Geochemical Controls on the Distribution of Carbonate Cement in the Middle Member of the Bakken Formation, Williston Basin

Abstract

Operators within the Williston Basin have expressed increasing desire to evaluate the distribution of hydrocarbon charge in the middle member of the Bakken Formation, Williston Basin. Irregular carbonate cement appears to have grown authigenically prior to the migration of hydrocarbons from the Upper and Lower Bakken Shales. Where cement is present, charge is absent. This is a controlling factor on both the producibility of the Middle Bakken and stakeholders' resultant profit margins. The distribution of UV fluorescence on Middle Bakken cores portrays a clear distinction between the presence or absence of carbonate cement. This qualitative assessment has guided further work into understanding the geochemical controls on the genesis of such cement. An understanding of the genetic (and diagenetic) history may lead to our ability to predict the distribution of cement across the Williston Basin and improve operators' performance. Isotopic and mineralogical fingerprints may unlock the complex geochemical story of the Middle Bakken. Authigenic cementation is often attributed to the anaerobic oxidation of biogenic-methane in the shallow subsurface (methanogenic origins). Oxidation generates bicarbonate, which reacts to calcium in the subsurface waters and forms cement. By analyzing carbon isotopes of Middle Bakken cement it may be possible to determine whether the early release of biogenic methane from the Lower Bakken Shale led to the formation of carbonate cement. A detailed understanding of the formation and distribution of carbonate cement among a select group of North Dakota cores may then be extrapolated throughout the Williston Basin. This will aid operators in the optimization of their developmental initiatives.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015