LACUSTRINE PETROLEUM SYSTEMS: A HISTORICAL PERSPECTIVE OF EXPLORATION CONCEPTS AND RESULTS IN BRAZIL

Lacustrine petroleum systems have played a major role since the onset of exploration in Brazil. The first oil discoveries in the onshore Recôncavo Basin in NE Brazil in the 1930’s were associated with syn-rift lacustrine black shales of the Early Neocomian. Later, oil fields were discovered in the clastic pre-rift and syn-rift sections of the Sergipe and Potiguar onshore basins.

Since the late 1960’s, petroleum exploration in Brazil shifted to the offshore marginal basins. Starting with the Campos Basin in 1974, large oilfields have been found in Neocomian to Miocene reservoirs.

The confirmation that most petroleum was sourced by Early Cretaceous syn-rift lacustrine source rocks was made possible with the application of Organic Geochemistry in the 70’s. Geochemical data allowed to identify source rocks with type I kerogens, with slightly older ages in onshore NE Brazil than in offshore SE Brazil.

Sophisticated geochemical analyses in the 80’s allowed greatly improved oil-rock correlations. Biomarker and carbon isotopic data led to the recognition that source rocks in NE Brazil were deposited in anoxic freshwater lakes, as opposed to brackish-saline water anoxic lakes in the continental margin.

Numerical modeling of lacustrine petroleum systems became applicable with the advances in computing in the 90’s. Kinetics of kerogen cracking provided the input data to calculate the timing and volumes of generated petroleum. The focus later shifted to the investigation of migration pathways and to the prediction of petroleum phase and quality with compositional kinetics. Lacustrine-sourced oils in Brazil are mostly black oils, with API gravities ranging from 15 to 42. Biodegradation in shallow reservoirs is a major process affecting the composition and properties of lacustrine-sourced oils. Evaporative fractionation due to flushing of oils by late gas, with the formation of overlying gas condensates, has been identified in basins where lacustrine source rocks attained the gas window.

The integrated interpretation of better seismic and well data, coupled with petroleum systems modeling, ultimately led to the discovery of the giant pre-salt accumulations in the Santos Basin. Starting with the Tupi wildcat in 2006, several fields with oils generated by lacustrine source rocks have been found in the Pre-salt Province.