| Deep and ultra-deep dolomite is the target of oil and gas explortion in the world,which has gradually made significant breakthroughs in recent years.The property,structure,formation and evolution of storage space are key factors influencing the performance of reservoir.Therefore,it is of great significance to clarify the development and evolution mechanism of pore structures for the exploration and prediction of high-quality deep dolomite reservoirs.At present,a unified understanding has been formed around the origin mechanism of high-quality deep dolomite reservoirs:multi-phase dissolution during diagenesis is the main controlling factor for the development and evolution of secondary pores and reservoir quality.Among them,the important role of fluid-rock interaction during deep burial has gradually received attention.However,the complex pore structure and heterogeneity limited the study of deep dolomite dissolution mechanism,resulting in the understanding of the burial dissolution dynamic mechanism and the evolutionary differences of heterogeneity in deep dolomite reservoirs is still relatively weak,especially for complex pore structure type.As typical deep ancient dolomite oil and gas reservoirs in China,the Sinian-Cambrian formation in the Moxi-Gaoshiti area of the Sichuan Basin has undergone a complex process of diagenesis and superimposed modification by multi-phase dissolution.However,due to the large burial depth,high heterogeneity and complex pore structure,the evidence and main control factors of dissolution are still unclear,which restricts the understanding of reservoir space development,evolution and preservation mechanism,as well as the further exploration and development of deep ancient oil and gas reservoirs in China.Therefore,drilling cores of dolomite with typical pore structure characteristics of the Dangying Formation and Longwangmiao Formation in Moxi-Gaoshiti area were used as the research objects.Based on petrographic and geochemical analysis,the dissolution stages and fluid types of reservoir were clarified.Two-dimensional surface dissolution experiments(T=50℃,p=1 atm)based on a constant-temperature water bath apparatus,and three-dimensional internal dissolution experiments(T=40-160℃,p=89-520 atm)based on a high-temperature and high-pressure flow system apparatus were used to simulate the interaction process between dolomite reservoirs and organic acids in shallow to deep burial environments.In addition,scanning electron microscope(SEM)and X-Ray Computed Tomography were used to achieve in situ observation of rock surface micromorphology and internal pore structure,combined with ICP-OES chemical composition analysis and PHREEQC simulation to obtain the composition differential dissolution characteristics.By means of fractal and multifractal methods,the heterogeneity of chemical composition and microstructure changes in the process of dissolution was quantitatively characterized.The formation and evolution mechanism of pore structure in deep buried dolomite reservoirs under the action of complex fluids was comprehensively discussed,and the favorable regional accumulation conditions and further exploration directions of deep dolomite reservoirs were revealed.The major achievements are summarized below:1.The main rock types and reservoir spatial characteristics of dolomite reservoirs are clarified,and evidence of buried dissolution is identified.The dolomite reservoirs of the Dengying Formation and Longwangmiao Formation have various rock types and complex storage spaces,which have experienced four stages of dissolution.There are four principal rock types in the reservoirs:silty-fine crystalline dolomite,residual dolarenite,residual thrombolite dolomite,and algal-laminated dolomite.The storage types of the reservoir are classified as fracture type,fracture-pore type and fracture-vuggy type.A total of three stages of dissolution were identified in the reservoirs:penecontemporaneous dissolution,supergene karstification and burial dissolution(hydrothermal dissolution and organic acid dissolution),in which organic acids generated during kerogen maturation make a prominent contribution to the dissolution and ultimate preservation of deep reservoir space.2.The differential dissolution mechanism of surface micromorphology and composition in dolomite reservoirs is explored,and the compositional-structural selectivity of the dissolution process of dolomite is found.The dissolution pattern of dolomite with different genesis varies significantly,and their dissolution rate are significantly influenced by the density of microscopic defects and hydrothermal/clay minerals in the rocks.The high-energy positions of the dolomite crystal structure are the preferred sites for dissolution,and the release rate of Mg from these positions is significantly greater than that of Ca.Silty-fine crystalline dolomite formed by penecontemporaneous dolomitization and recrystallization tends to form honeycomb etch pits,while idiomorphic–hypidiomorphic medium–coarse crystalline dolomite cements of buried dolomitization genesis are more likely to form intracrystalline seams along the crystal.Differences in micro-defect density,heterogeneity and mineral composition caused by early diagenesis are the main controlling factors for the dissolution rate of dolomite reservoirs:specifically,the type of dolomite structure(grain/granular),grain size,degree of idiomorph,degree of recrystallization,crystal orientation,grain boundary contact relationship,structural/composition heterogeneity and hydrothermal/clay mineral infilling.Therefore,silty dolomite or residual dolarenite with strong recrystallization,which have more crystal defects,higher initial structural/composition heterogeneity,and with suitable amount of clay minerals(kaolinite,illite)are more significantly developed in microscopic pore structure and strong heterogeneity variation under burial dissolution,it may be potential high-quality deep dolomite reservoirs.3.The differential dissolution mechanism of internal pore structure in dolomite reservoirs is explored,the dissolution patterns and key evolution stages of different types of dolomite in organic acids are revealed,and the main control factors and further exploration directions of regional high-quality deep dolomite are predicted.The Early Triassic to Late Jurassic are the key stage of buried organic acid dissolution in dolomite reservoirs,which plays a significant role in improving the permeability of dolomite reservoirs.Reasonable configuration of fracture and karst cavities and the high heterogeneity are important prerequisites for the substantial enhancement of reservoir permeability during buried dissolution.The higher heterogeneity in the cavity-dominated fracture-vuggy type dolomite promotes the inhomogeneous dissolution pattern,and the number of large throats,pore structure heterogeneity and permeability are substantially enhanced after dissolution.It was subjected to more intense dissolution during the Late Triassic-Early Jurassic and Late Jurassic,making it a potential high-quality dolomite reservoir.In contrast,the dissolution of dolomite with predominantly fracture/pore structure is relatively uniform,which reduces the heterogeneity of pore structure,and increasing the number of small/medium throats at multiple fracture locations.Of these,the fracture-dominated dolimites were subjected to the strongest organic dissolution during the Late Triassic,while fracture-pore type dolomites with a large number of pore structures received more uniform organic acid dissolution during the Early Triassic to Late Jurassic.Based on the clustering analysis of multifractal parameters before and after dissolution,it can effectively and quantitatively distinguish the dissolution evolution characteristics of cavity-dominated dolomite and fracture/pore-dominated dolomite.Among them,the multifractal parametersΔα_L,Δf_L,Δfand the ratio of asymmetric index to porosity(R_p)are of great significance to the prediction of reservoir porosity and permeability,respectively.On the whole,a research method system combining chemical experiment,computational simulation and quantitative characterization has been formed,which can be used to study the evolution of dissolution dynamics in deep buried dolomite resrvoir.The study quantitatively answers the question of whether deep-buried dissolution can improve reservoir quality in a closed diagenetic system from chemical experiments and microscopic scale.It reveals the"dissolution window"effect of dolomite reservoir controlled by burial depth,and clarifies that the combination of early material base and later burial dissolution is the key to the formation and preservation of high-quality dolomite reservoirs in deep burial.Of which,the high-energy sedimentary facies(grain shoals),penecontemporaneous dissolution and supergene karstification caused by Tongwan/Caledonian movement laid the foundation for the formation of fracture-vuggy type reservoirs;the multi-phase tectonic fault and fractures provide effective"transport channels"for fluids and improved the connectivity of reservoir space;Finally,the buried organic acid dissolution associated with kerogen maturation plays a key role in promoting the formation of high-quality reservoirs,especially high-quality fracture-vuggy reservoirs.It also provides new evidences for the formation mechanism of early reservoir dissolution and late reformation.Overall,it provides a theory foundation and technical solution for the evaluation and prediction of deep to ultra-deep dolomite reservoirs in China. |
PDF Full Text Request