« Seismic-scale » normal fault in layered carbonates
Dealing with reservoir geology, karstology and/or fractured reservoirs implies to determine « where » and « how » the rock porosity is located. This needs to be assessed in both a qualititative and quantitative point of view. Based on conventional seismic analysis, even using the relevant seismic attributes, it appears challenging to localize structural heterogenities that do not evidence any 10m-scale shear offset or displacements (e.g. joints, m-fault, fractures). The outcrop study is of major interest in filling the gap between seismic and well/core data.
In fractured/faulted layered carbonate succession, can we predict where the structural porosity is located?
Can we orientate the anisotropy of permeability related to the fault/fracture network architecture?
Why this outcrop description?
In this article, we examine the spatial distribution of fractures and dissolution features at the vicinity of a seismic-scale normal fault zone affecting layered carbonates. These observations/interpretations can be interesting and inspiring for hydrogeologists as well as exploration geologists looking for structural porosity and dissolution markers in such subsurface rocks. The studied escarpment is around 250 m high.
NB: The depth during deformation (fracturing) is probably shallower than 3 km. This assumption would deserve to be properly quantified using field data (e.g. cross-section, Vitrinite reflectance sampling for host rock maximum burial depth or fluid-inclusion analysis on synkinematic cements).
The main studied fault
The studied fault-zone cuts and displaces a layered carbonate (mainly limestones) and shaly succession. Faults are mainly visible in carbonates. The displacement is around 30 meters for the R2 carbonate reservoir which is 15 meters-thick. The displacement value interestingly decreases towards the top of the hill (Reservoir 1). The deformation style into the carbonate layer appears « brittle » that is commonly described in fractured reservoirs.
The fault architecture appears complex into the shaly layers where 10-m scale deformed lenses can be individualized by fault segments (between R1 and R2). These structural complexities cannot be horizontally assessed using this outcrop.
The fault damage zone
We observe a significant asymmetry in the fault damage zone development, in particular in R1. The hanging-wall is more damaged than the footwall. Is this associated with a drag fold in R1? Is this zone of high fracture intensity related to a fault tip? The fault displacement seems to decrease upwards, therefore implying a tip damage zone (e.g. Kim et al., 2004). Could the top seal above R1 therefore be preserved (before erosion)?
The background fracturing
A strong background fracturing is believed to give an anisotropy of permeability in the system (N-S preferential fluid-flow), even far away from the fault. This hydraulic attribute can for instance be integrated into a DFN model and calibrated using dynamic data if available. Fracture direction, intensity and connectivity have to be properly adressed using a field work.
The dissolution features
Karstic landscape geomorphologies commonly exhibit sub circular depressions called sinkholes or dolines (e.g. Sauro, 2003). Sinkholes commonly originate from underlying subsurface dissolution cavities leading to surface collapse (e.g. Cavailhes et al., 2022). A necessary condition for forming large sinkholes seems to be a large mount of fluid-flow through the system, which allows and maintains the effective dissolution of the soluble rock (Hiller et al., 2014). Sinkholes or dolines are therefore efficient markers of dissolution that remain under-investigated using seismic data (e.g. see Lu et al. 2017, for an extensively documented case). In this case, sinkholes are clearly located into the fault damage zone where fracture intensity is highly developed, in particular into the footwall of the studied normal fault. The sinkholes are 80 meters-wide. Such collapse volumes related to dissolution have to be integrated when building a static reservoir model. The reservoir porosity is therefore not randomly distributed along such fault system, whatever the origin of the dissolution (hypogenic or epigenic karstification).
The constructive comments are always very welcome.
Cavailhes, T., Gillet, H., Guiastrennec-Faugas, L., Mulder, T., & Hanquiez, V. (2022). The abyssal giant sinkholes of the Blake Bahama Escarpment: evidence of focused deep-ocean carbonate dissolution. Geomorphology, 398, 108058. https://doi.org/10.1016/j.geomorph.2021.108058
Kim, Y. S., Peacock, D. C., & Sanderson, D. J. (2004). Fault damage zones. Journal of structural geology, 26(3), 503-517. https://doi.org/10.1016/j.jsg.2003.08.002
Lu, X., Wang, Y., Tian, F., Li, X., Yang, D., Li, Tao, Lv, Y., He, X., 2017. New insights into the carbonate karstic fault system and reservoir formation in the Southern Tahe area of the Tarim Basin. Marine and Petroleum Geology. 86, pp. 587–605. https://doi.org/10.1016/j.marpetgeo.2017.06.023
Sauro, U., 2003. Dolines and sinkholes: aspects of evolution and problems of classification. Acta Carsol 32 (2), 41–52. https://doi.org/10.3986/ac.v32i2.335