New Type of Kinematic Indicator in Bedding-Parallel Veins and Vertical Fracture Abundance and Timing in Vaca Muerta Formation, Argentina
In Late Jurassic-Early Cretaceous Vaca Muerta Formation, previously unrecognized, yet abundant structures constituting a new category of kinematic indicator occur within bed-parallel fibrous calcite veins (beef) in shales in outcrop. Domal shapes result from localized shortening and thickening of bed-parallel fibrous calcite veins and the intercalation of cm-thick wall-rock shale inclusions within fibrous calcite beef, forming thrust fault-bounded pop-up structures. Their orientation is consistent with regional E-W thrust transport and lateral shortening of the beef during beef growth. This new type of kinematic indicator is most likely present in deformed beef in the subsurface. The beef veins and kinematic indicators are cut by multiple sets of vertical fractures that have not been affected by stress release (fracture walls are intact). Vertical fractures with cement fill that cut into shale can be measured along 10+ m cliffs. 4 main fracture sets are present. The E-W set is the oldest set, followed by the NE-SW, NW-SE, and the N-S set is the youngest. The N-S set preserves abundant fracture porosity. The E-W set shows the highest micro-and macrofracture intensity. Two distinct power-law slopes in fracture aperture versus cumulative frequency log-log plots indicate two aperture populations of the N-S fracture set at Arroyo Mulichinco, but not at Puesto Gruñon. Cathodoluminescence images (SEM-CL) confirm 2 opening events of the widest N-S set at Arroyo Mulichinco. The E-W set has similar micro- and macrofracture intensities at both outcrops away from faults, but macrofracture intensity increases closer to faults. While macrofracture abundance is similar in beef and in shale, microfractures smaller than 0.1 mm are mostly absent in shale and dolomitic layers but are abundant in beef. Vertical fracture porosity in beef is considerable at least in some orientations (N-S) and could contribute to reservoir permeability if beef is voluminous. Temperatures of aqueous inclusions coexisting with hydrocarbon inclusions trapped in beef and vertical fractures indicate timing of beef and fracture opening and cementation. Beef stages formed between 94 and 62 Ma, corresponding with the time of maximum burial in the Late Cretaceous. The E-W set formed around the same time. Horizontal fractures that cut beef and the E-W set formed at ~60 Ma. The NE-SW and N-S sets formed in the Paleocene (~60-56 Ma). The E-W set was probably reactivated at this time.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017