Abstract Only subcommercial accumulations of heavy oil have been discovered in the Huasna Basin, which contains numerous active oil seeps and several source rocks. The petroleum geochemistry of a heavy oil sample from the only field in the basin and of five tar samples obtained at seeps has been determined and compared to the chemistry of core samples of Monterey and Rincon shales. The bulk chemistry and mineralogy of these Miocene shales can be used to identify several lithofacies, including phosphatic and siliceous shales containing variable amounts of detrital clay and carbonate minerals. Pyrolysis-FID and visual kerogen analysis demonstrate that Monterey and Rincon shales are good quality, oil-prone source rocks in the Huasna Basin. Thermal maturity modeling calibrated using bottom-hole temperature data and vitrinite reflectance measurements in several wells indicate that Monterey and Rincon source rocks are thermally mature in the axial syncline of the Huasna Basin. The tarry petroleum obtained at seeps is more severely biodegraded than a heavy oil sample from the Huasna field. The chemistry of these petroleum samples, especially their V/Ni ratios and sulfur concentrations, can be used to classify them into distinct groups. High-sulfur samples contain >4 wt. % sulfur and are enriched in V relative to Ni. Low-sulfur samples contain <2 wt. % sulfur and are enriched in Ni relative to V. One tar sample exhibits an intermediate chemistry. Sour oil and tars from the western flank of the Huasna Basin were generated by phosphatic Monterey shales or clay-poor siliceous Monterey shales containing sulfur-rich (Type II-S) kerogen. The sweeter petroleum seeping along the eastern margin of the basin was probably generated by more proximal, clay-rich Miocene shales containing Type II kerogen. Monterey lithofacies variations in the Huasna Basin and the distribution of subsurface oil and gas support this interpretation, as does the organic geochemistry of bitumen samples extracted from mature shales. Thus, inferred differences in Miocene oil quality within the Huasna Basin seem to have been influenced primarily by source effects (with a significant overprint of biodegradation) rather than by thermal maturity effects alone.