--> Prediction of Reservoir-Scale Sand Body Geometry From Mud Cap Thickness in Ponded Basins

AAPG ACE 2018

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Prediction of Reservoir-Scale Sand Body Geometry From Mud Cap Thickness in Ponded Basins

Abstract

Deepwater ponded basins contain a range of sand-prone stratigraphic architectures from simple turbidite sheets to more complex channelized frontal splay and lobe bodies. Key to understanding sedimentary architecture is the degree of flow confinement: flows that reach the basin margins should deposit more tabular sand-mud beds than less efficient smaller flows. Previous work has established bed correlations in the Peira Cava outlier of the Tertiary Annot system of SE France over distances of 10 km along depositional dip. Here we contrast several thick sandstone beds with markedly different sand distributions, inferred to represent more confined to less confined flow deposits. Bed 500 is the thickest continuous sand unit (9-14 m thick) across the outlier; stratigraphic marker MU5 in previous studies. It displays a relatively simple tabular bed geometry, albeit with local variations related to substrate erosion. The bed comprises mud-clast-rich basal sand succeeded by an extended normally-graded sand interval and a notably thick (1.5-4 m) mudcap. This unit is interpreted as a ponded ‘megabed’ deposited by a single flow event that covered the entire basin floor. This and a number of similar thick beds are interpreted to be deposited by voluminous, efficient flows that were highly confined, forming tabular sheets at the basin scale. The stratigraphically higher Bed 600 (MU6) occurs at the top of an amalgamated coarsening-and-thickening upward sequence. Whilst this unit has a similar sand thickness to Bed 500 in proximal and medial sections, the sand component thins by ~70% distally. Conversely its mudcap is relatively thin in proximal sections and thickens distally. This and other similar thickening upward units, are inferred to be prograding lobes, constructed by smaller, less efficient and more weakly confined flows. Thus ponded, tabular sheet sands deposited by exceptionally large flows have thick mudcaps that extend up-depositional dip into proximal sections, whereas downdip-thinning lobate sand packages have thin proximal mudcaps that thicken downdip, compensating for the underlying depositional topography, or bypassing to distal parts of the basin. Tabular sheets therefore tend to have significantly thicker mudcaps and lower bed NTG in proximal sections compared with units that thin downdip. Relative mudcap thickness is therefore a potentially useful predictor in analogous subsurface systems of downdip reservoir presence.