Datação relativa e potencial gerador de hidrocarbonetos em afloramentos pérmicos do Graben de Metangula

Abstract

This study uses data from routine techniques in the hydrocarbon industry such as rock-eval pyrolysis to determine the quantity and quality of organic matter and the potential for generating hydrocarbons, as well as the technique of transmitted white light microscopy to date the outcrops along which the study was carried out. The samples analysed came from the outcrops of Michunwa (K6 formation - Middle Permian) , Luíga (KSb formation - Upper Permian) Luchai and Nhamago (K4 formation - Lower Permian), located in the Metangula graben in Niassa province. In quantitative terms, the TOC (total organic carbon) values were moderate to high, indicating a good to excellent generation potential for all the outcrops analysed. In qualitative terms, there is an occurrence of type III kerogen (gas precursor) and a mixture of type II/III kerogen (oil or gas precursor). On the other hand, the Tmax and %Ro values show over-saturation of the OM, which may be associated with the tectonic events that accompanied the subsidence process in this basin. In terms of generation, in the K6 and KSb formations there is a potential for gas and oil generation respectively, along the levels composed of siltstones and claystones, and for the K4 formation the potential for gas generation is seen in the grey claystones. In palynostratigraphic terms, the pollen assemblage recovered from the Luchai outcrop shows a transition between the KK2 and KK3 biozones of Modie 2007 marked by the first appearance of the species Platysaccus papilionis , Striatopodocarpites fusus accompanied by Lueckisporites virkkiae, Striatopodocarpites gondwanensis, which indicate a Roadian age for this outcrop. This fact brings new elements if compared to the studies carried out by Verniers et al., 1989, bringing new species and suggesting new information concerning the geological age limits of the K4 Formation. However, for the Luíga outcrop, a stratigraphically important association of pollens and spores was recovered, marked by the appearance of the species Guttulapollenites hannonicus accompanied by Weylandites lucifer and Polypodiisporites mutabilis, which indicate the age of the Changhsingian (upper Lopingian) for this outcrop. On the other hand, for the Michunwa outcrop it was not possible to recover a stratigraphically important association due to the absence of guide species, as species from a high stratigraphic range were recovered. (TRADUÇAO NOSSA)