Study On Hydrocarbon Potential And Reservoir Features Of Shales From Lower Cambrian Shuijingtuo Formation In Yichang Region,western Hubei Province

In recent years,breakthroughs were made in the shale gas exploration in Yichang region,western Hubei Province.In this study,the shale collected from outcrops and boreholes of Lower Cambrian Shuijingtuo Formation was "microscopically" and"macroscopically" analyzed using sedimentological,petrological and geochemical theories and methods to establish a maturity assessment on high-post mature marine shales,and further,systematically evaluate the hydrocarbon potential of source rocks from Lower Cambrian Shuijingtuo Formation.In addition,a comprehensive evaluation model on pore structure features was established based on macroscopic lithofacies analyses and microscopic reservoir characterization.The Mo/TOC values of shales from Shuijingtuo Formation ranged from 7 to 20 with an average of 13,and the ternary plot of Al-Fe-Mn revealed that SiO2 in shales from Shuijingtuo Formation was not affected by hydrothermal activities,suggesting that shales in Shuijingtuo Formation was deposited in moderate closed-marine environment,and the sea level experienced a period of frequent fluctuation by a steady upwelling period,then downwelling to a steady upwelling period with an overall sea-level rise.The distribution of organic matter is mainly affected by the productivity and redox condition of paleo-ocean,revealing a close relationship with sea-level changes.A new in-situ method for maturity assessment of high to post-stage shales was developed based on field emission-scanning electron microscopy(FE-SEM)and laser Raman analyses,which made up the deficiency of the traditional index and applied well.The total organic carbon(TOC)content of shales in Shuijingtuo Formation ranged from 0.99%to 7.70%,with an average of 4.24%.And carbon isotope values of kerogens varied from-35.9‰ to-29.5‰,with an average of-32.6‰,showing the"in-situ" gas was mainly derived from type-I kerogen.The calculated reflectance values on laser Raman was from 2.83 to 3.27,with an average of 3.09.Therefore,the shale in Shuijingtuo Formation was evaluated as good-excellent source rocks,especially the siliceous shale in the Middle-Lower part,which had well hydrocarbon generation potential and would be worthy of shale gas exploration.Based on scientific,objective,applicable and unifiable principles,a two levels of classification scheme on "texture-composition" of shale lithofacies was established by selecting texture and composition as objective parameters.Five lithofacies,namely,silt-bearing argillaceous/siliceous mixed shale facies,(silt-silicon-calcium)-bearing claystone facies,quartzose silty shale facies,calcareous dolomite facies,and silt-bearing calcareous claystone facies,and three lithofacies assemblages were determined.The types and features of shale reservoirs with different lithofacies were inconsistent,including,the fractures and pores were relatively developed in silt-bearing argillaceous/siliceous mixed shale,whereas,fractures were not well developed in(silt-silicon-calcium)-bearing claystone and calcareous dolomite,which were mainly consisted of clay mineral interlayer-matrix pores,and intergranular pores within indigenous minerals,respectively.All data revealed that silt-bearing argillaceous/siliceous mixed shales and(silt-silicon-calcium)-bearing claystone were characterized by high TOC values,high porosity and high gas-bearing ratio,which would be the most favorable marine shale for gas exploration targetIn this study,a relationship between "microscopic" pore characteristics and"macroscopic" lithofacies analyses has been established to propose a model for shale-reservoir assessment.This model would both contribute to the prediction of reservoir-development and efficiently evaluate the potential of shale gas resources.