Poor Seismic Imaging Research Articles

Structural control of inherited salt structures during inversion of a domino basement-fault system from an analogue modelling approach

Abstract. The geometries of inverted rift systems are different depending on a large variety of factors that include, among others, the presence of decoupling layers, the thickness of the pre- and syn-extension successions, or structural inheritances. Our study focuses on the inversion of an extensional domino-style basement-fault system with a pre-extension salt layer using analogue models to understand the role of pre-existing structural features during inversion. Models investigate how different overburden and salt thicknesses, inherited extensional structures, and salt distributions condition the evolution during inversion. The experimental results show that models with thick salt can partially or totally preserve the extensional ramp–syncline basin geometry independently of the overburden thickness. In contrast, models with a thin salt layer result in a total inversion of the ramp–syncline basins with the development of crestal collapse grabens and extensional faults affecting the overburden. Inversion also triggered the growth or reactivation of salt-related structures such as primary weld reopening and/or obliteration, diapir rejuvenation, salt thickening, or thrust emplacement. The use of analogue modelling allowed us to address the processes that controlled the growth and evolution of these structural elements during the inversion. Experimental results also provide a template of different structural styles resulting from the positive inversion of basins with a pre-extensional salt layer that can help subsurface interpretation in areas with poor seismic imaging.

Reverse Time Migration Imaging Using SH Shear Wave Data

In this paper, we discussed the reverse time migration imaging of compressional wave (P-wave) and horizontally polarized shear wave (SH shear wave) seismic data, together with P- and SH shear wave constrained velocity model building. In the Sanhu area in Qaidam Basin, there are large areas of gas clouds, which leads to poor P-wave seismic imaging. The P and SH shear wave seismic data of a co-located survey line with the same acquisition geometry were used to access their imaging capability using reverse time migration. We first estimated the change in P-wave and SH shear wave velocity ratio using pre-stack time migration (PSTM) for constraining the overall depth domain velocity model. Additionally, we then used an eighth-order finite difference scheme for P-wave reverse time migration on a variable grid and used the sixth-order combined compact difference (CCD) wave field simulation method for SH shear wave reverse time migration on a regular grid. The results show that the constrained velocity model produces a good match in the overall geological structure shown in the P-wave and SH shear wave reverse time migration results. However, in the gas cloud areas, SH shear wave reverse time migration has obvious imaging advantages, which can clearly image the structure inside the gas clouds.

Mesozoic hydrocarbon accumulation model in the Northern South China sea

The ChaoShan Depression is the largest Mesozoic depression covering an area of 3.7 x 104 km2 where the relict Mesozoic strata are up to 5000 m thick. It has experienced disruption since the late Mesozoic. In early survey, the oil-gas migration condition and reservoiring mechanism are poorly expounded from the Meso-Cenozoic tectonic superposition due to poor seismic imaging to the deep Mesozoic layers. New seismic surveys using long streamers and quasi three-dimensional layouts has improved obviously the deep images of the Mesozoic formations, enabling analysis of tectono-stratigraphic features and petroleum geology. Correlating to the regional Mesozoic stratigraphic and facies characteristics from the well LF35-1-1 and onshore outcrops, two sets of source layer tested with high organic carbon content are interpreted within the upper Triassic to mid-Jurassic semi-closed gulf sequences which become thicker toward the east side of Dongsha Island. Three sets of potential reservoir beds are also interpreted, which are a basin-floor fan sandstone layer of the Triassic-Jurassic boundary, a limestone layer atop the mid Jurassic, and a sandy layer in the Upper Jurassic. Over the low bulge of the ChaoShan Depression, a big anticline is found bounded by two sets of faults which played as migration passage for hydrocarbon from the deep Mesozoic source to the reservoir layers. Likely, one set of the fault act reverse barrier to block the petroleum escape, thus form the fault-bounded trap, favorable for future exploration.

Improving seismic fault mapping through data conditioning using a pre-trained deep convolutional neural network: A case study on Groningen field

Seismic fault interpretation is a crucial and indispensable step in reservoir exploration that requires substantial time. As a result, much research has been dedicated to applying deep learning in this venture. Deep learning has shown significant progress in the identification of seismic faults. However, its applicability has been hindered given the lack of appropriate labelled fault data and poor seismic imaging. The deep convolutional neural network (DCNN) employed in this study is a cutting-edge deep learning technique for image improvement and identification.In this study, detecting seismic faults and improving seismic imaging using a pre-trained DCNN is proposed using the Groningen field that has complexity in accurately imaging sub-salt geological structures as a case study. The presence of salt in the field causes wave attenuation. The fault mapping procedure is considered a segmentation of the 3D seismic problem and trains an encoder-decoder architecture, using a Deep Residual U-net, to generate a fault probability volume. A decent fault prediction result is achieved on the Groningen seismic volume. The next step of this study involved seismic data conditioning, where DCNN is applied to denoise volumes to improve visualisation, significantly below the salt structure. The DCNN, when used to improve seismic imaging, achieved a signal-to-noise ratio (SNR) of 30.2, which is almost quadruple that of the original volume. Faults were mapped on the conditioned volume using DCNN, resulting in an improved seismic fault probability volume. This technique achieved distinctively interpreted faults illustrating the significant improvements DCNN brings to the seismic imaging process. In interpreting new seismic volumes with poor imaging and low signal-to-noise ratio, caused by a change in seismic frequency and amplitude propagating through an attenuating medium like the Groningen field, researchers and geophysicists may apply the DCNN for volume conditioning before mapping seismic faults using neural networks.

Overview of the exploration potential of offshore Argentina – insight from new seismic interpretations

Argentina's offshore sedimentary basins cover a vast area on one of the widest continental margins on the planet, yet they remain underexplored today. Previous exploration drilling has failed to encounter commercial volumes of hydrocarbons, in part due to the poor seismic imaging of legacy 1960s–1990s 2D seismic data, and to the majority of wells being drilled on structural highs outside of the source rock kitchens. In this study, we reviewed 52 000 km of recently acquired (2017–2018) regional 2D long-offset seismic data with broadband pre-stack time (PSTM) and depth migration (PSDM) processing. We identified five major structural domains with hydrocarbon prospectivity on the Northern Margin of Argentina and four on the Southern Margin, and the presence of previously unseen structural and stratigraphic traps involving sequences assigned to proven regional source rocks and reservoirs in Permian, Jurassic and Cretaceous rocks. The source and reservoir rocks, petroleum systems, and play types present in the deepwater of the undrilled Argentina Basin represent a true frontier for hydrocarbon exploration. Pseudo relief attribute seismic displays and amplitude v. angle (AVA) analysis are demonstrated to be valuable tools in predicting the stratigraphy of the basins. A new framework for understanding the oil and gas prospectivity of the study area is presented.

Deep-water reservoir distribution on a salt-influenced slope, Santos Basin, offshore Brazil

Studies of near-seabed datasets show that salt tectonics controls the distribution and architecture of deep-water reservoirs in many salt-influenced basins. It is typically difficult, however, to study the distribution and stratigraphic evolution of depositional systems preserved at deeper, economically significant depths, reflecting poor seismic imaging of steeply dipping strata flanking high-relief salt structures. 3D seismic and borehole data from the Santos Basin, offshore Brazil allow us to identify a range of depositional elements that form the building blocks of three main tectono-stratigraphic phases. During the first phase, channel systems and lobes were confined within updip minibasins and to the hangingwalls of salt-detached faults. During the second phase, channel systems and lobes filled updip minibasins to bypass sediment downslope, with coarse clastic deposition then occurring in downdip minibasins, >100 km from the coeval shelf margin. Syndepositional seafloor relief caused: (i) channel system deflection and diversion around salt-cored highs; (ii) channel system uplift and rotation on the flanks of rising salt structures; (iii) lateral and frontal confinement of channel systems. During the final phase, rising salt walls dissected previously deposited deep-water systems, with MTCs deposition becoming increasingly important. Our results have important implications for post-salt prospectivity in the Santos Basin and other salt-influenced sedimentary basins, with a range of reservoirs and trapping styles present in this underexplored interval. More specifically, we show that large volumes of clastic sediment were not trapped behind the ‘Albian Gap’, a salt-controlled depocenter dominating the north-western basin margin, but were instead delivered further basinward.

Sedimentology and reservoir architecture of a widespread siliciclastic intra-lava unit, Kangerlussuaq, East Greenland

ABSTRACT The Rosebank hydrocarbon discovery in 2004 proved that intra-lava sandstones form attractive reservoirs in the Faroe–Shetland Basin and the new volcanic play triggered the need for suitable analogues to describe and assess sedimentology, reservoir architecture, compartmentalization, and connectivity of intra-lava siliciclastic units. The onshore Kangerlussuaq Basin in East Greenland offers the opportunity to study Paleogene intra-lava siliciclastic sandstones and their interaction with lavas, on the scale of the Rosebank Field. The focus of this study is a siliciclastic-dominated intra-lava unit, 4–10 m thick, exposed in almost vertical cliff sections over distances of several kilometers. The unit reflects a short return to siliciclastic deposition following initiation of volcanic activity and extrusion of the first lava flows in the area. Deposition took place as shoreface and delta progradation in a marine-influenced, shallow embayment. Lateral variations in sedimentary facies distribution and geometry are prominent and were largely governed by an interplay of base-level variations and autocyclic processes, the surface roughness and type of substratum on which deposition took place, and differential block movements before and during deposition. Presence of local topographic barriers are of key importance and influenced the lava–sediment interaction and the resulting 3D-geometry of lava flows and sediment bodies. In addition, compartmentalization of the intra-lava sandstone unit is observed and is controlled by the offset across normal faults, intersecting dikes, and to a lesser extent by invasive and eroding lavas. A depositional model is suggested that incorporates the detailed sedimentological and 3D photogrammetric observations and presents a possible explanation for the contrasting architecture of the intra-lava unit observed in three areas located a few kilometers apart. The model embraces the complex interplay between siliciclastic and volcanic settings and reveals important aspects to consider when recoverable volumes of hydrocarbons are estimated in intra-volcanic subsurface reservoirs in volcanic rifted margins with poor seismic imaging of the relatively thin intra-lava reservoirs.

A new insight into the structural evolution of Rosetta Fault, eastern margin of Herodotus Basin, East Mediterranean

While exploration activities are flourishing in both the offshore Nile Delta Basin and the Levant Basin, Herodotus Basin remains unexplored. The study of the complex tectonic evolution of this basin is mainly hindered by large water depths, insufficient and poor seismic imaging, as well as the scarcity of well penetrations. The present study offers a new insight into the tectonic history of the eastern margin of Herodotus Basin through detailed structural study of the Rosetta Fault. Detailed structural mapping of Rosetta Fault using 3D seismic and borehole data reveals its complicated deformation history. Together with other NE-SW oriented faults in the Levant Basin and the northern Egypt onshore area, Rosetta Fault experienced different phases of slip at different times including normal slip due to extensional deformation at Triassic to early Cretaceous times and reverse slip at Late Cretaceous to Tortonian times due to the convergence of Eurasia and Afro-Arabia. Contrary to the findings of some plate tectonic models, which consider NE-SW drift of Afro-Arabia and Eurasia, the present study suggests that the direction of Mesozoic extension in Herodotus Basin (as well as the Levant Basin and the northern onshore areas of Egypt) was NW-SE based on the orientations of Mesozoic active faults. Rosetta Fault also shows deformation by sinistral transtension at Messinian and Holocene times intervened by Plio-Pleistocene normal slip. When the Rosetta Fault was affected by normal slip and transtensional deformation at Messinian to Holocene times, the Herodotus Basin lying to the NW of it was affected by NW-SE shortening associated with the Hellenic subduction. The obvious contrast in the deformation style of Rosetta Fault and the Herodotus Basin is attributed to the irregular shape of the convergent plate boundary extending from south Cyprus to SE Crete.

Seismic expression of miocene carbonate platform and reservoir characterization through geophysical approach: application in central Luconia, offshore Malaysia

A substantial proportion of proven oil and gas reserves of the world is contained in the carbonate reservoir. It is estimated that about 60% of the world’s oil and 40% of gas reserves are confined in carbonate reservoirs. Exploration and development of hydrocarbons in carbonate reservoirs are much more challenging due to poor seismic imaging and reservoir heterogeneity caused by diagenetic changes. Evaluation of carbonate reservoirs has been a high priority for researchers and geoscientists working in the petroleum industry mainly due to the challenges presented by these highly heterogeneous reservoir rocks. It is essential for geoscientists, petrophysicists, and engineers to work together from initial phases of exploration and delineation of the pool through mature stages of production, to extract as much information as possible to produce maximum hydrocarbons from the field for the commercial viability of the project. In the absence of the well-log data, the properties are inferred from the inversion of seismic data alone. In oil and gas exploration and production industries, seismic inversion is proven as a tool for tracing the subsurface reservoir facies and their fluid contents. In this paper, seismic inversion demonstrates the understanding of lithology and includes the full band of frequency in our initial model to incorporate the detailed study about the basin for prospect evaluation. 3D seismic data along with the geological & petrophysical information and electrologs acquired from drilled wells are used for interpretation and inversion of seismic data to understand the reservoir geometry and facies variation including the distribution of intervening tight layers within the Miocene carbonate reservoir in the study area of Central Luconia. The out-come of the seismic post-stack inversion technique shows a better subsurface lithofacies and fluid distribution for delineation and detailed study of the reservoir.

A numerical dispersion‐suppressed method for shallow seismic migration

ABSTRACTReverse time migration can accurately image underground earth structures. However, for shallow seismic exploration, the seismic wave velocity is often lower than the velocity in the middle‐deep layers, which causes numerical dispersion for finite‐difference schemes and leads to poor seismic imaging quality. Suppressing numerical dispersion by grid encryption or increasing the finite‐difference order seriously reduces computational efficiency, which is not the optimal solution. To improve the imaging quality without sacrificing computational efficiency, a regularization factor is added to the acoustic wave equation to correct the phase velocity of high wave‐number components. An appropriate regularization factor can eliminate numerical dispersion that results from large grid interval schemes and can reduce the size of computational grids and improve computational efficiency. For simulations in the frequency domain, reducing the grid size also means reducing computer memory requirements. Numerical experiments indicate that the regularization factor should match the degree of numerical dispersion. Larger regularization factors can suppress serious numerical dispersion. However, excessively large regularization factors may destroy the effective wave field. Different numerical models verify the effectiveness of the improved acoustic equation for suppressing numerical dispersion and maintaining amplitudes and provide a novel means to improve the shallow seismic image quality.

Integrating multigeophysical data to improve structural imaging in the Dayangshu Basin

We have used the integrated interpretation of gravity, magnetotelluric (MT) data, and seismic data to improve the structural imaging of the Dayangshu Basin. The Dayangshu Basin is mainly composed of clastic and volcanic rocks. The logging data in the basin show different degrees of direct hydrocarbon indication, suggesting that the Dayangshu Basin has good potential for exploration. However, the widely distributed volcanic rocks attenuate seismic waves and lead to poor seismic imaging. Thus, the seismic signal is weak in the Ganhe Formation (K1g) and reliable seismic images cannot be obtained below that formation. MT data can accurately obtain images of deep structures because the resistivity of volcanic rocks is significantly higher than that of sedimentary rocks. Therefore, to obtain a more reliable geologic model, we combine the traditional 3D MT inversion result with logging and seismic data to establish an initial model. The 3D MT fuzzy constrained inversion (FCI) produces a more reliable geophysical model and geologically meaningful results. The resistivity model inverted from FCI shows that volcanic rocks are widely distributed in the Ganhe Formation, and the resistivity value of the lower section of the Longjiang Formation is greater than that of the upper section of the Longjiang Formation. Finally, the 3D gravity inversion with structural constraints from 3D MT FCI method was performed to improve the model resolution in depth and to highlight the density variations within the Jiufengshan Formation, which can further optimize the geologic model. We have determined how the effective integration of gravity, MT, and seismic data can improve the structural imaging of the Dayangshu Basin.

Imaging with surface-related multiples to overcome large acquisition gaps

Abstract To get the best result for seismic imaging using primary reflections, data with densely-spaced sources and receivers are ideally preferred. However, dense acquisition can sometimes be hindered by various obstacles, like platforms or complex topography. Such areas with large data gaps may deter exploration or monitoring, as conventional imaging strategies would either provide poor seismic images or turn out to be very expensive. Surface-related multiples travel along different paths compared to primaries, illuminating a wider subsurface area and hence making them valuable in case of data with large gaps. We propose different strategies of using surface-related multiples to get around the problem of imaging in the case of a large data gap. Conventional least-squares imaging methods that incorporate surface-related multiples do so by re-injecting the measured wavefield in the forward-modelling process, which makes it still sensitive to missing data. We introduce a ‘non-linear’ inversion approach in which the surface multiples are modelled from the original source field. This makes the method less dependent on the receiver geometry, therefore, effectively exploiting the information from surface multiples in cases of limited illumination. However, such an approach is sensitive to the knowledge of the source properties. Therefore, we propose a ‘hybrid’ method that combines the non-linear imaging method with the conventional ‘linear’ multiple imaging method, which further improves our imaging result. We test the methods on numerical as well as field data. The results indicate substantial removal of artefacts in the image derived from linear imaging methods due to incomplete data, by exploiting the surface multiples to a maximum extent.

Obstructed minibasins on a salt‐detached slope: An example from above the Sigsbee canopy, northern Gulf of Mexico

AbstractSalt‐detached gravity gliding/spreading systems having a rugose base‐of‐salt display complex strain patterns. However, little was previously known about how welding of supra‐salt minibasins to the sub‐salt may influence both the downslope translation of minibasins on salt‐detached slopes and the regional pattern of supra‐salt strain. Using a regional 3D seismic reflection data set, we examine a large salt‐stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra‐canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip of the obstructed minibasin. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g., caught in a feeder) to mild (e.g., welded to a flat or gently dipping base‐of‐salt). By mapping both the presence of obstructed minibasins and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3‐D strain fields on a salt‐detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin‐rich salt‐detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our example is detached on allochthonous salt and where the base‐of‐salt is rugose, with the findings applicable to other such systems worldwide (e.g., Gulf of Mexico; Scotian Margin, offshore eastern Canada). However, our findings are also applicable to systems where the salt is autochthonous but has significant local basal relief (e.g., Santos Basin, Brazil; Kwanza Basin, Angola).

The pre-breakup stratigraphy and petroleum system of the Southern Jan Mayen Ridge revealed by seafloor sampling

The Jan Mayen Microplate Complex (JMMC) in the NE Atlantic is interpreted to mostly consist of continental fragments with possible interstitial embryonic oceanic crust. A complex Cenozoic rifting history accompanied by extensive extrusive and intrusive volcanism have made the geological characterization of the JMMC challenging especially due to poor seismic imaging beneath the breakup basalt succession. The presence of continental crust in the JMMC is inferred by seismic and magnetic data, but ground truthing evidence have yet to be provided. Here, we present the results from a seafloor sampling campaign undertaken in 2011 on the Southern Jan Mayen Ridge complex. Seabed samples were recovered using a gravity corer and a dredge along a 1000 m high escarpment with a 19° slope. Sampling locations were selected based on the interpretation of seismic profiles that suggest the presence of possible pre-breakup successions outcropping along this steep escarpment. Results include a sequence of samples with age diagnostic palynomorph assemblages ranging from Permian/Triassic to Eocene, and including igneous samples related to the Early Eocene breakup volcanism. Importantly, the samples were retrieved from hard substrate in an erosional gully lacking overburden sediments and have ages arranged in younging upward sequential order, supporting their near in-situ position. The sampling results were integrated into a lithostratigraphic pseudo-well that can be used to constrain the evolution and breakup of the JMMC. Additionally, evidence for active migration of Jurassic sourced hydrocarbons comprise the first indication of a working hydrocarbon system, with important implications for the petroleum prospectivity of the Dreki area. Finally, these results confirm that the Southern Jan Mayen Ridge is indeed a sliver of continental crust.

Reservoir interpretation of Rotliegend seismic reflections in the central part of the Mid-Polish Trough

The search for gas reservoirs in the deep central parts of the Mid-Polish Trough remains an open question because of fragmentary knowledge of the area caused by the small amount of available borehole data and poor seismic imaging of sub-Zechstein formations on vintage seismic data.2D seismic data acquired in 2011 reveals seismic events related to strongly indurated sandstone intercalation found within fine-grained playa-lake deposits in the axial part of the Mid-Polish Trough. Seismic full-wave modelling has been carried out along recorded seismic 2D line in order to analyse seismic signatures from potential reservoir layers of variable porosity and gas saturation. This has allowed to evaluate the possibility of use of seismic methods to detect potential reservoir sandstones located in the deep parts of the basin below a thick Zechstein-Mesozoic cover, in which the actual spatial distribution of Rotliegend formations is only hypothetical.The analyses were focused on impact of porosity changes and variations in content of minerals with high bulk modulus on the P-wave velocity and seismic response. 2D seismic modelling have revealed that under specific geological conditions, playa formation may contain a sandstone layer – with a porosity of approx. 5% and acoustic impedance values not distinctly different from those observed for the surrounding rocks. As the result sandstone reflections will be undetectable through seismic methods.Promising results have been obtained by comparison of synthetic sections against a seismic profile recorded in the other deep part of the Mid-Polish Trough. The similarity of seismic signatures on synthetic section calculated for layer with porosity of 9% and observed on actual section implies that these events are related to a porous sandstone layer. This interpretation is confirmed by borehole data showing a thin layer of aeolian sandstone with 11% porosity.

A sub-salt structural model of the Kelasu structure in the Kuqa foreland basin, northwest China

The Kuqa foreland basin, adjacent to the South Tianshan Mountains, is a major hydrocarbon accumulation basin in Western China. The Kelasu structural belt is the focus for hydrocarbon exploration in the basin due to the presence of ramp-related anticline traps and a thick salt seal. The model of the Kelasu sub-salt structure is still contentious because of the structural complexity and poor seismic imaging below the salt layer. The area–depth–strain (ADS) method is applied to the southern part of the Kelasu Fault, a regional fault that cuts basement rocks. The ADS results are consistent with the seismic data, which indicate that both thin-skinned thrusting and basement-involved deformation occur within the Kelasu structure, with the Kelasu Fault acting as the boundary between the two regions of contrasting deformation. The ADS results also suggest that the depth of the lower detachment of the thin-skinned thrust belt is 9.5–10 km, which may correspond to the base of the Triassic. The Kelasu structure has undergone approximately 8.15–10.76 km of horizontal shortening in the east and 16.34 km in the west of the structure.

Challenges of seismic imaging in complex media around Iran, from Zagros overthrust in the southwest to Gorgan Plain in the northeast

Seismic imaging is an old but immature technology in Iran. Several reasons for this act as obstacles in developing and applying newly introduced imaging methods for complex media. Ease of exploration of large reservoirs by geologic methods, dominance of petroleum geologists in seismic geologic interpretation, distrust between industrial technicians and academic researchers, and poor computational infrastructures are major reasons for the backwardness of seismic imaging methods in Iran. The high degree of complexity in the geologic condition of oil-field-bearing areas could be another reason for obtaining poor seismic images. In the general study of literature of seismic imaging in Iran, through time and over different geologic conditions, obstacles impeding the lack of development of seismic imaging can be classified in technical and nontechnical areas. In the first step, it is acknowledged that harsh topography and poor supervision of acquisition build the first and unfortunately irreversible ring in this chain. Subsequently, in processing as the second step, the application of very old-fashioned processing methods on data suffering from high levels of various noise and the application of simple and useless methods for migration prevent obtaining any appropriate final images for further realistic geologic interpretation. In the third step, interpretation is mostly performed on very low-quality images, which will result in an ambiguous interpretation with a large amount of uncertainty in defining geologic objects, location, and even correct and accurate recognition of geologic objects. The main challenge is to get rid of time imaging and geologic interpretation on time images, which is rather an unusual issue these days. Examples of the application of advanced imaging methods on data from different geologic conditions around Iran show that these obstacles could be removed if a proper imaging strategy were selected.

Broadband marine controlled-source electromagnetic for subsalt and around salt exploration

Salt basins, mainly Tertiary basins with mobilized salt, are notoriously difficult places to explore because of the traditionally poor seismic images typically obtained around and below salt bodies. In areas where the salt structures are extremely complex, the seismic signal-to-noise ratio may still be limited and, therefore, complicate the estimation of the velocity field variations that could be used to migrate the seismic data correctly and recover a good image suitable for prospect generation. We have evaluated the results of an integrated seismic-electromagnetic (EM) two-step interpretation workflow that we applied to a broadband marine controlled-source EM (mCSEM) research survey acquired over a selected ultra-deepwater area of Espirito Santo Basin, Brazil. The presence of shallow allochthonous salt structures makes around salt and subsalt seismic depth imaging remarkably challenging. To illustrate the proposed workflow, we have concentrated on a subdomain of the mCSEM data set, in which a shallow allochthonous salt body has been interpreted before. In the first step, we applied a 3D pixel-based inversion to the mCSEM data intending to recover the first guess of the geometry and resistivity of the salt body, but also the background resistivity. As a starting model, we used a resistivity mesh given by seismic interpretation and resistivity information provided by available nearby wells. Then, we applied a structure-based inversion to the mCSEM data, in which the retrieved model in step one was used as an input. The goal of that second inversion was to recover the base of the salt interface. The top of the salt and the background resistivities remained fixed throughout the process. As a result, we were able to define better the base of the allochthonous salt body. That was reinterpreted approximately 300–700 m shallower than interpreted from narrow azimuth seismic.

Velocity model building and imaging in the presence of shallow gas

Multiple shallow layers of low saturation gas charged sands cause poor seismic imaging and hamper exploitation of many oil and gas discoveries in SE Asia. These gas ‘clouds’ are characterized by bright high amplitude reflectors of low P-wave velocity beneath which the seismic reflections have delayed P-wave travel times, reduced bandwidth, reduced reflected and transmitted amplitudes and associated phase distortion (Figure 1). All gas clouds are not equal, the appearance of gas clouds varies worldwide and within regions. A single gas-charged sand may sometimes have almost no effect on P-wave propagation; a shallow region of limited spatial extent may sometimes be undershot resulting in an interpretable image or multiple shallow layers can result in complete ‘wipeout’ of the final image. We believe that both anelastic losses and multiple scattering phenomena (indicated by rays on Figure 1) contribute to weak reflected energy beneath the gas-charged sands. A similar problem in the North Sea led to the development of multi-component seabed seismic solutions utilizing converted waves to penetrate beneath the gas (Berg et al., 1994). These solutions have been successfully applied to the largest of the Malaysian producing fields and are the PETRONAS preferred technical solution (e.g. Radzi et al., 2015). As with many problems, a mixture of acquisition and processing related solutions produces the optimal result but sometimes processing alone can produce a cost-effective result. Despite cost reductions and technical advances, converted wave imaging is still expensive, difficult and time consuming. In this paper we attempt to build an efficient and accurate workflow for velocity model building of P-wave data in order to best exploit the latest imaging algorithms. We use a mixture of synthetic and real data examples from offshore Malaysia to illustrate this difficult and unresolved problem.

Merganser Field: managing subsurface uncertainty during the development of a salt diapir field in the UK Central North Sea

Abstract The Merganser Field is located in the East Central Graben of the UK Central North Sea, and consists of a gas/condensate column trapped in a structural attic on the flanks of a fully penetrating salt diapir. A large salt overhang obscures the field and structural definition is challenging owing to poor seismic imaging. Exploration drilling established a column of 1327 ft in Paleocene-aged deep-marine deposits of the Forties, Andrew and Maureen Sandstone members, and revealed significant geological complexity. Depositional styles record the relationship between salt tectonics and sedimentation, with variable reservoir distribution influenced by halo-kinetically induced palaeorelief and accommodation space. Re-mobilization of sediments is observed at multiple scales, and includes centimetre-scale de-watering structures, decimetre-scale sand injectites and kilometre-scale olistoliths. Whilst the hydrocarbon properties are consistent, contact depths are variable. Pressure data indicate compartmentalization across the field, which is likely to be caused by either radial faulting or hydrodynamic effects. Owing to the magnitude of subsurface uncertainty, the Merganser discovery could not sustain the investment required for standalone facilities. The development of the neighbouring Scoter Field provided the requisite local infrastructure to progress Merganser into production. The Field Development Plan (FDP) estimated recovery of 100 bscf gas and 3 mmstb condensate, and focused on delivering a low-cost development solution consisting of two horizontal wells and a subsea tie-back that would be robust against the downside, yet maintain flexibility to optimize in an upside outcome. Pilot holes were drilled to establish top reservoir with the subsequent horizontal well trajectories being re-designed to reflect structural geometry. The reservoir sections would maximize connectivity between fault compartments and stratigraphic units, and positioning was optimized with well-site biostratigraphy. Each reservoir section exceeds 4000 ft and maintains at least 1000 ft vertical stand-off from the gas/water contact. The facilities include a 5 km subsea tie back to the Scoter production manifold, with metering at the Merganser manifold for allocation purposes. Gas and condensate are commingled with Scoter, and transported 11 km to Shearwater for processing. The gas is transported onshore through the Shearwater Elgin Area Line and condensate through the Forties Pipeline System to Kinneil. Field performance to date has exceeded the FDP P50 both in terms of daily rate and cumulative production. Early production rates peaked at 100 mmscf/day of gas and 6000 stb/day of condensate and, to end-2014, Merganser has produced 161 bcf of gas and 10 mmstb condensate. This performance is due to a combination of better than expected connectivity, high reservoir k h , lower draw-down afforded by long horizontal wells and compression at the Shearwater platform. Subsurface uncertainties prior to development were considerable and the ‘appraisal through development’ strategy has demonstrated that success is achievable through meticulous planning and scenario analysis.