Martin C. Sinha

Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola

Detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry. Seismic methods have traditionally been used in this context, but the results can be ambiguous. Another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids. Modeling presented by Eidesmo et al. (2002) demonstrates that by using seabed logging (SBL), a special application of frequency domain controlled source electromagnetic (CSEM) sounding, the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified. Such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying.

Crustal trace of a hot convective sheet

The Iceland plume has played an influential role in the evolution of the North Atlantic Ocean and margins over the past 60 m.y. It is believed that this plume formed at the conjunction of a tetrad of hot, subvertical, convective sheets. The impingement of these hot sheets at the base of the lithospheric lid caused decompressional melting, generating substantial quantities of high-temperature magma that were injected into the cold overlying lid. Over the next 10 m.y., these sheets partly coalesced to form a crudely axisymmetric plume head. Here we analyze the lithospheric fingerprint of one of these hot convective sheets. By forward and inverse modeling of densely sampled wide-angle seismic data, in conjunction with gravity observations, we determined the three-dimensional shape of magmatic underplating trapped within the lithosphere. The injection of this melt into the lithosphere generated substantial permanent and minor transient uplift of Earths surface. Predicted and measured amounts of consequent denudation and sedimentation agree within error. Temporal variations in the patterns of deposition and oceanic circulation adjacent to the convective sheet show its evolution through time and space. Our results suggest that this linear sheet has probably been directly and indirectly responsible for cyclical events over 60 m.y. These events have 0.5–1 and 4–6 m.y. periodicities, the existence of which may help to elucidate the dynamic behavior of convective sheets during and after impingement. Thus, in particular circumstances, surficial geological processes yield an indirect record of mantle convection and melt-generation processes.

Role of fine-scale layering and grain alignment in the electrical anisotropy of marine sediments

Electrical resistivities of seafloor sediments determined by controlled source electromagnetic (CSEM) surveys have been found to be significantly greater than those measured by electrical well logging, in some instances by a ratio of as much as 5:1. Because borehole logging techniques invariably measure electrical resistivity using currents circulating in horizontal planes and CSEM surveys are sensitive to the currents circulating in vertical planes, a possible cause of this discrepancy is strong electrical anisotropy of the sediments. We have examined electrical log data from vertical exploration and appraisal wells and deviated production wells in the North Sea. By correlating the same sedimentary units between wells, we are able to compare resistivities measured at different borehole inclination angles. As the inclination angle changes, the amount of nvertical and horizontal resistivity contributing to the resistivity changes. Hence we can estimate the electrical anisotropy in the sediments. Results indicate that anisotropy ratios of 1:5 and greater are present within the shale-dominated units. We show that fine-scale horizontal layering can make only a relatively small contribution to this anisotropy, but that a model of horizontal alignment of highly oblate spheroidal grains can account for most or all of it.

An anisotropic model for the electrical resistivity of two-phase geologic materials

Electrical and electromagnetic surveys of the seafloor provide valuable information about the macro and microscopic properties of subseafloor sediments. Sediment resistivity is highly variable and governed by a wide range of properties including pore-fluid salinity, pore-fluid saturation, porosity, pore geometry, and temperature. A new anisotropic, twophase, effective medium model describes the electrical resistivity of porous rocks and sediments. The only input parameters required are the resistivities of the solid and fluid components, their volume fractions and grain shape. The approach makes use of the increase in path length taken by an electrical current through an idealized granular medium comprising of aligned ellipsoidal grains. The model permits both solid and fluid phases to have a finite conductivity useful for dealing with surface charge conduction effects associated with clay minerals and gives results independent of grain size hence, valid for a wide range of sediment types. Furthermore, the model can be used to investigate the effects of grain aspect ratio and alignment on electrical resistivity anisotropy. Good agreement was found between the model predictions and laboratory measurements of resistivity and porosity on artificial sediments with known physical properties.

The Potential of Controlled Source Electromagnetic Surveying in CO2 Storage Monitoring

Summary The capture and geological storage of CO2 is currently being investigated as a means of reducing CO2 emissions into the atmosphere. Before it can be accepted, the fate of the injected CO2 must be understood. Controlled Source Electro-Magnetic (CSEM) surveying could be used to monitor CO2 injection and migration, since the presence of CO2 causes resistivity within the reservoir to change. 1D EM modeling of potential storage sites can indicate whether CSEM monitoring would be viable. By altering the CO2 saturations in the models a range of CO2 storage scenarios can be investigated. The modeling shows that the changes in resistivity caused by the presence of CO2 are detectable by CSEM in some scenarios. However the vertical distribution of the CO2 plays a major role in determining the amount of change in the CSEM response.