Log Motif Applied to the Interpretation of Clastic Depositional Systems in the Subsurface
T. A. (Mac) McGilvery
Adjunct Professor, Univ. of Arkansas Geosciences
Seismic and well log data are the primary tools for evaluating subsurface stratigraphy and depositional systems. The application of seismic stratigraphy and seismic facies analysis became the “go to” techniques for such interpretations as a result of the publication of AAPG Memoir 26 in 1977 and the introduction of the famed Vail/Exxon model for sequence stratigraphy. There have been a host of additional Publications that address the various applications of seismic and sequence stratigraphy since. The application of log shape otherwise known as log motif has been formalized to a much lesser degree. Observations regarding log motif as applied to interpretation of depositional facies are commonly included in a supplementary way within publications focused on greater over all topics. One such publication is the text book Terrigenous Clastic Depositional Systems (Galloway and Hobday, 1996).
This course is designed to familiarize the participants with the application of log motif and cycle stacking patterns to the interpretation of stratigraphic basin fill architecture and clastic depositional systems. A systematic transition in log motif, both vertically (100’s to 1000’s ft) and laterally (1’s km to 100’s km) can be observed at the basin scale. In addition, there are a number of generic log shapes including funnel, bell, blocky, barrel, and serrate that commonly indicate specific depositional conditions such as confined or unconfined depositional settings. These may reflect depositional patterns such as upward coarsening, upward fining, channelized, unconfined sheets, etc. Having said this, log motif is a very non-unique response. There are many clastic depositional settings that may yield a similar log shape. For example, a funnel shape motif is common in progradational deltaic settings, basinward accreting shore zone systems and deepwater lateral and frontal splay complexes. Many workers avoid the application of log motif for this reason. This course is based on more than 35 years of industry experience in subsurface stratigraphic interpretation and the observation that the generic log motifs mentioned above occur in repetitive and systematic ways such that they can be effectively applied within the proper context. Integration of log shape and log attribute cut-offs can greatly enhance the effectiveness of interval thickness and Net:Gross interpretations. The goal is to give the participants a sound understanding of log motif analysis with emphasis on prediction of reservoir body geometry and degree of stratigraphic compartmentalization.