DUTTON, SHIRLEY P., SIGRID J. CLIFT, ROBERT L. FOLK, H. SCOTT HAMLIN, and BARARA A. MARIN, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Porosity Preservation by Early Siderite Cementation in Sonora Canyon Sandstones, Val Verde Basin, Southwest Texas

The Canyon sandstone play has produced almost 2 Tcf of gas from low-permeability, gas-bearing sandstone reservoirs in the Val Verde Basin. The Sonora Canyon interval comprises coalesced submarine-fan systems, forming a slope apron basinward (southwest) of a northwest-trending shelf margin. Reservoir sandstones are mainly slope-channel and fan-lobe facies deposited in continental slope and basin-floor environments in Virgilian to Wolfcampian time.

Sonora Canyon sandstones, which are fine-grained sublitharenites and litharenites (average composition Q<77>F<4>R<19>), have followed one of two main diagenetic pathways. Type l sandstones contain >10 % grain-rimming siderite cement that formed shortly after deposition of these deep-water marine sediments in an anoxic-nonsulfidic (post-oxic) geochemical environment. Bacterial reduction of iron accompanying organic matter decomposition increased the Fe{++} in the pore fluids, and in the absence of sulfide, siderite precipitated. Siderite-rich sandstones commonly occur in bedding-parallel layers that average 8 to 10 cm thick and are rarely >15 cm. Subspherical nannobacteriai bodies (0.05 to 0.15 (sym, mu)m) are revealed by etching siderite in warm HCl. These bodi s are locally abundant, ranging up to 100 per sq micrometer of siderite crystal surface; other portions of the crystals contain virtually no bodies. The bacteria presumably helped trigger siderite precipitation.

Abundant early siderite cement inhibited later porosity loss by compaction and quartz cementation; Type I sandstones average 33 % pre-cement porosity and 6 % quartz cement. Type II sandstones, which lack abundant siderite (<10 %), are extensively cemented by quartz (average = 11 %) and are much more compacted (16 % pre-cement porosity). Type I sandstones have higher porosity (7.9 %) and permeability (0.07 md) than Type II sandstones (average porosity = 6.4 %, geometric mean permeability = 0.01 md). Best matrix reservoir quality in Canyon sandstones occurs in siderite-cemented zones.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.