Understanding the Fundamental Processes Governing the Origin and Distribution of Clay-Coated Sand Grains in Petroleum Reservoirs Through a Modern Day Analogue

Abstract

Predicting the distribution of clay-coated sand grains within petroleum reservoirs is important to help exploit deeply buried, anomalously high porosity sandstones, resulting from the inhibition of the normally ubiquitous porosity-occluding quartz cement. The extent and completeness of clay coats are key factors that control the inhibition of burial-related quartz cement. At present, there is limited understanding of the origin of clay coats and no all-encompassing, high spatial resolution, predictive model of clay-coat abundance and distribution at a facies scale. To address this, we have focused on the distribution and origin of clay-coatings in modern sedimentary environments as a crucial step towards building a predictive capability. This study adopted a high resolution analogue approach using the Ravenglass marginal marine system, NW England, UK. The work involved detailed analysis of the modern sedimentary system, including mapping surface sedimentary bedforms and bioturbation- intensity and grain size analysis; shallow cores were also taken to develop transects. A range of scanning electron microscopy techniques were employed to characterise surface and core sediment samples in unison with Raman spectroscopy to study the distribution patterns and characteristics of clay grain coats on sand grains. These exceptional data sets have produced unique and highly detailed maps of spatial and stratigraphic variations of the distribution of clay grain coats, linked to depositional environment, grain size and bioturbation that can be applied to help prediction in the subsurface. This work has also focused specifically upon the mechanism of clay coat formation and attachment to framework grains. Through detailed and specialist analysis this work has started to reveal, for the first time, that clays are glued on to sand grain surfaces in estuarine sediments by biofilms developed through the extrusion of extracellular polysaccharides by the action of epipelagic diatoms. This work has begun to establish the fundamental controlling parameters governing the origin, mineralogy and distribution of clay coats on sand grains within marginal marine systems. The results can be applied to aid the prediction of advantageous grain coating chlorite in sandstone reservoirs and thus facilitate prediction where the degradation of reservoir quality in ancient, deeply-buried petroleum reservoirs by quartz cement has been inhibited.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016