The Carboniferous Lower Atoka outcrops in the Arkoma Basin of Central Arkansas contain turbidite deposits in a channelized and unchannelized submarine fan setting. The objectives of the study were to determine depositional characteristics and reservoir performance of these fine-grained submarine fan deposits. Four outcrops (Highway 5, Perryville, Chula, and Danville) were studied and described in detail emphasizing sedimentary facies, vertical succession of beds, sedimentary structures, and other small-scaled features that cannot be resolved in subsurface deposits. Five distinct sedimentary facies were recognized: Facies A - massive sandstones, Facies B - thin-bedded sandstones with a mudstone drape, Facies C - interbedded thin sandstones and mudstones, Facies D - chaotic deposits, and Facies E - mudstone. Facies and sedimentary characteristics of the outcrops provide clues as to which deepwater sub-environment the facies were deposited. On the basis of field criteria, the outcrops were characterized as channelized, channel or channel margin deposits, or unchannelized, sheet sand deposits. Channel deposits are massive fine-grained sandstones (facies A), with occasional scouring at the base and rip-up clasts throughout. Channel margin deposits consist predominately of interbedded thin sandstones and mudstones (facies C) with many of the sandstone beds containing small-scaled ripples and laminations. Sheet sand deposits are from lower-energy flows with the presence of more sedimentary structures, less scouring and a higher net-to-gross than the channelized deposits. Highway 5 and Perryville outcrops were deposited in a middle fan environment as channel and channel margin deposits. Danville and Chula outcrops were deposited in a lower fan setting as sheet sands. There are several potential source terranes for these deposits determined from point counting and detailed analysis of tourmaline crystals, which suggest that the main source consisted of sedimentary and metasedimetary rocks from the Appalachians with possible sediment transport from the craton interior: the Illinois Basin.

While the composition and origin of brines in deep sedimentary basins has been extensively discussed, the composition and origin of low salinity waters is not as well documented. Since the 1960s, the presence of deep, low-salinity waters has been observed in some sedimentary basins and is commonly present in overpressured sections. The episodic release of low salinity, overpressured fluids upward into sediments containing high salinity formation waters likely occurs at <100-year intervals. Because there is a growing body of evidence that suggests mixing formation waters of varying salinity could induce the dissolution and precipitation of minerals, it is important to have detailed information about chemical compositions of both end-member fluids to adequately predict such mixing results. This study concludes that low salinity waters (<35 g/l) are generally not unique in major solute composition when compared to high salinity waters (>35 g/l). On log-log plots, monovalent cations plot along 1:1 slopes with respect to salinity and total anionic charge, while divalent cations plot along 2:1 slopes. However, this study concludes that more compositional variability exists at salinities less than seawater and lower anionic charge values. Also, major cations and Cl correlates better with anionic charge than with salinity. While Cl plots on a 1:1 slope as a function of salinity in all waters in southwest Louisiana, other basins in this study begin to show variability at salinities <10,000 mg/l. Spatial analyses from this study suggest the origin of low salinity fluids in southwest Louisiana present at depths greater than 2500 m, are not likely meteoric in origin. Southwest Louisiana water compositions are most likely controlled by rock-buffering with ambient mineral phases. A calculation performed as part of this study showed that the smectite to illite transition could reduce salinity up to 43% in shales. Considering this and the documented presence of illite along the Gulf Coast as well as the stability of illite in the waters presented in this study, it is reasonable to conclude deep, low-salinity waters in southwest Louisiana originate in substantial part from the smectite to illite transition.

Overpressured Eocene Wilcox sandstones in the Newton County, Texas Sabine Tram field contain water with salinities ranging from 14 to 28 g/L. The solutes appear to fall loosely into three groups. Na, Ca, and Mg follow systematics previously known globally for saline formation waters whereby their concentrations are controlled by fluid-mineral equilibrium. Cl, Br, I, and B are conservatively diluted constituents of some saline endmember that is not solely connate marine. Potassium does not follow any known basinal systematic, and the formation water is supersaturated with respect to SiO2 by an order of magnitude. Meteoric water is not a probable source of fresh water in the production interval. Diluting water is suspected to be produced by illitization, but could in fact be any water low in dissolved constituents. Mass balance calculations indicate that mixing water of marine salinity with shale dehydration water could produce waters with salinities in the range of values seen at Sabine Tram. Isotopic compositions indicate a diagenetic or saline endmember that is depleted in deuterium and enriched in 18O. It is apparent that multiple contributing sources and reactions are progressing simultaneously in the study area. The possibility is proposed here that residence times are likely short, and the concentrations of the solutes represent multiple, sometimes spatially localized, reactions in progress. A downdip, higher temperature source may possibly be contributing constituents to the system. The data set for Sabine Tram provides an extension of knowledge regarding sedimentary basinal chemical systematics, and characterization of one possible endmember in the Gulf of Mexico sedimentary suite of formation water compositions.

The relations between overpressuring, diagenesis, and fluid flow in sedimentary basins are complex and multifaceted. The Matagorda Island 519 field (MI 519), offshore Texas, provides an excellent area for investigating some of these relations. The top of overpressure at MI 519 occurs at a depth of 3.5 to 3.8 km in a Lower Miocene deltaic sequence. On the basis of log-derived lithostratigraphy, the pressure seal does not appear to be lithologic in origin. Geochemical, mineralogical, and cuttings information indicate instead that the precipitation of diagenetic calcite and possibly quartz cements has been the major factor in seal development. Stratigraphic variation in mudstone chemistry indicates diagenesis has been an open-system process, with significant loss of Ca, Si, Mg, and Fe and gain of K in sediments below the pressure seal. Fluid pressures calculated from shale resistivities provide evidence for several vertically-stacked overpressured compartments at MI 519. Lateral sealing within the overpressured section may be provided by faults and precipitation of diagenetic cements within faults. In contrast to other areas of the Gulf of Mexico Basin, overpressure development at MI 519 does not appear to be due to compaction disequilibrium because of the lack of significant post-Miocene deposition and a lack of a reversal in mudstone porosity below the top of overpressure. More likely causes of overpressuring are clay mineral dewatering, petroleum generation, and the presence of a large column of natural gas. At least six stages of fluid flow and/or diagenetic development have occurred at the field: 1) calcite cementation within preferred intervals from fluids that originated by dissolution of updip salt domes, 2) deep overpressure development and upward focused flow of underlying Mesozoic brines and the development of secondary porosity in reservoir beds by carbonate dissolution, 3) precipitation of a seal by mixing of deeply-sourced and updip-sourced fluids, 4) hydrocarbon generation and shallow overpressure development, with hydrocarbons filling in porosity created by calcite dissolution, 5) hard overpressure development from smectite dehydration, and 6) development of a shallow freshwater lens during the Pleistocene lowstand.

Massachusetts Institute of Technology. Dept. of Geology and Geophysics. Thesis. 1965. M.S. / M.S.

Massachusetts Institute of Technology. Dept. of Geology and Geophysics. Thesis. 1966. B.S. / Includes 17 unnumbered pages. / Bibliography: leaves 18-20. / B.S.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Geology and Geophysics, 1968. / Bibliography: leaves 25-26. / by Laurence A. Wood. / B.S.

Thesis. (Ph.D.) Massachusetts Institute of Technology. Dept. of Geology and Geophysics, 1956. / Vita. / Bibliography: leaves [120-123]. / by Lawrence Strickland. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Geology and Geophysics, 1962. / Includes bibliographical references (leaves 53-54). / by Edwin G. Paulson. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Geology and Geophysics, 1963. / Includes bibliographical references (leaves 59-60). / by Harold Payson, Jr. / M.S.