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Abstract

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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