Soil-gas method for tracing neotectonic faults in clay basins: the Pisticci field (Southern Italy)

Two soil gas surveys were performed in 1991 and 1993 over an area of 95 km( 2) at Pisticci, Southern Italy. The research was aimed at assessing the rel ationship between soil gas distribution and fracture systems in a Plio-Plei stocene clay basin located in the Bradanic trough. More than 400 samples we re collected to measure the He, CO2 and Rn-222 concentration in soil atmosp here. Very large helium anomalies (Delta He), up to 21 000 ppb above atmosp heric reference, were detected in 1991 in correspondence with the youngest fractures reported in the literature. In order to verify the persistence of such soil gas anomalies, a survey was conducted again in 1993 in subareas where major helium anomalies were previously detected. The values of CO2 an d Rn-222 have shown about the same average as that in the 1991 survey, whil e Delta He values have revealed a sharp decrease with maximum values, not e xceeding 3000 ppb. This considerable difference is tentatively related to t he seismic activity that occurred just before the 1991 survey, whereas no a ppreciable seismic activity was reported in 1993. The results of the soil g as surveys show that the shape and spatial distribution of the gas anomalie s are related to the structural pattern of the area, whereas the magnitude of gas leakage is controlled by the occurrence of deep gas-bearing traps (h ydrocarbon reservoirs) and possibly triggered by seismic activity. (C) 2001 Elsevier Science B.V. All rights reserved.