Geopressured Zone Research Articles

Pore pressure prediction in offshore Niger delta using data-driven approach: Implications on drilling and reservoir quality

Despite exploration and production success in Niger Delta, several failed wells have been encountered due to overpressures. Hence, it is very essential to understand the spatial distribution of pore pressure and the generating mechanisms in order to mitigate the pitfalls that might arise during drilling. This research provides estimates of pore pressure along three offshore wells using the Eaton's transit time method, multi-layer perceptron artificial neural network (MLP-ANN) and random forest regression (RFR) algorithms. Our results show that there are three pressure magnitude regimes: normal pressure zone (hydrostatic pressure), transition pressure zone (slightly above hydrostatic pressure), and over pressured zone (significantly above hydrostatic pressure). The top of the geopressured zone (2873 mbRT or 9425.853 ft) averagely marks the onset of overpressurization with the excess pore pressure above hydrostatic pressure (P∗) varying averagely along the three wells between 1.06−24.75 MPa. The results from the three methods are self-consistent with strong correlation between the Eaton's method and the two machine learning models. The models have high accuracy of about > 97%, low mean absolute percentage error (MAPE < 3%) and coefficient of determination (R2> 0.98). Our results have also shown that the principal generating mechanisms responsible for high pore pressure in the offshore Niger Delta are disequilibrium compaction, unloading (fluid expansion) and shale diagenesis.

Impact of complex tectonics on the development of geo-pressured zones: A case study from petroliferous Sub-Himalayan Basin, Pakistan

In petroliferous basins, integration of regional and local tectonics along with well data is used to understand the generation of geo-pressured variations in different lithological units. In the last 40 years, abnormal geo-pressured zones is the main challenge during hydrocarbons drilling in the tectonically complex Kohat Sub-basin, Sub Himalayas. The trapped fluid in post-Eocene strata generated the over-pressure zones and triggered drilling problems. The present study is an integration of data, by use of regional maps, seismic lines, and formation pressure using Eaton’s equation to understand the possible causes for the development of geo-pressured zones. Where, regional maps and seismic lines were employed for deciphering the regional and local tectonics, respectively. Porosity function and Normal Compaction Trend (NCT) curve were utilized for the identification of abnormal pore pressure zones (over-pressure and under-pressure zones). Three major stress regimes were interpreted. The results show that the mechanism of over-pressure zones developed as a combination of effective horizontal stresses, lithological variations and rapid sedimentation. The Normal Compaction Trend (NCT) line and porosity function indicated that the 1st and 2nd stress regime consists of major abnormal geo-pressured zones. Therefore, it is concluded that these stress regimes experienced more overburden pressure and fracture pressure. Consequently, it is suggested that the results of geo-pressure estimation from this work could be used for the appropriate mud weight during drilling in analogous tectonic settings.

Pore and fracture pressure modeling using direct and indirect methods in Badri Field, Gulf of Suez, Egypt

Abnormal formation pressure is one of globally dominant drilling hazards because of its direct impact on the drilling operations and exploration risk. An adequate pore pressure and fracture gradient modeling are considered the heart of the well planning and execution. Safe drilling to reach total depth of any well is the main concern for both engineers and geoscientists. Consequently, the main objectives of this paper are evaluation of pore pressure and fracture gradients as well as identification of the geo-pressured zones (over-pressured) in the subsurface sedimentary section.The pore pressure and fracture gradients were evaluated using well logging data namely density, resistivity, gamma-ray and sonic logs obtained from offshore wells in Badri Field. The overburden (overburden stress) was calculated using Amoco overburden equation, while sonic and resistivity logs were used for pore pressure and fracture gradient calculations using Eaton's method.The results of this study revealed that: 1) integration of direct and indirect method can lead to adequate and accurate pore pressure modeling; 2) pore pressure regime shows over-pressured, normal pressure and sub-normal horizons; 3) over-pressured zones was detected in the base of Zeit Formation, South Gharib Formation, Hammam Faraun Member and Kareem Formation, pore pressure were 10.3, 10.5, 9.3 and 9.6 ppg equivalent density in arrange; 4) Reservoirs of Hammam Faraun Member and Kareem Formation indicated Sub-normal pressure zones where pore pressure values are varying from 5.5 to 6.2 ppg for Hammam Faraun reservoir and from 5.2 to 8 ppg in Kareem Formation in equivalent density; 5) Normal pressure regime was detected in the upper part of Zeit Formation and Feiran, Sidri and Baba members, where pore pressure value was recorded as 8.7 ppg; 6) Best fitting parameter for Eaton resistivity and sonic logs are 0.9 and 1.65 respectively while for Amoco overburden empirical equations, the Amoco exponent best fit is 0.85 where the mud line density is 2.15 g/cm3. Finally, it is highly recommended to use low mud weight against the under compaction zones and high mud weights against the identified over-pressured intervals.

New Pore Pressure Evaluation Techniques for LAGIA-8 Well, Sinai, Egypt as a Case Study

Drilling into a geopressured zone will generally cause a change in a number of basic formation/ drilling relationships. This change is usually seen as a reversal of a gradual depth related trend in a lithologically uniform formation. Of all the geophysical methods, the reflection seismic method is essentially the only technique used to predict pore pressures. The seismic method detects changes of interval velocity with depth from velocity analysis of the seismic data. These changes are in turn related to lithology, pore fluid type, rock fracturing and pressure changes within a stratigraphic column. When the factors affecting the velocity are understood for a given area, a successful pressure prediction can be made. For clastic environments such as the Tertiary section of the Gulf of Mexico or the Niger delta, the interval velocity of the rocks increases with depth because of compaction. In these areas, deviations from normal compaction trends are related to abnormally high pore pressures. The adapted methods provide a much easier way to handle normal compaction trend lines. In addition to well log methods, pressure detection can be obtained via drilling parameters by applying Eaton’s DXC methods. Seismic velocities have long been used to estimate pore pressure, indeed both these quantities are influenced by variations in rock properties such as porosity, density, effective stress and so on, and high pore pressure zones are often associated with low seismic velocities. Pressure prediction from seismic data is based on fundamentals of rock physics and seismic attribute analysis. This paper hence tries to assess the use of seismic waves as a viable means to calculate pore pressure, especially in areas where no prior drilling history can be found. Then we applied these methods on LAGIA-8 well, Sinai, Egypt as a case study. Pore pressure prediction from Seismic is a very essential tool to predict pore pressure before drilling operation. This could prevent the well problem as well blowout and to prevent formation damage, especially in areas where no prior drilling history can be found.

Horizontal stress anisotropy and effective stress as regulator of coal seam gas zonation in the Sydney Basin, Australia

Coal seam gas zonation in the Sydney Basin, NSW, Australia is related to basin hydrodynamics and hydrochemical facies evolution along the flow path from the subcrop to the basin center. Biogenic methane corresponds with meteoric water under hydrostatic pressure and persists down to the top of the geopressured zone (~800 to 1000m). Thermogenic gases, including wet hydrocarbons, can reach up to relatively shallow horizons of less than 500–600m depth. In the transition zone between the top of the geopressured and base of the hydrostatic zone, a mixed water and gas regime prevails, comprising brackish waters, and gases of mixed biogenic, thermogenic and inorganic origins, including CO2. Mechanisms for and the role of stress in the development of this layered hydrogeological and gas environment are investigated in this paper.The inverse relationship between effective horizontal stress and permeability in coals through regulation of cleat volumes is well documented, and there is evidence of regionally compartmentalized stress regimes with depth within the Sydney and other eastern Australian coal basins. This regional stress regime can be overprinted by the effect of localized geological features. It is hypothesized that the in situ stress regime plays an important role in the regulation of groundwater flow regimes and extents, resulting in the development of the reported gas content and compositional zonation.Analysis of regional gas and stress data obtained from public and private databases, as well as literature, supports this hypothesis. Changes in gas concentration and composition with depth correspond with discernable variations in horizontal stress anisotropy. Gas contents generally increase with depth down to a ‘peak gas’ horizon, below which concentrations decrease. This ‘peak gas’ zone is coincident with a horizontal stress anisotropy change from moderately high to low levels, associated with reverse to strike–slip faulting conditions, respectively. The stress release zone also marks the top of the thermogenic gas zone, identified by the first appearance of ethane in the vertical profile. This zone also hosts gases of mixed origins: biogenic, thermogenic and inorganic (CO2) and represents a mixed (transitional) groundwater flow environment. The base of the mixed gas zone is the top of the ‘geopressured-only’ flow associated with thermogenic gases and is signaled by the return to high stress reverse faulting conditions below 850–900m depth in the Sydney Basin.

Origin, chemical and isotopic evolution of formation water in geopressured zones in the Pannonian Basin, Hungary

In the central part of the Pannonian Basin we investigate formation waters from two deep sub-basins with sedimentary sequences up to 7000 m thick, and from fields above basement highs where the total thickness of the sediments is only 2000 m. In each study region the depth interval involves the Lower (lower part of the Upper Miocene) and Upper (upper part of the Upper Miocene and Pliocene) Pannonian sediments separated with a widespread aquitard, named the Algyő Formation, at the top of the Lower Pannonian. However, in the fields above basement highs, it occasionally contains sand lenses, sedimentological discontinuities, faults and fractures resulting in a more conductive character. We investigate the sources and evolution of the formation waters in two geological settings: in sub-basins and in fields above an elevated basement. We distinguish NaHCO 3-type water in the sub-basins and NaCl-dominated water in the fields above the elevated basement. The isotopic composition of most NaHCO 3-type samples indicates a pure paleometeoric origin, while a few samples are a mixture of paleometeoric and non-meteoric waters. The NaCl-type samples all have a non-meteoric contribution. We have developed a model for the origin and evolution of the formation water that operates for both water types and both geological conditions. The Cl − dominated water is a mixture of ascending pre-Pannonian evaporated water and freshening Lake Pannon water trapped during the Late Miocene and Pliocene sedimentation in the area above the elevated basement, where in some places the Algyő Formation is conductive. The source and evolution of the NaHCO 3 non-meteoric water in the sub-basins are similar to those of the NaCl water, with the main difference being the aquitard character of the Algyő Formation in the sub-basins. The aquitard consists of siltstone and clay-marl, which has resulted in overpressure in the layers under the Algyő Formation. The ascending pre-Pannonian water, which mixes with in-situ pore water squeezed through the compacted clay by overpressure, is exposed to microfiltration. The Cl − remains behind, and the squeezed water is depleted of the heavier isotopes. These processes are shown by recent chemical and isotopic data ( δ 18O and δD), as well as by archival chemical and hydrological data.

Regional overview of deep sedimentary thermal gradients of the geopressured zone of the Texas–Louisiana continental shelf

Nearly 600 bottom-hole temperature data from the northern continental shelf of the Gulf of Mexico, each corrected for drilling disturbance, yielded a regional map of geothermal gradient down to approximately 6 km (3.7 mi) sub–sea floor. Two geographic trends can be seen on the map. First, from east to west, the geothermal gradient changes from values between 0.025 and 0.03 K/m (0.014 and 0.016°F/ft) off the Alabama–Mississippi shore to lower values of 0.015–0.025 K/m (0.008–0.014°F/ft) off eastern Louisiana and to higher values of 0.03–0.06 K/m (0.016–0.033°F/ft) off western Louisiana through Texas. Second, thermal gradients tend to be lower toward the outer continental shelf (less than 0.02 K/m [0.0112°F/ft]). We believe that the observed variations are primarily attributable to the thermal effect of rapid and regionally variable sediment accumulation during the Cenozoic era, which resulted in the occurrence of the geopressured zone in the Texas–Louisiana shelf. In the eastern Louisiana shelf, where accumulation was fastest, sediments down to about 6 km (3.7 mi) are relatively young (about

Hydrodynamic convergence of hydropressured and geopressured zones, Central Texas, Gulf of Mexico Basin, USA

Freshwater moving downdip in the Carrizo–Wilcox aquifer, Central Texas, USA, and saltwater and hydrocarbons moving updip from a geopressured zone come together in a groundwater convergence zone, marked by (1) a hydraulic-gradient reversal, (2) “updip” oil fields, and (3) the downdip limit of potable water beyond which there is a marked increase in salinity. Data combined from groundwater-supply and petroleum-extraction industries document the interface between the hydropressured and geopressured zones. The hydraulic-head gradient updip of the convergence zone is 0.001 to 0.002, directed toward the coast; farther downdip it is −0.02 to −0.04, directed inland. Salinity increases from 100,000 mg/L in the geopressured zone. Upward-directed flow paths probably predominate in the convergence zone. The convergence zone in the study area lies within only 30–50 km of the outcrop because updip extensional faulting offsets permeable aquifer units against low-permeability strata and restricts the downdip flux of recharged water. The major elements of the convergence zone may have been in place since the Miocene development of circulation in the updip coastal aquifer following incision of river valleys and lowering of base level.

Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field

Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid, lower Na out of both the mineral and fluid, easier dissolution of the mineral. Similarly, lower-K fluid is more powerful to dissolve K-feldspar than the higher. The anorthite component of plagioclase is most easily dissolved in ground water-rock system, the albite is the secondary, and K-feldspar is the most stable. Selective dissolution of plagioclase occurs in diagenesis because of the plagioclase solid solution that is mainly composed of albite and anorthite end-members, Ca-rich part of which is preferentially dissolved by the pore water, in contrast to the Na-rich parts. Based on investigation of reservoir quality, secondary pore, dissolution structures of feldspar, clay minerals and ground water chemistry of the Kela 2 gas field of Kuqa Depression in the Tarim basin of Western China, we discovered that the secondary pore is very well developed in the highest quality section of the reservoir, and the plagioclase of which was obviously selectively dissolved, in contrast to the overgrowth of K-feldspar. Chemistry of the ground water out of the highest quality section of the reservoir is consistent with the complete dissolution field of anorthite, partial dissolution field of albite component of plagioclase, and the precipitation of K-feldspar on the temperature-logK phase equilibrium diagram drawn from this study. The thermodynamic modeling calculation gave us an insight into formation of the secondary pore within the reservoir, and the dissolution of plagioclase an important cause of the secondary pore. It is assumed that the solvent to dissolve the plagioclase frame grains originated from the origin-enriched compacted fluid acids out of the mudstone buried beneath the top of geopressured zones in the Kuqa Depression.

Salinity variation of formation water and diagenesis reaction in abnormal pressure environments

An understanding to the chemistry of formation waters in sedimentary basins is important for many geological processes, such as the fluid-rock interaction, the migrating paths of fluid and the entrapment mechanisms of hydrocarbon. This paper deals with the salinity variation of formation water and diagenesis reaction in the abnormally pressured system. The Shiwu depression of the Songliao basin and the Yinggehai basin are selected for case studies. The studies indicate that there is a distinct difference in the chemistry of subsurface water between hydropressured and abnormally pressured systems. The Shiwu depression of the Songliao basin is composed of terrigeneous clastics in fluvial-lacustrine environments, which contain brackish water with salinity ranging from less than 1000 mg/L to 12000 mg/L. Water composition varies with depth and hydrochemical regions. In the underpressured strata deposited during the rifting period, the formation water is characterized by CaCl2 type water with high salinity. NaHCO3-dominanted water with lower salinity occurs at the hydropressured strata deposited during the post-rifting period. In this halite-free basin, brackish water may be attributed to the condensation of meteoric water and water-rock interaction. In the deeply buried underpressured water, a predominated diagenesis reaction resulting in enrichment of Ca and Cl and reduction of Na may be related to the albitization of plagioclase following the basinal fluid line (BFL). The Yinggehai basin constitutes clastic deposits in nearshore, neritic, shelf environments and contains brackish and saline water with salinity ranging from less than 15000 mg/L to 50000 mg/L. Pore water in these marine deposits must have originated from seawater. In the overpressured system, however, the formation water has much lower salinity and ion concentrations (except bicarbonate and carbonate) than normal seawater. The low salinity indicates that interstitial connate seawater is diluted by the water released from the transformation of smectite to illite. High bicarbonate and carbonate concentrations may contribute to mineral dehydration and kerogen-cracking reactions. Furthermore, low salinity water also occurs near and above the top of the geopressured zone in the diapiric structures, which may indicate that pore water with normal seawater salinity has been mixed by fresher saline water from deeper portions of the overpressured stratigraphic section.

Texture, Mineralogy, and Petrophysical Properties of Geopressured Shales, Gulf of Mexico

ABSTRACT Geopressured shale samples from wells located offshore Louisiana, Gulf of Mexico were examined by scanning electron microscopy (SEM), and compared with well-log analysis to study the characteristics of fabrics, fractures, and changes in porosity and permeability caused by high fluid pressures. Well logs from this study area including neutron-density, conductivity and sonic logs show features typical of a geopressured zone at a depth of 2200 m. The active smectite-to-illite transition zone indicated by changes in CEC and total K also corresponds to the top of the geopressured zone. Geopressured shales with increasing burial depth become more illitic, better oriented, and less porous; they show a significant decrease in permeability and a rise in the fluid pressure gradient. The shale compaction curve as a function of burial depth was used to determine the maximum sealing depth for hydrocarbon. In this study shales effectively seal above 2400-m depth with the maximum sealing depth of 1260 m. Shales undergo slow-drained compaction down to 2400 m while below 2400-m depth shales undergo drained compaction and fractures begin to develop.

Electron microscopic observations of shale diagenesis, offshore Louisiana, USA, Gulf of Mexico

Scanning and transmission electron microscopic analyses of shale samples from offshore Louisiana, USA, Gulf of Mexico, reveal the relationship between mineralogical and microfabric changes during burial diagenesis. The local geopressured zone begins at 2200-m depth. Above that depth the shales are smectite-rich, generally lack particle orientation, and contain appreciable pores. Below the 2200-m depth, the shales become more illite-rich with increasing burial, more crystalline, and less porous. Microfabric changes are mainly caused by compaction during burial diagenesis; mineralogical changes (smectite-to-illite) and crystal growth also play an important role in fabric alteration during deep burial diagenesis.

Abstract: A Geological Exploration Model for Offstructure Gas Accumulations in Geopressured Lower Miocene Sandstones, Offshore Texas

Abstract: A Geological Exploration Model for Offstructure Gas Accumulations in Geopressured Lower Miocene Sandstones, Offshore Texas

Buoyancy‐driven propagation of isolated fluid‐filled fractures: Implications for fluid transport in Gulf of Mexico geopressured sediments

A large portion of the sediments within the northern Gulf of Mexico contain pore fluid pressures in excess of hydrostatic. Development of geopressure is generally attributed to compaction disequilibrium caused by rapid deposition of low‐permeability sediments in the Miocene and Plio‐Pleistocene. Numerous studies have examined the formation of overpressures and/or expulsion of geopressured fluids into overlying hydropressured strata. However, very little attention has been given to fluid flow within the geopressured zone itself. Movement of oils from Cretaceous or older source rocks into Plio‐Pleistocene reservoirs in the Gulf Basin requires as much as 10 km of vertical migration in a few million years. Precipitation of cements in some geopressured sediments also implies large‐scale fluid flow. New evidence from a deep well in the Eugene Island area, offshore Louisiana, indicates that geopressured sediments are mechanically very weak with a Poisson's ratio greater than 0.4 and a shear modulus or rigidity less than 1 GPa. In addition, large‐scale fluid flow either through interconnected pores or fractures is not occurring in this location, at least at present. An alternative hypothesis is that upward fluid transport in geopressured sediments is caused by buoyancy‐driven propagation of isolated fluid‐filled fractures. Using linear fracture mechanics, I show that vertical fractures with lengths of a few meters can propagate at velocities of 1000 m/yr. Mass flux rates (∼100 kg/m2/yr) are significant assuming a mechanism for formation of fluid‐filled fractures exists, such as hydrofracturing when fluid pressures exceeded the minimum confining stress. Fracture propagation velocity and mass flux rate are strongly dependent on the shear modulus of geopressured sediments.

High resolution three-dimensional seismic survey of a thin sand at depth : Geophysics V56, N12, Dec 1991, P2036–2047

A thin sand has been successfully imaged at great depth using a carefully designed, high-resolution, three-dimensional (3-D), seismic survey. The area of the survey was along a portion of the boundary between northeastern Vermilion Parish and southern Lafayette Parish about twelve miles south of Lafayette, Louisiana. Surface terrain was typically flat farmland at the southern edge of the Pleistocene Prairie Terrace and was ideal for this type of high-resolution survey. The greatest elevation difference between any two source or receiver locations was about 6 ft (1.8 m).The target for this survey was the Cib jeff sand at a depth of about 13,400 ft (4084 m). The Cib jeff sand is within massive shales and is within a geopressured zone. Relative isolation of the Cib jeff sand by the surrounding shales makes the sand a good candidate for seismic imaging.After some preliminary field tests a survey was designed which used the crossed-array method in which the source and receiver lines are at some angle to one another, usually orthogonal. Data were collected using a 1024-channel recording system with vibrators as sources. Receiver arrays were not used as it was possible to sweep with frequencies outside the frequency range of the ground roll. A total of 1.38 X 10 6 seismic traces was collected.Many tests were also carried out in the processing of these data and it was found that large variations of offsets in the data volume resulted in deterioration in the quality of stacked data due to nonhyperbolic moveout. The migrated data volume was restricted to traces with less than 7500 ft (2286 m) source-to-receiver offset, and to the time window containing the Cib jeff sand, namely from 2.5 to 5.0 s.The Cib jeff sand was successfully imaged and the migrated data volume was interpreted using paper sections. As one would expect, the interpretation based on the total volume is more complex than the interpretation using available conventional two-dimensional (2-D) lines. In particular the fault pattern interpretation based on 2-D seismic data and well logs is believed to be in error. The western bounding fault is placed further west and other faults were delineated through the reservoir when the interpretation was based on the total 3-D volume. Overall, we believe that this reservoir was mapped with more control than was possible with 2-D data.

High‐resolution three‐dimensional seismic survey of a thin sand at depth

A thin sand has been successfully imaged at great depth using a carefully designed, high‐resolution, three‐dimensional (3-D), seismic survey. The area of the survey was along a portion of the boundary between northeastern Vermilion Parish and southern Lafayette Parish about twelve miles south of Lafayette, Louisiana. Surface terrain was typically flat farmland at the southern edge of the Pleistocene Prairie Terrace and was ideal for this type of high‐resolution survey. The greatest elevation difference between any two source or receiver locations was about 6 ft (1.8 m). The target for this survey was the Cib jeff sand at a depth of about 13,400 ft (4084 m). The Cib jeff sand is within massive shales and is within a geopressured zone. Relative isolation of the Cib jeff sand by the surrounding shales makes the sand a good candidate for seismic imaging. After some preliminary field tests a survey was designed which used the crossed‐array method in which the source and receiver lines are at some angle to one another, usually orthogonal. Data were collected using a 1024-channel recording system with vibrators as sources. Receiver arrays were not used as it was possible to sweep with frequencies outside the frequency range of the ground roll. A total of [Formula: see text] seismic traces was collected. Many tests were also carried out in the processing of these data and it was found that large variations of offsets in the data volume resulted in deterioration in the quality of stacked data due to nonhyperbolic moveout. The migrated data volume was restricted to traces with less than 7500 ft (2286 m) source‐to‐receiver offset, and to the time window containing the Cib jeff sand, namely from 2.5 to 5.0 s. The Cib jeff sand was successfully imaged and the migrated data volume was interpreted using paper sections. As one would expect, the interpretation based on the total volume is more complex than the interpretation using available conventional two‐dimensional (2-D) lines. In particular the fault pattern interpretation based on 2-D seismic data and well logs is believed to be in error. The western bounding fault is placed further west and other faults were delineated through the reservoir when the interpretation was based on the total 3-D volume. Overall, we believe that this reservoir was mapped with more control than was possible with 2-D data.

Perched brine plumes above salt domes and dewatering of geopressured sediments

Previous studies have shown that there is a brine plume with more than 160 g l −1 total dissolved solids (TDS) in the subsurface, perched above Welsh Salt Dome in South Louisiana. The occurrence of this plume has been postulated to be due to the expulsion of geopressured fluids up a fault on the flank of Welsh Dome. The process that resulted in formation of the anomalous brine plume near Welsh Dome is of importance because hydrocarbon production is approximately coincident with the brine plume in map plan, although in cross-sections, the brine plume actually occurs a few hundred meters above hydrocarbon production zones. Using a variable density groundwater flow model, Sutra, we tested the hypothesis that a pulse of fluid flow up the fault could result in brine formation near the top of the dome. When the permeability of the geopressured zone was very low in our simulations, brine formation in the hydropressured zone occurred entirely by brine-density flow and flow was downward along the fault flanking the dome. Above a certain threshold permeability for the geopressured zone, between 10 −7 darcy and 10 −5 darcy in the vertical direction, geopressured fluids were forced up the fault and brine formation occurred above the dome rather than on the flank. Brines can theoretically form above salt domes in very short geologic times (10 −1 Ma) by expulsion of waters from the geopressured zone below, but the fluid effluxes required are gigantic and appear to be unsustainable over large time intervals such as 10 Ma.

A vein and disseminated Ba-Pb-Zn deposit in the Appalachian thrust belt, St.-Fabien, Quebec

The St.-Fabien deposit occurs in the Taconic thrust belt of the Quebec Appalachians. It consists of intimately associated veins and disseminations of barite, galena, and sphalerite hosted in interdigitated sandstones and limestone conglomerates comprising a proximal, deep submarine facies of the Upper Cambrian St.-Damase Formation.At a late stage in the tectonic evolution of the host rocks, hydraulic fracturing driven by the buildup of a geopressured zone took place in a dilatant stress field without mechanical relationships to folding. Episodic opening of the fractures enhanced corrosive fluid flow and led to the development of a secondary dissolution porosity followed by pervasive dolomitization of large volumes of host rocks and partial sealing of the fractures and pores by dolomite, followed by ferroan dolomite, in a reducing environment of low sulfur activity.We propose that an interconnected network of incompletely cemented fracture and dissolution porosities channeled oxidizing, sulfate-bearing fluids into this reducing environment. These oxidizing fluids mixed with deep orogenic metalliferous fluids, set in motion by the Taconic orogeny, and were partially reduced to aqueous sulfide. At this stage, barite, galena, and sphalerite precipitated in the remaining open spaces of varying redox potential and partially replaced precursor cements and framework constituents. Remaining open spaces were cemented by calcite.Rare earth element patterns for the various generations of gangue dolomite and late calcite are characterized by positive europium anomalies ascribed to the partial dissolution of detrital potassium feldspars before, during, and after mineralization. Detrital feldspars are suggested as a major source of lead and barium in the St.-Fabien deposit. Lead isotopes support this crustal source for the lead, and their model age is consistent with the precipitation of galena during the late stages of, or slightly after, the Taconic orogeny. Homogeneity of the lead isotope ratios of both vein and disseminated galenas throughout the St.-Fabien deposit suggests that the lead isotopes were efficiently averaged throughout this part of the Taconic belt by a large volume of fluid.

Density-driven groundwater flow near salt domes

Active halite dissolution occurs around numerous salt domes in Louisiana forming brines with as much as 300 g l −1 TDS (TDS = total dissolved solids), most of which is Na and Cl. We numerically modeled the possible dynamics of: (1) brine-density flow induced by solute concentration gradients; and (2) thermal convection induced by temperature gradients, around hypothetical salt domes. Calculated maximum flow velocities due to brine density flow range from 0.01 to 1 m yr. −1 even in low-permeability sediments, much greater than velocities due to thermal convection. Immense quantities of halite can be dissolved by brine-density flow, up to 3 km 3 Myr. −1, and formation of brine plumes can occur very rapidly, in some cases requiring <1 Myr. Maximum flow velocities increase with bulk permeability and decrease with increasing anisotropy. Geopressured waters bleeding upward into the hydropressured zone may affect brine formation in the hydropressured zone, provided permeability of the geopressured zone is sufficiently high.

Linear and nonlinear solutions for one‐dimensional compaction flow in sedimentary basins

We consider the effects of assuming linearity in solving the moving boundary problem that describes one‐dimensional groundwater flow in compacting sedimentary basins. The linear problem in which specific storage and hydraulic conductivity do not vary can be solved analytically. This solution should be applied to geological problems with caution because of the implications of assuming that specific storage is constant over the range of stresses typical of sedimentary basins. A nonlinear equation in which specific storage and hydraulic conductivity vary with effective stress describes compaction flow more realistically. This equation can be solved accurately by a numerical method. Solutions to the nonlinear equation show that the solution to the linear equation can significantly overpredict excess heads in compacting basins. Large excess heads develop by compaction in sediments of lesser conductivity and over a smaller conductivity interval than suggested by the linear solution. The nonlinear solutions predict that geopressured zones should develop in shaly basins undergoing rapid sedimentation but fail to explain the abrupt transition from hydrostatic to near‐lithostatic pressures observed in these basins. The transition is better explained as the result of a facies change or a discontinuity in conductivity caused by diagenetic reactions than a decrease in hydraulic conductivity during compaction.