Reservoir Characteristics And Sedimentary Control Mechanism Of The Marine-continental Transitional Mud Shale

To clear the control mechanism of sedimentary environment on the reservoir characteristics of mud shale,the marine-continental transitional shale of Upper Permian Longtan Formation in Zhina Coalfield,western Guizhou was takes as a subject.Large amount of geological data and field trips in the research area were launched,and the mud shale samples were also collected.Combined with the modern analytic techniques,the reservoir physical characteristics of mud shale were studied,including organic geochemistry,mineral petrology,pore structure and gas-bearing properties.The Longtan Formation mud shale mainly developed into the lagoon-tidal flat and delta sedimentary environments.Four sedimentary facies,including barrier-bar facies,lagoon facies,tidal flat facies and delta,two sedimentary subfacies and seven sedimentary microfacies were distinguished.Then,six lithofacies that favors the development of organic-rich mud shale were established.Based on the detailed analysis of the geochemistry characteristics of the mud shale,five key factors that control the enrichment of the organic matter of the Longtan Formation mud shale,including paleoclimate,detrital influx,redox,paleoproductivity and deposition rate.The various sedimentary environments have significant influence on the enrichment of organic matter.The dysoxic bottom water environment played a dominant role in the enrichment of organic matter in the lagoon-tidal flat sedimentary environment,while the warm and humid climate under oxidizing environment and high sedimentation rate jointly controlled the enrichment of organic matter in the delta sedimentary environment.The studies on pore structure and gas-bearing characteristics of the mud shale show that the TOC,mineral components and pore structure were controlled by the sedimentary environments,which will finally determine the adsorption capacities of the mud shale.According to the field emission scanning electron microscope(FESEM),the pores in the shale are classified to organic pore,intergranular pore,intragranular pore and microfracture,and the organic matter presents as isolated organic matter and multiple organic matter.The characteristics of pore structure is studied with the high pressure mercury injection and low temperature nitrogen adsorption,the pores are divided into four and two types respectively based on the pore volume and shape.The TOC and content of clay minerals contribute to the pore structure and maximum adsorption capacities of the shale.Based on the research above,the sedimentary control mechanism of sedimentary environment on the reservoir properties characteristics from the lithofacies can be concluded.The sedimentary environments control the lithofacies and components of the organic-rich mud shale,which further determines the reservoir properties characteristics of the organic-rich mud shale.