The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

2017-02-09T02:42:24Z (GMT) by Thompson, Daniel
Non-marine, Barremian-Aptian (Lower Cretaceous) Pre-Salt strata within South Atlantic rift basins represent a new frontier in petroleum exploration, where recent discoveries on the Brazilian continental margin have yielded billions of barrels. Nevertheless, little is known about the controls on the carbonate reservoir facies or the depositional processes and environments surrounding these enigmatic sediments, in part due to the scarcity of lacustrine carbonate reservoirs throughout the geological record, and, consequently, the paucity of research completed. Pre-Salt carbonates were deposited in unique Cretaceous greenhouse Earth environments and do not conform to traditional facies models of nonmarine carbonates, which are relatively simplistic and narrowly focused (being based on humid, hard-water lakes), hampering the development of accurate Pre-Salt exploration models. Extensive accumulations of coquina deposited on the margins of horst blocks and ramps are the primary focus of this study, which is based on stratigraphic successions in offshore drill cores from eight wells (extending across 180 km along strike) in the Campos Basin (southeast Brazil) and onshore outcrop in the Sergipe-Alagoas Basin (northeast Brazil). Based on detailed stratigraphic and sedimentological analysis of 130 m of core, the coquinas of the Campos Basin are dominated by bivalve molluscs that form wackestone to rudstone facies with subordinate gastropods and non-marine ostracod microfossils. Marine, venerid bivalves in local horizons are suggestive of intermittent marine incursions. Coquina is locally interbedded with thin beds of conglomerate, sandstone, mudstone, siltstone and marl, and, less commonly, talc-stevensitic oolites, which are indicative of magnesium-rich, alkaline (pH > 9) water chemistry. A wide variety of depositional environments were recognised in this study, including aeolian, mudflat and fan delta settings; however, lacustrine and marginal lacustrine facies are dominant.Coquina typically comprises allochthonous bioclasts that accumulated in shallow, high energy environments as bioclastic shoals and bars. Nonetheless, parautochthonous accumulations formed wackestone facies below wave base, basinward of the coquinas. Laminated siltstone was deposited in lagoons located shoreward of the coquinas. Coquina commonly forms fining- and coarsening-upward successions that range from 1 to 4 m thick. These are interpreted as shallowing-upward successions on the basis of localised subaerial exposure surfaces at the tops, characterised by silicification, in situ brecciation and iron-staining; however, they are most common in proximity to horst blocks. These successions reflect high-frequency lake-level fluctuations that are particularly prevalent due to the marginal lacustrine settings represented in these wells. Pre-Salt coquina seldom crops out onshore, but the Sergipe-Alagoas Basin contains thick successions that have not been studied in any detail for over 75 years. Detailed stratigraphic and sedimentological analysis of outcrops at the Morro do Chaves locality in the Sergipe Sub-basin led to the identification of three main facies associations (and several subassociations), including: 1) interbedded conglomerate and sandstone; 2) crossstratified coquina and sandstone; and 3) bioturbated micaceous sandstone. Strata at Morro do Chaves unconformably overlie phyllite basement and comprise coquina beds ranging from 20 cm to 2.5 m thick that exhibit tabular, trough and compound cross-stratification and locally contain up to 70% pebbles. Interbedded conglomerate and sandstone were deposited in north-northeast flowing braided channels on fan deltas, indicating an axial drainage pattern, or radial drainage pattern sourced from the border fault. Cross-stratified coquina and sandstone facies were deposited in shallow, high energy environments (with possible tidal influences) as bars and are dominated by allochthonous bioclasts. Episodic storm events promoted the development of washover deposits into lagoons located shoreward of the bars, where texturally immature, micaceous sandstone was deposited. Storm washovers are recognised by interbedded, coarse, bioclastic sediment that is often associated with impoverished Skolithos ichnofacies. Less commonly, thin beds of bioturbated micaceous sandstone were deposited in an open lacustrine environment, in association with rare parautochthonous bivalves Unlike the Campos Basin coquinas, there is no evidence to suggest that coquinas were exposed subaerially; however, localised coarsening-upward successions, 4 to 5.5 m thick, were identified and are characterised by open lacustrine micaceous sandstone at the base that passes up into cross-stratified coquina and conglomeratic sandstone. On a broader scale, the strata at Morro do Chaves exhibit retrogradational stacking patterns, where fan delta conglomerate is gradually replaced by lagoonal micaceous sandstone. A taxonomic revision of the molluscan fauna comprising the coquinas at Morro do Chaves was necessary due to significant advances in molluscan taxonomy since the previous studies over 75 years ago. This study identified a marine to brackish, bivalve-dominated fauna with subordinate gastropods and late Barremian ostracods from biozone 009. The molluscan fauna is dominated by small bivalves with stunted growth patterns (known as the “Lilliput effect” or dwarfism) due to brackish water chemistry, which is supported by the inclusion of oligohaline to mesohaline, non-marine ostracods (Theriosynoecum and Cypridea) within the coquina beds, and moderately diverse molluscan assemblages. The bivalve taxa identified in this study include Clisocolus, ?Izumicardia, Nicaniella, Astarte (?Leckhamptonia), ?Opis, Remondia, Arcopagella, ?Pollex, Eocallista, ?Resatrix, Cyprimeria, Legumen, Corbula, Corbula (?Flexicorbula), Thracia and several unidentified venerid bivalve genera. In comparison, gastropods are paucispecific and dominated by the neritid gastropod Lissochilus, with subordinate zygopleurid gastropods, and also exhibit dwarfism. The Pre-Salt molluscan faunas in the Sergipe-Alagoas Basin, and throughout the entire South Atlantic, have historically been assumed to be non-marine due to their association with non-marine ostracod microfossils; however, the identification of a brackish to marine molluscan fauna at Morro do Chaves indicates this supposition is no longer valid, and supports previous contentions that marine incursions episodically penetrated the rift during the late Barremian. This new research has greatly enhanced our knowledge on the depositional processes and environments of petroleum-bearing Pre-Salt carbonates by integrating drill core and outcrop studies. Moreover, this research has shown that while modern analogues for these environments may exist in harsh volcanically active rift settings, such as the East African Rift System today, the best analogues for the microbial carbonates may in fact be ancient ones, such as Proterozoic stromatolites and stromatolite-like structures deposited in soda oceans.