Subsurface Weathering Zone Determination Using Down-Hole Seismic Surveys, Majnoon Oil Field, Southern Iraq

Abstract

Detecting the weathering zone is one of the required data correction steps to process reflection seismic data collected at the Majnoon oil field, south of Iraq. Two subsurface layers represent the weathering zone in this field. Those two layers were inferred by primary seismic waves utilizing the down-hole seismic method. Five wells in the studied area were drilled and named DH-1, DH-2, DH-3, DH-4 and DH-5. The collected seismic data was analyzed to determine the depths, thicknesses and lithology of the subsurface layers. The primary seismic wave velocities were calculated and applied to eliminate two sub-surface layers. The average primary wave velocities and thicknesses of the first layer were 374, 240, 211, 277, 370 m/s, and 2.8, 2, 2, 2, 4.9 m, respectively. In the second layer, the primary waves velocities were 1125, 1284, 1201, 1395 and 1008 m/s, and the thicknesses were 13.2, 11, 13, 11.5, 21.4 m. The first weathered layer represented on the five wells is clay. The second layer has different geometry on different wells, which is represented by a sandy clay layer on DH-1, and muddy clay on DH-2. Then, it is changed to be a silt layer in DH-3, sandy clay in DH-4 and DH-5.