Imaging the geology subsalt and at the transition between extra-salt and subsalt has been a challenge at Mad Dog even with extensive seismic data coverage, including two WATS surveys and multiple NATS surveys. WATS acquisition and TTI velocity model processing generated major improvements in the image at Mad Dog. One of the observations of a previous TTI project is the presence of a strong orthorhombic anisotropic effect in a salt mini basin above the field. This finding led to the decision to reprocess the Mad Dog data with a tilted orthorhombic (TOR) velocity model. The main objective of this project is to build an orthorhombic velocity model with nine parameters compared to five with the TTI processing. The TOR anisotropic parameters are generated with the latest FWI and tomography techniques and take guidance from the stress field from a geomechanical model. The outcome of the project is very encouraging with results including better constructive imaging in crucial areas of the field, an incremental increase in signal-to-ratio everywhere and increased fault resolution. The TOR velocity model will be used to migrate a future ocean bottom nodes survey to address some of the remaining imaging challenges.
Presentation Date: Wednesday, October 17, 2018
Start Time: 8:30:00 AM
Location: 208A (Anaheim Convention Center)
Presentation Type: Oral
Subsalt imaging at the Thunder Horse Field in the Gulf of Mexico is challenging primarily because the salt canopy, overlying roughly 75% of the structure, greatly distorts subsalt illumination and causes imaging and resolution problems. Since the Thunder Horse discovery, advancements in seismic acquisition techniques and imaging technologies have significantly improved subsalt images. The latest successful application is from a tilted transverse isotropy (TTI) reverse time migration (RTM) project combining two wide azimuth towed streamer (WATS) data sets and three narrow azimuth towed streamer (NATS) data sets. The addition of an extra WATS data set and the application of the recent imaging technologies are key contributors to the dramatic structural image improvements with better defined three-way events and a higher signal-to-noise ratio (S/N).
The Thunder Horse Field has been producing since 2008 and is located in the south-central part of the Mississippi Canyon protraction area in the Gulf of Mexico. A large overlying allochthonous salt body causes rapid spatial and temporal changes in illumination and image quality, making interpretation difficult, especially near the steeply dipping three-way closure against the salt stock. During the course of discovery and development, BP has made continuous efforts to better understand and improve Thunder Horse’s subsalt image with new seismic data sets and more advanced imaging technologies (Pfau et al., 2002; Ray et al., 2002, 2005; Gherasim et al., 2012). The latest successful TTI RTM project with two WATS data sets and three NATS data sets is the continuation of this effort to improve Thunder Horse subsalt images.
This project aimed to improve the structural image in poorly illuminated areas and to maximize the usable vertical and horizontal resolution for well targeting and planning. The latest image shows a dramatic improvement over the previous TTI RTM image produced in the 2012 project for three reasons. First, the additional WATS data in the NE-SW direction illuminated some key areas that the NW-SE WATS and three NATS surveys did not. Second, the majority of the NATS traces were migrated rather than just used to infill missing traces in the NW-SE WATS shot gathers, as was done in 2012. Finally, more advanced imaging workflows and technologies were used to address specific problem areas in the data. Shot patch-based angle gather illumination weighting (AGILW) and input data selection technologies, which were applied in this project, effectively attenuate noise while preserving signal. Specular imaging using RTM dip gathers also helped enhance the S/N. We also discovered one of the reasons for frequency loss underneath the salt.