Early Siderite Cementation as a Control on Reservoir Quality in Submarine Fan Sandstones, Sonora Canyon Gas Play, Val Verde Basin, Texas
Shirley P. Dutton, H. Scott Hamlin, Robert L. Folk, Sigrid J. Clift, 1996. "Early Siderite Cementation as a Control on Reservoir Quality in Submarine Fan Sandstones, Sonora Canyon Gas Play, Val Verde Basin, Texas", Siliciclastic Diagenesis and Fluid Flow: Concepts and Applications, Laura J. Crossey, Robert Loucks, Matthew W. Totten, Peter A Scholle
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Early precipitation of siderite cement in Sonora Canyon sandstones (Wolfcampian) in the Val Verde Basin, southwest Texas strongly influenced later diagenesis and reservoir quality in these low-permeability gas reservoirs. Sandstones of the Sonora Canyon interval were deposited in water depths of 100 to 500 m in coalesced submarine fans basinward (southwest) of the northwest-trending shelf margin. Sonora Canyon sandstones are composed of hundreds of feet of fanlobe turbidites and local channel-fill facies deposited on the continental slope and basin floor.
Sonora Canyon sandstones are fine-grained sublitharenites and litharenites (average composition Q77F9R19). Grain-rimming siderite rhombs 1 to 2 μm long were the earliest major cement to precipitate, in volumes ranging from 0 to 38%. Siderite is concentrateti in bedding-parallel layers 8 to 10 cm thick or in irregular patches 3 to 8 cm in diameter. Isotopic composition of the siderite falls in a narrow range, 5I3C averaging 2.4‰ (PDB) and 515O averaging 31.1 ‰ (SMOW). The isotopic data indicate that siderite cement formed in a methanogenic geochemical environment at a burial depth of about 300 to 600 m (27°C) from sea-water-derived pore fluids (δ18O = 0‰). Bacterial reduction of iron accompanying anaerobic bacterial methanogenesis increased the Fe+2 in the pore fluids and, in the absence of sulfide, siderite precipitated. Subspherical nan-nobacterial bodies (0.05 to 0.15 μm) are revealed by etching siderite in warm HCl. These bodies are locally abundant, ranging to 100 per μm2 of siderite crystal surface; other parts of the crystals contain virmally no bodies. The bacteria presumably helped trigger siderite precipitation.
Abundant early siderite inhibited later porosity loss by compaclion and quartz cementation; siderite-rich sandstones (containing a 10% siderite) average 33% minuscement porosity and 6% quartz cement. Siderite-poor sandstones (<10%), are extensively cemented by quartz (average = 11%) and are much more compacted (16% minus-cement porosity). Siderite-rich sandstones retain higher porosity (7.9%) and permeability (0.042 md) than do siderite-poor sandstones (average porosity = 6.4%, geometric mean permeability = 0.006 md). Best matrix reservoir quality in Sonora Canyon sandstones occurs in siderite-cemented zones.
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Siliciclastic Diagenesis and Fluid Flow: Concepts and Applications
Research in the area of siliciclastic diagenesis has historically incorporated advances in related disciplines such as petrography and petrophysics, mineralogy, geochemistry, organic geochemistry, stratigraphy and basin analysis, and more recently, fluid flow. While the collection of papers in this publication covers a broad range of topics, an underlying theme is the importance of fluid flow in diagenetic processes. The mineralogy, texture and geochemistry of authigenic minerals provide constraints for fluid flow models, while formation waters provide modern snapshots of pore fluid evolution. Separated into two sections (Part I: Concepts and Part II: Applications), conceptual and practical applications are both represented.