ABSTRACT: Thermochemical Sulfate Reduction and Hydrothermal Dolomitization (TSR-HTD): A Diagenetic Process That Created and Modified Middle Devonian Reservoirs in Northeastern British Columbia

TEARE, MARK R., and JAMES D. REIMER, Home Oil Company Limited, Calgary, Alberta, Canada

Thermochemical sulfate reduction and hydrothermal dolomitization (TSR-HTD) is a diagenetic process that created and modified many Middle Devonian gas reservoirs in northeastern British Columbia. These reservoirs exhibit a similar suite of rock fabrics that are characterized by intense microfracturing, leached pinpoint to locally cavernous porosity, brecciation, replacement and occluding saddle dolomitization, occasional sulfide or sulfate mineralization and pyrobitumen plugging. The widespread and cross-formational distribution of these fabrics suggests that this process was active on a regional scale.

In our model, TSR-HTD begins at the locus of existing oil pools in response to continued heating during burial. The reaction chemistry involves sulfate reduction, hydrocarbon sulfurization and dehydrogenation, plus dolomitization. The principal products are methane and carbon dioxide gas, saddle dolomite, pyrobitumen, and reduced sulfur species. Process mechanics may be described using a furnace analogy, which relates reaction intensity to relative position within the furnace. The most intense reaction occurs at the furnace core, where virtually all of the host rock is consumed and replaced. In contrast, we observe only incipient evidence of the reaction at the furnace extremities. This gradational effect accounts for the diverse family of TSR-HTD rock fabrics and products.

We conclude that TSR-HTD is an important hydrothermal process that simultaneously created and sourced these gas reservoirs. These concepts may also have worldwide application, for example in the Canning and Delaware basins, and along the Appalachian-Arkoma belt.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)