No 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.

The fissility is the ability of some rocks to split along relatively smooth surfaces parallel to the bedding. This property observed mostly in fine-grained rocks is particularly expressed in outcrops, where rocks are subjected to weathering processes. Most authors associate the fissility to the abundance of clay minerals and their orientation parallel to the bedding. The horizontal fabric can be promoted by depositional conditions such as sediment composition, quantity of total organic carbon content (TOC) and depositional mechanisms, giving rise to a primary fissility. Alternatively, the alignment of platy minerals can be linked to the burial history of the rock, by processes such as mechanical compaction or secondary mineral growth, resulting in a secondary fissility. The present study aims to identify the main controls of fissility development at the micro- and macroscopic scale in rocks of the Vaca Muerta Formation exposed in the Cerro Mulichinco area and in a 121-meter-long core extracted from a well within the Neuquén Basin. In outcrops, fissility is related to fine-grained laminated facies with low carbonate content, revealing the strong control exerted by lithology. The TOC measurements allow establishing a positive correlation between organic matter content and fissility intensity. Moreover, the analysis of the transgressive-regressive cycles shows that fissility is higher around the maximum flooding surfaces. Regarding their mechanical characteristics, the different interfaces observed in core are classified into first and second-order, the last one including fissility planes. Some of these interfaces evolve from potential (partially open) to effective (totally open) discontinuities in response to changes of stress conditions during the core extraction and due to the stress relaxation through time: weeks (T1), months (T2) and years (T3) after extraction. The time evolution of the effective core discontinuities points out rock intervals that are variably broken and core segments that remain intact. The Drying Alcohol Discontinuities (DAD) methodology reveals potential discontinuities within apparently intact core segments. By using this technique, a 4-class index is established as a proxy for fissility degree. When integrated with geological, petrophysical and geomechanical data, this index enables characterizing the main mechanisms controlling rock fissility that express through discontinuities promoting the loss of competence of a rock. Consequently, this mechanical property is considered to influence the efficiency of hydraulic fracture in shale reservoir completion.