Deep Water Massive Sands : grain to depositional element scale analysis of their internal character

Abstract

Deep Water Massive Sands (DWMS) are ubiquitous in the modern and ancient sedimentary rock record, where they form important hydrocarbon reservoirs. Despite their economic importance and decades of research concerning their origin and internal character, they remain relatively enigmatic. This study, therefore, aims to shed light on their internal character and the processes responsible for their formation, and the implications this may have on hydrocarbon exploration and production. The study utilises outcrop (Grès de Peïra Cava and Numidian Flysch) and subsurface (East Brae Field) data to investigate the sedimentary character of DWMS at the grain, bed and element scale, by employing digital image analysis, vertical and lateral facies analysis, geostatistics (Markov chain and Entropy analysis) and static heterogeneity coefficients. From the analysis of the data, the following contribution to the field of deep-water sedimentology can be made: (1) three varieties of massive sands have been identified: ungraded and graded massive sands, and massive sands with patchy texture; (2) massive sands are characterised by a variety of grain size trends within the different grain size percentiles; (3) oblique-to-flow and high imbrication angle is the typical fabric character; (4) massive sands form the ‘core’ of the deep-water deposits in proximal and medial locations, but DWMS sensu stricto are primarily located in the distal locations of a basin; (5) graded and ungraded massive sands are laterally extensive, but those exhibiting ‘patchy’ texture are spatially restricted; and (6) massive sand facies associations are characterised by low heterogeneity, but distinct layering can be defined using petrophysical properties that are not observed at the macroscopic scale. Based on these findings, new models of DWMS deposition are presented for graded and ungraded massive sands, and massive sands with ‘patchy’ texture. These new modes of deposition for DWMS control grain- to element-scale heterogeneity in the sedimentological characteristics, which in turn controls petrophysical trends at different scales and ultimately affect how hydrocarbons hosted in massive sands are produced.