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ABSTRACT: Causes, Characteristics, and Implications of Pervasive Oil-induced Horizontal Fracturing in Bakken Source System Reservoir Rocks

An intense, but disappointing, industry horizontal-drilling ling effort to recover oil from the Lower Mississippian-Upper Devonian Bakken shales concentrated on vertical fractures in the shales themselves. However, the shales actually contain no mobile oil, because the real reservoir rocks of the Bakken Source System are the three rocks (lowermost Lodgepole, Bakken siltstone, upper Three Forks) adjacent to the two Bakken shales. Where the Bakken shales are immature, these three rocks are tight and extremely organic-poor. However, where the Bakken shales have generated hydrocarbons (HCS), the three reservoir rocks are heavily oil-stained, and laced with horizontal fractures, thus having fracture-enhanced permeabilities. Secondary microporosity is always developed adjacent to these fractures. The very well interconnected horizontal fractures were caused by a super-lithostatic injection of Bakken oil into the reservoir rocks, which in turn was caused by a large organic-matter volume expansion during HC generation in the Bakken shales. The secondary microporosity adjacent to the oil-induced fractures was caused by redox reactions between the injected Bakken oil and the indigenous pore water of the reservoir-rocks. These reactions largely yield CO2 and carboxylic acids which created secondary porosity both in this case and in conventional oil fields.

Pivotal implications result from this research in the Bakken Source System. For example, expulsion of HCS from source-rocks is assumed to be very efficient. Our data demonstrate that the Bakken shales are among the richest source rocks worldwide, and also are post-mature over large areas. Yet the Bakken Source System has formed no conventional oil fields. Thus, this, and other published, research illustrates that in most cases HC expulsion is actually highly inefficient, a conclusion with significant ramifications for HC exploration and assessment, the use of Petroleum Systems, and the existence of huge unconventional in-place oil resources around organic-rich source rocks.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico