Ian D. Hutcheon

Geochemistry of Early Carbonate Cements in the Cardium Formation, Central Alberta

ABSTRACT Authigenic minerals and diagenetic textures in the marine conglomerates and sandstones of the Cretaceous Cardium Formation in the northeast Pembina area preserve a complex sequence of diagenetic events. Textural relationships observed in thin section and under the scanning electron microscope were used to determine the relative timing of diagenetic events in these marine sediments. Paragenetic sequences are similar for the conglomerate, sandstone, and siderite nodules in the enclosing shales. Extensive cementation by siderite and calcite occurred early in the diagenetic history, before any significant compaction. The geometry and distribution of lithofacies in the Cardium may have influenced the diagenesis and internal stratigraphy of the conglomerate. Overlying shale matrix conglomerate may have trapped upward-migrating fluids increased in buoyancy by dissolved CO2 produced by decay of organic matter in the enclosing shales. These fluids could have infiltrated the upper portion of open matrix conglomerate, causing cementation. Petrographic evidence shows alternating precipitation of siderite and pyrite, implying fluctuating activities of dissolved carbonate and hydrogen sulfide, probably caused by bacterially mediated processes during early diagenesis. Early sequences of siderite and pyrite precipitation can be related to zones of iron reduction, sulfate reduction, and, possibly, decarboxylation during diagenesis. Carbon and oxygen isotopic data indicate a systematic change in the isotopic compositions of calcite and siderite from -25 to -3013C and 018O, for cements early in the paragenetic sequence, to 0 to -513C and -15%18O for cements which are interpreted to occur later in the paragenetic sequence. This observation is consiste t with the influence of meteoric water on later stages of cement deposition in the Cardium, a unit considered to be marine and deposited well offshore on a shallow marine shelf. The intrusion of meteoric water far offshore may be related to changes in sea level. Many variables must be accounted for, but it is feasible, using the estimated regional dip of the Cardium at the time of deposition (0.005°), for a 1-m drop in sea level to push the freshwater--seawater interface seaward on the order of 100 km.

- Geochemical monitoring of gas-water-rock interaction at the iea Weyburn CO 2 Monitoring and Storage Project, Saskatchewan, Canada

Publisher Summary The Weyburn Oil Field is the site of EnCanas C02-injection enhanced oil recovery (EOR) project in a carbonate reservoir in southern Saskatchewan. An IEA-sponsored program was initiated to evaluate the potential for geological storage of greenhouse gases. Geochemical monitoring and modeling plays an important role in assessing gas-fluid-rock interactions. The geochemical dataset is the primary input for quantifying the short- and long-term (years to thousands of years) CO2 storage potential in the basinal brine, non-recoverable oil, and in newly precipitated minerals. Three geochemical processes have been found occurring in the area of the reservoir as a result of CO2 injection: (1) rapid CO2 dissolution in the brine and oil; (2) the CO2 “sweep” previously assessing the inaccessible portions of the reservoir; and (3) carbonate mineral dissolution due to acidification of the basinal brine. Equilibrium relations among gases, brines, and liquid hydrocarbons in the Weyburn field demonstrated a close correlation among dissolved CO2, free gaseous CO2, and the amount of injected CO2.

Mineralogical characterization of the Weyburn reservoir, Saskatchewan, Canada: Are mineral reactions driving injected CO2 storage?

Publisher Summary The Weyburn Oil Field is the site of EnCanas large C02-injecfion enhanced oil recovery project in southern Saskatchewan, Canada. The Weyburn field is one of a number of large oilfields that lie along the Mississippian subcrop belt on the northern part of the Williston Basin. In close cooperation with the International Energy Agency, an international multi-disciplinary research initiative to study the short and long-term potential of geological storage of CO2 in a carbonate reservoir has been established. Although the Weyburn Midale reservoir is dominantly calcitic and dolomitic, significant amounts of potentially reactive silicate minerals are present to assist in CO2 storage have grouped CO2 trapping into three types. Type 1 capture is defined as occurring when a mineral is precipitated incorporating the anion formed by the dissolved gas. Type 2 capture is defined as occurring when an acid gas is neutralized in solution forming a nonvolatile soluble salt and subsequently leading to brine formation. Type 3 capture is characterized by CO2 trapping in both solid and aqueous phases. The detailed mineralogy of the Weyburn reservoir, with particular attention paid to EnCanas flow unit nomenclature, is described. Mineral reaction modelers, assessing the long-term fate of injected anthropogenic CO2, may use the mineral modes of flow units as input variables.