AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Integration of Evidence of Hydrocarbon Seepage from 3-D Seismic and Geochemical Data for Predicting Hydrocarbon Occurrence: Examples from Neuquen Basin Argentina
(1) dGB Earth Sciences, Sugar Land, TX.
(2) Capex SA, Buenos Aires, Argentina.
Detailed microbial surveys were acquired over several fields and discoveries in the Neuquén Basin of Argentina in order to detect overlooked oil and gas reservoirs. The technique involves analyzing shallow surface soil samples for a specific suite of hydrocarbon oxidizing microbes. Hydrocarbon micro-seepage, and the resultant microbial activity, is a very dynamic process. Depleted reservoirs quickly lose their microbial communities. Thus, these surveys can often detect non-depleted reservoirs. While microbial surveys are an effective measure of hydrocarbon micro-seepage, they have several limitations. First, while hydrocarbon micro-seepage is often vertical, it can be influenced by shallow faulting. Thus, the geochemical anomaly may be offset from the source of the anomaly. Second, we cannot tell from the microbial anomaly, the precise depth of the hydrocarbon reservoir. The Neuquén Basin has potential reservoir objectives, ranging from very shallow (<500 meters) to very deep (>5000 meters).
To provide a link from reservoir to surface we need to delineate these hydrocarbon migration pathways in the 3D seismic data. On seismic data, vertical hydrocarbon migration paths are generally recognized as vertically aligned zones of chaotic, often low amplitude reflectivity, described variously as gas chimneys or gas clouds. A chimney probability volume is produced by a neural network from multiple seismic attributes extracted at examples of gas chimneys picked by the interpreter. This volume can then be visualized in 3D, to determine where the hydrocarbons originated and where they migrated.Case studies from the Neuquén Basin in Argentina show a good correlation between strong geochemical anomalies and chimneys in which wells were drilled subsequent to the geochemical survey. However, many of the chimneys, and by inference, the geochemical anomalies, originate from very shallow intervals. These shallow zones may represent bypassed pay. There is also a good correlation between deeper pay and chimneys. Often these deeper chimneys do not have a direct correlation to the shallow geochemical anomalies. Shallow chimneys are also observed beneath areas with no geochemical signature. Chimneys leave a permanent imprint while microbial activity is very dynamic. Thus, the chimney can represent a seep which is no longer active or has been depleted by production.