| Aiming at the pore structure characterization of high-quality shale reservoir and the development mechanism of high-quality pore system as the core scientific issues,this paper conducts systematic qualitative-quantitative analysis to study the reservoir development characteristics,especially the nano-scale pore structure characteristics,based on the systematic investigation and previous research progress following the technical routes of field logging-laboratory experiment-data analysis-comprehensive comparison and theoretical analysis.Representative shale samples and geological data of the Wufeng-Longmaxi formation were collected from the East Sichuan area.A series of innovative methods were designed for solving the difficulties and hotspots of characterization.The pore structure of high-quality shale reservoir was comprehensively characterized by a variety of innovative methods and multi-quantitative characterization technologies,and the interval differences of pore structure were quantified.A multi-quantitative characterization technology system was formed.The particularity of high-quality pore structure reservoir was extracted through reservoir comparison,revealing the multi-scale and multi-level influencing factors of shale reservoir pore system.The genetic differences of pore system of different intervals are clarified,which helps the sumrization of the development mechanism of high-quality shale reservoir.A new approach of high-quality reservoir optimization based on high-quality pore-system development mechanism is formed to make combination with pore research and practical application.The results show that:Under the control of tectonic evolution,paleogeographic pattern change and sea level change,shale reservoirs of the Wufeng Formation and the lower part of the Longmaxi formation in the East Sichuan area was deposited in the sedimentary environment of detention basin and deep-water shelf facies,respectively,and was transformed into semi-deep to shallow water shelf sedimentary environment in the middle-upper Longmaxi formation.The shale layers composed of carbonaceous shale,siliceous-carbonaceous shale and other rock types were deposited under the control of different sedimentary conditions,forming the interval differences of material composition and rock structure between the Wufeng Formation and the bottom,lower,middle and upper Longmaxi formation.Graptolite,radiolaria,pteropoda and other paleontological fossils are found in shale layers,which provide the source of organic matter and siliceous matter for the reservoir.The graptolite-rich horizontal beddings are developed at the Wufeng-bottom Longmaxi formation,which is characterized by micro "stratabound structure".The major sedimentary structure was silty laminations in the middle and upper Longmaxi formation.The mineral components are mainly composed of clay minerals,brittle minerals and pyrites,among which the clay minerals are mainly illite,chlorite,and mixed-layer clay minerals.Siliceous,montmorillonite and organic matter are enriched at the Wufeng-bottom Longmaxi formation,and the influence of terrigenous clasts is enhanced upward.The organic matter(OM)occurs as matrix-kerogen,graptolite fossil OM,solid bituman and other types.OM mainly belongs to type I kerogen,and reaches high-over maturity stage.The high porosity and high gas-bearing intervals are overlapped with intervals with high brittle minerals and high TOC content.The full-scale pore observation(3nm to micro-fracture scale)was realized based on the innovative method of "micro-film-cover and no-spraying-gold observation technology" in combination with three-axis polishing methods to eliminate the impact of natural heterogeneity of sediments.A comprehensively systematic classification scheme including four major categories and over 20 subtypes was established for the Wufeng-Longmaxi formation shale reservoirs from the perspective of genesis,morphology and material composition combining development scale,spatial location,genesis and morphological characteristics of pores.The micro-reservoir space distributions of the Wufeng-Longmaxi formation shale reservoirs are revealed through the comprehensive tests of gas adsorption,high pressure mercury injection,image processing,nano-CT and low-field NMR,etc.,which shows the micro-pores is the most developed type,with limited but stable mesopores developed.Innovative methods such as NMR test under the condition of water-saturation,oil saturation and centrifugation,spontaneous imbibition-NMR and nano-CT modeling were designed and conducted aiming at the characterization difficulties of pore-system connectivity and differences between organic and inorganic pore structure.The micro reservoir space was divided into multi-level connectivity domains,which shows the three-dimensional distribution characteristics of pore network and the distribution of organic matter is the main factor affecting the distribution of pore network.It also indicates that mineral pores contribute to the connectivity of reservoir pore-system.On the basis of high-resolution images and classification,image mosaic technology and image processing technology are applied to extract the structural characteristic parameters of over 400000 pores,obtaining the non-intuitive characteristics of pore size distribution,fractal dimension,entropy value and shape factor of different types of pores.In this way,the special characterization of different types of pores was realized,providing a new approach for the quantification of pore proportion,pore morphology,pore type in different size range and pore proportion in different layers.The results show that the overall pore morphology of the Wufeng-Longmaxi formation reservoirs is nearly circular,with dominant pore size range of 10nm-30 nm.The proportion of organic matter pores in the Wufeng formation and bottom Longmaxi formation is 58% and 62%,respectively,and the proportion of OM-pores is significantly reduced upward.The interval difference of pore propotion and structure is significant in the Wufeng-Longmaxi formation reservoirs.Combined with qualitative and quantitative research methods and innovative technologes,this paper summarizes the multi-method quantitative characterization technology system for shale micro-reservoir space quantification to enhance the accuracy of reservoir pore characterization.The particularity of pore structure of high-quality shale reservoirs is extracted through correlation analysis,reservoir correlation and innovative experiments.The structural characteristics of high quality pore system are summarized.Factors including material composition,sedimentary structure,pore space distribution,compaction condition and the potential of forming micro fracture network affect the development degree of pore system.The nano-scale adsorption pores of the Wufeng-bottom Longmaxi formation are mainly organic pores.The noble metal nanoparticles injection experiment,in which unconventional reservoirs can be divided into five major types considering pore network connectivity in combaination with reservoir correlation,show that the seepage channels contributing to the percolation connectivity belongs to the "core percolation channel" type with directionality.The pore system is classified and characterized in terms of material combination,rock structure,adsorption pore characteristics,seepage pore characteristics and multi-scale connectivity.The best-quality pore-system belongs to siliceous-carbonaceous shale type,followed by carbonaceous shale type.The development of high-quality pore-system of reservoir is influenced by multi-factors,multi-scale and multi-level control factors.Occluded detention or deep-water slow sedimentation conditions provide the material basis for the reservoir,which lead to the interval differences of material composition and rock structure.Under the control of tectonic burial,the reservoir is comprehensively influenced by the diagenesis of organic matter,hydrocarbon generation,mineral transformation,etc.The carbonaceous-siliceous shale pore-system with high adsorption storage capacity,self sealing performance and brittleness at the Wufeng-bottom Longmaxi formation is formed by special diagenetic transformation process with different diagenesis degree and types on the material basis of rich biological source siliceous,organic matter,montmorillonite and well-developed horizontal bedding structure.The paper systematically reveals the basic geological characteristics and pore structure characteristics of the shale reservoirs of the Wufeng-Longmaxi formation in the East Sichuan area.The fine quantitative characterization of the reservoir pore-system was realized forming a series of innovative technical methods and multi-method quantitative technical system to study the formation and development mechanism of different pore-system.The formation and development mechanism of high-quality reservoir with high-quality pore-system and the optimal evaluation method was summarized from pore angle to provide scientific methods and basis for the deepening,refinement and practicality of pore research.This paper has 231 figures,66 tables and 428 references. |
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