Controls of reservoir quality in Carboniferous tight gas sandstone

Abstract

The UK Continental Shelf is one of the most mature offshore basins in the world, yet the existing Namurian tight gas reservoirs are still largely unexplored since permeability differences of several orders of magnitude over small areas of reservoir are a key exploration risk. It is therefore important to be able to identify sandstones with sufficient reservoir quality to optimize the drilling process and minimize exploration costs.

This study integrates sedimentological logging, quantitative petrographic data, burial history modelling, and quantitative analysis of the pore network using microCT to understand the controls on reservoir quality in Copernicus discovery and Cavendish fields of the Southern North Sea. These two fields display different reservoir qualities despite the fact that both lie on the same structure and show similar present day burial depths.

This study shows that a combination of multiple factors was responsible for reservoir quality distribution. The best reservoirs are associated with quartz rich sandstones that are less prone to adverse effects of diagenesis than the predominant, more feldspathic and more kaolinitised sandstones, which were sourced from a different area. Next, depositional environment was responsible for differences in detrital composition, grain size and different initial pore water chemistry that led to varying diagenetic pathways. Furthermore, differences in burial histories created better conditions for quartz cement development in Copernicus field, ultimately rendering it an economically non-viable resource.

Mapping of the pore networks with microCT on micron scale shows how small variations in diagenetic style result in permeability differences of several orders of magnitude over small areas of reservoir. An integrated approach utilising burial history modelling, analysis of diagenetic processes, 3D modelling of pore architecture, and modelling changes in pore connectivity through time, offer a methodology that allows reconstructing permeability evolution and the timing and mechanisms of reservoir tightening throughout the geological history.