Sea Bed Logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers in deepwater areas

Abstract

In this paper we describe a technique called Sea Bed Logging (SBL), an application of marine CSEM sounding, which can be applied to detect and characterize hydrocarbon bearing reservoirs in deep water areas. The basis of the approach is the use of a mobile horizontal electric dipole (HED) source and an array of seafloor electric field receivers. The transmitting dipole emits a low frequency electromagnetic signal both into the overlying water column and downwards into the seabed. The array of sea floor receivers measures both the amplitude and the phase of the received signal that depend on the resistivity structure beneath the seabed. A survey consisting of many transmitter and receiver locations can be used to determine a multidimensional model of subseafloor resistivity. In deep water areas the geological strata are generally dominated by shale or mud rocks with rather low resistivity. A hydrocarbon reservoir can have resistivity perhaps 10-100 times greater. With an in-line antenna configuration the transmitted electric field enters the high resistive hydrocarbon layer under a critical angle and is guided along the layer. Electromagnetic signals constantly leak from the layer and back to the seafloor. The guiding of the electric fields significantly alters the overall pattern of current flow in the overburden layer. A broad-line antenna configuration does not generate guided waves, and thus the two antenna configurations have different sensitivity to thin buried resistive layers. This so-called 'split' effect that is diagnostic for buried resistive layers is verified by 1-dimensional modelling, 1- and 2-dimensional modelling and real data acquired offshore West Africa also demonstrate that by careful positioning of transmitter tow tracks and receivers relative to a suspected hydrocarbon bearing structure, the SBL technique can provide detailed information on the presence and lateral extent of the hydrocarbon reservoir.