Origin and distribution of calcite concretions in Cretaceous Wall Creek Member, Wyoming: Reservoir-quality implication for shallow-marine deltaic strata

Calcite concretions reduce reservoir quality, but there are limited studies that examine 3D distribution and consequences for reservoir quality in outcrop analogs. We integrate petrography, diagenesis, and geochemistry with 3D ground penetrating radar and borehole data to investigate the timing, 3D distribution and origin of concretion growth within a mixed fluvial, tide influenced shallow-marine deltaic reservoir analog in Cretaceous outcrops in Wyoming.

Calcite concretions, varying in size and shape from 70 cm to 5.5 m in length and from 20 cm to 60 cm in height, fill up to 15% of the sandstone volume. Concretions range from almond shape, long, thin ellipsoids, associated with tidal bar facies, to short, thick ellipsoids, within more fluvial-dominated distributary- channel facies. 3D mapping shows concretions are moderate to highly connected forming an aggregate pattern with irregularly shaped branches.

Several concretions have clear nucleation sites that include carbonaceous muds, calcareous muds, marine shell material, and/or organic matter. Carbon-isotope values suggest carbon sources that include in-situ marine skeletal-fragments and organic carbon. Rather than reflecting an early or late origin, these concretions are much more complex and show a long-lived history of growth. Cements are confined to the middle parts of the sandstone body, suggesting that initial preferential flow paths become sites for later cementation and reduction of porosity. This will potentially reduce overall reservoir volumes and may impede fluid flow in both horizontal and vertical directions. The 3D distribution of concretion must be taken into account in reservoir modeling and fluid flow simulations to avoid overestimation in recovery factors.