LaP-Ramp geometry of fracturing in travertines: examples from the Acquasanta Terme deposits (Central Italy).

Travertines are continental carbonatic rocks characterized by a poor layering with a strong internal lamination. Travertines form by the interplay between chemical and microbial activity in a wide range of depositional environments (from ponds to cascades) reflecting different hydrodynamics regimes. The resulting geometry show a typical spatial anisotropy varying from decametric- to meter-scale. The internal lamination is provided by variations of the depositional facies and it is mainly displayed by alternation of more porous and less porous horizons. Despite the high primary porosity, these deposits present a low permeability, as attested by the presence of fossil gas bubbles. It follows that pores interconnection (secondary permeability) is strongly influence both by primary (depositional) and secondary (structural) factor. The presence of internal lamination deeply influences the fracture trajectories characterized by a dual attitude with parallel and cross-cutting segments. Travertine deposits represent today a primary task in many, newly discovered, oil reservoirs in the Southern Atalantic. The dimension of single fracture is under the resolution of seismic sections. Analogue field studies are thus well required to properly model the secondary permeability features. In this study we describe the patterns and the ways of fracturation in travertines. Fractures in Acquasanta-Terme travertines are characterized by portions cutting through the lamination (ramp, R) and portion running parallel to the internal lamination (lamination parallel, LaP). The combination of R and LaP results in a stairway geometry of the fractures. A Montecarlo approach was successfully used to provide a suitable model of fractures development. We found a relation between the average lamination dipping and the L/R ratio (computed dividing the cumulative length of LaP over R segments) and a relation between the average lamination dip and the angle formed by the intersection between R and LaP segments. These relations suggest that the lamination dipping of 25-30 represent a critical angle reflecting the depositional rheology of the different depositional environments. The found The found L-R fracture property assures a fracture connectivity within travertine deposit thus locally providing effective permeability increase.