THE INFLUENCE OF FOLD AND FRACTURE DEVELOPMENT ON RESERVOIR BEHAVIOR OF THE LISBURNE GROUP OF NORTHERN ALASKA

摘要

The Carboniferous 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 on fracture patterns. (3) The influence of deformation on fluid flow. (4) Lithostratigraphy and its influence on folding, faulting, fracturing, and reservoir characteristics. The Lisburne in the main axis of the Brooks Range is characteristically deformed into imbricate thrust sheets with asymmetrical hanging wall anticlines and footwall synclines. In contrast, the Lisburne in the northeastern Brooks Range is characterized by symmetrical detachment folds. The focus of our 2000 field studies was at the boundary between these structural styles in the vicinity of Porcupine Lake, in the Arctic National Wildlife Refuge. The northern edge of thrust-truncated folds in Lisburne is marked by a local range front that likely represents an eastward continuation of the central Brooks Range front. This is bounded to the north by a gently dipping panel of Lisburne with local asymmetrical folds. The leading edge of the flat panel is thrust over Permian to Cretaceous rocks in a synclinal depression. These younger rocks overlie symmetrically detachment-folded Lisburne, as is extensively exposed to the north. Six partial sections were measured in the Lisburne of the flat panel and local range front. The Lisburne here is about 700 m thick and is interpreted to consist primarily of the Wachsmuth and Alapah Limestones, with only a thin veneer of Wahoo Limestone. The Wachsmuth (200 m) is gradational between the underlying Missippian Kayak Shale and the overlying Mississippian Alapah, and increases in resistance upward. The Alapah consists of a lower resistant member (100 m) of alternating limestone and chert, a middle recessive member (100 m), and an upper resistant member (260 m) that is similar to Wahoo in the northeastern Brooks Range. The Wahoo is recessive and is thin (30 m) due either to non-deposition or erosion beneath the sub-Permian unconformity. The Lisburne of the area records two major episodes of transgression and shallowing-upward on a carbonate ramp. Thicknesses and facies vary along depositional strike. Asymmetrical folds, mostly truncated by thrust faults, were studied in and south of the local range front. Fold geometry was documented by surveys of four thrust-truncated folds and two folds not visibly cut by thrusts. A portion of the local range front was mapped to document changes in fold geometry along strike in three dimensions. The folds typically display a long, non-folded gently to moderately dipping backlimbs and steep to overturned forelimbs, commonly including parasitic anticline-syncline pairs. Thrusts commonly cut through the anticlinal forelimb or the forward synclinal hinge. These folds probably originated as detachment folds based on their mechanical stratigraphy and the transition to detachment folds to the north. Their geometry indicates that they were asymmetrical prior to thrust truncation. This asymmetry may have favored accommodation of increasing shortening by thrust breakthrough rather than continued folding. Fracture patterns were documented in the gently dipping panel of Lisburne and the asymmetrical folds within it. Four sets of steeply dipping extension fractures were identified, with strikes to the (1) N, (2) E, (3) N to NW, and (4) NE. The relative timing of these fracture sets is complex and unclear. En echelon sets of fractures are common, and display normal or strike-slip sense. Mesoscopic and penetrative structures are locally well developed, and indicate bed-parallel shear within the flat panel and strain within folds. Three sets of normal faults are well developed in the area, and are unusual for the Brooks Range. One set is parallel to and another is transverse to the strike of the folds. A single major normal fault has an intermediate orientation. The normal faults cut across folds, but may have been active late during folding because fold geometry differs across faults and some folding apparently continued after normal faulting.