Abstract

Previous work on fractures has shown that a relationship exists between their development and concentration and the local and regional structure upon which they occur. These areas of concentration can be illustrated through preparation of an isofracture map which shows maximum concentration to be in areas of highest rate of dip and/or strike change.

Field and subsurface work shows that tectonic dolomites have the same fabric relationship to local and regional structure and are related to fracture concentration areas on structure. These tectonic (secondary) dolomites constitute important oil reservoirs in the subsurface and include the Deep River field (26 million barrels) and the Scipio-Albion field (100 million barrels) of Michigan. Often, the crest of a structure may be barren (tight) while a major tectonic dolomite reservoir may be present on the shoulder or flanks of the structure.

Petrographic criteria are available to distinguish tectonic (secondary) dolomites from depositional dolomites. Once porosity due to the former tectonic cause is differentiated from the depositional fabric, considerable progress can be made in constructing the depositional fabric of an area in the search for stratigraphic traps. Depositional dolomites appear to be related to the crystal instability of aragonite and may in part be of facies significance (quiet water lagoonal facies). The increment concept of primary carbonate deposition illustrates lateral as well as vertical facies relationships. Oil traps related to these primary depositional changes include the Putnam fifeld of Oklahoma, the Hermosa (Aneth etc.) production of the Paradox basin in Utah, the Mississippian production in North Dakota (Rival pool). Canada (Midale pool) and the Cottonwood (Permian) field of Wyoming.

Many other oil reservoirs related to both tectonic and depositional dolomitization and carbonate facies can be cited. Recognition of these two entirely different origins and occurrences of dolomite and related porosity is necessary for exploration purposes.