Airborne hydrocarbon mapping

Multi-method airborne geophysical survey assisted a hydrocarbon exploration project in central Mozambique. The survey concept included the use of time-domain electromagnetics, magnetometry and gamma-spectrometry applied from rotary and fixed wing aircraft.

AeroTEM IV system

As an advisor to the Norwegian petroleum company DNO International ASA, NGI has been planning, supervising and interpreting an Airborne Electromagnetic (AEM) survey over an area of some 2.000 km2 in central Mozambique, North of Beira.

The survey target for the multi-method airborne geophysical survey was near surface resistivity anomalies caused by hydrocarbon seepage plumes. Survey concept included the use of time-domain electromagnetics, magnetometry and gamma-spectrometry applied from rotary and fixed wing aircraft.

Preliminary data interpretation, closely tied to 2D seismic information and the regional geological understanding, indicates a successful delineation of a seepage alteration zone, just one month after survey completion.

Our data analysis highlights an anomaly extending some tens of kilometers through the survey area along the eastern margin of the Urema graben. The area is imaged by AEM as a shallow resistive unit and shows high Uranium and low Potassium concentrations (normalized to mean Thorium ratios).

A seismic dimming zone on a 2D seismic line crossing the feature coincides with the resistivity and radiometric anomaly.

Radiometric data (left) presented as combined relative Uranium and Potassium anomaly normalized to Thorium draped over topography with shading from an azimut of 139 degees. Areas in red are likely to be caused by gas seepage. Right: Airborne EM response, secondary data; Early time apparent resistivity derived via pseudolayer half-space model draped over topography. The western resistive (blue) lineament appears to be linked to seepage.

Additionally, the geological exploration model expects seepage to be linked to the graben fault systems and an active seep has been sampled close to the anomaly. We thus interpret this anomaly to be the outline of a gas seepage zone. The combination of AEM resistivity sections and advanced radiometric maps appear as a useful tool for gas seepage alteration mapping.

DNO International entered Mozambique in 2003 by signing an Exploration and Production Contract with the authorities for the onshore Inhaminga block located north of Beira and south of the Zambezi River. The Inhaminga block covers an area of 16,500 square kilometres and is located some 200 km from Sasol's development of the Pande and Temane gas fields. The sheer size of the license and the relatively early exploration status makes it a suitable target for airborne geophysical mapping.

Survey Area
The AEM survey covers an area of roughly 50 x 40 km located in the northern area of the license, close to Inhaminga. The geological setting comprises a half-graben structure striking NE. Especially the flanks of the graben are heavily faulted (as seen on 2D seismic images) and the faults might be a pathway for hydrocarbon seepage. Gas and oil seeps have been observed and reported in the area.

Cross section through the Urema Graben indicating the geological setting of the survey area.

Target for AEM mapping

Fluids which migrate from buried hydrocarbon reservoirs all the way to the surface (seepages) interact with the near surface geology, e.g. cause alteration effects in mineralogy and pore water in limestones and clastic rocks. Further seepage associated bacterial activity appears to have the potential  of skewing the local geochemistry. These alterations potentially create anomalies in the physical properties of the rocks and can then be mapped by airborne geophysical methods.

AEM instrumentation
Given the expected geological background and target properties, we choose a time domain helicopter borne EM system provided by Canadian AEM contractor Aeroquest ltd (AeroTEM IV, see photo) in concert with a fixed wing radiometric and magnetic gradiometer provided by UTS Geophysics, Australia. The full instrument package provides earth magnetic field intensity & gradient, surface concentrations of Potassium, Uranium and Thorium, and finally EM data which leads to subsurface resistivity sections via sophisticated inversion algorithms.

NGI is cooperating with PetRosEikon Inc., an experienced EM imaging company in Toronto, to ensure best processing and inversion results. The survey is scheduled to commence in July 2008 and interpretation is to be concluded by September.



DNO International ASA
Aeroquest ltd
PetRosEikon Inc.