Limit Of Seismic Resolution Research Articles

Imaging along-strike variability in fault structure; insights from seismic modelling of the Maghlaq Fault, Malta

Seismic modelling studies of outcrop analogues have proven a powerful method to provide a link between geology as observed at outcrop, and the interpretation of subsurface geology from seismic data. Seismic imaging of fault zones is inherently challenging, as they may be associated with steep dips, narrow zones of heterogeneously deformed rocks, and complex geometries at the limit of seismic resolution. We here investigate how along-strike variations in fault geometry and structural style are imaged in reflection seismic images, based on seismic modelling of well-exposed outcrops of the Maghlaq Fault hosted in the Miocene-Oligocene carbonate rocks in Malta. Using two-dimensional seismic modelling, selected structural cross-sections have been modelled, focusing on variations in fault- and hangingwall bed geometries, and dominant seismic signal frequency. The seismic models show great variability in resolution and, thus, level of resolved geologic detail. The modelled seismic images show that, for all signal frequencies, the fault itself is not imaged, but is manifested by terminations of footwall reflections as well as fault-related strain in the form of drag folding of hangingwall beds, in addition to fault-proximal seismic imaging artefacts. Rotated and ‘drag-folded’ hangingwall beds along the fault produces a high-amplitude fault parallel reflection bundle in the immediate hangingwall of the fault. The illumination issues that dominate in low dominant frequency seismic models are less prevalent in the higher-resolution, higher dominant frequency models. The results of this outcrop-to-seismic study offer insight to the relationship between faults and seismic images, which may improve our ability to accurately interpret faults in the subsurface form reflection seismic data.

Møre–Vøring Composite Tectono-Sedimentary Element, Norwegian Rifted Margin, Norwegian Sea

Abstract The Møre and Vøring basins at the mid-Norwegian margin were affected by a series of rift phases separated by periods of tectonic quiescence from Caledonian collapse in the Devonian to final continental break-up in the earliest Eocene. The tectonic phases were mostly extensional in nature, and the pre-drift extension across the Møre and Vøring basins since mid-Permian time has been estimated to be of the order of 200–250 km. The pre-Cretaceous basin configuration is poorly constrained due to limited seismic resolution at great depths and a lack of borehole control. A fundamental change occurred in the crustal deformation regime: from Late Jurassic distributed stretching of the upper crust to Early Cretaceous thinning at lower-crustal levels located along the axis of the Møre and Vøring basins. A westward migration of Late Cretaceous–Paleocene depocentres in the Vøring Basin reflects the focus of deformation towards the time and line of break-up. The basin architecture and evolution are summarized in relation to tectono-sedimentary elements corresponding to the main tectonic and depositional phases, and represent characteristic megasequences in the seismic data. Finally, the exploration history and main petroleum systems of the Møre and Vøring basins are presented.

Geostatistical seismic inversion and 3D modelling of metric flow units, porosity and permeability in Brazilian presalt reservoir

Flow units (FU) rock typing is a common technique for characterizing reservoir flow behavior, producing reliable porosity and permeability estimation even in complex geological settings. However, the lateral extrapolation of FU away from the well into the whole reservoir grid is commonly a difficult task and using the seismic data as constraints is rarely a subject of study. This paper proposes a workflow to generate numerous possible 3D volumes of flow units, porosity and permeability below the seismic resolution limit, respecting the available seismic data at larger scales. The methodology is used in the Mero Field, a Brazilian presalt carbonate reservoir located in the Santos Basin, who presents a complex and heterogenic geological setting with different sedimentological processes and diagenetic history. We generated metric flow units using the conventional core analysis and transposed to the well log data. Then, given a Markov chain Monte Carlo algorithm, the seismic data and the well log statistics, we simulated acoustic impedance, decametric flow units (DFU), metric flow units (MFU), porosity and permeability volumes in the metric scale. The aim is to estimate a minimum amount of MFU able to calculate realistic scenarios porosity and permeability scenarios, without losing the seismic lateral control. In other words, every porosity and permeability volume simulated produces a synthetic seismic that match the real seismic of the area, even in the metric scale. The achieved 3D results represent a high-resolution fluid flow reservoir modelling considering the lateral control of the seismic during the process and can be directly incorporated in the dynamic characterization workflow.

Depositional environment and source rock distribution of the Late Cretaceous Rakopi formation in the deepwater Taranaki Basin, New Zealand

The interpretation of depositional environments and evaluation of potential source rocks in deepwater frontier basins are challenging because of the limited data availability. The Deepwater Taranaki Basin, the northwest extensional area of the Taranaki Basin in New Zealand, hosts the Late Cretaceous Rakopi Formation, which is one of the most important source rocks and potential reservoirs in the Taranaki Basin. This study utilized seismic data and well ties from the Romney-1 well to interpret three seismic units in the study area, representing different depositional environments and evolutions. These units were correlated with the regional Megasequences B2–B4 in the Deepwater Taranaki Basin. Sediment transport and progradation mainly occurred from the northeast to the southwest direction. Fluctuations in relative sea levels influenced the shifting of the shelf and marginal environments, potentially creating coal-bearing strata in the study area. The key source rock strata were identified within units 2 and 3 characterized by kerogen type III, coaly source rocks in a marginal environment. A minor source rock interval was found in unit 2, characterized by kerogen type II, marine organic matter in a bathyal environment in the depocenter of the study area. Coal seams were identified using seismic and well log data. However, thin coal seams were challenging to distinguish clearly, sometimes showing a similar response to sandstone beds, because of the limited seismic resolution. Thus, within limited data areas, source rock predictions can be made in the context of depositional interpretation.

Visualizing subtle structural and stratigraphic features on 3D seismic-reflection data: A case study from offshore Libya

Seismic-reflection data contain residual noise after processing, which can cause geologic misinterpretation of noise-sensitive seismic attributes. For detailed subsurface imaging, seismic data conditioning can enhance the visualization of subsurface reflection features down to the limit of seismic resolution. This study on the Gabes-Tripoli Basin of western offshore Libya demonstrates the potential of postprocessing seismic data conditioning using a standard industry 3D-seismic data set. The seismic data exhibit distinct reflection discontinuities and configurations interpreted as folds, reverse faults, and crustal- and gravity-driven normal faults. Other reflection discontinuities are interpreted as imaging the external form and internal architecture of buried carbonate platforms. The seismic-reflection interpretation finds that seismic data conditioning by filtering strongly supports the interpretation of the 3D geometry, type, and trend of distinct subsurface reflection features, particularly if used as input for a structural-attribute generation. Postprocessing seismic conditioning initially improved the signal-to-noise ratio by structure-oriented filtering with edge preservation. Application of this filter configuration emphasized subtle geologic features supporting, e.g., the detection of faults close to the limit of the seismic resolution. At the same time, the filtering resulted in a higher lateral continuity of the individual seismic reflectors, supporting the autotracking of the horizons. Structural attributes generated from the conditioned data such as the variance and curvature imaged more subsurface reflection detail when compared with the structural attributes generated from nonconditioned data. The filter-based workflow proposed can be applied in most seismic interpretation software packages and is recommended to be used as a standard procedure preceding a structure-attribute calculation and structural interpretation of the seismic-reflection data of limited quality.

Applications of high-resolution seismic frequency and phase attribute analysis techniques

Seismic prospecting for oil and gas exploration and development is limited by seismic data resolution. Improving the accuracy of quantitative interpretation of seismic data in thin layers, thereby identifying effective reservoirs and delineating favorable areas, can be a key factor for successful exploration and development. Historically, the limit of seismic resolution is usually assumed to be about 1/4 wavelength of the dominant frequency of the data in the formation of interest. Constrained seismic reflectivity inversion can resolve thinner layers than this assumed limit. This leads to a series of highresolution quantitative interpretation methods and techniques have been developed. Case studies in carbonates, clastic, and unconventional reservoirs indicate that the application of quantitative interpretation techniques such as high-resolution seismic frequency and phase attribute analysis can resolve and allow/or allow quantitative estimation of rock and fluid properties in such seismically thin layers. Band recovery using high resolution seismic processing technology can greatly improve the ability to recognize geological details such as thin layers, faults, and karst caves. Multiscale fault detection technology can effectively detect small-scale faults in addition to more readily recognized large-scale faults. Based on traditional seismic amplitude information, high-resolution spectral decomposition and phase decomposition technology expands seismic attribute analysis to the frequency and phase dimensions, boosting the interpretable geological information content of the seismic data including subsurface geological characteristics and hydrocarbon potential and thereby improving the reliability of seismic interpretation. These technologies, based on high-resolution quantitative interpretation techniques, make the identification of effective reservoirs more efficient and accurate.

Advances in seismic imaging of magma and crystal mush

Seismic imaging methods have provided detailed three-dimensional constraints on the physical properties of magmatic systems leading to invaluable insight into the storage, differentiation and dynamics of magma. These constraints have been crucial to the development of our modern understanding of magmatic systems. However, there are still outstanding knowledge gaps resulting from the challenges inherent in seismic imaging of volcanoes. These challenges stem from the complex physics of wave propagation across highly heterogeneous low-velocity anomalies associated with magma reservoirs. Ray-based seismic imaging methods such as travel-time and surface-wave tomography lead to under-recovery of such velocity anomalies and to under-estimation of melt fractions. This review aims to help the volcanologist to fully utilize the insights gained from seismic imaging and account for the resolution limits. We summarize the advantages and limitations of the most common imaging methods and propose best practices for their implementation and the quantitative interpretation of low-velocity anomalies. We constructed and analysed a database of 277 seismic imaging studies at 78 arc, hotspot and continental rift volcanoes. Each study is accompanied by information about the seismic source, part of the wavefield used, imaging method, any detected low-velocity zones, and estimated melt fraction. Thirty nine studies attempted to estimate melt fractions at 22 different volcanoes. Only five studies have found evidence of melt storage at melt fractions above the critical porosity that separates crystal mush from mobile magma. The median reported melt fraction is 13% suggesting that magma storage is dominated by low-melt fraction crystal mush. However, due to the limits of seismic resolution, the seismological evidence does not rule out the presence of small (<10 km3) and medium-sized (<100 km3) high-melt fraction magma chambers at many of the studied volcanoes. The combination of multiple tomographic imaging methods and the wider adoption of methods that use more of the seismic wavefield than the first arriving travel-times, promise to overcome some of the limitations of seismic tomography and provide more reliable constraints on melt fractions. Wider adoption of these new methods and advances in data collection are needed to enable a revolution in imaging magma reservoirs.

Seismic Thin Interbeds Analysis Based on High-Order Synchrosqueezing Transform

Thin interbeds, which are made of several thin layers, are critical geological structures in seismic exploration. Due to the limited seismic resolution, thin interbeds cannot be easily analyzed with the standard time analysis or frequency analysis from a seismic dataset. Time-frequency (TF) analysis is a powerful technique for analyzing a nonstationary signal in the time and frequency domains simultaneously. Although it is tough to use the TF tool to estimate the thin layer’s thickness, TF analysis still has much potential for analyzing seismic thin interbeds from seismic data. The high-order synchrosqueezing transform (SST) has a significantly higher TF resolution than the linear TF transform and some commonly used SSTs. In this study, we propose to utilize the high-order SST to extract the TF features of thin interbeds’ seismic response. The TF analysis on the seismic response of cyclic thin interbeds shows that the thickness variation in thin interbeds causes a clear increasing (or decreasing) frequency curve in the TF plane. A seismic data example based on the Marmousi2 model demonstrates the proposed method can reflect the thickness variation with the depth. One real seismic data example has shown that the high-order SST can specifically extract the frequency component and be used to characterize the thickness variation, which is verified by two well’s velocity logs.

Quantifying Uncertainty in Downscaling of Seismic Data to High-Resolution 3-D Lithological Models

Building high-resolution lithological models using seismic data can facilitate decision-makings for earth resources development, but they are subject to considerable uncertainties due to the limited seismic resolution. Traditionally, high-resolution lithological models are built deterministically or stochastic simulation is performed using seismic as “soft data” in geostatistical contexts. Many approaches have been developed to do the above, but in this work, we explicitly account for the uncertain relationship between seismic data and lithological models. We introduce a data-driven Bayesian approach to first quantify soft data uncertainty by generating multiple lithology proportions from seismic. The spatial lithology proportion uncertainty arises from the uncertain relationship between low-resolution seismic and lithology proportions. We quantify this proportion uncertainty by learning the statistical relationships with seismic using high-resolution borehole data. Once conditioning the statistical relationships to observed seismic, we can generate multiple realizations of spatial proportions. With the generated proportions as volume averages, a sequential indicator simulation (SISim) is then performed to build high-resolution lithofacies models. This approach is applied to a real channelized turbidite system with four lithofacies. We demonstrate that the generated high-resolution lithological models can preserve the global uncertainties captured by proportion trends while locally matching borehole observations. When compared with the conventional approaches that use deterministic soft data, our statistical learning approach avoids the problem of underestimating reservoir model uncertainty. More importantly, the lithofacies models from the proposed method are less likely to be falsified by observed data. Additionally, an open-source Python library for the uncertainty quantification is provided.

Why Mohr-circle analyses may underestimate the risk of fault reactivation in depleting reservoirs

Fault reactivation may lead to induced seismicity, damage to wells and loss of containment of potentially harmful and economically valuable fluids or gas in the subsurface. For this reason, fault reactivation is assessed in hydrocarbon development projects, in particular when injecting water, gas, or steam, but also when disposing of (contaminated) water or CO2 in depleted oil or gas reservoirs. In this paper, we compare the results obtained by a Mohr-circle evaluation and two-dimensional Finite-Element (FE) analysis that are commonly used to assess the likelihood of fault reactivation. It is found that Mohr-circle evaluations provide a non-conservative estimate of the allowable reservoir pressure in cases where the reservoir is offset by a fault. In practice, this may comprise virtually all imageable faults, as the limited seismic resolution does not allow interpretation of faults with no or small throws. Furthermore, we demonstrate that the maximum (vertical) element size for capturing the onset of fault slip with sufficient accuracy is about 2% of the reservoir thickness. Finite-element models with larger vertical element size increasingly over-estimate the depletion pressure required to cause fault reactivation or won't predict fault reactivation at all, thereby under-estimating the hazard associated with fault reactivation. This paper concludes with recommendations for evaluating onset of fault slip using Mohr-circles, and two- and three-dimensional finite-element analysis.

Controls on Reservoir Heterogeneity of a Shallow-Marine Reservoir in Sawan Gas Field, SE Pakistan: Implications for Reservoir Quality Prediction Using Acoustic Impedance Inversion

The precise characterization of reservoir parameters is vital for future development and prospect evaluation of oil and gas fields. C-sand and B-sand intervals of the Lower Goru Formation (LGF) within the Lower Indus Basin (LIB) are proven reservoirs. Conventional seismic amplitude interpretation fails to delineate the heterogeneity of the sand-shale facies distribution due to limited seismic resolution in the Sawan gas field (SGF). The high heterogeneity and low resolution make it challenging to characterize the reservoir thickness, reservoir porosity, and the factors controlling the heterogeneity. Constrained sparse spike inversion (CSSI) is employed using 3D seismic and well log data to characterize and discriminate the lithofacies, impedance, porosity, and thickness (sand-ratio) of the C- and B-sand intervals of the LGF. The achieved results disclose that the CSSI delineated the extent of lithofacies, heterogeneity, and precise characterization of reservoir parameters within the zone of interest (ZOI). The sand facies of C- and B-sand intervals are characterized by low acoustic impedance (AI) values (8 × 106 kg/m2s to 1 × 107 kg/m2s), maximum sand-ratio (0.6 to 0.9), and maximum porosity (10% to 24%). The primary reservoir (C-sand) has an excellent ability to produce the maximum yield of gas due to low AI (8 × 106 kg/m2s), maximum reservoir thickness (0.9), and porosity (24%). However, the secondary reservoir (B-sand) also has a good capacity for gas production due to low AI (1 × 107 kg/m2s), decent sand-ratio (0.6), and average porosity (14%), if properly evaluated. The time-slices of porosity and sand-ratio maps have revealed the location of low-impedance, maximum porosity, and maximum sand-ratio that can be exploited for future drillings. Rock physics analysis using AI through inverse and direct relationships successfully discriminated against the heterogeneity between the sand facies and shale facies. In the corollary, we proposed that pre-conditioning through comprehensive petrophysical, inversion, and rock physics analysis are imperative tools to calibrate the factors controlling the reservoir heterogeneity and for better reservoir quality measurement in the fluvial shallow-marine deltaic basins.

Deformation patterns of upper Quaternary strata and their relation to active tectonics, Po Basin, Italy

AbstractDespite increased application of subsurface datasets below the limits of seismic resolution, reconstructing near‐surface deformation of shallow key stratigraphic markers beneath modern alluvial and coastal plains through sediment core analysis has received little attention. Highly resolved stratigraphy of Upper Pleistocene to Holocene (Marine Isotope Stage 5e to Marine Isotope Stage 1) alluvial, deltaic and coastal depositional systems across the southern Po Plain, down to 150 m depth, provides an unambiguous documentation on the deformation of previously flat‐lying strata that goes back in time beyond the limits of morphological, historical and palaeoseismic records. Five prominent key horizons, accurately selected on the basis of their sedimentological characteristics and typified for their fossil content, were used as highly effective stratigraphic markers (M1 to M5) that can be tracked for tens of kilometres across the basin. A facies‐controlled approach tied to a robust chronology (102 radiocarbon dates) reveals considerable deformation of laterally extensive nearshore (M1), continental (M2 and M3) and lagoon (M4 and M5) marker beds originally deposited in a horizontal position (M1, M4 and M5). The areas where antiformal geometries are best observed are remarkably coincident with the axes of buried ramp anticlines, across which new seismic images reveal substantially warped stratal geometries of Lower Pleistocene strata. The striking spatial coincidence of fold crests with the epicentres of historic and instrumental seismicity suggests that deformation of marker beds M1 to M5 might reflect, in part at least, syntectonically generated relief and, thus, active tectonism. Precise identification and lateral tracing of chronologically constrained stratigraphic markers in the 14C time window through combined sedimentological and palaeoecological data may delineate late Quaternary subsurface stratigraphic architecture at an unprecedented level of detail, outlining cryptic stratal geometries at the sub‐seismic scale. This approach is highly reproducible in tectonically active Quaternary depositional systems and can help to assess patterns of active deformation in the subsurface of modern alluvial and coastal plains worldwide.

Seismic tuning of dispersive thin layers

ABSTRACTThe wedge model is commonly used to study the limits of seismic resolution where conventionally the thickness of sub‐resolution seismic layers can be determined from thin layer tuning curves. Tuning curves relate the layer temporal thickness to the wavelet frequency, but, to our knowledge, no systematic study has been done to date of the effects of velocity dispersion and attenuation on tuning. In this work, we study the tuning properties of a thin layer dispersing according to the standard linear solid model. We show that the first tuning curve is sensitive to the attenuation, relative polarity and magnitude of the two reflection coefficients at the top and base of the layer. We provide an analytic formula for the upper bound in the mismatch between the elastic and dispersive tuning curves. We conclude that highly attenuative thin layers can appear thicker or thinner than they actually are depending on polarity and relative magnitude of reflection coefficients at the layer interfaces. Our results are particularly relevant in the quantification of CO2 where knowledge of the temporal thickness of CO2 layers is reliant on their tuning curves.

Tomographic filtering via the generalized inverse: a way to account for seismic data uncertainty

SUMMARY Tomographic-geodynamic model comparisons are a key component in studies of the present-day state and evolution of Earth’s mantle. To account for the limited seismic resolution, ‘tomographic filtering’ of the geodynamically predicted mantle structures is a standard processing step in this context. The filtered model provides valuable information on how heterogeneities are smeared and modified in amplitude given the available seismic data and underlying inversion strategy. An important aspect that has so far not been taken into account are the effects of data uncertainties. We present a new method for ‘tomographic filtering’ in which it is possible to include the effects of random and systematic errors in the seismic measurements and to analyse the associated uncertainties in the tomographic model space. The ‘imaged’ model is constructed by computing the generalized-inverse projection (GIP) of synthetic data calculated in an earth model of choice. An advantage of this approach is that a reparametrization onto the tomographic grid can be avoided, depending on how the synthetic data are calculated. To demonstrate the viability of the method, we compute traveltimes in an existing mantle circulation model (MCM), add specific realizations of random seismic ‘noise’ to the synthetic data and apply the generalized inverse operator of a recent Backus–Gilbert-type global S-wave tomography. GIP models based on different noise realizations show a significant variability of the shape and amplitude of seismic anomalies. This highlights the importance of interpreting tomographic images in a prudent and cautious manner. Systematic errors, such as event mislocation or imperfect crustal corrections, can be investigated by introducing an additional term to the noise component so that the resulting noise distributions are biased. In contrast to Gaussian zero-mean noise, this leads to a bias in model space; that is, the mean of all GIP realizations also is non-zero. Knowledge of the statistical properties of model uncertainties together with tomographic resolution is crucial for obtaining meaningful estimates of Earth’s present-day thermodynamic state. A practicable treatment of error propagation and uncertainty quantification will therefore be increasingly important, especially in view of geodynamic inversions that aim at ‘retrodicting’ past mantle evolution based on tomographic images.

Application of geostatistical seismic inversion in reservoir characterization of Sapphire gas field, offshore Nile Delta, Egypt

Seismic reservoir characterization becomes challenging when reservoir thickness goes beyond the limits of seismic resolution. Geostatistical inversion techniques are being considered to overcome the resolution limitations of conventional inversion methods and to provide an intuitive understanding of subsurface uncertainty. Geostatistical inversion was applied on a highly compartmentalized area of Sapphire gas field, offshore Nile Delta, Egypt, with the aim of understanding the distribution of thin sands and their impact on reservoir connectivity. The integration of high-resolution well data with seismic partial-angle-stack volumes into geostatistical inversion has resulted in multiple elastic property realizations at the desired resolution. The multitude of inverted elastic properties are analyzed to improve reservoir characterization and reflect the inversion nonuniqueness. These property realizations are then classified into facies probability cubes and ranked based on pay sand volumes to quantify the volumetric uncertainty in static reservoir modeling. Stochastic connectivity analysis was also applied on facies models to assess the possible connected volumes. Sand connectivity analysis showed that the connected pay sand volume derived from the posterior mean of property realizations, which is analogous to deterministic inversion, is much smaller than the volumes generated by any high-frequency realization. This observation supports the role of thin interbed reservoirs in facilitating connectivity between the main sand units.

Channel identification using 3D seismic attributes and well logging in lower Shihezi Formation of Hangjinqi area, northern Ordos Basin, China

Channel identification in oil and gas exploration is a vital task due to the importance of fluvial reservoirs. The fluvial sandstone of Lower Shihezi Formation of Ordos Basin is the primary target layer of gas exploration and development which has the maximum potential of producing tight gas sand reservoir in China. Fluvial paleo-channels can be identified easily by core, outcrop and logs data, but due to development of the several faults and limited seismic resolution in the study area, it is difficult to identify them by conventional seismic methods. The study was conducted in Hangjinqi area on the regional level of about 4537.87 km2. The radical objective of this research is to recognize the horizontal distribution and geometry of the multiple channels. Based on the study of integrated analyses of geological cores, geophysical well-logs, and seismic attributes it is confirmed that multiple braided channels are present in the study area. Channel distribution based on seismic attributes and its overlapping on the paleogeographic map showed that the channels are coming from the North and are merging towards South direction.

Rock-physics based time-lapse inversion in Delivery4D: synthetic feasibility study for CO2CRC Otway Project

Conventional approach to 4D seismic inversion consists of parallel inversions applied to seismic vintages. Only then, the inverted changes of seismic attributes are converted into petrophysical properties using rock physics. The paper develops a robust approach to 4D seismic inversion based on a Bayesian approach along with rock physics constraints. This means that observed time-lapse seismic response along with the baseline amplitudes are inverted directly into rock properties via pre-defined relations to the seismic properties. To this end, we extend the functional of Delivery - an open-source stochastic inversion software. We illustrate efficiency of Delivery4D using synthetic 4D dataset generated for Stage 2C of CO2CRC Otway project, Victoria. Complexity of the synthetic wavefield resembles field data acquired for the Otway project while all unknown sources of noise/uncertainty are excluded and we have ‘ground-truth’ subsurface properties. Despite the relatively thin CO2 plume, the 4D inversion reduced detected time-lapse anomaly to the location that closely corresponds to the actual CO2 plume. Estimated distributions of the plume characteristics (thickness, saturation and CO2 mass) are overall similar to the static and dynamic geomodels. However, the values inverted at a particular trace may differ significantly. We attribute these discrepancies to the limited seismic resolution and imperfections of the amplitude-preserved seismic processing.

Seismic depth imaging of sequence boundaries beneath the New Jersey shelf

Numerical modelling of fluid flow and transport processes relies on a well-constrained geological model, which is usually provided by seismic reflection surveys. In the New Jersey shelf area a large number of 2D seismic profiles provide an extensive database for constructing a reliable geological model. However, for the purpose of modelling groundwater flow, the seismic data need to be depth-converted which is usually accomplished using complementary data from borehole logs. Due to the limited availability of such data in the New Jersey shelf, we propose a two-stage processing strategy with particular emphasis on reflection tomography and pre-stack depth imaging. We apply this workflow to a seismic section crossing the entire New Jersey shelf. Due to the tomography-based velocity modelling, the processing flow does not depend on the availability of borehole logging data. Nonetheless, we validate our results by comparing the migrated depths of selected geological horizons to borehole core data from the IODP expedition 313 drill sites, located at three positions along our seismic line. The comparison yields that in the top 450 m of the migrated section, most of the selected reflectors were positioned with an accuracy close to the seismic resolution limit (≈ 4 m) for that data. For deeper layers the accuracy still remains within one seismic wavelength for the majority of the tested horizons. These results demonstrate that the processed seismic data provide a reliable basis for constructing a hydrogeological model. Furthermore, the proposed workflow can be applied to other seismic profiles in the New Jersey shelf, which will lead to an even better constrained model.

Seismic interpretation of sill complexes in sedimentary basins: implications for the sub-sill imaging problem

Application of 3D seismic reflection data to igneous systems in sedimentary basins has led to a revolution in the understanding of mafic sill complexes. However, there is considerable uncertainty on how geometries and architecture of sill complexes within the subsurface actually relates to geometries in seismic reflection data. To provide constraints on how sill complexes in seismic data should be interpreted, we present synthetic seismograms generated from a seismic-scale (22 × 0.25 km) outcrop in East Greenland constrained by abundant field data. This study highlights how overlying igneous rocks adversely affect imaging of underlying intrusions and rocks by decreasing seismic amplitude, frequency and making steeply dipping features near-impossible to image. Furthermore, seismic modelling in this study shows that, because of the high impedance contrast between siliciclastic host rock and dolerites, very thin (1–5 m) intrusions should in principle be imaged in reflection seismic data at 3 km depth. However, comparison with actual seismic data with well data shows significant amounts of unimaged sill intrusions, and this is likely due to limited seismic resolution, overburden complexity, inadequate velocity-models, and interference between reflections from closely spaced sills and sill splays. Significant improvements to sill imaging and interpretation could be made by better predicting occurrence and geometry of sill intrusions and including these in velocity models. Supplementary material : A scaled version of the input panel (Fig. 5d) used for seismic modelling (A1), and very high resolution versions of subfigures in Figure 11 (A2–3) are available at https://doi.org/10.6084/m9.figshare.5607160

Seismic attribute analysis for fracture detection and porosity prediction: A case study from tight volcanic reservoirs, Barmer Basin, India

The Raageshwari Deep Gas Field is situated in the southern part of the onshore Barmer Basin in Rajasthan, India. The field contains gas condensate with excellent gas quality reservoired dominantly within the tight volcanic rocks. Optimal development and production of the tight reservoirs requires characterization of faults and natural fractures that are important to fluid flow and production. Reservoir quality of the volcanic complex is governed by the distribution of matrix pore spaces and fractures. Use of seismic attributes for detecting effective reservoir pore spaces such as fractured zones and vesicles is challenging considering the limits of seismic resolution in the tight volcanic reservoirs. Poststack seismic attributes used in this study for fracture detection include geometric attributes such as coherency for detection of seismically resolved faults and reflection curvature attributes for discrimination of subseismic faults. Discrete frequency components extracted from seismic data are also used to analyze geologic discontinuities. Besides geometric attributes, seismic wave attenuation from the basic instantaneous seismic attributes — instantaneous amplitude, phase, and frequency — are also key indicators of fractured zones and vesicles that create effective pore spaces. In fractured intervals within the volcanic reservoirs, a decrease in density and slowdown in acoustic propagation velocity yields a relative drop in P-wave impedance, which are important characteristics to determine fractured zones. Detected fractured zones from seismic attributes were validated with wells, image logs, and production data. In addition to fracture detection, predicting subsurface properties, such as porosity distribution away from wells, has always been a fundamental requisite for appropriate infill well planning. Full-azimuth seismic data and selected seismic attributes were used in multiattribute analysis to predict porosity and were correlated with “blind” wells. This study captures the heterogeneous volcanic rock porosity distribution in the field, improving the understanding of varied production behavior observed within the volcanic rock complex.