The elongated and sinuous mud volcanoesand their seismic expression (Gulf of Cadiz SWof Spain)

摘要

The Miocene Gulf of Cadiz basin, located offshore in SW Spain, can be geologicallydivided in two main areas: the eastern area, characterized by the presence of olistostromicmasses indented amongst the Miocene sediments, and the western area, free of olistostromes,and where biogenic gas is produced from Messinian turbiditic sands. Exploration in thewestern part has been made since the 80′s searching for bright spots supported by amplitudeversus offset (AVO) class III anomalies, with a success ratio of an 80%.In the late 90′s the revision in the eastern part of the old 2D seismic led to the interpretationof the possible presence of isolated Messinian turbiditic channels. This new play for the SEarea revived the interest for this under explored sub-basin (with only two wells drilled untilthe year 1982 with no sand reservoirs and numerous gas shows), and a 3D seismic was shotwith the idea to use AVO anomalies as indicative of the presence of gas charged sands.After the first new exploratory well, this work will show how misleading was this idea, sincethe interpreted Messinian channels sands turned out to be narrow, elongated and sinuousmud volcanoes emerging from the over pressured olistostrome.