Reservoir heterogeneity in the Triassic Sherwood Sandstone Group: new insights from the Larne Basin, Northern Ireland

The Sherwood Sandstone Group (Lower Triassic) is being evaluated for geoenergy applications across northwest Europe. This study investigates the interplay between depositional environment, heterogeneity, provenance and its impact on reservoir quality, utilising the only continuously-cored Sherwood Sandstone Group borehole (Kilroot GT-01) on the island of Ireland. Core analyses, wireline logs, QFL and grain size results were integrated with local outcrops in the Larne region of Northern Ireland to develop a lithofacies scheme and establish textural controls on porosity-permeability. Results indicate that the best reservoir quality occurs in fine-to-medium grained, high net-to-gross (>70%) sandstones within stacked fluvial systems. The Lower Triassic can be divided into two broad coarsening upward packages, possibly related to cycles of increased fluvial discharge. Comparing results to previous provenance studies from Kilroot GT-01, the best primary reservoir quality appears to be linked to the extension of the Budleighensis river system which flowed northwards from the Variscan highlands. The finer grained sediment, with reduced reservoir quality, appears to be sourced more locally from Laurentian basement. Constraining the spatial extent of the Lower Triassic river systems sourced from Variscan terrains may, therefore, be of critical importance in understanding the distribution of the best quality reservoir sands for geoenergy applications.