Mixing Zone and Deep Burial Diagenesis: Relevance to Reservoir Porosity and Oil Migration, Middle Jurassic Great Oolite Formation, Wealden Basin, England
Roger K. McLimans, Patricia E. Videtich
The Middle Jurassic Great Oolite Limestone of the Wealden basin, southern England, is a target of active exploration owing to recent discoveries of oil, such as the Conoco No. 1 Storrington location.
Throughout the basin, similar lithologies in the Great Oolite have porosities ranging from less than 5% to greater than 20%. Hence, the principal control on porosity is diagenetic history. In the Storrington No. 1 production interval, the porosity is mainly primary. Cement, isotopic, and trace element stratigraphies of that interval indicate that primary porosity is preserved through early mineralogical stabilization of limestones in the mixing zone between freshwater and marine phreatic environments. At the nearby Southwater No. 1 location, studies of the same types of data show a similar early diagenetic history.
Following mixing zone diagenesis, further subsidence resulted in deep burial diagenesis, principally the occlusion of porosity by coarse calcite spar. The deep burial origin of the spar is based on fluid inclusion data, which also indicate that uplift followed the precipitation of spar. The amount of uplift varies depending on basin location. Oil-filled fluid inclusions also occur in the spar, indicating oil was in a mobile phase at the time of cementation. Reconstruction of basin structure and maturation modeling show that cementation-oil migration was in the Cretaceous. That, together with the distribution of oil-filled inclusions, indicate the primary areas of the basin where migration and accumulation of oil occurred.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.