56th EAEG Meeting

Localized deposits of Upper Jurassic sands are an important exploration target in the North Viking Graben of the North Sea. These excellent reservoir quality Munin sands can be found encased within the widespread Upper Jurassic shales, preserved in places on the flanks of eroded paleo-highs. The challenge is to be able to locate these sands which are often situated in structural lows and poorly represented by the well control. The ability to use seismic data to differentiate these sands from the more common shales provides a powerful exploration tool.

The Danish Zechstein evaporites and carbonate deposits of the Southem Zechstein Basin, which have been the target of intense hydrocarbon investigation, are characterized by distinct velocities and densities and by a cyclic distribution (Fig . 1). This distinct distribution is utilized to calculate unique average velocities and densities for the individual lithologies by subdividing according to thickness, facies association and porosity. By assigning the calculated values to lithological data from wells and calculate synthetic seismograms, that correlate with both the synthetic seismograms calculated from the original logs and the corresponding seismic sections, the validity of the calculated average velocities and densities are found to be good. The values have a wide range of applications in seismic modelling and significantly enhance the seismic interpretation and prediction of porous carbonate intervals in southem Jylland, Denmark.

Lower Permian-Carboniferous carbonates are the main hydrocarbon sequence in the offshore area of the Timan-Pechora oil and gas province (Pechora Sea). Their- prospectivity has been proved by drilling results. The reservoirs are vuggy cavernous and fractured limestones. Their development is related to: - zones of high porosity in areas of long-lived reverse, overthrust and strike- slip faults; - bioherms; - zones of redeposited detrital limestones related to local uplift and erosion of some offshore areas; - zones of secondary leaching of carbonates.

During the design stage for the acquisition of 3D seismic surveys all essential parameters are tested and evaluated using available software packages. However, some key acquisition parameters can be objectively assessed only through field tests. Results from such tests assist in optimising the acquisition parameters and minimise the operational costs.

A transportable computer system for field processing of 3D marine seismic surveys has been developed by Amoco Production Company Research, and used aboard contractor vessels since November 1992. Through January 1994, full fold 3D binned stack volumes have been processed for ten 3D surveys, with deployments on five different vessels.

We are to describe the method used to produce onboard the survey vessel, a full-fold 3D DMO stack data volume immediately available at the completion of the acquisition of a 3D seismic survey The Camelot gas field is located in the southernmost portion of the U.K. southern gas basin. The trapping structures are a complex of rotated fault blocks. The reservoir is the Permian Rotliegendes formation. Reservoir depths average 1850m (1.2 TWT). Seismic imaging of the reservoir is complicated by the Rotliegendes fault pattern, lack of 2D seismic coverage over shallow sand banks, and the presence of a major Jurassic graben in the southern half of the area of interest. Production from the field began in 1988. Initial delineation of the field was based upon numerous vintages of 2D seismic program.

Typical near surface geology in a transition zone area consists of a water layer, a section of muddy deposits and underlying layers of more consolidated sediments (Fig. 1). Depending upon the water depth and the water bottom reflectivity, multiply reflected and refracted energy can be trapped in the water layer and degrade the seismic data quality. This trapped energy is traditionally dealt with in the data processing, which, depending upon the severity of the problems, may or may not be sufficient.

We examine the implementation aspects of the elastic linearized inversion in the k-w domain introduced by Rocca et. al (1993) and De Nicolao et al. (1993). The method is based upon the extension to the elastic case of the diffraction tomography technique of Wu and Toksoz (1987) and the singular value decomposition of the transfer function between data and parameters. The conceptual steps are: - Linearization of the forward problem through the Born approximation and linearized scattering of point diffractors (Wu and Aki, 1985). - Analysis of the response of the medium in the k-w domain (decomposition in sinusoidal perturbations of a uniform background). This leads to a diagonal relation between data and parameters as the incident and reflected waves are linked to the medium wave number by the Bragg resonance condition. - Inversion and analysis of the information of the reflections by means of singular value decomposition technique.

Elastic inversion of seismic data by modeling and data fitting with finite-difference modeling is attractive, as no restriction is put on the model geometry, and because information is naturally gained from all types of waves propagating in the subsurface model to be recovered.

Bottom simulating reflectors (BSRs) occur within oceanic sediments of the shelf, slope and rise systems of the worlds oceans. These reflectors mimic the water bottom reflection, and are thought to represent the phase boundary between solid gas hydrate above and methane below. Research into these gas deposits has been motivated by their potential as an energy resource. Because BSRs are pressure and temperature dependent, they are found in water depths greater than about 500 meters. These reflectors occur in most marine environments. However, due to gas expulsion associated with sediment compression, BSRs commonly occur near convergent margins. They represent isotherms and have been used to determine heat flow (Yamano and Uyeda, 1982).