Controls on Hydrothermal Dolomites and Their Reservoir Properties
Stratigrapher and Exploration Geologist, Cobalt International Energy, Houston, Texas
Hydrothermal dolomites occur in Precambrian to Cenozoic strata with many models for hydrothermal dolomite emphasizing proximity to faults. Although some hydrothermal dolomites occur adjacent to significant faults, many do not. In this presentation, hydrothermal dolomite are described in three intervals and locations – Wabamun Group (upper Devonian) in western Canada, Swan Hills Formation (middle Devonian) in western Canada, and the upper Pennsylvanian at Reinecke Field in west Texas. In all three areas, petrographic and stable isotope data indicate dolomitization at high temperatures after moderate to deep burial.
Porous dolomites are surrounded by impermeable Wabamun limestones creating stratigraphic traps that are scattered across the southern Peace River Arch in western Alberta. Many hydrothermal dolomites in the Wabamun follow depositional facies and early dolomitization. Some oil fields are adjacent to mappable faults, but many are not. Many of the Wabamun fields were discovered by 3D seismic data targeting anomalies away from faults.
Hydrothermal dolomites in and around Rosevear Field in western Alberta occur in grainstones and grain-rich stromatoporoid boundstones. Adjacent micrite-rich facies are generally not dolomitized creating the stratigraphic trap at Rosevear Field. Hydrothermal brines apparently moved up into platform margin grainstones and then moved long distances along the permeable platform margin and connected embayments.
At Reinecke Field in west Texas, hydrothermal dolomites occur in an upper Pennsylvanian limestone buildup. The hydrothermal dolomites created high-permeability horizontal and vertical “raceways” within the largely limestone reservoir. Those “raceways” fundamentally affected oil production during primary, secondary and CO2 recovery at Reinecke Field.
Hydrothermal dolomites are important hydrocarbon reservoirs in many parts of the world. They have excellent reservoir characteristic because of their large crystal sizes, vugs, and fractures. Many factors other than faults can control their distribution including depositional facies, early dolomite, highly saline brines in the basin, and convective flow. Careful petrography, collecting stable isotope data, and a good understanding of the basin history can help predict these types of reservoirs in the subsurface.
AAPG Search and Discovery Article #90159©2012 AAPG Foundation Distinguished Lecturer Series 2012-2013.