Is there a petroleum system here? How extensive and effective is it? How is it defined? Although we had presented a 2005 AAPG poster to address these questions, we now have performed an exploration look-back or case study demonstrating basin-wide presalt charge across Brazil’s Santos Basin. Santos has been a disappointing gas province with meager results compared to the adjacent Campos Basin for the past two decades. We have reviewed and expanded presentations at AAPG and SEG conferences from 1998 to 2005, which were followed 17 months later by the supergiant Tupi discovery, now Lula Field. We document the progression of analyses and revision of interpretations as a case history for multidisciplinary work in a frontier region with, at the time, scant coverage of key data types. Despite our access to a broad range of material (oil and cuttings samples, piston core extracts, slicks analysis, regional seismic lines, potential field coverages, and published literature), only a handful of point samples directly fitted our hypothesis of a mature oil-prone presalt source, supported by our inference, from leakage at the basin margins, of basin-wide migration and charge. Although the volumes of data collected across the Santos Basin are orders of magnitude larger in 2019, with a concomitant improvement in understanding the petroleum system and overall basin evolution, we take pains to limit our focus to what was known as of mid-2005 (although perhaps published later), which still sufficed to point to the future success. Because the source presence and effectiveness are the first consideration in evaluating frontier basins, our methodology provides one template for understanding a key geologic risk. We emphasize the importance of careful screening of inputs when information is scant and thus erroneous inferences are easily reached, with the need to take an exploration inference wherever data, once cross-validated, direct the explorer.

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