High-Resolution Analysis to Determine Controls on Carbon Sequestration and Mineralization in Ordovician Shale Gas Targets

The controls on shale gas reservoir quality are subtle and rely on rocks being organic carbon rich to generate gas, and cemented to be susceptible to hydrofracturing. In these fine-grained units carbon cycling between the spheres is significant, as it is responsible for defining the balance between organic carbon sequestration and mineralization. This study compares two Ordovician-aged carbon rich mudstone successions 1) the Utica Shale in southern Quebec and 2) the Winterhouse Formation on the Port Au Port Peninsula of Newfoundland, to determine the controls on carbon cycling in these potential unconventional reservoir units. Accurately evaluating fine-grained reservoirs requires investigating rock properties on a sub-centimeter scale; therefore, lithofacies variability was measured using a combination of petrographic (transmitted light, SEM) and geochemical methods (XRD, XRF, TOC and δ¹³C) in addition to field logging.

The Utica contains stacked successions of thin-bedded, normally graded calcareous mudstones and more argillaceous calcareous mudstones. These units contain clay to silt-sized fragments of broken calcite debris (>50 %) as well as smaller proportions of quartz (4.2 to 17.6 %), feldspars (0.1 to 7.5 %), clays (4.2 to 19.7 %), apatite (0 to 16.6 %), a partial carbonate cement (0 to 16.6 %) and minor amounts of chlorite, pyrite, and rutile. These mudstones exhibit significant textural variability (e.g. varying preservation of ripples, lamination onlap and erosional surfaces), and are variably bioturbated. Overall the Winterhouse fines upwards from calcareous sandstones to calcareous mudstones interbedded with clay mineral-rich calcareous mudstones. These units are composed of calcite (69.4 %) (primarily in the form of an extensive cement), quartz (14.1 %), feldspar (7.2 %), dolomite (6.2 %), and minor amounts of rutile, pyrite and chlorite.

In the Winterhouse pH and redox conditions at the time of deposition favored microbially mediated carbonate precipitation. Mineralization decreased organic carbon contents and porosity reducing the amount of potential gas generation. In contrast the Utica, which contains similar quantities of carbonate retained a greater amount of both porosity and organic carbon due to the fact that carbonate is dominantly in the form of fine-grained reworked skeletal debris. The lack of an extensive carbonate cement resulted in a greater proportion of the organic carbon fraction being preserved in the Utica Shale.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California