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

Abstract

Cretaceous source rocks in Colombia are characterized by significant variations in organic matter input and depositional environments. Organic matter input, sedimentary environments, redox conditions and lithology of the source rocks as interpreted from bulk properties, trace elements, biomarker and isotopic composition of oils correspond to changes in the organic facies geographically and through time. Marginal marine/tidal Aptian–Albian units found in the Putumayo and Upper Magdalena Basins contain marine algal/bacterial and higher-plant organic matter. Coeval units in the Middle Magdalena Basin (MMB) and the Eastern Cordillera contain less terrestrial input because 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 Cordillera and Llanos Basin contain higher-plant terrestrial input derived from the Guyana Shield, mixed with marine organic matter. Although similar to the oils generated from Lower Cretaceous rocks, Upper Cretaceous-derived oils can be distinguished by the presence of oleanane and other angiosperm biomarkers.