In the past decade a great deal of high-quality research on the sequence stratigraphy of unconventional resource plays has been conducted by teams at Exxon Mobil (Bohacks, Lazar, Ottman et al), Bureau of Economic Geology (Kerans et al), Colorado State University (Egenhoff et al), Colorado School of Mines (Sonnenberg et al) and Indiana University (Schieber et al) to name a few. This aim of this post is not to summarize what has already been done but to comment on a few common myths.1. Most unconventional resource plays are “shales”Unconventional resource plays are very diverse in terms of lithologies and depositional environments from basin-centered gas to diatomites. “Traditional” Exxon or Catuneanu/Embry-style sequence stratigraphic methods work just as well for tight-gas sandstones, coalbed methane and basin-centered gas since the majority of these are found in fluvial to shallow-marine sandstone successions. The majority of “shale” resource plays are actually NOT shales at all (see ternary diagram below). For example the Eagle Ford “shale” is actually a chalk, whereas the Three Forks “shale” is every facies one would expect along an arid carbonate ramp from nodular anhydrites to dolomudstones.

2. Unconventional Resource Plays are homogeneous

A quick look at core or thin-sections from any unconventional resource play is a wake-up call in diversity of processes. Cm-scale turbidites of phosphatic grains occur in the Barnett Shale, whereas debris flows several meters thick occur in the Bone Springs of the Permian Basin. Since fracturing is largely a function of rheology these heterogeneities make a difference. Many (Naccio et al., 2005) have demonstrated a strong relationship between small-scale mechanical stratigraphy and depositional environments (some facies associations are more fracture-prone than others).

3. “Special” expensive techniques need to be used to perform sequence analysis in unconventional resource plays

Based on ten years of working several onshore unconventional resource plays we would argue that nothing super-expensive or “out-of-routine” needs to be done to perform sequence analysis. Key-sequence stratigraphic surfaces may be subtle but are easily picked-out in most cores. Elemental analysis logs and XRD which companies have used for decades can be used to pick out changes in mineralogy. Borehole image logs that are run routinely to detect fractures can also be used for to identify ravinement surfaces and coarsening and fining trends. Biostratigraphic data are just as important as in any conventional play.

4. Sequence Stratigraphy is not useful in unconventional resource plays

Most unconventional resource plays are exploited using long horizontal wells – geosteering in the right zone is key. Sequence stratigraphy helps resolve internal architecture such that the drill bit stays within the most organic-rich and fracture-prone zones. We have helped several companies achieve this in the Bakken and Three Forks Formations. Many of the “deep water” shale-plays in the Permian Basin are dominated by sediment gravity flows (SGFs). Depending on relative sea-level stand these can bring in either carbonate-rich or silica-rich material (reciprocal sedimentation). The mineralogy and organic-content of the SGFs and intercalated pelagic units dictate the distribution of sweet spots. Therefore it is imperative to go back to the fundamentals and resolve systems tracts and high-order cyclicity.

With current oil prices and the slow pace of drilling operations, we now have the time to step back and think about the big picture and what sequence stratigraphy can do for us.