High heterogeneity, high potential: petrophysical proxy failure and CO 2 storage in paralic sediments

Abstract

Rapid expansion of the global geological carbon storage capacity required to meet net-zero emissions targets necessitates the continued evaluation of saline aquifers beyond hydrocarbon provinces. Arid-marine transition zones, and paralic systems in general, where continental sediments are overprinted by marine marginal processes, represent a potentially important and under characterised class of storage target. These systems combine high primary porosity with a wide variety of facies, and pore-scale heterogeneity, which can be unfairly simplified or penalised during early-stage Carbon Capture & Storage (CCS) screening exercises. This study investigates the extent to which commonly used petrophysical proxies capture this heterogeneity, using the Middle Jurassic Temple Cap Formation of central and southern Utah as a well-exposed analogue for arid-marine marginal deposition. Integrated outcrop-based gamma ray (GR) and magnetic susceptibility (MS) measurements are combined with sedimentological analyses and wireline petrophysical data to assess the relationships between facies and reservoir properties, both in outcrop and at reservoir depth. Results demonstrate a near-complete decoupling between GR and MS responses across facies and their associations, with GR magnitude, variance, and spectral composition reflecting multiple, non-unique mineralogical controls. Permeability showed no meaningful correlation with average grain size or porosity, instead scaling more closely with pore-throat radius, highlighting the dominant role of cementation in controlling injectivity. Facies with high pore volume but reduced permeability – traditionally regarded as poor hydrocarbon reservoirs – are shown to represent favourable CCS options owing to enhanced heterogeneity and trapping potential. The results of the study indicate that screening strategies based solely on single-proxy signatures risk systematically mis-characterising reservoir quality in paralic systems, underlining the importance of integrating multi-scale observations early on in the CCS site selection workflow.