Diagenesis and Burial History of Great Oolite Limestone, Southern England

Abstract

Oil is produced from the Middle Jurassic Great Oolite Limestone in the western Wealden basin, England. Additional production was discovered (150 STBOPD, 42° API) in 1986 at the Conoco 1 Storrington well. The Storrington discovery is of note because the porosity in reservoir grainstones and packstones is largely primary and averages 19.4%, the highest value yet found for the Great Oolite. Secondary porosity is of little significance. For all wells studied, porosity in Great Oolite grainstones and packstones ranges between 3 and 20%, and there is no relation between porosity and lithology. Locally, porosity may be largely occluded by coarse calcite spar, which fluid inclusion studies show precipitated in the deep-burial environment. Exploration in the Great Oolite, the efore, must seek diagenetic fairways wherein primary porosity has been preserved. Limestones in the producing interval of the Great Oolite at 1 Storrington contain zoned syntaxial cements, are generally high in magnesium content, and have stable isotope compositions that indicate the involvement of marine waters (relatively heavy ^dgr18O). Our interpretation is that the limestones were stabilized early in a mixing zone between freshwater and marine phreatic environments. Coarse calcite spar in the Great Oolite contains aqueous and oil inclusions. The results of geothermometry studies show that the precipitation of calcite cements occurred at depth, near maximum burial, and, at certain locations, was synchronous with oil migration. Inversion of the basin was a later event. The construction of burial history diagrams that incorporate fluid inclusion and maturation data shows that oil migration was during the Late Cretaceous. A distribution pattern for oil inclusions in the Great Oolite indicates migration was mainly in the western part of the basin and initially into pre-Late Cretaceous traps.