Paleoambiente deposicional e potencial gerador da formação Pojuca, Bacia do Recôncavo

Abstract

The present work aimed to evaluate the paleodepositional conditions and the potential for hydrocarbon generation in the Pojuca Formation, Recôncavo Basin. For this, an organic geochemical characterization of 48 originating from a borehole (9-FBA-79-BA well) was carried out, near the city of Aramari. The levels of total organic carbon (TOC) vary from 1.24 to 4.85%, denoting the heterogeneity in the conditions of production and preservation of organic matter. The results of the Rock Eval pyrolysis indicated kerogen predominantly type I / II, and type III in some of it. The hydrocarbon source potential (S2) varied from poor to excellent (1.26 to 26.56 mg HC / g rock), low to good concentration of free hydrocarbons (S1) and maximum temperature (Tmax) of Rock Eval pyrolysis indicate thermal immaturity for hydrocarbon generation (<440oC), validated by the ratios pristane/ n-C17 and phytane/ n-C18> 1, C29 steranes S/S+R, αββ/αββ+ααα, TS/(TS+TM), and by the greater abundance of NSO compounds. Such results were consistent with the TOC values, making it possible to identify that those with the highest TOC and S2 values have high IH (hydrogen index) and low IO (oxygen index) values, corroborated by kerogen type I / II thus validating the quality of organic matter. As a sponsor with low source potential, they have high IO values and low IH values, proportionally, configuring type III kerogen (oxidized organic matter). Through the stable carbon isotopes analysis (-33.83 ‰ to -21.54 ‰) and saturated biomarkers it was possible to infer that the depositional paleoenvironment from the Pojuca Formation was lacustrine with variations in water salinity, indicated by the positivity of the δ 13C values of some samples suggesting an arid environment with a lot of water evaporation, and that, possibly, in some moments during the deposition of the Pojuca Formation, there was an influx of saline waters in the basin, favoring greater proliferation of algae in relation to bacteria.