First Results From the Scientific Drilling at Colli Albani Volcanic Area (Italy)

Abstract

For the first time in Italy, a borehole was drilled SE of Rome in the Colli Albani volcanic district for scientific purposes. The 350m borehole contributes to understand the structure and the dynamics of the shallow crust in the Quaternary volcanic complex now quiescent, characterizing the rock units and defining the present stress field and its evolution in recent times. The borehole will host a broad-band seismometer for good seismicity records in a very urbanized area. We present the preliminary results of borehole data analysis and the ongoing researches. The well is located in the most interesting area of the volcanic complex where: 1) most seismic swarms occur; 2) geodetic and satellite investigations determined an ongoing uplift; 3) gas (CO2 and H2S) concentration in water is high; 4) local tectonics is peculiar and 5) stress data lack. Wireline drilling produced a complete stratigraphic record of the volcanic units down to the sedimentary basement. Biostratigraphic analysis are performed for a complete reconstruction of sedimentary sequence. Down-hole logs were carried out such as: gamma-ray, resistivity, sonic, borehole televiewer, caliper and magnetic susceptibility. Due to the shallow depth the whole logs are compared with cores. The Gamma-Ray provides a clear characterization of volcanic units and the definition of the intervals that most contribute to radioactivity. Resistivity and Sonic logs give a major definition of the different units and their characters. Caliper log and BHTV supply data (breakouts, fractures and faults) from which we interpret the main tectonic features and the present-day stress. Moreover a detailed magnetic susceptibility analysis is performed on shale samples. The blow-out occurred in the deeper sandy unit, allowed to sample deep fluids, that are now being analysed and will contribute to better understand the complex fluid flow. Mechanical analysis at high pressure and temperature are planned to understand the behavior of the volcanic rocks. Dynamic tests on Pliocene clays will determine parameters useful for the local seismic response; the result will be joined to the sonic log and volcanic laboratory measurements (Vp/Vs), providing data on real seismic behavior of these units.