Biofilm origin of clay-coated sand grains

Abstract

The presence of clay-sized particles and clay minerals in modern sands and ancient sandstones has long presented an interesting problem, because primary depositional processes tend to lead to physical separation of fine-and coarse-grained materials. Numerous processes have been invoked to explain the common presence of clay minerals in sandstones, including infiltration, the codeposition of flocculated muds, and bioturbation-induced sediment mixing. How and why clay minerals form as grain coats at the site of deposition remains uncertain, despite clay-coated sand grains being of paramount importance for subsequent diagenetic sandstone properties. We have identified a new biofilm mechanism that explains clay material attachment to sand grain surfaces that leads to the production of detrital clay coats. This study focuses on a modern estuary using a combination of field work, scanning electron microscopy, petrography, biomarker analysis, and Raman spectroscopy to provide evidence of the pivotal role that biofilms play in the formation of clay-coated sand grains. This study shows that within modern marginal marine systems, clay coats primarily result from adhesive biofilms. This bio-mineral interaction potentially revolutionizes the understanding of clay-coated sand grains and offers a first step to enhanced reservoir quality prediction in ancient and deeply buried sandstones.

Estuarine clay mineral distribution: Modern analogue for ancient sandstone reservoir quality prediction

“…() suggested an optimum range of 5 to 12% sediment volume as clays for the Tuscaloosa Formation and 4 to 7% for the Berea Sandstone. As a result, generalised models presented in this study (Figs and ), which predict estuarine sandstone reservoir quality, are based on the suggested volumes of clay necessary to form grain coats (stated above), as well as clay mineral distribution patterns (this study) and detrital clay coat distribution patterns reported in the Ravenglass Estuary (Wooldridge et al ., ,b). It should be noted that the generalised models assume that detrital chlorite, illite and kaolinite are direct precursors to burial‐diagenetic chlorite, illite and kaolinite, respectively.…”

“…), sits near the small town of Ravenglass located on the west coast of Cumbria. The Ravenglass Estuary, which encompasses the tidal reaches of the Rivers Esk, Irt and Mite, occupies an area of 5·6 km 2 , of which ca 86% is intertidal (Bousher, ; Lloyd et al ., ; Wooldridge et al ., ,b). It has been suggested (Bousher, ) that the Drigg and Eskmeals barrier‐spits developed at around 3000 bp , causing the coalescence of the previously separate and westward flowing Rivers Irt, Mite and Esk, into a singular complex estuary with one main channel out to the Irish Sea.…”

“…Diagenetic clay coats in sandstones may originate from the thermally‐driven recrystallization of low‐temperature detrital clay coats, or through in situ growth from the authigenic alteration of precursor and early‐diagenetic minerals, which interact with pore fluids during burial (Hillier, ; Aagaard et al ., ; Worden & Morad, ; Ajdukiewicz & Larese, ). As a result, to facilitate improved reservoir quality prediction, a new focus has arisen on the use of modern analogues to understand the origin, distribution and variable extent of detrital clay coats in clastic systems (Dowey et al ., ; Jones, ; Wooldridge et al ., ,b; Griffiths et al ., ; Virolle et al ., ). However, in addition to the extent and completeness (i.e.…”

“…Note that this article carefully discriminates between the term ‘clay mineral’, referring to aluminium‐rich sheet silicate minerals (phyllosilicates) and the term ‘clay’, referring to sediment particles that are smaller than 2 μ m in size. With reference to modern marginal marine environments, while it is possible to start to predict the distribution of clay grade material (Dalrymple et al ., ; Dowey et al ., ; Wooldridge et al ., ,b; Griffiths et al ., ; Virolle et al ., ), it is not possible to identify areas of enrichment of one specific clay mineral (for example, chlorite) relative to other clay minerals. Fundamentally, there is a lack of knowledge and understanding on how specific clay minerals are distributed in most modern sedimentary environments and in ancient, deeply buried sandstone reservoirs.…”

How To Quantify Clay-Coat Grain Coverage in Modern and Ancient Sediments

“…The need to explore, predict, and develop economically-viable, deeply buried petroleum prospects (. 3 km) has driven significant research in establishing a predictive capability for the distribution of clay-coated grains, via a range of core-based (Gould et al 2010;Saïag et al 2016;Skarpeid et al 2017), and modern-analogue approaches (Dowey 2013;Dowey et al 2017;Wooldridge et al 2017a;Wooldridge et al 2017b;Griffiths et al 2018). Experimental work (Lander et al 2008;Ajdukiewicz and Larese 2012) and core-based investigations (Ehrenberg 1993;Bloch et al 2002;Stricker and Jones 2016;Skarpeid et al 2017) have suggested that the completeness of the coat (here defined as the fraction of surface area of grains covered by attached clay material) is the principal factor governing the effectiveness of quartz cement inhibition and thus the preservation of good reservoir quality.…”