Breaching-related turbidites in fluvial and estuarine channels: Examples from outcrop and core and implications to reservoir models

An understanding of the paleoenvironment and the main sedimentary processes behind preserved deposits is crucial to correctly interpret and represent lithofacies and facies associations in geomodels that are used in the hydrocarbon industry, particularly when a limited dataset of cores is available. In this paper a fairly common facies association is discussed containing massive sands - here defined as thick (>0.5 m) structureless sand beds devoid of primary sedimentary structures, or with some faint lamination - deposited by mass failures of channel banks in deep fluvial and estuarine channels. Amongst geologists it is generally accepted that liquefaction is the main trigger of large bank failures in sandy subaqueous slopes. However, evidence is mounting that for sand deposits a slow, retrogressive failure mechanism of a steep subaqueous slope, known as breaching, is the dominant process. A model of breaching-induced turbidity current erosion and sedimentation is presented that explains the presence of sheet-like massive sands and channel-like massive sands and the sedimentary structures of the related deposits. Sheet-like packages of spaced planar lamination that are found together with massive sand bodies in deposits of these environments are identified as proximal depositional elements of breach failure events. The model, acquired from sedimentary structures in deposits in the Eocene estuarine Vlierzele Sands, Belgium, is applied to outcrops of the Dinantian fluvial Fell Sandstone, England, and cores of the Tilje and Nansen fms (Lower Jurassic, Norwegian Continental Shelf). The possible breach failure origin of some other massive sands described in literature from various ancient shallow water environments is discussed. Breach failure generated massive sands possibly also form in deep marine settings. The potentially thick and homogeneous, well-sorted sand deposits bear good properties for hydrocarbon flow when found in such an environment. However, in case of deposition in an estuarine or fluvial channel, these sand bodies are spatially constricted and careful facies interpretation is key to identifying this. When constructing a static reservoir model, this needs to be considered both for in-place volume calculations as well as drainage strategies.