Nanopore structure comparison between shale oil and shale gas:examples from the Bakken and Longmaxi Formations

In order to analyze and compare the differences in pore structures between shale gas and shale oil formations, a few samples from the Longmaxi and Bakken Formations were collected and studied using X-ray diffraction, LECO TOC measurement, gas adsorption and field-emission scanning electron microscope. The results show that samples from the Bakken Formation have a higher TOC than those from the Longmaxi Formation. The Longmaxi Formation has higher micropore volume and larger micropore surface area and exhibited a smaller average distribution of microsize pores compared to the Bakken Formation. Both formations have similar meso-macropore volume. The Longmaxi Formation has a much larger meso-macropore surface area, which is corresponding to a smaller average meso-macropore size. CO_2 adsorption data processing shows that the pore size of the majority of the micropores in the samples from the Longmaxi Formation is less than 1 nm, while the pore size of the most of the micropores in the samples from the Bakken Formation is larger than 1 nm. Both formations have the same number of pore clusters in the 2–20 nm range, but the Bakken Formation has two additional pore size groups with mean pore size diameters larger than 20 nm. Multifractal analysis of pore size distribution curves that was derived from gas adsorption indicates that the samples from the Longmaxi Formation have more significant micropore heterogeneity and less meso-macropore heterogeneity. Abundant micropores as well as mesomacropores exist in the organic matter in the Longmaxi Formation, while the organic matter of the Bakken Formation hosts mainly micropores.

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.