Broadband Q-Factor Imaging for Geofluid Detection in the Gulf of Trieste (Northern Adriatic Sea)

Aldo Vesnaver,Michela Dal Cin,Fabrizio Zgur,Martina Busetti,Gualtiero Böhm

doi:10.3389/feart.2021.640194

Aldo Vesnaver, Michela Dal Cin + Show 3 more

Open Access

https://doi.org/10.3389/feart.2021.640194

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Abstract

Seismic surveys allow estimating lithological parameters, as P-wave velocity and anelastic absorption, which can detect the presence of fracture and fluids in the geological formations. Recently, a new method has been proposed for high-resolution imaging of anelastic absorption, which combines a macro-model from seismic tomography with a micro-model obtained by the pre-stack depth migration of a seismic attribute, i.e., the instantaneous frequency. As a result, we can get a broadband image that provides clues about the presence of saturating fluids. When the saturation changes sharply, as for gas reservoirs with an impermeable caprock, the acoustic impedance contrast produces “bright spots” because of the resulting high reflectivity at its top. When the fluid content changes smoothly, the anelastic absorption becomes a good detector, as fluid-filled formations absorb more seismic energy than hard rocks. We apply this method for imaging the anelastic absorption in a regional seismic survey acquired by OGS in the Gulf of Trieste (northern Adriatic Sea, Italy).

Highlights

In the Adriatic Sea, the occurrence of fluids within the sediment and fluid seeps at the sea floor are well known
Using synthetic seismograms, Vesnaver and Lin (2019) showed that this method works nicely when the acoustic impedance contrast is modest in a hydrocarbon reservoir, so that the oil/water contact cannot be detected by a standard approach
The broadband estimation of anelastic absorption is an effective tool for characterizing fractures or fluid-saturated formations, or both

Summary

Introduction

In the Adriatic Sea, the occurrence of fluids within the sediment and fluid seeps at the sea floor are well known. Fluid accumulation in the shallow sediments and fluid seeps have been encountered by the high-resolution seismic, often associated to fluid seeps, in the Adriatic basin (e.g., Hovland and Curzi, 1989; Conti et al, 2002), including the Gulf of Trieste (e.g., Gordini et al, 2004; Busetti et al, 2020). The aim of this paper is characterizing some shallow formations by a broadband Q-factor imaging, as a low Q factor is often associated to rocks saturation and unconsolidated formations This method merges the macro-model obtained from Q tomography with the micro-model provided by the pre-stack depth migration of the instantaneous frequency. In this way, tiny local anomalies might be detected, pushing the resolution of seismic data up to its physical limits

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