Porosity modification during diagenesis: processes and products

Vuggy porosity within dolomitised rudistid grainstones of the Turonian Gattar Member of the Zebbag Formation, Southern Tunisia

Carbonate rocks are highly reactive, and therefore the volume, shape, size and connectivity of porosity can change dramatically from immediately after deposition into the deep burial environment, and during uplift. Methods for measuring pore volume and characterising pore type are in place, but only recently has it been possible to quantify the shape and size of carbonate pore networks. This has been facilitated by the advancement in image analysis technology and the resolution of X-ray CT imaging.

This theme focuses on building robust conceptual models that explain the mechanisms controlling post-depositional porosity modification, and the resultant geometry of the pore networks.  A suite of subsurface data from Aptian to Cenomanian carbonate reservoirs on the Arabian Plate provides the basis for determining both regional and local controls on porosity distribution.  Although there is evidence within these reservoirs for porosity formation during exposure at third-order sequence boundaries, there is also a clear phase of porosity modification in the burial environment, associated with basin inversion and hydrocarbon emplacement (Hollis, 2011).  

Through integrated petrophysical, rock physical and X-Ray CT imaging, the relationship between pore geometry, connectivity, permeability and velocity can be described (eg. Hollis et al., 2010; Jivkov et al., 2012; Lopez-Archilla et al., 2013a & b).  Through doing so, a database of pore geometrical data can be derived which will allow more accurate prediction of reservoir quality from subsurface datasets.  

Thin section photomicrograph showing corrosion of burial diagenetic, ferroan calcite cements to create vuggy porosity. Aptian, North Oman
Backscatter scanning electron microscope image of corroded microporosity adjacent to a fracture, Aptian, North Oman
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