Evaluation And Genetic Analysis Of The Shale Gas Source Rock In Northwest Region Of Guizhou

The source rocks of shale gas are widespread in the northeastern Guizhou.Lower Cambrian Niutitang Formation,Upper Ordovician Wufeng Formation,Lower Silurian Longmaxi Formation and Permian Longtan Formation are the main layers of the black shale.According to the field survey results,the three main sets of the shale gas source rocks in northeastern Guizhou are distributed as follows:1)Lower Cambrian Niutitang source rocks:mainly exposed in Zunyi,Guiyang and Bijie;2)Upper Ordovician Wufeng-Lower Silurian Longmaxi source rocks:mainly exposed in Zunyi Xishui,tongzi and so on.3)Lower Permian Longtan source rocks:mainly exposed in Guiyang and Bijie.From the perspective of geological features,Chinese shale gas has the features of multi-layer(Paleozoic,Mesozoic and Cenozoic),multi-field(marine,continental and transitional facies),multi-type(type I,type II and type III),multi-evolution(high,medium and low),multi-genesis(biological,thermal,pyrolysis).The source rocks in northwestern Guizhou has various organic matter abundance and organic matter types with high organic matter maturity.In terms of the organic matter type,the Niutitang source rocks are the best with type I kerogen,mainly algae and bacteria input;the Wufeng-Longmaxi source rocks are second with type I and type II kerogen;Longtan source rocks of transitional facies are mainly Type III kerogen.The organic matter abundance of the source rocks from Niutitang and Longtan Formation are better,the former have a better parent material type,while the latter are mostly developed at the intersection of land and sea swamp,more input of higher plants.The abundance of organic matter of Wufeng-Longmaxi Formation is lower.Maturity of organic matter is in line with the common characteristics of the southern marine shale,mostly in high ripe or overripe stage.The Longtan source rocks is lower in the mature stage with correcting.High mature hydrocarbon source rocks have been in the late stage of hydrocarbon generation with emissions.Because of the model of the gas generation is thermal-origin in the study area,the author think that high maturity can not affect the quality of the source rocks absolutely,with many investigations of the relationship between organic matter abundance,organic matter type,organic matter maturity and the quantity of shale gas.However,the preservation conditions become the key to the ability to form an industrial scale shale gas reservoirs.In northwestern Guizhou,different layers of shale gas source rocks has different causes.As follows the Cambrian Niutitang shales due to the role of hot water and upwelling factors,biological growth prosperous,generally have higher abundance of organic matter,always larger abundance of organic matter along the increasing affection of the hot water.While the Wufeng-Longmaxi shales mainly in deep-shallow shelf genetic model have better organic matter preservation conditions;Longtan shales are typical of transitional facies swamp mode.