First Break - Volume 13, Issue 1, 1995

Formation slowness is commonly estimated using first motion detection of the compressional headwave recorded with an acoustic tooI. This slowness estimation, in its simplest form, involves the difference in arrival time between two or more receivers. However, additional information is embodied in the traveltimes themselves. The measured traveltime at each receiver is actually a function of the path of the signal between the source and the receiver. In the presence of near-borehole changes in formation slowness, such as in altered or damaged zones (implying a reduction in the formation velocity in the vicinity of the borehole), refracted borehole arrivals may pass some inches to some feet from the borehole wall.

In recent years the integration of a predictive geological model with seismic data has become common in the oil industry in the search for the subtle trap, Such an integrated approach has been used by Saga Petroleum in order to define a well location in an Upper Jurassic stratigraphic play concept in the western part of NOCS Block 34/7. For the License it was important to investigate this possibility as relinquishment of acre was imminent, and the area could only be retained if hydrocarbons were proved by drilling. This article describes the main procedures used in the study. The main implications following the result of the exploration well 34/7-21 and further work related to the H-Discovery are also mentioned.

Interest in reservoir characterization continues to grow as emphasis within the petroleum industry shifts from exploration to improved recovery of existing fields with complicated heterogeneity, to contact mobile oil unrecovered or bypassed by previous injection procedures. The optimization of production using advanced secondary recovery programmes is critically dependent upon the detection of reservoir bodies, together with the distribution, alignment and density of heterogeneities on a variety of scales. Seismic measurements are of value in inferring spatial variability, using various assorted attributes derived from the wavefield, such as impedance, instantaneous phase and instantaneous frequency.