Don L. Kissling, James R. Ehrets
Winnipegosis pinnacle reefs range between 150 and 300 ft in height and up to 3 mi across, and their density distribution is several per township. They had accumulated as codiacean algae and peloid calcarenite mounds, capped by massive stromatolite boundstone, and fringed by stromatoporoid coral assemblages and detrital flanks during accelerating basin subsidence that had accompanied Winnipegosis platform-basin decoupling. Pinnacle reefs were terminated by increasing hypersalinity and toxicity during late Winnipegosis evaporative draw-down of the shelf sea. Postdepositional processes most responsible for preservation, enhancement, or loss of porosity and permeability in specific reef facies include prolonged vadose-zone leaching, early cementation or compaction, shallow-bu ial dolomitization, seepage reflux dolomitization and cementation, overdolomitization, and deep-burial brine invasion.
Pinnacle reef exploration and exploitation must consider a hierarchy of riddles. These are (1) to locate relatively small structures within the vast basin regime, (2) to distinguish porous, oil-charged reefs from non-porous, wet, or salt-plugged reefs, and (3) to locate favorable reservoir bodies within heterogeneous reef structures. The initial task of finding reefs integrates geological and geophysical data, but remains a "shotgun" approach without a reef distribution model that comprehends epeirogenic, paleotectonic, and paleohydrographic factors. Selecting the "right" reef might be attempted through understanding Winnipegosis-Prairie diagenesis, or by formulating techniques for recognizing remote signposts of overdolomitization, salt plugging, and oil emplacement. Because diagenet c history and resultant reservoir characteristics are strikingly different among pinnacle reef facies, predicting the distribution of porous and permeable reservoir within reefs might be possible if spatial depositional facies and diagenetic models were developed.
AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988