Characterization of lithology using crosshole methods

Abstract

A crosshole data log has been recorded in a carbonate reservoir in the Permian Basin of west Texas. This log consists of traces at 3 m depth increments recorded from horizontally propagating waves between two wells spaced 304 m apart in a carbonate reservoir. Even at this long offset the recorded data contain frequencies up to the cutoff frequency of 3000 Hz. Using first arrival times, Vp, Vs, Vp/Vs, and Ap/As calculations were made. Vp and Vs values plotted on a "Pickett diagram" indicate a limestone or dolomite lithology. A plot of Vp/Vs and porosity from the receiver well shows an inverse correlation indicating that crosshole Vp/Vs has the potential for high resolution of lithological parameters. Amplitude (Ap/As) calculations from the crosshole log show two distinct zones. The zone of higher Ap/As values has been interpreted as a fluid-filled region. Q values have been calculated from a set of high frequency crosshole data recorded in a shallow clastic sequence in an oil field in south Texas. A piezoelectric bender and a single hydrophone receiver were used to record data over a depth interval from 152 m to 180 m in 3 m increments. Data were recorded from three inline wells giving well offsets of 15 m and 61 m. The spectral ratio method was used to make Q calculations. Q averaged over frequency at the various depths ranges from 43 to 128. A comparison of Q with a resistivity log from the source well shows an inverse correlation. The two high Q regions have been interpreted as shaly zones and the lower Q zones as less shaly or sandy layers.