La Paz oil field is one of the most famous, well-documented fractured reservoirs in the world. Since 1945 it has produced more than 830 million bbl of oil from both low porosity Cretaceous limestones and underlying granitic basement. The field is situated on a large inversion structure, partially uplifted in the late Eocene, having the majority of the inversion occurring in the Miocene-Pliocene. Fracture distributions, initial rate and cumulative production distributions, and trends in formation water chemistry all suggest that reservoir quality and reserves are controlled by natural fractures associated with Miocene-Pliocene and younger strike-slip faults and possibly by secondary microporosity related to the geometry of the earlier Eocene block uplift. Production levels within the area of Eocene uplift are anomalously high, with one well producing 59 million bbl of oil. Typical wells in other parts of the field produce 1 million bbl of oil. Analysis of the distributions cited previously suggests that production has occurred from highly elliptical to linear drainage areas surrounding faults. These restricted zones of fault-related fractures allow for communication of hydrocarbons stored in the low porosity rock carbonate matrix to the wellbore. In addition, diagenetic microporosity may be an important component in matrix storage in the carbonates and may be due to local subaerial exposure during the Eocene uplift.

This article shows that analysis of the natural fracture system with respect to the production characteristics in even old fields can give rise to new reservoir models, leading to new infill locations within the field limits or additional exploration opportunities in the area.

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