GNGTS 2013 - Atti del 32° Convegno Nazionale

are used to infer the subsidence of the Plain. The Bagnoli Plain (Fig. 1) was, thus, selected to perform the drilling activities for different reasons: a) it is the less investigated area, because the larger part of previous deep drillings were located in the eastern and central-northern sector of the caldera (AGIP, 1987; Mormone et al. , 2011; Carlino et al. , 2012 and references therein); b) the site is located about 1km west from the NYT volcano-tectonic structure (Posillipo cliff) and, therefore, new well stratigraphic data could represent a further constrain to understand the amount of sin-caldera collapse and post-caldera volcanic events; c) the site is less than 2 km from the eastern part of the La Starza marine terrace and from the most active Solfatara crater; and d) the site hosts the western part of the city of Naples, consequently it represents the highly exposed area to volcanic risk. Moreover, from technical point of view, the drilling site meets an important safety requirement, since it is located within an iron factory (ILVA), which is now dismissed, and therefore is far enough to residential buildings (>400 m) to guaranty the absence of risk straightforwardly related to the drilling activity. Finally, the borehole has been designed in order to be equipped with technologically advanced instruments for the seismic and geochemical monitoring the plain which houses part of the city of Naples, consequently it represents one of the highly exposed area to the volcanic risk in the caldera. Drilling operations. The drilling of pilot hole was conducted in to two phases in late sum- mer and late fall 2012, interrupted by three months of drill stop. Borehole diameters are 16 inch down to 33.5 m and 121/4” down to 222 m. In the second phase, the hole was deepened to 434 m (8 ¼ inch borehole diameter) and finally drilled to 506 m with 6 inch diameter (all depths are meter below Kelly Bushing). Almost the entire hole was drilled for cuttings, which were separated from the drilling mud at the shale shakers, analysed in a field laboratory and sampled for further studies. The drilling was carried out with circulating drilling mud having densities from 1.10 kgl -1 in the shallower part (222 m) to 1.20 kgl -1 at greater depth. In the last stage of drilling, from 423 m to 506 m the bentonite-based mud was replaced by a mud based on wa- ter and organic additives. At the bottom of the well, two cores were retrieved from the depth interval 438.0-438.9 m (0.9 m recovery) and 500-501 m (1 m recovery), respectively. The well is completely cased, with exception of the lowermost 80 m, where a slotted liner is installed. This will promote fluids and gases to flow into the well for future geochemical monitoring and sampling. At the end of the drilling, mud was replaced by water with anti-corrosion additive (Brineback). The Baker Hughes Company provided all monitoring services during the drilling, includ- ing pressure, mud temperature, rate of penetration, environmental and drill mud gas concen- tration in the mud, etc. The drilling was also accompanied by geophysical downhole logging, performed by Schlumberger Company, from 0 m to 422 m of depth. Analytical methods. We have analyzed mud samples from deep drilling exploiting of the 506 m pilot-borehole in the Bagnoli Plain. Cutting samples were collected every 3 m in the first 225 m and every 5 m down to 501 m. Samples were washed, sieved and dried at 80°C. Measurements were also performed on cored rocks collected at 438 m and 501 m of depths. The different grain size fractions and the cores were observed under a binocular microscope to define type, texture, mineralogy and alteration of the drilled materials; single crystals or fragments were hand-picked for further investigations. Thin sections of cored rocks were studied under a reflex-light-equipped optical microscope and by Scanning Electron Microscope (SEM) and Energy dispersed scanning microscopy (EDS). SEM observations and semi-quantitative and quantitative EDS analysis were carried out at CISAG Laboratory (Università di Napoli Federico II), by using a JEOL-JSM 5310 SEM, equipped with a Link EDS and a Inca 4.08 software. Operating conditions were 15 kV accel- erating voltage, 50-100 mA filament current, 5-10 µm spot size and 50 s net acquisition time. X-Ray Diffraction (XRD) intensity data were collected on selected grains, minerals or whole-rocks. We have used a X’ Pert Powder diffractometer by PANalytical, at the Istituto 265 GNGTS 2013 S essione 1.3