Expanding the Use of Seismic Amplitude and AVO From Fluid Prediction to Stratigraphic Architecture Definition

Abstract

One of the key challenges in exploration is understanding the lithology and fluid characteristics that cause seismic anomalies, since seismic responses are non-unique. The magnitude and direction of change in amplitude versus offset (AVO) is traditionally employed to predict fluids in reservoirs, but is also affected by variations in reservoir architecture and lithology. In cases where reservoir variability adds complexity to the seismic response, AVO and full-stack amplitude anomalies do not always yield the expected drilling result, so it is critical to understand how the seismic response could be affected by the interplay of reservoir characteristics and fluids. This presentation will describe a new method for using AVO and full stack data to depict stratigraphic stacking patterns, expanding the usefulness of seismic character beyond the prediction of hydrocarbon presence. In this methodology, the solution is driven by the integration of two key elements: forward modeling of reservoir stacking patterns, followed by the transformation of Intercept-Gradient volumes into facies maps. The forward modeling uses synthetic seismograms from likely facies models for the reservoir of interest. Geological constraints from well control provide the appropriate rock properties, but in frontier exploration, analogs can also be used. Modeling the variability in stratigraphic stacking, vertically and laterally, provides insight into their effects on the seismic response. Applying this approach to an offshore Falkland Island's well, synthetic gathers revealed systematic changes in AVO character as a function of facies combinations. Two distinctive facies modeled corresponded to two AVO classes: a massive sand facies that corresponded to AVO class IIP, and a thin-bedded facies that corresponded to a class II-III. Using an Intercept and Gradient Volume over the prospect, combined with the forward modeling, a map of AVO classes could be generated, which in turn was transformed into a facies map. This not only explained the variable AVO response, but revealed complexities of a channelized architecture, which increased confidence that the proposed reservoir model was internally consistent. By bridging AVO, seismic attributes, well data and reservoir analogs to test multiple reservoir architecture scenarios, this methodology can be exported to other basins, and should ultimately increase the success rate based on pre-drill predictions, in both frontier and mature exploration.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017