Abstract: Velocity Problems not Related with Thrusting or Salt Tectonics

Ferraris, O.; Salinas, A.; Koremblit, M.; and Soubies D. - YPF

The oil industry use to take structure in time maps like structure maps. This custom means that a relationship at least laterally constant should exist between time and depth. If we consider now that this relationship is a function of the lithological column, any variation can be an advice that a lateral velocity gradient is present. New techniques like 3D seismic usually improve the data quality but the problems related to a velocity variations remain unvariable because they are characteristic of each particular place.

In the case history two independent oil fields were discovered using independent seismic lines, later a 3D survey including both structures was shot and the resulting structure in time maps showed the two anticlines but with the opposite relationship comparing with an depth map. Comparing the velocity sets (sonic log and check shots) of two wells allocated in different structures separated 10 Km, the first interesting consequence is that the upper interval has a typical velocity value, so we can avoid the possibility of surface anomalies with influence in statics, besides the other interval velocities are higher in the northern well, the second one is that the velocity effect not only changes or makes null but also reverses the slope in time domain. As a result of these tests the recommendation was depth migration of time data.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil