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Variations in bed- and bedset-scale permeability in bioturbated siliciclastic rocks result in heterogeneity that is difficult to model at a reservoir scale. This paper presents a technique to upscale permeability, such that the permeabilities of the bioturbated zones are explicitly included in the upscaled values. Study of 28 cored wells of the Lower Cretaceous Viking Formation in the Provost Field, Alberta, Canada integrated sedimentologic and ichnologic features to define recurring bed-/bedset-scale hydrofacies (HFs) characterized by a geometric mean permeability derived from plug samples. Permeability was upscaled by calculating equivalent vertical and horizontal permeabilities for composite hydrofacies (CHFs) using expressions for layered media. The equivalency of the vertical and horizontal permeability values assigned to the CHFs against those of the HFs was evaluated using a simple numerical flow model. Reservoir-scale flow was then simulated along a hydrogeological cross-section that was constructed using the CHFs. The resulting flow regime was consistent with flows simulated using published permeability estimates from tight reservoir units of the Viking Formation. This approach has the potential, therefore, for upscaling permeability for reservoir-scale modeling in tight oil and gas reservoirs, and more broadly in reservoirs where permeability is highly variable at the bed/bedset scale.

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 S HCS ) consist of alternations between intensely bioturbated beds and sparsely bioturbated/laminated beds. Trace fossil assemblages in bioturbated beds of Facies S HCS 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 S B ) 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 M B ) contain ichnofossils representing an archetypal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur throughout Facies M B that are similar to Facies S B ; 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.

Aquifer characteristics and water use data for 43 widely distributed small island states indicate that 44% are in a state of water stress. While recharge is projected to increase on 12 islands it is projected to decrease by up to 58% on the other 31. The majority of naturally occurring freshwater on small islands is groundwater, which is primarily recharged by precipitation1. Recharge rates are therefore likely to be impacted by climate change2. Freshwater resources on small islands are particularly vulnerable to climate change because they are limited in size and easily compromised3,4,5. Here we have compiled available aquifer system characteristics and water-use data for 43 small island developing states distributed worldwide, based on local expert knowledge, publications and regional data sets. Current vulnerability was assessed by evaluating the recharge volume per capita. For future vulnerability, climate change projections were used to estimate changes in aquifer recharge. We find that 44% of islands are in a state of water stress, and while recharge is projected to increase by as much as 117% on 12 islands situated in the western Pacific and Indian Ocean, recharge is projected to decrease by up to 58% on the remaining 31 islands. Of great concern is the lack of enacted groundwater protection legislation for many of the small island developing states identified as highly vulnerable to current and future conditions. Recharge indicators, shown alongside the state of legal groundwater protections, provide a global picture of groundwater supply vulnerability under current and future climate change conditions.

Bioturbation is generally perceived to be detrimental to bulk permeability by reducing primary grain sorting, homogenizing sediment, and introducing mud as burrow linings and feces. Recent studies show, however, that some ichnogenera and biogenic fabrics serves to increase porosity and permeability. In tight hydrocarbon reservoirs, subtle changes in sand and silt distributions, such as may be generated by bioturbation, can greatly affect the resulting porosity and permeability distribution. Despite this, permeability across unfractured sedimentary reservoirs is commonly assessed solely on the basis of average grain size. This study of the Lower Cretaceous Viking Fm integrates sedimentary and ichnologic features to define recurring “hydrofacies” that possess distinct permeability grades. Grain size, lithology, bioturbation index, and trace fossil suites were described from a cored section of well 14-30-22-16W4. The k max values from small plugs and full-diameter core samples were used to represent each hydrofacies. Hydrofacies were qualitatively defined at the bed/bedset scale, based on sedimentary, ichnological and permeability attributes, all of which affect flow pathways in heterolithic facies. The Markov chain method was employed to compare the vertical transitions of permeability (k max ) within a borehole against grain size and hydrofacies at the bed to bedset scale. This provided an intuitive framework for interpreting facies relationships such as coarsening-upwards successions. The results show that in the studied core, grain size only correlates to permeability in homogeneous rock units. The transiograms show that the volumetric proportions of different k max classes show a 15% correlation with grain size, compared to a 97% correlation with the hydrofacies, indicating that variations in permeability down the well are strongly related to variations in the hydrofacies. The hydrofacies approach potentially can be used as a conceptual framework for the spatial modeling of permeability in tight hydrocarbon reservoirs, where grain size may not be the primary factor on permeability distributions.