55th EAEG Meeting

Optimal imaging of reflection energy in steep dip structural environments requires that the data be acquired and processed in a way to retain, and position (migrate) the seismic reflection information. In certain structural situations, such as salt diapirs, special acquisition geometries can be employed which substantially contribute to the overall goal of enhanced imaging of the subsurface . However, these acquisition geometries can cause unique problems in the subsequent processing of the data. Overcoming these difficulties and producing a high quality product requires retention of steeply dipping energy throughout the processing sequence, specially adapted seismic processing techniques, integration of prior knowledge and a degree of creativity.

In the last few years great effort has been put into the development of quality control processes, which are applied in connection with marine seismic data collection. This is made possible by both a better understanding of the acquisition proces and the introduction of more powerful and cost-effective computer technology. An increasing amount of quality control data is available to the seismic crew and the client representative in order to made decisions about the further course of the seismic operation. Saga Petroleum has been pioneering novel quality control techniques, which have been presented at previous meetings. However, in order to handle all this information its staff of representatiees onboard the seismic vessel had to be increased .

A quantitative analysis has been performed to show the effectof navigation errors on marine seismic data . It is found that in some cases large systematic navigation errors can lead to negligible degradation or mispositioning of the seismic data, though this is case dependent. The analysis accounts for the type and magnitude of the mispositioning error, streamer feather, acquisition parameters and subsurface geology. The effects on the seismic data are reported in terms of stack degradation, error in NMO velocity and misposition of the seismic data.

The explorationist must select 3D acquisition parameters that will best image the subsurface target objectives for each potential prospect. The usual starting point in parameter selection is to derive acquisition geometries based on a planer earth model with no dip or with constant dip. It has been found, through ray trace modeling, that acquisition parameters derived this way will not always meet the needed exploration objectives when dealing with a complex geologic structure. In examining numerous cases, complex geological structures scatter the primary reflections in such a manner that what is thought to be imaged and what is "truly" imaged are usually quite different. The explorationist must include, as part of the parameter selection list, the location, orientation, and size of survey that will best image the subsurface objectives in complex geologic settings.

The Troll field, situated in the northern part of the Viking Graben offshore Norway, is primarily a giant gas field. However, underneath one sub-area ,the Troll West gas Province (TWGP), there is a 12m thick oil column, constituting approximately 450 x 10 6 m³ oil in place! In order to maximize recovery from this oil volume, horizontal wells satisfying specific geological requirements are needed.

Amoco recently developed proprietary techniques to quickly extract horizon or interval based response attributes from seismic data residing on interpretative workstations. Following their extraction, maps of the various attributes are generated using vendor software. This new technology enables quick analysis of large volumes of data to ascertain which attributes show significant variation. Forward modelling of well data and physical property data provide a means of "groundtruthing" the attribute variations to lithologic changes, changes in reservoir phase, or to processing artifacts.

Investigation of the migrated seismic data acquired on the Heimdal Field shows that the seismic correlations between two wells in the gas field are not obvious and the well-to-seismic ties are dubious, mainly on the top of gas hearing sands. In order to solve these misties and picking problems, detailed Amplitude Versus Offset (AVO) processing and analysis are performed on several seismic lines.

Due to overpressure below the Upper Miocene in the North Sea, seismic velocities below this level are low relative to depth in the central North Sea. The pronounced increase of chalk velocities with depth makes it possible to use deviations in velocities from a normal velocity-depth trend to outline the areal extent of overpressure in the tentral North Sea.

Standard seismic processing provides an image of the subsurface which is band-limited. Although the migrated stack is an important tool for structural evaluation, limited resolution creates difficulties in its use for small scale stratigraphic interpretation. Resolution is conventionally defined in terms of Rayleigh's criterion, requiring that two equal amplitude pulses are separated sufficiently to have their centres coincident with the others' first sidelobes of opposing sign. However, it has been noted by many authors (Kallweit and Wood,1982; van Riel and Berkhout, 1985) that the actual discrimination is much better, provided there are known to be a small number of reflectors in the model when compared with the number of data samples.

A new spectral-statistical approach to seismic processing and inversion has been developed based on multi-level decomposition of the wave field. Its main application is to obtain details of geological/geophysical characteristics of the medium within a local target zone. Pore pressure and lithological prediction is one of several successful applications of the CSD method. In this paper we will review the theoretical aspects of the method.