ESTUDO DE CICLICIDADE ORBITAL COM BASE EM DADOS SÍSMICOS - EXEMPLO PARA O MIOCENO DA BACIA DE CAMPOS

Abstract

The feasibility of using seismic data to evaluate the effects of variations of main orbital factors over the depositional cyclesis evaluated, in order to tum seismic into a major stratigraphy refinement tool. The concepts were tested in seismic and well data in Campos Basin, Brazil. First, it was performed a visual interpretation of the well sonic log, along with the biostratigraphic data, in order to split the section in segments with same accumulation rate. Spectral analysis treatments were applied to each segment to identify its most significant frequencies. The seismic trace that coincides with the position of the well was converted to depth, divided in interval sequivalent to those segments defined along the sonic log, and also submitted to spectral analysis. Next it was evaluated the correlation between the frequencies found in each data set and those of the phenomena (eccentricity, obliquity and precession) that affect the Earth orbit (MilankovitchTheory). The studied material was integrated, and the results were evaluated, considering the limitations of the tools used. Different levels of success were achieved in each segment, depending on the thickness of the prevalent cycle and the seismic resolution. The cycle thickness depends directly on the sedimentary accumulation rate, while the seismic data resolution varies conversely to the investigation depth, due to the absorption of high frequencies. It was possible, through the sonic well log spectral analysis, to identify cycles related to eccentricity (410 and 100ka), obliquity (41 ka) and precession (23 and 19 ka). The most influent phenomenon on the sedimentary accumulation seems to be precession, except for the sediments accumulated between the tops of biozones N-570 and N-580, where obliquity prevails. Accumulation rates varied from 4.5 cm/ka (N-560a) to 40 crn/ka (N-570b). The seismic trace spectral analysis allowed the identification of cycles related to eccentricity (100 ka). Where the segment was long enough, despite the low accumulation rate, it was possible to identify also the cycle related to the 410 ka period (N-560a). When there is a significant improvementin the accumulation rates, it is possible to correlate cycles even to the 41 ka period (N-570b), despite the relatively low seismic resolution (12.7m).