An ichnological-assemblage approach to reservoir heterogeneity assessment in bioturbated strata: Insights from the Lower Cretaceous Viking Formation, Alberta, Canada

Abstract

Facies-scale trends in porosity and permeability are commonly mapped for reservoir models and flow simulation; however, these trends are too broad to capture bed and bed-set heterogeneity, and there is a need to up-scale detailed, bed-scale observations, especially in low-permeability reservoir intervals. Here we utilize sedimentology and ichnology at the bed- and bedset-scale to constrain the range of porosity and permeability that can be expected within facies of the Lower Cretaceous Viking Formation of south-central, Alberta, Canada.Three main facies were recognized, representing deposition from the middle shoreface to the upper offshore. Amalgamated, hummocky cross-stratified sandstone facies (Facies SHCS) consist of alternations between intensely bioturbated beds and sparsely bioturbated/laminated beds. Trace fossil assemblages in bioturbated beds of Facies SHCS are attributable to the archetypal Skolithos Ichnofacies, and are morphologically characterized by vertical, sand-filled shafts (VSS). Bioturbated beds show poor reservoir properties (max: 10% porosity, mean: 85.1 mD) compared to laminated beds (max 20% porosity, mean: 186 mD). Bioturbated muddy sandstone facies (Facies SB) represent trace fossil assemblages primarily attributable to the proximal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur in varying proportions, namely sediment-churning assemblages (SC), horizontal sand-filled tube assemblages (HSF), VSS assemblages, and mud-filled, lined, or with spreiten (MLS) assemblages. Ichnological assemblages containing horizontal (max: 30% porosity, mean: 1.28 mD) or vertical sand-filled burrows (max: 10% porosity, mean: 2.2 mD) generally have better reservoir properties than laminated beds (max: 20% porosity, mean: 0.98 mD). Conversely, ichnological assemblages that consist of muddy trace fossils have lower porosity and permeability (max 10% porosity, mean: 0.89 mD). Highly bioturbated, sediment churned fabrics have only slightly higher porosity and permeability overall (max: 15% porosity, mean: 1.29 mD). Bioturbated sandy mudstone facies (Facies MB) contain ichnofossils representing an archetypal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur throughout Facies MB that are similar to Facies SB; SC, HSF, VSS, and MLS assemblages. The SC (max: 15% porosity, mean: 21.67 mD), HSF (max: 20% porosity, mean: 3.79 mD), and VSS (max: 25% porosity, mean: 7.35 mD) ichnological assemblages have similar or slightly lower values than the laminated beds (max: 20% porosity, mean: 10.7 mD). However, MLS assemblages have substantially lower reservoir quality (max: 10% porosity, mean: 0.66 mD).Our results indicate that the most likely occurrence of good reservoir characteristics in bioturbated strata exists in sand-filled ichnological assemblages. This is especially true within the muddy upper offshore to lower shoreface, where vertically-oriented trace fossils can interconnect otherwise hydraulically isolated laminated sandstone beds; this improves vertical fluid transmission. The results of this work largely corroborate previous findings about ichnological impacts on reservoir properties. Unlike previous studies, however, we demonstrate that the characteristics of the ichnological assemblage, such as burrow form and the nature of burrow fill, also play an important role in determining reservoir characteristics. It follows that not all bioturbated intervals (attributed to the same facies) should be treated equally. When upscaling bed-scale observations to the reservoir, a range of possible permeability-porosity values can be tested for model sensitivity and to help determine an appropriate representative elementary volume.