COMBINED EFFECTS OF OVERPRESSURE AND BED-PARALLEL CONTRACTION ON THE FORMATION OF BED-PARALLEL AND VERTICAL FRACTURES IN THE VACA MUERTA FORMATION, ARGENTINA

Natural fractures in unconventional reservoirs, such as shales, provide flow pathways between source and reservoir layers during gas charge, or between matrix pores and hydraulic fractures and the wellbore during production. Even where sealed, fractures may significantly influence engineering operations. Fractures form in sedimentary basins as a result of various combinations of tectonic, burial and thermal loading (stress), pore fluid pressure, and evolving rock strength. In naturally cemented fractures, fluid inclusions are a key to unlocking the history of fracture growth. Correlated with burial and thermal history models, fluid inclusion temperature (T) and pressure (P) sequences provide evidence for the history of fracture opening and cementation.

The black shales of the Late Jurassic–Early Cretaceous Vaca Muerta Formation, Argentina, contain widespread bed-parallel, fibrous calcite-filled (beef) fractures (BPVs) as well as bed-normal fractures. The wide (as much as 10 cm) BPVs in outcrop contain zones of curved fibrous calcite and kinematic indicators on the top and bottom surfaces that indicate bed-parallel contraction during BPV growth. BPVs are cut by four sets of vertical opening-mode fractures: E-W (oldest), NE-SW, NW-SE, and N-S (youngest). BPVs are also cut by late bed-parallel microfractures. All fracture cements contain coexisting aqueous fluid and hydrocarbon gas inclusions, therefore, T and P obtained from inclusions represent trapping conditions.

BPVs formed at 175 °C to 208 °C. The later, bed-parallel microfractures that cut fibrous calcite formed at 165 °C to 183 °C, whereas vertical fracture sets formed at 142 °C to 161 °C. Fluid salinity in all fractures is ~15 wt% NaCl equivalent. Gas inclusions are liquid at room T and homogenize at -75 °C to -65 °C, suggesting the presence of a methane-dominated gas.

Coexisting gas and aqueous inclusions in cements indicate the presence of two free fluid phases during cementation, and that the reservoir was overpressured during fracturing. The unusually wide BPVs probably formed due to the combined effects of overpressure and bed-parallel contraction during BPV opening and cementation in the Late Cretaceous (94-62 Ma). Bed-parallel and vertical fractures that cut BPVs formed in the Paleocene (60-56 Ma).