Reservoir quality of the Middle Jurassic (Ravenscar Group) sandstones along the Yorkshire coast

Abstract

An attempt was made to identify the degree and main control of early diagenesis. It was evident that the more porous, channel sandstone units provided a better pathway for fluidflow and were thus subject to a greater degree of meteoric water flushing. The channels sandstones were completely depleted of potassium-feldspar, and had relatively high amounts of kaolinite. It can therefore be assumed that related sandstones at greater depths and temperature would not form illite due to this lack of potassium. The fact that illite cannot form, may enable these deeper sandstones to preserve adequate porosities and permeability for production. Whereas in places where there was an available potassium source, the combination of kaolinite-illite transformation and quartz cementation would have a greater chance of destroying any remaining porosity. In contrast, the crevasse splay sandstone had remaining potassium-feldspar, and overall lower kaolinite fractions. Due to their tendency to pinch-out into a matrix of overbank fines, crevasse splay sandstones do not offer an ideal pathway for fluid-flow, but rather a ‘dead-end’, thus experiencing a lesser degree of meteoric water flushing and leaching of potassium feldspar. It is also evident from the occurrence of both kaolinite and potassiumfeldspar that these units were not buried deep enough (temperature of 130 degrees Celsius) to undergo illitization. Due to substantial quartz overgrowths, we can assume that these units probably experienced a minimum temperature of approximately 80 degrees Celsius (Walderhaug 1994) but due to the lack of illite (or presence of kaolinite and potassiumfeldspar), we can conclude that it had not been exposed to temperatures equal to or greater than 130 degrees Celsius, for any significant period of time. It is evident by these results that facies and sandstone body architecture play a significant role in fluid-flow properties and early diagenesis, which in turn can have dramatic effects on reservoir quality.