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Dolomite Reservoir Rocks: Processes, Controls, Porosity Development

By
Graham R. Davies
Graham R. Davies
Applied Geoscience And Technology (AGAT) Consultants Ltd. 3650 Twenty-First Street N.E. Calgary, Alberta, Canada T2E 6V6
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Published:
January 01, 1979

Abstract

Dolomitization of pre-existing limestones commonly enhances permeability and improves porosity. Dolomitization may have a “homogenizing” effect on a carbonate reservoir, so that vertical and lateral permeability within the reservoir is improved. Because of its low ductility relative to limestone (and sandstone), the reservoir characteristics of dolomite often are enhanced by fracturing. Secondary dolomitization may create porosity and permeability by post-burial diagenesis closely preceding or perhaps coincident with onset of hydrocarbon migration. For all of these reasons, dolomite rocks far outweigh limestones as the major carbonate reservoir rocks in North America.

Dolomite may be formed by penecontemporeous replacement of unconsolidated carbonate sediment (“primary” dolomites, often characterized by peritidal rocks), but more frequently by replacement of pre-existing limestones (“secondary” dolomite). Dolomitization is controlled by permeability, composition and particle size of the host, and by physico-chemical parameters including temperature, pressure?, and ionic concentration and composition of the pore fluid. Models for dolomitization include evaporative reflux, capillary concentration and evaporative pumping, fresh water-brine mixing, connate water expelled by shale compaction, cannibilization of Mg-calcite sediments, and others. Major dolomite units may be the product of variations on the mixed-water and connate water models. However, most dolomitization models ignore or overlook the potential impact of temperature (particularly post-burial geothermal) and the role of organic reactions (on alkalinity, etc.) that may favor dolomitization by post-burial diagenesis.

Patterns of dolomitization are controlled by microscale and macroscale factors, in turn controlled by permeability. Replacement dolomite commonly shows strong fabric selectivity. Typically, fine matrix carbonate is replaced first, followed either by later clast replacement or moldic/vuggy solution porosity enhancement. Conversely, early cementation such as pervasive submarine cementation or early spar cementation of a grainstone, may destroy permeability in a limestone and prevent or inhibit later dolomitization. Fracture development under differential loading or tectonic stress also characterizes low-ductility dolomite.

On a larger scale, patterns of dolomitization are controlled by:

  1. Paleogeographic setting of originally permeable limestone units relative to a source of dolomitizing pore fluids (eg. porous shelf edge reef between hypersaline lagoon and shale basin; tidal flat deposits).

  2. Exposure at unconformities.

  3. Paleogeographic distribution relative to tectonic highs or regions of subareal exposure (sites for fresh water recharge).

  4. Timing of eustatic or tectonic fluctuations controlling movement and evolution of ground water and connate water.

  5. Relationship to paleo-groundwater conduits, or water tables.

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Contents

AAPG Continuing Education Course Notes Series

Geology of Carbonate Porosity

Don Bebout
Don Bebout
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Graham Davies
Graham Davies
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Clyde H. Moore
Clyde H. Moore
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Peter S. Scholle
Peter S. Scholle
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Norman C. Wardlaw
Norman C. Wardlaw
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American Association of Petroleum Geologists
Volume
11
ISBN electronic:
9781629811888
Publication date:
January 01, 1979

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