Detrital clay grain coats in estuarine clastic deposits: origin and spatial distribution within a modern sedimentary system, the Gironde Estuary (south‐west France)

Abstract

Porosity and permeability may be preserved in deep sandstone reservoirs by clay coating (mainly chlorite) which limits quartz overgrowths. Chloritization around quartz grains results mainly from mineralogical transformations of pre-existing clays. It is fundamental to study those clay precursors to better understand and predict the location and distribution of clay coatings in subsurface sandstones for petroleum or geothermal prospecting. This paper reports a high-resolution analysis of the composition, distribution and fabric of clays along a modern estuary, the Gironde (south-west France). The scale of the study ranges from thin sections, through sand bar bodies, up to the entire estuary. Results show that clays are detrital and deposited at the same time as sand grains despite strong hydrodynamic conditions. Clays bind to medium-grained sands forming detrital clay grain coats. On average, 26% of detrital sand grains are coated along the entire length of the estuary. Coat thickness varies from 1 lm to more than 200 lm, and coat coverage exceeds 30% in some samples. The turbidity maximum zone position (surface water turbidity from 1 to 10 g l À1 ) in the estuary, which is controlled by seasonal variations in hydrodynamic processes, significantly impacts the location of the maximum clay content and the abundance of coated grains in sandy facies along the estuary. Flocculation in the water column results in the accumulation of suspended matter and contributes to the high turbidity in the estuary. Exopolymeric substances produced by diatoms are observed both in the field and by cryo-scanning electron microscopy, suggesting that they may play a major role in binding detrital clay around sand grains. Finally, tidal bars and heterolithic point bars in the estuary funnel and estuarine channels are prime sedimentological targets for finding clay coatings and potentially good reservoir quality in deeply buried sandstones.

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

“…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.…”

“…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.…”

Origin and Sources of Minerals and Their Impact on the Hydrocarbon Reservoir Quality of the PaleogeneLulehe Formation in the Eboliang Area, Northern Qaidam Basin, China

“…Since berthierine is rarely detected and volcanic rock fragments are widely developed in the Lulehe sandstone, smectite, which is commonly derived from alteration of volcanic rock fragments, may be the primary clay mineral precursor of chlorite. In addition, the Lulehe sandstones were mainly deposited in fluvial deltaic environments, and proximal and distal delta fronts are particularly rich in Fe [65]. Moreover, the dissolution of volcanic rock fragments and feldspar grains also provides rich Fe and Mg ions as the main source for chlorite development.…”

Chlorite coating patterns and reservoir quality in deep marine depositional systems – Example from the Cretaceous Agat Formation, Northern North Sea, Norway

“…Other studies have reported that emplacement of detrital clay on sand grains occurs due to bioturbation, either in response to the sand grains passing through the digestive system of the organism or in response to the burrowing action of organisms (Needham et al., 2005; Wilson, 1992). Numerous studies (Griffiths et al., 2018; Virolle et al., 2019; Wooldridge, Worden, Griffiths, Thompson, et al., 2017; Wooldridge, Worden, Griffiths, & Utley, 2017) have also shown that estuaries are suitable precursor clay factories where the emplacement of clay coatings are facilitated by extracellular polymeric substances secreted by microorganisms, forming biofilms (Wooldridge, Worden, Griffiths, Thompson, et al., 2017), which causes clay particles to be attached to the sand‐sized sediment fraction.…”