Characteristics Of Pore Structure And Its Evolution In Organic-rich Shale Reservoir

The commercial exploitation of shale oil and gas breaks through the conventional oil and gas exploration theory,and promoted the growth of global oil and gas reserves and production.Shales,long thought of by the oil and gas industry as hydrocarbon source rocks and seals in a conventional petroleum system,have emerged as effective hydrocarbon reservoirs for the self-generating and self-preserving of shale oil and gas.Shales have received renewed attention in recent years because of their emergence as effective hydrocarbon reservoirs.By referring to the successful exploration experience of shale gas in North America and combining with the actual geological conditions of organic-rich shales in South China and a series of key technological breakthroughs,the commercial exploitation of shale gas in Sichuan Basin has been successfully realized in China.Meanwhile,the Mesozoic and Cenozoic lacustrine strata widely developed in China,which have huge exploration potential in shale oil.However,the lack of understanding of reservoir capacity in lacustrine shale hindered the exploration of shale oil.The Wufeng Formation and Longmaxi Formation shales were collected from shale gas mature exploration area（southern and eastern Sichuan Basin）and the promising exploration area of the periphery of Sichuan Basin（Yichang area in middle Yangtze）,and the basic geochemical,mineralogical and reservoir petrophysical analysis were carried out.The influence of shale material composition and pore composition on shale reservoir properties was researched.Then,an integrated analysis of mineralogy,organic geochemistry,thermal simulation and scanning electron microscope（SEM）was conducted to characterize the reservoir capacity of Chang 7shale in Yanchang Formation of Ordos Basin and calcareous shale in Lucaogou Formation of Santanghu Basin in the early oil window.Moreover,the medium and low mature samples were artificially matured to the end of the oil window/gas window.The pore composition and evolution characteristics of shale in the oil window/gas window and the oil bearing capacity of calcareous shale in the oil window were studied.In summary,the main results and achievements were as follows:（1）SEM images show that a large number of OM pores were developed in Longmaxi Formation shales in southern and eastern Sichuan Basin,which could be honeycomb,bubbly and irregular in shape.The results of focused ion beam-scanning electron microscope show that the organic pores have a better pore connectivity than those developed in other matrix minerals,but the pore clusters display connectedness on a scale smaller than 5μm even within sample W4-3,which has the highest TOC value.The development of pores in Longmaxi Formation shale in southern Sichuan Basin is controlled by TOC and clay mineral content,while calcite and quartz may dilute the TOC content and cuase a negative correlation with porosity.The controlling effect of clay mineral content on permeability is stronger than that of TOC in southern Sichuan Basin,whereas high clay content（>30%）will block pore throats and result in low permeability.Moreover,a higher percentage of mesopores also increased the permeability of Longmaxi Formation shale in southern Sichuan Basin.（2）The vertical variation of elemental geochemical in the Well YH-1 shown that high paleoproductivity,dysoxic to anoxic bottom water conditions and low terrigenous input are beneficial to the development of organic-rich shale.A positive correlation between TOC and porosity was established in the Wufeng Formation and lower Longmaxi Formation（TOC>2%）.Pores（OM pores）are not only developed in organic matter,but also widely developed in organo-clay complexes,while amount of interp pores were developed in clay platelets and the development of mesopores and macropores was controlled by clay minerals.Therefore,organic matter and clay minerals controlled the pore development of shale in the Wufeng Formation and lower Longmaxi Formation.A negatively correlation between TOC and porosity was established in the upper Longmaxi Formation（TOC<2%）,while the clay mineral content and porosity are positively correlated in the upper Longmaxi Formation.Therefore,the pore development of shale in the upper Longmaxi Formation is mainly controlled by clay minerals.The correlations of pore volume and specific surface area with TOC and clay mineral content in the Well YH-1 indicating that micropores are the main pores in organic matter,while clay minerals are mainly contributed to the development of mesopores and macropores.（3）The decrease of pore volume of Chang7 shale during early oil window is mainly caused by the clogging of pores by residual oil,while the increase of pore volume is due to the enhanced hydrocarbon expulsion efficiency,the formation of OM pores and dissolution pores,and the development of pyrite interparticle and intraparticle pores in the late oil window.The secondary OM pores are associated with the pyrite interparticle and intraparticle pores,which may be related to the hydrocarbon generation catalyzed by pyrite.Compared with oil window,the stronger gas generation and fluid activity in shale are the main factors effect the pore evolution in Chang7 shale during wet and gas window.SEM images show that more extensive dissolution pores,OM pores,pyrite interparticle and intraparticle pores were developed in wet and dry gas stage than in oil window.Moreover,the scale of development of dissolution pore was further expanded at the highest maturity and widely developed in the area where OM are associated with pyrite.（4）In general,the residual oil content and oil saturation index（OSI）of the pyrolyzed Lucaogou Formation lacustrine calcareous shale increased gradually in the oil window,and the（S₁×100）/TOC increased significantly from 88.2 to 159.6 at a maturity range of 1.2–1.29 easy%Ro.The blocking of pores by generated oil and intense compaction caused a decrease in the nanopores volume during easy%Ro=0.79–1.2;while abundant OM pores formed due to significant hydrocarbon generation and expulsion in the lacustrine calcareous shale at 1.29 easy%Ro.Moreover,micron-scale dissolution pores and shrinkage OM pores were developed at1.0–1.29 easy%Ro,which contributed considerably to the total porosity and improved the performance of the lacustrine calcareous shale reservoir.In summary,the development and evolution of pores in the lacustrine calcareous shale were controlled by various factors,viz.,OM maturity,hydrocarbon generation and expulsion,and organic-inorganic interactions.A maturity range of 1.2–1.29 easy%Ro is favorable for shale oil exploration in lacustrine calcareous shale,due to a high OSI（S₁×100/TOC）and favorable reservoir capacity.