FRACTURE LOGGING INTO A FOLD
Forewords
Understanding and quantifying both fracture distribution and attributes remain key issues in terms of naturally fractured reservoir characterization. Questionning ourselves on outcrops is crucial to propose alternative fracture logging interpretations. In this example, we simply discuss the relation between folding, faulting, fracturing and cementation into a fictive borehole. The aim of this analysis is to decipher the different origins of structural heterogeneities occurence along a vertical scan-line.
Outcrop description (FIGURE 1)
The host succession is made of cm-thick grey marls and cm-thick grey layers of calci-turbidites. The host succession has probably been buried to 3 km of depth before deformation. The structural fabric (cleavage) developped into the marls suggests that the folding phase postdates the cleavage acquisition. The 10 cm-thick layer (R1) of carbonates have been also deformed under a brittle regime ( thrusting/fracturing/cementation) most probably contemporaneously with folding. Strong lithologic mechanical contrasts are also recognized.
FIGURE 2
Fictive structural logging through the studied fold showing some of the structural attributes of (i) bedding and (ii) fractures. Fracture intensity at least changes as a fonction of the stiffness, thickness, curvature of the unit layers, the distance to the thrust and the structural location (inner or outer part of the fold).
How could we use this outcrop in exploration?
We show that the final structural architecture of the reservoir would be a function of the relative input of : (1) the fold shape i.e. crest signal, hinge signal, intrado, (2) the fault-related fracturing i.e. core zone/ damage zone attributes and related structural diagenesis, (3) the lithomechanical contrasts related to the mechanical stratigraphy and (4) the regional background fracturing that is not obvious at this scale of investigation (Al Darmaki et al. 2014; Cavailhes et al., 2015).
Both the structural bedding/strata dips and the fracture attributes have to be considered synchronously in order to fully capture the different origins of the fracture sets. Interestingly, the fracture dips related to thrusting are easily recognizable based on their low angle dip (20°) and their cementation. They are expected to compartimentalize, at the production time-scale at least, the R1a and the R1b connectivity, therefore leading to two different plays (?).
The calcite cementation, significantly reducing both the structural porosity and permeability, is clearly located into the fault (thrust) and core damage zones. The fracture porosity is preferentially located into the extensional deformation of the fold, this latter being related to the lithologic mechanical contrast between « thick/brittle carbonate » and the marls. These fractures seem to be only partly-cemented, therefore keeping a structural porosity.
The fluid storage, injection and production preferential area would be located, based on a 2D analysis at least and despite the partial cementation, at the top of R1b.
The constructive comments are always very welcome.
References:
Al Darmaki, Mattner, J., Bouzida, Y., Cavailhes, T., Burreson, M., Lawrence, D., Lucas, N., 2014, Identification, Upscaling and Modeling Strategy for Multi-Scale Fracture Networks in Variable Lithology Reservoirs: EAGE, Borehole Geology Workshop, 13–15 October, Dubai, UAE.
Cavailhes, T., Funk, E., Monstad, S., Paulissen, W., Marré, J., Riva, A., Losser, M., Chalabi, A., Figa, M., Morset-Klokk, H., Canner, K., Bang, N., 2015, Investigating the hierarchical impact of polygenetic fracture populations in compressive settings and their implications on static fracture modeling, 2015, Conference: Banf Alberta, Mountjoy meeting: Advance in characterization and modeling of complex carbonate reservoirs.