Sedimentary Characteristics Of The Lower Cambrian Niutitang Fm Shale And Evaluation Parameters Of Shale Gas In Zunyi-Qijiang Area

Based on investigating the status of research on shale gasaccumulation conditions and evaluation parameters and analyzing the existingproblems, this paper selected the Lower Cambrian Niutitang Formation in theZunyi-Qijiang area as the target of study. Field outcrop observation and samplingwere carried out, available drilling data and core samples were collected, andlaboratory microscopic observation and various tests were performed. Based on theseworks, this paper systematically studied the sedimentary features of the dark shale inthe Lower Cambrian Niutitang Formation, analyzed the parameters for evaluating theshale gas and delineated three favorable areas for shale gas exploration. The followingresults and conclusions were obtained:1. Analysis of pyrite and trace elements shows that the Niutitang Formation shalewas developed in shallow and deep shelf anoxic environments. The major lithologiesinclude siliceous shale, carboniferous shale, dark shale and silty shale. They arecontinuous as a whole and are generally over 20 m thick and over 40m in thesouthwestern part. Its burial depth increases from southeast to northwest as a wholeand is 2000-5000m in Xishui area and 1000-4500m in northern Guizhou.2. The Niutitang Fm was studied systematically according to the evaluationparameters. Its TOC is relatively high with a general value over 2% and a high valueof 3% in some locations. The organic matters are dominated by kerogens of type-â… .The maturity is relatively high with Ro generally over 2% and locally being up to 4%.The minerals of the shale are mainly siliceous (average content 47.5%) and clays(average content of 40%). Calcareous minerals are rare. The clay minerals aredominated by illite with no kaolinite. These data indicates that the shale show goodbrittleness.3. Systematic laboratory studies show that vesicular micropores resulted fromhydrocarbon generation of organic matters are common. Intergranular micropores arewell developed and dissolution pores also occur locally. Micropores resulted fromtransformation of other clays mineras into illite are also common. Joints and microfractures occur in the shale and are in networked distribution. The existence ofmicopores and microfractures provide space for shale gas accumulation and also isfavorable for expulsion of shale gas during development.4. Systematic studies reveal that the sealing conditions of shale gas in the studyarea are good as a whole. Salt layers exist in the Middle Cambrian Shilengshui Fmand the Lower Cambrian Qingxudong Fm and they are relatively thick and continuousin the study area. The shale in the Niutitang Fm and other overlying strata commonlyhas an OCR value below 2.5, the threshold for shale breaking. Seismic data analysisindicates that faults are not highly developed and are commonly located in a subzoneof very slow flow of underground water, being also favorable for shale gasaccumulation.5. Three favorable areas for shale gas accumulation, including Bijie, Zunyi andXishui, were identified based on comparison analysis.