ABSTRACT: Diagenetic History of Ordovician Carbonates from Hydrocarbon Reservoirs and Outcrops in Southern Ontario: Unanswered Questions

Mario Coniglio, Department of Earth Sciences, University of Waterloo, Waterloo, ON, Canada

Ordovician rocks exposed in the Manitoulin Island-Lake Simcoe area have received attention for well over a century yet fundamental questions concerning their post-depositional alteration remain unanswered. Significant petroleum discoveries in Ordovician strata in the subsurface of southwestern Ontario and the adjacent USA highlight the importance of understanding these rocks regionally, paying particular attention to factors which controlled their different diagenetic pathways. Ordovician reservoirs are closely associated with the late-stage dolomitization of limestones along fracture systems in the Michigan Basin. This tectonic control resulted in dolomitization patterns that are markedly heterogeneous on several scales and non-stratiform overall. Preferentially dolomitized beds in the subsurface and in the outcrops of the Manitoulin Island area on the northeastern margin of the Michigan Basin, however, clearly indicate the presence of subordinate, but unidentified stratigraphic controls. In outcrop, variations in depositional facies and abundant submarine-cemented hardgrounds both indicate that early on in the burial history of these rocks, permeability and porosity were markedly heterogeneous, and therefore possibly important in controlling later dolomitization. In contrast, fractures in the Lake Simcoe area, which is located east of the Michigan Basin and north of the Appalachian Basin, rarely have associated late-stage dolomitization. Economically-important dolomitization, therefore, appears to be spatially linked to unidentified basinal controls. Such controls, however, were evidently not important during subsequent diagenesis, as indicated by the similar suite of latest diagenetic minerals (e.g., calcite, ferroan saddle dolomite, barite, fluorite, celestite, various sulphides) in the Michigan Basin and correlative strata in the Lake Simcoe area.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada