Porosity Zones Research Articles

Elastic-wave and related properties of clastic rocks from a part of Niger Delta, Nigeria

Elastic-wave velocities of drill-core specimens with mineral compositions and textures of elastic rocks at Niger Delta, Nigeria, can be useful for identifying large zones of high porosity or high clay content (such as some fault zones, or the clay-rich haloes surrounding known uranium ore bodies). The overall objective of this study is to identify zones of high porosity and high clay contents using elastic-wave velocities Vp and Vs, and the ratio Vp/Vs. Thin sections were studied, and measurement of Vp, Vs, and porosity were made from cylindrical core specimens representing the Niger Delta Agbada Formation. Multiple linear regression has been employed to determine 1\Vp and 1\Vs, and the ratio Vp/Vs as a function of porosity (Φ) and clay fraction (Fc) for all specimen from the study area. The seismic velocities Vp and Vs, and the ratio Vp/Vs for Agbada sediment are found to be influenced by changes in porosity more strongly than by changes in clay content by a factor of approximately four. For clay fractions less than 0.15, simple linear relationships appear to exist between the reciprocal velocities 1/Vp and 1/Vs, and the ratio Vp/Vs, and porosity. Keywords: compressional, shear velocity, porosity, permeability, clastic rock Nigeria Journal of Pure and Applied Physics Vol. 4(1) 2005: 82-89

Deeply buried, Early Cretaceous paleokarst terrane, southern Maracaibo Basin, Venezuela

Cretaceous carbonate rocks formed an extensive passive-margin section along the northern margin of the South American plate and are now found in outcrops in elevated and deformed ranges like the Mrida Andes and Sierra de Perij. Regional seismic profiles correlated with well data show that a 300-m (984-ft)-thick Cretaceous carbonate platform underlies all of the Maracaibo Basin of western Venezuela. We examined the Cretaceous carbonate section beneath the southern Maracaibo Basin using a 1600-km2 (617-mi2) area of three-dimensional (3-D) seismic reflection data provided by Petrleos de Venezuela, S. A., along with wells to constrain the age and environments of seismic reflectors. Well data allow the identification and correlation of the major lithologic subsurface formations with formations described from outcrop studies around the basin edges. Seismic reflection time slices at a depth range of 3.7–4.5 s (5–7 km; 3.1–4.3 mi) reveal the presence of a prominent, irregular reflection surface across the entire 3-D study area that is characterized by subcircular depressions up to about 600 m (1968 ft) wide and about 100 m (328 ft) deep. We interpret the subcircular features as sinkholes formed when the Lower Cretaceous carbonate platform was subaerially exposed to weathering in a tropical climate. The scale of the observed circular features is consistent with dimensions of limestone sinkholes described from modern karst settings. Correlation of the inferred karst horizon with well logs shows that the paleokarst horizon occurs within the shallow-water carbonate rocks of the Aptian Apn Formation. We infer that the karst formed during an Aptian eustatic sea level fall described from Aptian intervals in other parts of the world, including the Gulf of Mexico. The Aptian paleokarst zone provides a previously unrecognized zone of porosity for hydrocarbons to accumulate beneath the Maracaibo and perhaps other basins formed above the extensive passive margin of northern South America.

Abundance and mineralogical association of arsenic in the Suwannee Limestone (Florida): Implications for arsenic release during water–rock interaction

Arsenic (As) is being released from the Suwannee Limestone, Upper Floridan aquifer, during aquifer storage and recovery (ASR) cycle testing with concentrations of up to 130 μg/L in the recovered water. To determine the abundance and mineralogical association of As in the Suwannee Limestone matrix we have conducted a detailed mineralogical and geochemical study of 306 core samples. In addition to random sampling of core material, we also collected “targeted” samples of core material that contained organic material, hydrous ferric oxide, pyrite, phosphate minerals and clays. Bulk As concentrations were determined by acid digestion followed by hydride generation-atomic fluorescence spectrometry (HG-AFS). Specific mineral phases were analyzed using scanning electron microscopy (SEM) and electron-probe microanalyses (EPMA). The average As concentration for all 306 samples is 3.5 ppm, but only 1.7 ppm after exclusion of the “targeted” samples. The average for the “targeted” samples is 9.5 ppm. The detailed lithologic, mineralogical, and geochemical study of As in the Upper Floridan aquifer, Suwannee Limestone shows that: (1) Arsenic is present in the Suwannee Limestone in low concentrations, but is concentrated in minor mineral phases, particularly pseudo-framboidal pyrite. (2) Pyrite is generally As-rich and can contain concentrations between 100 and 11,200 ppm As (average 2300 ppm, n = 25). (3) Compared to pyrite, other trace minerals contain much less As. (4) Pyrite is ubiquitous throughout the Suwannee Limestone, but is most abundant in high porosity zones. (5) Previously suggested hydrous ferric oxide, clay minerals and apatite are not an important source of As. This study also provided some insight into the question whether the interaction (reaction) of water with an aquifer matrix that contains only a few part per million As is capable of producing high-As concentrations in groundwater. Our findings indicate that it can be relatively easy to generate a high-As groundwater despite a low bulk As concentration in the aquifer matrix. A change in physico-chemical conditions that selectively affects the stability of As-bearing minerals is sufficient to increase As concentrations by several orders of magnitude.

Contribution of dual phase (α, G, B) heat treatments to fatigue properties of SG cast irons

Ferritic spheroidal graphite (SG) cast irons are used to make parts that require high ductility and fracture toughness and good high-cycle fatigue resistance. Ferritising anneals have an all-ferritic matrix favouring high fracture toughness and ductility but this does not improve fatigue strength, and even lowers it in some cases. The purpose of the present study was therefore to optimise these heat treatments for the best compromise between fracture toughness and fatigue life. To attain this, the fatigue behaviour of cast irons was studied in the as cast and heat-treated conditions. A comparison of the lives and mechanisms of ferritisation of these metallurgical conditions made it possible to evaluate the influence of various parameters (characteristics of the initiation sites, properties of the matrix). From the present study, a method is proposed to optimise the heat treatments to produce ferrite in association with a stronger phase in the zones where microfissures are initiated. A study of heat treatments favouring the formation of a matrix made up of ferrite and bainite yielded encouraging results. The presence of bainite near the graphite spheroids and zones of porosity can increase resistance to crack initiation. This leads to a substantial improvement of fatigue life of the SG cast iron without at the same time embrittling the material.

Provenance and diagenesis of the evaporite-bearing Burns formation, Meridiani Planum, Mars

Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.

Reservoir property estimation in the carbonate reservoir with rock property inversion- a case study

One of the fundamental aims of the geophysics is to develop attributes which are understandable and acceptable to seismic interpreters, geologists and reservoir engineers. Elastic impedance inversion from far offset seismic reflection data can be considered as one of such desired attributes. It can used to derive elastic rock properties, which can be used for quick determination of lithology and fluid content. The role of the rock property inversion, as a tool for characterisation of carbonate reservoirs, has not been established. A carbonate reservoir in the South-West of IRAN is selected to examine the feasibility of rock property inversion for reservoir property estimation and lithological characterization. Of particular important was to apply this methodology in an area where reservoir rock is of a low reflectivity and acoustic impedance alone fails to separate high from low porosity zones. Lambda-Rho (??) and Mu-Rho (µ?) attributes were computed from both rock physics lab measurements and 2-D surface seismic survey. We show that the variation of µ? across a carbonate reservoir can be used to distinguish high porosity-permeability areas from non-reserve zones. The field data results correlate well with core sample measurements. We also show that in this case ?? cannot be reliably used to discriminate between oil and brine saturation.

Improving recovery from mature oil fields producing from carbonate reservoirs: Upper Jurassic Smackover Formation, Womack Hill field (eastern Gulf Coast, U.S.A.)

Reservoir characterization, modeling, and simulation were undertaken to improve production from Womack Hill field (eastern Gulf Coast, United States). This field produces oil from Upper Jurassic Smackover carbonate shoal reservoirs. These reservoirs occur in vertically stacked, heterogeneous depositional and porosity cycles. The cycles consist of lime mudstone and wackestone at the base and ooid grainstone at the top. Porosity has been enhanced through dissolution and dolomitization. Porosity is chiefly interparticle, solution-enlarged interparticle, grain moldic, intercrystalline dolomite, and vuggy pores. Dolostone pore systems and flow units have the highest reservoir potential. Petroleum-trapping mechanisms include a fault trap (footwall uplift with closure to the south against a major west-southeast–trending normal fault) in the western area, a footwall uplift trap associated with a possible southwest-northeast–trending normal fault in the south-central area, and a salt-cored anticline with four-way dip closure in the eastern area. Potential barriers to flow are present as a result of petrophysical differences among and within the cycles, as well as the presence of normal faulting. Reservoir performance analysis and simulation indicate that the unitized western area has less than 1 MMSTB of oil remaining to be recovered, and that the eastern area has 2–3 MMSTB of oil to be recovered. A field-scale reservoir management strategy that includes the drilling of infill wells in the eastern area of the field and perforating existing wells in stratigraphically higher porosity zones in the unitized western area is recommended for sustaining production from the Womack Hill field.

Crosswell electromagnetic and seismic imaging: An examination of coincident surveys at a steam flood project

We studied crosswell electromagnetic and seismic images of three oil‐saturated intervals within a southern California heavy oil field undergoing steam flood. The crosswell survey is located in a portion of the field where one well is in a “cold spot,” resulting in differing steam propagation within the three units. Log analysis shows linear or second‐order polynomial relationships (with correlation coefficients greater than 0.7) between electrical conductivity and water saturation, porosity, and clay content; whereas only a weakly linear relationship can be found between velocity and temperature in the lower unit studied. Crosswell seismic data is used to produce a velocity tomogram and a reflection section, and crosswell electromagnetic data is used to produce a conductivity section and derived porosity and water saturation. The seismic velocities from the tomograms show lateral variations consistent with the lateral variations in temperature seen in observation wells on either side of the crosswell section. The continuity and disruption of seismic reflections coincide with zones of continuous and variable porosity and water saturation as produced from the crosswell electromagnetic inverted conductivity section, and the derived regression fits between conductivity and porosity/water saturation. Seismic velocities, reflections, electrical conductivity, and the derived porosity and water saturation sections all contribute to explaining the observed lateral temperature variations between the wells within the three reservoir units. The unit with high steam content has low water saturation, high porosity, and laterally continuous low‐velocity and seismic reflections, consistent with no barriers to flow. The upper unit, where steam breaks through later in the experiment, also has high porosity and laterally continuous seismic reflections. However, it shows a velocity gradient between the hot and cold wells consistent with the lack of steam in the cold well at the time of the experiment. The middle unit, in which steam never reaches the cold well during the experiment, has the highest water saturation and the largest zone of reduced porosity and disrupted seismic reflections, indicating a possible barrierto flow.

Modeling of non-reactive solute transport in fractured clayey till during variable flow rate and time

Fractures and biopores can act as preferential flow paths in clay aquitards and may rapidly transmit contaminants into underlying aquifers. Reliable numerical models for assessment of groundwater contamination from such aquitards are needed for planning, regulatory and remediation purposes. In this investigation, high resolution preferential water-saturated flow and bromide transport data were used to evaluate the suitability of equivalent porous medium (EPM), dual porosity (DP) and discrete fracture/matrix diffusion (DFMD) numerical modeling approaches for assessment of flow and non-reactive solute transport in clayey till. The experimental data were obtained from four large undisturbed soil columns (taken from 1.5 to 3.5 m depth) in which biopores and channels along fractures controlled 96–99% of water-saturated flow. Simulating the transport data with the EPM effective porosity model (FRACTRAN in EPM mode) was not successful because calibrated effective porosity for the same column had to be varied up to 1 order of magnitude in order to simulate solute breakthrough for the applied flow rates between 11 and 49 mm/day. Attempts to simulate the same data with the DP models CXTFIT and MODFLOW/MT3D were also unsuccessful because fitted values for dispersion, mobile zone porosity, and mass transfer coefficient between mobile and immobile zones varied several orders of magnitude for the different flow rates, and because dispersion values were furthermore not physically realistic. Only the DFMD modeling approach (FRACTRAN in DFMD mode) was capable to simulate the observed changes in solute transport behavior during alternating flow rate without changing values of calibrated fracture spacing and fracture aperture to represent the macropores.

Mobilizing particles in a saturated zone during air sparging.

The mobilization of soil particles changes the porosity of saturated zone during air sparging. Soil porosity is shown to be correlated with soil electrical resistivity. This study performs porosity-resistivity tests to establish the relationship between porosity and resistivity of quartz sand. Experiments, involving a large sandbox to simulate the saturated zone, are then performed to compare the resistivity of compacted sand before air injection with that after air injection. The relevant data enable the mobilization of quartz sand particles to be quantified. Results of the experiments indicate the mobilization of sand particles and an increase in porosity directly proportional to the rate at which air is injected. Besides, a layer of fine-grained particles covered the compacted sand at the upper boundary of sandbox after each air injection experiment. This is direct evidence that finer particles were transported upward during air sparging. Two methods were applied to verify the results of this study. The first verification method indicated that changes in porosity increased directly proportional to the air injection rate, which is consistent with shear theory. The other validation method indicated that the mass of sand in the tank did not change after air sparging, which indicates that the resistivity-porosity method is unbiased.

Interplay of gouge, fluid pressure and porosity in fault zones

The objective of this study is to demonstrate how fault dynamics may naturally be placed in the context of incremental stress theory, rock textural evolution modeling and standard conservation laws. Casting the fault dynamics problem in this framework naturally introduces rock memory for failure, fluid pressure effects and the autonomous nature of fault evolution. Poroelasticity, nonlinear viscosity and gouge are combined in an incremental stress rheology approach to examine the effect of changes in particle size distribution on fluid pressure in fault zones.

Porosity and compaction trend in Okan field (Western Niger Delta) based on well log data.

Porosity and compaction trends in Okan Field have been highlighted based on the interpretation of well log data within the productive interval of 5000 to 10,000 ft. Six Porosity Zones have been delineated based on the interpretation of gamma – ray log from the representative wells. The zones that occurred between 5000 ft. and 8000 ft. appeared to be tectonically controlled whereas the deeper zones result mainly from diagenetic changes associated with the burial history of the sediments. Regionally, these zones represent from bottom to top, cyclic deposition of clays and sands which become increasingly sandy upward through the sequence. Porosity values are in the range of 15 and 25% and this is good enough to enhance hydrocarbon accumulation in the area. Sedimentation pattern is normal in most parts of the field except for a local zone of overpressure observed at a depth of 8300 ft. in the northeastern section of the field. The main hydrocarbon trap results from vertical stacking of porous reservoirs sands while the stratigraphic trap is evident from reservoir Pinchouts. Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 91-96 KEY WORDS: Porosity, Reservoir, Sedimentation, Overpressure, Hydrocarbon.

Density profile and flow of driven gas in an open porous medium with a computer simulation

A computer simulation model is used to study the flow of gas from a source through open porous media on a simple cubic lattice with a pressure bias. This model is used to represent marine sediments through which fluid flows. Isotropic and layered porous media are considered and the gas fluid is modeled by an interacting lattice gas. In isotropic porous medium with porosity p s =0.35, the density profile is linear in the absence of bias ( B=0) and oscillations develop in the presence of bias. At low porosity ( p s =0.32), the density profile becomes nonlinear in the presence of bias. The response of the flow rate to bias is linear in media with high porosity – consistent with the Darcy law. In porous medium with low porosity, the response of flow rate is linear only in low bias ( B⩽0.3) which crosses over to a different linear response at moderate values of bias (0.30⩽ B⩽0.7) before it becomes negative at extreme bias ( B⩽0.8). The presence of fault planes (i.e., linear zones of higher porosity within a low-porosity system) broadens the linear-response regime.

Permanent water level drop associated with the Spitak Earthquake: observations at Lisi Borehole (Republic of Georgia) and modelling

We present the results of a series of measurements that were made between 1988 and 1992 at Lisi (Georgia). Water level variations in the Lisi well, barometric pressure, precipitation (including rain and snow) and temperature measurements were made during this period. A hydraulic ‘slug test’ has been performed more recently in the well. Two major seismic events occurred during the observation period in the Caucasus area. The Spitak seismic event of 1988 December 9, 110 km from the Lisi borehole, left a clear post-seismic hydraulic signature, whereas the second event, that of 1991 April 29, 125 km from the borehole, did not seem to induce any detectable anomaly. First, we analyse the tidal and barometric responses of the water level in order to calibrate the borehole and to determine the hydraulic parameters of the aquifer. Then we develop a model for aquifer recharge by meteoric precipitation. Finally, we compare our model with the observed water level variations in the well. We highlight anomalous behaviour that correlates with the earthquake, with the following characteristics: the water level drops about 75 cm with a time constant of 6.6 days. The initial water level is never recovered and the change appears permanent on the scale of the period of observation. Since it is delayed in time, the anomalous water level cannot be attributed to coseismic deformation. Following the suggestion of some authors that the observed behaviour could be related to damage of the aquifer due to the passage of seismic waves, we attempted to take this process into account and to model the resulting water level variations in the aquifer. A double porosity model (including fractures and a porous medium) has been used to describe the modifications undergone by the medium. The medium is discretized at two different scales—(1) at the scale of a porous block and (2) at the scale of the fractured system (which may include a finite number of porous blocks). Using this basic model we have tested three interpretative models: (1) a variable-permeability model; (2) a locally varying porosity model; (3) a model combining (1) and (2). We show that an increase in permeability by a factor of 2 is needed in order to describe the permanent water level drop, whilst the locally varying porosity zones account for the observed time constants of the anomaly. Using this model, simulations show that we are able to describe water level variation associated with precipitation for the whole period of observation.

00/02425 A process and apparatus for treating solid fuel materials

LITHOPROBE is Canada's national, collaborative, multidisciplinary geoscience project directed toward an enhanced understanding of how the North American continent has evolved. Research in its eight transects, which span the country and geological time, are spearheaded by seismic reflection studies. These, combined with other studies, provide new insight into the varied tectonic processes that have been active in forming the continent. The Archean Kapuskasing Structural Zone, a prominent feature which cuts obliquely across east-west Superior Province structures, has been imaged as a low-angle thrust sheet above a major décollement, along which 55–70 km of crustal shortening has occurred to bring mid-crustal rocks to the surface. The shortening was accomplished by brittle faulting and erosion at levels above 20 km and ductile shear and flow in the lower crust. The GLIMPCE transect in Lake Huron shows a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville Front Tectonic Zone. A two-stage model involves terrane collision with the cratonic margin in the Early Proterozoic followed by crustal-scale stacking and ramping associated with the Middle Proterozoic Grenvillian orogeny. In Lake Superior, GLIMPCE data reveal the structure of the Middle Proterozoic mid-continent rift that almost splits North America. The Lithoprobe East transect covering the Paleozoic Newfoundland Appalachians reveals three lower crustal. blocks, defined seismically on the basis of reflectivity characteristics imaged along the seismic profiles. Reflective upper crust can be related to geological structures at the surface; the thin-skinned tectonics implied have yet to be reconciled with the block tectonics of the lower crust. Results from the southern Canadian Cordillera show that the Mesozoic-Cenozoic east-west shortening in the eastern and central Cordillera occurred by compressional deformation forming crustal-scale antiforms that overrode a westward thinning cratonic North America; and in the western Cordillera by subduction processes that have tectonically eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity is caused by a wide range of features, including fault/shear zones, lithologie contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

Changes in geometry and cortical porosity in adult, ovary-intact rabbits after 5 months treatment with LY333334 (hPTH 1-34).

The purpose of this study was to determine if the increased cortical bone porosity induced by intermittently administered parathyroid hormone (PTH) reduces bone strength significantly. Mature ovary-intact New Zealand white rabbits were treated with once daily injections of vehicle, or PTH(1-34), LY333334, at 10 or 40 microg/kg/day for 140 days. Geometry of the femoral midshaft was measured to evaluate changes in the cross-sectional moment of inertia (CSMI). Cortical porosity was measured in the midshaft of the tibia by dividing cortical area into three zones based on equal divisions of cortical diameter: near endocortical (Zone I), near intermediate (Zone II), and near periosteal (Zone III) regions. Total cortical porosity significantly increased after PTH treatment from 1.4% in the controls to 6.3% in the higher dose group, but the location of the new porosities was not randomly distributed. In the controls, porosity of Zones I and II (both 1.7%) was almost twice as much as that of Zone III (0.9%). In the lower dose group, cortical porosity of Zone I (5.5%) and II (1.8%) was greater than in Zone III (0.9%), but these differences were not statistically significant. In the higher dose group, cortical porosity of Zone I (11.5%) and II (6. 1%) significantly increased compared with Zone III (1.4%) (P < 0. 0005). Histomorphometric measurements showed that bone formation rate on both periosteal and endocortical surfaces increased, resulting in increased bone area and cortical area in the higher dose group. A model was developed to evaluate the effect of the changes in geometry and porosity on CSMI in the different zones. This simulation model indicated that CSMI in the higher dose group was significantly greater than in the other two groups, despite the increased porosity. We speculate the reason to be that porosity increased near the endocortical surface, where its mechanical effect is small. This increase was more than offset by apposition of new bone on the periosteal surface. These data suggest that (1) PTH increases cortical porosity in a dose-dependent manner, primarily near endocortical surfaces; (2) because of this nonhomogeneous distribution, the mechanical effect of increased porosity is small; (3) the increased cortical porosity associated with PTH treatment is more than offset by periosteal apposition of new bone, causing an overall increase in the bending rigidity of cortical bone; and (4) these changes cannot be accurately evaluated using noninvasive methods of bone densitometry, which cannot account for the location of bone gain and bone loss.

Detection of groundwater conduits in limestones with gravity surveys: data from the area of the Chicxulub Impact crater, Yucatan Peninsula, Mexico.

Small negative gravity anomalies are found in gravity data from along the northwestern shoreline of the Yucatan Peninsula. These anomalies are shown to be due to elongate, shallow anomalous porosity zones in the Tertiary carbonates. These zones are caused primarily by groundwater solution and are presently active conduits for groundwater flow. The association of these small gravity anomalies with known topographic and structural features of the area, which partially overlies the Chicxulub Impact crater, indicates their development was influenced by structures, faults and/or fractures, within the Tertiary and pre-Tertiary carbonates.

The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wall rock alteration

The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct.

Approaches to defining reservoir physical properties from 3-D seismic attributes with limited well control: An example from the Jurassic Smackover Formation, Alabama

Much industry interest is centered on how to integrate well data and attributes derived from 3-D seismic data sets in the hope of defining reservoir properties in interwell areas. Unfortunately, the statistical underpinnings of the methods become less robust in areas where only a few wells are available, as might be the case in a new or small field. Especially in areas of limited well availability, we suggest that the physical basis of the attributes selected during the correlation procedure be validated by generating synthetic seismic sections from geologic models, then deriving attributes from the sections. We demonstrate this approach with a case study from Appleton field of southwestern Alabama. In this small field, dolomites of the Jurassic Smackover Formation produce from an anticlinal feature about 3800 m deep. We used available geologic information to generate synthetic seismic sections that showed the expected seismic response of the target formation; then we picked the relevant horizons in a 3-D seismic data volume that spanned the study area. Using multiple regression, we derived an empirical relationship between three seismic attributes of this 3-D volume and a log‐derived porosity indicator. Our choice of attributes was validated by deriving complex trace attributes from our seismic modeling results and confirming that the relationships between well properties and real‐data attributes were physically valid. Additionally, the porosity distribution predicted by the 3-D seismic data was reasonable within the context of the depositional model used for the area. Results from a new well drilled after our study validated our porosity prediction, although our structural prediction for the top of the porosity zone was erroneous. These results remind us that seismic interpretations should be viewed as works in progress which need to be updated when new data become available.

Downflank hydrocarbon potential identified using seismic inversion and geostatistics: Upper Maastrichtian reservoir unit, Dan Field, Danish Central Graben

Porosities considerably higher than anticipated from porosity/depth trend models are encountered in a Maastrichtian reservoir unit on the western flank of the Dan Field. Because there is a good correlation between seismic impedance and well log porosity, inverted seismic data are used to infer that highly porous zones are widely distributed. The distribution of these high porosity zones is predicted using geostatistical methods based on the inverted seismic data. These predictions contradict the general assumption that porosity deteriorates with depth in the study area. Annealing cosimulation is applied using inversion-derived seismic impedances as soft data and well log porosities as hard data. The sensitivity of the porosity characterization to hydrocarbon in-place estimates is investigated through the calculation of water saturations using height above free water level and simulated porosity as input parameters. Multiple realizations show that calculated hydrocarbon in-place estimates are more sensitive to the location of the free water level than to uncertainties related to the geostatistical reservoir characterization.