Tectonic Controls on Very Thick and Laterally Confined Microbialites in the Pre-Salt Petroleum System of the South Atlantic
Zalán, Pedro V.
In the last six years explorationists became familiar with an unusual type of carbonate reservoir that yielded several discoveries of giant/super-giant fields in the Brazilian margin of the South Atlantic, namely microbialites. Recently, discoveries were announced in the Angolan margin. Several depositional models have been constructed in order to explain the distribution of so many uncommon carbonate facies, their high porosities and permeabilities, their occurrence just below thick packages of salt and their widespread distribution in huge sag basins, which were erroneously thought to be post-rift in nature. Models vary from classical carbonate ramp depositional systems, where sedimentary facies is a function of energy level and carbonate production, to more variable settings specific for different stromatolite occurrences throughout the world. The different models share two aspects: First, they all agree that the microbialites were deposited in hypersaline lakes in very arid climates. Second, they all lack a deep comprehension of the importance of tectonic control on the facies distribution, far beyond the common wisdom of syn-depositional structural highs controlling higher energy carbonate facies.
Recent studies have shown that the breakup of Gondwanaland in the vicinities of the Santos and Campos Basins occurred just after the deposition of the salt, around 111 Ma. They also demonstrated that the process of rupturing a mega continent (the rift phase) involves several episodes of intensive faulting, which are transgressive in time and space. Faulting can be switched off in a certain area while continuing far away. Rifts may be coeval and laterally adjacent to thermal sag basins. Sag basins may be sandwiched between fault-related growth strata. In this sense, both the microbialites and the salt were deposited during the rift phase, in different geometrical settings: in sag basins, in active grabens or in sag basins reactivated by normal faulting. It is exactly this high degree of faulting synchronous with the deposition of microbialites that can lead to the formation of huge vertical carbonate buildups related to bacteria thriving on thermal springs flowing along active faults. Various minerals can occur: the dominant carbonates, silica and barite. Flows may also be highly corrosive and could be responsible for the high degree of secondary porosity present in microbialites deposited in "normal" carbonate environments.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013