Grain-Scale Heterogeneity in Deep Water Massive Sands – Implications for Depositional Processes and Reservoir Quality

Deep water massive sands are ubiquitous in the deep-marine record and form excellent reservoirs for hydrocarbons. At the bed-scale, the sedimentologically homogenous nature of these sands has been the root cause of the controversy concerning their emplacing mechanism(s). However, sedimentary deposits are intrinsically heterogeneous at a variety of scales. This heterogeneity can be investigated to shed light on the very mechanism(s) responsible for depositing massive sands, as well as assessing the reservoir quality within the beds. To this end, vertical grain-scale heterogeneity (grain size, sorting and fabric) within six massive beds from the Grés de Peïra Cava (SE France) and the Numidian Flysch (northern Tunisia) was investigated in back-scatter electron images using digital image analysis. Images parallel to the bedding plane and perpendicular to the apparent grain long-axis orientation were acquired to minimise the uncertainty in the grain size and fabric, and increase the statistical significance of the data. Hypothesis testing was employed to reduce the subjectivity in assigning vertical trends within each bed. Results show that the majority of the massive sands contain statistically significant vertical variation in grain size and fabric (p-values <0.01). A vertically diverging followed by converging grain size profile involving the coarse- (90th) and fine-tail (10th) components is seen three of the beds. The remaining beds reveal an anomalous ‘undulating’ or statistically ungraded trend across the various percentiles. Grain fabric in the bedding parallel sections show a consistently flow-oblique trend with an average azimuthal deviation of 47°, but becoming increasingly flow aligned at the top of most beds. However, there is little consistency in the mean vector rotation about the palaeocurrent direction. All bedding perpendicular sections show imbrication angles greater than 15°, with an average imbrication angle relative to the horizontal of 80°. Both up- and down-current imbrication polarities were observed in equal abundance. These grain-scale heterogeneities are interpreted as a product of differential impact of near-bed sedimentation processes that are controlled by sediment fallout rates from a concentrated, but turbulent sediment gravity flow. The resulting geometric character of the grains can be used as a tool to determine high porosity-permeability zones at the bed-scale, and ultimately as predictive inputs during the development phase.