Stratigraphic Imaging Research Articles

A decade of technology advancement in seismic processing: A case study from reprocessing legacy sparse OBC data in Kashagan Field

Ceaseless advancements in seismic data processing technology and workflow enable the extraction of increasing amounts of subsurface information essential for derisking and optimizing natural resource development. The Kashagan oil field, one of the world's largest carbonate reservoirs, is faced with significant development optimization challenges due to a combination of complex geology and suboptimal seismic data coverage. The latest processing technologies were applied to the 2001–2002 vintage sparse ocean-bottom cable data and produced step-change improvements over the 2010–2011 legacy processing and imaging results. Such significant image uplift is representative of technology advancements over the span of a decade fully leveraging the newly preprocessed input data, including (1) an integrated model building workflow, (2) adaptive application of full-waveform inversion, (3) geologically constrained reflection tomography, and (4) least-squares imaging. The new seismic results improved the structural and stratigraphic imaging of multiple layers of shallow carbonates, mitigated fault shadow and other complex overburden effects, increased the resolution of subsalt carbonate reservoir features such as karst and internal fractures, improved well marker depth misfit, and ultimately influenced the placement of upcoming wells and reservoir development plans. Further improvements in seismic imaging would be feasible with a modern acquisition of more densely sampled seismic data, which would allow the full potential of the latest seismic processing technologies and workflows to be unlocked.

Wavefield decomposition in arbitrary direction and an imaging condition based on stratigraphic dip

In reverse time migration (RTM), wavefield decomposition can play an important role in addressing the issue of migration noise, especially low-frequency noise. The complete wavefield decomposition based on the Hilbert transform is a commonly used method in RTM, but it is accompanied by extra wavefield simulation and wavefield storage. We have developed three distinct methods. The first is a convenient method for wavefield decomposition, which is based on Poynting vectors. Only the unit vector in one direction is needed to realize the wavefield decomposition in an arbitrary direction by this method. It breaks through the limitation that the Hilbert transform-based method is applicable only to the up- and downgoing wave or left- and right-going wave decomposition, and the calculation cost is negligible compared with RTM. The second is a method based on the instantaneous wavenumber, which we developed for calculating the wave propagation direction. On the basis of wavefield decomposition, the imaging angle gather from the new method performs better than that of the Poynting vector method. Meanwhile, it also is used for generating the incident angle gather and dip angle gather. The latter expresses the dip angle of underground strata. More importantly, the above methods allow us to control the wavefield decomposition direction and three angles at any position underground. The third adopts a stratigraphic imaging condition method, and we briefly analyze the relationship between the new method and the inverse-scattering imaging condition. The stratigraphic imaging condition maps the results to the dip angle of the stratum through a spatial gradient wavefield, which can enhance the effective imaging information. The above three kinds of angle gathers also can be constructed by the stratigraphic imaging condition. Numerical experiments demonstrate that the imaging results and the angle gathers obtained by our proposed method have higher accuracy and resolution.

Detecting 1-D and 2-D ground resonances with a single-station approach

SUMMARYThe vibration modes of the ground have been described both in the 1-D and 2-D case. The 1-D resonance is found on geological structures whose aspect ratio is low, that is on layers with a lateral width much larger than their thickness. A typical example is that of a horizontal soft sediment layer overlying hard bedrock. In this case, the 1-D resonance frequency, traditionally detected by means of the microtremor H/V (horizontal to vertical spectral ratio) technique, depends on the bedrock depth and on the shear wave velocity of the resonating cover layer. The H/V technique is thus used both to map the resonance frequencies in seismic microzonation studies and for stratigraphic imaging. When 2-D resonance occurs, generally on deep and narrow valleys, the whole sedimentary infill vibrates at the same frequency and stratigraphic imaging can no longer be performed by means of the 1-D resonance equation. Understanding the 1-D or 2-D resonance nature of a site is therefore mandatory to avoid wrong stratigraphic and dynamic interpretations, which is in turn extremely relevant for seismic site response assessment. In this paper, we suggest a procedure to address this issue using single-station approaches, which are much more common compared to the multistation synchronized approach presented by research teams in earlier descriptions of the 2-D resonances. We apply the procedure to the Bolzano sedimentary basin in Northern Italy, which lies at the junction of three valleys, for which we observed respectively 1-D-only, 1-D and 2-D, and 2-D-only resonances. We conclude by proposing a workflow scheme to conduct experimental measurements and data analysis in order to assess the 1-D or 2-D resonance nature of a site using a single-station approach.

The value of VSP data through early phases of field appraisal and development: A modeling and acquisition case study in the Gulf of Mexico

The Gulf of Mexico is a rich hydrocarbon province that contains a diversity of petroleum systems play types. Often, identifying drilling targets can be challenging when solely using surface seismic data, particularly in areas with complex salt structures in the overburden. In this paper, we present a vertical seismic profile (VSP) modeling and acquisition case study for an oil field located in a subsalt, deepwater, ultrahigh-pressure high-temperature environment. Our objective was to model the subsurface to guide the acquisition of VSP data during the early phases of exploration and appraisal drilling. In the first exploration well, a salt-proximity VSP designed in a walkaway configuration was carried out to help better define the geometry of a salt overhang and verify anisotropy parameters, helping to reduce a critical uncertainty for imaging the subsalt structure across a large segment within our field area. In the first appraisal well, a zero-offset VSP was collected to establish a direct well tie and further calibrate our velocity model. In the second appraisal well, we utilized walkaway VSP data to form a high-frequency stratigraphic image between the two appraisal wellbores. These data were used to generate an enhanced image of the reservoir section that revealed subtle stratigraphic boundaries, another key subsurface uncertainty. Finally, we modeled both ambitious and conservative 3D VSP acquisition designs to understand the imaging area achieved through a 3D acquisition and undertook an assessment to understand the impact of PP and PS imaging for reservoir characterization. We conclude that VSP data are valuable tools in the early phases of field appraisal and development, and we demonstrate the business value of VSPs to optimize development drilling locations in our study area.

Model Based Inversion of Acoustic Impedance from Seismic Trace for Lithofacies Differentiation: an Application in Xy Field Offshore Niger Delta

This study presents the result of a Model-based seismic inversion technique which was used to invert an acoustic impedance structure within a reservoir interval by intergrating well logs and 3D post stack seismic data obtained from XY field offshore Niger Delta. The purpose was to delineate lateral and vertical alternations in subsurface rock properties which is caused by difference in lithofacies within the reservoir interval. This would help to define hydrocarbon fairways better and constrain the range of hydrocarbon zones for field development. The inversion workflow used in this study includes forward modelling of reflection coefficients from a low frequency impedance model driven from well logs and convolution of the reflection coeffiecients with a source wavelet derived from the seismic data. Acoustic impedance cross section obtained from the inversioin algorithim showed impedance values increasing from 4112 to 7539 (m/sec*g/cm 3 ) from top to bottom of the reservoir with gas filled sand facies observed at the top of the reservoir within time window 1900-2100msec. Below time window 2100msec, there is variation in impedance values observed within the anticlinal structures seen at this interval which suggests porous sand facies containing little shale intercalations. This is characteristic of sandstone reservoirs within the Agbada formation in the Niger Delta. These sands were most likely deposited through distributaries channel deposits, distributaries mouth bars, barrier bars, alluvial fans and crevasse which characterize the reservoir rocks (sandstones) in the Niger Delta. At time window 2100-2200msec, anticlinal structures containing porous sand facies with little shale intercalations was observed again. At time window 2200msec, water bearing sand facies (clean sandstone) was observed and at the bottom of the reservoir within time window 2300- 2500msec, the impedance was dominantly high which suggests the presence of shale facies at the bottom of the reservoir. Gas-oil contact (GOC) was observed between time window 2100-2200msec of the acoustic impedance section. These variations in acoustic impedance amplitude is due to lateral changes in lithofacies within the reservoir. The results obtained gave enhanced structural disposition of the reservoir and are important for accurate stratigraphic imaging interpretation to lower the risk in drilling of exploratory and development wells. Keywords : Acoustic impedance, Seismic inversion, Model-based, Lithofacies

Low-Frequency Noise Suppression Method Based on Improved DnCNN in Desert Seismic Data

High-quality seismic data are the basis for stratigraphic imaging and interpretation, but the existence of random noise can greatly affect the quality of seismic data. At present, most understanding and processing of random noise still stay at the level of Gaussian white noise. With the reduction of resource, the acquired seismic data have lower signal-to-noise ratio and more complex noise natures. In particular, the random noise in the desert area has the characteristics of low frequency, non-Gaussian, nonstationary, high energy, and serious aliasing between effective signal and random noise in the frequency domain, which has brought great difficulties to the recovery of seismic events by conventional denoising methods. To solve this problem, an improved feed-forward denoising convolution neural network (DnCNN) is proposed to suppress random noise in desert seismic data. DnCNN has the characteristics of automatic feature extraction and blind denoising. According to the characteristics of desert noise, we modify the original DnCNN from the aspects of patch size, convolution kernel size, network depth, and training set to make it suitable for low-frequency and non-Gaussian desert noise suppression. Both simulation and practical experiments prove that the improved DnCNN has obvious advantages in terms of desert noise and surface wave suppression as well as effective signal amplitude preservation. In addition, the improved DnCNN, in contrast to existing methods, has considerable potential to benefit from large data sets. Therefore, we believe that it can open a new direction in the area of seismic data processing.

3D seismic attributes for structural mapping and enhancement of deep gold mining: a case study from the West Wits Line goldfields, South Africa

Volumetric and horizon-based seismic attributes were carried out on the 3D reflection seismic data acquired on the deep crystalline rock environments of the Witwatersrand Basin (South Africa). This study aimed at improving the delineation of minor faults that cross-cut the gold-bearing quartz pebble conglomerate horizons (termed reefs) in the complex geology of the West Wits Line (WWL) goldfield using different seismic attributes. The 3D seismic data show delineation of faults that cross-cut and displace the target horizons by tens to hundreds of metres, leading to the delineation of the ore resources in faulted areas of the mines. In particular, the ant-tracking technique has provided better stratigraphic and structural imaging in complex faulted areas, relative to conventional interpretation methods. Other improvements include more accurate mapping of the depths, dip and strike of the key seismic horizon (Roodepoort shale) using the edge detection attributes, yielding a better understanding of the interrelationship between fault activity, reef distribution and the relative chronology of tectonic events. The integration of the 3D reflection seismic data, seismic attributes and information from borehole logs and underground mapping has provided better imaging and modelling of important fault systems that might have a direct effect on mining. This information could be used for future mining planning and designs to: (1) assess and mitigate the risks posed by mining activities, and (2) improve the resource evaluation of the gold-bearing reefs in the WWL goldfield.

Application of high-resolution 2D–3C seismic for characterization of the perspective Jurassic shale play in Central Poland

Here we show the application of broadband (4–120Hz) 2D–3C seismic for characterization of the perspective Jurassic shale play in Central Poland. Data were acquired along a network of 250km 2D profiles using single-point, densely spaced receivers (digital 3C sensors) and acquisition was focused on providing both high-resolution and broadband seismic that would enable structural imaging and quantitative interpretation of the key stratigraphic horizons in the Mesozoic sedimentary cover. Such acquisition parameters resulted in good quality data and allowed for more flexibility during processing, e.g., unaliased F–K filtering or digital group forming for ground-roll removal. Processing was oriented to preserve relative amplitudes and the broadband character of the dataset as the input for future quantitative interpretation. We obtained a high-resolution stratigraphic image of the target Upper Jurassic (Upper Kimmeridgian–Tithonian) sequence as well as overall structural portrait of this part of Mid-Polish Trough characterized by strong imprint of the salt tectonics. Lateral continuity of particular stratigraphic sequences has been determined and a more precise structural context for deposition and present-day structure of the Upper Kimmeridgian–Tithonian has been established.

Rapid Microstructure Analysis of Polar Ice Cores

AbstractWith climate change and its implications for society and the Earth being a frequent topic in both politics and science, measureable data on the influence of mankind on current and past climate has become essential information for making predictions and decisions about future climate. The polar ice sheet provides information about temperature, precipitation as well as gas and aerosol concentration as a unique depiction of climate throughout hundreds of thousands of years. The information obtained from ice cores enables future climatic events as well as general material properties of ice to be better understood. The longest ice core drilled in Antarctica has a length of 3270 m and contains climate information dating back more than 800,000 years. The rapid analysis (minimum scan time 3 s) provided by the large area scan macroscope (LASM; Fig. 1) with a resolution of 5 μm has proven to be an essential tool for analyzing the microstructures of ice cores, both in the field and in the laboratory. A stratigraphic image that supports dating the ice cores can be obtained using Intermediate Layer Core Scanner.

Continuous resistivity profiling survey in Mersin Harbour, Northeastern Mediterranean Sea

No detailed information has previously been available on the geological and geophysical characteristics of the sea floor and the underlying strata of Mersin Harbour, Northeastern Mediterranean Sea (Turkey). Continuous resistivity profiling (CRP) and borehole data from Mersin Harbour were used to interpret geoelectric stratigraphy of Neogene-Quaternary sediments in the area. This represents one of few such detailed case studies that have applied these valuable CRP techniques for the purpose of marine stratigraphic imaging. It was found that the Neogene-Quaternary sedimentary succession in the area consists of three geoelectric units (GU1, GU2, and GU3 from base to top). The lowest unit, GU1, has a resistivity value of greater than 20.0 ohm-m and consists of Miocene aged limestone and marl. The middle unit, GU2, is characterized by resistivity values ranging from 3.0 to 20.0 ohm-m. Its thickness is greater than 90 m, with the upper section being composed of stiff clay sequences which are Plio-Pleistocene in age. The uppermost unit, GU3, has resistivity values varying from 1.0 to 3.0 ohm-m. This unit displays a maximum thickness of 15 m, and is composed of Holocene muds together with gravel, sand, silt and clay (sometimes incorporating shells) materials of the Plio-Pleistocene age and their various mixtures, silty/clay limestone, and conglomerate sandstone. Comparisons of the geoelectric units with the depositional sequences interpreted from the available seismic data outwith, but close to, Mersin Harbour reveal that the geoelectric unit GU3 corresponds to the depositional sequences C (mainly Holocene) and B (mainly Plio-Pleistocene). The geoelectric unit GU2 partly correlates with the depositional sequence B which appears to be Plio-Pleistocene in age. The geoelectric unit GU1, which has not been encountered in previous seismic surveys, is a new discovery within Mersin Harbour. Limited correlation between the seismic and resistivity structures in the study area is attributed to differences in the acoustic impedance and resistivity contrasts of sub-bottom layers, as well as the penetration versus resolution performance of the systems.

Geostatistical analysis of an experimental stratigraphy

A high‐resolution stratigraphic image of a flume‐generated deposit was scaled up to sedimentary basin dimensions where a natural log hydraulic conductivity (ln(K)) was assigned to each pixel on the basis of gray scale and conductivity end‐members. The synthetic ln(K) map has mean, variance, and frequency distributions that are comparable to a natural alluvial fan deposit. A geostatistical analysis was conducted on selected regions of this map containing fluvial, fluvial/floodplain, shoreline, turbidite, and deepwater sedimentary facies. Experimental ln(K) variograms were computed along the major and minor statistical axes and horizontal and vertical coordinate axes. Exponential and power law variogram models were fit to obtain an integral scale and Hausdorff measure, respectively. We conclude that the shape of the experimental variogram depends on the problem size in relation to the size of the local‐scale heterogeneity. At a given problem scale, multilevel correlation structure is a result of constructing variogram with data pairs of mixed facies types. In multiscale sedimentary systems, stationary correlation structure may occur at separate scales, each corresponding to a particular hierarchy; the integral scale fitted thus becomes dependent on the problem size. The Hausdorff measure obtained has a range comparable to natural geological deposits. It increases from nonstratified to stratified deposits with an approximate cutoff of 0.15. It also increases as the number of facies incorporated in a problem increases. This implies that fractal characteristic of sedimentary rocks is both depositional process–dependent and problem‐scale‐dependent.

Quantitative stratigraphic image improvements through incremental processing advances

It is not surprising that divergent interpretations can often arise from different seismic data sets acquired over the same leases or concessions. Some reasons are that one data set may be more recent, more “state‐of‐the‐art,” or acquired in a preferred orientation when compared with another data set. However, it is surprising that there are instances where the same data set yields different interpretations. This is not because the interpreting geoscientists are vastly different in their interpretation styles or experiences but more often because this data set has been processed in different ways over the years. For this reason a good understanding of data processing is important for interpretation.

Using Borehole Images to Quantify Reservoir Quality and Stratigraphic Distribution: ABSTRACT

Understanding the distribution of good-quality reservoir rock in a prospective formation is essential to improved reserves computation and maximized production. High-resolution borehole images provide a rapid and efficient method to evaluate reservoir quality over extended sequences. Microconductivity curves from images are evaluated using modal analysis on histograms to establish specific populations. These populations are assigned to electrofacies, which are discriminated using other logs and core data to generate a lithofacies column. Net pay, or sand counts, are computed based on the distribution of log-derived lithofacies. Reservoir distribution is based on the integration of structural and stratigraphic image analyses. This method is applied to two different reservoirs, the first example is of a well developed, valley-fill sequence comprising thick, stacked fluvial-deltaic channel sands. These sands are over 100 ft thick, have >20% porosity, and over 200 md permeability. The distribution and thickness of optimum quality reservoir is random, however. The second example is of multiple, small scale, depositional units between 3 in. and 24 in., which have 15% porosity with >500 md permeability. The net thickness and exact position of high-quality reservoir intervals is not apparent from standard logs, but becomes clear after image analysis.

EFFICIENT 3‐D MIGRATION IN TWO STEPS*

AbstractThe addition of a third dimension to seismic data acquisition offers significant improvement of our reflection images of geologic structures, but potentially at a substantial increase in data processing cost. Here we study an approach to migration of three‐dimensional (3‐D) stacked data that can considerably reduce the burden of computation and data manipulation. Cascading two‐dimensional (2‐D) migrations in orthogonal lateral directions, we obtain an efficient 3‐D scheme that is exact for homogeneous media. As applied to examples of synthetic data from inhomogeneous media, the scheme introduces errors well below those attributable, in practice, to uncertainties in migration velocity. Application of this two‐step approach, using both Kirchhoff‐summation and finite‐difference algorithms, to a 3‐D seismic land survey over an area of known hydrocarbon traps additionally proves the importance of 3‐D migration in stratigraphic imaging.