Print this pageChemostratigraphy Applied to Paleozoic Black-Shale Intervals
The Brazilian Paleozoic basins show thick Silurian and Devonian shaly sections. These sections often display interlaminated mudstones/siltstones/very fine sandstones deposited in offshore conditions. Black-shales intervals with TOC values relatively high (> 4.0%) were detected in these sections. In homogeneous sections sometimes is difficult to identify the correct thickness of the black-shales intervals, using only facies analysis. The imprecise definition of the black-shale intervals make difficult the quantification of oil and gas generated in these basins. To solve this problem, a high-resolution chemostratigraphy study was initially carried out in sections located in the Paraná basin. Major, minor and trace elementar analysis, as well as organic geochemistry and isotopic studies were performed. The results point out to an anoxic/dysoxic depositional condition for the TOC rich intervals, which are related to flooding surfaces. This depositional condition is characterized by high TOC and HI values, rare bioturbation, low δ13Corg values and relative high concentration of organophyle chemical elements. The gas accumulations related to this interval normally display a dry composition, which indicate a generation at high maturation stage not reached by burial in the central area of the basin and, therefore, suggesting a non conventional process of generation related to heat effect of dolerite intrusions.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil