Estratigrafia química e potencial gerador da Formação Pimenteiras, Bacia do Parnaíba

Abstract

The hydrocarbon source potential of the Brazilian Paleozoic basins is related to hydrocarbon generation both by conventional and non-conventional or atypical processes. The atypical model is the most suitable to characterize the Parnaíba Basin. This basin located in the northeast of Brazil, presents different cycles of sedimentation from Pre-Cambrian to Mesozoic. In this context, this research aims to contribute to the reduction of exploratory uncertainties in this basin, using organic geochemistry in samples of shallow borehole drills located at the eastern edge of the basin. This tool allows identifying and tracing key surfaces in the Devonian section and evaluating its source potential. The intervals with total organic carbon (TOC) contents of more than 1% were submitted to Rock-Eval pyrolysis analyses, whole rock organic carbon (δ¹³C) isotope analyses, liquid chromatography and gas chromatography associated with mass spectrometry were performed on the intervals previously identified as important for oil and / or gas generation. The detailed stratigraphic analysis of the range of the Pimenteiras and Cabeças formations, based on profiles of deep and shallow boreholes, allowed the mapping four main marine flooding surfaces, ranging from Eifelian to Neofamenian (surfaces A, B, C and D), which limit three 3rd order genetic sequences (AB, BC and CD). The isopach map showing the migration of the depocenter from east to northwest during the deposition of the Pimenteiras Formation in the interval of time that extends from Eifelian to Frasnian. The isopach map of the diabase that affect thermally the Pimenteiras Formation, presents two areas of greater thickness, both concentrated in the central part of the basin. Fields of production of natural gas are located between these thicker areas of intrusive rocks. The association of TOC, pyrolysis, stable isotopes of total organic carbon, biomarkers and palynofacies allowed to subdivide the section of the eastern border of the Parnaiba Basin into four chemostratigraphic units, called LI to L-IV, from bottom to top, and the Southwest edge section in three units, called SO-I to SO-III, from the base to the top of outcrop A. The main floods at the eastern edge of the basin are present at the base of the L-I chemostatigraphic unit and the L-IV unit, being coincident with the surfaces A and B mapped on the gamma-ray profiles delimiting the AB sequence at this edge. At the southwest edge, the main flood is represented by the SO-III chemostatigraphic unit. At the eastern border, good source potential for gas and condensate was recognized for a small group of samples with TOC contents between 1.5% and 3.2%. However, most samples with these same contents show a moderate potential for gas generation. On the other hand, the southwestern edge, samples with these same TOC contents a moderate potential for gas generation. In the north-central part of the basin it was possible to identify the radioactive shale B and C described in the literature, which presented, respectively, a moderate potential for oil generation and good potential for gas generation. The Pimenteiras Formation is thermally immature in the analyzed samples from the east and southwest edges of the basin. The presence of unsaturated compounds and the high relative proportion of moretanes in samples from the east and southwestern edges indicate that these edges were poorly buried after the deposition of the Lower Carboniferous. In the areas of boreholes 1-CT-02-PI, located to the north of the eastern edges, and 1-OGX-22-MA, located in the central-north of the basin, the organic matter of the Pimenteiras Formation presents a thermal evolution degree that fits with the conditions of oil generation and gas generation, respectively. This difference of thermal evolution is related to the presence of intrusive rocks. The new data contribute to the advancement of knowledge about the stratigraphy, geological evolution, potential and thermal maturation of the organic matter of the Pimenteiras Formation