西加拿大沉积盆地二白斑页岩孔隙空间对微生物活动的制约

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

运用激光粒度仪、氮气吸附实验和色谱—质谱联用方法,对西加拿大沉积盆地(西加盆地)阿尔伯塔东南气田(Southeast Alberta Gas Field,SAGF)A井二白斑(Second White Specks)页岩13个生物气源岩样品进行粒度、比表面、孔径分布测试和烃类分子组成分析,探究浅层烃源岩孔隙空间与页岩中烃类生物降解之间的关系。研究样品有机质含量丰富,埋深浅,处于适合微生物活动的未成熟阶段,孔径分布显示样品的孔隙主要集中在中孔(2~50 nm)范围。分子地球化学参数分析表明,页岩中有机质组成主要受有机质输入和沉积环境的影响,烃类生物降解作用不明显。结合页岩孔径分析认为,目前页岩绝大部分孔隙空间都无法为微生物提供可生存环境,仅少量大孔(>200 nm)可能为页岩中的微生物活动提供有限空间。能大规模产生物气的气源岩应具有较大比例的孔径大于200 nm的孔隙。传统的生物气源岩评价标准可能对页岩孔隙空间这一限制微生物活跃性的因素有所忽视。

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.