Reservoir-Relevant Parameters Of Naturally Fractured Tertiary Intrusives In The Southern Plunge Of The Sierra Azul Anticline, Nequen Basin: An Outcrop Analogue Study

The oilfields along the “Valle del Rio Grande” structural trend are located in the subandean foothills of the Malargue Foldbelt in the northern part of the Neuquen Basin. For over twenty years these fields have been producing oil and gas predominantly from naturally fractured volcanic intrusives hosted directly within the Jurassic source rocks. The motivation for this outcrop study was to better understand the reservoir characteristics of the fractured intrusives. Field work was conducted in the southern plunge of the Sierra Azul range (a —45km long breached first-order anticline) which was considered the closest and best geologic analogue with respect to the producing fields. 9 km of continuous “map view” outcrop of intrusive rocks were statistically examined for reservoir-relevant parameters, such as fracture orientation, length, aperture, spacing, open fractures and mineral fill. A total number of —1200 individual fracture data were registered and new findings were made: It was observed that the lateral terminations of the intrusives are sharp edges and not plumose features. General fracture geometries (i.e. orientation, length and width) do not comprise significant variation along the entire outcrop. Open fractures exist in several directions (of all observed fracture sets), not one preferential direction. 21% of all fractures were found to be open. Furthermore, it was observed that in the vicinity (up to —60 m away) of secondary faults, with throws less than 50m, most of the fractures have lost their permeability due to calcite impregnation