Pinch Outs Research Articles

Prestack diffraction separation by parameterizing the reflection local slope

Diffracted waves provide an opportunity to detect small-scale subsurface structures because they give wide illumination direction of geologic discontinuities, such as faults, pinch outs, and collapsed columns. However, separating diffracted waves is challenging because diffracted waves have greater geometric amplitude losses and are generally weaker than reflections. To retain more diffracted waves, we have developed a prestack diffraction separation method based on the local slope pattern and plane-wave destruction (PWD) method. In general, it is difficult to distinguish between hyperbolic reflections and hyperbolic diffractions using data-driven local slope estimation in the shot domain. Therefore, we transfer slope estimation in the shot domain to velocity analysis in the common-midpoint domain and ray parameter calculation in the stack domain. The connection between local slope and the normal moveout velocity and the surface-ray parameter is known, which provides a novel approach for estimating the local slope of hyperbolic reflected waves in the shot domain. The estimated slope can provide an exact slope-based operator for the PWD method, thus allowing the PWD to separate diffracted waves from reflected waves in the shot domain. Synthetic and field data tests demonstrate the feasibility and effectiveness of our prestack diffraction separation method.

Subsurface geologic imaging of northeastern Tunisia during the Middle to the Upper Eocene: Insights from integrated geophysical interpretation

The Middle to Upper Eocene series are characterized by multiple hiatuses related to erosion, nondeposition, or condensed series in the Cap Bon and Gulf of Hammamet provinces. We have performed an integrated study taking advantage of surface and subsurface geology, faunal content, borehole logs, electrical well logs, vertical seismic profiles, and surface seismic sections. Calibrated seismic profiles together with borehole data analysis reveal unconformities with deep erosion, pinch outs, normal faulting, and basin inversion that are dated Campanian, intra-Lutetian, and Priabonian compressive phases; these events were also described at the regional scale in Tunisia. Tectonics, sea-level fluctuations, and climate changes closely controlled the depositional process during the Middle to Upper Eocene time. The depositional environment ranges from internal to outer platform separated by an inherited paleo high. We determine eight third-order sequences characterizing the interaction among tectonic pulsations, sea-level changes, and the developed accommodation space within the Middle to Upper Eocene interval. We correlate the obtained results of the Cap Bon-Gulf of Hammamet provinces with the published global charts of sea-level changes, and we find a good correspondence across third-order cycles. Model-based 3D inversion proved to be a solution to model the lateral and vertical lithologic distribution of the Middle to Upper Eocene series.

Tectonic controls on the salt diapir-related Mississippi Valley-type lead-zinc mineralization of Fej El Adoum ore deposit (Northern Tunisian Atlas): Constrains from detailed gravity and drill hole data

Relationship between salt domes and base metal mineralization is widely recognized across many salt-bearing basins (e.g., US Gulf Coast, Southern Europe and North Africa). In the northern Tunisian Atlas, the Mississippi Valley-type Fej El Adoum lead-zinc deposit represents one of these salt-related ore deposits. It consists of a combination of stratabound, veins, stockworks and cavity filling ore bodies, hosted by dolostone and limestone units, formed along the transition between Triassic evaporites and Cretaceous limestone.In the aim to better understand the concealed complex geological structures across the area, new-acquired detailed gravity data, constrained with field investigation and drill hole data were analyzed. Particularly, edge source detection techniques such as vertical gradient, total horizontal gradient and theta angle map were applied to outline structural contacts related to subsurface density variations. The main detected lineaments are in N–S, E-W and NW-SE directions. The N–S direction represents the deepest and most important lineament in direct contact with the evaporitic Triassic, while the transverse E-W direction is related to dextral strike-slip faults that subdivide the area into blocks. These major deeper lineaments have controlled the growth of the Triassic salt dome mainly during the Albo-Aptian as revealed by pinch outs of these series in direction of the crest of the structure. The detected lineaments seem to play a significant role in: (1) providing favorable pathways for the migration and ascent of mineralized fluids and (2) creating open space filling mainly at the interferences of N–S and E-W trending faults present at the contact between Triassic evaporitic unit and Cretaceous limestone. Our results suggest further reserves mainly in the E-W Ain Said occurrence and West of the Dar N'Hal closed mine.Our study also revealed the importance of structural study in the understanding of complex structures such as highly deformed salt dome cap rocks such as the Fej El Adoum diapiric structure.

A statistics-based descriptor for automatic classification of scatterers in seismic sections

Discontinuities and small structures induce diffractions on seismic or ground-penetrating radar (GPR) acquisitions. Therefore, diffraction images can be used as a tool to access valuable information concerning subsurface scattering features, such as pinch outs, fractures, and edges. Usually, diffraction-imaging methods operate on diffraction events previously detected. Pattern-recognition methods are efficient to detect, image, and characterize diffractions. The use of this kind of approach, though, requires a numerical description of image points on a seismic section or radargram. We have investigated a new descriptor for seismic/GPR data that distinguishes diffractions from reflections. The descriptor consists of a set of statistical measures from diffraction operators sensitive to kinematic and dynamic aspects of an event. We develop experiments in which the proposed descriptor was incorporated into a pattern-recognition routine for diffraction imaging. The obtained method is useful for performing the automatic classification of image points using supervised and unsupervised algorithms, as a complementary step to Kirchhoff imaging. We also develop a new type of filtering, designed to address anomalies on the diffraction operators caused by interfering events. We evaluate the method using synthetic seismic data and real GPR data. Our results indicate that the descriptor correctly discriminates diffractions and shows promising results for low signal-to-noise-ratio situations.

Enhancing interpretability with diffraction imaging using plane-wave destruction aided by frequency-wavenumber f-k filtering

A conventional processing workflow favors only the specular reflections, reducing or removing other wavefield interactions. These specular reflections are unsuitable for directly imaging sharp corners, such as those in fault zones and pinch outs; therefore, diffractions are used instead in a technique known as diffraction imaging. Plane-wave destruction (PWD) is a well-established method for removing reflections and imaging diffractions. However, this method assumes a gently variable slope; therefore, it fails to remove energy in areas that do not follow this assumption such as curved interfaces. To remove the remnant energy in these areas and thus enhance the overall interpretability of the diffraction images, we have adopted a simple spatial-variable filter in the frequency-wavenumber f-k domain based on the calculated dip field used for PWD, applied post PWD. To demonstrate the method, we have examined this on a range of synthetic data, complex synthetic data, and real data. The created diffraction images have then been interpreted to evidence the benefit of diffraction imaging in seismic interpretation, helping to delineate pinch outs, faults, and rugose surfaces.

Play distribution and the hydrocarbon potential of the Mannar Basin, Sri Lanka

The Mannar Basin is a frontier failed rift basin between India and Sri Lanka. The Sri Lankan part has an area exceeding 42,000 km2. Although the recent two gas discoveries have confirmed the existence of an active petroleum system in the Mannar Basin, a major portion of the basin is still poorly explored. This article summarized the progress of current exploration activities and the hydrocarbon potential of the Mannar Basin. This basin began to evolve since the Upper Jurassic and experienced two rifting events; an early Late Jurassic syn-rift phase associated with East–West Gondwana break up; and a later, earliest Cretaceous syn-rift phase associated with Antarctica separation from greater India around 142 Ma. Rifting was followed by a post-rift phase comprising a thermal sag period and an inversion period. Three potential source rocks intervals have been interpreted at Maastrichtian–Campanian, Albian–Aptian, and Late Jurassic stratigraphic levels. The basin modelling work has confirmed that (1) mature potential source rocks (mainly Type II) exist below the Maastrichtian–Campanian strata and (2) the best potential source rocks (mainly Type II) exist at Albian–Aptian stratigraphic levels. The Late Jurassic source rocks have more potential for gas, while other sources have potential for both oil and gas. According to basin modelling results, Maastrichtian–Campanian and Albian–Aptian source rocks reach the oil window in the present-day depocentre around 45 Ma and 80 Ma, respectively. The Late Jurassic source rocks (mainly Type III) reach the gas window around 112 Ma in the present-day depocentre. Five play levels were defined for the whole stratigraphic section of the Mannar Basin. Tertiary play level is dominated by submarine fans, mounds and rollover anticline like structures. The Upper Cretaceous play is dominated by forced-fold structures, intra-basalt turbidite sands, and sub-volcanic sand-rich systems. The Lower Cretaceous play is dominated by reefs and abrupt margin pinch outs. The Upper Jurassic play is dominated by abrupt margin pinch outs. The Basement play consists of weathered basement rocks. The main challenge of the Mannar Basin is imaging below the flood volcanic layer, which inhibits the penetration of seismic energy and results in low-quality seismic data. Therefore, hydrocarbon potential assessments have become a major challenge below the Upper Cretaceous. The interpretation shows that the basin has a low risk for the source and reservoir, and high risk for seal and traps. New exploration activity would unlock more potential areas for hydrocarbon accumulations. Finally, the findings of this study can help for better understanding of hydrocarbon potential areas and current progress of exploration activities in the Mannar Basin, Sri Lanka.

Mesh generation for thin layered domains and its application to parallel multigrid simulation of groundwater flow

The generation of detailed three dimensional meshes for the simulation of groundwater flow in thin layered domains is crucial to capture important properties of the underlying domains and to reach a satisfying accuracy. At the same time, this level of detail poses high demands both on suitable hardware and numerical solver efficiency. Parallel multigrid methods have been shown to exhibit near optimal weak scalability for massively parallel computations of density driven flow. A fully automated parameterized algorithm for prism based meshing of coarse grids from height data of individual layers is presented. Special structures like pinch outs of individual layers are preserved. The resulting grid is used as a starting point for parallel mesh and hierarchy creation through interweaved projected refinement and redistribution. Efficiency and applicability of the proposed approach are demonstrated for a parallel multigrid based simulation of a realistic sample problem.

Interpretation of Depositional Environment of Rho Field Using Sequence Stratigraphic Analysis, Shallow Offshore Depobelt, Niger Delta

Candidate Maximum Flooding Surfaces (MFS) identified on Rho-1, Rho-2 and Rho-3 were the basis of correlation across the wells for sequence Stratigraphic analysis in the field. Three (3) candidate MFS which include MFS-1, MFS-2 and MFS-3 were identified using the method adopted by Okoli [1]. Erosional surfaces were also delineated in wells from the observed stacking pattern between a sequence from Galloway model (1989) leading to the correlation of SB-1 and SB-2 across the wells. Using the Galloway model, four (4) depositional sequences were inferred SEQ (1-4). In the first sequence (SEQ 1), deposition occurred in a transgressive episode. Depositional environment was interpreted from electrofacies and revealing stacked sequences of reservoirs predominantly composed of fluvial channels which incised the Upper and Lower Shorefaces on a delta front system. In SEQ 2, looking at the aggradational pattern of the sequence, the formed channels were under the influence of both tidal and fluvial systems. In SEQ 3 and 4, based on electro-facies, the stacked reservoir sands were deposited predominantly in a shoreface/delta front environment and channel incisions in a wave subjugated high energy deltaic setting.
 The entire well section showed deposition in a regressive phase. Proposed conceptual models were generated using Petrel software and could be used as an input reconstruction of subsurface geological models. Indicating correct orientation of geo-bodies and facies belts (pinch outs of sands and shales), depositional dips and gross permeability architecture.

Shallow tunnel detection using SH-wave diffraction imaging

Clandestine tunnels, used for drug or human trafficking and tactical operations, pose a security threat worldwide and remain elusive targets for detection with geophysical methods. P-wave diffraction imaging is an increasingly common technique for detecting subsurface discontinuities that are smaller than the seismic wavelength (such as faults, pinch outs, and small voids) and has been successfully used to detect shallow tunnels. P-wave diffractions from tunnels typically have very low signal-to-noise ratios and are therefore challenging wavefield components for imaging. Mode-specific amplitude characteristics of theoretical diffractions from a shallow tunnel were evaluated using 9C seismic modeling. Results indicate that SH-wave diffraction has the largest amplitude and coherent phase characteristics along the traveltime hyperbola, making it ideal for diffraction imaging. In real data acquired over a 9.2 m deep tunnel, amplitudes of SH-wave diffractions are 20 dB greater than P-wave diffractions. The tunnel signature on the P-wave diffraction section is of low amplitude relative to the background. The SH-wave diffraction section has a high-amplitude signal focused at the horizontal location and a traveltime consistent with the tunnel location, indicating that the SH-wave may be optimal for diffraction imaging to detect shallow tunnels.

Grid design and numerical modeling of multiphase flow in complex reservoirs using orthogonal collocation schemes

An advanced grid system design was developed to capture accurately the effects of geometrically complex features such as geological features (faults, pinch outs and inclined beddings) and well-related phenomenon (multilateral wells of general orientation) in triangular coordinates. Modeling these effects can have significant impact on the accuracy of the simulation and prediction of reservoir performance as well as reservoir fluid flow using conventional grid designs. The finite difference method provides additional difficulty in capturing geological features in typical reservoir flow and grid model simulators. Hence, the orthogonal collocation method was used for simulating multiphase reservoir flow equations in triangular curvilinear coordinates left[ {xi left( r right),xi left( theta right),xi left( z right)} right] of domain [0,1] that were derived from Cartesian coordinates left( {x,y,z} right). This was to accommodate general three-dimensional deviated wells and complex reservoir geometry for multiphase flow of hydrocarbon in complex reservoir formations. Based on preliminary field data obtained from multinational oil and gas operator in Nigeria, the proposed model was used to predict saturation, production and petroleum productivity with time and distance in a MATLAB environment. The simulated plots revealed that pressure is parabolic at the center of the reservoir with coordinates xi (r) =0.4257, reflecting the impact of geological features in the pressure and production flow performance.

Visualization of three dimensional earth fissures in geological structure

Abstract. This paper proposes a new method for visualizing the earth fissures of geological structure in three dimensional (3-D) domains on the basis of the seismic data and features information of earth fissures. The seismic data were interpreted for obtaining the stratagraphic data with various lithological information and the depth of the earth fissures. The spatial distribution of the ground fissures including the dip, strike and width were digitalized on an ArcGIS platform. Firstly, the 3-D geological structure was rebuilt using the Generalized Tri-Prism (GTP) method which is a real solid method for displaying geological structures. The GTP method can reflect the inner material of the strata and can simulate complicated geological structures such as faults and stratagraphic pinch outs. The upper and lower surfaces of each stratum consist of Triangle Irregular Networks (TIN). The inner solid between the two surfaces are a series of triangular prisms. Secondly, since the width of the ground fissure gradually decreases with depth, multiple edge lines of the earth fissures on the bottom stratum surface are deduced on the basis of the fissure characteristics. Then, the model of the earth fissures consisting of a series of triangular pyramids can be constructed using these points and the edge lines. A cutting operation was carried out on the 3-D geological structure using this ground fissures model. If the surfaces of the ground fissures model intersects with the GTPs in the geological structure model, new GTPs were generated within the local regions. During this process, the topological relations between TIN, triangular prism and lines were reconstructed so that the visualization of ground fissures in the geological structure model is realized. This method can facilitate the mechanism for studying fissures and avoid the gaps between the fissure solid and the geological structure to accurately reflect their 3-D characteristics.

Techniques and best practices in multiattribute display

All color monitors display images by mixing red, green, and blue (RGB) components. These RGB components can be defined mathematically in terms of hue, lightness, and saturation (HLS) components. A fourth alpha-blending (also called opacity) component provides a means to corender multiple images. Most, but not all, modern commercial interpretation workstation software vendors provide multiattribute display tools using an opacity model. A smaller subset of vendors provide tools to interactively display two or three attributes using HLS, CMY, and RGB color models. I evaluated a technique (or trick) to simulate the HLS color model using monochromatic color bars and only opacity. This same trick only approximates true color blending of RGB or CMY components. There are three basic objectives in choosing which attributes to display together. The first objective is to understand the correlation of one attribute to another, and most commonly, of a given attribute to the original seismic amplitude data. The second objective is to visualize the confidence or relevance of a given attribute by modulating it with a second attribute. The third objective is to provide a more integrated image of the seismic data volume by choosing attributes that are mathematically independent but correlated through the underlying geology. I developed the interpretation value of the HLS display technique on a 3D data volume acquired over the Central Basin Platform of west Texas exhibiting faults, fractures, folds, channels, pinch outs, and karst features. To be a useful “technique,” I need to demonstrate these workflows within a specific package. Although I implemented the workflow in Petrel 2014, similar images can be generated using any software with flexible opacity capabilities. I also developed a short list of attribute combinations that are particularly amenable to corendering in HLS.

Modeling multiple coal seams using signed distance functions and global kriging

An approach is developed to simultaneously model the geometry of multiple coal seams in 3-D without directly modeling surface elevations or isochore thicknesses. The approach truncates a locally refined 3-D model of estimated signed distance function values. The signed distance function measures the sign dependent distance to a coal/non-coal contact; negative in coal and positive outside. Global kriging is used to estimate the distance function at unsampled locations so that the resulting model has the correct anisotropy and is free of search artifacts. This approach is aimed at relatively large modeling volumes where the number of coal seams varies and the seams show complexities such as pinch outs and bifurcation. The technique aims to locate all of the coal boundaries with high precision through local grid refinement. The results can be used for resource/reserve evaluation.

Structure and evolution of mass transport deposits in the South Caspian Basin, Azerbaijan

ABSTRACTThe Quaternary to late Pliocene sedimentary succession along the margin of the South Caspian Basin contains numerous kilometre‐scale submarine slope failures, which were sourced along the basin slope and from the inclined flanks of contemporaneous anticlines. This study uses three‐dimensional (3D) seismic reflection data to visualise the internal structure of 27 mass transport deposits and catalogues the syndepositional structures contained within them. These are used to interpret emplacement processes occurring during submarine slope failure. The deposits consist of three linked structural domains: extensional, translational and compressive, each containing characteristic structures. Novel features are present within the mass transport deposits: (1) a diverging retrogression of the headwall scarp; (2) the absence of a conventional headwall scarp around growth stratal pinch outs; (3) restraining bends in the lateral margin; (4) a downslope increase in the throw of thrust faults. The results of this study shed light on the deformation that occurred during submarine slope failure, and highlight an important geological process in the evolution of the South Caspian Basin margin.

Influence of the Ainslie Detachment on the stratigraphy of the Maritimes Basin and mineralization in the Windsor Group of northern Nova Scotia, Canada

The Late Devonian to Lower Permian Maritimes basin of eastern Canada developed as a late to post-tectonic extensional basin following the Early Devonian Acadian orogenic event and is subdivided into five main lithostratigraphic groups. The principal host to base metal mineralization is the Visean Windsor Group, which is also the only marine-dominated interval within the basin. The Windsor Group is a thick accumulation of evaporites, carbonates, and siltstone, deposited during passive regional subsidence and intracontinental submersion of a tectonically thinned crust. Earlier rift deposits consist of the nonmarine coarse clastic sediments of the underlying Tournaisian Horton Group, and the Horton to Windsor succession defines a typical rift and sag transgressive event. The Macumber and stratigraphically equivalent Gays River Formations are the marine basal carbonate units to the Windsor Group and contain numerous Pb-Zn-Ba occurrences in central and northern Nova Scotia. These carbonate units are overlain by thick evaporite deposits of the Lower Windsor Group, consisting of gypsum, anhydrite, and salt. In northern Nova Scotia the evaporite-carbonate contact at the top of the basal carbonates is the locus of intense shear and brecciation which developed in relation to the Ainslie detachment. This detachment is a stratigraphically controlled regional-scale flat-lying extensional fault that affected the hydrodynamic regime and mineralizing environment in the Carboniferous Maritimes basin. Movement on the detachment has stripped away thick evaporitic units and excised the entire Windsor Group across wide areas, effectively breaching a regional aquiclude. With shearing, permeability was locally enhanced through brecciation, creating a favorable environment for mineralization. Significant thickness variations within underlying coarse clastic rocks and pinch outs of the Horton Group aquifer also focused basin fluids.The detachment is rooted in a master basement normal fault which occurs offshore along the southeast margin of the Gulf of St. Lawrence, with the basement fault displaying up to 10 to 12 km of offset. Structural evidence from onshore exposures in the uplifted or horsted regions of mainland Nova Scotia and Cape Breton Island indicate that much of this movement was transferred horizontally along the Ainslie detachment. Thin-skinned gravitational sliding produced large rafts of allochthonous strata which progressively draped over the edge of basement normal faults during extension, resulting in tectonic salt build-up in the main offshore graben and a salt massif. Buttressing at the front end of the raft within the graben produced seismically imaged large amplitude buckle folds above the detachment.As well as a regionally extensive planar low-strength or low-viscosity layer, large-scale detachment faults require high fluid pressures to sustain motion of the allochthonous sheets. Abnormal fluid pressures are known to occur beneath evaporites, which is the site of nucleation for the Ainslie detachment. Fluid focusing along the detachment is suggested from the structural style, and evidence is provided by widespread synkinematic calcite + or - fluorite-barite-pyrite veins which occur in the calc-mylonite and breccia.

Processing and Interpretation of Seismic Reflection data from Ogaden Basin, Ethiopia: ABSTRACT

The Ogaden basin is believed to be an extensional basin created during the early rifting of Gondwana and the development of the western Indian Ocean. Preliminary studies conducted by various oil companies and national geoscientists suggest that the area is prospective for hydrocarbon accumulation. Prior to these studies, however, very little was known of the surface geology in this area and virtually nothing was known of the subsurface. The objectives of the study were to implement state-of-the-art seismic and gravity data processing and interpretation techniques in order to identify structural and/or stratigraphic trap sequences and indicate their significance in the hydrocarbon exploration of the basin. In this study, high resolution multi-channel reflection seismic data, acquired in 1993 from the Ogaden Basin, are used. The data were acquired for Hunt Oil Company using Vibroseis as an energy source. Various data enhancement techniques were applied to these data in order to produce an interpretable final seismic section for the identification of possible hydrocarbon habitats. These included discrimination and filtering of coherent noise from the signal, enhancing, focussing of the identified reflectors and static corrections. Statics solutions from shallow wells and automatic refraction statics were compared, with the best results obtained when automaticmore » refraction solutions were applied. Our study has revealed various reflection horizons that can be potential stratigraphic traps at pinch outs and may be porous strata which are terminated by lateral transitions or interconnected lines of either elastic sediments or carbonates. We discuss these results in the light of gravity modelling designed to estimate maximum sediment thickness along the line.« less

Using the pseudospectral method on curved grids for 2D elastic forward modelling1

AbstractWhen applying the conventional Fourier pseudospectral method (FSM) on a Cartesian grid that has a sufficient size to propagate a pulse, spurious diffractions from the staircase representation of the curved interfaces appear in the wavefield. It is demonstrated that these non‐physical diffractions can be eliminated by using curved grids that conform to all the interfaces of the subsurface. Methods for solving the 2D acoustic wave equation using such curved grids have been published previously by the authors. Here the extensions to the full 2D elastic wave equations are presented.The curved grids are generated by using the so‐called multiblock strategy which is a well‐known concept in computational fluid dynamics. In principle the sub‐surface is divided into a number of contiguous subdomains. A separate grid is generated for each subdomain patching the grid lines across domain boundaries to obtain a globally continuous grid. Using this approach, even configurations with pinch outs can be handled.The curved grid is taken to constitute a generalized curvilinear coordinate system. Thus, the elastic equations have to be written in a curvilinear frame before applying the numerical scheme. The method implies that twice the number of spatial derivatives have to be evaluated compared to the conventional FSM on a Cartesian grid. However, it is demonstrated that the extra terms are more than compensated for by the fewer grid points needed in the curved approach.

Recognition of Subaerial Exposure and Flooding Surfaces in Carbonate-Siliciclastic Eolianites and Marine Carbonate Sequences in Southwestern Kansas

Recent work on the St. Louis Limestone in southwestern Kansas has demonstrated that these units contain a significant eolian facies component (up to 80-90% of total unit thickness). Reservoir intervals within the St. Louis are confined to relatively thin subtidal grainstones that, in turn, are capped by a muddy carbonate and shale facies. Critical to exploration and development of these grain-shoal reservoirs is an understanding of their spatial and stratigraphic distribution. Core through the St. Louis and St. Genevieve limestones has been examined and features have been recognized at the top of the eolianites. These surfaces are interpreted as long-term exposure surfaces. The contact between the subtidal grainstone shoals and the overlying muddy carbonate and shale facies is relatively sharp and is interpreted as representing a flooding surface separating shoal from muddy-open shelf facies. In the St. Louis Limestone, the subtidal carbonate grainstone reservoir intervals consist of primary interparticle porosity. Initial geologic models have viewed the reservoir zone as a single blanket-like layer with small structures and relatively random changes in facies thickness. However, a detailed integrated analysis of facies, well logs, and reservoir data within the framework of high-resolution sequence stratigraphy has resulted in a new model of St.more » Louis reservoir geometry. Development and exploration concentrates on recognizing and mapping the updip pinch outs of porous grainstone facies related to maximum landward extent of paleoshorelines caused by relatively major fluctuations in relative sea level. These shoal facies build-ups and pinch outs should form viable exploration targets as they cross structural elements formed or reactivated during the Pennsylvanian.« less

Laser microprobe measurement of chlorine and argon zonation in biotite

Neutron activation of chlorine yields 38Ar which can be liberated by the laser microprobe (LMP) in sufficient quantities to be measured by gas mass spectrometry. When used in conjunction with electron microprobe (EMP) analyses, this technique can precisely measure low concentrations of Cl in minerals. The accuracy is limited by lack of an adequate standard. LMP analyses of standard Mmhb- 1 indicate its Cl concentration is quite heterogeneous and is not a useful standard for LMP Cl analyses. In natural phlogopite and hydrothermally treated (HT) biotite Cl concentration gradients have been measured with the LMP. The zonation probably reflects partial re-equilibration by diffusive anion exchange with Cl transport occurring more rapidly parallel to the silicate sheets than perpendicular to the sheets. The LMP also detected 40Ar gradients in the phlogopite and HT biotite, suggesting that the rate of Cl transport is comparable to that for Ar diffusion. LMP analyses also indicate that 40Ar migration is more rapid parallel to the silicate sheets than perpendicular to the sheets and that the radius of diffusion is on the order of the grain size. The latter observation apparently contradicts the relatively much smaller estimates of the radius of diffusion for Ar in biotite (∼ 150 μm) inferred from 40Ar analyses of bulk separates of hydrothermally treated biotite. The discrepancy may be related to the numerous pores occurring at layer terminations and pinch outs and at the edges of Guinier-Preston zones detected by high-resolution transmission electron microscopy (HRTEM) and which appear to be more abundant in the HT biotite than in the original untreated sample biotite. If these pores are sufficiently large to permit a conduit for rapid Cl and Ar transport via pipe diffusion, then they may be the cause of the apparently enhanced diffusivity of 40Ar observed in large-grain-size fractions of hydrothermally treated biotite.

The Impact of 3-D Seismic in Exploration and Production

The use of 3-D seismic in the US grew dramatically during the decade of the 1980s. Examples are given showing that the 3-D tool has become an indispensable aid in developing new reserves optimally. Not only can the field extent be structurally defined, but detailed fault patterns can be used to optimize drainage points. Attribute analysis can define stratigraphic variations and pinch outs within each reservoir unit. Timely incorporation of 3-D results in the development cycle reduces cost and adds additional reserves. Three-dimensional seismic is not also being broadly used as an exploration tool either prior to leasing or prior to drilling a prospect. Large speculative 3-D surveys have been conducted over parts of the Gulf of Mexico. The economics and results of these surveys, as well as future trends, are discussed.