Oil families in the eastern cordillera and llanos basin, Colombia: A comprehensive characterization using advanced geochemical technologies

Abstract

Three major age-related oil families, clustered into seven groups, were identified in the Llanos basin based on advanced geochemical technologies that unveiled complex mixtures throughout the basin. The analyses included the conventional whole oil and biomarker analysis and the more refined and advanced GC-MS/MS analysis. The latter provided for terrestrial diterpane-based age-specific arrays that discriminated between Cretaceous and Cenozoic floral types. The analysis of asphaltene hydrous pyrolysates revealed the initial charges to the reservoirs, often of early maturity, but also strongly biodegraded and overprinted by fresh oil charges. Extensive analysis on diamondoids allowed the characterization of the late maturity charges, differentiation between mixed and singly-sourced oils and the ability to tie together earlier with later generated oil components of oil mixtures. Oils from Family A are the most widespread in the basin, found in the southern Llanos and in the Foothills and are derived from Lower Cretaceous to Cenomanian marine shale with a terrigenous input. Within Family B, of Upper Cretaceous age, two main source facies were identified, one with a siliciclastic marine input, corresponding to Groups 4 and 5. They have the highest concentrations of the C25 highly branched isoprenoid (HBI) and occur in the Northern and Eastern Llanos and in the Axial Zone of the Eastern Cordillera. The other facies has a more calcareous aspect, identified mainly in the Southern Llanos designated as Groups 2 and 3. Family C, of Cenozoic age, is restricted to the Foothills and two oil Groups were distinguished: Group 6 as the only endmember of a terrigenous Cenozoic facies, identified in Oil 7, and Group 7 shows a Cenozoic terrigenous facies and varying degrees of mixing with a marine Cretaceous input. These families are mixed in a variety of ways, having mixtures from different facies, while others show multiple charges from the same facies.