An investigation of porosity-velocity relationships in faulted carbonates using outcrop analogues

Porosity and permeability are notoriously difficult to predict in carbonates, especially prior to drilling when there is a lack of direct petrophysical data. The aim of this paper is to document the initial results of an integrated outcrop and laboratory study designed to investigate the relationships between pore systems and acoustic velocities in faulted Oligo-Miocene carbonates on the Mediterranean islands of Malta and Gozo. Depositional facies is shown to have a significant effect, with velocities in grain-dominated carbonates up to 1000 m s21 higher than those in micrite-dominated carbonates. Based on outcrop structural data, the fault zones can be separated into three architectural components: a fault core; an intensely damaged zone; and a weakly damaged zone, with the last passing into undamaged protolith. Our data suggest that only the fault core component can be identified using porosity-velocity data, with P-wave velocity (Vp) values of 5000-6500 m s21 at helium porosities of less than 5%. Our study is novel in that the prediction of elastic properties and acoustic velocities across fault zones is anticipated by linking laboratory-scale measurements with seismic-scale predictions through quantitative rock physics modelling.