Mass transport events are virtually ubiquitous on the modern continental slope, and are also frequent in the stratigraphic record. They are commonly very large (volumes >10 3 km 3 , areas >10 4 km 2 , thicknesses >10 2 m). They extensively remould sea-floor topography on the continental slope and rise. Turbidity currents are highly sensitive to topography, thus turbidite reservoir distribution and geometry can be significantly affected by subjacent mass transport deposits or their slide scars. Given the abundance of mass transport deposits, we should expect that many turbidite systems are so affected. In fact several well-known deepwater outcrops may represent examples of MTD-influenced sedimentation. Turbidites may be captured within slide scars and on the trailing edges of MTDs. They may also be ponded on and around mass transport deposits, in accommodation developed when the mass movement comes to rest, or subsequently due to compaction or creep. The filling of such accommodation depends on the properties of the turbidity currents, their depositional gradient, and how they interact with basin floor topography. The scale of supra-MTD accommodation is determined largely by dynamics of the initial mass flow and internal structure of the final deposit, and typically has a limited range of length scales. We discuss the implications for reservoir location, geometry and facies distribution, and subsurface identification.
The Tolmount Field is a lean gas condensate accumulation located in Block 42/28d of the UK Southern North Sea. The field was discovered in 2011 by well 42/28d-12, which encountered good-quality gas-bearing reservoir sandstones of the Permian Leman Sandstone Formation. The discovery was appraised in 2013 by wells 42/28d-13 and 42/28d-13Z, which logged the gas–water contact on the eastern flank of the field. The Tolmount structure is a four-way, dip-closed, faulted anticline, orientated NW to SE. The reservoir comprises mixed aeolian dune and fluvial sheetflood facies deposited within an arid continental basin. Dune sands display the best reservoir properties with porosities around 22% and permeabilities exceeding 100 mD. Only minor diagenetic alteration has occurred, primarily in the form of grain-coating illite. Superior reservoir quality is observed at Tolmount compared to adjacent areas, due to the preservation of dune facies, a hypothesized early gas emplacement and a relatively benign burial history. Current mapped gas initially-in-place estimates for the field are between 450 bcf and 800 bcf, with an estimated recovery factor between 70 and 90%. An initial four-well development is planned, with first gas expected in 2020.
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