Characteristics Of Continental Shale Laminae Pore Structure And Its Controlling Mechanism On Shale Oil And Gas Enrichment

Laminae are widely developed in continental shales,which are closely related to organic matter enrichment,shale oil and gas reservoir space,development and production effect,etc.At present,the process of shale oil and gas exploration and development is restricted by the unclear understanding of the formation environment of shale laminae,pore structure and its influence on oil and gas enrichment.Aiming at the scientific problem of the controlling mechanism of shale oil and gas enrichment by the characteristics of shale laminae pore structure,the types and characteristics of laminae structure of continental shale in the Middle and lower Jurassic Qianfoya Formation and Da’anzhai member of Ziliujing Formation were studied by means of geochemical analysis,petrological analysis and element geochemical analysis,and the development models of shale laminae and lithofacies were established.Combined with scanning electron microscopy,gas adsorption,high-pressure mercury injection and high-temperature and high-pressure thermal simulation experiments,the differences of shale reservoir characteristics,such as pore structure and reservoir capacity with different lamellar structures developed from mid-maturity stage to high-maturity stage,were clarified.Methane isothermal adsorption and two-dimensional nuclear magnetic experiments were used to identify the occurrence state of oil and gas in different lamellar combination structures.Finally,the controlling mechanism of continental shale laminar pore structure on oil and gas enrichment under different lacustrine sedimentary models is revealed.The continental shales of the Middle and Lower Jurassic Qianfoya Formation and Da’anzhai Member of Ziliujing Formation in northeastern Sichuan have relatively high organic carbon content,high clay mineral content and low brittle mineral content.The shale is in the mature to high mature stage,the organic matter types are mainly typeⅡand typeⅢ,and the clay minerals are mainly mixed layers of illite and Aemon.The shales of the Qianfoya Formation and the Da’anzhai member of the Ziliujing Formation are characterized by self-generated and self-stored shale oil and gas with weak secondary migration.The laminae of continental shale in the study area are widely developed,mainly siliceous clastic laminae,clay laminae,carbonate laminae and organic-rich laminae,forming four laminae combination structures:thin laminae interlaminae,thick laminae interlaminae,thin laminae interlaminae and weak laminae.Siliceous clastic laminae were formed by strong clastic flow and influenced by terrigenous input.The laminae were formed in shallow water,strong hydrodynamic and anoxic environment during the deposition period.The carbonate laminae were less affected by terrigenous input,and the sedimentary period was dry and hot.The laminae underwent the action of high-energy water and formed in shallow water oxidation environment.The organic-rich laminae were influenced by terrigenous input and formed in a warm and humid climate under anoxic-reduction conditions under weak hydrodynamic conditions,which were controlled by terrigenous input paleo-productivity.Clay laminae are less influenced by terrigenous inputs and are formed in warm and humid climates.The laminae are formed under anoxic-reductive conditions in deep water and weak hydrodynamic environment.In siliceous clastic laminae,residual compacted pores(61.2%)were mainly developed,followed by clay mineral intergranular pores(34.3%).Clay mineral pores(54.76%)and dissolution pores(24.84%)are mainly developed in clay laminae.The carbonate laminae were dominated by dissolution pores(71.50%),followed by clay mineral pores(19.08%).Clay mineral pores(49.8%),followed by organic matter pores(33.5%)are mainly developed in organic-rich laminae.The difference in pore proportion and reservoir space of different laminae leads to the difference in reservoir capacity of different laminae.In the middle to late maturity stage(R_o:0.8%～1.3%),shales with thin interbedded,thin interbedded and weak laminated structures have better reservoir capacity.At the high maturity stage(R_o:1.3%-2.0%),the shale with thin laminae and weak laminae has good reservoir capacity,followed by the shale with thick laminae and interbedded shale structure.Based on the Langmuir equation and the difference of reservoir space of laminar structure,the effective occurrence space of shale free gas with different laminar combination structure is controlled by the total porosity and formation water saturation,which determines the free gas content of shale reservoir.TOC and water saturation mainly affect the effective adsorption sites within the laminar,and play a decisive role in adsorbed gas and total gas content in the case of a large amount of water.Based on the distribution difference of bound oil and free oil in the laminae of two-dimensional NMR image,the total pore volume,pore diameter and TOC mainly affect the effective occurrence space of free oil in different laminae combination structure,and determine the pore size range and free oil content in the laminae.The high TOC content determines the enrichment of free oil at 10～80nm.TOC and clay minerals affect the pore development ability of 2～10nm in laminar assemblage structure,and affect the location and content of bound oil.In the laminar assemblage structure of Qianfoya Formation formed by terrigenous clastic shallow lacustrine sedimentary model,free oil mainly exists in organic pores and large clay mineral pores.The shales with thin laminar interbedded assemblage structure and weak laminar interbedded assemblage structure are the main enrichment strata of shale oil,and the thin laminar interbedded assemblage structure with strong sealing ability can form local high pressure and lead to large accumulation of hydrocarbons.In the shale of Da’Anzhai Member of Ziliujing Formation formed by the sedimentary model of carbonate lake,the gas phase is mainly enriched in the shale section with thin laminar structure and weak laminar structure,and the shale with high organic matter abundance and thick laminar interbedded combination structure formed under the condition of deep water can also be used as the favorable shale gas enrichment interval.