Fine Characterization And Sweet Spot Prediction Of The Wufeng-Longmaxi Formations In Weiyuan Area,Sichuan Basin

Fine characterization and sweet spot prediction of shale gas reservoirs are the cores of shale gas exploration,which runs through the entire exploration and development process.In this study,the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation marine shales in the Weiyuan area of the Sichuan Basin was the main research object based on direct observation,geochemical analysis,physical testing,petrophysical model establishment and imaging analysis.The sedimentary environment and laminas development characteristics,the occurrence of organic matter and interactions with minerals,and the pore-fracture systerms of gas shales were analysed.On this basis,using the logging curves to establish a double standard for gas shales evaluation,a fine evaluation of the Wufeng Formation-Longmaxi 1st submember of the study area was carried out.The Wufeng Formation-Longmaxi 1st submember of Weiyuan area is mainly deposited from the calcareous shelf environment to siliceous shelf environment.The organic-rich shale has a high TOC content and a sedimentary thickness of 30-50 m,which is continuously distributed.During the whole sedimentary stage(Katian to early Rhuddanian stage),the sea level firstly declined rapidly,and fell to the minimum level during the Hirnantian glacial period(Kuanyinqiao Bed),and then slowly rose to the maximum level and then slowly decreased.Through the comprehensive study of main,elements,trace elements and the size of framboidal pyrite,it is found that during the deposition period of the Wufeng Formation,the water masses are in an oxic to dysoxic environment,and the organic matter content is low.The sedimentary water masses in the first section of the Longmaxi Formation are in dysoxic to euxinic conditions.Due to the ocean current surges and volcanic eruptions,the initial productivity of the water masses are higher.Because the supply of terrigenous debris is small and the water masses are in a semi-closed state,the organic-rich shale is deposited.From the bottom to the top of Wufeng-Longmaxi shales,the water masses with increasing oxygen are gradually closed,and the supply of terrigenous debris increases,resulting in a decrease in organic matter content.Through fine biostratigraphic comparison,it is found that organic-rich shales are maily deposited in the sulfide environment of water masses.And the deposition rate and the paleo-uplift of Weiyuan area control the distribution of high-quality shales.Graptolite zones LM1-LM5 are the main sedimentary periods of high-quality shales.TOC content and hydrocarbon generation potential(S₁+S₂),and petrological characteristics are used to mainly divide the shales in the study area into six types of lithofacies.The various lithofacies have different organic matter occurrence modes and inorganic-organic interactions.Organic matter is mainly distributed between the siliceous minerals,the clay minerals and the pyrite particles,and dissolved pores in the calcium minerals partially are filled with organic matter.The high-quality shale facies are organic-rich/medium clay-bearing siliceous shale facies distributed in the graptolite zones LM1-LM5.The lamina development in the shales is not obvious.There are three main sources of quartz.The biological source and the self-generated quartzs can form a concealed fracture system coupled with organic matter.The high-quality shale facies are mainly controlled by the depositional environment and deposition rate.Pore-fracture systems in shales can be investigated by macroscopic core observation and thin section analysis to microscopic scanning electron microscopy analysis.The combination of atomic force microscopy and imaging software identified three types of organic matter pores.The pore development of organic matter is closely related to hydrocarbon-forming organism and organic-inorganic interaction.Combined with the quantitative characterization of the petrophysical model,it is found that the high TOC siliceous shale is mainly composed of“trans-formational fracture-bedding fracture-large organic matter pores”,with best connectivity,high-efficiency accumulating hydrocarbons.The medium TOC calcareous shale is mainly composed of bedding fracture-intermediate organic matter pores,with medium connectivity.The medium TOC,argillaceous mixed shale is mainly composed of“small amount of bedding fracture-small organic pores”and the connectivity is relatively poor.On this basis,using logging curves and geological parameters,the gas shales in the study area are finely characterized,and it is found that the high quality shale is mainly distributed in the LM1-LM5 graptolite zones.And drilled wells in the underwater paleo-uplift development zone,the target should be placed in the middle-upper part of the“sweet spot”in the 1st layer.The lower-middle part of the“sweet spot”in the 1st layer should be set in the wells drilled in the absence of the ancient uplift.