Seismic characterization analyses of the Bulgarian Black Sea shelf for assessment of reservoir properties

In the Bulgarian Black Sea shelf, several gas fields have been discovered in Maastrichtian–Paleocene sediments to date. In this study, amplitude versus offset analysis (AVO) was carried out, and the extended elastic impedance (EEI) was also used, to predict fluid and lithology properties. Thus, in these special cases, AVO (or reflection angle) can significantly de-risk the existing Paleocene prospects and allow to focus on the Miocene–Oligocene successions, where some prospects have also been identified. By examining the variations in angle amplitude (or offset), it is possible to unravel lithology and fluid effects on the top of a reservoir. Seismic modeling is used to determine what type of seismic response, in terms of AVO, to expect at the depth of interest in the study area. There are five classes or types of AVO responses, and in the study area classes 1 and 2 were defined in the Miocene–Oligocene prospects. Brine-filled reservoirs and conglomerates  exhibit class 1 AVO response, whereas gas-filled reservoirs show class 2 AVO responses. EEI has the ability to give estimates for elastic parameters, such as Vp/Vs ratio, bulk modulus, shear modulus, Poisson’s ratio and other parameters. In the current study, simultaneous inversion testing was done and, analyzing all the wells together, Poisson’s ratio is the most promising fluid and lithology indicator, showing good separation between water and gas responses, although there is some overlap. Projection of EEI at an angle of 38° through brine points gives a useful separation of brine, 10% gas and 90% gas reservoirs. However, the amplitude of the resulting projection is still low, which may make it difficult to resolve fluid variation once seismic noise has been added.