The Sedimentary Environment And Reservoir Characteristics Of Upper Permian Shale In The Lower Yangtze Area Of Southern Anhui

In order to clear the sedimentary environment and reservoir characteristics of the upper Permian shale in southern anhui,the upper Permian Longtan formation and Dalong formation in southern Anhui was taken as research objects.A large number of field exploration and drilling core observation were carried out in the study area,and a large number of samples were collected.The organic geochemical test data,elemental geochemical test data and shale reservoir test data were obtained through the modern analytic techniques.The upper Permian Longtan formation was marine-continental transitional sedimentation,mainly composed of sedimentary clastic rocks such as mud shale,sandstone and coal,and only a set of carbonate rocks with small thickness was developed at the top.The sedimentary facies include lagoon facies,coastal swamp facies,shallow sea facies,delta facies and open platform facies.Dalong formation has a single lithology,mainly consisting of mud shale,siliceous shale and siliceous rock.The sedimentary facies include continental shelf facies and deep-water basin facies.By analyzing the organic geochemical characteristics of the Upper Permian shale in the southern Anhui region,the following understandings were obtained:?1?The Longtan Formation shale is a medium-deviation source rock,The Longtan Formation is medium-deviation source rock,and the organic matter is mixed input of lower aquatic organisms?such as algae and bacteria?and terrestrial higher plant sources,the kerogen type are mainly humic kerogen???,as well as sapropelic-humic type??₂?;?2?Dalong Formation shale is mainly medium source rock,and the source of organic matter is mainly lower aquatic organisms such as algae and bacteria.the kerogen type are humic-sapropelic type??₁?and sapropelic-humic type??₂?,and a small amount of humic type???;?3?The Upper Permian shale in the southern Anhui is in the maturity-high maturity stage in most areas,and some areas are in the early stage of over-maturity,which has great shale gas potential.The elemental geochemical data was used to analyze the characteristics of the sedimentary environment of the Upper Permian in southern Anhui and its relationship with the content of organic carbon suggest that organic matter enrichment of longtan Formation is the result of combined action of terrigenous clastic input,paleo-redox conditions,paleoproductivity,paleosalinity,paleoclimate,the organic matter enrichment mode of the Dalong Formation is the“preservation mode”,and the redox conditions are the key geological factors for organic matter enrichment.Combined with the characteristics of sedimentary facies,it is believed that the organic-rich shale of the Longtan Formation was mainly developed in areas such as lagoons,delta plains and tidal flat swamp,and the organic-rich shale of the Dalong Formation was mainly developed in areas such as deep-water shelf and deep-water basin.According to X-ray diffraction analysis,argon ion field emission scanning electron microscope observation,nitrogen?N₂?isotherm adsorption and carbon dioxide?CO₂?isotherm adsorption analysis,it is believed that the upper Permian shale in southern anhui has a high content of brittle minerals and a high brittleness coefficient,which is conducive to the formation of natural fracture and later fracture-making.Among the clay minerals,the content of illite is the highest,which is beneficial to the adsorption of shale gas.The clay mineral content of the Upper Permian in the Anhui area is generally too high,which has a negative impact on the later fracture-making.The shale in the study area mainly develops several types of pores such as intergranular pores,intragranular pores,organic pores,and microfractures.The pore structure is mainly composed of slit mesopores and micropores,and also has certain macropores.Mesoporous pores and micropores less than 10nm are the main storage sites of shale gas.