Grain-coating clay distribution and early diagenesis – Evidence from a modern coastline in Parnaiba, Brazil

Clay mineral coating on sand grains (e.g. illite, chlorite) can preserve and/or destroy porosity and permeability in reservoir rocks. Improved understanding of the distribution of clay minerals in depositional environments, the origin of precursor clay coatings and the pore water chemistry controls in early diagenesis will contribute to deciphering factors controlling reservoir properties.

Reservoir quality evolution in sandstone is of importance for all fluid resources: oil, gas and water. The origin of chlorite type coatings, textures and crystal morphology is, however, of special relevance in studies of water-filled sandstone reservoirs for CO2 storage – as it is a mineral which may dissolve and provide cations (Fe) for mineral trapping of carbon in the subsurface.

As part of the GeoSource project (, we have collected sediment and water samples from the deltaic and estuarine sands on the Brazilian coast, in Parnaiba.

Preliminary studies show that clays are sticking to grains in some sedimentary facies, while they are completely absent in other areas. New samples from below the surface, down to 5m, may shed light on how clay is redistributed during early diagenesis, depending on physical factors: e.g. tidal pumping and geochemical conditions: such as the supply of organic matter, pore water chemistry, and possibly microbiology.

Aims (research questions):

  • Are there biofilms on sand grain surfaces– aiding the emplacement of clay precursors?
  • Are clay precursors altered and/or redistributed within the first few meters (comparison of two different facies)?
  • Could the facies belts and clay distributions observed in Parnaiba be used as analogues for understanding North Sea reservoirs?


  • X-ray diffraction - mineralogy analysis
  • Scanning electron microscopy – characterization of clay coats
  • Ion chromatography – water chemistry
  • Possibility to obtain a scholarship to travel to Brazil (Belem) to participate in a hydrogeology-field course, or to do an internship*.


Fig 1: Beach deposits at Mangrove / Coastal dune facies interface in Parnaiba. Click here for a bigger version
Fig 2: Element map from SEM showing precursor clay coats on sand grain from beach sands in Parnaiba. Click here for a bigger version.


Published Oct. 18, 2019 12:37 PM - Last modified Oct. 18, 2019 12:37 PM

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