Steven Losh

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.

Vertical and Lateral Fluid Flow Related to a Large Growth Fault, South Eugene Island Block 330 Field, Offshore Louisiana

Data from sediments in and near a large growth fault adjacent to the giant South Eugene Island Block 330 field, offshore Louisiana, indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time. Core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux. Sediments within and adjacent to the fault zone in the U.S. Department of Energy-Pennzoil Pathfinder well at about 2200 m SSTVD (subsea true vertical depth) showed little paleothermal or geochemical evidence for throughgoing fluid flow. The sediments were characterized by low vitrinite reflectances (Ro), averaging 0.3% Ro, moderate to high d18O and d13C values, and little difference in major or trace element composition between deformed and undeformed sediments. In contrast, faulted sediments from the A6ST well, which intersects the A fault at 1993 m SSTVD, show evidence for a paleothermal anomaly (0.55% Ro) and depleted d18O and d13C values. Sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well; this effect diminishes with distance from this gouge zone. Cuttings from other wells in South Eugene Island Block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone. Overall, indicators of mass and heat flux indicate the main growth fault zone in South Eugene Island Block 330 has acted as a conduit for ascending fluids, although the cumulative fluxes vary along strike. This conclusion is corroborated by oil and gas distribution in downthrown sands in Blocks 330 and 331, which identify the fault system in northwestern Block 330 as a major feeder. Simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by A6ST well was short-lived, having a duration less than 150 yr. The anomaly could have been produced by a 2 ´ 106 m3 pulse of fluid ascending the fault at an actual velocity of over 1 km/yr (Darcy flux of 330 m/yr) from 3 km deeper in the basin. Simple Darcy law computation indicates a transient fault permeability on the order of 110 md during this flow. Pulsing of fluid up the fault was probably the norm, although most flow did not produce such strong thermal anomalies as the one detected in the A6ST well. Analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it. The hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault. Fault permeability is best understood in terms of effective stress. Under ambient conditions, the fault is at high pressure relative to downthrown reservoirs. A pulse of high-pressure fluid ascending End page 244---------------- the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability. If the fluid is in an area of the fault adjacent to downthrown, relatively low pressure reservoir sands, the fluid will discharge into them. Permeability in and adjacent to the fault then decreases, such that fluid cannot reenter the fault zone and escape from the reservoir.

Stable isotope and modeling studies of fluid-rock interaction associated with the Snake Range and Mormon Peak detachment faults, Nevada

Stable isotope and fluid-inclusion data were obtained from rocks from traverses within and above the Snake Range and Mormon Peak detachments in Nevada in order to evaluate fluid sources and the nature of fluid flow associated with detachment faults during faulting, and to determine whether the initial depth of the detachment fault influenced the nature of syntectonic fluid flow. Oxygen and hydrogen isotope data indicate the detachment faults were infiltrated by meteoric water over a range of structural levels; however, only the upper-plate rocks and brittlely deformed portions of the faults exhibit significant O-isotopic shift. Although all traverses included limestones, δ 18 O of detachment fault breccia, veins, and upper-plate rocks differed significantly depending on the specific limestone involved. In one traverse in the Snake Range, where thin-bedded limestone of the Cambrian Lincoln Peak Formation was sampled, the δ 18 O of detachment fault breccia, veins, and stylolitically deformed upper-plate limestone near the detachment fault is typically 15‰±2‰ standard mean ocean water (unexchanged limestone has δ 18 O of ≈20‰), and the matrix was in O-isotopic equilibrium with vein fluid. Elsewhere in the Snake Range, where the detachment fault lay in massive medium-grained Cambrian and Ordovician limestones, δ 18 O values of detachment fault breccia and veins was much lower, typically 2‰±3‰, whereas δ 18 O of limestone matrix was between 16‰ and 20‰, far out of O-isotopic equilibrium with vein fluid. The sampled portion of the Mormon Peak detachment lay in medium-bedded Cambrian and late Paleozoic limestone: early veins have high δ 18 O values of 23‰–28‰ (unexchanged limestone has δ 18 O of ≈28‰), whereas later veins and detachment fault breccia have δ 18 O between 6.6‰ and 17.9‰. Thus, in the Snake Range, fluids were either in isotopic equilibrium with wall rock throughout the sampled fault history, implying intergranular flow, or were far out of equilibrium with it, implying channeling via a fracture network. The fluids in the Mormon Peak detachment were initially in isotopic equilibrium with wall rock, becoming increasingly 18 O depleted and out of O-isotopic equilibrium with wall rock with time. The difference in isotopic exchange history in the detachment faults and related rocks is evidently not a function of initial structural depth, but of permeability and its distribution between matrix and fractures. Combined thermal, fluid flow, and oxygen isotope exchange modeling demonstrate that the observed isotopic composition of rocks in and associated with the detachment fault could have been produced either by influx of meteoric water from topographically high areas downdip of the sampled area during detachment faulting, or by convection in the upper plate induced by elevated geothermal gradient and deformation-enhanced permeability. Locally derived meteoric fluids also infiltrated the detachment fault system in places, but are not required in order to account for low δ 18 O detachment breccias and veins. The model computations indicate that cumulative syntectonic fluid flux along the detachment faults was between 1700 and 11 000 kg/cm 2 , depending on location. Time-averaged fault permeabilities are estimated by modeling to have been between 2 and 20 mD. Thus, the model results not only verify fluid source, but also provide insight into fluid flow mechanism and important hydrogeologic properties of the detachment faults during deformation.

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.

Temperatures and fluids on faults based on carbonate clumped–isotope thermometry

We present results from a carbonate clumped-isotope thermometric study of 42 carbonate samples collected within ∼1 m or less of the Mormon Peak detachment, a large-slip Miocene normal fault in the Basin and Range province of southern Nevada. Samples include cataclastic rocks, narrow vein fillings and larger void-filling carbonates. Our results are consistent with earlier measurements of O and C isotopic ratios and fluid inclusion temperatures, and provide independent constraints on the isotopic composition and temperature of both syntectonic and post-tectonic pore waters. The results reveal a wide range of precipitation temperatures (24 to 137 °C) associated with deformation, and indicate that the pore waters were meteoric, with δ18O as low as −11.6 permil (VSMOW) and δ13C as low as −8.0 permil (VPDB). The results do not provide any direct evidence for high-temperature thermal decarbonation reactions (∼500 to 800 °C) that are widely expected to result from flash heating along upper crustal faults, although they do not rule them out so long as recarbonation occurs at very low temperature, or the products of these reactions are volumetrically minor. The results are difficult to reconcile with recent suggestions that the detachment is the base of one or more catastrophically emplaced, surficial landslides. In concert with other lines of evidence, the data are most simply interpreted as recording deformation and precipitation events through a long history of slip, accompanied by relatively deep (>3 km) circulation of meteoric pore waters along the detachment plane.

Reservoir fluids and their migration into the South Eugene Island Block 330 reservoirs, offshore Louisiana

This study in the well-documented Pliocene-Pleistocene South Eugene Island Block 330 (SEI330) field, offshore Louisiana, unravels a complex petroleum system by evaluating both the inorganic and organic geochemical characteristics of reservoir fluids. The brines at SEI330 dissolved halite prior to entering the reservoirs and equilibrated with reservoir sediments, exchanging sodium for calcium, magnesium, and other cations. The systematically varying extent of brine sodium depletion in two reservoirs defines south to north flow in those sands. These sands were filled from a fault that bounds the reservoirs on the south. Oil compositional parameters also show north-south variation across these reservoirs. The SEI330 oils and gases each had different sources. In contrast to published Jurassic sources for oil, carbon isotope data indicate that SEI330 hydrocarbon gases probably sourced from early Tertiary or Cretaceous sediments, after oil had migrated through them. Distribution of biodegraded vs. unbiodegraded oils indicates the reservoirs filled much more recently than formation of the salt weld beneath the field. Oil compositions indicate that some SEI330 oils were partially stripped of low-molecular weight compounds by their dissolution in a mobile vapor phase (gas washed) by large volumes of gas several hundred meters below the deepest reservoir. Modeling of this gas-oil interaction aids in identifying deep potential targets in which gas washing occurred. Hydrocarbon distribution, combined with oil chemistry and reservoir pressures, indicate reservoirs filled from fault systems on both the north and south sides of the field. The fault feeders are wide (>100 m), structurally complex zones that can direct different types of fluids into different reservoirs.