Tectonic model and three-dimensional fracture network analysis of Monte Alpi (southern Apennines)

The Monte Alpi (southern Italy) is well known as an outcropping analogue of the Val d'Agri subsurface units which host a number of oil fields of econo mic importance. This paper presents an example of a detailed study which in tegrates stratigraphy, structural geology, tectonic analysis and fracture n etwork study. The new structural model presented is based on a detailed 1:1 2,500 field survey of the area, whose results are illustrated in a comprehe nsive outcrop map, geological map and structural cross-sections, and stereo graphic projections and palaeostress inversion calculations of a mesostruct ural data set. The Monte Alpi Unit is characterised by a 2000-m-thick sequence of Jurassic -Cretaceous platform carbonates, conformably covered by Middle-Upper Miocen e calcarenites. This sequence is unconformably covered by Upper Messinian t errigenous clastics, mainly along a number of major growth faults. Our deta iled mapping revealed that the Monte Alpi Unit is tectonically overlain by the Lagonegro basinal Unit (thin siliceous carbonates and remnants of pelag ic pelitic and carbonate deposits, all highly deformed) and by the Monte la Spina Unit (cataclastic Mesozoic carbonates and dolomites). This solves ea rlier raised questions concerning the palaeogeographic provenance of the ca rbonates outcropping in the area, which can thus: without any doubt be corr elated with the external carbonate domains known from the subsurface. The p resent-day structure shows a structural inversion along the major Late Mioc ene growth faults which acted as sinistral transpressive faults. The latter faults are part of a regional set dissecting the southern Apennines, activ e up till at least Middle Pleistocene times. A fracture study was performed on the Mesozoic carbonates of the area in or der to collect data regarding length distribution, orientation, density and spatial heterogeneity of the fracture network to be used for consideration s regarding exploration and development of the Val d'Agri reservoirs. The data were collected on 37 horizontal (strata-parallel) and vertical out crop surfaces through photographic restitution and digitising of fracture l ineament traces, and measurements of orientations of all visible fractures. Furthermore, three major outcropping rock walls along the flanks of Monte Alpi and Santa Croce were photographed and fracture and fault zone lineamen ts were digitised. All data were inserted and calibrated in a three-dimensi onal (3D) Geographic Information Systems and Computer Aided Design (GISCAD) environment consisting of a database and a 3D geographical framework compr ising topographic map, Digital Terrain Model (DTM), geological map, aerial photograph lineament map and a 3D reconstruction of major fault surfaces. M aking use of the potential of the 3D working environment, outcrop surfaces and rock walls were reconstructed at their correct 3D position and orientat ion. Data analysis comprised stereographic contour diagrams and rosette diagrams of 1800 fracture orientations in order to recognise main fracture sets. Th ree corrections were performed on these data: length weighting, Terzaghi co rrection (removal of sample orientation bias) and tilt removal. From a comp arison between orientation data on various scales combined with field evide nce, it can be deduced that fault and fracture sets are part of the same st ructural pattern, and that fracture sets were generated prior to Plio-Pleis tocene tectonics and passively rotated. Fracture and fault length distribution was analysed by the generation of di agrams which plot cumulative frequency versus length in log-log space, norm alising for area of observation. These diagrams, composed of about 30,000 m easurements and constituting the most complete data set presented in the li terature up till now, show that small-scale and large-scale features form p art of one scale invariant system with fractal properties. No scale gap is present in our data set which comprises rock wall data in the 'subseismic' scale range. It appears that the 3D fracture network can best be described by a fractal dimension of 2.98 and a coefficient of 0.25. The outcome of the study places important constraints on fracture network s imulation exercises in hydrocarbon reservoir simulation studies, which use numerical input parameters in order to make predictions of the tectonic tex ture present in the subsurface. (C) 2001 Elsevier Science B.V. All rights r eserved.