Origin of late cements in oolitic reservoir facies: a fluid inclusion and isotopic study (Mid-Jurassic, southern England)

Abstract

During Bathonian times a carbonate ramp was constructed over southern England by the progradation of oolitic, skeletal and oncoidal grainstones and packstones. The grainstones provide a significant hydrocarbon reservoir facies. Although these carbonates suffered early diagenesis under marine and, more significantly, meteoric conditions, later diagenetic cements such as saddle dolomite and, more particularly, mildly ferroan calcite are volumetrically more important in causing the occlusion of porosity on a regional scale. We have analysed systematically these late cements employing a number of techniques: standard petrography (including CL and UVL); C and O isotopes; Sr isotopes and fluid inclusion studies. Integration of the results of these approaches constrains both the pore-fluid composition and the temperature regime at the time of cement precipitation. Saddle dolomite was emplaced before the mildly ferroan calcite at temperatures between 75 and 85°C and from highly saline brines (18–19.5 wt.% NaCl equivalents). Mildly ferroan calcite precipitated over a generally lower temperature range and from less saline brines (6.5–15 wt.% NaCl equivalents). Five types of fluid inclusion have been recognised: (1) monophase aqueous (liquid); (2) monophase oil (liquid); (3) two-phase aqueous (liquid+vapour); (4) two-phase oil (liquid hydrocarbon +vapour); (5) three-phase aqueous-oil (liquid hydrocarbon+aqueous liquid+vapour). During burial warm (> 75°C) brines passed through the limestones and precipitated saddle dolomite. Later, slightly cooler oil-rich brines precipitated mildly ferroan calcite wherever hydrocarbon accumulation had not already occurred. Our Sr isotopic studies suggest that both these brines were derived from underlying, and more deeply, buried Lias shales and that these brines were generated from Late Jurassic times onwards. Brine movements are likely to have been largely complete by the Late Cretaceous.