Reducing Risk in Lithology and Fluid Discrimination with Attributes Derived from Seismic Inversion

Pre-stack simultaneous inversion of seismic reflection data, a methodology based on amplitude variations with offset (AVO), presents a more flexible and detailed tool than traditional post-stack amplitude analysis. Given an initial earth model from well logs and structural geology interpretation, the simultaneous extraction of acoustic (AI) and shear impedances (SI) from seismic data allows for an effective transform of the seismic reflectivity series to layered rock properties. In the High Island Area, offshore Gulf of Mexico, the sand and shale lithologies show relatively equal acoustic impedance values and Class 2 AVO in the presence of hydrocarbons. To extract a thorough suite of rock property information from seismic in this environment, I use inverted AI and SI measurements to derive other seismic attributes, such as Lambda-Rho (λρ), Mu-Rho (μρ), and water saturation (Sw). By deriving these attributes, rock property information such as rock type, porosity, and pore fluid content can be translated from localized well data to the larger seismic volume. These derived attributes assist in defining relationships between properties that distinguish the sedimentary rock type that AI and density from post-stack amplitude analysis alone cannot. The results of the experiment showed a successful establishment of relationships to separate sand and shale lithology, as well as the extraction of structurally-conformable hydrocarbon-bearing sand in conformance with seismic AVO attributes and termination against faults in the study area which act as seals to the gas sand reservoirs.