Estuarine Clay Mineral Distribution: Modern Analogue for Ancient Sandstone Reservoir Quality Prediction
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Sedimentology (2019) doi: 10.1111/sed.12571 Estuarine clay mineral distribution: Modern analogue for ancient sandstone reservoir quality prediction JOSHUA GRIFFITHS*†, RICHARD H. WORDEN*, LUKE J. WOOLDRIDGE*‡, JAMES E. P. UTLEY*, ROBERT A. DULLER* and RHIANNON L. EDGE§ *Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, UK (E-mail: joshua.griffi[email protected]) †BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UK ‡BP Upstream Technology, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UK §Department of Health and Medicine, Lancaster University, Bailrigg, Lancaster, LA1 4YW, UK Associate Editor – John Howell ABSTRACT The spatial distribution of clay minerals in sandstones, which may both enhance or degrade reservoir quality, is poorly understood. To address this, clay mineral distribution patterns and host-sediment properties (grain size, sorting, clay fraction abundance and bioturbation intensity) have, for the first time, been determined and mapped at an unprecedentedly high-resolution in a modern estuarine setting (Ravenglass Estuary, UK). Results show that the estuary sediment is dominated by illite with subordinate chlorite and kaolin- ite, although the rivers supply sediment with less illite and significantly more chlorite than found in the estuary. Fluvial-supplied sediment has been locally diluted by sediment derived from glaciogenic drift deposits on the margins of the estuary. Detailed clay mineral maps and statistical analyses reveal that the estuary has a heterogeneous distribution of illite, chlorite and kaolinite. Chlo- rite is relatively most abundant on the northern foreshore and backshore and is concentrated in coarse-grained inner estuary dunes and tidal bars. Illite is relatively most abundant (as well as being most crystalline and most Fe–Mg- rich) in fine-grained inner estuary and central basin mud and mixed flats. Kaolinite has the highest abundance in fluvial sediment and is relatively homogenous in tidally-influenced environments. Clay mineral distribution patterns in the Ravenglass Estuary have been strongly influenced by sediment supply (residence time) and subsequently modified by hydrodynamic pro- cesses. There is no relationship between macro-faunal bioturbation intensity and the abundance of chlorite, illite or kaolinite. Based on this modern-analo- gue study, outer estuarine sediments are likely to be heavily quartz cemented in deeply-buried (burial temperatures exceeding 80 to 100°C) sandstone reser- voirs due to a paucity of clay grade material (<0Á5%) to form complete grain coats. In contrast, chlorite-enriched tidal bars and dunes in the inner estuary, with their well-developed detrital clay coats, are likely to have quartz cement inhibiting authigenic clay coats in deeply-buried sandstones. Keywords Clay mineral, estuary, modern, prediction, sandstone reservoir quality. © 2018 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of 1 International Association of Sedimentologists This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 2 J. Griffiths et al. INTRODUCTION prediction, a new focus has arisen on the use of modern analogues to understand the origin, dis- Clay minerals may enhance or degrade sand- tribution and variable extent of detrital clay coats stone reservoir quality, depending on type and in clastic systems (Dowey et al., 2017; Jones, abundance. For example, pore-filling illite signifi- 2017; Wooldridge et al., 2017a,b; Griffiths et al., cantly lowers permeability, whilst grain-coating 2018b; Virolle et al., 2018). However, in addition chlorite may preserve porosity through the inhi- to the extent and completeness (i.e. fraction of bition of pore-filling quartz cement (Ehrenberg, the sand grain-surface covered by clay minerals) 1993; Ajdukiewicz & Larese, 2012; Stricker & of a clay coat, the mineralogy also exerts a strong Jones, 2016; Skarpeid et al., 2017; Griffiths et al., control on the ability of clay coated sand grains 2018a,b). Chlorite grain coats are found in sand- to inhibit quartz cementation (Bloch et al., 2002; stones deposited in a range of depositional envi- Billault et al., 2003; Lander et al., 2008; Ajdukie- ronments but are especially common in marginal wicz & Larese, 2012). Consequently, there is an marine settings (Dowey et al., 2012). Despite additional need to map out clay mineral distri- growing interest in the petroleum industry, there bution patterns and understand the dominant is no workflow to predict the spatial and strati- controls on clay mineral distribution patterns at graphic distribution of clay minerals in the a scale relevant to oil and gas reservoirs. The pri- subsurface. mary objective of this study is to better under- Volumes of sediment, modern or ancient (as stand the distribution of clay minerals in an sedimentary rock), are rarely homogeneous; estuary, which will enable the prediction of clay instead they typically display significant hetero- mineral type, abundance and distribution in geneities in terms of total clay content, primary ancient marginal-marine sandstones. grain mineralogy, grain size and grain sorting, In order to implement modern analogue stud- and varying degrees of bioturbation, soil devel- ies of clay mineral distribution patterns, to facil- opment and infiltration. It can also be antici- itate improved prediction of authigenic clay coat pated that the relative abundance of specific and clay mineral distribution patterns in petro- types of clay minerals are not homogeneously leum reservoirs, typical burial diagenetic path- distributed throughout modern and ancient sand ways need to first be considered. Even during bodies. Note that this article carefully discrimi- the long timescale of burial diagenesis, the main nates between the term ‘clay mineral’, referring components of clay minerals (for chlorite: Fe- to aluminium-rich sheet silicate minerals (phyl- oxides, Al-oxides and Si-oxides) have extremely losilicates) and the term ‘clay’, referring to sedi- low aqueous solubility and therefore clay min- ment particles that are smaller than 2 lmin eral, and especially chlorite, diagenesis during size. With reference to modern marginal marine burial can be assumed to be an isochemical pro- environments, while it is possible to start to cess; i.e. closed-system diagenesis (Worden & predict the distribution of clay grade material Morad, 2003; Worden et al., 2018). Chlorite, and (Dalrymple et al., 1992; Dowey et al., 2017; possibly other clay mineral presence, is thus not Wooldridge et al., 2017a,b; Griffiths et al., 2018b; the result of mass influx of materials into sand- Virolle et al., 2018), it is not possible to identify stones during diagenesis, rather it represents areas of enrichment of one specific clay mineral either primary chlorite presence or the diage- (for example, chlorite) relative to other clay min- netic transformation of precursor components erals. Fundamentally, there is a lack of knowl- that were initially present in the primary sedi- edge and understanding on how specific clay ment. A study of modern environments is thus minerals are distributed in most modern sedi- an appropriate way of developing an under- mentary environments and in ancient, deeply standing (by analogy) of the distribution of chlo- buried sandstone reservoirs. rite in sandstones, since the distribution of Diagenetic clay coats in sandstones may origi- chlorite, or the key components of chlorite (Fe- nate from the thermally-driven recrystallization oxides, Al-oxides and Si-oxides) in the original of low-temperature detrital clay coats, or through sediment, controls where chlorite will be found in situ growth from the authigenic alteration of in the sandstone after burial diagenesis (Worden precursor and early-diagenetic minerals, which & Morad, 2003; Worden et al., 2018). The first interact with pore fluids during burial (Hillier, stages of sediment burial should however be 1994; Aagaard et al., 2000; Worden & Morad, considered carefully, since during this phase 2003; Ajdukiewicz & Larese, 2012). As a result, sediment may be part of an open-system. For to facilitate improved reservoir quality example, widespread kaolinization has been © 2018 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Clay minerals in modern estuarine sands 3 reported to be typical in marginal marine set- Rateev et al., 2008). Kaolinite has been reported tings due to meteoric water-flushing; especially to be most abundant in low-latitude marine in lowstand system tract sandstones (Ketzer environments adjacent to land masses with et al., 2003). Consequently, clay mineral distri- warm and humid conditions that permit intense bution patterns have been linked to sequence chemical weathering (Windom, 1976; Eberl et al., stratigraphy (Morad et al., 2013). Furthermore, 1984; Chamley, 1989; Rateev et al., 2008). Smectite the co-deposition or diagenetic formation of is generally typical of weathering from semi-arid other minerals may also inhibit or promote the continental sources, subject to only the early development of authigenic chlorite. For exam- stages of chemical weathering conditions (Salem ple, iron is necessary for Fe-chlorite growth but et al., 2000). It has been reported that prove- may preferentially be locked up at the expense nance plays