Introduction
We lay out a case history of a multi-method airborne geophysical survey performed to assist a hydrocarbon exploration project in central Mozambique. The dedicated targets of the survey are near surface resistivity anomalies caused by hydrocarbon seepage plumes. The geophysical survey concept includes 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 and regional geological information, results in the successful delineation of a seepage alteration zone just one month after survey completion.
Airborne Electromagnetic (AEM) resistivity mapping has been used by the oil industry to some extent since the 1990´s when airborne multi-method surveys were conducted to outline near surface alteration zones caused by hydrocarbon seepage plumes (Smith & Rowe, 1997). The correlation between hydrocarbon seepage and anomalies of the electrical properties of near surface sediments has been investigated broadly using of land-based EM and resistivity methods (Sternberg, 1991 and Hughes, 1983). No significant activities in this field have been reported since then, however. AEM exploration for base metals has been booming over the last decades, mainly driven by a steady rise in commodity prices. Although oil and gas prices have increased significantly as well, few AEM developments have been reported in this field. Nevertheless, high energy-prices enabled oil and gas companies to widen their horizons and investigate the use of unconventional methods such as AEM for hydrocarbon exploration.
In early 2008 we had the opportunity to initiate a multi-method airborne geophysical survey in a petroleum exploration license in Mozambique based on the evidence of gas seepages in the area. As a multidiscipline group consisting of a Norwegian oil & gas exploration and production company, a geoscience research and consulting institute as well as geophysical service and software development providers in Canada, we planned and successfully conducted this survey over an area of some 2000 km2. This survey combined helicopter-borne time-domain EM with a magnetic and gamma-spectrometry survey. Here we describe the survey planning, processing and initial interpretation phases highlighting the findings and value we obtained from the data.
The AEM survey area is located in the Republic of Mozambique within the onshore Inhaminga block situated north of Beira and south of the Zambezi River. The Inhaminga block covers an area of 16,500 square kilometers and is located some 200 km from the Pande and Temane gas fields. The AEM survey covers an area of roughly 50 x 40 km located in the northern area of the license, close to Inhaminga (Figure 1). Main flight lines are about 45 km long heading NW-SE with 500 m line spacing.
Geology
The Urema Graben was formerly interpreted to represent the southernmost part of the East African Rift, but new seismic surveys show the main rift period to be Mesozoic in origin. Only a thin portion of the graben fill can be ascribed to the East African Rift episode (the sequence above the Oligocene in Figure 2). The Urema Graben forms a half-graben with the main fault located just west of the Inhaminga High.
