The impact of facies heterogeneity on the seismic properties of carbonates: forward modeling and reservoirs potential

Forward modeling is a fundamental prospecting method to understand reservoirs structure and architecture in the subsurface. Moreover, it also helps maximizing the hydrocarbons extraction of petroleum systems. Carbonate reservoirs result in non-univocal seismic response caused by the facies heterogeneity and due to the possible presence of infilling fluids. The carbonate ramp outcropping in the Majella Massif (Central Italy) is an excellent surface analogue of buried structures. It offers the opportunity to directly analyze a carbonate reservoir which clearly shows facies variations and natural hydrocarbon-impregnations allowing to quantify the induced petrophysical changes. In this study field and laboratory measurements are used to carry out 1D and 2D forward seismic models of the reservoir. Density and porosity of samples were measured through a helium pycnometer on both hydrocarbon-saturated and not-saturated natural samples. Data show density from 2,35 g/cm³ to 2,64 g/cm³ and porosity from 5,9% to 21%. Seismic velocity data from previous works and ongoing measurements show values from 3,24 km/s to 5,93 km/s and they are clearly related to porosity. After building a seismic velocity, density and acoustic impedance model, a low-frequency (40Hz) synthetic 1D seismogram was carried out simulating facies (porosity) and hydrocarbon-saturation variations. The presence of hydrocarbon causes an increase in acoustics impedance by 16,2% and it increases the amplitudes by 13,6% at the reservoir boundaries. A 12 km long synthetic profile from the platform top to the basin, oriented SSE-NNW, was then carried out simulating the outcropping architecture and spatial distribution of the facies. The synthetic 2D model demonstrates how Vp variations related to facies association and distribution slightly influence seismic facies. Sensitivity tests show that Ricker wavelet with 25 Hz of central frequency is the frequency that produces best seismic images. Ongoing laboratory measurements and simulations are expected to help in validating facies interpretations of real seismic profiles and detect the presence of hydrocarbons in carbonate-ramp petroleum systems.