Earth Model Building Research Articles

Rejuvenating exploration opportunities in Egypt's Mediterranean region using basin-scale 3D seismic reprocessing and subsurface evaluations

The Eastern Mediterranean region has proven to be a key province for natural gas resources over the last two decades. Gas discoveries in the Nile Delta Basin, Levantine Basin, and over the Eratosthenes platform have proven that an efficient working petroleum system extends to the deepwater areas and into the distal parts of the Nile Delta and Levantine basins. While the suprasalt biogenic post-Messinian play is adequately imaged in various areas in the Eastern Mediterranean region, seismic imaging of the deeper subsalt pre-Messinian plays is challenged by legacy seismic acquisition and processing techniques. Moreover, the complexity of the Messinian salt structures and the heterogeneity of the lithology of the Messinian layer across the Mediterranean basins represent a key subsurface challenge. In phase 1 of the study presented in this paper, we reprocessed 12 legacy 3D seismic data sets over 12,000 km2 and constructed a consistent basin-scale earth model from the shelf areas offshore Sinai to the deepwater areas around the Zohr carbonate platform. The data processing and imaging sequences incorporate full-waveform inversion and tomography in the earth model building process followed by a basin-to-lead evaluation to delineate exploration opportunities in phase 1. The evaluations resulted in delineating Cretaceous and Oligo-Miocene untested structures. Furthermore, potential deepwater Lower Miocene sand fairways were outlined, supported by improved regional seismic imaging and consistent seismic attributes over multiple legacy surveys. The authors of this paper expect to update the results of their work after completing the earth model building and the prestack depth migration workflows of an additional 13 legacy surveys over 35,000 km2 in phase 2 of the study.

Delve deep: Seismic revolution in acquisition and model building for exploration

Sparse-node acquisition is the latest approach to improve the earth model through the inclusion of ultra-long offsets and low-frequency generating sources. The new designs of node data collections with simultaneous shooting can be deployed on a regional basis, covering thousands of square miles in a cost-effective manner. In complex geologic settings such as salt environments, salt interpretation has a direct impact on subsalt imaging. However, salt interpretation can be quite time intensive and challenging. Full-waveform inversion (FWI), as a data-driven optimization algorithm with full-wavefield modeling, has become an essential tool for earth model building. However, its use in salt tectonic contexts, especially with streamer data collection, is limited because the frequencies acquired are not sufficiently low and the offsets are not sufficiently long. The recent advancement of the objective function enables the application of an FWI workflow, coupled with sparse-node acquisition in the deepwater settings of the Gulf of Mexico, to refine the salt geometry, correct the background velocity error in subsalt, and uncover a basin's structural framework. The increased frequency content of the FWI-updated fields enables us to extract the FWI-derived reflectivity (FDR), which is an image created by the FWI nonlinear iterative process with the full wavefield. Additionally, the classic extraction of amplitude-versus-angle/amplitude-variation-with-offset parameters employs the elastic Zoeppritz formulation to obtain rock-physics volumes and explain the amplitude on the prestack image gathers, which are typically generated by Kirchhoff or reverse time migration (RTM) depth migration. The next step is to substitute these migrations with FWI and output not only the poststack FDR but use it in a prestack mode to aid amplitude analysis at an early stage of model building.

Sparse‐node acquisition for data fitting velocity model building

AbstractThe salt interpretation can be quite time‐intensive and challenging. Full‐waveform inversion, as a data‐driven optimization algorithm with full wavefield modelling, has become one of the essential tools for earth model building. However, full‐waveform inversion application in the complex salt geology, especially with streamer data collection, is limited, whereas the ocean‐bottom node with ultra‐long offsets and low frequencies unleashes the power of full‐waveform inversion. Sparse ocean‐bottom node acquisition has become a standard approach to improve the earth model building by the addition of ultra‐long offsets and possibly low frequencies. The new survey designs with ocean‐bottom node coupled with simultaneous shooting can be deployed on a regional basis covering thousands of square kilometres in a cost‐effective manner. In complex geological settings, including irregular salt geometry, salt interpretation has a direct impact on subsalt imaging. The recent development of robust objective functions allows full‐waveform inversion work with sparse ocean‐bottom node surveys in the deep‐water environment as the Gulf of Mexico to build and refine the salt geometries and correct the background velocity error in subsalt, uncovering the structural configuration of the basin that has not been seen before. Although full‐waveform inversion mostly employs the acoustic wave equation, we know that the high velocity boundaries in the earth model may require elastic wavefield propagation. Increased physics in full‐waveform inversion allows us to interrogate the power of elastic full‐waveform inversion versus acoustic full‐waveform inversion, and here, we demonstrate that elastic full‐waveform inversion has an advantage over acoustic full‐waveform inversion despite the extra cost associated with implementing the more complete physics.

Improving hydrocarbon exploration with pore pressure assisted earth-model building

In shale-dominated clastic lithology environments, a rock-physics model relating velocity and pore pressure (PP) can be calibrated and used to convert velocity to PP properties. The crossvalidation between velocity and overpressure, which follows the geology, can be used to better understand the model, help to build an initial velocity model, and allow selecting tomography solutions with more confidence. The velocity model developed using this approach is more plausible and more suitable for subsequent PP analysis. We highlight the application of this method in areas with poor seismic illumination and insufficient well control.

Technology Focus: Seismic Applications (March 2016)

Technology Focus From time to time, I am asked to address general audiences. The mission is to describe what we do in the seismic business. Typically, the first slide I show is a prenatal ultrasound display of my daughter. I explain that using reflected sound waves to create such an image is precisely what we do with the Earth. The world’s first reflection seismic field tests were conducted near Oklahoma City in 1921, and, ever since then, the industry has endeavored to improve that seismic imaging process. So, indeed, one of the papers selected for this Technology Focus section and one of the papers recommended for additional reading deal with case histories in which imaging is improved through better velocity-model building. In the first paper, integration of microgravity data, resistivity measurements, and seismic is the key in the onshore case history from Qatar. In the first additional-reading paper, full-waveform inversion of seismic travel times and amplitudes is the key in the case history from offshore Australia. Advances in seismic applications are not just confined to imaging, though. In another additional-reading paper, amplitudes are used for identifying porous zones in an otherwise tight-sandstone gas reservoir in Oman. And, in the third additional-reading paper, the authors use the amplitudes in 4D analyses for identifying new reservoir drive mechanisms in a field offshore Brunei. Both imaging and amplitude inversion benefit greatly from broader-band data. Therefore, this is the topic of the second paper presented. In this example from offshore Malaysia, the marriage of a new acquisition technique with new processing algorithms yields broader frequency content, enabling more-accurate estimates of gas in place to be derived. But even further advances in seismic applications are taking place now, especially in the world of unconventional resources. The authors of the third selected paper discuss the integration of miscroseismic data with 3D-seismic attributes, well-log data, and completion data to understand the geomechanical rock properties in the Midland basin of Texas. This information was important for planning the spacing of new wells. I hope this journey is successful in showing that substantial advances continue to be made globally in seismic applications. But, perhaps even more importantly, I hope the journey shows that some of the more exciting advances are actually arising from the integration of seismic with other technologies and that, while imaging may still be king in seismic (and in some fields of medicine), the applications of seismic to building 3D mechanical Earth models, for example, are gaining prominence. JPT Recommended additional reading at OnePetro: www.onepetro.org. IPTC 17905 High-Resolution Anisotropic Earth-Model Building on Conventional Seismic Data Using Full-Waveform Inversion: A Case Study Offshore Australia by Bee Jik Lim, Schlumberger, et al. SPE 177552 Seismic Reservoir-Quality Prediction, Khazzan Field, Oman by T. Chris Stiteler, BP, et al. IPTC 18491 4D Seismic in Stacked Reservoirs—From Puzzles to Insights on Production Drive Mechanisms by Denis Kiyashchenko, BSP, et al.

Basin-scale integrated earth-model building using rock-physics constraints

Earth modeling plays a decisive role in seismic imaging. Presently, methods such as tomography and full-waveform inversion (FWI) are widely used to generate 3D high-resolution velocity models across sedimentary basins. However, given the nonunique nature of the solution, the relative fidelity of these velocity models remains low. Moreover, earth-model building must incorporate a dual strategy. In particular, it must derive a velocity model for migration and yield a series of rock properties such as density, temperature, effective stress, and pore pressure. These large-scale, basin-sized rock properties are used for drilling purposes and as an initial model for small-scale reservoir property inversion. Hence, one must inquire: How can we improve an earth model built at basin-scale using seismic data? Traditional workflows will simply convert seismic-derived velocity into rock properties, thereby propagating the uncertainty without addressing the issue. To alleviate this problem and fully exploit the potential of seismic data, an alternate workflow will be discussed. The new workflow involves using rock physics to link rock properties with velocity, and the physical range of the rock properties is used to constrain velocity when derived from surface seismic data; this way we reinforce the reliability of the final earth model. The application of this new workflow is demonstrated using Gulf of Mexico examples.

Toward a closed loop from seismic imaging to earth-model building

Velocity-model building is the first task of seismic inversion and the foundation of the subsequent data-processing workflow. When the earth velocity becomes multivalued with respect to the propagating direction of the waves, velocity-model building becomes severely underdetermined and nonunique. The traditional workflow separates velocity-model building from lithologic inversion, which hampers both processing steps. An integrated model-building scheme is demonstrated to simultaneously consider prestack seismic data and its structural and lithologic inversion results from a previous iteration. The prestack seismic inversion is performed using wave-equation migration velocity analysis (WEMVA) for vertical transverse isotropic (VTI) models. To constrain the seismic inversion, the geologic information is integrated as spatial-model correlations, and the rock-physics information as lithologic-model correlations. This feedback step completes the loop from seismic imaging to lithologic-model building, where previous rock-physics estimations and geologic interpretations can be validated further and updated in order to constrain the next WEMVA iteration. Improvements from the integrated inversion scheme are shown on a Gulf of Mexico field data set.

Imaging through shallow gas: Integrating broadband acquisition, processing and high-end model building for improved imaging of deeper targets

SUMMARY We present a case study offshore Malaysia, shallow gas features in the overburden distort the seismic imaging at the target level. While a multifaceted approach involving a combination of seismic acquisition and processing strategies were used to improve the bandwidth of the seismic data, particularly for the low-frequency content of the seismic image, several distortions still existed at the target level. The prominent structural sag evident at the reservoir level is a typical indication that the overlying shallow gas velocity model needed to be resolved and incorporated into a depth migration algorithm. To resolve the transversally and laterally variant velocity features in the shallow gas areas, a solution that consisted of full waveform inversion (FWI) and high-resolution reflection traveltime tomography was utilized to produce an accurate compressional velocity model. To further resolve the amplitude and phase distortions at the reservoir level due to shallow gas effects, Q tomography was incorporated into the model building phase to derive a space-variant 1/Qmodel and Q compensation was integrated within depth migration. The integrated approach of broadband receiver acquisition, data processing strategies and high-end Earth model building has cumulatively improved the imaging of the reservoir below the shallow gas anomalies.

Earth-model building using rock physics and geology for depth imaging

Modern depth imaging is a model-driven technology. At the heart of the process is the challenging task of building an accurate earth model — one that yields a reliable and geologically consistent image at the right depth and accurate earth properties such as pore pressure, lithology, and pore fluid at those depths.