Pore Structure And Thermal Evolution Of Yanchang Shales From Southeastern Ordos Basin

The porosity and pore structure of shale is significant for acknowledging the gas/oil occurrence,movement and potential.During the thermal maturation of shales,changes of the organic matter(OM)occurrence have great effect on the pore development.However,the factors that mainly control pore development of shales of different thermal stage need more research.In this work,shales from Yanchang formation of soureastern Ordos Basin are the main research object.By integrating the low pressure N2,CO2 adsorption,He pycnometry,high pressure mercury intrusion and field emission scanning electron microscope(FE-SEM)observation,the pore structure of geological shale samples and samples got through thermal pyrolysis are investigated to illustrate the relationship between thermal maturation and pore development of OM rich shale.Some conclusions are drawn:(1)The OM type of chang 7 and chang 9 member shales of Yanchang Formation from the southeastern Ordos Bain is ?.Chang 7 shales are mainly in the primary and peak oil generation stage and Chang 9 shales are mainly in the late oil generaion and condensate oil period.As the methane adsorption capacity of extracted samples is higher than kerogens and there is positive correlation between clay minerals and methane adsorption capacity,it seems like that clay minerals have greate effect on the methane adsorption capacity of dry shales.(2)Based on the FE-SEM observation,the pore development of OM could be subdivide into four types: non-porous,poor porous,porous and excellentlly porous.The nonporous OM are mainly woody OM,solid bitumen and laminae OM.Pores mainly present in small pieces of OM which rest in the inter-mineral pores.The excellently porous OMs are nano-fiber shapped.These OMs might be extracellular polymeric substances.(3)Pore volume of shales with total organic carbon(TOC)content larger than 5% decreases with increasing TOC.Two factors could explain this phenomenon: one isthat shales with high OM content generate more solid bitumen occupying the pores between minerals.The great pore volume increase of shales after extraction proves that solid bitumen occupies pores of shales in the oil window.The other is the ability of minerals protecting OM from compaction is weak when the OM volume is large.(4)Thermal simulation of one shale sample in two apparently different simulation systems(one semi-closed system with high pressure exerted and the other closed system without pressure)indicates that the high pressure of the semi-closed system retards OM cracking resulting in the intense hydrocarbon expulsion.In the oil window,the pore volume of both simulation systems decreased.Different N2 adsorption capacity of samples before and after extraction reveals the causes of pore decrement and that there is pore development in oil generation window.Oil cracking stage is the main pore development period.The high fluid pressure results from oil cracking leads to the intense hydrocarbon expulsion of the high pressure semi-closed system.For samples with similar thermal maturity,the meso,macro-pores of samples from the semi-closed system is larger than that of samples from closed system.While,the micropore volume of the two systems increases in similar way and volume.(5)Thermal simulation of shales with different OM type and TOC content presents that samples with better OM type and higher TOC content tend to have higher expulsion efficiency and thus higher pore volume increment.Compared with shales with type ?/? OM,shales with type ? and ? OM tend to generate larger macropore pore volume during thermal pyrolysis.Compression restrains the pore development of shales with larger macropore volume.