Origins of Quartz in Organic-Rich Shales of Upper Devonian Duvernay Formation, Central Alberta, Canada: Implications for Geomechanical Behavior

Abstract

Quartz cementation in shale or mudstone hydrocarbon reservoirs has received much less attention than in sandstone reservoirs, largely because this finely crystalline cement is difficult to image within a dark-colored, organic-rich mudstone matrix. Recent studies have suggested, however, that it can have significant influence on geomechanical properties. In this study, we investigate the organic-rich shales of the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, a major shale oil and gas target that spans a wide range of thermal maturity and has considerably variable rock compositions. Bulk and clay mineralogy analyses by X-ray diffraction, thin section examination, and scanning electron microscopy combined with cathodoluminescence analysis were integrated to identify quartz types and their origin. Leco combustion, Rock-Eval pyrolysis and ICP-MS techniques were used to characterize the organic and inorganic geochemical composition. Geomechanical parameters such as Young's Modulus, Possion's Ratio, brittleness calculated from wire-line log data, and hardness measured on core samples were used to document the stratigraphic variations of geomechanical properties and the effect of quartz cementation on geomechanical behavior.

Quartz in the Duvernay Formation exists as detrital silt, authigenic overgrowths on the detrital silt grains, pore-filling cement in the intragranular pores (e.g. radiolarian and styliolinid chambers) and microcrystalline quartz distributed throughout the clay-size matrix. Radiolaria and sponge spicules are locally abundant but largely replaced by calcite and pyrite. The positive correlations between calculated biogenic SiO₂ and authigenic quartz content from point-counted SEM-CL images indicates that dissolution and re-precipitation of biogenic silica from radiolaria and sponge spicules was the principal source for quartz cementation in the Duvernay Formation.

Hardness and Al₂O₃ concentrations are strongly negatively correlated, suggesting that clay minerals are the most significant factor controlling geomechanical properties. Biogenic silica is positively correlated to hardness. Detrital silica is negatively correlated to hardness, an artifact of the positive correlation between detrital clay minerals and detrital quartz. We conclude that quartz cements sourced from biogenic silica rather than smectite-to-illite transition strongly contributed to the rock strength in the Duvernay Formation

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018