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

The distribution of mineral cements, total porosity, microporosity and perm eability have been determined for the Humbly Grove oolitic carbonate reserv oir (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, w ith a pronounced bimodal distribution. Within the oil leg, average permeabi lity 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. Petr ography and log-derived water saturation data indicate that this transition corresponds to a significant increase in pore filling burial diagenetic ce ments (ferroan calcite and ferroan dolomite). This is accompanied by a chan ge of the effective pore system from a combination of primary intergranular mesoporosity plus secondary intragranular microporosity (above 3395') to p redominant intragranular microporosity (below 3395'). The ferroan cements c ontain 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 plac e at depth, after stylolite formation. The diagenetic and reservoir quality heterogeneity within Humbly Grove is attributable to an early episode of o il emplacement and the establishment of a syn-diagenetic oil-water contact at 3395'. Burial diagenesis was strongly inhibited in the palaeo oil leg, b ut precipitation of ferroan carbonate cements and solution enhancement of m icroporosity was able to continue in the palaeo-aquifer. A second oil charg e entered the Humbly Grove field during Tertiary basin inversion, depressin g the oil-water contact through the diagenetically altered zone to the base of the permeable facies. The field consequently preserves a vertical (stra tigraphic) layering of reservoir quality that is the result of differential late diagenetic modification rather than facies and/or syndepositional dia genetic variability. Regional modelling of oil emplacement with respect to burial diagenetic processes may permit ranking of reservoir quality in simi lar Jurassic reservoirs of the Weald Basin. (C) 2000 Elsevier Science Ltd. All rights reserved.