Geological Interpretation of a Weakly Confined Channel Complex Integrating Seismic Amplitudes and Elastic Inversion Attributes (Offshore Ghana)

摘要

During the last decade, seismic-based studies have become fundamental for detailed and objective descriptions of reservoir depositional elements, a necessary condition for building robust stratigraphic frameworks and sedimentological interpretations unbiased by questionable analogues or existing models. A close integration of sedimentary geology and geophysical interpretation is presented here to demonstrate that these two disciplines can support each other to provide a powerful tool for predicting reservoir sandstones architecture. The work was carried out on the deep-water sequences of the Ghanaian offshore, which have been interpreted as weakly confined channel complexes (WCCC) deposited at the base of slope. WCCC are approximately 150-300m thick and 10-15km wide and made of stacked, laterally amalgamated channels and lobes vertically separated by shale-prone intercalations. Detailed interpretation of individual seismic reflections followed by an elastic inversion and Bayesian facies classification calibrated with 3 wells allowed to identify several distinct small-scale composite channel fills and lobes forming sand bodies 1-2km wide and 15-25m thick. High and low-sinuosity sand bodies compensate each other and stack vertically, migrating progressively from West to East. Furthermore, some channel-fills show a marked aggradational pattern. The overall depositional pattern is believed to represent a progressive decrease in sediment supply leading to the de-activation of the system. Despite the key sand bodies were correctly identified on seismic amplitudes and well-logs through conventional, “qualitative” approaches, their subsequent “quantitative” imaging by elastic inversion and facies (shale, brine and gas) probabilities allowed to clarify many uncertainties in the stratigraphic and sedimentological framework, particularly where the marked AVO fluid effect inevitably interfered with the initial, purely sedimentological interpretation. High-res biostratigraphic data further confirmed the inferred physical-stratigraphic and litho-fluid framework established through the integration of seismic sedimentology and quantitative geophysics.