Geochemistry of Natural Gases from Mud Volcanoes and Surface Gas Seeps in NW-Colombia

Abstract

Mud volcanoes and natural gas seeps are common in the southwest-northeast-trending Sinu Atlantico basin (SAB) and San Jacinto Fold Belt (SJFB) of NW-Colombia. The structural subunits are part of a highly complex active continental margin where the sediments become increasingly younger to the west from Late Cretaceous to Early Tertiary in the SJFB to Late Tertiary in the SAB. Some of the mud volcanoes are permanently active and form huge structures and are often aligned along major faults. Both seep and mud volcano gases are with low C{sub 2}-C{sub 4} and CO{sub 2} contents. Carbon and hydrogen isotope concentrations allow the subdivision into four groups of gas. These gases can be divided into four genetic based on their carbon and hydrocarbon isotope compositions. The isotopic variability of the different groups attests to the fact that very different gas-forming processes are or have been operating in the subsurface ranging from low-temperature thermogenesis. The various groups show, with few exceptions, a distinct regional distribution: Mud volcanoes and seeps with bacterial gases are restricted to the southern part of the SAB where the greatest thickness of young sediments is observed; gases of mixed thermogenic and bacterial origin are found in the coastal areasmore » of the northern SAB; gases of thermogenic origin are predominantly observed in the SJFB; the thermogenic gases of Group 3 are restricted to two locations in the north of the SJFB. This regional association of genetic gas types with specific geotectonic units reflects different thermal histories of the respective tectonic areas and allowed the authors to delineate prospective areas for oil in NW-Colombia.« less