Geological Characteristics And Evaluation Of Upper Carboneous Benxi And Upper Permian Shanxi Shale Gas In Yanchang Area,Ordos Basin,NW China

Yanchang area in Ordos basin,as one of the most important terrestrial shale gas production area,has been proved to be potential and productive.During recent years,the marine-terrestrial shales of upper Carboneous Benxi and upper Permian Shanxi shales,with relatively higher burial depth and maturity,have been targeted as new potential for shale gas exploration.To better understand characteristics and evaluate shale gas capacity of upper Carboneous Benxi and upper Permian Shanxi shales,detailed investigation on the basis of geochemical and physical experimental anasis were conducted on shale samples picked from Q14 well and Q25 well in the study area.The results show that organic matter of upper Carboneous Benxi and upper Permian Shanxi shales,respectively average TOC of 1.91 wt.% and 1.58 wt.%,have a large number of Vetrinite,61.7% and 74.6%,belonging to type III.The maturity of organic matter of Benxi and Shanxi shales is 2.74% and 2.6%,which indicates that Benxi and Shanxi shales have been at the stage of dry gas or over-mature.One of the most significant characteristics of Benxi and Shanxi shales is a larege proportion of clay minerals,average 68.9% and 54% of shale composition.The content of quartze of Benxi and Shanxi shales,average 16.7% and 39.7%,rank second.Low temperature nitrogen adsorption experiments shows that Benxi and Shanxi shales mainly comprise of silt-shaped pores,characterized by relatively low porosity and low permeability.According to results of high temperature and pressure methane adsorption experiments,adsorbed gas consists majorly of shale gas content of Benxi and Shanxi shales measured by United States Bureau of Mines Method(USBM),average of 81% of 2.27m3/t and 2.455m3/t respectively.Based on experimental comparison of three shale samples picked from Benxi and Shanxi shales,the large number of vetrinite and clay minerals of Benxi and Shanxi shales is likely to provide abundant adsorption sites for shale gas,which might be the accumulation mechanism of Benxi and Shanxi shales in Yanchang Area,Ordos basin,NW China.Basically,the area around Q14 well has higher shale gas content than that aournd Q25 well.Specifically,Benxi shale gas in the area around Q14 well,of average 3.73 m3/t,is the most potential target in Yanchang area,and Shanxi shale gas from Q14 well of 3.26 m3/t seconds,then Shanxi shale gas from Q25 well of 1.65 m3/t and Benxi shale gas from Q25 well of 0.81 m3/t.