John M. Sharp

Energy transport in thick sequences of compacting sediment

This investigation synthesizes the theories of energy transport and gravitational compaction of sediment to develop a deterministic model capable of generating pore-fluid pressure, porosity, and temperature distributions throughout the accumulation of basin sediment. Abnormal pore-fluid pressures develop with increasing rates of sediment accumulation and decreasing hydraulic diffusivity. Sediment temperature distributions depart from typical linear profiles characteristic of steady-state conduction and have increasing rates of sediment accumulation and decreasing hydraulic diffusivity. A comparison of model output with data from the Gulf of Mexico geosyncline demonstrates that gross fluid pressure, porosity, and temperature distributions are explained by the model. Lateral movement of pore fluid to faults in combination with lithically induced hydraulic and thermal parameter variations explain cases of departure from the general patterns. Sediment in the Gulf of Mexico geosyncline may have been subjected to abnormal pressure since Cretaceous time; sediment presently in the near offshore may now be at its maximum pore-fluid pressure and minimum temperature at any given depth.

Wide Azimuth Streamer Imaging of Mad Dog; Have We Solved the Subsalt Imaging Problem?

Summary Forming an image of the Mad Dog field is critical for the efficient development of the reservoirs. BP has applied a variety of depth migration methods on standard surface streamer data to improve the image. However, the severe velocity discontinuities due to the complex salt which is proximate to the rugged water bottom causes gaps in the illumination of the structure that can not be filled with conventional surface streamer data. In 2004 BP implemented a “first of its kind” marine wide azimuth streamer survey. Depth migrating the wide azimuth seismic data yielded a substantially improved image of the reservoirs. This paper discusses the motivation behind the seismic acquisition redesign and some of the challenges of processing wide azimuth surface streamer data.

Monitoring pumping test response in a fractured aquifer using ground-penetrating radar

This is the published version. Copyright 2010 by the American Geophysical Union. All rights reserved.

Porosity and permeability variations in fractured and liesegang-banded Breathitt sandstones (Middle Pennsylvanian), eastern Kentucky: diagenetic controls and implications for modeling dual-porosity systems

Abstract Middle Pennsylvanian fluvial sandstones in eastern Kentucky (Breathitt Group) manifest visible evidence of alteration related to fluid flow localized through near-vertical joints. Fracture-related alterations involve both physical and chemical modifications that together create dramatic permeabilisty variations at the outcrop scale. On the fracture surface, infiltered detritus combined with mineral and organic coatings have reduced pore sizes and, hence, permeabilities (0.03–0.44 md) by an order of magnitude over values characteristic of the adjacent sandstone (0.32–1.53 md). Prominent zones of orange-brown discoloration contain evidence of oxidation reactions and form an envelope of variable thickness around the fractures. Authigenic iron oxides are not uniformly distributed within these zones, but rather are concentrated as local bands of pervasive mineralization commonly known as liesegang bands. Petrographic evidence suggests that most of the iron that now resides in oxidized authigenic phases was derived from solutes mobilized through dissolution of older iron-bearing authigenic minerals. Permeability and pore sizes within the oxidation zone are bimodal and vary from high values similar to those in adjacent unoxidized sandstone to low values, associated with the zones of pervasive mineralization, that approach those values observed for the fracture skin. The large magnitude of permeability variation around fracture systems in these sandstones documents the presence of a dual porosity system and suggests that fluid and contaminant transport cannot be realistically modeled using average rock properties.

Permeability Controls on Aquathermal Pressuring

Aquathermal pressuring refers to fluid pressures in excess of the hydrostatic which have been created by fluid thermal expansion against a less expansive sediment matrix. Two commonly stated objections to this mechanism are: (1) hydraulic gradients and the finite, but very low, permeabilities now existing in the Gulf of Mexico are sufficient to dissipate aquathermal pressures, and (2) the increasing temperatures decrease fluid viscosity at a rate so that hydraulic conductivity increases fast enough to dissipate these pressures. Evaluation of the momentum transport equations in the Eulerian frame of reference indicates the first objection is not applicable because it treats the problem as steady flow and not transient flow. Furthermore, existing data indicate that under no mal diagenetic conditions the decrease in intrinsic permeability caused by consolidation is greater than the viscosity effect. Therefore, the two commonly stated objections to aquathermal pressuring do not by themselves prove the mechanism is ineffective in settings such as the Gulf of Mexico. Permeability increases by fracturing and faulting may, however, be sufficient to bleed off aquathermally produced excess pressures. The measured, very low permeabilities of Gulf Coast fine-grained sediments indicate that aquathermal pressuring is a potentially important mechanism if the fine-grained sediments do control the hydraulic response.

Temperature Variations in South Texas Subsurface

Above-average thermal gradients and temperature are observed in portions of the Gulf Coast basin. In this study, over 1,600 bottom-hole temperature measurements from south Texas were collected and analyzed. Our analysis resulted in a three-dimensional characterization of temperature distributions in a large portion of the basin in south Texas and numerical models of representative cross sections of this area. In three dimensions, the most prominent thermal feature in the south Texas subsurface is a band of high temperatures that corresponds to the Wilcox growth-fault trend. A similar but greatly subdued band corresponds to the Vicksburg-Frio growth-fault trend. Indications from temperature profiles and from modeling are that these temperature bands are controlled by the a vection of fluids from the deep basin upward along the growth-fault zone. The source of the fluids appears to be quite deep, over 12,000 ft (4,000 m) below the surface. In addition, temperatures generally increase from the northeast to the southwest within the area. This trend is presumably created by variation in basement heat flux.

Fracture control of regional ground-water flow in a carbonate aquifer in a semi-arid region

We integrate fracture mapping and numerical modeling to assess the role of fractures in regional round-water flow. Although the importance of fractures in ground-water flow and solute transport is accepted generally, few studies have addressed quantitatively the regional hydrogeological implications of fractures. The field-study area in west Texas and southeastern New Mexico consists primarily of subhorizontal Permian carbonate rocks cut by extensional faults and fractures. Air-photo analysis and field mapping reveal a broad fracture zone extending from the Sacramento Mountains of New Mexico to the Salt Basin near Dell City, Texas. Most fractures are subparallel to major normal faults. The most intense fracturing coincides with a prominent trough in the potentiometric surface and an apparent “plume” of relatively fresh ground water. Flow models, corroborated by geochemical data, indicate that fracturing has created a high-permeability zone that funnels recharge from the Sacramento Mountains at least 80 km southeastward to its discharge zone. A steady-state finite-element flow model uses fracture data to predict the spatial transmissivity distribution. Given the probable range of recharge, discharge, and other hydrologic parameters, fractures are the most important factor affecting the potentiometric surface configuration. Our study implies that: (1) fractures can control ground-water flow over large (>1000 km 2 ) areas; (2) effective recharge areas and regional ground-water chemistry trends are strongly influenced by fractures; and (3) a priori inferences about aquifer properties and regional flow are possible by means of fracture studies. This study demonstrates that the timing and nature of fracturing can affect regional subsurface fluid flow, as well as related processes such as hydrothermal mineralization, diagenesis, and hydrocarbon transport and entrapment.

Modification of the Local Cubic Law of fracture flow for weak inertia, tortuosity, and roughness

The classical Local Cubic Law (LCL) generally overestimates flow through real fractures. We thus developed and tested a modified LCL (MLCL) which takes into account local tortuosity and roughness, and works across a low range of local Reynolds Numbers. The MLCL is based on (1) modifying the aperture field by orienting it with the flow direction and (2) correcting for local roughness changes associated with local flow expansion/contraction. In order to test the MLCL, we compared it with direct numerical simulations with the Navier-Stokes equations using real and synthetic three-dimensional rough-walled fractures, previous corrected forms of the LCL, and experimental flow tests. The MLCL performed well and the effective errors (δ) in volumetric flow rate range from −3.4% to 13.4% with an arithmetic mean of |δ| ( ) equal to 3.7%. The MLCL is more accurate than previous modifications of the LCL. We also investigated the error associated with applying the Cubic Law (CL) while utilizing modified aperture field. The δ from the CL ranges from −14.2% to 11.2%, with a slightly higher  = 6.1% than the MLCL. The CL with the modified aperture field considering local tortuosity and roughness may also be sufficient for predicting the hydraulic properties of rough fractures.

Tracing regional flow paths to major springs in Trans-Pecos Texas using geochemical data and geochemical models

Abstract San Solomon, Giffin, and Phantom Lake Springs, located in Trans-Pecos Texas, have a high TDS, Na–Cl–SO 4 baseflow component derived from a regional flow system and a low TDS, mixed cation–mixed anion stormflow component derived from local precipitation events. The hypothesis that the regional flow system maintaining baseflow spring discharge originates in the Salt Basin and flows through the Apache Mountains towards the springs is tested with historical geochemical data from wells and springs. Data from over 1400 wells in the study area over a 50-year period were analyzed and used to delineate 11 hydrochemical facies based on the predominant ions. Geochemical data from samples along the hypothesized regional flow path indicate a trend of increasing dissolved solids and Cl–HCO 3 ratios and decreasing Na–Cl ratios. These are consistent with evolution of groundwater in an unconfined regional system dominated by carbonates and evaporites. In the bicarbonate facies, the waters represent recent recharge modified by mineral dissolution and cation exchange. In the sulfate zones, the hydrochemical facies are controlled by gypsum, anhydrite, and halite dissolution, cation exchange, and mixing with Na–Cl waters. In the chloride zones, the hydrochemical facies are controlled by halite dissolution and irrigation return flow. Spring discharge chemistry is most similar to chloride zone waters; Na–Cl and Ca–SO 4 ratios suggest that baseflow is derived from the chloride zone waters upgradient along the hypothesized flow path. PHREEQC modeled groundwater evolution along the hypothesized flow path and spring discharge under stormflow and baseflow conditions. Results indicate that: (1) hydrochemistry along the regional flow path is controlled by dissolution of halite, gypsum, dolomite, and CO 2 and by precipitation of calcite; (2) baseflow spring discharge is derived primarily from this regional flow system; and (3) spring discharge after major storm events can constitute as much as 72% local recharge that is further modified by dissolution of calcite, gypsum, and CO 2 . Data analysis and model results suggest that cave formation in this system is occurs during major storm events.

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.