Abstract Detailed microbial surveys were acquired over the Agua Del Cajon and El Salitral fields in the Neuquén Basin of Argentina to detect bypassed oil reservoirs. Depleted reservoirs quickly lose their associated microbial anomalies. Thus, these surveys can often detect nondepleted reservoirs. Although microbial surveys are an effective measure of hydrocarbon microseepage, they have several limitations. First, hydrocarbon microseepage is predominantly vertical, but it can be influenced by faulting and offset from the source of the anomaly. Second, we cannot determine the depth of the suspected reservoir. To provide a link from reservoir to surface, we need to delineate the hydrocarbon migration pathways. On seismic data, vertical hydrocarbon migration paths are characterized by vertically aligned zones of chaotic reflectivity, called gas chimneys. These chimneys are detected by training a neural network using multiple seismic attributes and examples of gas chimneys picked by the interpreter. The resulting chimney volume is visualized in 3D seismic volume to determine where the hydrocarbons originated and where they migrated. Results of the study show a good correlation between strong surface microbial anomalies and shallow chimneys. Many of the chimneys have a deep origin in the gas-prone Jurassic Molles interval. There is also a good correlation between deeper Jurassic gas pay and these chimneys. The Late Jurassic Vaca Muerta, a thermally mature oil-prone source shale, provides an effective top seal for these deep pays. Chimneys, associated with the major east–west-trending thrust fault and northwest-trending shear faults, penetrate the Vaca Muerta interval. Cretaceous Quintuco carbonates and Centenario clastic oil reservoirs are closely associated with these chimneys. The deep chimneys are the means to expel oil from the Vaca Muerta shales into the shallow reservoirs. The combination of microbial surveys and chimney detection is being used to successfully discover bypassed pay in this area.

Abstract The lower Valanginian–lower Barremian Agrio Formation in the Mendoza region of the Neuquén Basin (Argentina) accumulated in a hemipelagic outer ramp/basin setting. The formation records the alternation of periods of fine-grained carbonate accumulation and periods of fine-grained clastic deposition, which resulted in a strongly rhythmic succession in which dark clastic hemicycles alternate with light carbonate hemicycles. Clastic and carbonate hemicycles show distinct geochemical signatures that reflect cyclic fluctuations in terrigenous influx to the basin. Clastic hemicycles have relatively high percentages of Fe, Al and K, and less than 60% CaCO 3 , whereas carbonate hemicycles contain relatively low quantities of detrital elements (Fe, Al and K), and more than 60% CaCO 3 . Fluctuations in terrigenous supply to the basin occurred in response to the alternation between two contrasting climate regimes and resulted in the formation of dilution cycles. Warm temperate (winter-wet) climate conditions led to the accumulation of clastic hemicycles, whereas arid conditions promoted the deposition of carbonate hemicycles. The ultimate mechanism that controlled the alternation of the dark and light intervals was orbital forcing, as the latitudinal shifting of climate zones was driven by the c. 21 000 years precessional cycle.