Abstract

A large gas reservoir was discovered in the previously unproductive Jurassic-aged Abenaki carbonate margin in 1998. Most of the reservoir porosity is developed in dolostones. These dolostones replaced preexisting wackestones, packstones, and grainstones(?) associated with reefal and adjacent depositional environments. Many dolomites were subsequently recrystallized or dissolved, accounting for much of the preserved secondary porosity. Subsequent fracturing helped enhance reservoir permeabilities.

Enhanced petrographic techniques established that dissolution of previously dolomitized fabrics generated much of the secondary porosity in these dolostones. Diffused plane-polarized light revealed relict grains and textures invisible with standard microscopic observations. Petrographic and geochemical observations also confirmed that dissolution occurred under deep-burial conditions after incipient pressure solution.

Dissolution was not confined to the centers of dolomitized grains, as is commonly seen when remnant calcitic grains dissolve out during the advanced stages of replacement dolomitization.

Instead, dissolution was random within relict grains, as isolated dolomite crystals were also variably dissolved. The geochemistry of these dolomites and associated late-stage calcites implied precipitation from basinal hot fluids, as well as hydrothermal fluids. Later diagenetic fluids, either acidic or calcium rich, or perhaps both at different times (based on associated mineralization), seemingly promoted dolomite dissolution.

The presence of tectonic fractures and stylolites, helium gas, and faults observed in seismic data implied that dolomitization and subsequent dissolution along the Abenaki platform margin were controlled by reactivated wrench faults tied to basement. On a finer scale, diagenetic fluids moved through fractures and pressure-solution seams.

The data collected to date support our contention that the dolomitization and dissolution process, which has created most of the porosity in the Abenaki reservoir, was poststylotization and deeper burial in origin. Given the timing of tectonic activity in the area and its inferred connection to diagenesis, it is probable that at least a part of the diagenetic fluids were hydrothermal in nature.

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