AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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The Sergipe-Alagoas Basin (NE Brazil) as analogue of coquina reservoir

Abstract

The permo-porosity analysis of carbonate rocks formed by coquinas is of major importance to characterize the pre-salt oil reservoirs from the Brazilian coastal basins. In this context the Sergipe-Alagoas Basin is particularly relevant due to the completeness of its depositional sequences. This paper presents the results obtained from petrographic and petrophysical analysis developed for the Morro do Chaves Formation (Lower Cretaceous). This unit outcrops at the InterCement Quarry, located near São Miguel dos Campos (Alagoas state, northeast of Brazil; fig. 1). Its best outcrop is a large mining with 1000 meters long, 300 meters wide and 100 meters deep. The unit can be also studied through sample cores from wells, which enable its exploration up to 300 meters deep. In this work, stratigraphic profiles data were integrated with petrographic and petrophysical data using stochastic and deterministic methods in order to generate equally probable models. This approach is a helpful tool in the establishment of the distribution pattern of the main reservoir properties and the behavior of fluids, and therefore in evaluation procedures of its exploration potential. The studied rocks described as grainstone/rudstone present 5 main types of porosity: moldic, vug, intergran, intragran and intraparticle with 12.6% of average porosity (fig. 2). Petrophysical analyzes provided the best values for the calcarenitic facies in all the wells, with porosity values ranging between 15.53% and 18.28%. For the sandstone, represented by calcarenite and calcirrudite facies, permeability values of the studied wells range between 841.42 and 5839.03 mD. An interpolator deterministic algorithm was used to estimate the distribution pattern of the permo-porosity properties of the recognized facies within the unit. Results show that the best reservoir facies of the Morro do Chaves Formation is the carbonate rocks of the calcirrudite and calcarenite facies. Moreover, these facies display good porosity (15%) as a result of early cementation. However, connection between pores is scarce thus affecting the production costs. As so, the coquina carbonate rocks of the Morro do Chaves Formation display medium reservoir potential. Nevertheless, the Morro do Chaves Formation is being used as an analogue for the Lagoa Feia Group of the Campos Basin, namely for the Coqueiros Formation, currently subject of oil exploration by different companies. Other studies are being developed in the unit. They include the taphonomic analysis of the recorded bioclastic components - namely the interpretation of sin-depositional (biostratinomic) and post-deposicional (diagenetic) processes - aiming at inferring the factors which originated the resulting distribution pattern of the permo-porosity properties.