Abstract: Molecular and Isotopic Oil-Source Correlation in the Primary Bahia Sul (Offshore Brazil) Petroleum System

Curiale, Joseph; and Steven Sperry - Unocal; Craig Schiefelbein - GeoMark

The Camamu Basin is located in the central portion of the Bahia Sul region, offshore Brazil. Numerous half-grabens characterize the structure of the basin (Mello, et al., 1995), and extremely organic-rich lacustrine source rocks were deposited during the Neocomian. As in other basins of coastal Brazil, Cretaceous lacustrine shales are the primary petroleum source rocks for the waxy, high pour point oils of this region. The lower Neocomian Morro do Barro Formation is considered to be the primary source rock in the basin (Mello, et al., 1995).

We have examined oils and rocks of the Bahia Sul, in an effort to define the main petroleum system by establishing a geochemically supported oil-source rock correlation. Selected Jurassic-reservoired oils of the Camamu basin (1-BAS-64 and 3-BAS-74 wells) were assessed using standard molecular and isotope geochemical techniques. In addition, rock samples from the sedimentary section in the southern part of the basin (well cuttings of the 1-BAS-71 well) were evaluated using geochemical logging methods, and using detailed molecular and isotopic techniques in selected organic-rich intervals.

Substantial oil source rock potential occurs in the Neocomian (TOC 2-5%; HI 400-900 mg/g), which is marginally mature in the 1-BAS-71 well (VR 0.5-0.8%). Although the oils of the Camamu basin are molecularly and isotopically similar to the extracted organic matter of the most organic-rich portion of the 1-BAS-71 well (approximately 100 km to the south of the oils), oil-source rock correlations between any single oil and any single extract are not exact. Minor molecular (predominantly biomarker) differences (Fig. 1), and much more striking d¹³C differences in n-alkane isotope profiles, show substantial variability within the 600+ m organic-rich portion of the Morro do Barro Formation in the 1-BAS-71 well. Such isotopic variability is consistent with previous studies of lacustrine source rocks (Curiale and Gibling, 1994). In the case of the Camamu Basin, contributions from several isotopically distinctive source horizons within the source interval must be invoked to reach a reasonable oil-source rock correlation conclusion. We present our results in the context of the relationship between the tectonic evolution of the area, and the development of the petroleum system that is defined by the oil-source rock correlation described here.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil