Petrologic and Geochemical Attributes of Dolomite Recrystallization: an Example from the Mississippian Pekisko Formation, West-Central Alberta

JoAnn Adam and Ihsan S. Al-Aasm
University of Windsor, Windsor, ON

Carbonate rocks of the Pekisko Formation make up an important reservoir in west-central Alberta, especially in fields along the Pekisko subcrop edge. They represent a transgressive-regressive carbonate platform sequence comprised of upward shallowing facies, which subsequently underwent extreme erosion leading to the development of karst topography. As a result diagenetic alteration mainly through dolomitization has affected most of the carbonate facies.

There are four types of dolomite identified in the Pekisko Formation based on petrographic and geochemical analyses: (1) Pervasive, fine to coarse crystalline, subhedral to anhedral replacive dolomite, (2) Void-filling, coarse crystalline, euhedral dolomite cement, (3) Selective, fine to coarse crystalline, euhedral to anhedral dolomite and (4) Dissolution seam associated, fine crystalline, euhedral dolomite. Dolomite types (1), (3) and (4) are interpreted to have formed early in the diagenetic history and subsequently recrystallized whereas void-filling dolomite formed later. Petrographic evidence for possible recrystallization of dolomite types, excluding void-filling dolomite, includes: etching, displayed mainly on euhedral crystals; dedolomitization, dissolved cores on many crystals of varying shapes; non-planar crystal boundaries, exclusively in pervasive dolomites; and coarsening crystal size, evident in both pervasive and selective dolomite types.

Preliminary stable oxygen and carbon isotope results show a deviation from the original postulated Mississippian dolomite isotopic signatures with depleted oxygen values (means; δ¹³C=2.97‰, δ¹⁸O=-3.85‰). There is a definite negative trend whereby wells closest to the subcrop edge have the most depleted values and those farthest away show the least depletion. This trend in δ¹⁸O isotope values may point to recrystallization of the earlier dolomites.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005