Continental Carbonates as Hydrocarbon Reservoir, an Analogue Case Study from the Travertine of Saturnia, Italy

The Pleistocene Saturnia travertine represents an analogue of the pre-salt continental carbonate reservoir discovered on both sides of the South Atlantic margin. Two sub-horizontal travertine plates, few tens of metres thick and extended over an area of several kmsq, have been studied in 3D quarries exposures. The aim of this study was to reconstruct the facies variations and associate petrophysical properties applying an integrated multidisciplinary approach. The Saturnia travertine complex, due to warm waters spring still active in the area, is made of various stacked banks, separated by emersion erosive phases, showing alternation of lacustrine tabular bodies, terraces and sills. The facies association includes crystalline crust, pisoid, paper-thin raft, coated bubble, reed, lithoclast-breccia, micritic travertine. The δ13C, ranging between +4‰ e +8‰, supports the CO2 volcanic mantel source, while the δ18O from -5‰ and -9‰ is in agreement with warm meteoric water as for other travertine in the area. The Sr ratio isotopic signature is agreement with the carbonate derived from dissolution of Mesozoic deep seated carbonate. The facies reservoir properties have been studied through an integrated approach that includes porosity and permeability analysis on plugs, 3D X-Ray Computer Tomography, which evidenced the porosity connectivity, and image analysis on micro scale under thin section (microporosity) and macroscale on large rock slabs to define various porosity indexes (shape, roundness, pore size). A strong heterogeneity of the pore system and variable connectivity were pointed out. The porosity ranges from 4 to 30% and permeability is highly variable reaching hundreds of mD. The average pore diameter is between 1 and 4 mm, microporosity is low and the prevailing pore type is the inter-granular one. The study highlighted also that in travertine, the investigation should be conducted at a large scale in order to measure the large pores and wide scale connectivity.