Abstract

Merging of data from seismic surveys and well logs in geophysical exploration is a part of synergistic approach for better understanding of geological model of the field. Sonic log provides continuous records of different formation velocities especially at shallow and intermediate depths, which may not be interesting from reservoir point of view. Exact knowledge of velocities is used for time to depth conversion, computation of synthetic seismic sections, migration and processing of geologically complex areas. Once hydrocarbon trap is proved by a discovery well, the whole seismic interpretation is to be re-looked for a new range of possibilities, resolution and detailed study. Delineation of known producing formations then becomes more discernible on seismic sections studied in conjunction with geophysical borehole measurements. While using sonic logs for seismic purposes, one has to adjust the sonic logs according to seismic measurements in the boreholes because sonic times obtained through the integration of sonic logs usually differ from those obtained by means of a well seismic. The reasons for drift range from basic discrepancies between two approaches due to different geometry / frequency of measurement principles etc. to more trivial disturbances in the formation i.e. alteration / invasion, which affects the sonic logs resulting in cycle skipping, detection of mud arrivals in large holes. Since the check shot / VSP method is simpler and corroborates with reflection survey results, sonic is more susceptive. Therefore, it is necessary to calibrate the sonic logs in order to eliminate these possible errors to use it for seismic application for a particular area. In the present study interval velocity, average formation velocity computed from both one way seismic time and integrated sonic time data available over sixteen wells spread all over Upper Assam oil fields show negative drifts for the calibration of sonic transit time. Amount of drifts is more in Girujan Clays about more than half of the total drift. Interval velocity values derived from seismic one way time are showing finer variations, which are some times cyclic in nature i.e. higher velocity followed by lower one, as compared to interval velocity values derived from sonic. These variations will result in creating fictitious reflections on synthetic seismograms, which are not seen on log measurements. Block shifting of sonic log ensures integrated travel time at par with seismic one-way time by preserving formation signatures required for seismofacies mapping. It is worth mentioning that sonic log has not lost even a bit of its grace as continuous velocity log even with recording of well seismic data / VSP. This method of correcting sonic logs for seismic applications will certainly provide a value addition for the realistic interpretation of seismic data in future study of Upper Assam fields and the method may be extended for other area too.