DETECTION OF HYDROCARBON RESERVOIRS USING THE CONTROLLED-SOURCE ELECTROMAGNETIC (CSEM) METHOD IN THE “BETA†FIELD, DEEPWATER OFFSHORE NIGER DELTA, NIGERIA

Abstract

The Controlled-Source Electromagnetic method (CSEM) detects and characterizes hydrocarbon-bearing reservoirs buried beneath a conductive sea bed by the use of electromagnetic (EM) energy. The survey was performed by towing a mobile Horizontal Electric Dipole source (HED) with a square pulse of 0.25Hz from NW-SE above an array of receivers deployed in two sets across the study area. A detailed data interpretation using a combination of forward modelling, geophysical inversion and imaging showed indications of shallow and deeper high-resistive anomalies. The shallow and deeper anomalies observed were interpreted to be caused by shallow and deep-seated hydrocarbon bearing reservoirs respectively which contained residual gas saturations. It was discovered that hydrocarbon reservoirs can also be detected at electromagnetic frequencies as low as 0.25Hz in the deepwater environment. At this frequency range (0.25 – 1.25Hz), residual hydrocarbons were detected. CSEM data were acquired and interpreted, confirmed that the shallow anomalies were actually residual gas sands with resistivities of 2 to 3ohm-m and that the deeper high-resistive anomalies were indeed hydrocarbon-bearing sands with a resistivities of 10 to 100ohm-m.