Combined method of petrography, geochemistry and petrophysics to characterize bitumen-bearing reservoirs; application to a North Sea sandstone

Reservoir rocks are known to display large variations in their petrophysical properties even within the same formation. This methodological paper shows the influence of the post-sedimentary organic phases (bitumen) on porosity and permeability, using digital image analysis to quantify all constituents and to characterize the porous network. Five samples originating from a Kimmeridgian sandstone reservoir of the North Sea were studied. They consist of fine-grained and poorly sorted sandstones which display similar porosity (about 20%) but highly contrasted permeability (ranging from 17.6 to 344.0 mD). The whole rock samples were observed in reflected white light, by UV illumination, and by stereomicroscopy, and all images were digitized. The quantitative measures of area and perimeter allow to characterize the morphology through the shape factor (SF) and the specific surface (SS). The organic phase, including both kerogen (1 to 10% of the organic content) and bitumen, is exclusively located in the primary porosity. The relative proportion of bitumen increases from 0.9 to 3.6% as permeability increases. Two main types of bitumen were observed: dominant amorphous patches and scarce spherules. For the opaque areas, corrections based upon geochemical carbon and sulfur analysis of extracted samples, the density of each phase, the porosity, and the visual estimation of kerogen and bitumen were made. A first group of samples with low permeability values comprises very small-sized, subspherically-shaped organic particles, and small-sized, few elongated pores. A second group of samples with high permeability values comprises large-sized, irregularly-shaped organic particles, and large-sized, elongated and tortuous pores. The presence of bitumen within the porous network results in the increase of the specific surface and of the shape factor of pores. This is due to the organic phase which coats the pore walls, inducing restriction of throats. The specific surface versus the shape factor of the porous network with its organic content points out two trends. The first trend is a marked evolution of the specific surface for a constant shape factor. The second one is a marked evolution of the shape factor for a constant specific surface. The shape factor, which reflects the elongation and tortuosity of the porous network, is linked to high porosity, permeability, and connectivity of pores. This study shows that the combination of several petrographical methods coupled with digital image processing is able to provide an analytical tool that allows to improve the knowledge of in situ bitumen within reservoirs.