Lithofacies types and geochemical characteristics of carbonate source rocks in Taiyuan Formation of middle and eastern Ordos Basin
With the discovery of high-yield gas reservoirs in the inclined limestone and Maoergou limestone of the Upper Paleozoic Lower Permian Taiyuan Formation in the central and eastern Ordos Basin,it is very important to evaluate the hydrocarbon generation potential of carbonate rocks other than the coal-measure source rocks of Taiyuan Formation.In this study,the Taiyuan Formation limestone in the central and eastern Ordos Basin was taken as the research object.Through the comprehensive application of core observation,logging data and geochemical experimental data,the organic geochemical characteristics of limestone in the tidal flat environment of sea-land mixed deposition were analyzed,and the lithofacies types conducive to the development of source rocks were divided.The results showed:①In general,the organic matter abundance of Taiyuan Formation limestone is lower than that of mud shale,and the organic matter abundance of Xiedao section limestone is the highest,followed by Maoergou section.The organic matter type is mainly type Ⅰ,and some samples show type Ⅱ kerogen characteristics.Organic matter is generally in the high maturity evolution stage,pyrolysis peak temperature(Tmax)average up to 483 ℃,vitrinite reflectance(Ro)is generally higher than 1.2％.②Based on the differences of geochemical parameters and petrological characteristics,Taiyuan Formation limestone is divided into four lithofacies types:Ⅰ-Ⅱ type kerogen high organic matter laminated argillaceous micritic limestone facies,Ⅰ-Ⅱ type kerogen medium-high organic matter massive micritic limestone facies,Ⅰ-Ⅱ type kerogen organic matter massive bioclastic micritic limestone facies and Ⅰ type kerogen low organic matter massive algal clot limestone facies.③The vast area in the middle of the study area is dominated by Ⅰ-Ⅱ type kerogen medium-high organic matter massive micritic limestone,and the source rock thickness is not large:the western part is dominated by organic matter massive bioclastic micrite limestone lithofacies in type Ⅰ-Ⅱ kerogen,and the effective source rock thickness is large.The limestone deposits in the eastern part are widely distributed and have a large thickness.Although they are dominated by type Ⅰ kerogen low organic matter massive algal clot limestone,the lithofacies are variable and the thickness of effective source rocks is not small.
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