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Joe S Brinton1, Catherine L Hanks1, John C Lorenz2
(1) Geophysical Institute, University of Alaska, Fairbanks, Fairbanks, AK
(2) Sandia National Laboratories, Albuquerque, NM

Abstract: Fracturing in carbonate rocks as a function of lithology and position in detachment folds: examples from the Brooks Range, Alaska

Lisburne Group carbonates form an important naturally fractured hydrocarbon reservoir in the North Slope of Alaska. The Lisburne is relatively undeformed in the Lisburne field, but future exploration plays in the Lisburne may be closer to the mountain front, where the Lisburne is both detachment folded and thrust faulted. Detatchment folds in the Lisburne Group are widely exposed in the northeastern Brooks Range, making it an ideal natural laboratory for study of the character and distribution of fractures as a function of mechanical stratigraphy and position within a detachment fold.

Relatively undeformed or mildly deformed Lisburne display two distinctive fracture sets: an early NNW-oriented set of extension fractures and a later EW-oriented set of extension and shear fractures. The fracture character and distribution in both sets are clearly linked to carbonate lithology, with fine-grained lithologies having higher fracture densities than coarser lithologies. Within each set, the fracture orientation is very consistent. With folding, the relationship between lithology and fracture density remained unchanged. However, when compared to fold limbs, the fracture densities in fold hinges are higher in all lithologies, and show greater vertical extent. Both fracture sets become more variable in orientation and character in fold hinges, with greater numbers of conjugate fracture sets and shear fractures. The relative timing of the two fracture sets becomes more ambiguous, with NNW-oriented fractures not always clearly predating E-W-oriented fractures. Other structures that would potentially destroy permeability, such as dissolution cleavage, also occur in hinge areas. These observations suggest that structural position in detachment folded carbonates play a major role in determining reservoir character.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana