Aptian Carbonates of Carmopolis Field, Sergipe-Alagoas Basin: stratigraphy and depositional model

Abstract

There are occurrences of microbial carbonates in the basal portion of Ibura Mb., Muribeca Fm. Located at the Aracaju High, Late Aptian of Sergipe Basin. These carbonates were deposited over the basement and were described as spatially restricted to the area informally known as the Rosary Inlet. The goal of this paper is to present the depositional carbonate system related to the Ibura Mb., these carbonates were observed in the base section of the cored well. The interval of core description corresponds to the Upper Aptian (P-270 biozone) composed of carbonatic, siliciclastic, evaporitic and hybrid facies. It has been also described early diagenesis related to litoral and evaporitic environment based on rock and thin sections of cuttings. The middle diagenesis is characterized by fracturing, dissolution, prismatic anhydrite, silicification, dolomitization and piritization. Based on core description (facies and facies association) it was performed log facies analysis and it was done the composite logs for all the wells in the area. The microbial carbonate layer is stratigraphically correlated in the entire Aracaju High. A fundamental observation is the thickening trend of the microbial carbonate on the lower areas, with thinning on the higher areas. The Muribeca Fm. (specifically the Ibura and Oiteirinhos members) is interpreted as deposited in a transitional environment with some marine evidences. For the Ibura Mb, at the base of the described core, the depositional environment was interpreted as restricted evaporitic lacustrine lagoon due to: (a) rare presence of fossils, which are limited to some specimens of ostracods, (b) microbial carbonates in the form of microbial mats or carpets, (c) lack of sedimentary structures related to flow and tides, (d) evidences of high salinity and intense evaporation during the deposition in the form of diagenetic anhydrite and evaporite dissolution breccias, (e) presence of nodular and stratified anhydrite layered with carbonaceous mudstone as evidence of shallow water, (f) presence of exposure features like teepees and desiccation cracks, (g) dominance of low-energy lithologies. Thus, it is proposed the term Lake-Lagoon Complex of the Rosario for the facies associations found in the area of Aracaju High.