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Título

THERMAL EFFECT OF IGNEOUS INTRUSIONS AND SEDIMENTARY OVERBURDEN ON PETROLEUM SOURCE ROCKS FROM THE CRETACEOUS OF THE MUNDAÚ SUB-BASIN IN THE CEARÁ BASIN, BRAZILIAN EQUATORIAL MARGIN.

Texto do resumo

The Ceará Basin, located in the eastern portion of the Brazilian Equatorial Margin, covers an area of approximately 65,000 km2, of which about 50,000 km2 are submerged. This basin is divided into four sub-basins, with Mundaú being the one that concentrates the main oil and gas exploration activities. In this basin, there are shales with high organic matter content conducive to hydrocarbon generation (Ubarana, Paracuru, and Mundaú formations). The thermal effect of igneous intrusions together with sedimentary overburden significantly influences the maturation of the petroleum source rocks, affecting their potential hydrocarbon generation. In certain regions, these shales only have reached thermal maturity due to the influence of heat from igneous intrusions related to Macau Formation magmatism. Data provided by the National Agency of Petroleum, Natural Gas and Biofuels (ANP) from wells encompass a variety of information, including mineralogy, petrography, petrophysics, and geochemistry, such as total organic carbon (TOC) content, Rock-Eval pyrolysis indices, and vitrinite reflectance. Of these wells, three revealed the presence of igneous intrusions. This preliminary study aims to evaluate the thermal effects of these intrusions on kerogen in the source rock organic facies of these specific wells. 184 samples were analyzed from the Ubarana, Paracuru, and Mundaú formations. The TOC and Hydrocarbon Generation Potential (S2) data from the source rocks of the Ubarana, Paracuru, and Mundaú formations in wells 1-CES-8-CE reveal significant variations. In the Paracuru Formation, TOC and S2 vary from 0.70 to 8.65% and from 0.07 to 67.01 mg HC/g rock, respectively, indicating excellent potential for hydrocarbon generation. While the Ubarana and Mundaú formations are located in the poor hydrocarbon generation intervals. In wells 1-CES-29 and 1-CES-0111B-CE in the Ubarana and Paracuru formations contain intrusive rocks and correlate with the lowest S2 and HI values. These values can be considered residual values, as they are related to the high degree of thermal evolution (Ro values > 0.85%) resulting from the heat generated by the intrusive rocks. The Hydrogen Index (HI) values of the three wells indicate that kerogen is predominantly type I in the Paracuru Formation, reaching HI values of up to 928 mg HC/g TOC, indicating a oil prone source rock, except for those samples with low values of the TOC, S2, and HI, both in the Paracuru formation and in the Ubarana and Mundaú formations, which show type IV kerogen, resulting from continental organic matter or, more probably, residual due to depletion caused by hydrocarbon generation due to high temperatures, caused by both intrusive igneous rocks and sedimentary overburden. The thickness of the igneous intrusions in the three wells varied from 10 to 600 m. These intrusions caused a significant impact on TOC, HI, and S2 values, which values decrease near the intrusions. This indicates a progressive increase in maturation, resulting in overmaturity towards the igneous body. Preliminary results suggest the need for further studies integrating petrographic data, seismic profiles, and geochemical modeling to understand the generating potential of the Ceará Basin, including the effects of igneous intrusions on kerogen maturation and transformation rate.

Palavras Chave

Hydrocarbon generation; Total Organic Carbon; Rock-Eval pyrolysis