the mature stage and peak period of hydrocarbon generation and expulsion.Since the Mesozoic,the regional multi-stage strong tectonic movement provided a good accumulation site for oil and destroyed the early oil reservoirs.Based on the history of hydrocarbon generation,expulsion and tectonic evolution,it is determined that the oil sand in Northern Qaidam Basin was formed by the preservation of crude oil secondary migrated to the surface which derived from oil reservoir destructed by the tectonic.The evaluation of oil sand resources in Northern Qaidam Basin is estimated based on the determination of oil content,density of oil sand,measure of different depths of oil sand,calculation of true burial thickness,and minimum mining depth according to economic and technical indicators for oil sand development.The calculation of oil sand resources in Northern Qaidam Basin lays a foundation for further exploration and development.