Influencing Mechanism Of Gypsum-salt Minerials On Shale Pore Evolution In The Dongpu Depression

The 115 of oil and gas resources in the world are related to saline sediments,which mainly occur in sedimentary strata in the form of caprock shale or salt-associated shale.A large number of shale oil resources have been found in the saline shale sediments of the Cenozoic terrestrial lake basin in China.The oil and gas generation is controlled by the hydrocarbon generation capacity and reservoir capacity of shale.The reservoir capacity is mainly characterized by the pore type,pore structure and porosity.Many of the shale oil and gas resources found in China belong to saline shale formations,and has considerable resource potential.The role of gypsum-salt rocks in the formation and evolution of organic matter（OM）in such formations has received extensive attention,some achievements have been made in hydrocarbon generation and enrichment mechanism of organic matter,but no systematic understanding has been formed.The influence of gypsum-salt rock sediments on the mechanism of pore formation and evolution in shale is especially weak.In this paper,the shales in the third member of the Shahejie Formation（Es₃）of the Bohai Bay Basin were used to study the influence of halite on the formation and evolution process of pores.By means of geochemistry,X-ray diffraction（XRD）,field emission scanning electron microscopy（FE-SEM）,CO₂/N₂adsorption,high-pressure mercury injection,combined with the autoclave hydrocarbon generation thermal simulation experiment,the morphology and pore characteristics of 9 saline shale samples from the Es₃were analyzed.The genesis of salt rock and the mechanism of halite minerals in pore evolution are discussed and the pore variation characteristics in different thermal evolution stages are defined.The results showed that the TOC ranges from 0.47 wt.%to 3.26 wt.%,orangic matter type is II₁,the content of halite minerials ranged from 0.6%to 6%.Halite and gypsum minerals were obviously associated with OM,which made them more likely to develop OM pores.The samples with a high halite mineral content（HC）are more developed in terms of the pore volume and specific surface area than samples with a low HC.The formation of gypsum and halite minerials are influenced by a combination of deep thermal brine upwelling,sea erosion and arid environments.The frequent alternation between humid and arid environments led to the outbreak and death of organisms and the precipitation of gypsum-salt rock,which formed the simultaneous deposition of OM and halite minerals.In summary,according to the particularity of halite minerials,a pore evolution model with the participation of gypsum-halite minerials was established.In both stage II and stage IV,halite minerals directly and indirectly promoted the development of pores.On the one hand,halite minerals increased the pore space of shale due to their own characteristics;on the other hand,halite minerals increased the pore space of shale due to their special physical properties.It promotes the formation and expulsion of hydrocarbons and indirectly promotes the development of various pores.This study deepens the theoretical understanding of gypsum and hailte minerials on the formation of organic matter,provides a theoretical basis for the pore structure characteristics and functions of global saline basins,provides strong microscopic evidence for the study of global salt basins,and has guiding significance for shale oil exploration and target favorable area selection in this study area.