Deep Imaging Seismic Data used for Maturation Modeling along the Brazilian Equatorial Margin

Estimating the present day oil generation window in the deepwater along continental oceanic boundaries (COB) is difficult due to rapid changes in crustal type and thickness. In this study on the Brazilian Equatorial Margin , we have used a method that avoids the assumption of steady state, which typically requires calibration to wells (BHT, Ro, surface HF). We have interpreted deep (15-40 Km) reflectors imaged on Greater BrasilSPAN (18 sec twt, 40 Km) pre-stack depth migrated (PSDM) reflection seismic data to determine crustal type and thickness (depth to Moho). Our modeling matches the available well data and conforms to the deep geology. This adds confidence to the modeling of the oil window in deepwater which lacks wells.

The last segment of continental crust to break in the separation of South America from Africa was along the equatorial margin. The crust here is very old (~1-2 Ga) and thick (>40 Km), resulting in brittle breaks initiated along continental strike slip faults which subsequently developed into the transform margin. Along this COB, thick sections of cold continental crust (25-40 Km) remain adjacent to thinner (7-12 Km) oceanic crust. In this deepwater setting that is generally without wells, conventional methods of calculating the oil window based on spot estimate of surface heat flow, do not adequately account for these rapid changes in crustal type and thickness.

We have interpreted deep (15-40 Km) reflectors within the continental crust for estimating the upper and lower crustal thickness which are used for calculating radiogenic heat generation. The Moho (base of crust) under both the continent and oceanic crust has been imaged, and the 1330o C isotherm within the upper mantle has been used as a lower boundary condition and the temperature of the seawater as the upper boundary. Conventional seismic data (~8-12 sec twt), even in combination with potential fields data, can’t provide the resolution to model these variations within the crust.

This method of modeling is providing new areas where potential source rocks are in the present day active oil window. Older Jurassic sediments that are on thick continental crust have remained immature until recent burial. A band of Turonian sediments deposited as the first sediments on oceanic crust appear to be in the present day oil window due to the higher temperature gradient associated with the more shallow Moho under oceanic crust.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California