Using gas geochemistry to delineate structural compartments and assess petroleum reservoir-filling directions: A Venezuelan case study

Here we examined the light hydrocarbon and nitrogen content and isotopic signatures of eleven gaseous samples in order to evaluate lateral intra-reservoir continuity in a Venezuelan reservoir in the central area of Lake Maracaibo Basin. At least three single compartments, located in the northern-central and southern parts of the reservoir, are revealed by nitrogen concentrations showing clear step-like compositional breaks. The occurrence of step-breaks was also supported by the isotopic signature of individual hydrocarbon compounds in the range of C1–C4 alkanes. Samples presented only slight differences in N2 and hydrocarbon gas compositions within the central and northern parts of the reservoir, and therefore it was not possible to infer structural barriers in coherence with the geological section. Some oil bulk parameters corroborate gradual changes that provide additional information on the reservoir-filling history, thus suggesting that the lateral physical–chemical equilibrium of fluids was not reached in this reservoir.

► This research study aims to understand lateral continuity of a Venezuelan reservoir. ► This work examines the compositional and isotopic signatures of C1–C4 alkanes. ► Some compartments are revealed by N2 and C1–C4 alkanes showing compositional breaks. ► The north-central part of the reservoir shows similar compositions in N2 and alkanes. ► Oil S contents and gravities confirm changes related to the reservoir-filling period.