| Shale gas is an efficient,clean and high-quality energy and an important strategic energy resource in China.Carboniferous Dawuba formation is an important target formation for shale gas in Guizhou Province,it develops in the Ziyun-Yadu rift trough and spreads in the NW-SE direction.The facies transition is very fast,which brings great difficulties for shale gas selection and exploration.In order to advance the shale gas exploration work of the Carboniferous Dawuba formation,enhance the basic work degree and support the shale gas constituency,several typical sections and drilling data of the early Carboniferous,which can represent each facies area,were found in the study area,The petrological characteristics,sedimentary facies and geochemical characteristics of the shale gas are studied in detail,and then the sedimentary environment changes are analyzed and compared with the regional data,which will provide scientific reference for the next research on the exploration and development of shale gas in the region.In this study,we selected Ziyun Luogang section and Ziyun Sidazhai section as the key research objects in Ziyun aera.Field measurements and sample collection have been carried out,using sedimentology,petrology,element geochemistry,organic geochemistry and so on,The sedimentary environments and facies belts of Dawuba formation in the study area are analyzed in detail.it is considered that the deposition period of Ziyun Luogang section Dawuba formation is in a deep-water reduction,a relatively stable and sloped sedimentary environment with a relatively stable water,in a weak redox-reduction depositional environment,close to the land source,both the land-source organic matter input andthe marine organic matter input.Form the bottom to the top,it can be divided into Sea-level rise period,maximum sea-level period,sea-level decline period.The whole section can be delimited as the slope subfacies under the slope facies of the marginal slope of the trough basin.it is considered that Ziyun Sidazhai section Dawuba formation is quiet water,deep-water reduction environment dominated by suspended sediment,in located in the reduction environment,far from the land,mainly in the marine organic matter input.and can be divided intoSea-level rise period?maximum sea-level period?sea-level decline period from bottom to top,The whole section can be classified as compensted basin-under compensted basin-compensted basin.On the basis of the study of the petrology,sedimentology,geochemistry of two key sections,a combination of study area and several sections around it,the paper makes a comparative analysis and finds that the water body from NE-SW is deepened,and the sedimentary facies is changed into shore-tidal flat facies?trough basin marginal slope facies?trough basin facies.Based on the crosswise distribution characteristics of facies,the lithofacies palaeogeography was divided into Sea-level rise period and sea-level decline period.During the Sea-level rise period,the paleogeographic pattern was high in the north-east,low in the south-west,and the water was rapidly deepened from the north-east to the south-west.The distribution of the depositional environment from NE to SW in general is:shore-tidal flat facies ?upper slope subfacies ? lower slope subfacies ? compensted basin ? under compensted basin.The palaeogeographic pattern,water depth,facies and distribution of the sea level decline period are consistent with the rising stage of sea level.The difference is that the whole sedimentary facies moved south-west due to the sea regression after the highest sea level period.The carbon mudstone in the Lower Carboniferous fi Ziyun area during Sea-level rise period and maximum sea level period that the continuous thickness is big,the interlayer is less,it is anoxic reduction environment,the content of TOC is high,the horizontal distribution is stable,and it has good shale gas geological conditions,so it can be used as high-quality shale section. |
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