The main purpose of a critical basin analysis is the discovery of those factors which can be held accountable for the presence or absence of hydrocarbons in a given basin or group of basins. The word "critical" which was inserted in the title of the panel stresses the fact that one has not only to look for those factors which are known to have contributed positively-or for that matter negatively-but that we also must remain aware of the possibility that there are numerous factors we have not even thought of in the presently prevailing theories.
The first three papers of the panel discuss distinct hydrocarbon habitats, and describe and analyse the parameters which have controlled the presence and distribution of their hydrocarbon accumulations. The last two papers attempt to compare various basins, and to come to an understanding which factors control in the various settings the size and type of the accumulations, in the hope that such an analysis might lead to a useful predictive tool in the search for hydrocarbons.
The first paper, by C. W. M. CAMPOS, K. MIURA and L. A. N. REIS, read by K. Miura, dealt with the East Brazilian continental margin, where several basins occur separated by basement arches. The common tectono-sedimentary evolution of these basins was stressed, and the influence of the different stages of development on the hydrocarbon accumulations was illustrated by several examples.
Of particular interest is the development of source rocks corresponding to the successive tectonosedimentary stages, and on a question by G. D.
ROBERTS (U.K.), Mr Miura specified that the best source rocks are apparently the lacustrine beds of the Neocornian, deposited in taphrogenic depressions. A corresponding concentration of the accumulations is observed in the pre-Aptian section. The post-Aptian sequence contains source rock horizons in the marine shale section, to which the accumulations in this zone are attributed.
The author said that the geochemical criteria used to define these source rocks are the quantity and quality of organic matter and kerogen.
In reply to a question by R. J. WEIMER (U.S.A.), Mr Miura affirmed that undercompacted overpressured shales do occur at the base of the up to 3700 m thick Upper Cretaceous-Tertiary section, associated with growth faulting.
The second paper, by I. P. ZHABREV, I. P.
ZUBOV, N. A. KRYLOV and V. V. SEMENOVICH, and read by T. LOKHOVA, dealt with the comparative evaluation of the hydrocarbon potential of the Epi-Paleozoic basins of the USSR. In this paper an outline was given of the sedimentary sequences filling the various basins, and their common tectonodepositional setting. The repartition of the hydrocarbon accumulations was described, and some controlling fa
