Distinguishing Scales of Fracturing in Lisburne Field, Prudhoe Bay, Alaska

Abstract

This talk illustrates the value of combining geological and engineering data in the description of reservoir fracture systems. Examples are taken from a nearly ten-year study of Lisburne field, a middle Carboniferous, shallow marine, shelfal, carbonate structure. Fractures occur on many scales ranging from millimeter size, stratabound microfractures to tectonic faults with decameter offsets. Patterns are complex due to several stages of burial, uplift, erosion, and tilting. Most fractures are filled with calcite. Performance data shows that two fracture sets dominate reservoir behavior: (1) small-scale fractures developed along an unconformity (the subunconformity alteration zone: SAZ); and (2) large tectonic faults. SAZ fractures are open due to dissolution associated with late burial dolomitization. Thus, distribution of SAZ is related to a mappable diagenetic event. SAZ fractures provide rapid interwell communication along the unconformity and crossflow from underlying pay layers. On a much larger scale, pressure information and tracers reveal that major faults provide both vertical and lateral communications throughout many parts of the reservoir. Lisburne fracturing has both beneficial and adverse effects on reservoir behavior. Faults act as natural collectors of oil, reducing the number of development wells, depleting low permeability pay, and providing vertical pressure support. On the negative side,more » SAZ fracture systems create unwanted pressure gradients, high gas-to-oil ratios, and have led to deferral of waterflooding. Because heterogeneities like those in the Lisburne field can have drastic effects on a reservoir, early recognition is essential for developing planning.« less