Evolution of the Cretaceous organic facies in Colombia: implications for oil composition

Cretaceous source rooks in Colombia are characterized by significant variat ions in organic matter input and depositional environments. Organic matter input, sedimentary environments, redox conditions and lithology of the sour ce rocks as interpreted from bulk properties, trace elements, biomarker and isotopic composition of oils correspond to changes in the organic facies g eographically and through time. Marginal marine/tidal Aptian-Albian units f ound in the Putumayo and Upper Magdalena Basins contain marine algal/bacter ial and higher-plant organic matter. Coeval units in the Middle Magdalena B asin (MMB) and the Eastern Cordillera contain less terrestrial input becaus e they were deposited on a broad platform that locally developed evaporites . Cenomanian-Coniacian marine-shelf shales and marls in the Putumayo, Upper and MMBs contain marine organic matter deposited on a marl/carbonate-prone shelf resulting in a type II kerogen. Upper Cretaceous offshore and lower shoreface mudstones preserved in the present-day position of the Eastern Co rdillera and Llanos Basin contain higher-plant terrestrial input derived fr om the Guyana Shield, mixed with marine organic matter. Although similar to the oils generated from Lower Cretaceous rocks, Upper Cretaceous-derived o ils can he distinguished by the presence of oleanane and other angiosperm b iomarkers. (C) 2001 Elsevier Science Ltd. All rights reserved.