ABSTRACT Natural fractures are abundant in the Vaca Muerta Formation and are important because they may affect hydraulic-fracture growth during well stimulation. They contribute to anisotropic mechanical behavior of the reservoir rock and may cause hydraulic fractures to arrest or divert along them by opening or shear. In the subsurface, the Vaca Muerta Formation contains bed-parallel veins (BPV) of fibrous calcite (beef) and bed-perpendicular, completely or partly calcite-filled, opening-mode fractures in multiple orientations. In outcrops of the Vaca Muerta Formation in the Agrio fold-and-thrust belt, BPV and bed-perpendicular fractures are also common. Fracture cement geochemistry (including stable isotopes) and fluid inclusion and clumped isotopic thermometry indicate that the outcrops are similar to the most mature parts of the Vaca Muerta reservoir and can be used as guides for this part of the basin. In outcrops near the Cerro Mocho area, two main bed-perpendicular, opening-mode fracture sets are oriented east–west (oldest) and north–south (youngest), and two additional sets (northeast–southwest and northwest–southeast) are locally present. Fluid inclusion microthermometry, combined with burial-history curves, indicates that BPV in the area of Loncopué formed in the Late Cretaceous during bed-parallel contraction and in overpressure conditions, whereas bed-perpendicular sets formed in the Paleocene. Similar ages were obtained for Puerta Curaco outcrop on the basis of clumped isotope temperatures, although BPV opening may have lasted until the Miocene in this area. BPV are the most common and some of the oldest types of fracture sampled by vertical cores, and stable isotope analyses indicate that they formed deep in the subsurface, probably under conditions similar to those inferred for outcrops. In cores of the Loma Campana block, bed-perpendicular fractures show orientations similar to those in outcrops, although the youngest, north–south set is generally missing. Without appropriate fluid inclusions for microthermometry or oriented cross-cutting relationships in core, fracture timing was established on the basis of a tectonic model. Our model indicates that in the Loma Campana block, fractures preferentially formed in east–west and northeast–southwest orientations in the Early Cretaceous, northeast–southwest in the Late Cretaceous, northwest–southeast in the Cenozoic, and east–west and east-northeast–west-southwest at present. Fracture timing and orientations from this tectonic model, fracture aperture from core, fracture height and length measured in outcrop, and fracture intensity from a geomechanical model calibrated with core and image logs were used to construct discrete fracture network (DFN) models of the subsurface and build specific reservoir development plans.

Abstract Shale gas reservoirs are commonly produced using hydraulic fracture treatments. Microseismic monitoring of hydraulically induced fracture growth shows that hydraulic fractures sometimes propagate away from the present-day maximum horizontal stress direction. One likely cause is that natural opening-mode fractures, which are present in most mudrocks, act as weak planes that reactivate during hydraulic fracturing. Knowledge of the geometry and intensity of the natural fracture system and the likelihood of reactivation is therefore necessary for effective hydraulic fracture treatment design. Changing effective stress and concomitant diagenetic evolution of the host-rock controls fracture initiation and key fracture attributes such as intensity, spatial distribution, openness and strength. Thus, a linked structural-diagenesis approach is needed to predict the fracture types likely to be present, their key attributes and an assessment of whether they will impact hydraulic fracture treatments significantly. Steep (>75°), narrow (<0.05 mm), calcite-sealed fractures are described in the Barnett Shale, north-central Texas, the Woodford Formation, west Texas and the New Albany Shale in the Illinois Basin. These fractures are weak because calcite cement grows mostly over non-carbonate grains and there is no crystal bond between cement and wall rock. In bending tests, samples containing natural fractures have half the tensile strength of those without and always break along the fracture plane. By contrast, samples with quartz-sealed fractures do not break along the fracture plane. The subcritical crack index of Barnett Shale is >100, indicating that the fractures are clustered. These fractures, especially where present in clusters, are likely to divert hydraulic fracture strands. Early, sealed, compacted fractures, fractures associated with deformation around concretions and sealed, bedding-parallel fractures also occur in many mudrocks but are unlikely to impact hydraulic fracture treatments significantly because they are not widely developed. There is no evidence of natural open microfractures in the samples studied.

Abstract Fracture geometries and fracture-sealing characteristics in dolostones reflect interactions among mechanical and chemical processes integrated over geological timescales. The mechanics of subcritical fracture growth results in fracture sets having power-law size distributions where the attributes of large, open fractures that affect reservoir flow behaviour can be accurately inferred from observations of cement-sealed microfractures and other microscopic diagenetic features, which are widespread in dolostones. Fracture porosity is governed by the competing rates of fracture opening and cement precipitation during fracture growth and by cements that post-date fracture opening. Combined analysis of structural and diagenetic features provides the best approach for understanding how fracture systems influence fluid flow. We review previous work and integrate new data on fractures and diagenetic features in cores from the Lower Ordovician Ellenburger and Permian Clear Fork formations in West Texas, and the Lower Ordovician Knox Group in Mississippi, together with outcrop samples of Lower Cretaceous Cupido Formation dolostones from the Sierra Madre Oriental, Mexico, in order to illustrate our approach.