A Seismic-Structural Interpretation on the Identification of Possible Causes in the Formation of Gas Chimneys in Colombia’s Offshore

The presence of hydrocarbon seepages, both liquids and gases in the Colombian offshore, is a phenomenon that occurred naturally and continuously. In fact, it is a dynamical characteristic of hydrocarbon system due mainly to the vertical and/or lateral hydrocarbon migrations. Recently, this migration process has been approached by means of seismic attributes, specifically Gas Chimney. Geologically speaking, they could be treated as valves that release gases throughout faults and permeable sediments. Gas chimney has been matched with geochemical analysis coming from gases in piston core samples, thus generating a strong correlation with hard data de-risking offshore exploration analysis in most cases.

The main goal was to develop an integrated and comprehensive geological and geophysical analysis throughout 3D structural interpretation and attribute calculations, expecting to improve the knowledge and understanding of the nature of gas chimney formation and its possible relation with a petroleum system in a Colombia’s offshore area. Previous work in the area let the exploratory process with a moderate possibility of a hydrocarbon reservoir, and the gas chimney analysis played a relevant role on that positive statement, however the gas chimney seems to have some relation with the complex geological structure. A 3D structural interpretation was one of the focus of effort, trying to identify the role of faults in hydrocarbon migration, to detect the migration routes, and to find the seismic events that could be the source rock location. Other evidences were collected by means of sea floor observations, like pockmarks, mud volcanoes and mud diapirs.

Based on the seismic data, combined with the regional geological knowledge, we could determine a break-forward, shear imbricate fault-bend fold system. From the integrated analysis, we observed that the chimney didn’t have a large lateral extent, and exists along the whole thrust fault, which reveals that there is a large influence of the faults in the migration routes. Moreover, through the application and combination of seismic attributes, we could differentiate a preferential anomaly in the low frequency ranges, by the application of spectral decomposition analysis. This kind of anomaly has been found to be associated with gas reservoir in many cases worldwide. Finally, we detected on the seafloor the presence of typical sea floor morphological features, usually related with seepages of gas from the subsurface.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California