Aplicação da estratigrafia de sequências na seção albiana da porção sul da Bacia de Santos / Aplication of the stratigraphy of sequence on the albian section of the south portion of the Santos Basin

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

The main goal of this work is to detail the lower / middle Albian carbonatic section of Santos Basin based on the principles of Chemostratigraphy and Sequence Stratigraphy. Because this section was deposited under intense stress conditio ns (high salinity), its fossil content is very poor . It has limited the chronostratigraphic subdivision based only on biostratigraphic data. In order to get this detailing and also a better understanding of the depositional history of this section, conventional (well log data, lithological and biostratigraphic data from drill cuttings, cores and cuttings and seismic data) and non conventional (carbon and oxigen isotopes curves; total organic carbon; sulfur; major, minor and trace elements det erminations and analysis of sedimentary cycles) data used in the petroleum industry were integrated in the current study. An area with 16.000 km, where the equivalent section of Lower / Middle Albian (about 7 million years) is well developed, located in the southern part of Santos Basin, was selected to evaluate the proposed methodology. The Albian Andar itself extends to approximately 12 million years (ICS, 2006) and its sedimentary section reaches almost 1500 meters of t hickness, of which ninety percent belong to the lower / middle Albian section that remains without a proper biostratigraphic subdivision. On the other hand, the Upper Albian (about 5 million years) and the Cenomanian (about 6 million years) are represented by very thin equivalent sections in the study area. The first one is represented by a condensed section, typically deposited in deep waters, resulting from a marine transgression that completely drawn the carbonatic platform during the end of the Middle Albian. Fine grained siliciclastics sediments remaining from the well known Turonian unconformity characterize the Cenomanian section. In the southern part of Santos Basin, the Turonian section is represented by a radioactive shale related to the well known and wor ld wide anoxic event, which is the source rock for most of the petroleum already found in the world. For this work, well log, core and drill cuttings data from 12 wells, where the studied chronostratigraphic interval is better represented, were selected, as well as seismic data connecting these wells. The chemostratigraphic studies were concentrated in 2 wells, named well X and Y, which sampled the most complete lower / middle Albian section in the study area, including the top of the deeper Aptian evaporitic section. Samples for chemical analysis were collected from drill cuttings and cores in both wells. As a result of the present work, a subdividion of the Albian Cenomanian section into 6 depositional sequences was obtained through the int egration of chemical analysis data with well log data, based on the basic concepts of sequence stratigraphy. As these sequences represent a geological time relatively long, they can be classified as third orders sequences, since they have correspondence on seismic data. Correlation of the identified sequence boundaries in the wells X and Y, via synthetic seismograms, showed that these boundaries have a good seismic representation, which allowed the picking of these events towards the others selected wells and confirmed the proposal classification.The additional data obtained by the chemical elements analysis, besides to corroborate previous interpretation, were particularly important for the paleo environmental characterization of t he identified sequences. This shows that the proposed method is particularly appropriated for areas where the exploratory risk assessment requires a better understanding of the environmental onditions present durin the deposition of a given sedimentary section. The encouraging results obtained by this work also point to the strong potential of drill cuttings samples, which are always available in all wells drilled by the oil industry and which can bring a great amount of analytical information to the exploration process at low additional cost.

ASSUNTO(S)

estratigrafia geochemistry geologia stratigraphy santos basin geoquímica bacia de santos

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