Depositional environments, diagenesis and porosity relationships of the Canyon Sands, Edwards and Sutton Counties, Texas
Huang, Fu-wen Frank
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Upper Pennsylvanian sandstones on a portion of the Northern shelf of the Val Verde Basin serve as common reservoirs for hydrocarbons. They occur primarily in stratigraphic traps and are not related to structure. The sandstones occur within a thick sequence of clastics that was deposited as parts of submarine fans occupying the shelf slope basinward of the Northern Shelf of the Val Verde Basin. The sandstones were studied in fourteen core samples from four wells: Berger No. 1 and 3, Ward No. 4, and Shanklin No. 1 - 10 in Sutton and Edwards counties, Texas, at depths ranging from 2521.0 to 6401.0 feet. In addition to cores, caliper, gamma ray and compensated density logs of three wells and core analyses from two wells also were studied. The sandstone show a suite of sedimentary structures and facies associations which suggest deposition by turbidity currents. Several individual fans have been delineated by drilling, and facies relationships within these fans are recognizable. Outer and inner fan facies with channelized intervals are established on bedding types. The outer fan facies of Shanklin No. 1-10 Canyon sandstone is characterized by generally complete Bouma sequences (Tabcde and Tabce) fan units less than 1.0 foot thick. In the inner fan facies of the Berger No. 3 and Ward No. 4 wells, Canyon sandstones are represented by thicker beds of incomplete Bouma sequences (Ta Te, Ta Tb Te) that range in thickness from 1.0 to 5.0 feet. These sequences are interpretated as channelized turbidites. Compositionally, the Canyon sandstones are characterized as subarkoses to sedlitharenites. Detrital composition is relatively constant within the basin, However, the ceraents found in the Canyon sandstone vary significantly due to diagenesis. Calcite, silica, clay minerals, and siderite are important cementing agents. The inner-fan channel sandstones of Ward No. 4 and Berger No. 3 wells have the highest porosities and permeabilies. Study reveals that the precipitation of siderite graincoatings has impeded growth of quartz overgrowths and thus preserved most of the primary sandstone porosities and permeabilities. Reservoir properties of porosity and permeability are primarily controlled by sedimentary environraents and secondarily controlled by diagenesis. The paragenetic sequence infered is: 1. siderite filras, 2. quartz and feldspar overgrowths, 3. kaolinite and other authigenic clays, 4. microquartz, 5. ferrocalcite and doloraite cements, and 6. hydrocarbon emplacement.
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