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Título

ORIGIN AND DISTRIBUTION OF CRETACEOUS PETROLEUM SOURCE ROCKS IN THE MUNDAÚ SUB-BASIN OF THE CEARÁ BASIN, BRAZILIAN EQUATORIAL MARGIN

Texto do resumo

The Ceará Basin covers an area of approximately 65,000 km2 and is subdivided into four sub-basins, with Mundaú being one of them, covering an area of 12,000 km2. It is located at the eastern extreme of the basin and is bounded to the west by a fault inflection, which is where the main oil and gas exploration activities are concentrated. This preliminary study aims to present the geochemical contrast of the source rocks of the Paracuru (Aptian) and Ubarana (Albian to Present) formations in two wells (1-CES-26-CE and 1-BRSA-1114-CES) located on the mid-shelf and outer shelf of the basin, approximately 38 and 75 kilometers from the coastline, respectively. For this study, data from Total Organic Carbon (TOC), Rock-Eval Pyrolysis, and Vitrinite Reflectance (Ro) of 112 samples were used, provided by the National Agency of Petroleum, Natural Gas and Biofuels (ANP). In well 1-CES-26-CE, in the Ubarana Formation, the TOC content and generation potential (S2) range from 0.65% to 2.08% and 0.46 to 3.22 mg HC/g rock, respectively, indicating fair to very good potential for hydrocarbon generation. Hydrogen Index (HI) values reveal that the kerogen is Type III and Type IV (HI: up to 191 mg/HC g COT), and therefore gas-prone. The rock is immature with an average Ro of 0.59%. In the Paracuru Formation, the TOC and S2 contents range from 0.98% to 3.81% and 0.19 to 16.49 mg/HC g rock, respectively, indicating good to very good potential for hydrocarbon generation. HI values indicate that the kerogen is Type II, III, and IV (HI: up to 495 mg/HC g COT), oil/gas-prone. Ro% analysis indicates immaturity at the top of the formation, with a progressive increase in organic matter maturity with depth (%Ro up to 0.80), and consequently a decrease in TOC, HI, and S2 values. In well 1-BRSA-1114-CES, in the Ubarana Formation, TOC contents ranged between 0.7% and 2.9%, with S2 up to 15.49 mgHC/g rock, very good potential for hydrocarbon generation, and HI values of 720 mg/HC g COT, indicating kerogen types I, II, and III. Oil and gas prone source. The average %Ro value is 0.43%, indicating immature rock. In the Paracuru Formation, the TOC content reached 6.26% and S2 up to 22.33 mgHC/g rock, indicating excellent potential for hydrocarbon generation. HI values of up to 406 mg/HC g COT suggest Type II kerogen (top) and Type III, Type IV (base). %Ro analysis between 0.61% and 1.2% shows an increase in maturity with well depth, indicating the transition from the oil window to the gas and condensate zone. The results of this preliminary study indicate that well 1-BRSA-1114-CES, positioned in ultra-deep waters, demonstrates substantially higher values of rock generation potential and thermal maturity, especially evident in the Paracuru Formation. Thus, there is room for further exploration of relevant information about the Ceará Basin through future investigations, integrating organic geochemistry and computational modeling, to enhance understanding of its generating potential.

Palavras Chave

Cretaceous Petroleum source rocks; Mundaú Sub-basin; Ceará Basin; Brazilian Equatorial Margin Basin.