The role of airborne geophysics for detecting hydrocarbon microseepages and related structural features: The case of Remanso do Fogo, Brazil

Abstract

The first direct indication of hydrocarbon occurrence in Remanso do Fogo area (Minas Gerais State, Brazil) was the existence of microseepages. However, Quaternary sedimentary cover in the area made the identification of new occurrences and associated controlling structures quite difficult. This study investigated the spatial distribution of hydrocarbon related gases in shallower to intrasedimentary depths in Remanso do Fogo area, using airborne gamma-ray spectrometry and magnetic data. The geophysical data were processed using techniques designed to suppress the influence of regional geological signatures. Known and possible new occurrences of microseepages were detected by mapping low residual potassium values and high uranium residual values in relation to potassium, termed as DRAD values, which resulted from the subtraction of potassium from uranium residual values, using a Thorium (Th)-normalizing approach. The validation of these occurrences was done based on existing gas geochemistry data in the soil. For the magnetic data, the amplitude of the analytic signal, combined with the total horizontal gradient of the subtraction between the 1200- and 400-meter upward continuations, enhanced the northwest–southeast and east–west magnetic lineaments, which are partially related to the microseepages and the drainage of the area. The distinction of near-surface and deep signatures also allowed the general identification of intrasedimentary and basement structures, which are potentially controlling the occurrences of seepages in the area.

Magnetotelluric imaging integrated with seismic, gravity, magnetic and well-log data for basement and carbonate reservoir mapping in the São Francisco Basin, Brazil

“…The area is cut by faults, possibly putting the potential reservoir in communication with the surface along fracture zones that could explain the apparent structural control on the distribution of the known gas seepages (Curto et al 2012). Indeed, several NNW-SSE-trending thrust faults crop out near MT station P41 in Figure 1 (see also Porsani & Fontes 2001).…”

“…Romeiro-Silva & Zalán 2005). Recently, Curto et al (2012) applied airborne radiometric and magnetic geophysical methods to the detection of hydrocarbon seepages and their possible structural controls in the Remanso do Fogo area at the confluence of the Paracatu and São Francisco rivers (Fig. 1).…”

“…This should complement seismic information, especially River. The red box is the Remanso do Fogo area marked by the structural high (Hercos et al 2008) and hydrocarbon microseepages (Curto et al 2012). Shell block SSF-T-93 is shown by the dark grey dotted box.…”

“…1) is one such basin (e.g. Curto et al 2012). This paper presents a two-dimensional (2D) reinterpretation of an MT survey conducted in this basin by Porsani & Fontes (1993, who carried out only a 1D modelling study.…”

“…Our goal is to determine whether 2D MT resistivity imaging alone or in conjunction with well log, seismic, gravity and magnetic data can provide useful information on the structure of the basement and its relationship to the cover rocks, and, in particular, the carbonate reservoirs and the occurrence of gas seepages (e.g. Curto et al 2012) in a selected part of the São Francisco Basin.…”