Cement distribution in a carbonate reservoir: recognition of a palaeo oil-water contact and its relationship to reservoir quality in the Humbly Grove field, onshore, UK

The distribution of mineral cements, total porosity, microporosity and permeability have been determined for the Humbly Grove oolitic carbonate reservoir (Middle Jurassic Great Oolite Formation, Weald Basin, onshore UK) using a combination of optical petrography, electron microscopy, fluid inclusion analysis, quantitative XRD, wireline data analysis and core analysis data. Grainstone reservoir facies have porosities ranging between 5 and 24%, but are mostly between 11 and 24%. Permeabilities vary from <0.1 to 1000 mD, with a pronounced bimodal distribution. Within the oil leg, average permeability decreases by two orders of magnitude below 3395' from about 100 mD to about 1 mD. Average porosity declines by only 1.3% over this interval. Petrography and log-derived water saturation data indicate that this transition corresponds to a significant increase in pore filling burial diagenetic cements (ferroan calcite and ferroan dolomite). This is accompanied by a change of the effective pore system from a combination of primary intergranular mesoporosity plus secondary intragranular microporosity (above 3395') to predominant intragranular microporosity (below 3395'). The ferroan cements contain petroleum and aqueous fluid inclusions, the latter yielding elevated homogenisation temperatures consistent with cementation at or near maximum burial depth. Enhancement of early diagenetic microporosity also took place at depth, after stylolite formation. The diagenetic and reservoir quality heterogeneity within Humbly Grove is attributable to an early episode of oil emplacement and the establishment of a syn-diagenetic oil-water contact at 3395'. Burial diagenesis was strongly inhibited in the palaeo oil leg, but precipitation of ferroan carbonate cements and solution enhancement of microporosity was able to continue in the palaeo-aquifer. A second oil charge entered the Humbly Grove field during Tertiary basin inversion, depressing the oil-water contact through the diagenetically altered zone to the base of the permeable facies. The field consequently preserves a vertical (stratigraphic) layering of reservoir quality that is the result of differential late diagenetic modification rather than facies and/or syndepositional diagenetic variability. Regional modelling of oil emplacement with respect to burial diagenetic processes may permit ranking of reservoir quality in similar Jurassic reservoirs of the Weald Basin. (C) 2000 Elsevier Science Ltd. All rights reserved.