Characteristics Of Nanometer Scale Pores Associated With Solid Bitumen And Its Influences On Pore Development Of Shales

Organic matter?OM?pores are crucial for reservoir space of highly-over mature gas shales and have significant influences on the evaluation of flowability of shale gas.Since oil potential and conversion of type I and type II kerogens are relatively high,abundant solid bitumens could be formed as a result of secondary cracking of residual oil after the oil generation and expulsion.Characteristics,development processes and relative contributions of nanometer-scale pores in kerogen and solid bitumen are key issues in studies on shale porosity.As Lower Paleozoic strata in south China is generally over mature,it is difficult to accurately discriminate solid bitumen from kerogen,which makes resolving related scientific problems challenging.In addition,OM in carbonate reservoirs associated with giant gas fields is mainly solid bitumen,resulting from severe cracking of oil.Its nanometer-scale pore characteristics are of great value in understanding the development of OM pores,but related investigations were seldom performed.This thesis has first studied effects of sample preparation,such as crushing,sieving and acid treating,on low-pressure gas adsorption analysis of shales and solid bitumen by condition experiments.Geochemical and nanometer-scale pore characteristics of solid bitumens in carbonate reservoirs of the Sichuan Basin were then investigated.Finally,the influences of solid bitumen on the development of nanometer-scale pores of over mature,OM-rich shales were studied,by combining natural shale samples with pyrolysis-derived shales containing different contents of residual hydrocarbons.Main conclusions are as follows:1)Sample preparation procedures were demonstrated to have important effects on low-pressure gas adsorption analysis of shale and solid bitumen samples.N₂adsorption on samples is more affected by particle size than the CO₂ adsorption.Since a small particle size is more favourable for pore connectivity of shales and the gas diffusion,samples with the size smaller than 80 mesh are suggested for the measurements of N₂ and CO₂ adsorptions.In addition,results obtained after treating solid bitumen with acid illustrated that a part of fine mesopores of OM may have been destroyed by the treatment with hydrochloric acid.2)Solid bitumens in carbonate reservoirs of the Sichuan Basin show very large variations in the development of nanometer-scale pores.N₂ adsorption capacity,specific surface area and meso-macropore volume of solid bitumens vary greatly.Meso-macropore volumes of solid bitumens are mainly contributed by the pores from10 nm to 100 nm.In addition,micropores developed abundantly in solid bitumens of reservoirs.CO₂ adsorption volumes?P/P_o=0.03?of relatively pure solid bitumens are in the range of 14–20 cm³/g.3)The Maoming oil shale rich in type I kerogen has a very large amount of clay minerals,which is associated with most of primary pores of the shale as shown by the slight oxygenation and removal of OM in the shale.Within the oil generation stage,dehydroxylation and dehydration reactions could have resulted in an increase of micropore volume of clay minerals;however,clay minerals have few effects on the pore development of bulk shale at high to over maturity stages.4)The formation of solid bitumen affected the development of nanopores in shales,which maybe depend on types and contents of OM of shales.At high to over maturity stages,a large amount of residual hydrocarbon in the shale with type I kerogen have been converted to solid bitumen,which facilitated the development of mesopores and macropores,whereas only slightly influenced the micropore evolution.For the shale with type II kerogen,residual hydrocarbons rich in polar compounds may have recombined into kerogen to form a new kind of solid organic matter having few pores,which only has very slight influences on pore evolution of shale.