Why is it important to understand deep-water reservoirs?
The inaccessibility of deep-water siliciclastic systems, combined with their enormous hydrocarbon potential, make these deposits unique. By definition, they form as a result of gravity flow processes in a deep-water setting. In a haze of fluid and suspended particles, an instability at the shelf edge, triggered by any number of causes (seismic loading, over steepening, rapid sediment accumulation, etc.) delivers vast amounts of siliciclastic material to the depths of marine systems and deep-water lakes.
Though often heterogeneous, inaccessible and difficult to characterise, it is the interlayering of oil-prone source rocks, high quality reservoir facies and impermeable caps that make these systems so attractive to the oil and gas industry.
It is well documented that the reservoir quality of deep-water sandstones can be among the best of all reservoir sedimentary rocks, and the scale of the sedimentary process involved can result in very extensive reservoirs. They are known for their high permeabilities, porosities and net-to-gross ratios. They often produce very profitable reservoirs, which need few wells to be produced, because given the right depositional conditions the sandstones will pond and stack.
Having a detailed picture of the architectural and facies properties of deep-water systems is vital. It is precisely that internal heterogeneity which is so crucial to understanding the hydrocarbon potential of a given deposit. It determines how a reservoir is plumbed, how oil and gas is distributed within it, and how it will flow when attempts are made to produce it.
But the remoteness of the environments in which they form makes this hugely challenging. Rarely are submarine gravity flow processes, such as turbidites, observed in real-time. Present day deposits accumulate mainly in water depths in excess of 500 m. Though it is possible to investigate aspects of depositional mechanics through experimental equivalents in the lab and numerical simulations, the processes can’t be fully replicated faithfully or to scale.
The uppermost part of the Gres d’Annot (Eocene-Oligocene) at Tete de l’Áuriac in SE France. This section shows a range of deep-water elements including amalgamated turbidite sheets, debris flow deposit and a channel-fill.
In the absence of the ‘real-deal’, geoscientists often turn to comparable sequences in the rock record – analogues – to better characterise the architecture of a given system. By comparing deep-marine outcrops exposed in the rock record, geoscientists are able to better define system architectures and, therefore make more informed decisions about the uncertainty space between wells. However, this approach is not flawless either. More and more tools are becoming available which allow visualisation of 2D outcrops in three dimensions, but rarely can the problem of scaling outcrop dimensions to sedimentary system dimension be overcome. Valuable too is the comparison of varying deep-water systems but varying data collection methods, along with use of different terminology and classification approaches means characterisation of the internal architecture is hindered.
Nevertheless, the huge potential, wide-geographical distribution and availability of better technology which allows us to tap into these resources means that deep-water siliciclastic reservoirs continue to be of enormous economic importance and will play an important role in the global energy mix.
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