Complex Petroleum Compositional Heterogeneities in Deep Offshore Settings in Brazilian Marginal Basins

Penteado, Henrique B.¹; Araújo, Laury M.; Hamsi, Gilvan P.; Santos, Rennan O.; dos Reis, Maria Anna A.; Garcia, Sávio F.; and Cupertino, José Antonio
¹[email protected]

Petroleum composition and physical properties are known to vary according to several processes. Source-rock and kerogen properties determine the composition of the expelled petroleum. API gravities, gas-to-oil ratios (GOR) and gas dryness tend to increase with source-rock maturity. Along upward migration pathways, petroleum phase separation might take place. During and after accumulation in a reservoir, processes like mixing, compositional vertical segregation, biodegradation, evaporative fractionation and remigration might lead to a very complex range of petroleum compositions.

Recent drilling results in deep offshore Brazilian marginal basins have provided evidence of petroleum systems with such large compositional heterogeneities. Contributions from more than one source rock (usually Lower Cretaceous lacustrine shales, and Albian to Turonian marine marls and shales) lead to the difficult task of unravelling mixtures using information from oil (gas chromatographic, biomarker and isotopic data) and gas (composition, carbon and hydrogen isotopes). However, even in areas where one marine source-rock level accounts for most of the generated petroleum, widely different compositions have been identified. Early-generated black oils might originally accumulate in shallow reservoirs, thus receiving a variable contribution of biogenic methane and being subject to a reduction in GOR depending on the lack of seal quality. Later pulses of lighter, more mature oils can mix to variable extents with previously accumulated oils, causing an increase in API and GOR. Further on, with source rocks generating gas in the late-oil and gas windows, upward moving gas interacts with earlier black-oil and volatile-oil accumulations, leading to complex gas composition and phase separation. Gas caps might break through seals and gas condensates pool in shallower reservoirs, and their final GOR will be a function of PVT properties and seal integrity. The overall petroleum composition and properties in each of a series of nearby stacked reservoirs will be the end-result of this complex interplay of processes. The distribution of fluid types as a function of migration distance from the source rock is far from straightforward. Therefore, predicting petroleum composition and phases in this type of setting by petroleum systems modeling is a very challenging task, requiring not only refined facies maps and petrophysical data, but also detailed compositional kinetic and PVT models.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013