--> Abstract: Organic Geochemistry of Oil Seep from Sinu-San Jacinto Basin, Colombia, by Cortes, Jorge E.; Nino, Jaime; Tobo, Giselle; Wilches, Omar; Aguilera, Roberto; Jimenez, Miguel F.; #90166 (2013)

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Organic Geochemistry of Oil Seep from Sinú-San Jacinto Basin, Colombia

Cortes, Jorge E.; Niño, Jaime; Tobo, Giselle; Wilches, Omar; Aguilera, Roberto; Jimenez, Miguel F.1
1[email protected]

The molecular organic geochemistry characteristics of 31 oil seeps from Sinu-San Jacinto Basin were studied in order to determine its origin, depositional environment, maturity and biodegradation.

Bulk parameters show a medium to highly paraffinic, non-waxy crude oils with API of 12.69°-43.17°, sulfur values of 0.110- 1.651wt.%, saturate fraction was found >52.1%, aromatic fraction >4.1%, resins >1.4%, while asphaltene fraction varies between 0.1% to 9.4%. The δ13C isotopic composition of saturate fraction ranges between -28.9% and -22.2‰, while the aromatic fraction has a range of -28.8% to -22.7%, and C.V. between -5.54 to -0.65, indicating typical non-waxy marine oils.

Most of the samples analyzed by GC/FID in both WOGC and C15+ fraction show a severe biodegradation characterized by the total absence of n-alkanes and isoprenoids, the presence of an intense UCM, regular steranes in low concentration, low ββ-steranes, presence of 25-norhopanes, and other demethylated tricyclic terpanes and diamondoid ratios.

Biomarkers showed higher concentration of hopanes over tricyclic terpanes, Ts/Ts+Tm is >0.25, the O.I. ranges between 0.26-6.38 suggesting that the organic matter was deposited in a marine or marine deltaic environment during the Tertiary and/or Late Cretaceous. The C25T/C26T ratio indicates both marine and non-marine organic sources. The alternate predominance of Sterane distribution indicates that samples have marine, terrestrial or mixed characteristics. Diasterane/Sterane ratio suggests both a carbonate and silisiclastic lithology. Thermal maturity, according to the 20S/20S+20R-C29 and ββ/αα+ββ-C29 steranes, adamantane, diamantane distribution and aromatic parameters, such as MPI, Rc%, MDBR, indicate a moderate to high maturity.

The onshore rock samples from the Paleocene Arroyo Seco Formation have type II oil-prone kerogen. The rest of Cenozoic formations indicate type III gas-prone kerogen to type IV kerogen.

The presence of a source rocks with type II and III kerogens in the basin as shown by the pyrolysis data and the transitional marine to continental depositional conditions of the Cenozoic units formation are in accordance with the present crude oil geochemical data and suggest that many of the oil seeps reported in the basin could have origin, at least in part from the Arroyo Seco Formation and other Cenozoic units buried in the deeper depocenters surrounded by mud diapirs at which most of the oil seeps are found in this basin.

 

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013