Present Day Fracture and in Situ Stress Distribution in the National Petroleum Reserve of Alaska (NPRA): Implications for the Evolution of a ‘Fracture Window’ Through Time

Joshua Payne², Catherine Hanks¹, Alfred Kleck², Alec Duncan², Paul O'Sullivan³, and Marty Parris⁴
1Geophysical Institute & Petroleum Engineering, University of Alaska, Fairbanks, AK
²Department of Geology and Geophysics, University of Alaska, Fairbanks, AK
³Apatite to Zircon, Inc, Viola, ID
⁴Kentucky Geological Survey, Lexington, AK

Hydrocarbon migration in a basin may be partially controlled by the existence and orientation of open fractures near or in the oil and gas generation window. In the Colville basin of NPRA, field observations, microthermometry, geochronology and in situ stress analysis can be integrated to provide constraints on the areal and temporal distribution of open fractures that are optimally positioned to serve as migration pathways.

Surface observations in the central Brooks Range foothills suggest early extension fractures that formed perpendicular to the active thrust front and within the oil generation window were the fractures most likely to have served as past migration pathways. Borehole breakout concentrations suggest that similar, northwest-striking open fractures may be forming in the Colville basin today in a ‘fracture window’ at depths >~10,000 feet (Sv >80 mPa).

Geohistory models of selected NPRA wells suggest that Mississippian through Triassic Ellesmerian sequence rocks first entered this ‘fracture window’ in the southernmost parts of the Colville basin during middle Cretaceous time. Open fractures at this time were most likely oriented northnortheast, perpendicular to the then-active thrust front. This zone of potential subsurface fracturing widened and extended to the northnortheast during the remainder of the Cretaceous as the Colville basin deepened. Overlying basal Cretaceous Brookian sediments (i.e., the Torok Formation) did not enter the ‘fracture window’ in the southern parts of the Colville basin until Late Cretaceous. Younger Cretaceous and Tertiary Brookian sediments (i.e., Nanushuk and Colville Groups) were never buried deeply enough in most parts of NPRA to experience this fracturing.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas