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

BEEUNAS, M. A., Chevron U. S. A., New Orleans, LA, M. SCHOELL, Chevron Oil Field Research Company, La Habra, CA, and C. BEROIZ, Chevron Overseas Petroleum Inc., San Ramon, CA

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 very few exceptions predominantly composed of methane with low C(2)-C(4) and CO(2) contents. Carbon and hydrogen isotope concentrations allow us to subdivide four groups of gas. These gases can be divided into four genetic groups based on their carbon and hydr carbon isotope compositions as follows:

                          C(13)%                D(%)
(1)Bacterial            -67 to -63          -200 to -180
(2)Mixed                -60 to -53          -195 to -175
(3)Thermogenic 1        -53 to -44          -160 to -200
(4)Thermogenic 2        -43 to -39          -162 to -128

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 bacteriogenesis to high-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 (possibly a mixture of Groups 1 and 3) are found in the coastal areas of the northern SAB; gases of thermogenic origin (Group 3) 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 asso iation of genetic gas types with specific geotectonic units reflects different thermal histories of the respective tectonic areas and allowed us to delineate prospective areas for oil in NW-Colombia.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)