Recognising Top of Overpressure Zone from Resistivity and Penetration Rate data: NW Niger Delta, Nigeria

Abstract

The NW Niger Delta contains overpressure zones in the lower portion of its Agbada Formation, where prolific petroleum bearing sands exist. The bit has often penetrated the sands within the overpressure zone without warning and contingency preparation. This sometimes in the past resulted in loss of drilling rigs, logging equipment, oil wells and human life. Minor problems have included hole caving and stuck pipe. For petroleum development activities to continue safely and at reduced total cost, the top of the overpressure zone must be recognized early and contingency drilling mud prepared in time at the well-site. Reversal in trend of resistivity and penetration rate in shales, augmented by increased content of methane, ethane, propane and butane in gas liberated from shales, clearly reveals the top of the overpressure zone. This top is a pressure seal constituted by a thick marine shale associated calcite precipitation. This shale is the shallowest regional marine transgressive shale, and is lithostratigraphically Uvigelinella-5. When mud-log data and LWD (logging while drilling) resistivity data are analysed integratedly, the top the overpressure zone will be predicted and recognized in real time at the well-site.