STRATIGRAPHIC AND SEDIMENTOLOGIC CORRELATIONS BETWEEN SURFACE EXPOSURES AND SUBSURFACE INTERSECTIONS USING BULK AND SPECTRAL GAMMA RADIATION

The Heath Formation is a Late Mississippian (Serpukhovian) mixed carbonate and siliciclastic cyclothemic succession preserved in the Big Snowy Trough of central Montana. This unit has been extensively investigated as both a source rock and recently, as an unconventional, standalone petroleum system. Correlation of intraformational elements within the Heath Formation has remained enigmatic both in core and in surface exposures where outcrop is commonly highly weathered.

This study entails a sedimentologic and stratigraphic investigation of the Heath Formation using coupled outcrop and synthetic gamma ray (GR) logs in conjunction with paired core and well GR logs in an effort to correlate surface and subsurface intersections using GR signatures.

A handheld RS-230 BGO Super-SPEC was used to collect both bulk GR and spectral [U (ppm), Th (ppm), & K (%)] values from measured outcrop sections. Measured sections were trenched to produce fresh exposure and accurate measurements. Bulk and spectral data (assays) were acquired at 10 cm intervals for 120-second periods throughout the vertical extent of each exposure from stratigraphically older rock to younger. Raw data from assays was then converted to synthetic GR logs and compared to detailed descriptions of cored Heath intervals paired with down hole GR logs using Petrel E&P software.

In outcrop, GR signatures increase with greater organic content and decreasing grain size. These attributes are mainly shared by offshore marine, variably fossiliferous, dark grey, calcareous mudrocks (<150 API). Coarser grained siliciclastic, and fossiliferous carbonate rocks with shallow marine ichnofossils contain the lowest bulk GR (>75 gAPI) and spectral values. Conversely, some algal limestones contained elevated GR signatures. Analyzed wireline logs indicated similar trends with GR signatures between 15 gAPI and 165 gAPI in similar facies identified in core.

This investigation provides a controlled example in which synthetic GR logs acquired from surface exposures can be used to create a geophysical fingerprint for the purpose of identifying similar facies and lithologies in GR well logs. This result has specific implications for subsurface GR log correlation where, in the absence of core, outcrop is the only available analogue.