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Fluid Venting System - Hydrocarbon Leakage Indicators
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Calibration of positive high amplitude anomalies (PHAAs) on 3D seismic with drill-hole samples & comparison of PHAAs with outcropped methane-related carbonates


Conference proceedings

Seep carbonates: seismic expression vs. outcrop observations


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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. 

Key words: Sutieng Ho pockmark Sutieng Ho gas chimney seismic Sutieng Ho polygonal fault Sutieng Ho methane authigenic carbonate Sutieng Ho Lower Congo Basin Angola
Figure above: A positive high amplitude anomaly in 3D seismic has been confirmed its hard carbonate origin. Figure below: A methane-related carbonate outcrop has a size comparable with the positive high amplitude anomaly in seismic.

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