ABSTRACT: The Dunvegan Field, NW Alberta, Canada: Dolomitization and Porosity Preservation in Mississippian Sabkha and Contiguous Subtidal Sediments

PACKARD, JEFFREY J., Esso Resources Canada Ltd., Calgary, Alberta, Canada

The Dunvegan field has over 1.2 tcf of sweet gas in place. The reservoir is found within the 150 m thick Upper Debolt Formation of mid-Visean age (Middle Mississippian), which in the west-central part of Alberta is composed of a succession of stacked, thin (generally less than 2 m) sabkha cycles. The reservoir is highly stratified with isolated pay zones restricted to portions of single sabkha cycles. Porosity is micro-intercrystalline and occurs predominantly in dolomitized subtidal muds that have their fine primary fabric preserved.

The dolomite forms isotropic planar-E fabrics with crystal sizes ranging from 1 to 20 microns, averaging 10 microns with associated reservoir quality of up to 38% porosity and 75 md permeability. The dolomite is nonferroan, Ca-rich (average 56 mole % CaCO3), with a lattice-ordering index of 0.31 (poorly ordered). Stable isotopes range from +0.5 to +2.2 Oxygen 18 PDB and +1.7 to +4.3 Carbon 13 PDB. These parameters are strongly reminiscent of Holocene protodolomites and are hence suggestive of a shallow seepage reflux dolomitization process. This hypothesis is further strengthened by radiogenic Sr determinations (87/86 = 0.7077) from the dolomites that coincide precisely with estimated Visean sea water values.

The outstanding issue with respect to these dolomites is not so much when and how they formed, but rather how the porosity has been preserved, and how the dolomite has remained relatively unaltered for an estimated 275 Ma. This despite the nonstoichiometry and poor ordering of the dolomite and the fact that the strata have been buried to depths in excess of 4000 m. The possible role of early biogenic methane as a diagenetic inhibitor and porosity preserving agent has been investigated.

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