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

Abstract

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 (C1/iC4) in gas extracted from these organic-rich mudrocks reveals a wide range of expulsion efficiency. C1/iC4 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.A key control on the degree of petroleum expulsion in the Upper and Lower members that have served as sources for the Middle Bakken reservoir is the extent of cementation by authigenic microquartz. The grain assemblages in the organic-rich mudrocks are mixed, but are mostly dominated by grains of extrabasinal derivation (tarls). At the time of deposition only a minority of the mudrocks contained a substantial component of grains of intrabasinal derivation, notably radiolarians, which reacted to form authigenic microquartz. The higher-maturity mudrocks cemented by microquartz display relatively high retention of generated hydrocarbons whereas the associated tarls show a wide range of expulsion behavior. The contrast in expulsion state displayed across Bakken Formation mudrocks suggests that compaction, where not inhibited by cementation, persists into the oil window and acts together with overpressuring as a driver of hydrocarbon expulsion. Where cemented early in the diagenetic history, mudrocks cease to compact and are able to retain more of their generated oil. These results for the Bakken Formation have implications for unconventional systems more generally. The cementation status of mudrocks may be one of the key factors for predicting expulsion efficiency and for determining the efficacy of organic-rich mudrocks as hydrocarbon sources (where compaction-dominated) versus reservoirs (where cemented early in the burial history).