Unravelling the Devonian Lower Clair Group of the Clair Oil Field, West of Shetland: an integrated reservoir quality and sedimentology study

The Clair Field is thought to contain the largest hydrocarbon accumulation on the UK continental shelf, with the principal reservoir comprising low matrix-permeability, naturally fractured, Devonian clastic red-beds of the Lower Clair Group (LCG). The pre-2020 development plan on Clair Ridge focused on achieving production rates via targeting natural fractures. However, recent challenges with open fracture prediction (from early-water-breakthrough to low productivity wells) has led to a renewed focus on understanding the matrix. This study summarises the predictability of matrix character across the LCG, specifically to (i) optimise well placement (matrix property trends), (ii) understand completion requirements (rock strength trends), and (iii) improve pre-drill deliverability predictions and well sequences. The study integrated a range of reservoir quality analysis with sedimentology, core observations, petrophysical log, and heavy minerals. Analysis unveiled a relationship between the resulting sedimentological characteristics (facies), diagenetic overprint, primarily calcite cement, and the contrasting styles of reservoir quality across the LCG units which denote fluctuating climatic conditions. Furthermore, constraining spatial trends in clay mineral volumes (source of calcite cement) within a resolved depositional model has produced a pre-drill predictive capability in cement volumes and facies (matrix character). The availability of a predictive model for matrix character has increased confidence in well performance prediction and impacted well sequencing, reservoir targeting and completionsstrategies. Together with a successful production-increasing drilling strategy, this has increased production on Clair Ridge from 40 mboed (thousand barrels of oil equivalent per day) to 80 mboed in 2021 alone.