Facies and depositional environments for the coquinas of the Morro do Chaves Formation, Sergipe-Alagoas Basin, defined by taphonomic and compositional criteria

Abstract

Lacustrine carbonate rocks form important hydrocarbon accumulations along the Brazilian continental margin, some of which are contained in oil fields in which coquinas are one of the main reservoirs (viz. Campos Basin). The complexity and heterogeneity of these deposits make them a challenge in terms of reservoir description. For the necessary classification and paleoenvironmental interpretation of the coquinas, it is essential to evaluate many aspects including biological (such as carbonate productivity), sedimentological (energy regime in the depositional environment, transport of bioclasts, terrigenous supply), taphonomic (fragmentation of shells, abrasion) and diagenetic processes. The facies analysis applied in this study is considered a more appropriate classification approach to understand these coquinas, since it is more flexible and comprehensive than the existing classifications for carbonate rocks. The material investigated here consists of rock samples of the coquinas from the Atol Quarry of the Morro do Chaves Formation (Barremian/Aptian), Sergipe-Alagoas Basin. These rocks that crop out in the Atol quarry complex can be considered as a case study for similar coquinas reservoirs found in the Brazilian continental margin basins. Six sedimentary facies were described, using the main taphonomic (fragmentation of shells) and compositional (presence of micrite and siliciclastic matrix) features as a diagnostic criteria. Two carbonate facies, two mixed carbonate-siliciclastic facies and two siliciclastic facies (mudstones) were identified. From the facies succession, combined with a review of the literature on the subject, the following depositional paleoenvironments were defined: high-energy lake platform, lacustrine delta in a high-energy lake platform and lake-centre. In this paper, a new facies model for the studied coquinas succession is proposed.

Computational and experimental pore-scale studies of a carbonate rock sample

“…6f). Results suggest that the studied rock is composed mainly of calcium, oxygen and carbon (most likely Ca-CO 3 ), indicating that the coquina was probably formed by calcite as previously shown by Tavares et al (2015) from evaluations of coquinas from the same Morro do Chaves formation.…”

“…The coquinas were formed by bivalves and non-marine ostracodes with varying percentages of terrigenous deposits. As detailed by Tavares et al (2015), shells have different sizes and degrees of fragmentation, with facies in some cases containing bivalves in living or articulated positions. They noted that coquinas from Morro do Chaves have calcite as the main mineralogical component, while containing mostly mollusc shells.…”

“…The rock matrix is composed of clay, siliciclastic sand (mainly quartz), as well as some lithoclasts of igneous, sedimentary and metamorphic rocks. According Tavares et al (2015), six lithofacies can be identified in the Fig. 1.…”

“…For this many have used carbonate rocks samples from an outcrop in the northeast region of Brazil (Azambuja Filho et al, 1998), mostly to study their porosity and permeability properties. These samples, called coquinas (Tavares et al, 2015), contain large amounts of shells and as such exhibit considerable heterogeneity and anisotropy.…”

Petrophysical Rock Typing of Coquinas From the Morro Do Chaves Formation, Sergipe-Alagoas Basin (Northeast Brazil)

“…As noted by Câmara (2013), most of these carbonate reservoirs, responsible for a large portion of world's hydrocarbon production, originated from carbonates deposited in marine environments. In Brazil, the study of lacustrine carbonate depositional environments, mainly the Coquinas, has a long history with the discovery of Pampo, Badejo, Linguado and Trilha reservoirs in Campos Basin during the 70's, and are described by several authors with respect to various sedimentological, stratigraphic and petrophysical properties (Feijó & Pereira, 1994;Azambuja & Arienti, 1998;Campos Neto et al, 2007;Câmara, 2013;Tavares et al, 2015).…”