Gas geochemistry evaluation of onshore Colombian basins: New insights for natural gas and liquified petroleum gas prospectivity

This study examines the molecular and isotopic composition of 193 gas samples collected from oil and gas fields across Colombia's onshore basins with active hydrocarbon production. Comparative analyses were conducted on both isotopic and molecular compositions across the Lower Magdalena Basin (LMB), Middle Magdalena Basin (MMB), Upper Magdalena Basin (UMB), Putumayo Caguán Basin (PUTCAB), Catatumbo Basin (CATB), Eastern Llanos Basin (LLAB), and Eastern Cordillera Basin (ECB). The primary objectives were to classify the gases produced, characterize their origins, assess transformation processes such as biodegradation and migration, and analyze the statistical distribution patterns of their components. This geochemical characterization aims to support the discovery of new reserves for both natural gas (NG) and liquefied petroleum gas (LPG), given Colombia's potential risk of diminished energy self-sufficiency in gas resources. The basins under study produce dry gas, wet gas, and liquefied petroleum gas (LPG/C₃₊), all associated with oil and gas fields of commercial hydrocarbon production. Notably, the LLAB contains the highest proportions of heavy isotopic carbon and C₃₊ (LPG) concentrations, whereas LMB is characterized by isotopically lighter methane, indicative of dry gas predominance. Results suggest a predominantly thermogenic origin for the gases studied, generated within the oil and gas windows, with several samples originating from secondary oil cracking, while some samples from LMB display a likely biogenic origin. Additionally, evidence of gas migration and biodegradation was observed in a significant subset of samples. The analysis of statistical distributions and compositional trends reveals a prevalent high methane content, with substantial C₂-C₅ (C₂₊) gas concentrations across all basins studied. This composition underscores the potential for both natural gas (NG) and LPG production. The C₃₊ (LPG) content varies between 1 % and 92 %, with 35 % of the samples containing less than 5 % LPG. High original gas-in-place (OGIP) volumes and substantial LPG content in the Eastern Llanos foothills, encompassing fields such as Cusiana and Cupiagua, highlight the prospective potential of this region. Near-field exploration could further add reserves of both NG and LPG.