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Subsalt imaging at Stampede field in the Gulf of Mexico has remained challenging for decades due to the existence of a large and complex sediment inclusion inside thick tabular salt. Recently, appropriate full-waveform inversion (FWI) algorithms have been developed for automatic salt model updating (Shen et al., 2017; Zhang et al., 2018). By applying this technology to newly acquired ocean bottom node (OBN) data with good low-frequency content and ultra-long offsets, we are able to invert both the shape and velocity of this complex sediment inclusion at Stampede and provide significant improvement to the subsalt image. A good starting model for FWI is still needed in this workflow, but detailed interpretation efforts are not necessary. Moreover, we expect further improved subsalt imaging if data with even longer offsets becomes available.

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GepoSpec Cameroon Terms of Reference for Proposals on free blocks

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This study is based on work performed on horizontal wells in the Cleveland Sandstone Formation. Previous work describes the technique of using automated, quantitative mineralogy (RoqScan) to analyse drill cuttings and derive rock property and elastic pseudo-logs to customize completion designs. We expand on these findings to utilize the estimated elastic rock properties in a proprietary coupled reservoir-geomechanical simulator. The geometry of the individual cluster fractures can then be computed during pumping to investigate different treatment stage designs. The study supports the idea that optimized stage spacing will result in the development of a larger SRV and higher production.

Complex overburdens often distort reservoir images in terms of structural positioning, stratigraphic resolution, and amplitude fidelity. One prime example of a complex overburden is in the deepwater Gulf of Mexico, where thick and irregular layers of remobilized (i.e., allochthonous) salt are situated above prospective reservoir intervals. The highly variant salt layers create large lateral velocity variations that distort wave propagation and the illumination of deeper reservoir targets. The salt layers also induce large transmission losses that make the subsalt events weak in amplitude, and therefore easily overpowered by strong multiples from shallow multiple generators and other types of noise. In subsalt imaging, tools such as reflection tomography, full-waveform inversion and detailed salt interpretation are needed to derive a high-resolution velocity model that captures the lateral velocity variations. Once a velocity field is obtained, and after the best attempt at multiple suppression, reverse time migration (RTM) can be applied to restore structural positioning of events below and around the salt. However, RTM by nature is unable to fully recover the reflectivity for desired amplitudes and resolution, especially for deeper subsalt reservoir images. This shortcoming is well recognized by the imaging community, and it has propelled the emergence of least-squares RTM (LSRTM) in recent years. In simple terms, LSRTM inverts for the reflectivity that best fits the recorded data through modeling (i.e., demigration) and migration processes. LSRTM can be performed either by an iterative gradient-based local search, or, for better efficiency and applicability, by an approximate single-iteration approach. We investigated how current LSRTM methods perform on subsalt images. First, we compared the formulation of data-domain vs. image-domain least-squares migration (LSM), as well as methods using single-iteration approximation vs. iterative inversion. Next, we examined the resulting subsalt images of several LSRTM methods applied on both synthetic and field data. Among our tests, we found that image-domain single-iteration LSRTM methods, including an extension of Guitton’s (2004) method in the curvelet domain, not only compensated for amplitude loss due to poor illumination caused by complex salt bodies, but also produced subsalt images with fewer migration artifacts in the field data. By contrast, an iterative inversion method showed its potential for broadening the bandwidth in the subsalt, but was less effective in reducing migration artifacts and noise. Based on our understanding, we summarize the current state of LSRTM for subsalt imaging, particularly in terms of single-iteration and iterative LSRTM methods.

Reservoir characterization methods require seismic data processed for this purpose. We propose a QC methodology for AVO and low frequency. We use a small set of attributes with which we identify signal preservation and pre-stack data consistency across the full data bandwidth. The methodology is demonstrated on two examples. In a first example, we outline the QCs necessary to monitor pre-stack data continuity, AVO model quality and wavelet stability. In a second example, we show that looking at correlation between angle stacks at low frequency is a good indicator of the low frequency quality of the data.

The use of existing geological and structural maps, previous 2D seismic profiles, boreholes and correlation models between these data is sufficient to understand basin structure and thermal systems on a regional scale. However, this is not sufficient on a scale of a geothermal site....

Reflection-based full waveform inversion (RFWI) is increasingly used to recover long wavelengths of the background velocity model and provide updates that extend beyond the reach of diving waves. In our case study, we use an RFWI method that first updates the density using the high-wavenumber components of the decomposed full waveform inversion (FWI) gradient and then updates the velocity using the low-wavenumber components. We show on a deep water example from the Mexican side of the Perdido fold belt that RFWI improves the velocity inside the sediment mini-basins and thus the interpretability of the underlying salt. We also apply this method for the intra-salt and subsalt velocity updates and show how it can improve imaging of the deep targets.