Geological Parameters
in Winnipegosis Pinnacle Reef
Exploration, North Dakota and Saskatchewan
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