Distribution of diagenetic minerals in Lower Cretaceous sandstones and their relationship to stratigraphy and lithofacies: Glenelg, Thebaud and Chebucto fields, offshore Scotian Basin

Abstract

Diagenetic cements are of decisive importance in determining reservoir quality. It is therefore important to understand how such cements form. Only a few studies of diagenetic alteration have been published for the Scotian Basin. The spatial and temporal distribution of diagenetic cements has been interpreted in relationship to lithofacies and sequence stratigraphy of the Lower Cretaceous sandstones from the Glenelg, Chebucto, and Thebaud fields in the Sable Subbasin. Chlorite and illite coated grains occur in transgressive systems tracts (TST) in Glenelg N-49 and Thebaud I-93, and are cemented by Fe-calcite. Early kaolinite occurs as booklets and with vermicular stacking textures principally in sandstones immediately beneath the TSTs, particularly in cross-bedded, coarse-grained, channel sandstones. Illite occurs as fibrous crystals, which in the Chebucto K-90 well are included in ankerite. Fe-rich chlorite rims, found principally in samples from the Thebaud field, have developed from earlier Fe-rich clay. Pore-filling chlorite occurs in contact with detrital quartz grains, on which diagenetic quartz cement has not developed and this chlorite is commonly associated with illite. Quartz cement, well developed in medium- and coarse-grained sandstones, postdates kaolinite and predates most other cements. Calcite, Fe-calcite, Mg-calcite, ankerite and siderite are the major cementing minerals in the studied wells. In those wells, two siderite cements, rich in Mg (15-30 mol.%), were defined; the earlier one (siderite I) occurs in lithofacies 0 as large, corroded grains. The late microcrystalline siderite II (< 10 ?m) occurs in lithofacies 3 of TST. It forms the tiny crystals that fringe detrital grains and fill intercrystalline micropores. Mesodiagenetic ankerite is present only in the Glenelg E-58, Thebaud #5, and Thebaud #3 wells, whereas Fe-calcite is absent only from Thebaud #5 well. These differences might be related to the presence of different eogenetic minerals, or to differences in hydrocarbon charge history. In samples from the Glenelg field perthite is replaced by Fe-calcite. Late framboidal pyrite in carbonate cement indicates burial under both reducing and alkaline conditions. Rare traces of a P-rich mineral, presumably francolite, are found in the Glenelg wells, associated with illite and calcite cements. This study demonstrates that the distribution of diagenetic minerals and their impact on reservoir-quality evolution can be better elucidated when linked to a sequence stratigraphic framework.