GNGTS 2018 - 37° Convegno Nazionale

572 GNGTS 2018 S essione 3.1 formations showing a good continuity only at the main lithological boundaries (i.e. carbonates/ marls, marls/sand and clays), which correspond also to the main unconformities. Porosity sections do not show any relevant characteristics in term of anomaly, being not representative for the identification of gas. Results obtained by applying the multi-attribute analysis to predict P-velocity and resistivity panels demonstrated that only resistivity anomalies can be considered as an indicator of gas within sediments. Fig. 2 shows gas concentration on the seismic line STENAP 08. The first 2500 CDPs appear to be characterized by a sub-horizontal gas distribution from the surface down to 1.2 s, while the rest of the line shows local increases in gas concentration along vertical paths. The most evident vertical path matches with a velocity pull-down, located in proximity of an abandoned gas field. Some of the bright spots recognizable in the seismic profile are not associated to high gas concentration. This is because porosity estimation fails in a few isolated points, leading to exclude these points from the quantification. Gas can reach considerable values of concentration and it is very difficult to give an estimate of related errors (especially in the shallowest part, where seismic is very noisy). However, relative variations show a specific distribution that can be justified through the geology of the area. The occurrence and distribution of gas within the sedimentary succession, in fact, reflects the stratigraphic setting, being characterized by the presence of thick sandy and clayey turbidite layers, permeable for gas-charged fluid accumulation. Gas appears to be hosted at various levels within the Plio-Quaternary succession and to migrate upward through high-permeability paths and, in places, through sub-vertical faults. Only part of it would reach the surface and partially contribute to gas seepages (Donda et al. , under review). Further studies will be assessed in better understanding the gas dynamics in the subsurface, especially in relation with the present tectonic activity and past fluid industrial extraction. References Donda, F., Forlin, E., Gordini, E., Panieri, G., Buenz, S., Volpi, V., Civile, D., and De Santis, S.; 2015: Deep- sourced gas seepage and methane-derived carbonates in the Northern Adriatic Sea . Basin research, 27(4), 531-545. Donda, F. Tinivella, U., Gordini, E., Panieri, G., Volpi, V., Civile, D., Forlin, E., Facchin, L., Burca, M., Cova, A. and Ferrante, G. M.: The origin of gas seeps in the Northern Adriatic a Sea . Italian Journal of Geosciences, under review. Adelinet, M. and Le Ravalec, M.; 2015: Effective medium modeling: How to efficiently infer porosity from seismic data?. Interpretation, 3(4), SAC1-SAC7. Coren, F., Volpi, V. and Tinivella, U.; 2001 : Gas hydrate physical properties imaging by multi-attributes analysis – Black Ridge case history . Marine Geology, 178, 97-210. Fig. 2. Gas concentration, in % of volume, of STENAP 08 seismic line. The SE part is characterized by a deep and horizontal gas distribution, while the rest of the line is interested by an increase of concentration along vertical paths. Where it is not able to escape through vertical paths, gas seems to accumulate in the shallower 200 ms, probably thanks to clayey turbidite layers. Between CDP 4500 and CDP 5000 a long vertical path is associated to a velocity pulldown.