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Abstract: Balanced Filled Lakes Worldwide: Insights for Optimum Source Character and Distribution in Brazilian Continental Margin Basins

Bohacs, Kevin M., and Neal, Jack E. - Exxon Production Research Co.

Balanced-filled lake systems contain the most prolific lacustrine source rocks and beneficent facies juxtapositions for hydrocarbon accumulation, based on observations of lacustrine strata of many different ages and basins (e.g. Quaternary of east Africa, Tertiary of USA & Asia, Cretaceous of China & Africa). Published data from Early Cretaceous lakes of the Brazilian continental margin indicate that, although each area contains various lake-basin types, their dominant source facies are in Balanced-filled lake systems. Organic-rich rocks are best developed in this lake-basin type due to an optimal interaction of productivity, preservation, and concentration.

Balanced-filled lakes are one of three lake-basin types (Overfilled, Balanced-filled, and Underfilled) recognized from recurring lithofacies associations and stratal stacking patterns. Lake type derives from interpreted interplay between potential accommodation and sediment+water supply over sequence-set time scales. These factors are approximately equal over time in a Balanced-filled lake system. The concept of lake-basin type is useful for sorting out the complexities of lacustrine deposition to derive a predictive framework. Each lake type has distinctive facies and stacking patterns at the sequence and para-sequence scale that reveal the link of sediment flux to changing lake level. Understanding this link leads to predictability in the sequence stratigraphy and distribution of hydrocarbon system play elements in each lake type.

Many limnogeologists have noted the common occurrence of lake basins in the rock record with depositional environments that evolve from fluvial, through deep, then shallowing lake, back to fluvial. This "lake-sandwich" evolution is usually attributed to tectonics or climate. We observe lake type to evolve as a function of both tectonics and climate, with climate especially controlling the type of lake during maximum subsidence rate. Basins with dry climates commonly evolve from fluvial (Overfilled) to deep lake (Balanced-filled) to evaporitic lake (Balanced-filled to Underfilled). The Campos Basin fits this evolution as paleoclimate was dry during the rift phase. Composite well logs through the Campos Basin clearly show this evolution and its impact on source facies (Fig. 1). The organically enriched Late Jiquiá interval represents the Balanced-Fill phase. From seismic data, the rift fill evolves from a chaotic, low amplitude, areally restricted seismic facies to a higher amplitude, continuous to semi-continuous, parallel-bedded and more widespread package (Fig. 2). This transition marks the evolution of lake types from Overfilled to Balanced-filled and Underfilled. Basins with more humid climates may evolve only to Balanced-filled at peak subsidence (e. g., Recôncavo Basin). Balanced-filled lakes can be shallow or deep with thick or thin sequences but they share similar geochemical, biofacies, sedimentological, and sequence-stratigraphic attributes. Sequence boundaries are formed by a mix of erosion and desiccation- erosion may be best developed during transgression. Lake-water chemistry varies systematically between fresh and saline/alkaline. Most organic-rich rocks are deposited in the profundal zone, with subordinate amounts in the lake plain behind mixed biogenic- clastic shorelines. Organic matter is dominantly algal Type I, typically with TOC < 30% and HI < 750 mgHC/g C. Organic facies are relatively constant laterally, changing only relatively close to shore.

Comparisons of geochemical, geological, and geophysical data from Balanced-filled lakes worldwide with Brazilian basins provides a greater understanding of source rocks in the lacustrine systems responsible for most of the giant oil accumulations in offshore Brazil. This integrated geological-geochemical model allows prediction of not only hydrocarbon quality, but also source character and distribution, to serve as input for analysis of migration and trap fill.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil