Palaeoenvironmental and Palaeogeographic Implication of Morro do Chaves Formation (Low Cretaceous of Sergipe-Alagoas Basin, Northeastern Brazil) to Marine Influence in Northeastern Brazilian Basins from the North.

Abstract

Calcarenitic and calcirrudaceous deposits (“coquinas”) of Morro do Chaves Formation are important carbonate potential reservoir, developed in the end of rift phase in Sergipe-Alagoas Basin, Northeastern Brazil. They have been studied for several years due to their palaeoenvironmental and stratigraphic similarity with bioclastic reservoir of Lagoa Feia Group, in Campos Basin (Southeast Brazil) and Toca Formation, west Africa, important oil reservoirs in these basins. This work is part of a multi-scale reservoirs characterization (macro, meso and micro scales studies) over Morro do Chaves outcrops occurrences, in order to better understand the permoporous systems distribution in this type of carbonate rock reservoir, as analogue to similar buried reservoir of pre-salt sequences in southeastern coastal Brazilian basins. An important aspect to consider on analogues outcrops reservoirs analysis consists in understand the palaeogeographic (tectonic-sedimentary aspects) and palaeoenvironments similarities between the considered deposits. Based in field studies and descriptions of 6 stratigraphic wells in the studied area associated with petrographic and palinological studies of 229 interbedded coquinas and shales' samples is possible conclude that this deposits were generated in a coastal subaqueous environment, controlled by cyclic tectonics and/or climate events that supplied terrigenous sediments aports. This conditions promoted a relative shallowing environment, allowing in these times the bioclastic carbonate deposition of Morro do Chaves Formation. The bioclastic deposits were reworked and redeposited in calcareous and calcirrudaceous beds interbedded with marls and mudstones deposits in deeper environments, under anoxic dominated conditions. Preliminary petrographic analysis identified crystalline carbonates, grainstones and packstones with bivalves mollusks and ostracods, bioclastic wackestones and hybrid sandstones (carbonate cemented). The presence of siliciclastic rock grains is related to proximity of the igneous-metamorphic source area. In palaeoenvironmental and paleogeographic terms, Azevedo (2004) and Arai (2009) indicate a marine influence from the north for the deposits situated in the north area of Sergipe-Alagoas Basin, where the outcrops of Morro do Chaves Formation were sampled for this work. Palinological studies identified, integrated with sulfur isotopes data, permitted interpreted the palaeoenvironmental context of these deposits as situated in a transitional position between continental to marine environments with sporadic marine ingressions from the north. Possible marine palinological elements identified as Prasinoficeas, palinoforaminifers, scenedesmus and undetermined algalic vesicles, will be applied to test the possible Tetian/North Atlantic invasion. Hot climate conditions can be identified by palinoflora, characteristic of the Brazilian continental margin during the lower Cretaceous.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015