A Preliminary Assessment of the Hydrocarbon Potential of Kerio Valley Basin: Gravity and Magnetic Interpretation

ABSTRACT The aim of the study was to assess the hydrocarbon prospectivity of the Kerio Basin in the Kenya Rift. An Isostatically corrected anomaly map produced from a Bouguer anomaly grid was filtered using a Hanning low pass filter of order 2 to remove low wavelengths. Four profiles were extracted from the grid to give 1D interpretation along straight lines. Magnetic grid was corrected for IGRF, diurnal, filtered using a 1 Hz low pass 10km Hanning filter to reduce noise, later, reduced to equator to place all anomalies directly over underlying sources and make anomalies less complicated. Tilt derivative of the magnetic grid was used to estimate depth to basement. The residual analytic signal anomaly map derived from the magnetic grid was used to capture the response of existing near surface magnetic signatures even the reversely magnetized ones. Kerio basin is characterised by low gravity anomalies ranging between 35mGals to −100mGals related to variations in quantities of sediments deposited. Gravity profiles show that sediment thickness gradually increases to the south where we expect hydrocarbon accumulation. The magnetic anomaly map reveals low susceptibility rocks of between −20nT to −200nT to the south of the basin. Magnetic tilt depth indicates sediment thickness of 2.0-3.5Km above the basement. This corresponds to both gravity and magnetic interpretation of the same area. Integration of these data with seismic and other constraints may help gauge the hydrocarbon potential and reduce exploration uncertainty in the southern area of the Kerio Basin.

Attributes of the magnetic field, analytic signal, and monogenic signal for gravity and magnetic interpretation

Many of the transforms and attributes used in gravity and magnetic interpretation can be expressed as a 2D or 3D vector. The horizontal gradient and the 2D analytic signal are 2D vectors. The gravity or magnetic field, the 3D analytic signal, and the monogenic signal are defined by a 3D vector. In practice, we prefer to interpret the amplitude and/or phase of a 2D or 3D vector, but we often forget that a meaningful interpretation requires a magnetic reduction-to-the-pole operation when these techniques are applied to magnetic anomaly data and the source body is 3D. Furthermore, the gravity or magnetic anomaly has an unknown constant level that may affect the amplitude and phase. The horizontal gradient, the analytic signal, and the monogenic signal can be applied to not only the gravity or magnetic anomaly but also any [Formula: see text]th-order derivative or a filtered version of the anomaly. They can be related to each other and to the magnetic field vector. We do not introduce new attributes. Instead, we have explained the relationships among different transforms (or vectors) and addressed precautions and requirements for their practical use.