Fractured reservoirs and underlying/overlying ductile layers (salt analogue?)
Understanding naturally fractured reservoirs is crucial for optimizing subsurface fluids injection, production or storage. This kind of reservoir is particularly targeted by hydrogeologists as well as reservoir geologists; they have often the particularity to initially appear highly productive but have a clear ability to decline rapidly (Bratton et al., 2006). NFR commonly contain fault-zones, fracture corridors and background fracturing that all need to be understood in terms of distribution and fluid-flow properties (e.g. aperture, porosity, permeability). Fracture systems commonly gives an anisotropy of permeability into the reservoir (Nelson, 2001); this hydraulic parameter remains problematic to quantifiy/predict without any well dynamic data. Fracture intensity in layered formations is often controlled by the rock mechanical stratigraphy (Laubach and Olson, 2009). Defining the locations where mechanical strength contrasts occur help to identify the mechanical units where hydraulic properties have to be modified compared to the surrounding « preserved » host rock (e.g. structural porosity for storage capacity; structural permeability for injection rates). This outcrop is proposed to partly highlight some concepts related to the mechanical stratigraphy in NFR overled and underled by ductile geological layers (e.g. salt).
The mechanical stratigraphy? Can we use self-similarity?
Laubach and Olson (2009) accurately define the mechanical stratigraphy as: « Mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength, elastic stiffness, brittleness, and fracture mechanics properties. Fracture stratigraphy subdivides rock into fracture units according to extent, intensity, or some other observed fracture attribute. Mechanical stratigraphy is the by-product of depositional composition and structure, and chemical and mechanical changes superimposed on rock composition, texture, and inter-faces after deposition. Fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure. Because mechanical property changes reflect diagenesis and fractures evolve with loading history, mechanical stratigraphy and fracture stratigraphy need not coincide. In subsurface studies, current mechanical stratigraphy is generally measurable, but because of inherent limitations of sampling, fracture stratigraphy is commonly incompletely known. To accurately predict fractures in diagenetically and structurally complex settings, we need to use evidence of loading and mechanical property history as well as current mechanical states. »
A simple/short outcrop description
The studied outcrop is a made of metamorphic rocks. The lower and the upper units are made of interbedded quartzite and marble layers (grey) whereas the middle layer is made of rubaned quartzites. The thickness of R1 is around 10 cm.
The main interest of this outcrop is the difference in structural style between R1 and the underlying/overlying layers, mainly due to a difference of rheology during deformation (e.g. stiffness). Both the lower and the upper units seem to have « flowed » according to the observed ductile deformation exhibiting overturned and recumbent folds. Could we partly use both of these units as salt/evaporite layers analogous?
The structural style in the middle unit (R1) appears to be « brittle », displaying numerous antithetic normal faults and related fracture systems. The overall deformation is homogeneously and widely distributed; the deformation architecture is consistent with a mechanical extension. The fault architecture displays « Riedel type » growing structures increasing the numbers and the connectivity of the structural heterogeneities at the vicinity of the the fault-zone. No evidence for damage zone asymetry is observed. Could we partly use the R1 unit as an analogous of naturally fractured carbonate reservoir in local secondary extensive setting?
How could we use this outcrop in exploration?
The well A would deserve to be designed as perpendicular as possible to the fracture strikes in order to fully capture the fracture hydraulic potential (structural porosity and permeability). Fault-zones are expected to be highly permeable (dilatant fault rock), therefore increasing the risk of draining the lower part of the R1 unit (water?).
NB: Possible cementation phases reducing the fracture permeability have not been discussed.
The constructive comments are always very welcome.
Bratton, T., Canh, D. V., Van Que, N., Duc, N. V., Gillespie, P., Hunt, D., … & Sonneland, L. (2006). The nature of naturally fractured reservoirs. Oilfield Review, 18(2), 4-23.
Laubach, S. E., Olson, J. E., & Gross, M. R. (2009). Mechanical and fracture stratigraphy. AAPG bulletin, 93(11), 1413-1426. https://doi.org/10.1306/07270909094
Mattauer, M. (1973). Les déformations des matériaux de l’écorce terrestre (Vol. 2). Hermann.
Nelson, R. (2001). Geologic analysis of naturally fractured reservoirs. Elsevier.