Depositional Environments, Stratigraphic Packages, and Petrography of the Basal Cambrian Sandstone

The Basal Cambrian Sandstone (BCS) has become the focus of increasing Carbon Capture and Sequestration (CCS) efforts in Western Canada. Shell’s Quest Facility has been operating since 2015 and is actively capturing and sequestering CO2 within the BCS near Fort Saskatchewan. In addition, prospective Cambrian intervals exist at Cold Lake, where the Oil Sands Pathways to Net Zero initiative has indicated that they have applied to the government for pore space (DOB, 2022). Despite the increasing activity within this interval, little work has been presented in the public domain on the sedimentological and stratigraphic characteristics within the BCS (Desjardins et al., 2013; Herbers et al., 2022). Multiple cores collected across Alberta at Fort Saskatchewan, Redwater, Leduc-Woodbend, and Lethbridge form a robust dataset from which a facies scheme can be used to erect a depositional model.

A variety of cores (1400 m–3600 m) were sampled for porosity, permeability, thin-section, and XRD to develop a comprehensive burial diagenesis framework for the Basal Cambrian Sandstone. XRD and thin section samples were tied to the plugs from which porosity and permeability were originally measured, to produce an internally consistent dataset that can relate mineralogy to the original core analysis data. Grain size histograms were produced through photomicrographs analyzed within JmicrovisionTM.100 grains per slide were measured to quantify sorting as a control on porosity and permeability. Detailed petrographic work allowed for the production of intergranular volume-cement plots that quantify the effects of compaction and cementation on porosity and permeability (cf. Houseknecht, 1987).

Feldspar content was found to vary greatly with certain facies containing up to ~10% potassium feldspar. The BCS becomes increasingly feldspathic up-section, which can be tied to depositional cycles within a transgressive system. Critically, potassium feldspar alteration to kaolinite during burial reduces the porosity and permeability. Dolomite cements with sweeping extinction are present, consistent with precipitation from circulating hot fluids rich in Mg2+. Additionally, evidence for secondary porosity is observed in abundant embayed quartz grains suggesting the circulation of highly alkaline fluids capable of leaching quartz. Petrographic and XRD results show that cements, mechanical and chemical compaction, and grain size sorting play a significant role in controlling the distribution of porosity and permeability.