Maceral evolution of lacustrine shale and its effects on the development of organic pores during low mature to high mature stage:A case study from the Qingshankou Formation in northern Songliao Basin,northeast China

Organic matter(OM)hosted pores are crucial for the storage and migration of petroleum in shale reservoirs.Thermal maturity and macerals type are important factors controlling the development of pores therein.In this study,six lacustrine shale samples with different thermal maturities from the first member of the Qingshankou Formation in the Songliao Basin,of which vitrinite reflectance(R_(o))ranging from 0.58% to 1.43%,were selected for a comparative analysis.Scanning electron microscopy(SEM)and reflected light microscopy were combined to investigate the development of organic pores in different macerals during thermal maturation.The results show that alginite and liptodetrinite are the dominant primary macerals,followed by bituminite.Only a few primary organic pores developed in the alginite at the lowest maturity(R_(o)=0.58%).As a result of petroleum generation,oil-prone macerals began to transform to initial-oil solid bitumen at the early oil window(R_(o)=0.73%)and shrinkage cracks were observed.Initial-oil solid bitumen cracked to oil,gas and post-oil bitumen by primary cracking(R_(o)=0.98%).Moreover,solid bitumen(SB)was found to be the dominant OM when R_(o)>0.98%,which indicates that SB is the product of oil-prone macerals transformation.Many secondary bubble pores were observed on SB,which formed by gas release,while devolatilization cracks developed on migrated SB.Additionally,at the late oil window(Ro?1.16%),migrated SB filled the interparticle pore spaces.With further increase in temperature,the liquid oil underwent secondary cracking into pyrobitumen and gas,and spongy pores developed on the pyrobitumen at higher levels of maturity(Ro=1.43%),which formed when pyrobitumen cracked into gas.Vitrinite and inertinite are stable without any visible pores over the range of maturities,verifying their low petroleum generation potential.In addition,it was concluded that clay minerals could have a catalytic effect on the petroleum generation,which may explain why organicclay mixtures had more abundant pores than single OM particles.However,after R_(o)>0.98%,authigenic minerals occupied the organic pore spaces on the organic-clay mixtures,resulting in fewer pores compared to those observed in samples at the early to peak oil window.

Microstructure Characterization of a Biogenic Shale Gas Formation——Insights from the Antrim Shale,Michigan Basin

Biogenic gas shales,predominantly microbial in origin,form an important class of organicrich shale reservoirs with a significant economic potential.Yet large gaps remain in the understanding of their gas generation,storage,and transport mechanisms,as previous studies have been largely focused on mature thermogenic shale reservoirs.In this study,the pore structure of 18 Antrim Shale samples was characterized using gas adsorption(CO2 and N2).The results show that most of the Antrim Shale samples are rich in organic matter content(0.58 wt.%to 14.15 wt.%),with highest values found in the Lachine and Norwood members.Samples from the Paxton Member,characterized by lower organic content,have smaller micropore surface area and micropore volume but larger meso-macro pore surface area and volume.The deconvolution results of the pore size distribution from the N2 adsorption indicate that all of the tested Antrim Shale samples have similar pore groups.Organic matter in the Antrim Shale hosts micro pores instead of meso-macro pores,while clay minerals host both micro and meso-macro pores.Mineralrelated pores play a primary role in the total porosity.The biogenic Antrim Shale,therefore,has different pore structures from other well-studied thermogenic gas shales worldwide.