Controls on Reservoir Quality in the Lower Triassic Montney Formation

  • Author / Creator
    Vaisblat, Noga
  • The Lower Triassic Montney Formation is a world class hydrocarbons reservoir in Alberta and British Columbia. Although commonly described as a shale, it is a siltstone over most of its subcrop, for which predictive models for reservoir quality properties such as porosity and permeability, geomechanical properties, and water saturation are generally lacking. Reservoir properties are generally functions of rock fabric, rock composition and diagenetic processes, which in turn depend on sediment provenance, depositional environment, burial history of the rock, and paleo fluid-flow. This study is aimed to identify the compositional controls on reservoir quality properties in the Montney siltstone and their relationship to the lithostratigraphy. In addition, we compare controls on reservoir quality of the siltstone to those found for shale formation.
    Variations of Young’s Modulus, Poisson’s Ratio, and relative brittleness from well logs were compared against indentation measurements of the four long cores and against rock composition in all wells. Young’s modulus, brittleness, and hardness showed similar trends in each well, whereas Poisson’s ratio demonstrated a trend with depth opposite to all other elastic parameters. Statistical analysis indicated that clay content, and to a lesser extent organic matter content, had the strongest control on elastic moduli in the Montney Formation, reducing Young’s modulus, brittleness, and hardness, but increasing Poisson’s ratio.
    Clay content is the most significant parameter affecting the mechanical properties of the Montney Formation, an observation consistent with several other fine-grained reservoirs. An increase in clay content leads to an increase in dynamic Poisson’s Ratio and a decrease in dynamic Young’s Modulus values in all the formations examined in this study. In addition, water saturation and the number of bedding planes are positively correlated with a decrease in rock brittleness in the Montney Formation.
    Quartz content exerts a positive control on both porosity and permeability, related to strengthening of the rock framework and reduced porosity loss due to compaction. Elevated clay content is also associated with higher porosity and permeability, possibly in part because of an association with quartz, but also because clays may shelter interconnected primary porosity that promotes permeability. Organic matter content is negatively correlated to porosity and permeability despite the presence of organic matter porosity, because relict oil (now pyrobitumen) plugs primary pore space and because other pore types contribute more to the total pore volume of the rock.

    Sedimentological analysis identified 16 lithofacies across the basin, with depositional environments ranging from tidal flat to offshore sediments. No clear distinctions were found between petrophysical or geomechanical properties of different lithofacies in each well. More importantly, the same lithofacies commonly displays significantly different geomechanical or petrophysical properties in each of the wells. We attribute the weak influence of depositional environments on the sediment to the size and compositional homogeneity of detrital material that entered the basin, and to massive cementation at shallow burial depth that further homogenized the rock and eliminated any compositional differences between lithofacies.
    Water saturation is calculated for eight wells in the Pouce Coupe field of the Montney Formation. We use the software GAMLS© to perform a probabilistic multi-variate cluster analysis of well-log data that identifies rock types endmembers (electroclasses) and assigns matrix and fluids properties to them. A matrix and fluids mass-balance is performed, accounting for the assigned fluids properties and rock composition. Our model is computed and calibrated against well-logs and core data. Results show that modeled mineralogy and computed total water saturation fit core measurements in the model well and the validation well and are consistent with previously published data for the Montney Formation. The probabilistic model allows for better estimations of hydrocarbons reserves and permits more credible decision-making when targeting low Sw reservoir zones.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
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