Occurrence and distribution of barite and sphalerite cement in the Mesozoic sandstones of the Scotian Basin

Abstract

Cementation of sandstone by minor barite and sphalerite is widespread in the Scotian Basin at burial depths > 2 km. These minerals provide further constraints on diagenetic processes and the nature of reservoir-quality risk in the exploration of this basin. This report documents the mode of occurrence, geochemistry, and paragenetic sequence of diagenetic barite and sphalerite and the structural character of veins of these minerals. The texture and geochemistry of these minerals were analysed by scanning electron microscopy and electron microprobe on samples from conventional core from 17 wells, sampling the geographic and stratigraphic range of the basin. Barite and sphalerite post-date silica and carbonate cementation, occurring in veins or occupying secondary porosity. They occur with diagenetic chlorite, pyrite, and titania minerals. Sphalerite is common in Tithonian rocks of the Eastern Scotian Basin and in Early Cretaceous rocks of the Central Scotian Basin. Barite is also common in the Eastern Scotian Basin, particularly in the Tithonian, but is less common and shows no systematic variation with stratigraphy in the Central Scotian Basin. Late barite veins cut Lower Cretaceous sandstones in the wells Wenonah J-75, South Desbarres O-76, and Onondaga O-95. The Sable Island C-67 well contains an unusual diagenetic assemblage including kutnohorite, late Mn-siderite and pyrite. Late dolomite is present in the Onondaga O-95 well. Mississippi-Valley-type ore deposits show many similarities to the barite-sphalerite diagenesis described here, in the association with dolomitizing and Mn-rich fluids, the evidence for synchronous emplacement of hydrocarbons, and the ability of the fluids to create secondary porosity in quartz. The distribution of barite and sphalerite can be used to constrain models of fluid flow in the basin.