THAYER, PAUL A., Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, NC

ABSTRACT: Petrology and Reservoir Characteristics of Upper Triassic Sandstones, Subsurface Dunbarton Basin, SC

Study of core from five wells in the buried Dunbarton basin indicates that the sandstones are fluvial rift valley deposits of continental block provenance. Point counts of 45 thin sections show mean QFL = 51:38:11, Q/F = 1.05, and P/F = 0.73. Average chemical composition is: SiO<2>, 72.8%; M<2>0<3>, 11.6%; CaO, 4.44%; MgO, 0.34%; NO, 4.13%; K<2>O, 0.48%; Fe<2>O<3>, 1.95%; MnO, 0.04%; TiO<2>, 0.30%; and P<2>O<5>, 0.08%. The sandstones are moderately to poorly sorted, fine to very coarse-grained plagioclase arkose, lithic arkose, and feldspathic litharenite. Textural and mineralogical immaturity, chemical composition, and framework mineralogy indicate derivation from a nearby metamorphic terrane composed of gneiss and schist with min r amphitolite, granite, quartzite, and metavolcanic rock.

Total porosity of 36 sandstones averages 7.2 +/- 5.2%, whereas mean effective porosity >1 micrometers is 4.1 +/- 2.4% Geometric mean permeability (horizontal) is 0.1 mD. Study of mercury injection curves, SEM photomicrographs, and blue dyed, epoxy-impregnated thin sections demonstrates that most pores are micro-sized with pore-aperture radii less than 0.5 micrometers. Isolated grain dissolution pores comprise less than 30% of total pore space and formed by removal of feldspar, unstable heavy minerals, calcite replacements of feldspar, and labile rock fragments. Low matrix permeability results from the high percentages of allogenic and authigenic clays, tight grain packing, squeezing of ductile rock fragments into primary intergranular pores, and cementation.

Triassic sandstones have undergone considerable burial and are in the advanced stage of diagenesis. Diagenetic events include physical and chemical compaction, grain dissolution, and cementation. Authigenic replacements and cements include chlorite, illite, kaolinite, calcite, hematite, quartz, albite, pyrite, and laumontite.

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