Rock Fabric, Rock Composition, and Reservoir Quality in the Montney Formation, Western Canada

Abstract

In conventional reservoirs, reservoir quality is primarily determined by porosity and permeability. In unconventional plays, quality also depends on geomechanical properties, which dictate hydraulic stimulation success rates. This study investigates the influence of rock composition and fabric on the petrophysical and geomechanical properties of the Montney Formation siltstone. The Montney Formation of Western Canada holds estimated reserves of 450 TCF of gas, 14,520 MMB of gas liquids, and 1125 MMB of oil. The Montney was deposited as a westward dipping wedge of silt-size siliciclastics and carbonates. Depositional environments include deltaic facies, upper and lower shoreface facies, offshore successions and turbidites.

Porosity, permeability, and mechanical properties of the Montney were compared to rock composition and lithofacies in four cores on a down-dip transect into the basin. Data include detailed core descriptions, mineralogical composition from QEMSCAN, TOC, porosity (Hg injection, He-porosimetry on crushed samples, and macroporosity from QEMSCAN analysis), and pulse decay permeability on core plugs. Hardness profiles were obtained with a rebound velocity device, and dynamic elastic moduli and brittleness indices were calculated from sonic and density well-logs, and from mineralogical data.

Core analyses identified 16 lithofacies which varied little in mineralogical composition. Dominant minerals in the Montney include quartz, feldspars, micas, dolomite, Fe-dolomite and calcite, and clay minerals in various concentrations (1-30 wt.%). Maximum and average TOC values were 4.5 and 1.3 wt.%, respectively. He porosimetry values decrease downdip, averaging 16% and 7.5% for the shallowest and second shallowest cores, respectively, and 5.1% for both deep cores. Matrix permeability ranged from 300 nd to ~1000 nd for the shallow Montney cores and from <10 nd to ~80 nd for the deep cores. Young’s Modulus increases with burial depth and ranged from 20-82 GPa, while Poisson’s ratio shows no clear depth trend and ranged from 0.15-0.3.

Our study concludes that petrophysical and geomechanical properties are not related to lithofacies, probably because detrital material entering the basin had little compositional variation, and was not strongly influenced by depositional environment. Diagenetic minerals, clay minerals, and organic matter were identified as the controlling factors on reservoir quality in the Montney Formation.

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