XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA...
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XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA 2-6 de Noviembre de 2008 Isla de Margarita, Venezuela 1988 1988 1988 1988- - -2008 2008 2008 2008 20 AÑOS DE ALAGO 20 AÑOS DE ALAGO 20 AÑOS DE ALAGO 20 AÑOS DE ALAGO ORGANIC FACIES OF THE ALMADA CANYON DEPOSITS, ALMADA BASIN - BRAZIL Taíssa Rêgo Menezes 1 , Carla Viviane Araujo 1 , Igor Viegas A. F. de Souza 1 1 PETROBRAS/CENPES/PDEXP/GEOQ, Rua Horácio Macedo nº 950, Cidade Universitária, Ilha do Fundão, Rio de Janeiro, 21941-915, Brazil, [email protected] INTRODUCTION The Almada Basin, located in Bahia State, is associated with the rifting that resulted in the break-up of South America and Africa and the development of the Brazilian marginal basins. It is limited to the north by Itacaré High that separates it of Camamu Basin, and to the south by Olivença High, that separates it of Jequitinhonha Basin. The onshore Almada Basin’s area has 200km 2 and maximum thickness of sediments of 1.800m, and offshore area reaches a larger extension, with about 1300km 2 and sedimentary column with 6000m of thickness (Brunh & Moraes, 1989). This study comprises palynofacies and bulk geochemical analyses from the onshore portion, which is characterized by canyon filling deposits. This sedimentary section range in age from Campanian to Maastrichtian and are represented by deposits of turbidite channels from Urucutuca Formation (Brunh & Moraes, 1989; d’Avila et al., 2004). According to d’Avila et al. (2004), the Almada canyon is a submarine conduct of tectonic origin that was enlarged by repeated passing of turbidity currents. The sedimentary section presents moderately to high efficient turbidites intercalated to prodelta pelites and low density plumes deposits. METHODS The palynofacies characterization was based on qualitative and quantitative analyses of the particulate organic matter according the procedures described by Tyson (1995) and Mendonça Filho (1999). Microscopy analyses were performed on 47 pelites core samples from Almada-F1 well using transmitted white light and incident blue light on strewn slides of isolated organic matter. Palynofacies analysis involves the identification of the individual particulate components (palynomorphs + phytoclast + amorphous organic matter - AOM), their absolute and relative proportions and their preservation states, which is integrated study of all aspects of the organic matter assemblage (Combaz, 1964; 1980). In addition, bulk geochemical analyses (Total Organic Carbon - TOC%, Rock-Eval pyrolysis) were carried out to correlate with microscopic data with the objective of aiding in the paleoenvironmental assessment. RESULTS AND DISCUSSION The three main groups of morphological constituents can be recognized within kerogen assemblage: phytoclasts, palynomorphs and AOM (Figure 1). The studied samples were characterized by the dominance of the phytoclast components, which derived from the ligno-cellulosic tissues of terrestrial macrophytes (> 80%). The high values of terrestrial macrophytes debris dilute all other components.
Transcript of XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA...
XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA 2-6 de Noviembre de 2008 Isla de Margarita, Venezuela
1988198819881988----2008200820082008 20 AÑOS DE ALAGO20 AÑOS DE ALAGO20 AÑOS DE ALAGO20 AÑOS DE ALAGO
ORGANIC FACIES OF THE ALMADA CANYON DEPOSITS, ALMADA BASIN - BRAZIL
Taíssa Rêgo Menezes1, Carla Viviane Araujo1, Igor Viegas A. F. de Souza1
1PETROBRAS/CENPES/PDEXP/GEOQ, Rua Horácio Macedo nº 950, Cidade Universitária,
Ilha do Fundão, Rio de Janeiro, 21941-915, Brazil, [email protected]
INTRODUCTION The Almada Basin, located in Bahia State, is associated with the rifting that
resulted in the break-up of South America and Africa and the development of the Brazilian marginal basins. It is limited to the north by Itacaré High that separates it of Camamu Basin, and to the south by Olivença High, that separates it of Jequitinhonha Basin. The onshore Almada Basin’s area has 200km2 and maximum thickness of sediments of 1.800m, and offshore area reaches a larger extension, with about 1300km2 and sedimentary column with 6000m of thickness (Brunh & Moraes, 1989).
This study comprises palynofacies and bulk geochemical analyses from the onshore portion, which is characterized by canyon filling deposits. This sedimentary section range in age from Campanian to Maastrichtian and are represented by deposits of turbidite channels from Urucutuca Formation (Brunh & Moraes, 1989; d’Avila et al., 2004). According to d’Avila et al. (2004), the Almada canyon is a submarine conduct of tectonic origin that was enlarged by repeated passing of turbidity currents. The sedimentary section presents moderately to high efficient turbidites intercalated to prodelta pelites and low density plumes deposits.
METHODS
The palynofacies characterization was based on qualitative and quantitative analyses of the particulate organic matter according the procedures described by Tyson (1995) and Mendonça Filho (1999). Microscopy analyses were performed on 47 pelites core samples from Almada-F1 well using transmitted white light and incident blue light on strewn slides of isolated organic matter.
Palynofacies analysis involves the identification of the individual particulate components (palynomorphs + phytoclast + amorphous organic matter - AOM), their absolute and relative proportions and their preservation states, which is integrated study of all aspects of the organic matter assemblage (Combaz, 1964; 1980).
In addition, bulk geochemical analyses (Total Organic Carbon - TOC%, Rock-Eval pyrolysis) were carried out to correlate with microscopic data with the objective of aiding in the paleoenvironmental assessment. RESULTS AND DISCUSSION
The three main groups of morphological constituents can be recognized within kerogen assemblage: phytoclasts, palynomorphs and AOM (Figure 1). The studied samples were characterized by the dominance of the phytoclast components, which derived from the ligno-cellulosic tissues of terrestrial macrophytes (> 80%). The high values of terrestrial macrophytes debris dilute all other components.
XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA 2-6 de Noviembre de 2008 Isla de Margarita, Venezuela
1988198819881988----2008200820082008 20 AÑOS DE ALAGO20 AÑOS DE ALAGO20 AÑOS DE ALAGO20 AÑOS DE ALAGO
The components of the Phytoclast Group are present along the entire analyzed section and it was observed that there are variations in its content. These variations are well recorded in each palynofacies interval marked by the presence of the disconformities reported in d’Avila et al. (2004) (Figure 2). However, the relative abundance of non-opaque phytoclast is higher than opaque woody material in all section, reflecting the selective preservation of phytoclasts in more proximal-trend.
The Palynomorph Group is characterized mainly by sporomorphs and freshwater algae (Botryococcus + Pediastrum), the marine elements occur in very low concentration (autotrophic and heterotrophic dinocysts, prasinophytes and foraminiferal test linings). Beside this, it was observed the presence of the spore massulae from water-fern (Azolla -fossil remains of the aquatic pteridophyte).
The great majority of phytoclast particles are reflecting their high supply and their preferential preservation or preferential sedimentation. It is clear that on geological timescales, global peaks of terrestrial organic material coincide with major sea-level lows when the shelf is relatively narrow or the shelf edge margin is cut by submarine canyons. The highest phytoclast abundance is also strongly related to the proximity to fluvial-deltaic source and during the high runoff period (Muller, 1959; Habib, 1982; Summerhayes & Masran, 1983; Summerhayes, 1987; Tyson, 1984, 1995).
The percentage of phytoclast is also significant in oxidizing situations, where the highly resistant kerogen component is the only organic material preserved.
The presence of the water-fern Azolla has an important implication, because it shows a pronounced proximal-trend, once these elements occur in calm waters, tropical-temperate warm climate. The proximal-trend is also indicated by the occurrence of Homotryblium (dinocyst genera), that suggests restricted neritic to inner neritic environment (Brinkhuis, 1994; Dale, 1996; Van Mourik & Brinkhuis, 2001; Ramírez, 2004).
In general the TOC values are average (>1%). These values associated with enriched plant debris have been also found in sediments on submarine canyons. This may represents both higher sediment accumulation rates and the bypassing of terrestrial debris from shelf to the deep sea via these channels (Gross et al., 1972). Beside this, TOC contents are also related to an abundant organic matter input to the section. The Hydrogen Indices are low and Oxygen Indices are high reaching up 330 mg CO2/g TOC, characterizing Type III kerogen.
The fluorescence characteristics are useful in identifying organic facies. In this study, it was observed that some samples reveal moderate percentage of non-fluorescing or weak fluorescing AOM, that indicate oxic conditions and/or it could be derived from degraded terrestrial macrophytes. In these samples, the palynofacies association may suggest a low-energy deltaic facies, according to criteria described by Tyson (1993).
According to Jones (1987) and Jacobson (1991), the bulk geochemical parameters correspond to kerogen found in all systems tract incluing the fine-grained parts of turbidites in lowstand systems tracts and the silts and muds on highstand prograding sediments. CONCLUSION
Integration of data has shown a typical organic facies deposited in a highly proximal marine paleoenvironment under oxic conditions.
XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA 2-6 de Noviembre de 2008 Isla de Margarita, Venezuela
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ACKNOWLEDGEMENTS The authors wish to thank the geologist Roberto Salvador Francisco d’Ávila for
providing the samples for this study.
Figure 1: Organic components recognized in kerogen assemblage from Almada-F1 well. A to D: Predominance of Phytoclast Group; E-F: Pollen grain; G: Spore; H: Pediastrum - freshwater algae; I-J: Heterotrophic dinocysts; K- L: Autotrophic dinocysts; M: Foraminiferal test lining; N-O: Spore massulae from water-fern (Azolla). All photomicrographs were taken using transmitted white light, in exception E, H, K O, in fluorescence mode.
XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA 2-6 de Noviembre de 2008 Isla de Margarita, Venezuela
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Figure 2: Palynofacies and TOC parameters showing variations on the percentage of some groups of kerogen along the analyzed section. REFERENCES
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