| Medium-high maturity lacustrine shale oil is an important replacement for oil and gas reserves and production in China.The Triassic Chang 7 Member in the Ordos Basin bears abundant shale oil resources with huge development potential.There are two kinds of shale oil in the basin,“silty interlayer type” and “pure shale type”.Among them,the latter one is mainly composed of shales,with the total thickness of siltstones less than 5%.The “pure shale type” has been the focus of exploration since its huge volume and total resources.However,because of the complex resource preservation characteristics,and the inapparent “sweet spots” distribution,determining the “sweet spots” and the main production layer(s)is difficult in the exploration and development of shale oil.Conventional geochemical methods determining sweet spots and main producing layer(s),including carbon isotopes,biomarkers,etc.,do not apply to shale oil,and thus it is urgent to develop new approaches for shale oil reservoirs.Recently,Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS)technology has developed rapidly,being an effective method for detecting the composition and content of non-hydrocarbons in crude oil.Shale oil contains a rich variety of non-hydrocarbons with strong heterogeneity in the formation,which is sensitive to the formation and evolution process of petroleum.It can effectively trace the generation,migration,accumulation,and exploitation process of shale oil,and has advantages in identifying sweet spots and the main production layer(s).Taking the Chang7 Shale of the Yanchang Formation in the Ordos Basin as the research object,this study selects 2 typical shale oil wells,namely Well Z22 and Well CY1,to conduct systematic petrological,mineralogical,and organic geochemical analysis.This study focuses on 3key points:(1)the organic geochemical characteristics and differential enrichment of shale oil,(2)the composition and fractionation characteristics of non-hydrocarbons,(3)the designation of sweet spots by non-hydrocarbons.The main achievements and understandings are described as follows:(1)The thick shale in the Ordos Basin is widely developed.In the thick shales,the high-TOC shale intervals generate hydrocarbons,among which the mobile components migrate and enrich to the low-TOC shale intervals,forming “in-source sweet spots” which can be characterized by a high content of free hydrocarbons,high extract yield,a high proportion of saturated hydrocarbons and good mobility,being the exploration target for the “pure shale type” shale oil.There are thick pure shale sections in Well Z22 and Well CY1.Among them,the organic geochemical analysis reveals strong heterogeneity in the abundance and composition of shale oil.The intervals with high shale oil content and a high abundance of light hydrocarbons are usually developed in lower TOC sections.The results of mass balance calculation show that the high-TOC sections have high hydrocarbon generation quantity and positive hydrocarbon expulsion efficiency,suggesting a large amount of hydrocarbon generation and outward expulsion during geological history.While the lowTOC shale section has low hydrocarbon generation quantity and negative hydrocarbon expulsion efficiency,suggesting that the amount of retained hydrocarbons exceeds the theoretically generated ones in geological history.and it receives crude oil migrating from the high-TOC sections.Combined with the geo-fractionation effect,the mobile components are enriched in the low TOC shale section,forming the “in-source sweet spot”with abundant free hydrocarbons,high extraction yield,high saturated content,and favorable mobility.The existence of “in-source sweet spot” suggests the migration and redistribution of shale oil within the pure shales,and the low-TOC shale which is adjacent to the high-TOC sections may be the key exploration target in the thick pure shales.(2)The non-hydrocarbons of shale oil retained in “in-source sweet spots” is characterized by long alkyl side chains and low aromatization degrees.The nonpolar alkyl side chain can weaken the polarity of functional groups and reduce the adsorption capacity of rocks to molecules.Besides,the fewer π-π bonds between aromatic rings of non-hydrocarbons are not easy to cause precipitation,which facilities the mobile of these molecules underground.To explore the fractionation characteristics and mechanism of non-hydrocarbons when migrating,three kinds of samples,including the high-TOC shales,“in-source sweet spots” and siltstones,are selected to characterize the composition of non-hydrocarbons.The results show that compared with the high-TOC shales,“in-source sweet spots” and siltstones are relatively rich in non-hydrocarbons with long alkyl side chains and a low degree of condensation.The reason lies in the replacement of alkyl side chains to the hydrogen atom around the polar functional group,which could shield the polarity of functional groups,and thus hinder the adsorption of crude oil molecules by rocks,and improve the mobility of molecules.At the same time,the low aromatization molecules weaken the π-π bond between the aromatic rings,which makes the crude oil difficult to form nano aggregates,and reduces the precipitation among molecules,making petroleum easy to flow.In addition,the non-hydrocarbons with long alkyl side chains and low condensation degree are the early products of kerogen hydrocarbon generation.At the early stage,the formation is water-saturated with favorable porosity and permeability,which is conducive to the migration of the early-generated molecules,and thus results in the migration and accumulation of the compounds with long side chains and low aromaticity.Owing to that the kerogen releases the compounds with long alkyl side chains and low condensation degrees early,crude oil migration can lead to a low maturity indicated by non-hydrocarbons.Besides,it is discovered that there is a similar fractionation process of non-hydrocarbons when exploited.Compared with the rock extracts,the crude oil is relatively rich in non-hydrocarbons with long alkyl side chains and low condensation degrees.(3)By calculating the compositional similarity of non-hydrocarbons,oil-source correlation can be carried out to identify the distribution of “in-source sweet spots”,the migration direction of shale oil,and the main production layer(s).The compositional similarity calculation of non-hydrocarbons suggests the production contribution of “in-source sweet spots” may be greater than that of thick siltstone layers in the same sequence.By vectorizing the composition data of non-hydrocarbons,the similarity of the compounds can be semi-quantitatively compared by Pearson correlation coefficient,cosine similarity,and Euclidean distance,which can be applied to identify the “in-source sweet spots”,the migration direction of shale oil,and the main production layer(s).(1)The composition of non-hydrocarbons in "in-source sweet spots" is highly similar to siltstone intercalations,thus by comparing the similarity of non-hydrocarbons in thin shale and siltstone layers,potential “in-source sweet spots” can be identified.(2)In pure shales,the similarity of the retained oil composition in the high-TOC shale intervals is high,but as the oil migrates to the “in-source sweet spots”,the compositional similarity gradually decreases,which can be applied to indicate the migration direction of shale oil within the source.(3)The petroleum retained in the production layers with high producibility has high compositional similarity to the oil from the wellhead.In Well CY1,“in-source sweet spots” is one of the main production layers,which production contribution is greater than that of some thick siltstone sections,indicating that “in-source sweet spots” may have a bright prospect in exploration and development.The method that calculating the compositional similarity of non-hydrocarbons is simple and effective in oil-source correlation,which is of potential value in the identification of shale oil sweet spots and main production layer(s). |
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