Abstract

During June and July 1976 the Texaco carbon/oxygen (C/O) log was run for P.T. Caltex Pacific Indonesia in eleven wells in central Sumatra, Indonesia, in order to evaluate the applicability of C/O logging in that area. The C/O technique uses inelastic gamma rays produced by bursts of fast neutrons into the formation to directly indicate the presence of liquid hydrocarbons. Since C/O interpretation does not rely on a measurement of the salt content of the formation waters, the log is designed for low salinity areas such as central Sumatra, where waters in most reservoirs contain less than 3500 ppm NaCl.

Cased-off sands, perforated sands, shales, and shaly sands constituted the vast majority of the formations investigated. The C/O log appeared to clearly distinguish cased-off oil bearing sands from depleted and water saturated sands. In many perforated intervals, however, ambiguous log interpretations resulted from borehole fluid contamination of these formations. Residual oil saturations in most logged depleted sands appeared low, indicating that primary reservoir sweep efficiencies were relatively high.