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DEVELOPMENT OF DEEP EXPLORATION IN THE GEOTHERMAL AREAS OF ,

Guido Cappetti', Romano Celati2, Ugo Cigni', Paolo Squarci2, Giancarlo Stefani' and Learco Taffi2

ENEL, Unita Nazionale Geotermica P.zza Bartolo de Sassoferato 14 54100 Pisa, Italy

'Istituto lnternazionale per le RiceJche Geotermiche CNR Via del ~u~izg~~to1 541 00 Pisa, Italy

3ENEL, Uizita Nazionale Geotermica Lirrderello (PI), Italy

ABSTRACT information, based mainly on drilling data, is made available. A more accurate picture has thus been attained The first deep well (2703 m) in the Larderello field utilizing new data on the stratigraphic, structural and was drilled in 1961, with the target of improving hydrogeological characteristics of the rocks and the physical knowledge on the structural setting of the formations and chemical characteristics of the fluids present in the underlying the shallow exploited reservoir, their underground. permeability and the physical-chemical characteristics of the fluid encountered. This borehole proved the existence Prior to deep exploration and the seismic reflection of deep fractures and of increases in temperature and prospectings, our knowledge was limited to the first 700 to pressure with depth. Production from deep wells was, 800 m b.g.1.' The structural-hydrogeological model however, considered uneconomic until the oil crisis of the developed for that depth interval in the three Tuscan 1370~~when the deep drilling program was given renewed geothermal fields includes: impetus. Serious drilling problems were encountered in the A- a practically impermeable cover of Neogene clastic central part of the Larderello field, where closely spaced formations (upper Miocene-Pliocene) and fractures and corrosive fluids are encountered for very long all~hthonousflysch (Ligurian Nappes: Cretaceous- depth intervals. These difficulties have so far prevented the Eocene) (Figure 1, units 2 and 3); development of a systematic production-oriented deep B- a reservoir of Mesozoic evaporitic and carbonate drilling program in this area. Deep drilling gave formations overlain locally by an Oligocene arenaceous commercially interesting results in the peripheral areas of series (Tuscan Nappe) (Figure 1, units 4 and 5). Larderello and in the Travale and Mt. Amiata areas, where Permeability in this complex is tied mainly to tectonic great thicknesses of impermeable formations are found movements, and preferentially occurs close to the planes before reaching the deep productive horizon. separating different tectonic units, such as the contact between units 3 and 4 or 3 and 5 (Figure 1); C- a Triassic-Paleozoic basement of terrigenous (phyllite INTR~~UCTIO~ and quartzite) formations of varying grades of The structural and hydrogeological model of the metamorphism and generally low permeability. Tuscan geothermal fields (Larderello, Travale and Mt. The first step in the direction of deep exploration, Amiata) has been continually revised and improved as new directed at the deeper levels of the Triassic-Paleozoic

303 Deep Exploration in Tuscany, Italy

W E~

81.0 - -0

- 1000- - .. 1000

- 2000 - - - 2060

- 3000 - - - 3000

- 4000- - - 4000

-5000 -I-A A'

a ~ b I.rt.:-l R-ql I"J mj 4 5 6 7 8 9 10 11

Figure 1. Geological cross section through the ~rdetellogeothermalfield. 1. Mr. Amiata rhyodacitic volcanites (0.4-0.2 Ma). 2. Neogene sediments (upper Miocene- Pliocene). 3. Fl ysc h nappes: shal y-ma rl y-a renaceous- formations (Cretaceous- Eocene). 4. Tuscan Nappe: a-sa nds tone, b-pol ych rome shale and calcarenite. 5. Tuscan Nappe: limestone, magnesian limestone and breccia, dolostone and anhydrite (Upper Trias-Lower Cretaceous). 6. Tectonic slices complex: quartzite, phyllite,anhydrite and dolostone (Upper Trias) with metamorphic basement rocks (Paleozoic). 7. Filladi Inferiori Groups: phyllite and quartzite (Ordovician-Silurian) (Larderello basement). 8. Formation: carbonatic quartzite, phyllite, metagreywacke, basic metatuffite with dolomitic marble and anhydrite (~vonian-SiIurian)(Travale basement). 9. ~etagreywac~eand metapelite (~rboniferous-Devonian(?))(Mt. Amiata basement). 10. Marble (Paleozoic) (Mt. Amiata basement). 11. Micaschis~,gneissand amphibolite (Lower Paleozoic-Precambrian(?)). 12. Deep reflecting horizon. 13. Isotherm. For location see Figure 3.

basement, was taken as far back as 1951; assuming that the dimensional problem, i.e. determining the areal extension geothermal fluids were of magmatic origin, a drilling of a well-defined productive horizon, but at this point a program was drawn up for a 3000 m well aimed at third dimension, depth, attained equal importance. recovering a higher temperature and higher pressure The geological studies of the basement formations, steam. This project was postponed for financial reasons either outcropping or crossed by wells, led co their until the early 1960s, when a 2703 m well was drilled in the stratigraphic reconstruction and to an interpretation of the Larderello zone. This well crossed several fractured structural setting (Gianelli, Puxeddu and Squarci, 1978). horizons within the basement, whose temperatures and The structural and stratigraphic data suggested that pressures increased with depth. The well, however, proved exploitable fractured horizons could exist at depth. This noncommercial. hypothesis was also corroborated by the results of a seismic The unsatisfactory results in terms of production, survey at Larderello, which revealed the existence of a drilling problems and the low cost of the conventional series of deep reflecting horizons, one of which extends energy sources caused the initial project to be abandoned over the entire region at depths between 3000 and 5000 m and replaced by one with more limited objectives. The wells (Batini and others, 1978). This horizon is also present in programmed from 1968 on were directed at exploring the the Travale and Mt. Amiata areas, but at greater depths. layers of the reservoir immediately below those already On the basis of the available data, well Sasso 22 was exploited, in densely drilled zones whose reservoir drilled in the period 1978 to 1980, reaching a final depth of pressures were close to the rating value of the power plant. 4092 m. This is the deepest geothermal borehole in Italy, Even a small increase in pressure would have improved and the first to encounter temperatures in the order of conversion efficiency. 400" C. The first of these wells, Sperimentale 1, was drilled at During this same period deep exploration also began Larderello in 1968-1969 and reached a depth of 1097 m, in the Mt. Amiata geothermal region, with one well drilled cro'ssing about 750 m of the 'basement' format~ons.The in the field and another in the ~iancastagnaio well revealed the existence of three productive levels at field, in 1978-1979. Both wells discovered a water- 300 m, 370 m and 750 m, with pressures increasing from dominated reservoir within the basement, 2000 m below 4.8 to 6.9 bar (Ferrara, Panichi and Stefani, 1970). the already exploited horizon, with a pressure of about 200 The oil crisis of the early 1970s brought a renewal of bar and temperatures above 300°C. interest in the geothermal resource and offered new Deep exploration began in the Travale field in 1981. prospects for deep exploration. Hitherto exploitation of The first two wells were drilled in the 'old' field, which is no the geothermal fields had been considered a two- longer exploited. They encountered a vapor-dominated

304 Cappetti and others

S.Pompeo 2 S

s.1. 0. 0

- 1000 ' -1000

-2000- - 2000

- 3000, - 3000

- 4000

- 5000 B

Figure 2. Geological cross section through the Larderello geothermal field. (For symbols see Figure 1. For location see Figure 3.)

Figure 3. Temperature distribution at the top of the geothermal reservoir (A) and at 3000 m depth (B) in the Larderello-Travale geothermal region

productive horizon at a depth of about 1800 m, separated metamorphic basement. This complex (6) consists of from the shallower water-dominated reservoir by about "tectonic slices." Beneath it, and separated by a tectonic 1300 m of practically impermeable terrains. surface,is the top of the metamorphic basement, consisting predominant~yof phyllites (7) and of micaschist and gneiss DEEP EXPLORATION IN THE (11). These formations are presumed to belong to the Mid LARDERELLO AREA Paleozoic-Precambrian. According to petrologic and The structural situation beneath tectonic units 3,4 and radiometeric data, the basement rocks have undergone a 5 (Ligurian and Tuscan Nappes) is complicated (Figures 1 Hercynian polyphased metamorphism, followed by late and 2) by tectonic repetitions of series, consisting of Hercynian and Alpine thermal events (Puxeddu, 1984). Triassic detrital me tasediments (phyllite, quartzite), The thickness of this unit varies from 1.5 to 3-4 km anhydrites, dolostones, with rocks of the Paleozoic (Figures 1 and 2).

305 Deep Exploration in Tuscany, Italy

The first deep well, Sasso 22 (Figure 3), provided to the “Filladi Inferiori” formation, with reservoir information on permeability distribution and on the pressures of about 30 bar. thermodynamic characteristics of the fluids within the Beneath this zone the well crossed impermeable basement (Bertini and others, 1780). Some of the relevant phyllites and micaschists to 2730 m, at which depth it characteristics of the well are shown in Figure 4. A sequence encountered a fractured horizon. The high temperatures of permeable horizons were detected within the basement; and pressures of the fluid in the latter impeded further permeability was found in the 1500 to 1700 m interval in drilling and measurements. Reservoir pressure and the “Filladi Inferiori” formation and in the 2400 to 3800 m temperature could not be measured, as each time the bit interval in the micaschist and gneiss. Reliable values of reached this horizon violent explosions caused the reservoir pressure were obtained by transient test analysis formation to cave in and hundreds of meters of debris filled at 1200 m (25 bar) and 3000 m (55 bar). Reservoir the borehole. An indirect evaluation of pressure can, temperatures were obtained’ by temperature transient however, be made for the deep horizon if we consider that: analysis at bottom-hole, in impermeable intervals. -the well blew out during drilling with a return Temperature and pressure data reveal that steam is the circulation of water; dominant phase in the reservoir, at least to depths of -after the last violent blowout 2560 m of the well were almost 300 m. accessible; beyond that depth it was filled with debris. The well was then shut-in in order to record pressure ABSORPTION PROFlL E RESERVOIR TEMPERATURE (“C ) (% OF FLOW) build up. When wellhead pressure reached 150 bar, it was 20 40 80 80 10 6 200 300 400 P I I 212 bar at 2560 m. Extrapolation to 2730 m gave about 240 bar. No further pressure build up could be recorded at wellhead due to leakages in the surface equipment and in the casing. Shortly afterwards pressure and tempera- ture logs were run while the well was open and emanating only small quantities of gas and steam: there

I was no liquid in the well and a temperature of 334” C was 4 I measured at 2560 m. \ \\ On the basis of these data, we can infer that fluids with \ pressures above 240 bar and temperatures above 400” C are ! \ present in the fractured horizon encountered at 2730 m. It \ \ would appear that the deep fractured horizon is isolated \ \ from the overlying productive horizons. \ \ \ Deep exploration in the marginal areas began in the \ \ northwest zone of the field and revealed productive \ \ \ horizons at depths between 2000 and 3000 m, in the \ \ micaschist formation; pressures of about 70 bar and \ \ temperatures of 280 to 330°C were recorded. These wells \ \ produce dry steam at rates of 25 to 30 t/h. \ \ Exploration will also be extended to the southeast . \. margins of the field, near the absorption areas of meteoric Figure 4. Stratigraphy and completion of well Sasso 22, and results of waters. The upper part of the reservoir in these areas is flowmeter log, run with bottomhole at 2960 m and undisturbed reservoir affected by cold water circulation but high temperature temperature (estimated from temperature buildup). (For geological gradients have been found in the basement, resulting in symbols see Figure 1.) temperatues of about 300°C at a depth of 2000 m. Due to the results of Sasso 22, and particularly to the Deep exploration confirms that the areas with high difficulties encountered in drilling densely fractured temperature in the upper parts of the reservoir are mainly formations, the deep exploration programs were continued limited in size by circulation of the meteoric waters. The along two different lines: thermal anomaly at 3 km depth is much more widespread, -research activity in the central part of the field, aimed at as shown in Figure 3 and in the cross sections in Figures 1 improving reservoir knowledge and drilling technology; and 2; the maximum on the southern margin of the -exploitation activity, directed at recovering fluids of geothermal field corresponds to the peak of the deep higher temperature and pressure in the marginal areas reflecting horizon. of the field,.where low permeability had been detected in DEEP EXPLORATION IN TRAVALE AREA the upper parts of the potential reservoir formations. THE As part of the research program, well San Pompeo 2 In the Travale area the complex of tectonic slices is was drilled about 5 km s,outhwest of Sasso 22, where the missing and the Tuscan Nappe lies directly over the highest temperatures were recorded in the upper part of basement. The latter is characterized by a thick Paleozoic the reservoir (>300”Cat 1200 m), and where the main series of alternations of carbonate quartzites, phyllites, reflecting horizon detected by seismic prospecting was metagreywackes, graphitic phyllites and basic metatuffites nearest the surface (-3000 m)(Batini and others, 1983). with recrystallized dolostones and anhydrites (Castellucci, Dense fracturing in the basement was also encountered by Minissale and Puxeddu, 1983) (“Boccheggiano this well down to a depth of 2300 m, in correspondence Formation”) (Figure 5). Beneath the “Boccheggiano

306 Cappetti and others

w E 500 - -500 ‘

s. I. 0 - -0 - -

- 1000- - -1000 - -

- 2000- - -2000 I -

Figure 5. Geological cross section through the Travale geothermal field. (For symbols see Figure 1. For location see Figure 3.)

Formation” there are probably micaschists identical to those of the Larderello zone, as found in a deep well drilled between Castelnuovo and Travale. They could, therefore, form a continuous horizon below both areas. Deep exploration began in the Travale field in the 1980s and so far only two 2000 m wells have been drilled in the “old field” zone, near the outcrops of the reservoir formations (Burgassi and others, In the upper levels vvvvv, 1975). vw v VN of the reservoir in this area complex interactions between vvvv the vapor-~ominatedsystem and recent meteoric waters (Cataldi and others, 1970;Celati and others, 1977) led to the abandonment of exploitation of the shallow reservoir in the 1960s. The deep wells encountered fractured horizons at 1700 to 1800 m depth, with temperatures of 280” to 290°C and pressures of 65 to 75 bar. The deep horizon seems to be connected with the now exploited vapor-dominated system, whose initial pressures were 60 to 70 bar. The isotherms drawn in Figure 5 show a rise in the carbonate-evaporitic shallow reservoir in correspondence to the uplifted sector of the structure. This rise was caused Figure 6. Geological sketch map of the Mt. Amiata geothermal region. 1. Mt. Amiata rhyodacitic volcanites (0.2-0.4 Ma) and Radicofani by a flow of steam that, ascending through the fracture trachybasalts (0.97 Ma). 2. Neogene mainlyclayey sediments (Pliocene). 3. system on the western margin of the graben, spread into Flysch nappe: shaly-marly-arenaceousformations (Cretaceous-Eocene).4. the shallow fractured horizon. The temperature inversion Tuscan Nappe: polychrome marl, shale and calcarenite (Upper beneath the carbonate-evaporitic reservoir can be ascribed Cretaceous-Eocene). 5. Tuscan Nappe: limestone, magnesian limestone and breccia, dolostone and anhydrite (Upper Trias-Lower Cretaceous). 6. to low permeability. The deep thermal anomaly also has a Location of the geological cross section. 7. Geothermal field. 8. Area of maximum in correspondence to the uplifted structure on deep exploitation. 9. Deep geothermal well. the western margin of the graben.

w Bagnore Piancastagnaio E

2 3 4km L - 4000 - 4000 A??..1

Figrrre 7. Geological cross section through the Mt. Amiata geothermal field (For symbols see Figure 1.)

307 Deep Exploration in Tuscany, Italy

DEEP EXPLORATION IN THE MT. AMIATA AREA 2500 and 3500 m; these should permit the installation of In this area (Figure 6), as in Travale, the complex of seven 20 MW power plants, totaling 140 MW. tectonic slices is missing (Figure 7). Beneath the evaporitic TECHNOLOGICAL PROBLEMS OF formation of complex 5 is a thick sequence of metapelite DEEP DRILLING and metagreywacke of probable Carboniferous- Devonian(?) age (Bagnoli and others, 1980). There are also The deep wells drilled in the peripheral areas of the lithotypes, in complex tectonic structures, that seem to Larderello field, and in the Travale and Me. Amiata areas, belong to the “Boccheggiano Formation.” Underlying the presented no serious technological problems as, once the Carboniferous-Devonian( ?) phyllitic series is a predomi- shallower reservoir had been isolated by the casing, the nantly carbonate metamorphic formation that has been deeper ‘basement’ formations were usually compact crossed by a few wells for about 500 m. enough for drilling to proceed to the deep productive The Bagnore and Piancastagnaio fields (Figure 6) horizon with mud or water circulation. were discovered in the late 50s and early 60s in the Mt. Deep drilling in the central area of the Larderello field, Amiata area (Calamai and others, 1970) and are still being on the contrary, met with a series of problems caused exploited. Production in these fields was obtained from the mainly by the intensely fractured reservoir rocks, their high upper part of the Triassic carbonate-evaporitic formation temperatures and corrosive fluids. The presence in the underlying the cover. The wells drilled here led to the areal basement of rocks of very different drilling coefficients and delimitation of the shallow thermal anomalies, so that the complex schistosity patterns made it very difficult to policy was essentially to exploit the identified resource. maintain the well verticality required to reach the deep layers. In these conditions, a specially stabilized drill string At the end of the 1970s, in an attempt at recovering had to be used regularly so as to control deviation of the well fluids of higher temperature at greater depths, two axis and avoid friction, wear and fatigue on the drill string. exploratory wells were drilled, one at Bagnore and the This methodology, commonly used in drilling, is other at Piancastagnaio, at the same time as deep explor- particularly hazardous in geothermal wells when drilling ation was beginning at Larderello. These and later wells without return circulation. In these circumstances, drilling revealed the existence of a mainly phyllitic formation, long stretches of open hole is a risky endeavour. practically impermeable to a depth of 2.6 to 3.5 km beneath the carbonate-evaporitic formation. At these depths a Consequently, the fractured zones have to be isolated by water-dominated reservoir was found with temperatures casings and the problem arises of drilling to great depths with industrially acceptable diameters. The presence of between 300 and 350°C and pressures of 200 to 260 bar. fractures also creates problems in achieving a successful Despite the limited transmissibility of the productive cementation of the casings. layer (0.1 to 0.5 darcy.m) and great depth, the wells are Corrosion becomes a serious problem when drilling commercial producers because of high reservoir without return circulation, as there is no possibility of temperatures and pressures. Flashing occurs in the controlling the chemistry of the fluid in the well. formation after a short production period. Until now only Generalized corrosion and pitting on drillpipes were one of the Mt. Amiata deep wells (PC 26) has produced for observed in many wells when crossing highly fractured a long period (2 years),giving about 40 t/h of steam with 5 formations; it is thought to be caused by a water-rock percent noncondensable gas and 5 m3/h of liquid water interaction in a high-temperature environment (leaching) with 20 g/1 TDS (predominantly boron, sodium chloride with dissolution of the most soluble salts, and by the and silica). presence of reservoir fluids that generally contain gases So far 10 deep wells have been drilled. All the wells such as C02 and H2S and such notoriously aggressive ions as drilled at Piancastagnaio are productive. The relatively low chloride and ammonium. As these corrosion phenomena gas content (about 5 percent) permits their utilization in usually develop at a rate comparable to ordinary mechanical condensing power plants. The noncommerical wells are wear, they can be kept under control. Any extensively located in the Bagnore area, Poggio Nibbio area and in the damaged material can be replaced whenever necessary. graben east of Piancastagnaio, where reservoir Stress corrosion (SCC), on the other hand, is a far iemperature is lower than’at Piancastagnaio. All these more serious problem as, in this case, it is impossible to wells did reach the deep permeable horizon, so it predict failure of the drill string. The consequences to the presumably extends beneath the entire area of Bagnore, well are at times disastrous. Stress corrosion was Poggio Nibbio and Piancastagnaio. The cooling at depth in particularly severe in the tool joints and in the slip area of the Poggio Nibbio and Bagnore areas seems to be caused by the drillpipes. the presence, in the vicinity, of important absorption areas These phenomena were particularly aggressive in well of meteoric waters connected with the deep reservoir. The San Pompeo 2. Generalized corrosion was noted well drilled in the graben east of Piancastagnaio also throughout the entire thickness of the drill pipes and encountered the permeable horizon at about 2600 m depth, decarburization of the casing steel caused by atomic but the temperature there was below 250°C. Temperatures hydrogen attack. All these phenomena are due to high of 350°C were found at about 4 km depth, but in temperatures (>400” C) and extremely aggressive reservoir impermeable formations. fluids. Metallographic analysis of some fragments of casing The development program for the Piancastagnaio showed that carbon had dropped from 0.35 to 0.08 percent area based on these results includes the drilling of another and the original ferrite + perlite structure of the material 47 wells over an area of about 30 km2, at depths between had been transformed to ferrite only. Research is now

308 Capetti and others

directed at solving these corrosion problems, at Batini, F., Bertini, G., Bottai, A., Burgassi, P.D., Cappetti, G., Gianelli, G. constructing special instrumentation and equipment, and and Puxeddu, M., 1983, European Geothermal Update: 3rd Intern. at developing muds and cements for use in deep drilling. Seminar on Results of EC Research and Demonstration Projects in the Field of Geothermal Energy, Munich, 29 Nov.-1Dec. 1983, p. CONCLUSIONS 34 1-353. Batini, F., Burgassi, P.D., Cameli, G.M., Nicolich, R. and Squarci, P., 1978, (1) Deep exploration on the inside of the Larderello Contribution to the study of the deep lithospheric profiles: deep field revealed the existence of a series of fractured horizons reflecting horizons in the Larderello-Travale geothermal field: Mem. in the basement to the investigated depth of about 4000 m, SOC.Geol. It., v. 9, p. 477-484. and the presence of the vapor-dominated system down to Bertini, G., Giovannoni, A., Stefani, G.C., Gianelli, G., Puxeddu, M.and about 3000 m. Squarci, P., 1980, Deep exploration in the Larderello field: Sass0 22 Well San Pompeo 2 reached the lower layers of the drilling venture: Advances in European Geothermal Research, Proc. metamorphic basement and encountered within it a 2nd International Seminar on Results of EC Geothermal Energy fractured zone containing abnormally high temperature Research, Strasbourg, 4-6 March 1980, Riedel, Dordrecht, p. 303-31 1. and high pressure fluids, separated from the overlying Burgassi, P.D., Stefani, G.C., Cataldi, R., Rossi, A.,Squarci, P. and Taffi, L., fracture systems. 1975, Recent developments of geothermal exploration in the Technological problems ensuing from drilling into Travale-Rad~condoliarea: Proceedings Second U.N. Sym~siumon intensely fractured formations (Le. without return the Development and Use of Geothermal Resources, San Francisco, circulation), as well as corrosive fluids, have so far May 1975, v. 3, p. 1571-1592. prevented actuating a systematic production-oriented Calamai, A., Cataldi, R., Squarci, P. and Taffi, L.. 1970, Geology,geophysics drilling program in this zone. and hydrogeology of the Monte Amiata geothermal fields: (2) Deep drilling gave commerically interesting Geothermics, special issue 1, p. 1-9. results in the marginal areas of the Larderello field, at Castellucci, P., Minissale, A. and Puxeddu, M., 1983, Nature and tectonic Travale and at Monte Amiata. Great thicknesses of setting of Travale-Radicondoli basement in the Larderello impermeable formations were encountered beneath the geothermal field (Italy): Mem. SOC. Geol. *It., v. 25, p. 237-245. shallow exploited reservoir in these areas, so that deep Cataldi, R., ROSSI, A., Squarci, P., Stefani, G.C. and Taffi, L., 1970, productive horizons could be reached without particular Contribution to the knowledge of Larderello geothermal region: d r i lli ng problems . remarks on the Travale field: Geothermics, spec. issue 2, v. 2, pt. 1, p. In the Mt. Amiata area, where the exploitable areas of 587 -602. the shallow reservoir were defined, deep drilling revealed Celati, R.,Squarci, P., Stefani,G.C. and Taffi, L., 1977,Studyof water levels the existence of a very wide productive horizon; the in Larderello region geothermal wells for reconstruction of reservoir development program is expected to increase the present pressure trend: Geothermics, v. 6, p. 183-198. installed capacity tenfold. Ferrara, G.C., Panichi, C. and Stefani, G.C., 1970, Remarks on the (3) Deep exploration revealed the presence of much geothermal phenomenon in an intensively exploited field. Results of wider productive horizons than the fields exploited at an experimental well: Geothermics, special issue 2, v, 2, pt. 1, p. 578-586 present, so that exploitation programs can also be drawn up Gianelli. G,, Puxeddu, M. and Squarci, P., 1978, Structural setting of the for zones that were formerly noncommercial. Larderello-Travale geothermal region: Mem. SOC.Geol. It., v. 19, p. 469-476. REFERENCES Puxeddu, M., 1984, Structure and Late Cenozoic evolution of the upper Bagnoli, G., Gianelli, G., Puxeddu, M.,Rau, A., Squarci, P.and Tongiorgi, lithosphere in southwest Tuscany (Italy): Tectonophysics, v. 101, p. M., 1980, Segnalazione di una potente successione clastica di eta 357-382. probabilmente carbonifera ne1 basamento della Toscana meridionale: Mem. SOC.Geol. It., v. 21, p. 127-136.

309