ABSTRACT: Dolomite Neomorphism and Porosity Evolution, Wahoo Fm., Lisburne Field, Prudhoe Bay, Alaska

JAMESON, JEREMY, Exxon Exploration Company, Houston, TX

Differences in the extent of neomorphic recrystallization are largely responsible for the observed variation in reservoir properties of Lisburne dolomites. Depositional texture is less important. This paper summarizes results from a nearly ten year evaluation of the Pennsylvanian age Lisburne field. It illustrates the importance of understanding the origins of dolomite fabrics as part of reservoir description.

Lisburne dolomites show a range in texture and trace element composition, reflecting a long period of dolomitization in changing diagenetic environments. Early dolomites formed in a mixing zone formed beneath a regional unconformity. Iron gradients within overgrowths and cross-cutting relationships reveal neomorphism occurred later, during shallow burial. Externally derived fluids moved downward from the unconformity surface and laterally as roll fronts. Neomorphism accompanied fluid movement.

Early dolomites are cloudy, consisting of tightly interlocking, fine (<20 micron), nonferroan, sub-to-anhedral crystals. Early dolomites have poor porosity and permeability. Best porosities and permeabilities are found in coarser (30-100 micron) mosaics of clear, sub-to-euhedral crystals. These coarser fabrics have arisen by a combination neomorphic processes, including overgrowth enlargement and replacement of early, fine dolomite. Overgrowths occur as syntaxial replacements and cements. Neomorphic recrystallization has occurred in irregular streaks, resulting in a more heterogeneous reservoir lithology. Continued enlargement of dolomite crystals in some parts of the field has given rise to an interlocking fabric of relatively large crystals (>200 microns) with little porosity r permeability.

Unraveling the fabric changes associated with neomorphism and fluid migration forms the basis for the dolomite reservoir model.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)