Robert M. Holt

Conceptual model for transport processes in the Culebra Dolomite Member, Rustler Formation

The Culebra Dolomite Member of the Rustler Formation represents a possible pathway for contaminants from the Waste Isolation Pilot Plant underground repository to the accessible environment. The geologic character of the Culebra is consistent with a double-porosity, multiple-rate model for transport in which the medium is conceptualized as consisting of advective porosity, where solutes are carried by the groundwater flow, and fracture-bounded zones of diffusive porosity, where solutes move through slow advection or diffusion. As the advective travel length or travel time increases, the nature of transport within a double-porosity medium changes. This behavior is important for chemical sorption, because the specific surface area per unit mass of the diffusive porosity is much greater than in the advective porosity. Culebra transport experiments conducted at two different length scales show behavior consistent with a multiple-rate, double-porosity conceptual model for Culebra transport. Tracer tests conducted on intact core samples from the Culebra show no evidence of significant diffusion, suggesting that at the core scale the Culebra can be modeled as a single-porosity medium where only the advective porosity participates in transport. Field tracer tests conducted in the Culebra show strong double-porosity behavior that is best explained using a multiple-rate model.

Gravity anomalies and deep structural controls at the Sabah-Palawan margin, South China Sea

Abstract The hydrocarbon-rich southeastern margin of the South China Sea is divided by NW-SE lineaments into a series of sharply contrasting segments distinguished by, among other things, abrupt changes in gravity patterns. The Sabah segment is bounded to the SW by the Tinjar or West Baram line and to the NE by the Balabac line at the southwest margin of the Sulu Sea. The most prominent gravitational feature of this segment is the strong freeair gravity low associated with the Sabah Trough which lies about 150 km offshore. Seismic reflection data suggest that loading by prograding sediments and gravity driven thrust sheets has depressed the extended continental crust of the South China Sea below its level of local isostatic compensation, producing the trough as a foreland basin in which sedimentation has failed to keep pace with subsidence. The load masses themselves, supported in part by the rigidity of the underlying crust and lithosphere, are above their levels of local compensation and deep Neogene sedimentary basins lie on the flanks of Bouguer and free-air gravity highs. Gravity values decrease across the Sabah coast so that the Crocker Ranges, including Mt Kinabalu, rest in rough isostatic equilibrium on presumably weaker lithosphere. The free-air gravity anomaly associated with the Sabah Trough is smaller than would have been predicted from the thickness of the water column, suggesting crustal thinning beneath the trough axis. This is not a characteristic of normal foreland basins and can therefore be assumed to predate basin formation. It can be concluded that the NE-SW trending belts of parallel gravity anomaly and geomorphology, of which the Sabah Trough is the most obvious, have been controlled by the pre-existing fabric of the crust and lithosphere of the South China Sea since they are discordant to the Palaeogene geological trends in Sabah. Reconstructions of the Tertiary history of the Sabah segment can be based on this assumption, which also suggests that sediments deposited in rift basins formed during the Palaeogene break-up of the South China margin were the source for much, if not all, of the hydrocarbon reserves of the area.

The Salt That Wasn't There: Mudflat Facies Equivalents to Halite of the Permian Rustler Formation, Southeastern New Mexico

Four halite beds of the Permian Rustler Formation in southeastern New Mexico thin dramatically over short lateral distances to correlative clastic (mudstone) beds. The mudstones have long been considered residues after post-burial dissolution (subrosion) of halite, assumed to have been deposited continuously across the area. Hydraulic properties of the Culebra Dolomite Member have often been related to Rustler subrosion. In cores and three shafts at the Waste Isolation Pilot Plant (WIPP), however, these mudstones display flat bedding, graded bedding, cross-bedding, erosional contacts, and channels filled with intraformational conglomerates. Cutans indicate early stages of soil development during subaerial exposure. Smeared intraclasts developed locally as halite was removed syndepositionally during subaerial exposure. We interpret these beds as facies formed in salt-pan or hypersaline-lagoon, transitional, and mudflat environments. Halite is distributed approximately as it was deposited. Breccia in limited areas along one halite margin indicates post-burial dissolution, and these breccias are key to identifying areas of subrosion. A depositional model accounts for observed sedimentary features of Rustler mudstones. Marked facies and thickness changes are consistent with influence by subsidence boundaries, as found in some modern continental evaporites. A subrosion model accounts for limited brecciated zones along (depositional) halite margins, but bedding observed in the mudstones would not survive 90% reduction in rock volume. Depositional margins for these halite beds will be useful in reconstructing detailed subsidence history of the Late Permian in the northern Delaware Basin. It also no longer is tenable to attribute large variations in Culebra transmissivity to Rustler subrosion.

Depth indicators in Permian Basin evaporites

Abstract The Permian Basin of West Texas and New Mexico contains one of the worlds best-preserved and most extensively studied evaporite basin-to-platform sequences. From analysis of fabrics and small-scale cycle patterns, reconstruction of the position of these elements in the basin-filling sequence and comparison to laboratory-grown and modern evaporite fabrics, we created a table of fabrics that serve as water-depth indicators. Evaporites formed in deeper water (more than a few to hundreds of metres) in both halite- and gypsum-precipitating settings in the Permian Basin are characterized by cumulate fabrics. Cumulates are fine crystals or rafts of fine crystals formed at the air – brine interface that fall though the water body and accumulate on the basin floor with fine lamination, draping relationships, dark colours and minimal early diagenesis. Intervals of coarser crystals precipitated on the basin floor are interpreted as evidence for episodic transport of saturated surface water to the basin floor during perturbation of stratified conditions. Shallow water evaporites in the Permian Basin are dominated by bottom-growth fabrics such as halite chevrons and near-vertically oriented gypsum crystals. Bands of fluid and other inclusions record high frequency changes in depositional rate. Truncated crystals document flooding by undersaturated fresh or marine water under shallow conditions where mixing was adequate to cause undersaturated low-density water to contact the basin floor. Formation of base-of-cycle insoluble residues is a strong indicator of shallow water during the flooding event that initiated each sedimentary cycle. In the Permian Basin, exposure is documented by formation of synsedimentary evaporite karst pits and pipes, truncation, dissolution and recrystallization of earlier fabrics, and precipitation of cements. Red siliciclastic mudstones are associated with the late stages of depositional cycles when the surface was subaerially exposed. Repetition of alternately exposed and saline water table conditions created an array of distinctive fabrics including chaotic mudstone–salt mixtures, karst fills, replacement and recrystallization fabrics, and cracks and saltpolygons.

Spatial bias in field‐estimated unsaturated hydraulic properties

Hydraulic property measurements often rely on non-linear inversion models whose errors vary between samples. In non-linear physical measurement systems, bias can be directly quantified and removed using calibration standards. In hydrologic systems, field calibration is often infeasible and bias must be quantified indirectly. We use a Monte Carlo error analysis to indirectly quantify spatial bias in the saturated hydraulic conductivity, K{sub s}, and the exponential relative permeability parameter, {alpha}, estimated using a tension infiltrometer. Two types of observation error are considered, along with one inversion-model error resulting from poor contact between the instrument and the medium. Estimates of spatial statistics, including the mean, variance, and variogram-model parameters, show significant bias across a parameter space representative of poorly- to well-sorted silty sand to very coarse sand. When only observation errors are present, spatial statistics for both parameters are best estimated in materials with high hydraulic conductivity, like very coarse sand. When simple contact errors are included, the nature of the bias changes dramatically. Spatial statistics are poorly estimated, even in highly conductive materials. Conditions that permit accurate estimation of the statistics for one of the parameters prevent accurate estimation for the other; accurate regions for the two parameters do not overlap in parameter space. False cross-correlation between estimated parameters is created because estimates of K{sub s} also depend on estimates of {alpha} and both parameters are estimated from the same data.

Discussion of a Miocene collisional belt in north Borneo: uplift mechanism and isostatic adjustment quantified by thermochronology

Scientific editing by Alex Maltman. John Milsom & Robert Holt write: Knowledge of the gravity field is vital to any study of isostasy. This was acknowledged by ⇓Hutchison et al. (2000) when they noted that the work of ⇓Holt (1998) placed constraints on their geological model for Sabah. However, they went on to ignore these constraints, arguing that the depth of the crustal root beneath the Western Cordillera might have been overestimated due to ‘the model assumption of an excessively thick ophiolitic basement’ and that the presence of thick crust beneath Sabah was ‘in conflict with the outcropping geology’. The Hutchison et al. interpretations of the deep structure of Sabah were illustrated in a series of cross-sections in which some features (notably the topography) were amplified for the sake of clarity. There was, however, no suggestion that any features would be underemphasized, and the depth scales, although described as only ‘rough guides’, indicated normal 5–10 km thicknesses for oceanic crust. Therefore, and in the light of the comments quoted above, the thin crust shown throughout Sabah and the generally somewhat greater thickness of the extended continental crust of the Dangerous Grounds must be regarded as essential elements of the model. The Late Miocene cross-section (⇓Fig. 1), showing the end of orogeny in Sabah, is a guide to the Hutchison et al. view of the present-day situation. Crust is less than 30 km thick beneath the Western Cordillera and less than 20 km thick elsewhere. We contend that these values are incompatible with the gravity field, and that it is no more acceptable to present a geological interpretation that ignores geophysical data than a geophysical interpretation that ignores geology. Some way must be found of reconciling the two. Fig. 1. Sabah in the Late Miocene, from ⇓Hutchison et al. (2000, fig. 6b). …

Predicting fractured zones in the culebra dolomite.

Fracturing in the Culebra Dolomite Member of the Permian Rustler Formation exhibits a high degree of spatial variability in the vicinity of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico. The WIPP is the U.S. Department of Energys deep geological repository for transuranic and mixed wastes resulting from the nations defense programs. The WIPP repository is located 655 m below ground surface in bedded halite of the Permian Salado Formation, which underlies the Rustler and Culebra. Culebra transmissivities (Ts) in the vicinity of the WIPP vary over six orders of magnitude, with higher Ts (log 10 T (m 2 /s) > -5,4) reflecting zones of well-interconnected fractures. We develop, test, and refine a conceptual model for predicting fracture zones within the Culebra. We define three regional-scale controls on Culebra fracturing, including the dissolution of salt from below the Culebra, the presence of halite above and below the Culebra, and overburden thickness. We also identify two local-scale controls on Culebra fracture zones including fracture-filling cements and localized deformation due to ductile flow of the mudstone that underlies the Culebra. The spatial distribution of the regional-scale controls is easily predicted. However, the influence of local controls can only be uniquely identified in hydraulic test data. A drilling program initiated in 2003 tests aspects of this conceptual model and leads to minor revisions of our conceptual understanding of the geologic controls on fracturing in the Culebra.

Uncertainty in Vadose Zone Flow and Transport Prediction

This special section of Vadose Zone Journal considers uncertainty in vadose zone flow and transport prediction. The recent emergence of this topic is a consequence of a major change in the motivation for predictive modeling in the vadose zone. Until the 1980s, predictive modeling of vadose zone flow

Evaluation of halite dissolution at a radioactive waste disposal site, Andrews County, Texas

Waste Control Specialists (WCS) has been granted permits to dispose of radioactive waste at their surface facility in western Andrews County, Texas, U.S.A. The facility is located over Permian-age, halite-bearing formations, and the possibility of dissolution and its effects on the long-term performance of the disposal site must be considered. We compare data from the WCS site and vicinity to three conceptual hydrologic models of dissolution processes (shallow, deep, and stratabound). Geophysical logs yield stratigraphic and lithofacies data from the underlying halite-bearing units, the Permian Rustler and Salado Formations. The halite units are continuous, and thickness and lithofacies vary by depositional process, with no discernible postdepositional dissolution. Modest structural trends in these units differ from local changes in evaporite thickness. Therefore, deformation is not related to thickness differences. Local thickness changes in the middle of the Salado Formation indicate depositional variations. Core obtained from upper evaporite-bearing formations at the WCS site show normal depositional features and no brecciation that would be associated with postdepositional dissolution. Deeply buried bedded halite behaves as a ductile material, and the effective stress is close to zero, so that the pressure in pore fluids approaches lithostatic. Hydraulic gradients are outward from halite toward overlying and underlying rocks, eliminating access for low-salinity fluids. Formation fluids at depth are commonly saline and slow moving, further limiting the dissolution process. The hydrologic systems at the site limit the potential for future dissolution.

Generalized radial flow in synthetic flow systems.

Traditional analysis methods used to determine hydraulic properties from pumping tests work well in many porous media aquifers, but they often do not work in heterogeneous and fractured-rock aquifers, producing non-plausible and erroneous results. The generalized radial flow model developed by Barker (1988) can reveal information about heterogeneity characteristics and aquifer geometry from pumping test data by way of a flow dimension parameter. The physical meaning of non-integer flow dimensions has long been a subject of debate and research. We focus on understanding and interpreting non-radial flow through high permeability conduits within fractured aquifers. We develop and simulate flow within idealized non-radial flow conduits and expand on this concept by simulating pumping in non-fractal random fields with specific properties that mimic persistent sub-radial flow responses. Our results demonstrate that non-integer flow dimensions can arise from non-fractal geometries within aquifers. We expand on these geometric concepts and successfully simulate pumping in random fields that mimic well-test responses seen in the Culebra Dolomite above the Waste Isolation Pilot Plant.