Lithofacies Characteristics And Their Impacts On Pore Structure And Petrophysical Properties Of Lacustrine Shales In The Lower Cretaceous Shahezi Formation In The Changling Fault Depression,Songliao Basin

The Shahezi Formation,which is the primary source rocks in the Changling Fault Depression(CFD),demonstrates great potential for lacustrine shale gas exploration and development,considering its large thickness and hydrocarbon generation potential.From the perspective of shale lithofacies,this study takes the Shahezi Formation in the CFD as an example,understanding of reservoir space characteristics,pore structure characteristics,wettability and connectivity characteristics of different lithofacies.The purpose of this paper is to determine the controlling factors of lithofacies types for reservoir physical properties and pore structure,and to select the dominant shale lithofacies types of the Shahezi Formation in CFD.Combined with the logging data and multiple qualitative(Quantitative evaluation of minerals by scanning electronic microscopy(QEMSCAN),field emission-scanning electron microscopy(FE-SEM),a large-view FE-SEM splicing technology and quantitative methods(CO2 and N2 adsorption,high pressure MIP and Nuclear magnetic resonance)have been used to analysis and test these samples,to comprehensive evaluation of pore structure and reservoir capacity was carried out in continental shale,Songliao Basin.The results show that(1)Based on the different structures and sedimentary characteristics the sedimentary filling in the southern region of CFD is summarized as domino type semi-graben type sequence sedimentary filling pattern,asymmetric graben type sequence sedimentary filling pattern and rolling semi-graben type sequence sedimentary filling pattern.(2)The total organic carbon(TOC)contents are mainly between 0.5% and 2%,and the types of organic matter are mostly ?2 and ? kerogen with the mature-over-mature stage,and the mineral composition is mainly clay minerals and quartz.The principal component analysis method and BP artificial neural network model were used to predict the lithofacies distribution in a single well;(3)The majority of pores in the Shahezi shales are organic matter(OM)pores,clay minerals intragranular pores,and dissolved pores.OM pores have the large numbers,small pore diameter and roundness;the distribution range of pores within clay mineral grains has the largest scale and high roundness.(4)With complexed pore size distribution and strong heterogeneity,the pore size distribution ranges are 0.5 ? 0.9nm,3 ? 6nm,10 ? 40 nm and 200?2000nm.Among them,organic-rich clay shale has the largest proportion of micropore volume and micropore surface area.(5)The Shahezi formation shale has mixed wetting and limited pore connectivity characteristics,the rules following calcareous shale> argillaceous shale>siliceous shale.(6)The analysis result shows that the TOC content and clay mineral composition play a key role in the pore structure and physical properties.As the most favorable type of lithofacies,Organic-rich clay shale lithofacies has a high proportion of micropores and the specific surface area,which is conducive to shale gas accumulation.