Geochemical Characteristics And Genesis Of Tight And Shale Oil From The 7^th Member Of Yanchang Formation In Ordos Basin

The tight and shale oil resources in the Ordos Basin,which is one of the most important basins for producing unconventional petroleum in China,are of great exploration potential,due to that excellent source rocks have been widely developed in the 7^th Member of the Yanchang Formation.As reservoir rocks,the shale and tight sandstone in the member are generally poor with respect to physical properties and have great heterogeneity.Crude oil reserved in these rocks is characterized by generally high content of light components,low density and good flowability,which thus facilitate the great success of commercial exploitation of unconventional oil.Attentions have been mainly paid on the study of physical property and genesis of shale and tight sandstone.However,controlling factors for the relatively high content of light components and good flowability of oil are still poorly understood.In addition,restricted by conventional evluation methods for shale oil potential and due to relatively large variations in depositional environment of shale,the oil potential of the 7^th Member is another key issue needed to be carefully addressed.The 7^th Member is the most important section for both shale oil and tight oil explorations of Yanchang Formation.Therefore,the oil samples in tight sandstones and shales have been systemically collected from the 7^th Member of the basin in this study,as well as shale and tight sandstone samples.Tight oil samples from the 6^thh Member were also collected for a comparative study.After a series of oil analyses on physical property,group composition,molecular characteristics,carbon isotopes,etc.,further studies on tight sandstones and shales were also carried out,concerning with mineral-organic compositions,pore distribution and oil occurrence state.Meanwhile,an experimental simulation of hydrocarbon-generation in the oil window has been performed with an organic-rich shale from the 7^th Member.Main conclusions of this study are listed below:?1?The oil samples from tight sandstones and shales are of similar compositions,with high content of hydrocarbon fraction and low content of polar compounds.The viscosity,density and freezing point of both tight oil and shale oil,at a basin scale,are relatively low,and the flow ability is pretty good.This is related to very high content of saturated hydrocarbons and very low content of polar and aromatic compounds in the oil,which is normally greater than 75%and less than25%,respectively.Carbon isotopic compositions of n-alkanes in the oil show relatively small variation?-33‰to-30‰?,indicating that the oil source was mainly derived from lacustrine type I-II organic matter with minor input of wax compounds from the high plant,which is also supported by the characteristics of molecular markers and the pyrolyzates of low mature shale.Regarding the property of source rocks,these have provided favorable conditions for the formation of light,tight oil.?2?Large amount of analysis of methylphenanthrene isomerization ratios for oil and extracts of tight sandstone and shale have suggested a main R_o range of 0.8–1.3%for the 7^th Member of Yanchang Formation.Mean?¹³C value of n-alkanes from tight oil and extracts of tight sandstone varied little with increasing maturity,limiting the oil maturity as high as EasyRo 1.4%on the basis of results of thermal simulation experiments.These all suggested that the member is mainly within maturity intervals varying from the oil peak to the late oil-generation stage.The increase of maturity of tight oil from the seventh Member has shown evident relationships with the increase of saturated/aromatic hydrocarbon ratios of oil and with the decrease of oil viscosity.Thus relatively high maturity is also important for the formation of light,tight oil.?3?A compositional comparison of extracts of the Chang7 shale with solvents of different polarity has suggested a similarity of group compositions between shale oil and the extracts obtained with petroleum ether.By contrast,the extracts obtained with a solvent mixture of dichloromethane and methanol contain relatively abundant resins and asphaltenes.Migration fractionation happened within shale layer is mainly responsible for the the decreases of oil density and viscosity.Quantification of n-alkanes of shale oil and tight oil has demonstrated different degrees of loss of long-chain alkanes,which has happened during the migration of the products of hydrocarbon generation either within shale layer or from shale to tight sandstone reservoirs.This process would further lower the oil density and viscosity.?4?Free oil dominated the extractable organic matter in tight sandstone,and is similar to the tight oil with respect to their group compositions.Free oil has much higher contents of saturated hydrocarbons but lower contents of aromatic and polar compounds than the adsorbed oil.The maturity of free oil is slightly higher than that of adsorbed oil.Free oil was charged later than adsorbed oil and has been affected by migration fractionation with a larger extent relative to adsorbed oil,suggesting that the sandstone reservoir became more and more tight accompanying the oil infilling.?5?The amount of shale extracts obtained with petroleum ether show relatively poor relationships with the TOC content and the amount of free hydrocarbons of Rock-Eval analysis.However,relatively good relationships would be observed,once these data were separately evaluated according to the sedimentary sub-facies.Therefore,evaluations of shale oil potential,when using the extracts amount and S₁values,should be carefully performed by taking the variation in sedimentary facies into account.