Recôncavo Basin

Integrated Study of The Petroleum Systems

This study presents an integrated analysis of the petroleum systems of the Recôncavo Basin based on geochemically oriented studies that were recently performed by Geochemical Solutions International (GSI) in that area. The goal of this study is to provide a consistent and objective analysis of the petroleum systems of that region to be used as an exploration tool.

The spec surveys in which this work is based include a soil gas campaign, which collected 630 soil samples, geochemical data from 18 crude oil samples representing the major field in the Recôncavo Basin, and a basin modeling study performed along a East-West geological section.

The Recôncavo Basin is located in the Bahia State, northeastern Brazil, and currently produces about 55,000 barrels of oil equivalent per day, with estimated reserves of 564 million bbl oil. The Recôncavo Basin is divided into northern, central, and southern sub-basins by two major faults: the Itanagra-Araças and Mata-Catu faults. Petroleum system studies indicate that hydrocarbon generation has occurred within the lacustrine shales of the Candeias Formation, and that migration occurred mainly through fault planes and/or permeable zones from the Agua Grande System, as well as through direct contact between source and reservoir intervals.

The soil gas spec survey collected and analyzed 630 surface soil samples in ANP blocks BT-REC-1, BT-REC-2 and BT-REC-3. The sampling depth ranged from 1.2 to 1.5 m below the surface, in order to minimize interferences from biologically derived hydrocarbons that occur naturally in the soils. Soils were analyzed to determine the content and composition of "free" light hydrocarbon (C1-C5) gases at the Center of Excellence in Geochemistry (CEGEQ) in Rio de Janeiro. These data are treated statistically in order to identify "outliers" indicative of geochemical anomalies.

Onshore geochemical surveying is one of the oldest and most successful surface prospecting methods. It is based on the fundamental premise that light hydrocarbons generated from deep subsurface sources can migrate vertically to surface and near surface soils. The migrating hydrocarbons are manifested as chemical, physical or biological anomalies in the soils, which can be detected using a variety of means. When integrated with surface geological structures derived from analysis of satellite images, this approach represents a powerful tool for evaluating the exploration prospectivity of different regions within a basin. This approach has been used successfully in several of the onshore Brazilian petroliferous basins.

Results of Recôncavo soil gas survey include: 1. Samples Logs with location, sample depth, soil type, soil color, soil humidity, location relief and vegetation for each site. 2. Geochemical Data for 630 soil samples documenting light hydrocarbon concentrations and composition. 3. Maps of sample locations (including interpreted surface faults) and various parameters indicative of geochemical anomalies. 4. Data CD-ROM with database (MS Access 97) containing all analytical results and sample log information. 5. Interpretative Report in both printed and HTML formats describing the locations and magnitude of geochemical anomalies.

The results of these individual studies have been integrated in a petroleum systems context to define the principal source rocks and kitchen areas, migration pathways and hydrocarbon compositions expected for the Recôncavo basin.