Abstract

An Upper Jurassic (Oxfordian) reef mound in the Palmers Wood field area of the Weald basin, southern England, clearly shows the relationship of facies, diagenesis, and porosity development to relative sea level changes. The coral-microbial reef mound was initiated over a drowned oolite shoal during a third-order marine transgression and exhibits changes in coral morphology from base to top as the reef mound caught up with sea level during the subsequent highstand. During the highstand, extensive encrustation of the reef mound took place by microbial, stromatolitic cements, with subsequent porosity loss. During the following lowstand of relative sea level, extensive leaching of the aragonitic corals took place, and a lowstand wedge accumulated down- ramp. Porosity was lost during burial-related cementation and compaction. Coral-microbial reef mounds of this type have moderate potential for porosity formation, unlike tighter, deeper ramp reefs in the Jurassic, which are less prone to subaerial leaching and have less potential for secondary porosity development because they are less rich in aragonitic components. Although the Upper Jurassic reef mound is only a minor contributor to reservoirs in the Palmers Wood field, it may provide a model for other Upper Jurassic reef plays.

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