Flow heterogeneities in the UK Sherwood Sandstone Group : resolving the role of tectonic vs. sedimentary structures

• Main Author: Medici, Giacomo
• Other Authors: West, Jared ; Mountney, Nigel
• Published: University of Leeds 2017
• Subjects: 551.3
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.721815

Fluvial and aeolian sedimentary successions represent porous media that can host both groundwater and hydrocarbon resources. Study of their heterogeneities provides a better understanding of both contaminant dispersal in aquifers and techniques for enhancing recovery in oil reservoirs. This research investigates the hydraulic properties of a fluvial succession deposited in a continental rift setting: the Triassic St Bees Sandstone Formation, which represents the basal part of the UK Sherwood Sandstone Group in the eastern Irish Sea Basin. These fluvial deposits have also been compared to the anologous fluvial deposits of the more slowly subsiding shelf-edge basin of the eastern England Shelf aiming to constrain the effect of subsidence rates on preservation of low permeability heterogeneities. The Triassic deposits of the St Bees Sandstone Formation were investigated from the plug up to the field-scale combining a range of sedimentological, structural, petrophyisical and hydro-geophysical tecniques. The aim of this research is to characterize the impact of sedimentary and tectonic heterogeneities on flow in the continental deposits of the Sherwood Sandstone aquifer assessing the validity of the findings up to reservoir depths. The hydraulic properties of the St Bees Sandstone Formation are compared with those of other formations of both aeolian and fluvial origin within the Sherwood Sandstone Group, and similar siliclastic formations worldwide to achieve a more general understanding of flow behaviour in siliciclastic sedimentary successions. In the relatively shallow (< ~100-200 m BGL) saturated zone of the St Bees Sandstone aquifer, acidic meteoric waters have enlarged fractures to create karst-like features resulting in very high field-scale hydraulic conductivity (K~10-1-100 m/day). Here, contaminant dispersal likely occurs at a relatively high rate. A deeper investigation (> 150m depth) demonstrates that the aquifer has not been subjected to rapid groundwater circulation at these depths; hydraulic conductivity is substantially lower, decreasing from K~10-3 m/day at 150-400 m BGL, to 10-4 m/day down-dip at ~1 km BGL. Pore-scale permeability becomes progressively more dominant with increasing depth. Thus, this sandstone aquifer at ~ 1 km depth approximates the hydraulic properties of analogous hydrocarbon reservoirs which are dominated by intergranular flow. The succession contains a variety of fine-grained and relatively low-permeability units including mudstone beds, interbedded with highly permeable channel deposits. Where present, a higher frequency of occurrence and greater lateral extent of mudstone units impede flow, reducing the field-scale permeability. Zones characterized by higher preservation of mudstone layers also show higher field-scale permeability anisotropy (Kh/Kv) due to a significant reduction of flow perpendicular to these fine-grained heterogeneities. In contrast, normal faults represent preferential flow pathways up to ~1 km depth, due to presence of highly connective open fractures. Continental successions in rift settings are also characterized by aeolian deposits in the NW Triassic realm, which typically possess higher matrix permeability due to a relatively paucity of integranular clay with respect to fluvial deposits. Thus, reservoir quality rises with increasing content of preserved aeolian sediments due to the dominance of intergranular flow in silicilastic successions buried at depths > ~1 km. Deposits of aeolian-dune versus fluvial origin also exhibit contrasting hydraulic behaviour where intersected by fault zones: normal faults deform aeolian deposits and are dominated by granulation seams which would partially impede flow to production wells in analogous hydrocarbon reservoirs.