Lawrence M. Cathles

The Viscosity of the Earth's Mantle

Approximately 12,000 years ago, at the end of the last ice age, the three kilometers of ice that covered Canada, the large European glaciers in Fennoscandia and Siberia, and many other minor glaciers melted quickly. The resulting meltwaters increased the depth of the worlds oceans by about 110 meters. The earths response to this redistribution of loads was one of fluid flow. By studying the way in which that flow occurred, much can be learned about the viscosity structure of the earths mantle: that is, how the fluid properties of the earth vary with depth. In this volume Lawrence M. Cathles III sets out to lay the theoretical foundations necessary to model the isostatic (fluid) adjustment of a self-gravitating viscoelastic sphere, such as the earth, and to use these foundations, together with geological evidence of the way the earth responded to the pleistocene land redistributions, to study the viscosity of the mantle.The author argues that the viscosity of the entire mantle is very close to 1022 poise, except for a low-viscosity channel, about 75 kilometers thick, in the uppermost mantle. This conclusion differs sharply from the common view that the earths mantle becomes very viscous (1027 poise) below a depth of about 1000 kilometers./pOriginally published in 1975.The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Electrical conductivity in shaly sands with geophysical applications

We develop a new electrical conductivity equation based on Bussians model and accounting for the different behavior of ions in the pore space. The tortuosity of the transport of anions is independent of the salinity and corresponds to the bulk tortuosity of the pore space which is given by the product of the electrical formation factor F and the porosity ϕ. For the cations, the situation is different. At high salinities, the dominant paths for the electromigration of the cations are located in the interconnected pore space, and the tortuosity for the transport of cations is therefore the bulk tortuosity. As the salinity decreases, the dominant paths for transport of the cations shift from the pore space to the mineral water interface and consequently are subject to different tortuosities. This shift occurs at salinities corresponding to ξ/t(+)f ∼ 1, where ξ is the ratio between the surface conductivity of the grains and the electrolyte conductivity, and t(+)f is the Hittorf transport number for cations in the electrolyte. The electrical conductivity of granular porous media is determined as a function of pore fluid salinity, temperature, water and gas saturations, shale content, and porosity. The model provides a very good explanation for the variation of electrical conductivity with these parameters. Surface conduction at the mineral water interface is described with the Stern theory of the electrical double layer and is shown to be independent of the salinity in shaly sands above 10−3 mol L−1. The model is applied to in situ salinity determination in the Gulf Coast, and it provides realistic salinity profiles in agreement with sampled pore water. The results clearly demonstrate the applicability of the equations to well log interpretation of shaly sands.

Streaming potential in porous media: 2. Theory and application to geothermal systems

Self-potential electric and magnetic anomalies are increasingly being observed associated with hydrothermal fields, volcanic activity, and subsurface water flow. Until now a formal theoretical basis for predicting streaming potential of porous materials has not been available. We develop here a model giving both the macroscopic constitutive equations and the material properties entering these equations. The material properties, like the streaming potential coupling coefficient, depend on pore fluid salinity, temperature, water and gas saturations, mean grain diameter, and porosity. Some aspects of the model are directly tested with success against laboratory data. The streaming potential increases with temperature, grain size, and gas saturation, and decreases with salinity. At the scale of geological structures the model provides an explanation for the presence of kilometer-scale dipolar self-potential anomalies in geothermal systems and volcanoes. Positive self-potential anomalies are associated with fluid discharge areas, whereas negative self-potential anomalies are associated with fluid recharge areas. Self-potential anomaly maps determined at the surface of active hydrothermal fields appear to be a powerful way of mapping the fluid recharge and discharge areas. In the case of free convection the vorticities of the convection pattern generate a magnetic field. The greater these vorticities, the greater the associated magnetic field. It follows that hydrothermal systems act as natural geobatteries because of the flow of pore fluids in the subsurface of these systems.

A commentary on “The greenhouse-gas footprint of natural gas in shale formations” by R.W. Howarth, R. Santoro, and Anthony Ingraffea

Natural gas is widely considered to be an environmentally cleaner fuel than coal because it does not produce detrimental by-products such as sulfur, mercury, ash and particulates and because it provides twice the energy per unit of weight with half the carbon footprint during combustion. These points are not in dispute. However, in their recent publication in Climatic Change Letters, Howarth et al. (2011) report that their life-cycle evaluation of shale gas drilling suggests that shale gas has a larger GHG footprint than coal and that this larger footprint “undercuts the logic of its use as a bridging fuel over the coming decades”. We argue here that their analysis is seriously flawed in that they significantly overestimate the fugitive emissions associated with unconventional gas extraction, undervalue the contribution of “green technologies” to reducing those emissions to a level approaching that of conventional gas, base their comparison between gas and coal on heat rather than electricity generation (almost the sole use of coal), and assume a time interval over which to compute the relative climate impact of gas compared to coal that does not capture the contrast between the long residence time of CO2 and the short residence time of methane in the atmosphere. High leakage rates, a short methane GWP, and comparison in terms of heat content are the inappropriate bases upon which Howarth et al. ground their claim that gas could be twice as bad as coal in its greenhouse impact. Using more reasonable leakage rates and bases of comparison, shale gas has a GHG footprint that is half and perhaps a third that of coal.

Scales and Effects of Fluid Flow in the Upper Crust

Two of the most important agents of geological change, solar energy and internal heat from the mantle, meet and battle for dominance in propelling aqueous and related fluids in the earths upper crust. Which prevails and how they interact are subjects of active research. Recent work has demonstrated that both agents can propel fluids over nearly continental-scale distances in a fashion that influences a host of important geological processes and leaves a record in chemical alteration, mineral deposits, and hydrocarbon resources.

Phase fractionation at South Eugene Island Block 330

Persistent gas flux can dissolve, remobilize and alter reservoired or migrating oil through a process of phase fractionation. Moving gas, when flowing through an oil, can dissolve large fractions of that oil. The composition of the oil dissolved in the gas is dependent on the pressure-temperature conditions of the oil and the fluid flow history of the basin. The composition of the residual oil can be interpreted to yield both the depth at which the oil fractionated and the volume of gas required to fractionate the oil. South Eugene Island Block 330 in the U.S. Gulf Coast is a hydrocarbon province that has recently experienced large gas fluxes. Some of the oils in the region show signs of progressive fractionation and remobilization by gas transport. For example, the oils are more aromatic and less paraffinitic than unfractionated oils of similar maturity from the same area. The altered oils are also depleted of light n-alkanes. We have developed a computer-based model of oil alteration based on a fluid phase equilibria algorithm to simulate progressive fractionation of oil by gas. Application of the model to the South Eugene Island Block 330 area shows that several of the oils in the area have compositions that are compatible with alteration caused by equilibrating with approximately 12 to 14 mol of gas per mol of oil (2 to 2.7 g of gas per g of EI oil). The oils appear to have fractionated at approximately the depths of their present reservoirs. The model has great potential to examine hydrocarbon fluids for evidence of past migration and mixing.

Gas washing of oil along a regional transect, offshore Louisiana

Abstract Gas chromatogram data for 219 oils in a 190 km N–S transect offshore Louisiana reveal a spatially coherent pattern of compositional change which is caused by gas washing. Near the Louisiana shoreline, as much as 91% of the original n-alkanes have been removed from the oils. The maximum intensity of depletion decreases southward in a nearly regular fashion to nil at the Jolliet field 190 km offshore. The oils show a parallel change in the maximum carbon number of the removed n-alkanes, implying that the pressure at which gas washing took place also decreased in the offshore direction. The systematic change in maximum extent of depletion crosscuts tectonostratigraphic boundaries as well as oil source provinces. Models of gas washing suggest that the maximum depth of washing reflects the distribution of deeply buried continuous sands, suggesting deep sands may have provided sites for efficient gas–oil interaction.

A compositional kinetic model of hydrate crystallization and dissolution

Hydrates are crystallizing near and at the seafloor from gas vents on shelves where the sedimentation rate is high and hydrocarbons are being generated. When seafloor temperature, vent rate, or vent gas composition changes, these hydrates may become unstable and decompose. We have constructed a compositional kinetic model of hydrate crystallization and dissolution that can address these issues. The model crystallizes hydrate in compositional bins and allows each to dissolve at either a kinetically or compositionally controlled rate if vent gas composition or temperature causes it to become unstable. We empirically calibrate the model to venting at the Bush Hill hydrate mound in the offshore Louisiana Gulf of Mexico, show how variations in venting rate crystallize hydrate of diverse composition in the subsurface, and investigate how bottom water temperature variations similar to those measured could increase the rate of gas venting by destabilizing hydrates within a few meters of the seafloor. We show that increases in bottom water temperature can cause gas venting rates to increase similar to100%, as suggested by recent measurements, only if the dissolution kinetics are fast compared to the empirically calibrated crystallization kinetics and dissolution gases are removed rapidly enough that they do not thermodynamically inhibit the rate of dissolution. Model characteristics required to further investigate hydrate mound construction are identified.

Prediction of thermal conductivity in reservoir rocks using fabric theory

An accurate prediction of the thermal conductivity of reservoir rocks in the subsurface is extremely important for a quantitative analysis of basin thermal history and hydrocarbon maturation. A model for calculating the thermal conductivity of reservoir rocks as a function of mineral composition, porosity, fluid type, and temperature has been developed based on fabric theory and experimental data. The study indicates that thermal conductivities of reservoir rocks are dependent on the volume fraction of components (minerals, porosity, and fluids), the temperature, and the fraction of series elements (FSE) which represents the way that the mineral components aggregate. The sensitivity test of the fabric model shows that quartz is the most sensitive mineral for the thermal conductivity of clastic rocks. The study results indicate that the FSE value is very critical. Different lithologies have different optimum FSE values because of different textures and sedimentary structures. The optimum FSE values are defined as those which result in the least error in the model computation of the thermal conductivity of the rocks. These values are 0.444 for water-saturated clay rocks, 0.498 for water-saturated sandstones, and 0.337 for water-saturated carbonates. Compared with the geometric mean model, the fabric model yields better results for the thermal conductivity, largely because the model parameters can be adjusted to satisfy different lithologies and to minimize the mean errors. The fabric model provides a good approach for estimating paleothermal conductivity in complex rock systems based on the mineral composition and pore fluid saturation of the rocks.

Mississippi Valley-type deposits. Products of brine expulsion by eustatically induced hydrocarbon generation An example from northwestern Australia

Mechanisms for fluid flow such as compactive dewatering, gravity-driven hydrologic flow, and seismic pumping are not geologically plausible for Mississippi Valley-type (MVT) mineralization in the Lennard Shelf, Canning Basin, northwestern Australia. Sulfides at Cadjebut and the other Lennard Shelf MVT deposits precipitated from overpressured, hydrocarbon-rich brines. The deposits lie below a major mid-Carboniferous unconformity that is linked to a global sea-level fall. Independent evidence at Cadjebut also constrains the time of mineralization to the mid-Carboniferous. A sea-level fall at this time could have raised the sediment-water interface above the thermocline, thereby increasing the temperature of the basin by [minus]16[degree]C. Model calculations show that a rise in basin temperature would induce sufficient hydrocarbon gas maturation to expel 1005 km[sup 3] of brine. This brine volume would be more than sufficient to transport and deposit the metal budget of the known MVT deposits on the Lennard Shelf. The existence worldwide of MVT deposits below unconformities suggests that sea-level-induced hydrocarbon generation could have assisted MVT mineralization in other provinces. 33 refs., 2 figs.