Calibration of positive high amplitude anomalies (PHAAs) on 3D seismic with drill-hole samples & comparison of PHAAs with outcropped methane-related carbonates
Presented by: Sutieng Ho, Jean-Philippe Blouet, Patrice Imbert, F. Tampilang, J. Podia, B. Paternoster
in 30th IAS Meeting of Sedimentology 2013 Patches of positive high-amplitude anomalies (PHAAs) are commonly observed on seismic data in mud-dominated deep-water siliciclastic series, like mud-rich turbidite systems along passive continental margins. They are generally interpreted as methane-derived carbonates 1) in the absence of any other convincing mechanism to produce such local anomalies and 2) by reference to present-day seafloor observations of seep carbonates. The absence of direct physical calibration however leaves some uncertainty on this interpretation. Better characterization would help interpreting the conditions of their formation, itself related to the understanding of the evolution of seepage vs. time in buried series. Two occurrences of PHAAs were recently drilled, providing the first direct calibration of these anomalies. One of the boreholes recovered a 10 cm-scale sample from an isolated slightly depressed PHAA “thanks to” borehole caving; the other drilled through a series of stacked subcircular PHAAs forming depressions up to 7 m deep with a maximum diameter of 300 m. The logs recorded show that the corresponding sediment bodies are characterized by a high sonic velocity and a high resistivity, along with a local decrease in the rate of penetration indicating harder material. Sampling in the second case is limited to ditch cuttings recovered every 10 m. About 10% of carbonate cuttings were observed in the interval where the amplitude anomalies develop, whereas none were seen above or below. This suggests that carbonates occur as relatively thin beds or scattered nodules rather than making a continuous, thick layer. Examination of the cuttings from the second borehole show that the ca. 10 % of limestone fragments in the interval of PHAAs consist of dark gray micrite with several generations of calcitic cement including sparite and botryoidal calcite, while the encasing muds contain foraminifera and abundant pyrite framboids indicating a reducing environment of emplacement. On the other hand, methane-derived carbonates at the outcrop have been observed in the Vocontian basin, SE France. The role of methane in their formation is evidenced by the depletion in δ13C isotope with respect to normal marine values, while continuous carbonate layers in the same series show normal marine isotopic signatures. Methane-derived carbonates occur as nodules, tubes and irregular concretions a few cm to a few m in diameter. They are typically clustered in preferential layers a few meters thick, where individual concretions make from a few % to ca.20 % of the interval. Petrographic examination shows the same lithological and diagenetic characteristics as in the boreholes. Put together, these observations allow proposing a geometric model for the positive high-amplitude anomalies observed in the Lower Congo Basin: they are likely to correspond mainly to a local enrichment in subseismic-scale nodules, tubes and concretions, many of them potentially emplaced below seafloor, in the sulphate-methane transition zone. The seismic response would thus result from the averaging effect of the seismic signal over a domain locally enriched in carbonate nodules and concretions.
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