Palinologia do cretáceo superior da formação urucutuca - Bacia de Almada (BA): interpretações bioestratigráficas e paleoambientais

Abstract

The present work addresses results obtained with the integration of palynology, palynofacies, and organic geochemistry data (COT / S and Rock-Eval Pyrolysis) performed on 26 core samples from well SST-01. This 250-meters well was drilled through the Urucutuca Formation in the onshore portion of the Almada Basin, located in the State of Bahia, Brazil. Through the characterization of palynofaciological variations based on the content of organic matter, associated with organogeochemical, and palynological data, it was possible to distinguish three types of palynofacies, with characteristics of different marine influence environments. Palinofacies 1 is characterized by a high percentage of phytoclast (70%) and TOC of 1%; Palinofácies 2, on the other hand, has the highest content in marine palynomorphs (40%), and the lowest value for TOC 0.67%. While in Palinofácies 3, the group of amorphous organic matter (MOA) predominates with percentages of up to (54.1%) and TOC of 0.75%. From the ICE analysis, values between 4 to 5 were found, indicating that Urucutuca Formation, in the investigated area, is in the immature stage, although quite close to the hydrocarbon generation window. The Total Organic Carbon (TOC) analyzes presented values ranging from 0.04% to 1.85%, heterogeneously throughout the whole well. Through the results, acquired with the Rock-Eval Pyrolysis was found that the whole well has a low oil generating potential, with S2 values below 2 (mg/g), and low values of Hydrogen Index (HI), and Oxygen Index (OI), altogether indicating that the organic matter present is composed by Type IV kerogen. Consequently, even the levels with TOC above 1%, the area generating potential is extremely low. The palynological succession shows a predominance of continental origin representatives. However, the presence of marine forms (dinoflagellates and/or palynoforaminifera) is present throughout the entire section. Throughout the section, two expressive occurrences of dinoflagellates are evident, both in terms of quantity and diversity, in the basal and middle portions, suggestive of the presence of transgressive events or, at least, of variations in the sea level. Through palynological analysis, 46 species of palynomorphs, including pollen grains, spores, dinoflagellates, and algae were identified. In biostratigraphic terms, it was possible to recognize and correlate the section as fitting in the Tricornites elongatus (P-470) Biozone of the Crassitricolporites brasiliensis (P-450) Superzone and assign the Eomaastichtian aged. These biostratigraphic units are correlated to proposed zoning for other basins on the Brazilian Atlantic Margin. The integration of palynofaciological, organogeochemical and palynological data, allowed the observation of constant variations in a predominantly platform paleoenvironment, now distal and now more proximal, with possible transgressions.