--> Abstract: Identifying Abnormal Geopressures Through Gas Chimney Detection, by David L. Connolly and S. Renee Bourque; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Identifying Abnormal Geopressures Through Gas Chimney Detection

David L. Connolly1; S. Renee Bourque1

(1) dGB Earth Sciences, Sugar Land, TX.

It has long been recognized that many of the Tertiary, Mesozoic, and Paleozoic basins are dominated by vertical hydrocarbon migration. This vertical migration is recognized in seismic data as vertically aligned zones of chaotic, often low amplitude, reflectivity described variously as gas chimneys, seepage pipes, blowout pipes, mud volcanoes, gas clouds or acoustic turbidity zones. The weak expression of these gas chimneys in seismic data make them difficult to map. A method for the detection of gas chimneys in post stack 3D seismic data was developed to map their distribution and to allow them to be visualized in three dimensions. This chimney probability volume is produced by a neural network from multiple seismic attributes extracted at examples of gas chimneys picked by the interpreter.

A major application of gas chimney processing is in detecting abnormal geopressure in areas where seismic velocity is unreliable. Seismic velocities, which are an important method for predicting pressure, are often observed in the well-imaged stratigraphic section outside of the gas chimney, rather than in the poorly imaged gas chimney itself. Gas clouds or gas chimneys are a mechanism for transferring deep pressures into shallower stratigraphic intervals through fractures in the overlying seal, and they can indicate the top of overpressure. We will show examples from the Gulf of Mexico, West Africa, and the North Sea, which show the close correlation between gas chimneys and excess geopressure.