Abstract
Our knowledge of subsurface structures often derives from seismic velocities that are measured during seismic acquisition surveys. These velocities can greatly change due to lithological, fracture frequencies and/or effective pressure/temperature variations. However, the influence of such intrinsic lithological properties and environmental conditions at the large scale is poorly understood due to the lack of comprehensive datasets. Here, we analyze 43 borehole-derived velocity datasets of 3 end-member tight carbonate sequences from Central Italy, including massive pure limestone (Calcare Massiccio, CM), thick-layered (20–50 cm) pure limestone (Maiolica, MA), and thin-layered (2–20 cm) marly limestone (Calcareous Scaglia, CS). Our results show that the main rock parameters and environmental conditions driving large scale velocity variations are bedding and paleostresses, while mineralogical composition and current tectonic stress also play a role. For each of the 3 end-members, measured VP values vary differently with depth, as the thin-layered CS units show a clear increase in Vp, while velocity slightly increases and remains constant for the thick-layered MA and massive CM units, respectively. Such observations show that velocities are affected by specific characteristics of lithological discontinuities, such as the thickness of bedding. Counterintuitively, larger Vp values were recorded in the deformed mountain range than in the undeformed foreland suggesting that higher paleo-stresses increase velocity values by enhancing diagenesis and healing of discontinuities. Our results thus demonstrate that large scale velocity variations are strictly related to variation of lithological properties and to the geological and tectonic history of an area. We suggest that such lithological and environmental controls should be taken into account when developing velocity and mechanical models for tectonically active regions of the Mediterranean Area, where earthquakes mostly nucleate and propagate through carbonate formations, and for resource exploration in fractured carbonate reservoirs.
Highlights
Our knowledge of subsurface structures often derives from seismic velocities that are measured during seismic acquisition surveys
We refer to the velocity derived from the sonic log arrival time with the terms “Sonic Log Velocity” (SLV)
Interval velocity derived from check shots (IV) and sonic logs velocities (SLV) give comparable results for the tight carbonate rocks investigated in this paper despite the different volume of rock sampled
Summary
Our knowledge of subsurface structures often derives from seismic velocities that are measured during seismic acquisition surveys. Our results demonstrate that large scale velocity variations are strictly related to variation of lithological properties and to the geological and tectonic history of an area We suggest that such lithological and environmental controls should be taken into account when developing velocity and mechanical models for tectonically active regions of the Mediterranean Area, where earthquakes mostly nucleate and propagate through carbonate formations, and for resource exploration in fractured carbonate reservoirs. Crustal scale conditions such as stress and temperature[15,16,17,18], fluid p ressure[19,20] or lithology lateral v ariations[21], can change resulting in complex seismic properties-depth relations according to lithology, rate of compaction, grain types and amount of cementation[22,23] These variations are relevant to carbonate rocks that play a strategic role for resource exploration accounting for more than 60% of the world’s proven hydrocarbon reserves[24]. We compared the seismic-velocities derived from each borehole to obtain the mean velocity variations for the different carbonate facies and tectonic areas[31,32]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
