Horizon Interpretation Research Articles

Developing a robust geologic conceptual model using pseudo 3-D P-wave seismic reflection data

As part of a multiscale hydrogeophysical and modeling study, a pseudo three-dimensional (3-D) seismic survey was conducted over a contaminant plume at P area, Savannah River site (South Carolina), to enhance the existing geologic model by resolving uncertainties in the lithostratigraphic sequence. The geometry of the dissolved phase trichloroethylene plume, based on initial site characterization, appears to be confined to a narrow corridor within the Eocene sand overlying a clay unit approximately 25 m (82 ft) below land surface. Processing the seismic data as a 3-D data volume instead of a series of closely spaced two-dimensional lines allowed for better interpretation of the target horizons, the lower clay, and the sand above the clay. Calibrating the seismic data with existing borehole geophysical logs, core data as well as vertical seismic profiling (VSP) data allowed the seismic data to be inverted from two-way traveltime to depth, thereby facilitating full integration of the seismic data into a solid earth model that is the basic part of a site conceptual model. The outcome was the production of realistic horizon surface maps that show that two channel complexes are located on the section, which are not present in the conceptual model, and that the upper and middle clays are not laterally continuous as previously thought. The geometry of the primary channel has been transposed over the map view of the plume to investigate potential relationships between the shape of the plume and the presence of the channel.

Integrated 2D 4-C OBC velocity analysis of near-seafloor sediments, Green Canyon, Gulf of Mexico

Using 2D four-component ocean-bottom-cable (2D 4-C OBC) seismic data processed in common-receiver gathers, we developed robust [Formula: see text] and [Formula: see text] interval velocities for the near-seafloor strata. A vital element of the study was to implement iterative interpretation techniques to correlate near-seafloor P-P and P-SV images. Initially, depth-equivalent P-P and P-SV layers were interpreted by visually matching similar events in both seismic modes. Complementary 1D ray-tracing analyses then determined interval values of subsea-floor [Formula: see text] and [Formula: see text] velocities across a series of earth layers extending from the seafloor to below the base of the hydrate stability zone (BHSZ) to further constrain these interpretations. Iterating interpretation of depth-equivalent horizons with velocity analyses allowed us to converge on physically reasonable velocity models. Simultaneous [Formula: see text] and [Formula: see text] velocity analysis provided additional model constraints in areas where data quality of one reflection mode (usually [Formula: see text] in the near-seafloor environments) would not provide adequate information to derive reliable velocity information.

Modelagem volumétrica e simulação de fluxo em arenitos análogos a reservatórios profundos: exemplo da Formação Campo Mourão, Bacia do Paraná

Analog modeling of high resolution data was done into an approach for flow simulation (gas). High resolution seismic, outcrop information and shallow wells were used in an integrated framework to generate computational geological models. The study area is located near Ponta Grossa city, where rocks of Itaraté Group, Paraná Basin, outcrops with good analog relationship to real hydrocarbon reservoir from depth portions of the same basin. The 2D seismic, acquired with gun-fire seismic source, proved to be helpful to build a volumetric model. The seismic lines are between 250 and 700m of length and about 200m of depth penetration. Seismic data was load into a computational environment for interpretation of faults and horizons. Solids models are filled with petrophysical values obtained in outcrops samples analysis. The flow simulation runs within a freeware simulator after some translations in files formats done by an in-house software.

Translation invariant attributes for effective lithofacies discrimination

This study introduces translation-invariant attributes using the Hadamard and Rapid transforms of seismic data to discriminate lithofacies. Unlike the Fourier transform, which projects the data onto a set of orthogonal sinusoidal waveforms, the Hadamard transform projects data onto a set of square waves called Walsh functions. The Hadamard transform is particularly good at finding repeating, stacked vertical sequences. Crossplot analysis indicates translation-invariant attributes for horizon-based time windows are less sensitive to horizon interpretation errors in the reservoir interval. Therefore, translation-invariant attributes can be used to find a particular geologic pattern of interest in a reservoir interval. These attributes have been applied successfully to discriminate the lithofacies in a reservoir interval of interest for a 3D seismic survey in the upper Assam basin of India. The sand-shale thickness ratio at the drilled locations within the reservoir interval is used to define the lithofacies categories. Using the discriminant function analysis of each of the conventional-seismic, principal-component, and translation-invariant sets of attributes, we create a corresponding discriminant score map. The probability density function (PDF) of the discriminant score at the drilled locations transforms the PDF to lithofacies categories. The comparative analysis presented in this study indicates that translation-invariant attributes are superior to conventional-seismic and principal-component attributes in discriminating the lithofacies.

Volumetric curvature attributes add value to 3D seismic data interpretation

Horizon-based curvature attributes have been used in seismic data interpretation for predicting fractures since 1994 when Lisle demonstrated the correlation of curvature values to fractures measured on an outcrop. Different measures of curvature (such as Gaussian, strike, and dip) have been shown by different workers to be highly correlated with fractures, and many more applications are also possible. By definition, all such applications need the interpretation of a seismic horizon, which may be simple if data quality is good and the horizon of interest corresponds to a prominent impedance contrast.

Unsupervised seismic facies analysis using wavelet transform and self-organizing maps

Unsupervised seismic facies analysis provides an effective way to estimate reservoir properties by combining different seismic attributes through pattern recognition algorithms. However, without consistent geological information, parameters such as the number of facies and even the input seismic attributes are usually chosen in an empirical way. In this context, we propose two new semiautomatic alternative methods. In the first one, we use the clustering of the Kohonen self-organizing maps (SOMs) as a new way to build seismic facies maps and to estimate the number of seismic facies. In the second method, we use wavelet transforms to identify seismic trace singularities in each geologically oriented segment, and then we build the seismic facies map using the clustering of the SOM. We tested both methods using synthetic and real seismic data from the Namorado deepwater giant oilfield in Campos Basin, offshore Brazil. The results confirm that we can estimate the appropriate number of seismic facies through the clustering of the SOM. We also showed that we can improve the seismic facies analysis by using trace singularities detected by the wavelet transform technique. This workflow presents the advantage of being less sensitive to horizon interpretation errors, thus resulting in an improved seismic facies analysis.

Applications of Love Wave Dispersion for Improved Shear-wave Velocity Imaging

Love wave dispersion and associated analytic partial derivatives are theoretically derived using the reflection∕transmission (R∕T) method. The numerical implementation is then applied to linearised inversion of synthetic and field data. For simple cases dominated by the fundamental mode, Love wave sensitivity and inversion stability is higher than the Rayleigh wave dispersion. However, in general, Rayleigh wave inversion converges more rapidly than Love waves, although similar low misfits can be achieved. When assumed interfaces are used, the inversion of fundamental-mode Love wave dispersion of the normally dispersive profile provides a more accurate result, because the Love wave dispersion is independent of the Poisson’s ratio. In more realistic, irregularly dispersive profiles, deep structure is only interpreted using higher-mode Love and Rayleigh wave dispersion. A field test over a shallow geologic fault with coincident Love and Rayleigh wavefields shows the fundamental—mode Love wave dispersion above [Formula: see text] to have at least 10% higher phase velocities than Rayleigh waves. However, at lower frequencies, Love wave dispersion is at least 10% slower. The resulting inverted models show up to 25% difference in shear-wave velocity. This is attributed to transverse isotropy of [Formula: see text] and [Formula: see text] in shallow fluvial sediments and allows improved geological horizon interpretation and soil discrimination over conventional, single-component surface wave inversion. A second test at a seismograph station site shows the Love wavefield less scattered and mode identification is simple, unlike Rayleigh waves over the same line, and the inversion results correlate well to downhole shear-wave velocity logs.

AUSTRALIAN MEGASURVEYS—THE KEY TO NEW DISCOVERIES IN MATURING AREAS?

In a maturing province such as the North West Shelf, it is time-critical to find remaining hydrocarbon resources as well as to develop small finds before existing big field installations and their associated infrastructure are decommissioned. Finding the remaining smaller fields with subtle geophysical expression is a challenge, and a thorough understanding of the petroleum geology is essential. To achieve this, the subsurface structure and depositional systems must be understood in a regional as well as a local context.To date, exploration companies’ regional models have been based on a mixture of 2D and 3D seismic of varying vintages, orientations, and quality. Consequently they have been incomplete and lacking detail. To address this problem, PGS initiated the MegaSurvey Project, merging a number of 3D surveys into large, consistent 3D data sets. For the first time, the regional picture and prospect-size detail are both available from a single dataset.Two MegaSurveys for the North West Shelf are now available; the Vulcan Sub-Basin MegaSurvey (VMS) and the Carnarvon MegaSurvey (CMS).The MegaSurvey seismic data and consistent horizon interpretation (tied to released well control) enables asset- focussed oil companies to concentrate on the more detailed search-for-the-subtle-trap to find, understand, and develop remaining reserves. Interpretation of the first MegaSurvey (Vulcan Sub-Basin) was completed in 2004 and work is focussed on the Carnarvon MegaSurvey, the interpretation of which will be completed in March 2005.The PGS 3D MegaSurveys allow visualisation of the subsurface both on a scale and resolution that has hitherto been unavailable. They provide an essential new tool to help fully unlock the remaining potential of the North West Shelf.

Integrated 3-D Reservoir Modeling Based on 3-D Seismic: The Tertiary Malampaya and Camago Buildups, Offshore Palawan, Philippines

Three-dimensional (3-D) seismic interpretation and drilling results indicate complex sedimentary geometries of the Malampaya and Camago build ups (offshore Palawan, Philippines) with localized progradation due to unidirectional offbank transport alternating with vertical aggradation. Successive reduction of size during buildup growth and backstepping of the protected landward margin in response to transgression ultimately appear to have triggered the demise of carbonate production and platform drowning. The shallow-water platform top sediments repeatedly show signs of subaerial exposure before reflooding. A modeling functionality was developed to allow development of multiple-scenario 3-D reservoir models in an exploration or appraisal stage. The model enables merging of seismic-scale observations based on 3-D volume and horizon analyses with subseismic scale information from well data; however, inherent noise within the seismic data introduced by the complex buildup morphology has resulted in inconsistent attribute distribution and fault dimming. These difficulties are compounded by erratic velocity distribution within the limestone, nonhyperbolic move out, and a narrow relatively low-frequency spectrum, all of which prevent the use of the 3-D seismic volume as hard data but rather allow its use as a soft constraint for guiding the geological interpretation and ultimately the modeling process. Seismic data quality in such complex morphologic settings and scarcity of well data hamper greatly the use of geostatistically driven modeling approaches; therefore, a new functionality was developed within Shell's proprietary integrated 3-D modeling suite (GEOCAP), which allows deterministic model reservoirs using seismic horizon and volume interpretation, sequence- and cyclo-stratigraphic architecture, and the concept of reservoir rock type. Seismic velocity in clean carbonate formations is predominantly a function of porosity distribution. To assess time-to-depth conversion uncertainty, the reservoir rock type based models were first produced in the time domain. Only after differential 3-D depth conversion of these models could the scenarios be reconstructed in the depth domain. The depth models subsequently were used to derive permeability and saturation 3-D distortions, and thus hydrocarbon volumes for each deterministic scenario. The models were then used for simulation purposes.

Structural analysis of 3D seismic data, using the correlation attribute: a case study – Carboniferous of the Southern North Sea (UK)

With the advent of a new seismic attribute, generically termed correlation, interpreters have a non-specialist tool which enables decisions to be made on structural and stratigraphic patterns within a 3D dataset. The process behind the attribute may appear to be black box technology, however, the resultant data are straightforward and enable the interpreters to make their own conclusions about the seismic information presented. Correlation data are simply a quantified comparison of adjacent seismic traces. The value to be unlocked from this attribute lies in rapid delineation of fault and stratigraphic patterns. This requires a revised approach to the interpretation workflow, involving analysis of the correlation attribute volume during primary stages of any 3D-interpretation project. Using a technique termed ‘ribbon interpretation’, correlation data are analysed for information relating to lineaments and data inconsistencies. The identification of features using ribbons, which are short 2 point segments, can then be stacked and filtered by the interpreter to give direct information on faulting and stratigraphic features. The filtered ribbon data are then progressed through to horizon interpretation, where they can be integrated to provide a more robust structural or stratigraphic solution. Application of the ribbon interpretation technique within the mature environment of the structurally complex, Carboniferous play (UK SNS) (Fig. 1) has allowed faster and better results across exploration, appraisal and production scenarios.

3-D Visualisation and Interpretation of Seismic Attributes Extracted from Large 3-D Seismic Datasets: Subregional and Prospect Evaluation, Deepwater Nigeria: ABSTRACT

High resolution 3-D visualization of horizon interpretation and seismic attributes from large 3-D seismic surveys in deepwater Nigeria has greatly enhanced the exploration team`s ability to quickly recognize prospective segments of subregional and prospect specific scale areas. Integrated workstation generated structure, isopach and extracted horizon consistent, interval and windowed attributes are particularly useful in illustrating the complex structural and stratigraphical prospectivity of deepwater Nigeria. Large 3-D seismic volumes acquired over 750 square kilometers can be manipulated within the visualization system with attribute tracking capability that allows for real time data interrogation and interpretation. As in classical seismic stratigraphic studies, pattern recognition is fundamental to effective depositions facies interpretation and reservoir model construction. The 3-D perspective enhances the data interpretation through clear representation of relative scale, spatial distribution and magnitude of attributes. In deepwater Nigeria, many prospective traps rely on an interplay between syndepositional structure and slope turbidite depositional systems. Reservoir systems in many prospects appear to be dominated by unconfined to moderately focused slope feeder channel facies. These units have spatially complex facies architecture with feeder channel axes separated by extensive interchannel areas. Structural culminations generally have a history of initial compressional folding with late in extensional collapse and accommodationmore » faulting. The resulting complex trap configurations often have stacked reservoirs over intervals as thick as 1500 meters. Exploration, appraisal and development scenarios in these settings can be optimized by taking full advantage of integrating high resolution 3-D visualization and seismic workstation interpretation.« less

Transport and accumulation of sediment and contaminants in the Lagoon of Kuršiųmarios (Lithuania) and Baltic Sea

In this paper an overview is given of earlier work on transport, deposition AND transformation of suspended matter, and contaminants (metals, hydrocarbons) in Lithuanian coastal waters as well as in other parts of the Baltic Sea. The Lithuanian river Nemunas dischargesc. 600.106 kg suspended matter into the Kursių Marios Lagoon annually, of which two thirds are accumulated in the lagoon. The remainder is exported to the Baltic Sea through the Klaipeda Strait influencing biological activity in the coastal zone. Sedimentation rate in Kursių Marios Lagoon is estimated at 3.2 mm y−1. In the last 50 years the surface area of the lagoon decreased by 0.7 km2 y−1. Concentration profiles of metals and hydrocarbons were measured in the Kursių Marios Lagoon, the Baltic proper, and in the Gulf of Finland. Among hydrocarbons both anthropogenic and compounds from natural sources are present. It is argued that correct interpretation of concentration horizons in sediment may be hampered by various processes,e. g. early sedimentogenesis and diagenesis, and cyclic salinity changes in the Baltic Sea.

Effect of deep-reaching tectonics on conductivity structure in the Pannonian basin; Can magnetotelluric data constrain tectonic interpretations?

Magnetotelluric (MT) deep soundings carried out along the Pannonian Geotraverse detected anomalous conductivity structures below the narrow Szolnok flysch belt, with a characteristic width of about 20 km, both in the lower crust and the upper mantle. Here seismic investigations determined a strike-slip zone, the main fault zone of which penetrates below the Mohorovičić discontinuity to a depth of 35–40 km. Although the similarity in these two independent tectonic indications is high, the change in the conductivity structure is questioned, and is studied from the point of view of MT field distortions. It is concluded that the resolving power of MT methods is strongly reduced by the geologic noise, and so the tectonic interpretation of deeper horizons becomes uncertain.

Recycling Stacking Velocities for Better Seismic Processing

In processing seismic data, interpretation and quality control of the primary velocity field is one of the most laborious tasks undertaken by both the processing contractor and the client. The required level of effort is even greater in areas with poor signal to noise ratio and a high level of multiple contamination. In order to consistently achieve optimal stack quality, a simple automated velocity conditioning technique is used to allow for the continuous recycling of a 3-D modelled stacking velocity database into subsequent processing projects. This approach reduces time spent determining the primary velocity field and improves accuracy of the results, especially in difficult seismic areas. The modelled primary velocity field Is equally suited for use in both reprocessing and new infill acquisition projects.The velocity conditioning technique creates a horizon-consistent stacking velocity database by cross-interpolation between the interpreted seismic time and raw stacking velocity databases. The top down conditioning approach commences in the shallow part of the section where velocity determination has both far less uncertainty and increased sensitivity. Stacking velocities for the initial stratigraphic interval are converted to the interval velocity domain, midpoint time-normalised to remove compaction effects, heavily smoothed using knowledge of the spatial distribution of known lithologic variations, then reconstructed back to a new lower layer stacking velocity after inverse time-normalisation. This conditioned lower layer becomes the upper layer for the next stratigraphic interval and the process is repeated. The final three-dimensional stacking velocity model is strictly seismic-based, horizon-consistent, tied in both the strike and dip directions, and contains only geologically plausible interval velocities.The horizon-consistent aspect of this technique integrates geophysical interpretation with seismic processing. A preliminary horizon interpretation for all major chronostrati-graphic boundaries is available to the velocity interpreter as a set of inflection points in the supplied central function velocity curve. The velocity model also provides an accurate velocity field for process testing during the early stages of seismic processing projects. The same conditioning technique can be used to spatially check the final primary velocity field prior to committing to stack and migration.A case history from the Timor Sea demonstrates the application of this technique. This historically difficult seismic area has used two and sometimes three passes of velocity analyses. The conditioned stacking velocity database allows the processing sequence to be reduced to a single post-demultiple/post-DMO quality control velocity analysis step for both reprocessing and new infill acquisition programs — which yeilds a 50% reduction in the time spent picking and generating velocity analyses and eliminates the need to produce iso-velocity plots. The results show that proper conditioning of a stacking velocity database allows velocities to be recycled for improved quality and efficiency in pre-stack seismic processing, migration, and stacking velocity assisted depth conversion techniques.

The geometry of normal faults in a sector of the offshore Nile Delta, Egypt

Abstract This paper presents an interpretation of the geometry of normal faults in part of the offshore Nile Delta. The structure is dominated by north-dipping faults that are clearly imaged on seismic reflection profiles, and affect sediments of Pliocene-Pleistocene age, together with the underlying Miocene sequences that contain potential hydrocarbon reservoir rocks. The geometrical analysis is carried out in several stages. Firstly, time slice maps of the fault pattern are produced in order to establish the overall fault geometry of the area. Secondly, for each indivdiual fault identified, structure contours on the fault surface itself are mapped. This procedure serves to highlight any complexities in the geometry of the faults, and allows potential inconsistencies in the interpretation to be identified. Thirdly, horizon interpretations and correlations are made across the faults, and these are checked for consistency by constructing maps of the magnitude of the dip slip component of displacement within the fault surface. The complete geometric analysis makes it possible to extend the results from the good data areas in the upper parts of seismic sections to the poorer quality areas lower down. In this way it is possible to derive an interpretation of the structural geometry of the Miocene reservoir sequences. The seismic reflections from the Miocene are poor and irregular, and structural interpretation and modelling reduces the uncertainty associated with the reservoir geometry. The structural interpretation is presented as a model of tilted and fault bounded blocks, with considerable horizontal separation. This provides a much clearer and more consistent picture of the reservoir geometry, and contrasts with previous interpretations that have shown a more or less continuous reservoir sequence with little structure. The geometrical interpretation should form the basis for a new hydrocarbon play in the area because it predicts the locations of footwall crests, as well as areas where reservoir is missing because of separation across faults.

The Tectonic Evolution of Red Sea and Gulf of Aden

The Red Sea and Gulf of Aden, formed by rifting of the Arabian plate away from Africa, have been the subjects of numerous studies since the 1960s. Geophysical and geological data gathered during hydrocarbon exploration by several companies are being synthesized as part of the World Bank-executed Red Sea/Gulf of Aden Regional Hydrocarbon Study Project. This synthesis provides an opportunity to study the regional variation in tectonic history and structural style within the two basins, particularly toward the basin margins, where data coverage is most complete and a thinner sedimentary cover allows more reliable interpretation of deeper horizons. Based on this data, most of the Red Sea shows a similar tectonic history to that of the Gulf of Suez. Widespread normal faulting developed contemporaneous with deposition of a dominantly clastic sequence of Oligocene( ) to middle Miocene age. The faulting often dies out within or below thick middle to upper Miocene evaporite deposits with only few faults affecting the younger units. The Gulf of Aden, on the other hand, shows a strong contrast between largely west-northwest-east-southeast oriented extensional structures on land, and generally more east-west oriented extensional structures on the continental shelves and slopes. The onshore extensional structures oriented obliquemore » to the coast, and approximately perpendicular to transform fault trends in the oceanic crust of the central Gulf of Aden. This contrast is extensional structure orientations has important implications for the tectonic evolution of the Gulf of Aden.« less

Interactive broadband constrained inversion

Conventional algorithms for the inversion of seismic data generally produce inverted traces which are limited in bandwidth to that of the input seismic data. This necessitates that a low-frequency model be added to the inverted traces in order to produce full bandwidth seismic logs. A problem with this method is that there is usually a gap in frequency between the low end of the seismic and the high end of the model spectra. A further problem with conventional inversion methods is that it is difficult to incorporate well or geologic information into the inversion process. To overcome these problems a new broadband constrained inversion method which combines seismic, well, and geologic information is used. This method simultaneously satisfies constraints imposed by the well and geologic information, whilst inverting the seismic data. The output is an optimized broadband acoustic impedance model. In order to combine the diverse data types of seismic amplitudes, well logs, geologic models, and horizon interpretations, an interactive workstation is used. The interactive environment is ideal for this work as it provides the flexibility required to manipulate and display both the varying incoming data types and the output data model.

Three-Dimensional Mapping of Horizons Picked on Two-Dimensionally Migrated Seismic Sections

There are some well-known problems in interpretation and mapping of horizons picked on migrated sections:? horizons do not tie at intersections? traveltimes or depths as posted are erroneous in both position and value, as far as out of dip lines are concerned.

Paleoenvironmental interpretation of a Pliocene limestone horizon on the Red Sea coast, Egypt

Paleoenvironmental interpretation of a Pliocene limestone horizon on the Red Sea coast, Egypt

The usefulness of pollen analysis in interpretation of stratigraphic horizons, both Late-glacial and Post-glacial

In the English Lake District, it has been shown on chemical evidence that the lake sediments originated in the soils of the drainage basins. Changes in the stratigraphy of the sediments, in both La...