Pore space restrictions on microbial activities in the Second White Specks shale of the Western Canada Sedimentary Basin

A suit of thirteen shallow biogenic gas source rock samples taken from the Second White Specks Formation (2WS) in well A at South Alberta Gas Field (SAGF), Western Canada Sedimentary Basin (WCSB) were analyzed by a laser particle size analyzer, nitrogen adsorption and GC-MS to obtain grain size, surface area, pore size distribution, and hydrocarbon molecular compositions, in order to get an insight into the constraining effects of pore space on microbial activity within shallow buried source rocks. The samples are organic rich with high TOC values, shallowly buried and immature, which are suitable for microbial activity. Pore size distributions derived from nitrogen adsorption isotherms illustrate the dominance of mesopores (2-50 nm). Molecular geochemical parameter analysis indicates that the compositions of extractable organic matter are basically attributed to the variation of source input with no solid evidence of biodegradation. The analysis of pore size distribution shows that very limited portion of pores with diameter over 200 nm cannot sustain extensive microbial activity, which possibly accounts for the slight biodegradation of organic matter in shale. Biogenic source rocks capable of generating large amount of biogenic gas are supposed to have a large proportion of pores larger than 200 nm. Traditional schemes for potential biogenic gas source rock assessment may have neglected the decisive role of shale pore space in bacteria activity.