Chapter 7: Integration of Gas-chimney Detection and Surface Geochemistry to Predict Hydrocarbon Occurrence: Neuquén Basin, Argentina
David Connolly, Roberto Garcia, Jose Luis Capuano, Graciela Prestia, 2013. "Integration of Gas-chimney Detection and Surface Geochemistry to Predict Hydrocarbon Occurrence: Neuquén Basin, Argentina", Hydrocarbon Seepage: From Source to Surface, Fred Aminzadeh, Timothy B. Berge, David L. Connolly
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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.
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With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.