Abstract

A VSP survey was acquired in January 1987 in the Minas Field, the largest oil field in S.E. Asia which has already produced more than 3 billon barrels of oil. The source for the VSP was a 200 cu.in. airgun in a mudpit. The downhole sensor was a three component (3C) SAT tool. This led to the discovery that the airgun was an efficient source of energy for shear waves in addition to P-waves. The shear waves data was then used with the P-waves data to correlate to lithology. To increase the S-waves velocity data base, three additional wells were recorded using the airgun source in a pit. These data enabled the development of Vp-Vs relationships for Central Sumatra for further use in seismic amplitude versus offset modelling.

The interest generated by the new S-waves data led to surface recording of 3C data using single Galperin geophones. The Sikladi area was chosen for the first experiment as previous conventional recording showed two shallow bright spots associated with gas accumulations. The objective was to gather additional data via converted waves. The correlation of P-SV to P-P reflections, both generated by an explosive source, confirmed the extra potential of shear waves for discriminating hydrocarbon bearing zones.

Both examples of downhole and surface recording of 3C data demonstrate the usefulness of vector wavefield techniques in the Central Sumatra area.