The Influence of Fold and Fracture Development on Reservoir Behavior of the Lisburne Group of Northern Alaska

Abstract

The Lisburne Group is a major carbonate reservoir unit in northern Alaska. The Lisburne is detachment folded where it is exposed throughout the northeastern Brooks Range, but is relatively undeformed in areas of current production in the subsurface of the North Slope. The objectives of this study are to develop a better understanding of four major aspects of the Lisburne:1. The geometry and kinematics of detachment folds and their truncation by thrust faults. 2. The influence of folding and lithostratigraphy on fracture patterns. 3. Lithostratigraphy and its influence on folding, faulting, fracturing, and reservoir characteristics.4. The influence of lithostratigraphy and deformation on fluid flow. The results of field work during the summer of 1999 offer some preliminary insights:The Lisburne Limestone displays a range of symmetrical detachment fold geometries throughout the northeastern Brooks Range. The variation in fold geometry suggests a generalized progression in fold geometry with increasing shortening: Straight-limbed, narrow-crested folds at low shortening, box folds at intermediate shortening, and folds with a large height-to-width ratio and thickened hinges at high shortening. This sequence is interpreted to represent a progressive change in the dominant shortening mechanism from flexural-slip at low shortening to bulk strain at higher shortening. Structural variations in bed thickness occur throughout this progression. Parasitic folding accommodates structural thickening at low shortening and is gradually succeeded by penetrative strain as shortening increases. The amount of structural thickening at low to intermediate shortening may be inversely related to the local amount of structural thickening of the Kayak Shale, the incompetent unit that underlies the Lisburne.The Lisburne Limestone displays a different structural style in the south, across the boundary between the northeastern Brooks Range and the main axis of the Brooks Range fold-and-thrust belt. The steep forelimbs of angular asymmetrical folds typically have been cut and displaced by thrust faults, resulting in superposition of a fault-bend fold geometry on the truncated folds. Remnant uncut folds within trains of thrust-truncated folds and the predominance of detachment folds to the north suggest that these folds originated as detachment folds. Fold asymmetry and a more uniformly competent Lisburne Limestone may have favored accommodation of a significant proportion of shortening by thrust faulting, in contrast with the dominance of fold shortening to the north.Two dominant sets of fractures are present in the least deformed Lisburne Limestone: Early extension fractures normal to the regional fold trend and late extension and shear fractures parallel to the regional fold trend. These two major fracture sets remain as deformation increases, but they are more variable in orientation, character, and relative age. Compared to fold limbs, the fold hinges display greater density and extent of fractures, more conjugate and shear fractur...