Geoquímica orgánica del petróleo en la región sureste de la cuenca del lago de Maracaibo (Venezuela)

Abstract

This work presents a geochemical study performed on 48 crude oils from a Eocene reservoir (Unit B-4, Fm. Misoa), along with 18 channel 18 rock samples, they all from wells located in the Ceuta Southeast Area in Lake Maracaibo. The aim of this research work was to establish oil-oil and oil-source rock correlations, as well as to determine the lateral continuity and sediment deposition pattern in the reservoir under study. 31, 8, 6, and 3 crude oils from Lagotreco (offshore Tomoporo), onshore Tomoporo, Franquera, and La Ceiba fields, respectively, were analyzed. The crude oils the first two fields show API gravities averaging about 30°, while those samples from Franquera and the last field show values below 20° and approximately 23º, respectively. All the crude oils are mature and can be characterized by a high proportion of resins, asphaltenes, sulfur and trace metals. It is concluded that the crude oils under study can be grouped as a single genetic type after carrying out whole oil and maltene fractions carbon isotope analyses, as well as biomarker analyses by gas chromatography coupled to mass spectrometry; their common source rock (Fm La Luna) was deposited in a marine paleoenvironment defined by a low terrigenous input, and under a reducing environment associated with carbonate mud sedimentation. Furthermore, the study crude oils show n-paraffins and 25-norhopanes in their biomarker distributions, leading to infer a variable mixture of a first paleobiodegraded crude oil charge with a second unaltered oil pulse, which also explains the low API gravities of the study crude oils. The determination of the oil-oil correlations indicates that the samples under study can be classified into several compositional groups (offshore and onshore Tomoporo oils, samples from Franquera and crude oils from La Ceiba) on the basis of the different contribution of the two pulses aforementioned (one charge corresponding to the paleobiodegraded oil and another relative to the unaltered crude), which is recognized as the most important factor to distinguish the oils object of this study. In addition, it should be noted the existence of two outliers (�vagrants�) among the samples from the Lagotreco Field (W-62 and W-33), indicating the possible presence of at least two discontinuities or sub-compartiments separated from the rest of the reservoir at the level of the Unit B-4 (Misoa Formation). The foregoing suggestion has been corroborated by the study of the light fraction using both whole oil gas chromatography technique and Fourier-transform infrared spectroscopy. Finally, the 1D thermal modeling carried out in the study area based on the characterization of 9 channel rock samples from the well W-53, and other 9 ones from the well LC-1, confirmed the double hydrocarbon kitchen in the La Luna source rock. This fact has been previously reported by several research studies as the origin of the crude oils from the southeast part of the Lake Maracaibo Basin. After integrating all the geochemical data, it can be suggested that the Southeast Ceuta Area could have received two charges of oil; a first one of Eocene age, and the other from the second kitchen of Neogene-Quaternary age. Additionally, it is confirmed that the first charge of oil accumulated in the B-4 reservoir traps (Fm Misoa) in the Lagotreco field was generated by the Eocene kitchen northwards, undertaking a direction of migration northeast- southwest; while the second pulse would reach this reservoir undertaking a principal direction of migration southwest-northeast, being generated by another kitchen of post- Oligocene age and located south of the study area. A similar idea can be applicable to Tomoporo, Franquera and La Ceiba fields.