164 A new concept of karst development based on hydrogeology and geophysics Stratigraphy versus borehole electrical methods This stratigraphic framework has not yet been thoroughly correlated with borehole geophysical logging data, particularly natural gamma-ray (NGR) and resistivity logs (single point, short normal, and long normal). The main limitation lies in the difficulty of aligning the detailed stratigraphic interpretation with NGR signals, which generally provide low resolution in carbonate settings, and to a lesser extent, with short normal resistivity responses. As illustrated in Figure 12, a comparison between the C1 core log and the M1 geophysical log—the two boreholes exhibiting the least karstification—demonstrates this discrepancy. In the C1 borehole, discontinuities D7 to D9 are clearly identified and supported by core observations. However, in borehole M1, the gamma-ray curve differs significantly, and only the D8bis discontinuity can be reliably identified, primarily due to the clay-rich nature of the erosional surface, which produces a clear GRN peak. In contrast, resistivity logs provide more coherent results. For example, the D8bis discontinuity corresponds to a high-resistivity peak exceeding 3500 ohm·m, while the “Assise à terriers” Formation is characterized by a general increase in resistivity. A distinct peak around 3000 ohm·m is associated with the D7 discontinuity. These more consistent resistivity trends offer better stratigraphic correlation, although they remain locally disturbed. The difficulty in log interpretation is partly attributed to karstic features filled with younger sediments, such as Cenomanian clays, which can significantly alter geophysical signatures (Bassil et al., 2016). The first conductive horizon is identified on borehole M4, between 50.05 and 53.15 m depth, where a sequence of vertically stacked cavities up to 1 m in height is observed both on the OPTV log and on the long normal resistivity log (Fig. 13a). This karstified interval corresponds to a bioturbated interval that is also recognized in boreholes M09, M10, and M14. The second conductive zone, between 83 and 89 m depth, is attributed to the upper dolomitized interval. In borehole M11, a prominent cavity is located between the D7bis hardground and an overlying clayrich horizon. This dolomitized zone, clearly delineated by low resistivity values (Fig. 13b), is consistently observed in boreholes M7, M8, and M20 (Fig. 11) and corresponds to dolomitization within the Oolitic formation. In borehole M20, adjacent to M01, another cavity is detected from 96.4 to 102.2 m depth by acoustic data (Fig. 11). This feature lies above the D6bis discontinuity, which caps a sequence of well-preserved centimetric bioturbated strata. Based on a long normal resistivity log, the most pronounced porous interval is the upper dolomitized zone, situated below the D7bis hardground that defines the top of the Oolitic Formation. A secondary conductive zone, above the D7 discontinuity, occurs within the Oolitic and Oncolitic formations of the Concavum Biozone (Aalenian) and is correlated with the middle-dolomitized zone, bounded at its base by the D7 discontinuity.
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