Experimental Investigation Of The Role Of Rock Fabric In Petroleum Generation,Expulsion,Retention And Pore Evolution Within Marine Shales And Its Geological Implications

The processes of petroleum generation,expulsion,retention,and pore evolution within organic-rich marine shales are extremely complex under geological setting,which are key to unconventional shale oil and gas reservoirs.Although commercial production in shale gas has currently been achieved in both North America and South China,the development and improvement of geological theory on unconventional petroleum system remains a long-term process.In this study,four sets of type II kerogen-dominated organic-rich marine shales were selected from shale oil-and/or gas-producing basins in the United States as follow:one Barnett Shale,Eagle Ford Shale,Woodford chert and siliceous mudstone,which span over a wide range of organic richness and lithofacies variation.Each series of miniature core plugs with a diameter of 5 mm and a length of 2?3 cm,were drilled from the above four samples and performed for modified gold-tube pyrolysis with high-temperature and high-pressure conditions.By quantification and geochemical analysis on simulated products,combining with scanning electron microscope analysis with Ar ion milled subsamples,the main aim of this study is to investigate oil and gas formation,expulsion,retention,and pore evolution within organic-rich shales during thermal maturation,and further discuss the possible influences of maceral,mineral composition and petrographic characteristics on the process of hydrocarbon expulsion and retention,respectively.It is worth noting that these miniature core plugs used in pyrolysis experiments remain their original rock fabric with complete microscopic structure,texture and pore systems of these given shale samples,and therefore,organic matter in the rocks occurs in natural bedded states.They have this advantage over conventional kerogen samples and powdered rocks during pyrolysis.The main achievements and cognitions as follow:(1)By comparison of miniature core plugs and powdered rock samples from the same Eagle Ford Shale conducted in gold-tube pyrolysis,we have investigated the effect of rock fabric on gas generation,expulsion and retention within the given sample,and studied that the extent of gas retention from the rock fabric effect decreases with increasing thermal maturity.Excepting for the influence of hydrocarbon expulsion efficiency,it possibly implies that the evolution of pore network,especially organic-hosted pore development plays an important role in improving interconnectivity for gas migration and expulsion.Besides,as compared to powdered rock samples,miniature core plugs yields less inorganic CO₂ throughout the entire range of thermal maturity stages investigated by avoiding significant organic-acid reactions with carbonate minerals in presence of rock fabric.Meanwhile,yields of gaseous alkenes and molecular hydrogen from miniature core plug experiments are lower compared to rock powder,suggesting that the rock fabric plays an important role in promoting additive reactions between alkenes and hydrogen.Products from pyrolysis experiments involving samples of the Eagle Ford Shale with the rock fabric intact are more representative of naturally occurring gases than those generated from experiments with powdered rock.(2)Gold-tube pyrolysis experiments using miniature core plugs under high-temperature and high-pressure conditions,combined with Ar-SEM analysis,have successfully achieved the processes of pore formation and development in organic-rich shales during thermal maturation,which provided as a new means of investigating micro-nanometer scale pore evolution in organic-rich shales under geological setting.Experimental results show that the formation and development of pores within type II kerogen-dominated organic-rich shales are mainly controlled by thermal maturity and macerals.The abundance of Tasmanites algae observed in the studied Woodford siliceous mudstone sample have geochemical characteristics similar to type I kerogen,and therefore,both the formation of petroleum and the occurrence of pores related to Tasmanites algae in the Woodford siliceous mudstone sample significantly differ from amorphous organic matter in the Barnett and Eagle Ford Shale samples investigated.And nanometer scale spongy pores were developed in the late stage of oil window and oil cracking stages.However,we have also noticed that a few OM particles have no pores over the entire range of thermal maturity investigated,which are empirically recognized by inertinite.(3)On the basis of mass balance calculation,each quantitative model for evaluating petroleum formation,expulsion and retention within type II kerogen-contained marine organic-rich shales were established for comparison.Experimental results show that the processes of petroleum formation,expulsion and retention within type II kerogen-dominated organic-rich shales have been significantly affected by macerals and lithofacies variations.As compared to the Barnett Shale,Eagle Ford Shale,and Woodford chert samples investigated,the Woodford siliceous mudstone sample with the highest original TOC content has the lowest oil expulsion efficiencies,which is possibly related to the preserve of abundant Tasmanites algae and petrologic characteristics.Besides,as thermal maturity increase,at equivalent thermal maturity stages oil saturation indices of these miniature core plugs involving both the Barnett and Eagle Ford Shale sample during pyrolysis are fairly close to the highest free oil content for each the corresponding organic-rich shales under geological setting.It suggests that these miniature core plugs during pyrolysis in laboratory have comparable oil saturation indices with naturally matured sample under geological setting.(4)At each equivalent thermal maturity stage,generated gas yields from these studied samples are linearly proportional to their corresponding residual TOC contents,regardless of lithofacies variation,and therefore,empirical relationship between gas yields and residual TOC contents at different stages of thermal maturity are derived to establish a plot of gas generation on the type II kerogen.This empirical plot provides a simple and convenient means to describe gas generation yields and residual TOC contents of type II kerogen-dominated organic-rich shales at the varying stages of thermal maturity.When applying this plot to preliminarily estimate the extent of gas loss of the Wufeng—Longmaxi Formation shale gas play in the Sichuan Basin,it suggests that the total extent of gas loss is estimated as high as 70%?90%of the total gas generated because of both hydrocarbon expulsion and tectonic dissipation.In other words,it implies that only less than 30%of the total gas generated is retained and stored in shale reservoirs to form the high-over thermal maturity Wufeng—Longmaxi Formation shale gas play in the Sichuan Basin.