ABSTRACT: Implications of Stratigraphic Variability in Petroleum Source Rocks from the Potiguar Basin, Brazil, for Oil/Source Correlation and Migration Effects

TRINDADE, L. A .F., Stanford University, Stanford, CA, and Petrobras, S. C. BRASSELL, Stanford University and Indiana University, Bloomington, IN, and E. V. SANTOS NETO, Petrobras/Cenpas/Divex, Rio de Janeiro, Brazil

The Aptian stage of the Potiguar basin, northeast Brazil, is characterized by rocks deposited in a lagoonal environment subjected to marine incursions. One drilled core of this sequence recovered thermally mature organic-rich shales interbedded with carrier bed sandstones. Both gross organic geochemical properties and source-dependent biomarker parameters of the shale show major variations reflecting depositional conditions that, in turn, controlled the type and abundance of biological contribution of biomarker precursors. Such source rock heterogeneities preclude the recognition of depletion trends in hopane and sterane concentrations associated with primary migration from the center of the shale to the contact with the underlying sandstone. An oil show recovered from the sandstone d splays biomarker source-dependent ratios that approach values which are the average of those observed in the overlying shale source beds. However, their higher maturity parameters suggest contributions from a deeper and more mature source rock, or demonstrate the preferential migration of more mature components. Comparison of maturity-dependent biomarker parameters and the porosity of the carrier beds indicate that the higher porosity sandstones are filled with less mature oil, demonstrating their earlier migration to more porous sandstones. The sandstones overlying the shale are well cemented and tight, and therefore are not filled with oil. These results are consistent with previous observations that oil movement proceeds in the direction of least resistance.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)