Characterization Of Unconventional Reservoir Of Chang 7 Section Of Triassic Yanchang Formation, Xiasiwan Area, Central Ordos Basin

Ordos basin has plentiful low permeability oil and gas resources.Shales are extensively developed in MesozoicTriassic Yanchang formations andhave a favorable shale gas accumulation conditions.The Zhang Jiatan shale which typified the shale beddings at the bottom of Chang 7 section is characterized by considerable thickeness,widespread deposition,high organic matter content,and medium thermal maturity.This study analyzes the existing logging,core,tests data ofXiasiwan area wells first,combining reservoir geology,sedimentology,and petroleum geology theories,with core description and analysis of 4 wells,carried out geochemical,pysical property,and diagenesis research of unconventional reservoir of Chang 7 section in the research area.Summarizethe influence factors of reservoir development and the main factors influencing the physical property of theunconventional reservoir of Chang 7 sectionin the research area.In Chang 7 section,among the brittle minerals,the average content of quartz is 19.8%,the average content of feldspar is 15.9%.Clay content is high and varies greatly,the main clay are illite/smectite mixed-layer and illite,with a small amount of chlorite and kaolinite.Organic matters are mainly type ?,organic maceral is mainly vitrinite and inertinite,with a small amount of exinite.The total organic carbon of the shale is average 4.79%,and more than 85% of the samples are ranked as good hydrocarbon source rocks.The measuredRo reaches 0.92%,suggesting that rocks are mature for oil generation.According to the SEM photo-micrographs of pore structure characteristics of Chang 7 shale,a pore classification is presented in this paper which consisting 3 major pore types: interparticle pores,intraparticle pores,and organic-matter pores.From the standpoint of origin of pores,most of pores are secondary interparticle pores,including poresbetween clay platelets and intercrystalline pores within pyrite framboids,and also a fewdissolved pores.There aremore pores in the sandy mudstone laminations,both primary and secondary,and have dissolution-rim pores around feldspar which are key pathways for shale gas.There are complex organic matter pores which mainly are original biological cells of organic macerals and secondary pores formed during thermal maturation,commonly 1-200 nm in dimension.Porosity of Chang 7 section is very low,averages 2.1%.The mercury injection curves are characterized by low sorting coefficient,thinskewnesss which interpreted as thin throats and poor condition for free gas to accumulate.The study shows that the higher the feldspar content and the clay content.And also the higher the pororsity,and the higher the organic matter content,the lower the porosity which indicates the organic matter has no contribution to porosity.Unconventional Reservoir of Chang 7 Section of Yanchang Formation in Xiasiwan area of Ordos Basin is widespread and has a considerate thickness and advantages for hydrocarbon generation,shows a great potential for shale gas accumulation and development,but theheterogeneity of the shale and the relationship between clay and organic matter needs further reaserch.