The Developmental Characteristics And Controlling Factors Of Organic-associated Pores In Organic-rich Shales

Organic pore formed in the process of organic thermal evolution,is the trace of shale gas generation,diffusion and accumulation left behind,and is also the refection of gas generation and storage capacity for shale gas in shale reservoirs.The previous studies on organic pore system was mainly focused on certain sedimentary face shale.In this thesis,marine,continental,and marine-continental shales were selected to make a comprehensive analysis on the characteristics of organic pores and the thermal evolution.The main testing methods employed in this thesis are the characterizing methods for shale pore system and the related geochemical tests.It is analyzed that the organic pores are essentially determined by the alterability of the inner structure of organic macerals during thermal evolution and the porous regularity is controlled by gas–expansion-force.The formation stage of organic pores in structure-alterable liptinite(oil-prone macerals)is comparatively lagged behind the hydrocarbon-generation,namely organic pores are poorly developed in oil-window stage,and only in the stage of gas-window when the gas-expansion-force is large enough could the macerals breakthrough the OM surface and develop organic pores in large quantities.In contrast,the formation stage of organic pores is approximately synchronized with hydrocarbon-generation in huminite(gas-prone macerals)since they can directly generate gases and accumulate the gas–expansion-force in low maturity,but the organic pores cannot be widely developed confined to the difficulty of their structure to alter.Furthermore,worth noting that whichever maceral it is,the organic pores are not unlimitedly expanded with the increasing organic maturity.After entering metamorphism stage,the organic pores originally formed will be kept getting smaller and gradually turned into the linear or pin-point-like ones due to the drastic poly-condensation of kerogen and the decease of gas-expansion-force,and until to the deep metamorphic stage it will be lost in the residue carbon.Therefore,terrestrial shales in North China rich in structure-alterable macerals poorly develop organic pores due to the lower maturity,and the marine shales in South China largely develop the organic pores since they are still in the gas window.But for shales in metamorphic stage,organic pores are reduced sharply like Niutitang Shale.While transitional shales whatever in North China or in SouthChina can only develop the organic pores in small amount regardless of the level of maturity because they are dominated by the structure-unalterable humic macerals.Additionally,the organic pores are also strongly influenced by the mechanical properties and the occurrence pattern of organic macerals.The over-compaction of the organic matter in the over-mature stage will cause serious deformation of organic pores.In addition,the maceral that adhering the minerals generally has strong ability to resist compaction compared with the isolated maceral without mineral-supporting.This study can be helpful to better understand the hydrocarbon generation mechanism and the gas storage capacity of organic matters in shale reservoirs,and also can provide a selection index for favorable zone of shale gas.