Spyros Lazaratos

Encoded Simultaneous Source Full-Wavefield Inversion For Spectrally Shaped Marine Streamer Data

In this paper, we apply encoded simultaneous source fullwavefield inversion (FWI) to marine streamer data. FWI of large scale 3D data is a challenging problem, especially constraining the inversion using the high frequencies available in exploration seismic data. Two methodologies that make high-frequency FWI feasible for field data are: (a) applying encoded simultaneous source full-wavefield inversion (ESSFWI) and (b) shaping the data to provide a preferential weighting to the low-frequency components of the data. These two methods in combination provide us with the computational efficiencies needed for large 3D runs. To date, most encoded simultaneous source methods have been applied to fixed-receiver data; i.e., each receiver records data from all shots in the survey. We developed an approach that enables us to apply ESSFWI to marine streamer data that are non-fixed spread. The approach uses a normalized cross-correlation objective function with multiple realizations of the encoded data at each iteration of the nonlinear FWI. The method can be applied to 2D/3D data with any survey geometry. Here we demonstrate the methodology and discuss its details with synthetic examples. Although not presented here our initial investigations on 3D field streamer data look encouraging.

Deterministic Spectral Balancing For High-Fidelity AVO

A common problem for Amplitude Versus Offset (AVO) analysis is that the frequency spectra of near and far-offset data are different; near-offset sections contain more highfrequency energy than far-offset sections do. This variation in frequency content can significantly alter the AVO response, since the interference patterns between adjacent reflection events (for instance the top and bottom of a reservoir layer) will be different on the near and far-offset data. Difference in interference (tuning) can create false AVO anomalies or make valid AVO anomalies disappear. Because of their different frequency spectra, correlating reflection events between near and far-offset sections is sometimes difficult or ambiguous.

Case Studies of Using Far and Ultra-Far Seismic Data in Deep-Water Nigeria Erha North Field: Beauty and the Beast

This paper highlights the complexity and challenges associated with deep-water exploration and development in Nigeria. The Erha North field Reservoir-300 was discovered using the block-wide 2000 seismic survey. A shallow channel severely attenuated the seismic image at the reservoir level. A new 2005 seismic survey was shot perpendicular to the shallow channel and improved the imagibility of the reservoirs by undershooting the shallow channel. ExxonMobil’s Pre-migration Spectral Shaping (EM_PreSS) technique further enhanced the data quality. The EM_PreSS ultra-far 38-48° angle stack provided the best well tie in the Erha North Phase I development area. The significantly improved seismic data quality greatly increased interpretation confidence and impacted the business decision to not drill appraisal well Erha Far-East (EFE) and instead to drill the exploration well Erha North-East (ENE). The Erha NE prospect is located 7 km away from the center of the Erha North field, with the primary target in Reservoir-300. The seismic response at the up-dip of the prospect is similar to the hydrocarbon response observed at the Erha North field, but lacks conformance to structure - an expected DHI element. The original hole (OH) targeted an up-dip location and encountered multiple gas and oil sands, in both primary and secondary reservoirs. However, the primary target Reservoir-300 was penetrated with two separated thin gas-on-oil sands. No water sand was penetrated. A determination was made that such highly compartmentalized reservoir could not be developed economically. Down-dip from the Erha NE original hole discovery, a strong low-impedance amplitude anomaly was observed on the ultra-far (38-48°) angle stack. No other angle stack showed a similar strong low-impedance response and no other DHI attributes were observed. This amplitude response was neither similar to other known hydrocarbon responses nor to known water leg responses. Geophysical modeling results could not rule out the possibility of down-dip hydrocarbon presence by using reasonable rock properties assumptions. Given the high resource uncertainty, the operator determined it would be beneficial to test the down-dip location before Phase II development starts. The down-dip side-track (ST) well was drilled and penetrated a thick clean water-bearing sand. Post- drill modeling suggested that abnormally high anisotropy in the sand could be the cause of the ultra-far low impedance amplitude anomaly.

Application of Spectral Shaping to Improve Sub-Basalt Imaging

A principal impediment in sub-basalt imaging is the severe attenuation of high-frequency energy. Since low frequencies are relatively less attenuated, a plausible way to improve imaging is to apply spectral shaping early in the processing sequence. Reprocessing of conventionaltow data of a 2D seismic line at Rockall Trough, incorporating early spectral shaping, generated an improved result of quality similar to what was achieved with deep-tow data. In addition to early shaping, application of Radon de-multiple after migration (rather than before migration) can offer some advantages in this situation. Care should be exercised at the de-multiple stage, to fully preserve the low-frequency signal and avoid artifacts. In particular, low-cutting the multiple models at 2Hz can reduce artifacts and generate a reasonable result.