Controls on diagenesis of Lower Cretaceous reservoir sandstones in the western Sable Subbasin, offshore Nova Scotia

Abstract

Lower Cretaceous deltaic sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. The influence of several factors on diagenesis has been investigated: depositional lithofacies, sea level changes, chemistry of basinal sediments and basinal fluid flux during burial. The distribution and chemistry of diagenetic minerals was determined from nine wells located along a dip section of the Sable Subbasin. Mineral type and paragenesis were characterized using a combination of optical petrography, back-scattered electron images, and electron microprobe analyses. Siderite is unusually abundant in marine sediments of the Scotian Basin and has thus been studied in detail. Siderite occurs in several generations. Early and late siderites are similar in chemical composition, suggesting buffering by pre-existing siderite, but rare low-Mg siderite is related to a greater contribution of meteoric water. Siderite has locally dissolved to create microporosity and has suppressed quartz overgrowths. Siderite is most common in those muddy prodeltaic lithofacies where there is the highest availability of detrital ilmenite. Reactive Fe released by breakdown of this ilmenite is responsible for the unusual presence of early siderite in marine sediments, with the Ca and Mg content of the siderite indicating fully marine waters. Lithofacies have a strong influence on early diagenetic mineral assemblages. Lithofacies deposited in the transgressive system tract have abundant early Fe-calcite and siderite. Early kaolinite occurs principally in proximal (fluvial and river mouth) lithofacies, where meteoric water was most likely available during the deposition. Contrary to other studies, we find little impact of sequence stratigraphy on diagenetic minerals except in the transgressive system tract. Mesogenetic minerals are related to flux of formation water and maturing hydrocarbon products, resulting first in pyrite and siderite and later in ankerite and ferroan calcite. The principal controls are interpreted to be high Fe 2+ and low Ca 2+ of formation waters and vertical movement of the waters along faults. Overall, the bulk chemical composition of terrigenous sediments and the depositional lithofacies are the most important factors controlling diagenetic minerals in the Lower Cretaceous of the Scotian Basin.