Análise Microfaciológica e Geoquímica dos Folhelhos da Formação Morro do Chaves, Bacia de Sergipe-Alagoas

Abstract

Shales are fissile microclastic rocks that may occur as source rocks, reservoir rocks, or seals in a petroleum system. The Morro do Chaves Formation, Jiquiá in age (Barremian/Aptian) from Sergipe-Alagoas Basin, is formed of shales and sandstones interbedded in a coquina succession deposited in a lacustrine paleoenvironment, outcropping in the former Atol quarry, in São Miguel dos Campos (AL). This outcrop is considered an important analogue of carbonate reservoirs from the Pre-salt interval of Santos and Campos Basin. Thus, this study aims the microfaciological characterization of those shales present on this coquina stratigraphic section, with the objective of enrich the environmental interpretation proposed in the literature. To characterize the microfacies, 150 meters of a continuous well core were described macroscopically and 25 thin sections were analyzed, supported by XRD analyses, organic chemical analyses (TOC) and well logs (Gamma Ray and Caliper). Nine microfacies were characterized, eight siliciclastic, formed by claystones, siltstones and sandstones, composed basically of quartz, muscovite, feldspar and clay minerals from caulinite group, chlorite and illite, and calcite (from ostracode shells); and one carbonatic. The interpretation of those microfacies points to fluctuations in the lake level, alternating oxidizing and reducing conditions, that are associated with climate variations or, sometimes, to tectonic activities in the basin.