--> Low Pore Connectivity and Anomalous Fluid Migration in the Three Forks Formation

Southwest Section AAPG Annual Convention

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Low Pore Connectivity and Anomalous Fluid Migration in the Three Forks Formation


The Late Devonian Three Forks Formation, roughly at the thickness of 250 feet underlies the Bakken Formation in the Williston Basin, underlying parts of Montana, North Dakota, and Saskatchewan. Production in the Three Forks Formation has been from Units 5 and 4 which get their source from the organic rich Bakken shale. The most recent USGS assessment in 2013 estimates the Bakken/Three Forks play (US only) contains 7.375 billion barrels of oil, 6.723 trillion cubic feet of gas and 527 million barrels of natural gas liquids; the values double from an earlier 2008 assessment.

The main barrier to sustainable development of US shale, the pore structure of the nanopores storing and transporting hydrocarbons, as well as its implications in low recovery factor, has been quietly ignored. This work is to study nanopetrophysics, in terms of nanopore connectivity and associated fluid migration, of The Three Forks formation in the Williston basin. Mixed siltstone/sandstone/carbonate rock samples of the formation are obtained from well Round Prairie 1-17H from Williams County (API No. 18257) for all of five units of the Three Forks Formation.

However, the oil recovery factor in the Three Forks Formation is reported to be 8.9±5.32% according to North Dakota Industrial Commission. For these tight rocks, we have studied pore structure, edge-accessible porosity, and how wettability is associated with mineral and organic kerogen phases, from the following complementary tests: vacuum saturation with vacuum-pulling on dry shale followed with tracer introduction, fluid and tracer imbibition into partially-saturated shale, and tracer diffusion into fluid-saturated shale. These tests use tracer-bearing fluids (API brine or n-decane) to examine the association of tracers with mineral or kerogen phases, using a combination of elemental laser ablation-ICP-MS mapping and high-resolution SEM approaches. These innovative approaches indicate the limited accessibility (several millimeters from shale sample edge) and connectivity of nanopores in shales, which could lead to the steep first-year decline and low overall recovery because of the limited connection and migration of hydrocarbon molecules in the shale matrix to the stimulated fracture network.

Acknowledgment: We like to thank Julie LeFever of the North Dakota Geological Survey for her support in selecting and acquiring core samples from wells in the study area.