Multi-Resolution Graph-Based Analysis for Fissility Evaluation in the Vaca Muerta Formation (Neuquén Basin, Argentina).

Abstract

Fine-grained rocks exposed at outcrops can remain massive or may split along relatively smooth surfaces parallel to the bedding when the rock is fissile. Fissility is an intrinsic structural characteristic of the rock revealed by weathering through the opening along weakness planes. Those weakness (or fissility) planes depend on numerous factors such as mineralogical composition, rock fabric, diagenetic processes and organic matter content and can exert an influence in the hydraulic stimulation of unconventional plays such as Vaca Muerta Formation. The fissility analysis performed on four cores encompassing more than 300 meters of the Vaca Muerta Formation results in a 4-class index, designated DAD (Drying Alcohol Discontinuities), that represent different fissility intensities. This information is related to the well log response of the cored rocks through a supervised classification using the MRGC (Multi-Resolution Graph-based Clustering) algorithm. This methodology allows considering multiple fissility controls simultaneously: gamma ray and photoelectric factor logs represent the mineralogical composition; sonic log as a proxy of geomechanical behavior; and the shallowest resistivity, representative of the texture of the rock. This classification reproduces potential fissility classes with a reasonable match on the four cores. To improve the accuracy of the resulting model, a high-resolution gamma ray curve is calculated from the processing of the microresistivity image. A second model is built replacing the raw gamma ray log with the HR-GR with an outstanding improvement, especially in the case of high fissility rock classes. Finally, the analysis is complemented with rock mechanics measurements coming from laboratory test to establish the effect of fissility on the geomechanical behavior of the rock.