Salvatore D'Andrea, Roberto Pasi, Giuseppe Bertozzi, Paolo Dattilo
FlNA Italiana SpA
Milan
Tempa Rossa oil field is 50 km south of Potenza in the Basilicata region on the eastern side of the southern Apennine mountains at more than 1,000 m elevation.
It represents one of the most interesting oil fields discovered in the Apulian platform target beneath a regional thickness of allochthonous strata of 4,000 m.
Modern hydrocarbon exploration in the southern Apennines started during the late 1960s and resulted in small but significant oil fields in the following decade: Castelpagano with 31.1 gravity, oil; Benevento with 46 gravity oil, and Pieri e Mattavelli, 1986.
However, only a few companies were seriously involved in the exploration of the Campania-Lucania Apennines. This is due to poor quality seismic data, the depth of the expected reservoir, and major drilling difficulties related to the penetration of an overpressured allochthonous sequence before getting to the balanced or slightly overpressured target.
The small but encouraging AGIP Costa Molina oil field discovery (20.3 gravity oil) in 1982 only slightly increased the general interest for the region, which is inhibited by the relative high risk investments implied.
Relevant improvement of seismic 'acquisition and processing techniques raised the interest of the area in the early 1980s. A small group of companies, especially FINA Italiana, Total, and AGIP concentrated the efforts in testing and evaluating all the more advanced exploration techniques available so as to reasonably assess the southern Apennines plays and targets.
Helicopter seismic acquisition became routine in the area because the rough topography, and environmental problems did not allow access to land seismic crews.
The oil price drop caused a remarkable reduction of the number of exploration companies active in the area during the late 1980s just before the petroleum potential of the region was completely assessed.
In 1987 the Monte Alpi license joint venture, headed by Petrex, discovered Monte Alpi light oil field. In 1988 the Laurenzana license joint venture, led by FINA Italiana, started drilling the ID Tempa Rossa, the discovery well of Tempa Rossa oil field. It encountered at least 1,000 m of oil column, and the oil-water contact depth is still unknown.
EXPLORATION TECHNIQUES
A careful geological analysis, integrated with a wide and updated Knowledge of compressive belts case histories and structural, stratigraphic, and depositional models, is probably the major exploration technique for this area.
Southern Apennines seismic line interpretation, even if it is still the main tool, due to poor quality data is definitely insufficient for prospect evaluation if not driven by a predictive geologic model integrating all the available data.
Several papers dealing with southern Apennines structural and stratigraphic models have been published the last two decades.1 2 3 4 5
However, the complexity of structural and stratigraphic relationships and the lack of detailed structural analysis within well-defined stratigraphic constraints has resulted in reconstructions of tectono-sedimentary solution of the region that remain largely conjectural.
Therefore, getting deeply involved in the local complex geology seemed to FINA Italiana the only way to perform a reasonable assessment of the hydrocarbon potential of the southern Apennines.
During the first exploration phase, all the published and public subsurface data were collected and new data were acquired. These data were referred to surface data both from articles and direct observations.
The integration of surface and subsurface available data, most of which have been gathered during the last few years, led the Laurenzana license operator to generate its own general structural and stratigraphic model. Major efforts focused on establishing a qualitative stress regime model for the Apulian platform carbonates.
This work led through a new approach to the poor quality seismic data interpretation that resulted in the Tempa Rossa oil discovery within a previously explored area. Furthermore, this approach strongly enhanced the petroleum potential of previously neglected areas in this region.
Essential background references to our work include the already cited papers plus others. 6 7 9
GEOLOGICAL FRAMEWORK
The Southern Apennines fold-thrust belt resulted after the compressive deformation of the Tethys ocean southwestern margin during the Neogene convergence of the African and European plates.9
From west to east, three major elements can be recognized within the Campania-Lucania sector: the Apenninic carbonate platform, the Lagonegro deep marine basin, and the Apulian carbonate platform.
Foredeep Plio-Pleistocene deposits separate Apulian platform foreland carbonates from overlying, thrusted strata. Overthrusted deep marine sediments (Ligurian units) bound westward Apenninic platform outcrops (Fig. 1).
Neogene emplacement of tectonic units marks the end of a story started in the early Mesozoic (Fig. 2). From the late Triassic up to the Paleogene the paleogeography and the tectonic regime were monotonous; the Apenninic platform and the Apulian platform kept growing separated by the deep and starved Lagonegro basin (Fig. 2A).
The first relevant collisional phase could be attributed to the end of Paleogene: Ligurian units overthrust Apenninic platform. The Apenninic platform is thrusted over its eastern margin and the western sector of the Lagonegro basin. The resulting foredeep is filled by the "external flysch" strata (Fig. 2B).
A further, probably more intense, compressive phase occurred at the end of the early Pliocene. The "external flysch," overlain by part of the Lagonegro basin, thrusted over the Pliocene foredeep, above the Apulian platform (Fig. 2C).
APULIAN PLATFORM FORELAND CARBONATES
Inferred buried foreland structural style has to be related to the above mentioned tectonic phases. Compressive features along the Apulian platform western margin can be interpreted from the seismic lines, while within the eastern sector of the buried foreland and the Murge area outcrops extensional features occur.
Between these two zones a relatively deep belt is present. These subvertical reverse faults occur with major vertical- displacement (several hundreds of meters), interpreted as "upthrust" or flower structure.
Based upon these observations, an eastward decreasing horizontal compressive sigma-1 stress axis 10 can be inferred within the Apulian platform (Fig. 3). The western sector of Apulian platform, characterized by compressive features, could be interpreted as affected by a compressive regime with an horizontal northeast oriented a, and vertical a,. Compressive stresses presumably dissipated along the western margin thrust faults.
Eastward of this compressive belt, a still horizontal, even though reduced with respect to the other stress axes, 01 can be inferred: 03 axis should therefore be horizontal and 02 vertical. A transpressive tectonic regime is therefore interpreted as responsible for the observed structural style.
Easternmost extensional regime with vertical 01 and horizontal 02 and 03 is probably a consequence of the peripheral bulge migration during the different tectonic phases.
Excellent reservoir properties of wrenching induced positive structures highlighted by Harding," led us to concentrate our efforts toward this neglected main target.
The Tempa Rossa 1D discovery, confirmed by Tempa Rose 2 and Tempa Rossa 1D-sidetrack, supported our model pointing out the potential of previously explored but underestimated areas.
WELL HISTORIES
TR-1 directional was drilled as a deviated well - due to terrain problems in siting a vertical location - at 5,050 m measured depth, through the allochthonous sequence and encountered about 700 m of oil bearing limestone of Tertiary and Cretaceous age.
Technical problems that occurred after the third drillstem test didn't allow deepening or completion.
A second well, TR-2, was planned by the same group of companies on the neighboring license of Torrente Sauro in 1991 about 3 km south of the discovery well.
During 1992 the TR-2, which penetrated an oil column of more than 800 m, was put on a 135 day extended well test and produced at a controlled natural flow rate approximately 1,220 b/d of oil with no water and no reduction of flow rate or wellhead pressure.
Following these very encouraging results, TR-1 directional well was reentered and sidetracked to 5,401 m in the limestone and dolomite from Miocene and Cretaceous age and tested 16-22 gravity oil over three intervals at flow rates of 1,600, 2,560, and 3,470 b/d.
TEMPA ROSSA RESERVOIR
The unusually thick oil column and the various lithology ind porosity types characterize this nonconventional reservoir.
Modeling the Tempa Rossa reservoir is difficult due to fracture porosity not homogeneously distributed all through the section, microfracture and fissure, opened stylolite, vuggy interval solution enlargement, and moldic and matrix porosity (especially in dolomite strata).
In order to get the best reservoir knowledge, efforts had been focused toward a high quality acquisition and elaboration of well site data.
A special mud-logging unit was used, equipped with a small mud loss recording tool, a quantitative hydrocarbon occurrences indicator on the cuttings; the continuously recorded penetration rate was processed with the other Geological information to detect fractures and fracture behavior during drilling.
About 100 m of oriented core were collected with a low invasion coring system, when possible, in order, to evaluate this methodology in low porosity fractured reservoir.
Petrophysical measurements on cores concerned conventional and special core analyses on plugs and full size cores, The special core analysis was performed to obtain resistivity measurement, capillarity curves, and flooding tests; tomography was also performed on all the whole core blocks.
The logging program included the last generation of Schlumberger tools. Attention was devoted to fracture quantitative characterization using an azimuthal Laterolog as resistivity tool, a DSI (dipole shear imager) as sonic tool, and FMI (fullbore microimager) as micro-resistivity tool. The program was completed by radioactive logs. An MSCT mechanical coring tool and MDT (modular formation dynamic tester) were not technically possible to run even though planned.
Several processings of the data acquired were performed. For the FMI, a FLIP (fracture identification by normalized Fracview (aperture determination by calibrated imaging)12 was run. A prototype called SPOT (secondary porosity typing for vugs characterization is under evaluation (J.P. Delhomme, in press).
For the DSI, iii STC - SONFRA (Stonely wave analysis) was done. This product had compared with FMI results for fractures identification and for width determination. 13
A project study on CPI calculation is proceeding. Unfortunately the cementation factor 'm' experimental determination didn't supply appreciable results. In the future it has to be planned an 'n' calculation on core at reservoir condition.
The porosity calculation is another source of problems because of hole enlargement due to the fractures and probably to the larger vugs. Anyway the porosity response on logs has to be calibrated with core results even if in vuggy zones this procedure is not unequivocal. The unconventional reservoir complexity is strongly increased by the oil column thickness. Within a 1,000 m thick oil column, the various porosity types are expected to be distributed with an extreme vertical and horizontal heterogeneity.
The statistic modeling on the third dimension of the data, gathered in the few wells drilled, is therefore thought to be at this stage a risky task.
The history of fracturing assessment in situ, stress determination, diagenetic study will be performed to define a geological model that, supported by well test data, could reduce the risk of a statistical simulation.
CONCLUSIONS
Tempa Rossa oil discovery confirmed the remarkable petroleum potential of the southern Apennines.
There are still many structures to explore. They will probably be found only through improving and updating the geological model and testing the new geophysical processing techniques. Advanced techniques are also necessity to improve the acquisition and processing of the wellsite data so as to maximize the information gathered from the few and widely separated wells available.
FINA Italiana will continue its exploration activity, following and improving its regional integrated approach with the exaltation of new and old acreage, being presently involved in about 20 licenses and applications from the Gulf of Taranto offshore to the Benevento mountains.
ACKNOWLEDGMENTS
We acknowledge FINA Italiana, a subsidiary of the Petrofina Group, for permission to publish this article. We are indebted to Dr. E. Barsocchini for continuously sustaining our work; T. Lakew, who provided excellent petrographic studies on the Tempa Rossa reservoir; and W. Gabelli for drawings. We thank L.J. Laurent for reviewing a draft of the article.
REFERENCES
- D'Argenio, B., Pescatore, T., and Scandone, P., Schema geologico dell' Apennino Meridionale (Campania-Lucaina), Atti del convegno: Moderne vedute sulla geologia dell' Apennino, Ac. Naz. Lincei, Quad. 183, 1973.
- Mostardini, F., and Merlini, S., L'Apennino Centro-Meridionale, sezioni geologiche e proposta di modello strutturale, Memorie Societa Geologica Italiana, Vol. 35, 1986, pp. 177-202.
- Hill, K.C., and Hayward, A.B. Structural constraints on the tertiary plate tectonic evolution of Italy, Marine & Petroleum Geology, Vol. 5, 1987, pp. 2-16.
- Casero, P., Roure, F., Endignoux, L., Moretti, I., Muller, C., Sage, L., and Vially, R., Neogene geodynamic evolution of the southern Appennines, Memorie Soc. geol. Ital., Vol. 41, 1988, pp. 109-120.
- D'Argenio B., L'Apennino Campano-Lucano, Vecchi e nuovi modelli geologici tra gil anni 60 e gli inizi degli anni '80, Memorie Soc. Geol. Ital., Vol. 41, 1988, pp. 3-15.
- Scandone, P., 1967, Studi de geologia lucana: la serie calcareo-silico-marnosa e i suoi rapporti con l'Apennino calcareo, Boll. Soc. Natur. Napoli, Vol. 76, 1967.
- Ortolani, F., Alcune considerazioni sulle fasi tettoniche mioceniche e plioceniche dell' Apennino Meridionale, Boll. Soc. Geol. Ital., Vol. 97, 1878, pp. 609-616.
- Patacca, E., Sartori, R., and Scandone, P., Tyrrehian basin and Apenninic acrs: kinematic relations since late Torotoninan times, Memorie Soc. Geol. Ital., Vol. 45, 1990.
- Amodio-Morelli, L., bonardi, G., Colonna, V. Dietrich, D., guinta, G., Ippolito, F., Liguori, V., Lorenzoni, V., Paglionico, A., Perrone V., Picaretta, G., Russo, M., Scandone, P., Zanetti-Lorenzini, E., and Zupetta, A., L'Acro Calabro Peloritano nell'orogene Apenninico-Magrebide, Memorie Soc. Geol. Ital., Vol. 17, 1976, pp. 1-60.
- Mercier, J.L., Sorel, D., and Simeakis, K., Changes in the state of stress in the overriding plate of a subduction zone, the Aegean arc from hte Pliocene to the present, Annales tectonicae, Vol. 1, No. 1, 1987, pp. 20-39.
- Harding, T.P., Petroleum traps associated with wrench faults, AAPG Bull., Vol. 58, 1974, pp. 1,290-1,304.
- Luthi, S.MM., and Souhate, P., Fracture apertures from electrical borehold scans, Geophysics, Vol. 55, No. 7, 1990, pp. 821-833.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
