Enrichment Characteristics Of Organic Matter In The Chang 7 Oil Shale Section In Triassic Yanchang Formation In Southern Margin Of Ordos Basin

This paper takes the southern part of the Ordos Basin?the Weibei uplift area and the southern part of the northern slope of Shaanxi?as the research area.After the first national oil shale resource evaluation,it confirmed that the oil shale resources are abundant.This paper mainly studies the characteristics of oil shale and the characteristics of organic matter enrichment in the study area.Oil shale formations in the study area were formed in the Ordos Basin during the Middle-Late Triassic Yanchang Formation,which was the largest flooding period in the Ordos Basin.On the basis of previous data collection,this paper studied the organic matter enrichment characteristics of the Chang 7 oil shale in the target area of the study area through field surveys,measured profiles,core observations,sample collection,and test analysis.This study enriches the Ordos Basin.The metallogenic model and resource evaluation of oil shale in the Triassic Yanchang Formation are of great significance.The color of the oil shale in the study area is grayish black and grayish brown,with horizontal bedding,low hardness and greasy sheen.The main minerals of oil shale and shale are terrigenous clastic minerals and clay minerals.The terrigenous detrital minerals are mainly quartz,the clay minerals are mainly Eimeng and illite;the shale and oil shale contain pyrite,and the pyrite content in the oil shale is much higher shale;the trace element content of oil shale and mud shale is significantly enriched in Mo;the oil shale has high organic matter abundance and is an excellent source rock,the organic matter types are type I and type II₁,and in the low maturation-immature stage.It is of great significance for enriching the metallogenic model and resource evaluation of the oil shale of the Triassic Yanchang Formation in the Ordos Basin.Based on a detailed observation of the core of the fully coring wells and analysis of logging data,the Yanchang Formation?chang6-chang10?is divided into three third-order sequences?Sequence I,II,and III?.The lower part of chang 9,chang 10,and chang 8 is the sequence I,the upper part of chang 7 and chang 8 is the sequence II,and chang 6 is a part of the sequence III.In the Yanchang Formation of the study area,there are mainly lacustrine,delta and turbidite deposits.Among them,the lake facies recognizes three kinds of sedimentary microfacies:shallow sands,muddy shallow lakes,and semi-deep lake-deep lake mudstone.The delta facies identified four sedimentary microfacies of underwater distributary channels,interchannels,estuary dams,and sheet sands.Visible fossils,fish manure fossils and plant stems and plant leaf fossils are visible in the formation.The sequence II in the study area is the main development stratum of oil shale.Comprehensive analysis of the TOC and rock pyrolysis data of the ZK903 well and the He 3 well sequence II show that the organic matter abundance of the HST is the highest,the organic matter type is the best,and the organic matter type II1 is the best.Mainly,with a small amount of type I and type II;followed by RST and TST,dominated by type II1 and type II organic matter;the worst was LST,mainly type II2 organic matter.Through studying the distribution characteristics of organic matter in the sequence stratigraphic framework of this area,we can see that the organic matter in the oil shale section is mainly concentrated in the HST of sequence II,and a small amount is in RST and LST.Among them,HST develops semi-deep lake-deep lacustrine deposits,the most abundant and developed organic shale with good quality;RST and LST developed semi-deep lakes-deep lacustrine lakes and muddy shallow lake deposits,with a rich organic matter.Poor and poor quality oil shale developed in RST.The characteristics of the sedimentary tectonic evolution in the comprehensive study area and the ratio of TOC and trace elements in the extension group from 6 to 10were found in the vertical change.In the drastic phase of the depression in the lake basin,the overall basement was unbalanced and the depression was subsidence under the palaeotectonics.Warm and moist palaeoclimate conditions,semi-deep lakes-deep lake sedimentary environments,and reduced freshwater paleolake water conditions are conducive to the enrichment of organic matter.