Thomas E. McKenna

Thermal Conductivity of Wilcox and Frio Sandstones in South Texas (Gulf of Mexico Basin)

Thermal conductivity and petrographic data are presented for verifying mechanistic models of sandstone thermal conductivity. We measured the thermal conductivity of 83 Wilcox and Frio sandstones from south Texas in the Gulf of Mexico sedimentary basin, and correlated conductivity to petrographic variables. Thermal conductivities of water-saturated sandstones at 20°C (68°F) and 3 MPa (435 psi) were measured on core plugs using a divided-bar apparatus. Thermal conductivity ranges from 2.06 to 5.73 W/m/K over a porosity range of 2.4 to 29.6%. Because of a higher quartz content, Wilcox sandstones at a given porosity are more conductive than Frio sandstones. A grain-matrix conductivity of 5.9 W/m/K is estimated for Wilcox sandstones; matrix conductivity is adequately described with an arithmetic mixing model. Thermal conductivities of clean ( 35% of the solids) sandstones, the dependence on quartz content can be dropped and thermal conductivities can be predicted with a linear decrease in conductivity with increasing porosity. These sandstones appear isotropic with respect to thermal conductivity.

Radiogenic Heat Production in Sedimentary Rocks of the Gulf of Mexico Basin, South Texas

Radiogenic heat production within the sedimentary section of the Gulf of Mexico basin is a significant source of heat. Radiogenic heat should be included in thermal models of this basin (and perhaps other sedimentary basins). We calculate that radiogenic heat may contribute up to 26% of the overall surface heat-flow density for an area in south Texas. Based on measurements of the radioactive decay rate of a-particles, potassium concentration, and bulk density, we calculate radiogenic heat production for Stuart City (Lower Cretaceous) limestones, Wilcox (Eocene) sandstones and mudrocks, and Frio (Oligocene) sandstones and mudrocks from south Texas. Heat production rates range from a low of 0.07 ±0.01 µW/m3 in clean Stuart City limestones to 2.21 ±0.24 µW/m3 in Frio mudrocks. Mean heat production rates for Wilcox sandstones, Frio sandstones, Wilcox mudrocks, and Frio mudrocks are 0.88, 1.19, 1.50, and 1.72 µW/m3, respectively. In general, the mudrocks produce about 30-40% more heat than stratigraphically equivalent sandstones. Frio rocks produce about 15% more heat than Wilcox rocks per unit volume of clastic rock (sandstone/mudrock). A one-dimensional heat- conduction model indicates that this radiogenic heat source has a significant effect on subsurface temperatures. If a thermal model were calibrated to observed temperatures by optimizing basal heat-flow density and ignoring sediment heat production, the extrapolated present-day temperature of a deeply buried source rock would be overestimated.

Potential salinity-driven free convection in a shale-rich sedimentary basin: Example from the Gulf of Mexico basin in south Texas

We investigate the plausibility of salinity-driven free (thermohaline) convection in sedimentary rocks of the south Texas part of the Gulf of Mexico basin using salinity data, Rayleigh number calculations, and numerical models. Previous studies speculated that free convection could account for high fluxes evidently required for diagenesis in the basin, but low-permeability shales are calculated to be a barrier to free convection. In the study area, salinity inversions occur either above or within the transition zone from hydropressures to overpressures. The positioning of brines over less saline fluids provides a significant buoyancy force. Rayleigh number calculations and numerical simulations suggest that homogeneous shaly systems are unstable near the high end of the expected ranges of shale permeability (10-15-10-16 m2). Numerical simulations show that the influx of brine into the permeable layers and permeability heterogeneity in the shales are both conducive to free convection. Simulations indicate that salinity-driven free convection can occur at lower permeabilities (10-16-10-18 m2) that may approximate the permeabilities of shales in the zone of extreme overpressures over geologic time. Simple Rayleigh numbers are inadequate to predict the occurrence of free convection in heterogeneous systems. Salinity-driven free convection at depths in some large sedimentary basins, such as the Gulf of Mexico, may be more common than expected.

Quantitative techniques in isotope chronostratigraphy

Abstract Modern techniques of signal to noise identification are used with several Plio-Pleistocene δ 18 O records to quantify correlations between records and to reconstruct quantitative measures of paleo-oceanographic processes. A type δ 18 O record for the Plio-Pleistocene is constructed and used as a preliminary reference section against which other δ 18 O records are compared. Construction of the TYPE record and correlation methods involve: (1) interpolative filters, (2) semblance and coherence, (3) auto-correlation and cross-correlation in time, and (4) power spectra, cross spectra and spectral coherency in frequency. By using these quantitative techniques we minimize, and hopefully with ultimately remove, the subjectivity of visual correlation. We also demonstrate the need to interpret the records within the resolution constraints of each method of signal identification.

Quantifying Tidal Mud Flat Elevations From Fixed-Platform Long-Wave Infrared Imagery

A procedure for estimating tidal mud flat topography from fixed-platform, long-wave infrared imagery is presented. Shallow water and low bearing capacity on many mud flats hinder traditional surveying methods by water craft, walking, or land vehicle. The approach utilizes identification of the intersection of water with the surface of the mud flat through a rising tide. Waterlines on mud flats are often indistinguishable to the naked eye and in visible-band imagery. Long-wave infrared imagery, relying on emitted radiance, provides a more distinct delineation between an exposed mud flat and water. The waterline is identified via an image intensity threshold and transferred to real-world coordinates using a geometrical transformation model that accounts for potential imager sway. Elevation estimates, interpolated to a uniform grid, show excellent agreement (absolute error generally less than 0.02 m) with ground truth elevations obtained using a sled-mounted global positioning survey system.