Preliminary analysis of a crosswell seismic dataset: Noel tight gas field, British Columbia
Joe Wong, Robert R. Stewart
A crosswell seismic survey was acquired in the Noel oilfield, northeastern British Columbia, by Z-Seis Corporation for BP Canada Energy Company in 2004. The goal was to obtain data for high-resolution imaging of gas-bearing, but tight, sandstone channels about 2500 meters deep in the Cadomin and Nikanassin formations. Recording between two wells approximately 150 meters apart employed hydrophones and a piezoelectric vibrator source. Seismograms were produced post-acquisition by cross-correlating detected vibrations with the pilot sweeps. The dominant frequencies in the crosswell seismograms are about 500 to 1500 Hz, suggesting wavelengths on the order of 5 to 10 m. Compared to wavelengths of about 100 meters that are usual for surface seismic data, the shorter crosswell wavelengths provide significantly better resolution of beds 10 to 25 m thick. Preliminary processing of the data included sorting and display of gathers, band-pass filtering, and first-arrival time-picking. A P-wave velocity tomogram was created from observed first arrival times using an iterative back-projection algorithm and accounting for refracted arrival times in the low-velocity zones. Velocities were found to be in the range of 5800 to 6200 m/s for sandstone, and 5100 to 5500 m/s for the siltstones and shales. The seismic boundaries imaged in the tomogram correlate excellently with natural gamma ray logs and the known geology. Future analysis of the dataset will include migration of the up-going and down-going reflected events. In addition, we intend to investigate possible anisotropy in the strata along with attenuation. The velocity values will be compared to sonic logs and surface seismic analyses.