The Carboniferous Geological Condition Evaluation In Shihuigou Delingha Depression In Qaidam Basin

The shale gas resources in China are very rich,but most mudshale studies are on marine sediments,mainly in Sichuan Basin,while mudshale reservoir studies on transitional facies are rare.The object of this paper is the transitional organic-rich mudshale reservoir in the Carboniferous Lime Trough of Delingha Depression.Combined with the geological properties of the Carboniferous kerogen in Qaidam Basin and based on the measurement of outcrop sections,observation of well core samples,and analysis of lab samples,the thickness and distribution of mudshales,content and types of organic matters,thermal maturity,permeability of the reservoir,types and distribution of accumulation space(reservoir space),and gas-bearing property of mudshales are studied.The results demonstrate that the Carboniferous Keluke Formation in Lime Trough Area is dominated by interlayered dark shales and thin limestones;There are more mudshale layers,which thus have a large accumulation thickness;The mudshale reservoir has a high concentration of organic carbon(the average number is 4.03%);The kerogen types are mainly Type II2 and sepropelic-humic Type III;Organic matters have reached the catagenesis stage(Ro=1.3~2.0%)which can benefit the production of shale gas;Mudshales in Keluke Formation have a high content of clay minerals,while the content of brittle minerals is relatively low,which is good for later fracture modification of mudshales.Pore types are conplicated.Organopores are more developed.There are a large amount of pyrite on the surface of organic matters,and micro-fractures are widely developed as well.All these features can benefit the accumulation of free gas.Mudshales in Keluke Formation has a high gas adsorption content,and the average number of the maximum therotical absorbtion content is 2.52ml/g,which indicates a relatively good gas-bearing property.The content of organic carbon is the controlling factor of the content of gas adsorption of mudshales.Based on the domestic and overseas geological assessment standards of shale gas,five parameters including gas-bearing property,TOC content,organic matter maturity,porosity,and brittle minerals are picked to do the assessment.Mudshales in Keluke Formation are great for shale gas production,and strata from 970~1000m are speculated to be the best production layers.