Seismic Sections Research Articles

Seismic forward modeling for investigating and interpreting thin beds in a carbonate reservoir in SW Iran

High-frequency contents of reflections are essential in the investigation and interpretation of thin-bed reservoirs. These beds can be even more complicated in carbonate rocks, as pore geometries influence final seismic responses. To address these complexities, we propose a seismic forward modeling workflow to investigate several thin-bed reservoirs in a carbonate oilfield with variable pore geometries. The new workflow enhances the existing forward models for the investigation of thin beds by integrating seismic petrophysics, geological model building, and 2D finite-difference elastic modeling. We used seismic petrophysics to ensure the consistency between petrophysical well logs and seismic data using rock physics modeling. Then, we introduced a new high-resolution workflow for velocity modeling to build a reliable geological model. Finally, the 2D finite-difference elastic modeling is employed to generate synthetic traces based on our geological model to obtain seismic responses for the existing thin-bed reservoirs. The forward models used in this study are a powerful tool for investigating thin layers because they enable high-resolution investigation of the given geological model in distinguishing lateral and vertical lithofacies changes. The new velocity modeling workflow, implemented in this research, is more reliable and effective than the conventional velocity property modeling approaches, which resulted in synthetic seismic sections with increased lateral and vertical resolutions and enhanced data from a thin bed. The main features of this workflow are the incorporation of well-log data into geological model building, combining the high-resolution data of horizontal seismic stacking velocity with vertical well logging, and the incorporation of a residual model to improve the seismic stacking velocity. We produced a more coherent section resembling the acquired 3D seismic data by applying the proposed workflow to data from an oil carbonate reservoir in the Fahliyan Formation within the Abadan Plain in SW Iran. It is concluded that the higher frequency synthetic sections from the proposed workflow can assist in resolving the seismic interpretation challenges. By applying the proposed workflow to the current data set, four thin-bed carbonate reservoirs were investigated with corresponding thicknesses of approximately 25, and 17 m at peak frequencies of 60, and 90 Hz, respectively.

Research on the fluid dynamics interaction between submarine sand waves and seawater by seismic oceanography

IntroductionThe seafloor is an important interface between the lithosphere and the hydrosphere, where processes related to circulation and energy exchange happen along with various marine processes widely developed in the water column near the seafloor. These processes are still not yet completely understood as there are constraints of submarine detection technology and the interdisciplinary nature of these complex environments. Seismic reflection data have been a preferable tool to study and image these processes due to their characteristics in terms of spatial resolution. In seismic reflection data, submarine sand waves show hair-like reflection configurations with low continuity and wearing-hair style, appearing with an angle with the seafloor. Investigation of the relationship between the characteristics of submarine sand waves induced hair-like reflection configuration and hydrodynamics is crucial for understanding hair-like reflection configuration generation and spatiotemporal evolution. MethodsThis study combines fluid dynamics numerical simulation and seismic oceanography to discuss the seismic response characteristics and formation mechanisms of the hair-like reflection configuration. First, we create a seawater time-variant fluid-dynamical model followed by the numerical simulation of seismic oceanography data. This procedure results in seismic oceanography numerical simulation sections with hair-like reflection configurations for different constant flow conditions forced on the boundary. Optimal matching method is then applied to interpret field seismic reflection sections given the results obtained with the numerical experiments. Results and discussionAs consequence, the fluid dynamic explanation for the formation mechanism of the hair-like reflection configuration due to differences in seawater thermohaline is proposed. The study provides additional comprehension and further insights into the dynamic process of submarine sand waves induced hair-like reflection configuration using the seismic oceanography method.

Genetic mechanisms and control on hydrocarbon accumulation in transtensional structures: A case study of Gaoqing area, northern Dongying Sag, China

Broom-shaped structures are widely seen in transtensional basins, including those discovered in many sags of the Bohai Bay Basin. Broom-shaped transtensional structural zones are generally petroliferous and thus important targets for hydrocarbon exploration. This study analyzed the evolution and genetic mechanisms of the broom-shaped transtensional structures in the Gaoqing area of the Dongying Sag using the 3D seismic flattening technique and a physical simulation experiment. Furthermore, the control effects of the broom-shaped transtensional structures on hydrocarbon accumulation and distribution were discussed. The research results are as follows. The Gaoqing area is of a broom-shaped transtensional structure in planar view, composed of several arc-shaped secondary faults intersecting with high-level main fault in the same direction. On the seismic section, it appears as a typical semi-flower-like structure. In the early stage (that is, during the deposition of the Kongdian Formation to the lower submember of the 4th member of the Shahejie Formation, Es4(L)), single-fault pattern consisting of single or multiple faults in alignment was developed under extensional stress. While with the change of structural stress (that is, during the deposition of the Es4(U) to the Es2(L)), more secondary faults were formed, and came in a pattern of broom-shaped structure together with the major fault. In the late stage (that is, during the deposition of the Es1 to the Guantao Formation), the en echelon fault pattern was formed, as the major fault broke into multiple secondary faults. The divergent end of the broom-shaped transtensional structure has many secondary faults developed, the fault planes are gentle, and small-scale fan deltas are the predominant type of deposits. The hydrocarbon tends to laterally migrate far away and accumulate mianly in structural and lithologic-structural traps. On the other hand, in the convergent end, the fault planes are relatively steep, resulting in small-scale subaqueous fans or large-scale deltas deposited along the major fault, with hydrocarbon accumulating minaly in structural-lithologic traps proximal to the provenances.

Chapter 6. Seismic expression of North Sea Jurassic sequences

Abstract Many of the stratigraphic sequences recognized in North Sea Jurassic well sections correspond to mappable surfaces on seismic sections. Typically, however, sequences are only mappable seismically within individual sub-basins, and seismic correlation between sub-basins, or across highs, is generally impossible without independent control from wells. Particularly prominent seismic sequence boundaries occur at near-base J54 in the Inner Moray Firth (‘Intra-Oxfordian Event’) the Viking Graben (‘Top Heather’ in this area), base J62 (‘Top Heather’, Moray Firth), base J66 (‘Top Lower Hot Shale’, Inner and Outer Moray Firth), base J71 (East Shetland Platform), base J73 (‘Top Siltstone Member’, Moray Firth) and top J70/base K10 (‘Base Cretaceous Unconformity’ (BCU), basin-wide). The BCU is the most frequently mapped seismic horizon in the North Sea Basin in Jurassic–basal Cretaceous studies. This surface, at the base of the Cromer Knoll Group, separates synrift sediments from post-rift successions above and marks a major shift in the tectonic evolution of the North Sea Basin.

An integrated study on the reservoir potential of upper Cretaceous succession of the Anambra Basin (Nigeria) using the seismic-structural, petrophysical, and sedimentological approach

The Anambra Basin has a relatively high hydrocarbon production base compared with other Cretaceous basins in Nigeria. Structural deformation, overpressures, and faulting in the Cretaceous sequence of the Anambra Basin have resulted in significant uncertainties regarding reservoir parameters, necessitating accurate reservoir characterization. This study uses several subsurface data, including 2D seismic data, a report of core descriptions, and well log data from an exploration well drilled in the Anambra Basin to identify reservoirs and assess the petrophysical parameters of Cretaceous Formations for hydrocarbon accumulations. Three reservoirs were identified in the interpreted seismic section of the Cretaceous strata, and the structural framework was described as a system of synthetic faults dipping north-south that controlled the thickness and lateral extension of the deposited sediments. The shale volume, total and effective porosity, permeability, water, and hydrocarbon saturation were estimated using standard equations. The low shale volume, good-to-very good porosity values, fair-to-moderate permeability values, and low-to-moderate hydrocarbon saturation of the identified Cretaceous reservoirs suggest that the reservoir sands have the capacity to produce hydrocarbons. The density-neutron cross-plots of the identified reservoirs show a dual mineral system and also indicate the effect of gas in the Cretaceous reservoirs. This study found that the Anambra Basin has gas potential. However, the identified gas reservoirs should be further evaluated using reservoir pressure data, if available, to aid in accurate hydrocarbon fluid typing. The reservoirs should be developed and optimized in such a way as to keep production risk to its minimum.

Using a synthetic data trained convolutional neural network for predicting subresolution thin layers from seismic data

Numerous studies have demonstrated the capability of supervised deep learning techniques for predicting geological features of interest from seismic sections, including features that are difficult to identify using traditional interpretation methods. However, the successful application of these techniques in practice has been limited by the difficulty of obtaining a large training data set where the seismic data and corresponding ground truth labels are well-defined. Manually creating large amounts of labels requires a heavy workload, and the uncertainty of the interpretation and labeling process decreases the model’s ability for making accurate predictions. Using the chalk-flint sequence scenario onshore Denmark as an example, we have developed a novel workflow for predicting subresolution thin layers from seismic sections. It entails generating large quantities of synthetic training data with high-quality labels using stochastic geological modeling, training a convolutional neural network based on the synthetic data set, and applying it to real seismic data. This is, to our knowledge, the first example of using deep learning to predict subresolution thin layers from seismic data based on geostatistically generated training images. It is shown that a neural network trained on synthetic data can predict a realistic number of subresolution flint layers from the real seismic data that have been collected from the Stevns region in Denmark, which has value for the understanding of the overall geological characteristics of succession and engineering applications such as construction site evaluation.

Dual‐sensor wavefield separation in a compressed domain using parabolic dictionary learning

AbstractIn the marine seismic industry, the size of the recorded and processed seismic data is continuously increasing and tends to become very large. Hence, applying compression algorithms specifically designed for seismic data at an early stage of the seismic processing sequence helps to save cost on storage and data transfer. Dictionary learning methods have been shown to provide state‐of‐the‐art results for seismic data compression. These methods capture similar events from the seismic data and store them in a dictionary of atoms that can be used to represent the data in a sparse manner. However, as with conventional compression algorithms, these methods still require the data to be decompressed before a processing or imaging step is carried out. Parabolic dictionary learning is a dictionary learning method where the learned atoms follow a parabolic travel time move out and are characterized by kinematic parameters such as the slope and the curvature. In this paper, we present a novel method where such kinematic parameters are used to allow the dual‐sensor (or two‐components) wavefield separation processing step directly in the dictionary learning compressed domain for 2D seismic data. Based on a synthetic seismic data set, we demonstrate that our method achieves similar results as an industry‐standard FK‐based method for wavefield separation, with the advantage of being robust to spatial aliasing without the need for data preconditioning such as interpolation and reaching a compression rate around 13. Using a field data set of marine seismic acquisition, we observe insignificant differences on a 2D stacked seismic section between the two methods, whereas reaching a compression ratio higher than 15 when our method is used. Such a method could allow full bandwidth data transfer from vessels to onshore processing centres, where the compressed data could be used to reconstruct not only the recorded data sets, but also the up‐ and down‐going parts of the wavefield.

Sequence structure, sedimentary evolution and their controlling factors of the Jurassic Lianggaoshan Formation in the East Sichuan Basin, SW China

Based on the data of outcrops, seismic sections, thin sections, heavy mineral assemblages and detrital zircon U-Pb dating, the sedimentary characteristics, lake level fluctuation and provenance characteristics of the Middle Jurassic Lianggaoshan Formation (J2l) in eastern Sichuan Basin, SW China, were investigated to reveal the control of tectonic movements of the surrounding orogenic belts on the sedimentary systems. The J2l mainly developed a delta—lake sedimentary system, which contained a complete third-order sequence that was subdivided into four lake level up-down cycles (fourth-order sequence). The lake basins of cycles I and II were mainly distributed in eastern Sichuan Basin, while the lake basins of cycles III and IV migrated to central Sichuan Basin, resulting in the significant difference in sedimentary characteristics between the north and the south of eastern Sichuan Basin. The provenance analysis shows that there were three types of provenances for J2l. Specifically, the parent rocks of Type I were mainly acidic igneous rocks and from the proximal northern margin of the Yangtze Plate; the parent rocks of Type II were intermediate-acid igneous rocks and metamorphic rocks and from the central parts of the southern and northern Qinling orogenic belts; the parent rocks of Type III were mainly metamorphic rocks followed by intermediate—acid igneous rocks, and from the North Daba Mountain area. It is recognized from the changes of sedimentary system and provenance characteristics that the sedimentary evolution of J2l in eastern Sichuan Basin was controlled by the tectonic compression of the Qinling orogenic belt. In the early stage, the lake basin was restricted to the east of the study area, and Type I provenance was dominant. With the intensifying north-south compression of the Qinling orogenic belt, the lake basin expanded rapidly and migrated northward, and the supply of Type II provenance increased. In the middle and late stages, the uplift of the North Daba Mountain led to the lake basin migration and the gradual increase in the supply of Type III provenance.

Quantitative determination of fault slip magnitude: a review

In geology, a fault is a rock fracture with perceptible relative displacement of the opposite sides of the fracture. Quantitatively obtaining the activity history of faults depends mainly on the estimation of fault slip and the study of markers, which can be used to understand and analyze the tectonic evolution of the faulted regions. Fault slip is calculated by restoring points that were originally adjacent before the deformation, those points are named piercing points. In this paper, we review some published methods to determine fault slip, using: (1) the offset of contours on structural contour maps; (2) offset on seismic reflection sections; (3) a known slip vector (fault striae) and one marker; (4) two known markers. Cases (3) and (4) are commonly applied to field work and geological maps.

Multichannel Sparse Deconvolution of Teleseismic Receiver Functions With f − x Preconditioning

AbstractTeleseismic receiver functions (RFs) are an effective tool for structural imaging due to their simplicity and sensitivity to changes in rock elastic properties. Generally, RFs are estimated through trace‐by‐trace deconvolution. This individualized approach ignores the lateral coherency among RFs from neighboring raypaths, which could lead to rougher or incorrect seismic sections due to the presence of noise. This study presents a multichannel sparse deconvolution method that takes advantage of the cross‐trace coherency of RFs at individual stations for stable and accurate imaging outcomes. The proposed algorithm incorporates sparse inversion and frequency‐space prediction filters, which facilitate the retrieval of high‐resolution and spatially continuous conversion energy. Rigorous testing using synthetic and actual data typically suggests the higher robustness of multichannel deconvolution over single‐channel deconvolution, especially under low signal‐to‐noise ratios. The noise‐resistance property of the multichannel approach offers new opportunities to increase the volume of usable, high‐quality data for receiver function analysis.

Seismic sequence stratigraphy and depositional modelling of the Lower Fahliyan Formation in the northwestern Persian Gulf, SW Iran

This study presents seismic stratigraphic-based sedimentary conceptual modelling through the integration of well data, seismic data, and literature in order to identify the architecture of the Berriasian-Early Valanginian Lower Fahliyan Formation in the northwestern Persian Gulf. The applied methodology is configured by interpreting seismic sections in terms of sequence stratigraphy and adapting them using well data facies associations and fossil assemblages. The time framework of these sediments is identified by their faunal/flora assemblage and extrapolated by seismic data because of their extensive coverage. Three third-order depositional sequences (DS-I, DS-II, and DS-III) were distinguished based on the facies changes, depositional geometries, and fossil assemblages. Regional and local tectonic activities and global sea-level changes controlled the sedimentation of the depositional sequences. The local tectonic activities were of higher importance as they strongly affected the thickness and facies type of the studied interval. The depositional sequences of the Lower Fahliyan Formation are deposited in the inner to outer settings of a carbonate shelf. It was concluded that DS-I and DS-II over the platform (Surmeh Platform), consisting of progradational parasequences, deposited during the highstand phase in shoal settings that due to primary texture and subsequent diagenetic imprints yield the best reservoir quality. The relationship of the parasequences with good reservoir quality was determined on seismic sections and the prepared regional correlation panel over the study area. Furthermore, the paleoenvironmentof the area was constructed and modelled for major stratal surfaces (Sequence boundaries and maximum flooding surfaces). According to the generated models, the distal setting (Garau Basin) migrated back gradually northward during the Berriasian-Early Valanginian so that it was completely replaced by a homoclinal ramp at the end of the DS-III deposition. Subsequently, the relief of Surmeh Platform was also flattened and it was recognized as the end of the first phase of the basin's activity.

Subsurface Structural Image of Galabat Field, North East of Iraq Using 2D Seismic Data

This research had been achieved to identify the image of the subsurface structure representing the Tertiary period in the Galabat Field northeast of Iraq using 2D seismic survey measurements. Synthetic seismograms of the Galabat-3 well were generated in order to identify and pick the reflectors in seismic sections. Structural Images were drawn in the time domain and then converted to the depth domain by using average velocities. Structurally, seismic sections illustrate these reflectors are affected by two reverse faults affected on the Jeribe Formation and the layers below with the increase in the density of the reverse faults in the northern division. The structural maps show Galabat field, which consists of longitudinal Asymmetrical narrow anticline of Fatha and Jeribe formations, where the Southeastern limb is steeper than the Northeastern limb. The seismic interpretation shows that Galabat Field has a positive inverted structure, it is an anticline at the level of the Tertiary Period. The direction of the anticline axis and the major reverses faults are Northwest -Southeast. It is concluded from the study that reverse faults originated due to Zagros tectonism which is widespread in the area are a major conduit that channeled petroleum flow from source to Miocene traps. In addition, these faults were caused by the presence of salt accumulation within the Fatha Formation and led to high variation in the thickness in the crest and limbs of the Galabat structure.

New insights on the origin of Barra volcanic ridge system, offshore Ireland: A long-distance influence of the Iceland mantle plume

The Iceland Plume has significantly impacted the North Atlantic region. Igneous edifices and large seamounts in the Rockall region were linked to Iceland plume pulsations during the Late Cretaceous to Mid-Eocene. In the south of the Rockall Basin a chain of northwest-southeast trending volcanic ridges known as the Barra Volcanic Ridge System (BVRS) have been identified on seismic and magnetic datasets. However, the timing, morphology, extent, and emplacement mechanism of the BVRS are not well understood. To investigate the volcanic ridge system, we analyze a 360-km-long pre-stack time migrated seismic section along the margin, which covers the southern extent of BVRS. The high velocity (4.5–4.8 km/s) obtained by using first arrival travel time tomography of downward continued data set, high magnetic anomaly and the typical morphology of the ridges suggest that they could be volcanoes of basaltic compositions. The integration of gravity and seismic data constrain the crustal structure and thickness, which indicates that the crust could be as thin as 4 km beneath the BVRS, requiring a maximum stretching factor >6. Folded compressional structures and lava flows within the Early Paleocene to Mid-Eocene sediments draping the volcanic ridges, suggest that these volcanic intrusions possibly have occurred post Early Paleocene age, which is approximately the same time-period when Iceland mantle plume arrived in the Rockall region. These results suggest that the BVRS developed because of pre-existing lithospheric configuration in the Rockall Basin and could be one of the southernmost volcanic provinces of the North Atlantic Igneous Province (NAIP).

Detailed Geoseismic Structure of the Riphean Deposits in Buolkalakh–Lena Area Based on Deep Drilling and Seismic Exploration Data

Abstract —The work considered an integrated approach to the study of deep-lying Riphean strata of the Buolkalakh–Lena region, which have certain prospects in terms of oil and gas potential. The method of revision and synthesis of acoustic and density logs is described in detail using the example of wells drilled in 80–90 for the purpose of correct reference to seismic sections. It has been shown that the Khaipakh, Debengdin, Arymas, Kyutingdinsky, and Sygynakhtakh formations of the Early and Middle Riphean eras overlook the pre-Permian surface in the middle part of the Lena–Anabar trough. The region also developed older Riphean deposits that are not exposed on the Olenek uplift and are not studied by deep drilling. The prospects for the oil and gas content of Riphean strata in the region are mainly associated with carbonate reservoirs formed in the top of the Riphean sequence and cropping out on the pre-Permian erosive surface, the most promising rocks are those constituting large carbonate platforms that were partially eroded and subjected to processes of hypergenesis and karst formation.

The spectral stacking method and its application in seismic data resolution increase

AbstractPreconditioning is a practice adopted in the treatment of seismic data and, when properly applied, favours a resolution increase. However, retrieving all the frequency contents is still a challenge to overcome. Techniques to improve the seismic resolution should preserve the existing relations in the distribution of seismic events. Otherwise, it may cause undesirable situations, from the degradation of the signal‐to‐noise ratio to the creation of false events, leading to errors of interpretation. We developed a method called spectral stacking, which makes use of circular convolution in the frequency domain to extend frequency content, preserving the distribution of seismic events without adding artefacts. As an application of the spectral stacking method, we create a sharpening filter that resolves layers under Ricker's criterion. In addition, the sharpening filter preserves the amplitude using a variable‐window correction based on the local minima of the absolute values of amplitude. We validated our proposed method in synthetic and real tests. The results showed that the sharpening filter obtained a resolution increase, providing a sharper image of the seismic section.

A look at the blind Kumamoto experiment: combining active and passive seismic observations to avoid Rayleigh-wave mode misidentification

We present our pathway through participation in the blind Kumamoto exercise, particularly the Step 1 of site characterization. The combination of passive and active seismic imaging techniques is used to image the velocity profile beneath the KUMA site. The estimation of the broadband Rayleigh wave dispersion curve is based on cross-correlations of ambient seismic noise and analysis of active seismic shots. We calculate correlations from the entire time series (only vertical components) of each seismic array after classical pre-processing of ambient noise data. Then, a passive seismic section is constructed using all available stations pairs and stacking the cross-correlation traces with similar interstation distances. The obtained passive seismic section is analyzed using a high-resolution Radon transform to obtain the dispersion image of Rayleigh waves traveling through the KUM-LL, KUM-M and KUM-SM arrays. Then, the information is merged and interpolated to obtain the final broadband dispersion curve. In addition, active source seismic data are used with the high-resolution Radon technique to constrain the model at shallow depths (< 30 m). Then, a broadband dispersion image is constructed with significant energy from 0.9 Hz to 45 Hz. The final dispersion curve is inverted using the non-linear neighborhood algorithm. Using just the fundamental mode Rayleigh wave, a first model with normal velocity variation in depth is obtained that corresponds well with the preferred model provided by the organizing committee. The addition of a mHVSR curve in a joint inversion better constrains the deeper part of the model (> 1 km). After comparison of the submitted dispersion curve to the theoretical dispersion curve for the preferred model (Step 4 of the blind test), the authors note that there was a clear misinterpretation in the fundamental mode of their submitted results, especially at frequencies higher than 5 Hz. Using both fundamental (only visible in the passive data set) and first overtones of Rayleigh waves (only visible in the active seismic data set) a refined velocity model could have been inferred, but we decided to keep our first submitted result. This detailed interpretation should be further studied as dispersion images from forward and backward hammer shots are quite different, which may indicate strong variations in the geometry and/or shear-wave velocities of the first meters of the subsurface.Graphical

Lithosphere Density Model of the Middle Urals Segment

For the Middle and North Urals segments and adjacent territories of the East European Platform and West Siberian Plate (within the graticular trapezoid with geographic coordinates 53–65° N, 48–72° E), ten deep seismic sounding (DSS) profiles were reprocessed using two-dimensional (2D) seismic tomography method, and gradient sections of crustal velocities were constructed in the form of grid functions. In the same format, density sections were constructed. The coefficients of density–velocity empirical correlation were calculated using the solution algorithm of 2D inverse problem of gravimetry. The method and calculation technology of a three-dimensional (3D) density distribution with tying to 2D data from reference seismic sections are embedded in the procedure of quantitative interpretation of potential fields with construction of 3D geophysical models. A stable solution of the 3D inverse problem of gravimetry is sought on the correctness set of the family of horizontal layers with 2D density distribution.

Deep crustal structure across the Challenger Deep: Tectonic deformation and strongly serpentinized layer

The southern Mariana subduction zone features the Earth’s deepest trench, named the Challenger Deep, and tectonic deformation is obvious in the overriding and subducting plates. We conducted a 230 km-long wide-angle reflection/refraction survey across the Challenger Deep. Based on ocean bottom seismometer (OBS) data, we image the velocity structure using forward and inverse modeling. The velocity model shows that the Pacific plate entering the subduction zone has normal crustal thickness (6.5–7.2 km) but lower velocity than mature Pacific oceanic crust. PnP seismic phases with large offsets and strong seismic reflectivity are observed at eight OBS seismic record sections, which constrain the thickness (4.0–6.5 km) and velocity (7.1–7.5 km/s) of the upper mantle low-velocity layer. We suggest that the observed velocity reduction in the crust and upper mantle of the subducting plate is caused by bending-related normal faults, which not only fracture the crust but also provide pathways for water infiltration and serpentinization of the dry mantle. The upper mantle reflector constrained by PnP seismic phases possibly indicates a rheological and fault-slip interface derived from rapid variations in the mechanical strength of peridotite. The inner trench slope (ITS) near the Challenger Deep is characterized by indented topography, where the upper crust is obviously thinned and the lower crust is uplifted; the isovelocity contour of 6.5 km/s is 1.5 km below the seafloor. This may be caused by the difference in the degree of subducting plate rollback on either side of the western boundary of the diffuse deformation zone.

Offshore Fault-related Fold along the Eastern Marginal Zone of Ishikari Lowland, Hokkaido, Japan

Distributions, characteristics, and activities of offshore fault-related folds which comprise the southern part of the Eastern Boundary Fault Zone of Ishikari Lowland (EBFIL) are clarified by conducting a high-resolution multi-channel seismic reflection survey. To determine termination point of the fault zone, we placed seven survey lines perpendicular to the fault-related folds with a total line length of 107.5 km. We choose a compact water gun (15 cubic inch) as the seismic source to obtain high-resolution imagery of the Late Quaternary and investigate activity of the fault zone. Based on seismic facies, we distinguished packages of offshore sediment (unit 2, 3 and 4, in descending order), which separated by downlap surfaces and correlated to the Middle to the Late Pleistocene. They are progressively deformed by the active fault and compose series of fault-related folds (Fo1, Fo2 and Fo3). Traces of these folds are figured out from the seismic sections obtained by this study and existing surveys. Deformation related to these folds are clearly recognized in the topography of the unit 4 base. The reliefs of strata indicate that the Fo2 fold should have uplifted seafloor at a rate of 0.09-0.20 m/ky since 300 ka. The structures, spatial distributions and growth-rates of the folds suggest that Fo1 and Fo2 should compose series of anticline developing at the front of EBFIL. Besides, Fo2 fold would compose the southern termination of the fault zone. This study has successfully demonstrated that densely operated high-resolution seismic survey can figure out distribution and activity of offshore active faults in the Late Quaternary, even at the outside of shelf.

Structural deformation and its implication for hydrocarbon accumulation in the Wuxia fault belt, northwestern Junggar basin, China

Abstract With the great discovery of unconventional oil in the Fengcheng Formation in the Mahu sag, the Wuxia fault belt, which shows similar lithological characteristics and lithofacies, is considered the most favorable area for future petroleum exploration. However, the complicated structural patterns remain unclear and restrict the petroleum exploration and development. In this study, combined with new seismic data and some borehole data, we conclude the structural styles in the Wuxia fault belt and analyze their distribution characteristics, and further investigate their implications for tectonic evolution and hydrocarbon accumulation. Five typical seismic sections are captured from the west to the east of the fault belt. Decollement folds and fold accommodation faults superimpose on the underlying basement fault related folds. Structure patterns also show a zonation in S–N direction and a segmentation in E–W direction. The balanced section reveals that the present-day structure features were fundamentally formed by Late Permian. The structural deformation shows distinctive features of a foreland basin which may develop in Early Permian and continue until the Late Permian. The oil reservoirs in the Fengcheng Formation in deeper detachment fold and the autochthonous Fengcheng Formation in fault propagation folds are the most favorable regions for further unconventional petroleum exploration.