Diagenesis and reservoir quality in high-resolution sandstone sequences: An example from the Middle Jurassic Ravenscar sandstones, Yorkshire CoastUK

Abstract

A section of the Middle Jurassic Ravenscar sandstones exposed between Cloughton Wyke and Burniston Bay was studied to understand fine-scale variation in reservoir quality that can affect flow properties at the reservoir scale. The study has identified two 3rd-order depositional sequences consisting of twelve 4th-order high-resolution sandstone sequences deposited mostly in a lower delta plain. Although these sandstones exhibit a narrow range in porosity values, however, a sharp disparity exists in their permeability which can have implications at the scale of reservoir development. Diagenetic studies have revealed that mechanical compaction produced the same effect for most of these sandstones. However, dissolution and cementation vary, hence, are mostly responsible for the sharp contrast in their reservoir quality. The distributary channel sandstones developed during normal regression of the area in the FS9 sequence are considered the best reservoir sand bodies. However, these LST sandstones are compartmentalised by authigenic illite, quartz and kaolin into two reservoir segments. The upper portion lacks illite with resultant higher permeability values above 1100 mD (ave. 2217 mD) and classified as reservoir group 4. In contrast, the lower part contains illite with relatively lower permeability values in the range of 300–1100 mD (ave. 879 mD) and classified as reservoir group 3 together with the transgressive crevasse channel sandstones in the FS11 sequence. The presence of illite cement in these sandstones is linked to the occurrence of k-feldspar, hence, facilitated illitization of kaolin in contrast to group 4, which lacks this mineral. The other transgressive sandstones deposited in interdistributary bay, crevasse delta, back-barrier lagoon, lagoon, and estuarine environments which form reservoir groups 1 and 2 have permeability values in the range of 0–50 mD (ave. 6 mD) and 50–300 mD (ave. 184 mD) respectively. These sandstones have complex styles of diagenetic cement with variable higher volumes of total authigenic cement consisting of quartz, kaolin, illite, chlorite and calcite than reservoir groups 3 and 4, hence, have significantly reduced their reservoir potential. Therefore, for effective reservoir development of these sandstone reservoirs in the North Sea and elsewhere in the world with a similar geological setting, detailed reservoir characterisation involving linking lithofacies, sequence stratigraphy and diagenesis at high-resolution are essential for understanding the distribution of reservoir flow properties.