The Upper Cretaceous - Danian Chalk Group of northwest Europe is an important producing reservoir in the North Sea. Chalk is a soft fine grained rock composed primarily of calcium carbonate (principally coccoliths), and is highly porous (20% plus porosity values), although most of the pores are sub 1micron in size. Traditionally, silica has been recognised either as fossil tests within this calcareous matrix or as distinct chert layers. Recently, however, XRD analyses revealed unexpectedly high levels of silica in chalk samples that show no obvious presence of silica in SEM and petrological studies. The assumption from these analyses is that the silica is distributed throughout the sample at the microscopic level. As with other fine grained microporous rocks, improved recovery depends on establishing the mineralogical constituents of the rock and the ability of reservoir engineers to extract the hydrocarbons contained within the micropores or microfractures.
In chalk reservoirs, recovery by injection of water depends on the wetting conditions of the rock and compaction by overburden due to water weakening the formation. Recent studies (Strand et al 2007) have concluded that the silica content of chalk is related to differences in wetting conditions and mechanical properties. It is also suggested that silica content may have affected the retention of porosity by inhibiting clay formation, and can also form intra-reservoir barriers.
In order to understand the relationship between permeability and mineralogy (in particular silica content) in chalk reservoirs, two well sections from the North Sea have been studied: Well A and Well B. Core samples studied from these wells span the Ekofisk and Tor Formations. In addition to standard carbonate sedimentological methodologies, samples have also been analysed by QEMSCANTM and MagporeTM techniques. Preliminary results of this ongoing study are presented here.
QEMSCANTM is an automated mineralogical scanning tool which uses a combination of SEM and EDS to evaluate the mineralogy of rock samples. MagporeTM uses magnetic anisotropy to evaluate the pore fabric of the lithologies. By combining these two techniques, it has been possible to define the 3D pore orientation data and relate these to bedding structures and mineralogy. It has also been possible to define various levels of silica within each sample, to map these mineralogically, and then to provide downhole plots which quantify silica concentrations through the well sections.