Unconformity-Controlled Dolomitization, Exploration Plays: Examples from Western Canada and the Middle East

A. Hamid Majid

Dolomitization is one of the main factors controlling the distribution of hydrocarbon pools hosted by carbonate rocks in the Western Canada Sedimentary Basin (WCSB). Unconformity-controlled dolomitization is one of the major mechanisms of pervasive dolomitization recognized in the WCSB. This type of dolomitization occurs when precursor limestone is exposed to a continuous erosional episode during which meteoric waters percolate into the system and dolomitize the original limestone. In most cases, pervasive dolomitization occurs when any limestone system, regardless of facies type, is exposed to meteoric waters for extended periods. The thickness and composition of the geologic unit covering the original limestone prior to the unconformity episode, as well as the duration f the unconformity, help to determine the degree of dolomitization and the vertical and lateral extents of the dolomite reservoir. Examples of this unconformity-controlled dolomite plays in the WCSB are:

(a) Triassic Baldonnel Formation, northeastern B.C. Pervasive dolomite formed at the post-Triassic erosional surface.

(b) Mississippi an Debolt Formation, northeastern B.C. The Upper Debolt is pervasively dolomitized where the overlying Permian rocks were eroded.

(c) The majority of the Mississippi an fields of the Pekisko, Shunda, Debolt and Elkton formations of southern Alberta are post-Mississippian unconformity-controlled dolomite reservoirs.

Similar dolomitization patterns are observed in some Tertiary rock units of the supergiant oil fields in the Middle East. Identifying the unconformity-controlled dolomite type is a tool to provide a "road map" in predicting additional undiscovered pools in dolomite reservoirs within carbonate systems.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995