Submarine Fan Systems: Proximal to Distal Reservoir Quality Controls

Abstract

Submarine fans and related turbidite systems are important components of continental margins; they contain a stratigraphic record of environmental changes, host large accumulations of oil and gas, and offer potential sites for carbon capture and storage (CCS). The influence of grain size and sediment flux to submarine fans has been recognised as a primary control on the heterogeneity of deep-water facies. Predicting fan depositional facies, changes in clay matrix content from proximal to distal settings, and evaluating the role played by clay coatings on detrital grains are important challenges in the characterizations of submarine fan systems. In this study, a multi-technique approach is applied, including detailed petrography, burial history modelling, SEM, SEM-CL, SEM-EDS mapping, and hydrothermal reactor experiments. Extensive subsurface datasets (well logs, cores, and thin sections) from 13 wells and 191 samples are used from the Paleocene-Eocene Forties Sandstone Member of the Sele Formation, Central North Sea, UK. These data are used to assess the role played by clays (detrital and authigenic), depositional facies, and burial diagenesis in reservoir quality evolution in proximal to distal fan depositional settings. The study reveals that amalgamated sandstones facies of proximal fan of the Forties Sandstone Member have the best reservoir quality due to coarser grain size, less detrital clay matrix, and low ductile grains content (< 5%), whereas the distal fan, mud-prone heterolithic sandstones have the poorest reservoir quality due to finer grain size, high clay matrix, and high ductile grains content (> 5 %). The optimum pore-filling clays volume, which have a deleterious effect on reservoir quality, range from 10-30 %. Furthermore, detrital smectite, which was both inherited and emplaced during sediment dewatering, was the main source of grain-coating chlorite, illite-smectite, and illite. The optimum clay-coating coverage to inhibit quartz cementation ranges from 40 to 50 % (based on the present-day burial depths of 2200 - 3200 m TVDSS) and clay volume of up to 8 %. Average quartz cement volume decreases from 3.62 % in proximal fan to 1.78 % in distal fan settings, presumably due to impact of grain-coating and pore-filling clays. Grain size has no impact on clay-coating coverage, which suggests that the finer-grained distal fan sandstones could have good clay-coating coverage to arrest quartz cementation during burial and chemical compaction depths > 2500 m. Blocky kaolinite and fibrous illite as low as 2 % have been found to have detrimental effect on the sandstones reservoir quality, whereas pore-filling chlorite has a threshold value ranging from 3 to 10 %. A complimentary experimental study using an autoclave-engineers hydrothermal reactor has been undertaken using sand-rich turbidite channel facies samples from the Bute Inlet, British Columbia, Canada. The results demonstrate that the presence of detrital clay is crucial for the formation of authigenic clays. Channelised sand facies with < 1 % detrital clay have poorly-developed coatings coverage (max 47 %) post-experiment. In comparison, lobe facies with 6 % initial detrital clay formed well-developed coatings coverage (max 77 %), with clay volume ranging from 19 to 27 % post-experiment. These results can be used as input parameters for the assessment of reservoir potentials in carbon sequestration and storage, and in hydrocarbon exploration and production.