Mineral diagenetic control of expulsion efficiency in organic-rich mudrocks, Bakken Formation (Devonian-Mississippian), Williston Basin, North Dakota, USA

摘要

Organic-rich mudrocks that served as sources for the prolific unconventional system of the Bakken Formation display pronounced heterogeneity in composition and expulsion behavior. The dominant mudrocks of the Bakken Formation (tarls) contain few biogenic allochems and have a diagenetic history dominated by compaction. Mudrocks that at deposition contained relatively more siliceous biogenic debris display microquartz cement that reduced porosity and enhanced rock stiffness. Against a background of strong expulsion at the formation scale, the ratio of methane to iso-butane (C-1/iC(4)) in gas extracted from these organic-rich mudrocks reveals a wide range of expulsion efficiency. C-1/iC(4) in crushed rock gas is strongly controlled by maturity, with lower maturity samples displaying a range of iso-butane content, but little methane. At higher maturity a strong negative correlation between methane and iso-butane is interpreted in terms of expulsion efficiency, with higher proportional amounts of iso-butane corresponding to greater oil expulsion. The majority of mudrocks in the Upper and Lower Bakken Formation source rocks display strong expulsion efficiency, although a minority of samples display relatively high oil retention.