Print this pageHigh Definition Erosional Map from RecôNcavo Basin, Brazil — A New Approach Based from Old Concepts
The petroleum province of Recôncavo Basin, located in northeastern Brazil, is an aborted continental half-graben formed during the fragmentation of the Gondwana supercontinent in the Lower Cretaceous. One of the main events that modified its physiography was the erosional process implanted at the end of the rift phase. This process was materialized by the Aptian unconformity. The main goal of this study was to evaluate the eroded thickness on the top of the Rift Sequence in that basin using in a different way a consortium of sonic log and vitrinite reflectance methods. The equation that describes the behaviour of the transit time with depth (here modified from that proposed by Heasler and Kharitonova, 1996) is an improvement, by analogy, of Athy's formula for porosity, with the addition of a constant term that represents the transit time in the fully compacted rock (ttmatrix): tt = Dtt exp (- c z ) + ttmatrix, where tt = observed transit time; z = depth; c = decay constant; Dtt =ttsurf - ttmatrix; ttsurf = transit time at the unconformity level; ttmatrix = transit time of the matrix (or compacted rock). The approach used was to compare the sonic log curves from each lithotype, with those that represent the original compaction condition with a more complete and preserved sedimentary section. To obtain an original curve it was necessary to add at depth column of the reference sonic log the erosion calculated by means of vitrinite reflectance method.The transit time of Post-rift and Pre-rift sequences were excluded of the mathematical regressions that were performed with the Petrobras proprietary program: Erosion; the Post-rift unconformity were placed at z depth equal to 0 (zero). The parameters of the original curves - the decay (c) and the transit time of the matrix (ttmatrix) - were fixed for each regression. This procedure avoids an overestimation of the erosion calculation, especially in cases in which a significant part of the original curve has been suppressed by erosive processes. A high definition erosional map was obtained for the Aptian event by applying that methodology at 114 sonic logs. An apparent tectonic control was observed by comparing the regional erosive features with the structural framework of Recôncavo basin.
HEASLER, H. P., KHARITONOVA, N. A., 1996, “Analysis of sonic well logs applied to erosion estimates in the Bighorn Basin, Wyoming”. The American Association of Petroleum Geologist Bulletin, v.80, n. 5, pp. 630-646.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil