Pore water evolution in sandstones of the Groundhog Coalfield, northern Bowser Basin, British Columbia

The succession of sandstone cements in chert and volcanic lithic arenites a nd wackes from the northern Bowser Basin of British Columbia comprises a re cord of diagenesis in shallow marine, deltaic, and coastal plain siliciclas tic sediments that pass through the oil window and reach temperatures near the onset of metamorphism. The succession of cements is consistent with sea water in the sandstones mixing with acid waters derived from dewatering of interbedded organic rich muds. Sandstone cement paragenesis includes seven discrete cement stages. From earliest to latest the cement stages are: (1) pore-lining chlorite; (2) pore-lining to pore-filling illite; (3) pore-fill ing kaolinite; (4) oil migration through some of the remaining connected po res; (5) chlorite dissolution; (6) quartz cement; and (7) calcite cement. T hese seven cement stages are interpreted as a record of the evolution of po re waters circulating through the sandstones after burial. The earliest cem ent stages, as well as the depositional environments, are compatible with s eawater as the initial pore fluid. Seawater composition changed during tran sport through the sandstones, first by loss of Mg2+ and Fe2+ during chlorit e precipitation (stage 1). Dewatering of interbedded organic-rich mudstones probably added Mg2+ and Fe2+ to partially buffer the loss of these cations to chlorite. Acids produced during breakdown of organic matter are presume d to have mixed into sandstone pore fluids due to further compaction of the muds, leading to reduction of initial alkalinity. Reduction in alkalinity, in turn, favours change from chlorite to illite precipitation (stage 2), a nd finally to kaolinite (stage 3). Pore waters likely reached their peak ac idity at the time of oil migration (stage 4). Chlorite dissolution (stage 5 ) and quartz precipitation (stage 6) occurred when pores were filled by the se hydrocarbon-bearing and presumably acidic fluids. Fluid inclusions in fr acture-filling quartz cements contain petroleum, high-pressure methane, and methane-rich aqueous solutions. Homogenization temperatures from primary t wo-phase inclusions are consistent with quartz cementation during progressi ve heating between approximately 100 and 200 degrees C. Following quartz pr ecipitation, alkaline pore waters were re-established, as evidenced by late -stage calcite cement (stage 7). (C) 1999 Elsevier Science B.V. All rights reserved.