Geochemical and Biostratigraphic Characterization of an Upper Cretaceous Organic-Rich Condensed Section in the Santos Basin, Brazil

L. A. F. Trindade, E. Porsche, H. L. B. Penteado, J. Botelho Neto, C. V. Araujo, M. C. Viviers, and L. C. V. Oliveira

The Cenomanian-Turonian section is worldwide recognized as an organic-rich interval associated with a widespread anoxic event. Samples from this interval, representing proximal sedimentary conditions have been well characterized in the Santos Basin, Brazil, where it reaches 300m thickness. The main objective of this study is to characterize the geochemical and paleoecological attributes as well as to ascertain the biostratigraphic position of this organic-rich section in a more distal location, at ODP Site 356.

Biostratigraphic analyses identify the interval at ODP Site 356 as a condensed section ranging in age from Cenomanian to Coniacian (11m), deposited under anoxic conditions associated with a warm, arid climate in the adjacent continental areas. Geochemical analyses indicate high total organic carbon contents (up to 6.7%), dominance of algal derived material, high relative abundances of homohopanes and methylsteranes, ^dgr¹³C around -27^pmil, indicating a typical marine anoxic section. Despite the high content of oil-prone organic matter, this section has not attained thermal evolution appropriated for hydrocarbon generation due to its shallow burial.

In proximal areas of the basin, this section is interpreted as a transgressive system tract which is composed by a series of sequences, that are enriched in organic material at their bases. Basinward the interval is thinner, but presents higher organic carbon contents according to geochemical data of ODP Site 356, typical of a condensed section.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California