Fluid evolution and diagenesis of a Carboniferous channel sandstone in the Prince Colliery, Nova Scotia, Canada

Abstract

The subsea Prince Colliery of the Sydney Basin extracts coal at depths up to 330 m below sea level from beneath the Prince Sandstone, a channel body and local aquifer. The sandstone has up to 42.5 md permeability an d 19.5% porosity, mainly secondary porosity generated from near-complete dissolution of early formed calcite cement, remnants of which are present locally. Some pores are partially filled with quartz overgrowths on framework grains and aggregates of kaolinite, quartz, siderite, and minor illite. Formation waters within the Prince Sandstone are Na-Ca-C1 fluids with total dissolved solids (TDS) from 7,950 to 47,840 rag/L, increasing downdip, and high Br:C1 ratios. Salinity estimates using geophysical 10gs on wells drilled prior to mining agree well with the TDS values, confirming that the fluid samples are relatively prist!ne. The saline component originated as residual evaporative brines derived from the underlying Windsor Group, and probably entered the sandstone along faults during deep Permo-Triassic burial.The salirie formation waters have been diluted by fresh surficial fluids that, based on isotopic data, were warmer than present precipitation, suggesting interglacial or preglacial sources. These surficial fluids entered the Prince Sandstone aquifer along the nearby Mountain Fault and/or from the surface, and dilution may have taken place at any time since the mid-Mesozoic, when basin inversion brought the strata to a near-surface position. The dilute fluids may have promoted generation of secondary porosity within the sandstone, probably long after maturation of the coals, as there is little evidence in the coalfield that fluids released during maturation generated high porosity levels. RI~SUME La mine de charbon Prince, siture sous lamer, du bassin de Sydney extrait du charbon d'une profondeur allant jusqu'~t 330 m sons le niveau de lamer, ~ partir du dessous du Gr~s de Prince, une masse de chenal et un aquif~re local. Le grrs montre jusqu'a 42,5 md de permrabilit6 et 19,5% de porositr, surtout une porosit6 secondaire grnrrre a partir de la dissolution quasi complbte d'un ciment de calcite prrcoce, pour lequel on retrouve des restes localement. Quelques pores s0nt partiellement remplis d'excroissances de quartz sur la fraction principale des grains de quartz et d'agrrgats de kaolinite, de quartz, de sidrrite et d'illite en quantit6 mineure. Les eaux de formation ~t l'intrrieur du Grrs de Prince sont des fluides h Na-Ca-C1 montrant des solides dissous totaux (SDT) allant de 7 950 a 47 840 mg/1, s'accroissant en aval-pendage, et des hauts rapport Br:C1. Les estimations de salinit6 faites en utilisant les diagrammes grophysiques de puits forrs avant la mine, concordent bien avec les valeurs de SDT, confirmant que les 6chantillons de fluides sont relativement de premiere origine. Les composantes salines ont pour origine les saumures 6vaporatives rrsiduelles drrivres du Groupe de Windsor sous-jacent, qui ont probablement prnrtrres dans le gr~s en longeant des failles durant I'enfouissement profond d...

How important is carbonate dissolution in buried sandstones: evidences from petrography, porosity, experiments, and geochemical calculations

“…Instead, most intergranular pores without carbonate cements were interpreted as secondary pores formed by the dissolution of carbonate cements (Taylor et al 2010). Based on the CO 2 /organic acids leaching hypothesis, and the negative relationship between porosity and the amount of carbonate cements in reservoirs, extensive burial dissolution of carbonate minerals has also been suggested by many other authors in the last few decades (Dutton and Willis 1998;Gibling et al 2000;Harris and Bustin 2000;Higgs et al 2010;Irwin and Hurst 1983;Khidir and Catuneanu 2010;Kordi et al 2011;Mcbride 1988;Shanmugam 1984;Wilkinson et al 1997). Similar to Schmidt and McDonald (1979a, b), no convincing petrography evidence on carbonate dissolution was provided in these publications.…”