Sismoestratigrafia e geomorfologia sísmica dos depósitos fluviais a marinhos rasos da Formação Açu, Campo de Caraúna, Albo-Cenomaniano da Bacia Potiguar

Abstract

The Açu Formation, a unit of the stratigraphic framework of the Potiguar Basin, corresponds to a siliciclastic succession deposited between the early Albian and Cenomanian, encompassing fluvial, deltaic, estuarine, and shallow marine systems. The deposits within this unit hold economic value, as they contain significant hydrocarbon reserves and constitute an important freshwater aquifer widely utilized in Rio Grande do Norte. The successions of the Açu Formation have been analyzed by different researchers over time, largely using outcrop, core, and geophysical logs data. Furthermore, most previous studies have focused on its upper portion. Consequently, there remains a lack of research involving seismic data, particularly regarding more modern techniques, as well as studies that address the lower section of this formation. This research aims to characterize the stratigraphic framework and petroleum system of the Albian-Cenomanian deposits of the Potiguar Basin in the Caraúna Field region. For this purpose, the study utilizes seismic data (2D and 3D) and well data located on the continental shelf. The study's first phase involved the recognition of surfaces and sequence stratigraphy units. Subsequently, these surfaces and units formed the basis for applying seismic geomorphology. This was accomplished through the horizon and proportional slicing techniques, aided by seismic attributes such as RMS amplitude and sweetness. The seismicstratigraphic interpretations, combined with the analysis of geophysical logs and lithological profiles from the wells, led to the recognition of eleven seismic horizons representing significant stratigraphic surfaces (subaerial unconformities, maximum flooding surfaces, and maximum regressive surfaces). These horizons delineate three depositional sequences (1, 2, and 3), subdivided into lowstand, transgressive, and highstand systems tracts. Seismic geomorphology, in turn, enabled the identification of elongated features in plan view, interpreted as shelf sand ridges. The features identified in the study are associated with the transgressive systems tracts of sequences 1 (Albian) and 3 (late Cenomanian-middle Turonian). The ridges of Sequence 1 appear as a set of isolated, noncoalescent, and parallel features. In contrast, the feature in Sequence 3 results from the coalescence process of two others. This ridge occurs in isolation and exhibits greater robustness compared to those in Sequence 1. Tidal processes have been pointed as the main agents in forming the recognized shelf sand ridges. However, it is believed that waves and ocean currents also played a role in developing these features. The Albian deposits of Depositional Sequence 1, and consequently, the identified shelf sand ridges in these deposits, are particularly important for the petroleum system of the Caraúna field.