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Clay-Coated Sand Grains in Petroleum Reservoirs: Understanding Their Distribution Using an Analogue Holocene Estuarine Fill Succession

Abstract

Sand grains coated with chlorite, illite and mixed mineralogy clays have been shown, both within nature and laboratory-based studies, to inhibit the growth of porosity-occluding authigenic quartz cements. Post compaction, the major cause for porosity- and permeability-loss within deeply buried clastic sediment is the growth of quartz cements. Thus, the ability to predict the distribution of clay-coated sand grains is vital to help find and exploit anomalously good reservoir quality within deeply buried sandstone reservoirs. This study adopted a high resolution analogue methodology, focused on the Ravenglass Estuary, UK. The work involved detailed analysis of this modern sedimentary system, which included high resolution surface mapping of sedimentary bedforms, bioturbation-intensity, biofilm abundance, and grain size analysis. 15m cores were taken down to glacial till, to develop a sedimentary framework of the Holocene estuarine succession. A range of scanning electron microscopy, environmental scanning electron microscopy and automated SEM-EDS mineralogy techniques, were employed to characterise the distribution patterns and characteristics of clay grain coats on sand grains within surface and core sediment samples. Raman spectroscopy and microbial carbohydrate analysis were undertaken to study the origin of clay coated sand grains. This sedimentary framework of the complete Holocene estuarine succession has produced uniquely detailed maps of the spatial and stratigraphic distribution of clay grain coated sand grains and sediment heterogeneity, which can be applied to help prediction of clay-coated grains in the subsurface. This work has also placed specific focus upon the mechanism of clay coat formation to further constrain predictive models and understand the relatively limited geographical distribution of chlorite clay-coated sand grains, most common in fluvial to marginal marine sediments. Here we report evidence of a biofilm mechanism of clay coat formation and attachment to sand grain surfaces, which has the potential to revolutionise the way the hydrocarbon industry understand the distribution of clay coated sand grains within sandstones. The work as revealed for deeply buried prospects that, against common convention, the best porosity may be found in fine-grained, clay-bearing inner estuary tidal flat facies sands and not in coarse, clean channel fill and bar facies.