3D Pescara Basin - Case History of an Unconventional Seismic Approach

Abstract

Abstract A strong integration among planning, acquisition, processing and interpretation was the basic tool to design an unconventional approach for a large 3D seismic survey (1,600 full fold sqkm) with very complex explorative stratigraphic targets. Several 2D seismic surveys had been shot in the area during the past years. Two small 3D surveys with structural targets were also recorded on production fields, However, these data were not suitable for an advanced stratigraphic interpretation (amplitude mapping, Avo) aimed to assess the residual potential of a mature area. Deep water acquisition was carried out using the cross shooting technique, that consists of two separate datasets Shot along dip (61 degrees) and strike (151 degrees) direction respectively, Where water depth was less than 15m (but more than 5 m)" a standard acquisition methodology was also used acquiring parallel to the coast in the strike direction (210 sqkm). This unconventional acquisition methodology was fully exploited by the flexibility of 3D processing. The encouraging results achieved during this project urge to continue investigations and also to apply this approach to future surveys. Introduction The area under investigation is located in the central Adriatic Sea, offshore Italy (fig. I), It is characterized by a complex tectonic setting, as well as shallow amplitude anomalies related to the presence of gas. Due to operational and geophysical constraints (e,g.: complex acquisition methodologies, feathering, survey direction, etc.), 3D marine seismic surveys often present an irregular sampling of azimuths., offsets and fold of coverage. Such irregularities produce well-known acquisition footprints in the final migrated volume, which become more evident during the stratigraphic interpretation (amplitude maps, Avo). The cross shooting acquisition methodology was designed to attenuate these acquisition footprints and a to provide higher fold of coverage especially for short offsets, thus increasing the quality of Avo studies and preserving the high frequency content. Moreover, allowing a high flexibility in data processing and interpretation, it provided a wider azimuthal distribution, a better velocity control, and a reduction of the final bin size without severe interpolations. The bins of the stack and migrated volumes, the effective fold of coverage and the azimuth of the grid can be different from the nominal acquisition ones. Satellite data transmission and data compression decompression were mainly used for remote QCIQA of both acquisition and onboard processing from Company's headquarters. Seismic data processing was specifically designed to take full advantage of the acquisition methodology, to avoid duplication between onboard and onshore work and to guarantee amplitude preservation for stratigraphic investigations and Avo analyses. The final migrated volume provided to be very useful and suitable for an advanced stratigraphic interpretation, as well as the structural imaging was very well defined. Geological Setting and Targets Definition The Pescara Basin covers an area of approximately 1,800 sq km in the Adriatic Sea. It is represented by a subsiding foredeep elongated NNW-SSE. It is bounded West and Northward by the appenninic thrusts and southward by the 'Apulo-garganica' carbonate platform.