Diagenesis
In the study area, Ratcliffe oil is typically entrapped within dolomitized burrow-mottled mudstones and wackestones. Dolomites are fabric preserving and microcrystalline to cryptocrystalline. Most of the visible porosity is associated with burial diagenetic processes (moldic and vuggy pores). Intercrystalline porosity is poor to fair and associated permeability is the same.
Vuggy and channel pores commonly occur within dolomitic strata and possibly developed during burial diagenesis. Late-burial dissolution associated with chemical compaction, maturing hydrocarbons, or hydrothermal processes may be a common control in Ratcliffe reservoir development. This evaluation has shown that Ratcliffe dolomitic reservoirs were subjected to burial diagenesis which modified pre-existing reservoir conditions and both improved and reduced porosity and permeability.
Anhydrite and calcite replacement are locally present in Ratcliffe reservoirs. These occluding and diagenetic minerals decrease both porosity and permeability. Anhydrite commonly occurs as a late precipitating and occluding cement, and calcite is locally present as a pore occluding mineral. Both minerals precipitated during mesogenesis.
Natural fractures within the Ratcliffe are developed within limestones and dolomitic beds and are observed in each core studied. These fractures are nearly vertical and have fluorescence. Many fractures within limestones were subsequently occluded by sparry calcite. Fractures in dolomites are usually 0.15 to 0.60 m (0.5 to 2.0 ft) long and occur within the burrow-mottled facies (Facies 3). Swarms or closely spaced vertical fractures are also present, but within thin beds.
Pressure-solution features, ranging from micro-stylolites to high-amplitude stylolites, wispy seams, and individual or isolated solution seams are present in Ratcliffe beds. Stylolitization is more common in non-reservoir limestones and these compaction features do not appear to adversely affect dolomitic reservoirs.