Heterogeneity And Controlling Factors Of Lithofacies Of The Lower 3^rd Member Of Paleogene Shahejie Formation Lacustrine Shale In Jiyang Depression

A set of organic-rich lacustrine shale is preserved in the Lower 3^rd Member of Paleogene Shahejie Formation in Jiyang Depression.Since exploration began,several wells have acquired high-yield industrial oil in the Lower 3^rd Member of Shahejie Formation,which shows excellent shale oil exploration potential.Unlike the widely distributed marine shale in North America,there is a terrestrial facies lake basin in the Lower 3^rd Member of Shahejie Formation in Jiyang Depression,the distribution of lithofacies is greatly affected by lake area,provenance and lake level fluctuation.The heterogeneity is obvious,and its controlling factors are not clear,so North American shale oil experience cannot be directly used for the development of Chinese continental shale oil exploration and development.This study applies integrated geochemistry,petrology and sedimentary theory,on the basis of establishing sequence stratigraphy framework,elaborate schemes and lithofacies types,the macroscopic and microscopic heterogeneity and the change rule of the shale within sequence stratigraphic framework are analyzed.The relationship established between lithofacies and environmental control factors.Advantaged lithofacies are discussed,the favorable exploration target interval of shale oil is predicted.The conclusions are as follows.1.The sequence stratigraphic framework is established for the Lower 3^rd Member of Shahejie Formation in the study area.Based on well logs,well cores and element geochemistry,the Lower 3^rd Member of Shahejie Formation is interpreted as a third-order sequence according to the identification of sequence interface,a expanding systems tract?EST?,a early highstand systems tract?EHST?and a late highstand systems tract?LHST?are further identified in this sequence.The key wells were under sequence division,and their internal sedimentary structure and characteristics were summarized.Build up connecting-well section in Dongying Sag and Bonan Subsag.The thickness of the shale in different regions were made in systems tracts.The thickness of different systems tracts is equal and stable in the study area.2.This study sets up a lithofacies classification scheme that is suitable for the shale of continental fault basin.Based on integrating various factors,such as minera1 composition,sedimentary structure and organic matter abundance,sets up a lithofacies classification scheme that is suitable for the shale in Jiyang Depression.According to “organic matter abundance+ structure+ composition+ lithofacies”,more than ten lithofacies were indentified in the Lower 3^rd Member of Shahejie Formation in the study area.The most developed six types of lithofacies include organic-containing laminated argillaceous limestone,organic-rich laminated argillaceous limestone,organic-rich laminated calcareous clay,organic-rich lamellar calcareous clay,organic-rich lamellar argillaceous limestone and organic-rich massive calcareous clay.On the basis of summarizing characteristics of all kinds of lithofacies,we analyzed the vertical evolution law of the main lithofacies.The EST is characterized by organic-containing laminated argillaceous limestone and organic-rich laminated argillaceous limestone.The EHST is composed of organic-rich laminated calcareous clay,organic-rich lamellar calcareous clay and organic-rich lamellar argillaceous limestone.The LHST is dominated by organic-rich lamellar argillaceous limestone,organic-rich lamellar calcareous clay and organic-rich massive calcareous clay.3.The macroscopic and microscopic heterogeneity and the change rule of the shale within sequence stratigraphic framework in the Lower 3^rd Member of Shahejie Formation in the study area are analyzed.As for macroscopic heterogeneity,from the EST to the LHST,the shale changed from gray to ash black,with color deepening,the rocky structure shows an upward change trend from laminated to lamellar then massive,fissure decreases gradually.As for microcosmic heterogeneity,we also found that the carbonates content decreases vertically from the EST to the LHST,while clay minerals and terrigenous detrital content increases upward,the average value of pyrite is stable while reaches its maximum at the top.TOC content is lowest in the EST and increase upward.Algaes have the largest number and most development species in the LHST.The porosity and permeability are highest in the EST.Finally,the study proposes a heterogeneity model of the reservoir quality for the Lower 3^rd Member of Shahejie Formation shale in the study area.4.The controlling factors of lithofacies heterogeneity of the Lower 3^rd Member of Shahejie Formation shale are analyzed.The lithofacies heterogeneity of the Lower 3^rd Member of Shahejie Formation shale in Jiyang Depression was controlled by tectonic movement,paleoclimate,paleo-waterdepth,paleosalinity,paleoproductivity and paleoredox.Under the background of a certain tectonic movement,climate from relatively dry and cold to relatively warm and wet to warm and wet,depth of water increased gradually,salinity reduced persistently,paleoproductivity from higher to high to lower,water from strong reduction to reduction.This study established the relationship between sedimentary environment and facies.Each lithofacies has different characteristic,and so are the sedimentary environment and developing stratum,we can also speculate lithofacies from sedimentary environment.5.The favorable exploration target interval of shale oil is predicted within the sequence stratigraphic framework of the Lower 3^rd Member of Shahejie Formation in this study.This research mainly contrast and analyze the oiliness of each systems tract,discuss the influence from heterogeneity to oiliness and also the controlling factors.organic-rich laminated argillaceous limestone,organic-rich laminated calcareous clay,organic-rich lamellar calcareous clay and organic-rich lamellar argillaceous limestone were predicted as advantaged lithofacies in this area.On this basis,comprehensive consideration of mud shale oiliness and fracability,the EHST in Bonan Subsag and the whole Lower 3^rd Member of Shahejie Formation in Dongying Sag are the most potential shale oil target interval from both reservoir's perspectives and hydraulic fracturing.