Chris Page

Quantifying and increasing the value of multi-azimuth seismic

Conventional marine exploration offshore Nile Delta is challenged by shallow heterogeneities and a deep, complex anhydrite layer called the Messinian. These subsurface complexities cause variable illumination and strong diffracted multiples resulting in imaging challenges below the anhydrite on conventional narrow-azimuth towed streamer acquisition because the recording antenna is too small (Figure 1a). Additionally, it has proved difficult to build the detailed depth migration velocity model required to correctly image below the Messinian layer. Multi-azimuth (MAZ) acquisition using time-domain processing however, has been very successful in addressing both the noise and illumination problems in the Nile Delta. The increased azimuthal and crossline offset coverage—bigger antenna—is achieved by acquiring six conventional marine surveys over the same area at 30° sail-line increments (Figure 1b).

Multi-Azimuth towed streamer 3D Seismic in the Nile Delta, Egypt

A thin but complex layer of partially eroded anhydrite and other facies lie at a depth of around 3km across large areas of the Nile Delta in the Mediterranean. Wavefield distortion, attenuation and the generation of complex multiple diffraction noise cause the quality of the underlying seismic image to be highly variable. (Figure 1) In this paper we describe the problem and then demonstrate how multi-azimuth seismic is able to improve the PreMessinian image.

Multi-azimuth 3D provides robust improvements in Nile Delta seismic imaging

Since gas was first discovered in the Nile Delta in the late 60s, most exploration programmes have focussed on shallow Pliocene reservoirs, where gas can clearly be seen as bright events on excellent quality seismic data and where exploration success has been very high. The petroleum geology of the deeper pre-Pliocene section is fundamentally no different to that seen in the Pliocene, where potential reservoirs consist of sand prone channel systems originating from the Nile. Why then, have we been deterred from exploring in the deeper section? The problem is two fold: 1. Deeper burial and harder rocks mean that reservoir sands and hydrocarbons will be less visible on our seismic data. 2. Pre-Pliocene seismic quality is highly variable and often very poor. (Figure 1) Poor imaging being the result of wavefield distortion though the Messinian anhydrite layer, attenuation, and the presence of complex multiple diffraction noise.

Leveraging the Value of Multi-azimuth (MAZ) Seismic through MAZ-stack

The growing demand for energy requires that the search for hydrocarbons must extend into more challenging settings and become more reliant on technology, exposing the limitations of conventional 3D marine seismic. This decade has seen a major response to these challenges in the marine setting, in the form of Wide Azimuth seismic acquisition, like Multi-Azimuth, Wide Azimuth Towed Streamer, Nodes and OBC. These methods illuminate the sub-surface more completely, and sample problematic 3D noise across azimuth as well as offset for better attenuation during stack. These surveys are much more expensive to acquire and process, so it is thus important to lever all the value from the data. This paper discusses two seismic processing options to lever additional value from MAZ data. The first, MAZ-Stack, has been developed to weight up signal in areas of poor illumination, in other words, to favour strong signal over weak/absent signal. This approach has the effect of reducing fold and so decreases random noise suppression. The second technique, 3D warping, extends an existing method to align sub-surface seismic volumes, and so minimize registration errors between the azimuth stacks which result from imperfect knowledge of the sub-surface velocity field.

Experience with towed streamer multi-azimuth processing and acquisition

Multi-azimuth towed streamer acquisition is a technique whereby conventional marine 3D seismic surveys are acquired in several distinct acquisition directions and then combined in some way to produce an improved image. The method as described here has produced some impressive comparisons in the literature, e.g., Keggin et al. (2006), Page et al. (2006), and is drawing increasing interest especially for areas of poor signalto-noise ratio. Over the years that this method has been pioneered, some standard acquisition and processing procedures have been established which will be discussed here with examples from BP’s six azimuth Raven survey from the Nile Delta, courtesy BP Egypt.

Multi-Azimuth 3D provides robust improvements in Nile Delta seismic imaging

Since gas was first discovered in the Nile Delta in late 60’s, most exploration programmes have focussed on shallow Pliocene reservoirs, where gas can clearly be seen as bright events on excellent quality seismic data and where exploration success has been very high. The petroleum geology of the deeper pre-Piocene section is fundamentally no different to that seen in the Pliocene, where potential reservoirs consist of sand prone channel systems originating from the Nile. Why then, have we been deterred from exploring in the deeper section?

Velocity analysis and quality control from sub-stacks

We present a robust and accurate method of velocity analysis based on measuring any difference in timing between near and far offset stacks. Unlike semblance-based analysis this method identifies the correct moveout velocity of reflectors showing a class 2 AVO response with polarity reversal. If so desired, it can be run as a quality control in conjunction with semblance-based analysis and can flag the occurrence of a polarity reversal. Time shifts are measured by an optimally accurate maximum likelihood method. A criterion is established for detecting two special cases when the amplitude on either the near stack or the far stack is close to zero. A simple remedy is then to halve the substack fold. By including only the nearest or farthest offsets, the sub-stack sums only amplitudes dominated by the same polarity.