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The microscale mechanisms of hydrocarbons movement along faults and fault zones, the potential carriers of hydrocarbons toward the productive geological traps, remain largely unknown. The majority of previous studies inferred the hydraulic behavior of faults with respect to hydrocarbon movements without providing meso and microstructural observations from faults permeated by hydrocarbons. To fill this gap, we document meso-structures together with the first fossil microstructural portraits of hydrocarbon flow and pathways along two hydrocarbon-bearing carbonate-hosted normal faults exposed in the central Apennines, Italy. In particular, we show that hydrocarbons cyclically move within tectonically active carbonate normal faults possibly during interseismic and coseismic phases of the seismic cycle. Channelized structures, injection features, and clast-cortex grains suggest the occurrence of transient and localized pulses of pressurized hydrocarbons during coseismic slip. In particular, clast-cortex grains are very similar to microstructures that develop along fault planes for slip velocities between 0.0001 and 1 m/s. Patches of hydrocarbon-bearing breccias/cataclasites with lobate and irregular boundaries, crackle breccias filled by hydrocarbons, and hydrocarbons arrested against hydrocarbon-free discrete fault planes suggest hydrocarbon flow during the interseismic phases with hydrocarbon (over)pressure dissipation after coseismic phases. Fault permeability is created during the coseismic phase and hydrocarbons penetrate the most permeable and uncemented uncohesive fault rocks within the damage zone and arrest against and/or within low-permeability and cemented fault cores. Results are consistent with previous numerical simulations on hydrocarbon movements along faults and time-lapse periods within tectonically active faults and along permeable zones cyclically created by seismic activity. Results from this paper are crucial for modelling the hydraulic behavior of carbonate fault damage zone, which have progressively gained popularity as targets of hydrocarbon exploration and production and for CO2 or H storage.

We combined structural data collected in the field and those obtained from a virtual outcrop model constructed from drone imagery, to perform Discrete Fracture Network (DFN) modelling and to characterize the fracture distribution within the damage zone of the low-displacement (~50 m) carbonate-hosted Pietrasecca Fault (PF) (central Apennines, Italy). Both in the hanging wall and in the footwall damage zones, fractures are vertical and parallel to slightly oblique to the fault strike. Fracture length distributions in the footwall damage zone indicate a high degree of fracture maturity, while in the hanging wall damage zone they indicate a low degree of fracture maturity. Pervasive stylolitization in the hanging wall must have hindered the development of through-going fractures, favoring diffuse fracturing characterized by stylolite-bounded fractures. DFN models suggest that permeabilities are 1–2 orders of magnitude greater in the footwall damage zone than in the hanging wall damage zone. As permeability (10−12 to 10−15 m2) is comparable with those measured in large-displacement (up to 600 m) faults in carbonates, our results show that also damage zones accompanying carbonate faults with ~50 m of displacement could be fracture corridors for efficient fluid flow within subsurface reservoirs. Therefore, we propose that jumps in subsurface permeabilities occurring in many carbonate fractured reservoirs could be associated with to the occurrence of high permeability fracture zones developed within damage zones of low-displacement faults. As the recent advancement in seismic imaging allow the recognition of faults with displacement in the order of a few tens of meters, reservoir geologists and engineers can apply results of this study to better model the subsurface flow pathways near low displacement faults in carbonate reservoirs.

The extensional Val Roveto Fault, which is the longest exhumed potentially-seismogenic structure of central Apennines, Italy, is examined to constrain earthquake-related fluid circulation and fluid sources within shallow carbonate-hosted faults. The study focuses on fault-related comb and slip-parallel veins that are calcite-filled and cut through the principal surface of the Val Roveto Fault. We observe multiple crack-and-seal events characterized by several veining episodes, probably related to different slip increments along the fault plane. We show that vein calcite precipitated in Late Pleistocene time below the present-day outcrop level at a maximum depth of similar to 350 m and temperatures between 32 and 64 degrees C from meteoric-derived fluids modified by reactions with crustal rocks and with a mantle contribution (up to similar to 39%). The observed warm temperatures are not compatible with a shallow (

Fault slip and healing history may crucially affect the fault seismogenic potential in the earthquake nucleation regime. Here we report direct shear friction tests on simulated gouges derived from a carbonate fault breccia, and from a clay/carbonate fault-core gouge, retrieved from a surface exposure of the Longmenshan Fault Zone (LFZ) which hosted the 2008 Wenchuan earthquake. The experiments were conducted under dry and hydrothermal conditions, at temperatures up to 140°C, at an effective normal stress of 50 MPa, and involved sequential velocity-stepping (VS), slide-hold-slide (SHS), and velocity-stepping stages. Dry tests performed on breccia-derived samples showed no dependence of (quasi) steady state friction (μss) on SHS or VS history, and a log linear relation between transient peak healing (Δμpk) and hold time, or classical “Dieterich-type” healing behavior. By contrast, all experiments conducted under hydrothermal conditions were characterized by “non-Dieterich” healing behavior. This included (1) an increase in μss upon resliding after a hold period and (2) an increase in friction rate parameter (a − b), after SHS testing. Comparison with previous results suggests that the healing behavior seen in our wet tests may be attributed to solution transfer processes occurring during hold periods. Our findings imply that the shallow portions of faults with carbonate/clay-rich cores (e.g., the LFZ) can heal much faster than previously recognized, while the upper limit of the seismogenic zone may migrate to deeper levels during interseismic periods. These effects have important implications for understanding the seismic cycle in tectonically active carbonate terrains.

Destructive earthquakes are common in tectonically active regions dominated by carbonate cover rocks. The catastrophic Wenchuan earthquake that struck Sichuan, China, also affected a section of carbonate cover terrain. Numerous studies have focused on characterizing the compositional, transport and mechanical properties ofsilicate fault rocks, with many models being proposed for dynamic fault frictional and rupture processes, but little data are available on carbonates to date. In this thesis, I report on the properties of fault rocks collected from two carbonate-rich surface exposures on the surface rupture associated with the Wenchuan earthquake. The main conclusions are as follows: 1. The enrichment/depletion patterns, element partitioning and a very large implied volume loss (> 90%) found are quite different from those characterizing faults in granites and clastic sedimentary rocks, and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution. Increasing enrichment in smectite toward the principal slip surface was observed. Illitization of the gouges on the principal slip surfaces, caused by coseismic frictional heating, was found in both exposures studied. Results imply that coseismic de-watering reactions can be expected to be extensive at depth, and possibly helped generate excess pore pressure assisting dynamic slip weakening during the Wenchuan Earthquake. 2. High-velocity friction experiments revealed that slip weakening is more pronounced for water-dampened gouge than dry gouge, indeed pointing to thermal pressurization. The fault core gouge studied has a very low permeability (