DUPLEX STRUCTURES AND ASSOCIATED FRACTURED RESERVOIRS
Geologic context about this 10 cm – scale duplex structure:
Naturally fractured reservoirs occur worldwide, either in basement rocks, in deeply buried sandstones or in most of the carbonate reservoirs. Assessing and discussing their hydraulic attributes remain challenging, in particular using seismic data in which the accuracy of the resolution does not allow to address their distribution with confidence. Storage, injection or production are highly dependent of fracture connectivity, density, and apertures. Fracturing can occur in response to many parameters that need to be understood such as the regional tectonic setting, folding, faulting, deconfining phases etc. We propose this outcrop picture to discuss the relations between folding, fracturing and reservoir tectonic thickening in a layered succession where strong mechanical contrasts are observed and reservoir matrix porosity/permeability are probably very low.
Outcrop details:
The host succession is made of cm-thick brownish to yellow decalcification clays and cm-thick grey layers of flints. The slight structural fabric developped into the decalcification clays suggest that the deformation postdates the alteration. The cm-thick layer of flint have been deformed under a brittle regime (fractures) whereas the clays have been deformed under a relatively soft/ductile regime.
How could we use this outcrop structural interpretation?
Seismic-scale geometries:
The studied succession could conceptually reflect an alternation between fractured reservoirs (R1, R2 and R3) and seals, assuming a “self-similarity*” concept. R3 is located into the inner part of a compressive structure, where the deformation is highly developped and expressed by thrusting/fracturing. Could we consider this as a tectonic reservoir thickening? This part of the structure appears difficult to image using conventional seismic methods. Gentle folds that are commonly recognized and mapped in surface often correspond to complicated duplex and/or imbricated structures in depth. Indeed, the crest of the surface structure is not right above the subsurface crest. Optimizing drilling and well trajectory remains challenging and highly dependent on seismic data quality. Such outcrop geometries have to be integrated during subsurface well design phases.
* »Self-similarity » refers to the idea of repeating a similar shape (often at a different scale) over and over again. In other words, a self-similar image contains copies of itself at smaller scales (Mishra and Bhatnagar, 2014).
Subseimic-scale structural heterogeneities:
R1, R2 and R3 are highly fractured. In this study case, the fracture intensity does not seem to be clearly correlated to the reservoir mechanical thickness as it is commonly described in the literature (Huang and Angelier, 2009). Quantifying fracture intensity (thanks to core logging) and conclude about mechanical stratigraphy can be misleading in such study case. The fractures exhibit antithetic geometries where the reservoir thickness is important, there are crossing the entire thickness in most of the cases. We would not extrapolate this observation to the field scale where these fracture systems can be for instance upscaled to fracture corridors.
R1 has been tectonically elongated and its present shape cannot be easily extrapoled to exploration/production/storage real cases. Indeed, the reservoir is highly discontinuous, fractured and faulted in response to secondary stresses commonly recognized in such layered settings.
R2 appears to be faulted (reverse) where the layer curvature is maximum. In this case, the fold hinge is not located at the fold crest. This hinge deformation should not imply reservoir (R2) juxtaposition issues.
R3 tectonically thickens thanks to imbricated thrusts. Reservoir and fluid pressure compartimentalization is more than probable. A dedicated fault seal analysis is needed using static and dynamic data. Into the footwall of the duplex (left part), the fracture intensity seems at its maximum. Is it related to the fact that this structure has accommodated most of the deformation? The entire deformation? Was this structure the first to form? Whatever the answer, at this final stage of deformation, the maximum fracture intensity is clearly located in this particular area. For this latter reason, the fictive well is designed to reach this area where fracture porosity, permeability is expected to be high.
Feel free to constructively comment this open discussion. Feel free to provide relevant references where similar structural geometries, sedimentary or diagenetic contexts, have been recognized or forecasted.
The figure above shows the results of a naturally fractured reservoir characterization from onshore reservoir in the Zagros Foreland basin near the area of Erbil (Kurdistan). This seismic profile exhibits interesting similarities with the aforementionned outcrop where a tectonic thickening has been interpreted into the inner part of the structure (Cavailhes et al., 2015). Discussing such outcrop geometries during foreland basin reservoir characterization seems relevant to (i) calibrate and propose alternatives in seismic data interpretation, (ii) to address the fracture location/attributes that are completly missing in such dataset.
From bottom to top, from oldest to youngest, details of the stratigraphy are given as follows: Butmah formation, Najmeh formation, Shiranish formation, Gercus formation, Fars formation. The present fractured reservoir is located by the black star.
Scientific references:
Cavailhes, T.,Funk, E., Monstad, S., Paulissen, W., Marré, J., Riva, A., Looser, M., Chalabi, A., Figa, M., Morset-Klokk, H., Canner, K., Berner Mitchell, K., and N. Bang, 2015, Investigating the hierarchical impact of polygenetic fracture populations in compressive settings: Implications on static fracture modelling: Conference: Banff Alberta, Mountjoy meeting: Advances in characterization and modeling of Complex Carbonate Reservoirs.
Huang, Q., Angelier, J., 2009, Fracture spacing and its relation to bed thickness:Geological Magazine, 126, 4, 355-362, Cambridge University Press.
Mishra, P., Bhatnagar, G., 2014, Self-similarity: At right angles 3.2, 23-30