Velocity Investigation over a Seismic Reflection Anomaly in the Zelten Field, Libya

Geophysics, Petroleum geology, North African studies

Ben-Ayad, Najim Mohammed

University:University of Glasgow

关键词: Geophysics, Petroleum geology, North African studies;   

Others  :  http://theses.gla.ac.uk/78285/1/11008017.pdf

来源: University of Glasgow

【 摘 要 】

The Zelten field was discovered in 1959. The first well was drilled by ESSO Standard Libya. More than 178 wells have since been drilled, most of them into the reservoir (the Zelten member). The last few wells have been drilled by Sirte Oil Company who are now the owners of the field. Part of the aim of the study was to look at a 'reef' or velocity anomaly which had been drilled, and to see whether it is purely a geophysical artefact, or whether it is a real geological feature. A velocity study was made for the nine wells in the area which have velocity surveys. More than 150 velocity analysis tabulations available from 17 seismic lines were studied. Time and velocity-depth curves, rms velocity plots, Dix average velocity plots, and rms iso-velocity contour maps have been produced. Statistical work has been carried out on the velocity study, to demonstrate relationships between some of the parameters. Seventeen seismic lines (Vibroseis data), covering an area of 350 km2, have been chosen from 1984/85 seismic survey for the reinterpretation. Three horizons that are of interest have been chosen and correlated, namely the Sheghega, Domran, and Ruaga horizons. Two-way time, velocity, and depth contour maps have been produced. Most of them have been produced using automated contouring, but three of two-way time maps have been produced using hand contouring. A static correction study has been carried out on a part of the seismic line where some uphole data has been provided. New seismic processing software (SierraSEIS) available in the department was used, making the reprocessing more easier and more convenient. Nine kilometres of the seismic reflection data across a seismic reflection anomaly from line 6V256-85 were reprocessed. Some Fortran77 programs were written to handle the data to make the study more convenient and faster. It was concluded that the discontinuity of the reflectors underneath the anomaly is due to the distortion of the raypaths of the near traces by shallow features.