Michael J. Steventon, Christopher Aiden-Lee Jackson , Howard D. Johnson, David Hodgson, Sean Kelly, Jenny Omma, Christine Gopon, Christopher Stevenson, Peter Fitch

The geometry, distribution, and rock properties (i.e. porosity and permeability) of turbidite reservoirs, and the processes associated with turbidity current deposition, are relatively well known. However, less attention has been given to the equivalent properties resulting from laminar sediment gravity-flow deposition, with most research limited to cogenetic turbidite-debrites (i.e. transitional flow deposits) or subsurface studies that focus predominantly on seismic-scale mass-transport deposits (MTDs). Thus, we have a limited understanding of sub-seismic MTDs ability to act as hydraulic seals and their effect on hydrocarbon production, and/or carbon storage and sequestration. We investigate the gap between seismically resolvable and sub-seismic MTDs and transitional flow deposits on long-term reservoir performance in this analysis of a small (<10 km radius submarine fan system), Late Jurassic, sandstone-rich stacked turbidite reservoir (Magnus Field, northern North Sea), which is supported by a relatively long (c. 37 years) and well-documented production history. We use core, petrophysical logs, pore fluid pressure, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), and 3D seismic-reflection datasets to quantify the type and distribution of sedimentary facies and rock properties. A range of sediment gravity deposits are recognised: (i) thick-/thin- bedded, structureless and structured turbidite sandstone, constituting the primary productive reservoir facies (c. porosity = 22%, permeability = 500 mD), (ii) a range of transitional flow deposits, and (iii) heterogeneous mud-rich sandstone interpreted as debrites (c. porosity = <10%, volume of clay = 35%, up to 18 m thick). Results from this study show that over the production timescale of the Magnus Field, debrites act as barriers, compartmentalising the reservoir into two parts (upper and lower reservoir), and transitional flow deposits act as baffles, impacting sweep efficiency during production. Prediction of the rock properties of laminar and transitional flow deposits, and their effect on reservoir distribution, has important implications for: (i) exploration play concepts, particularly in predicting the seal potential of MTDs, (ii) pore pressure prediction within turbidite reservoirs, and (iii) the impact of transitional flow deposits on reservoir quality and sweep efficiency.