Synthetic seismic forward modeling as a tool to assess the seismic signature of carbonatebearing fault zones

Seismic images are fundamental to interpret subsurface setting on both continental and marine environments. However, in carbonate systems, seismic response can be very difficult to be analyzed due to the petrophysical heterogeneities, being the interpretation particularly complicated when faults are present. In fact, faults rocks are generally characterized by different petrophysical properties with respect to the host rocks. Synthetic seismic forward modeling is a powerful tool to quantifying the seismic response associated with fault-related petrophysical changes, since input parameters can be modified. In this study, field and laboratory measurements are used to analyze the spectrum of the carbonate-bearing fault seismic response. Here, we focused on the carbonate ramp setting outcropping in the Majella Massif since it represents an excellent surface analogue of buried porous and faulted carbonate reservoirs worldwide. Density and porosity were measured through a helium pycnometer on representative fault (damage zone) rocks sampled at increasing distances from fault planes. Investigated damage zone samples, show both increased and decreased porosity with respect to the host rock, depending on the single fault characteristics. A 2D (12 km long) synthetic profile from the platform top to the basin, oriented SSE-NNW, was then carried out in Matlab simulating the outcropping architecture and spatial distribution of the sedimentary facies and of the faults. Together with a proper porosity variation related to facies distribution, the porosity along the damage zones (DZ) of the simulated faults was set according to the laboratory measurements. The lowfrequency (40Hz) synthetic seismic profile shows marked diffraction hyperbolas in the modeled fault zones for faults characterized by higher porosity DZ where lower seismic velocity is expected. On the contrary, fault zone characterized by lower porosity DZ with respect to the host rock resulted to give a less clear seismic image. Going into the details, faults with decreased porosity and consequent increased seismic velocity of 1000 m/s in the damage zone respect to the host rock, do not show a visible difference in the seismic response if compared with the seismic image without modified petrophysical characteristics within the faults. On the other hand, faults with increased porosity and decreased seismic velocity of just 500 m/s respect to the host rock, show clear diffraction hyperbolas where their apexes correspond to the fault planes. Ongoing laboratory measurements and simulations changing geometries, thicknesses and orientations of the damage zones, are expected to help in improving the knowledge of fault seismic response. This work helps in bridging the gap between forward modeling and actual seismic imaging with large implications for hydrocarbon-reservoir characterization and the identification of potential CO2 or hydrogen storage sites.