Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Multi-scale, Brittle-Ductile Couplets in Unconventional Gas Shales: Merging Sequence Stratigraphy and Geomechanics
(1) Geology and Geophysics, University of Oklahoma, Norman, OK.
(2) Geology and Geophysics, University of Oklahoma, Norman, OK.
The words “ductile” and “brittle” have emerged as two key descriptors for characterizing unconventional gas shales. The former is usually considered to be relatively organic (TOC)- and clay-mineral rich, while the latter is considered to be more enriched in “silica” (i.e. biogenic and/or detrital quartz)- and/or carbonate (calcite/dolomite) minerals. Our studies of some gas shales have shown that such ductile and brittle rocks occur as alternating ‘couplets’ at a variety of scales. At the largest, sequence stratigraphic scale, ductile beds comprise condensed sections (CS) which lie on or stratigraphically near a combined sequence boundary/transgressive surface of erosion (SB/TSE). Detritus-rich beds prograde over the top of the condensed section (i.e. maximum flooding surface) during the ensuing highstand/regressive (HST/RST) depositional phase. The next smaller, temporally-shorter parasequence scale often consists of a ductile CS shale overlain by a ‘cleaning’-upward’ (i.e. on gamma -ray log) HST/RST shale. Vertical stacking of repetitive parasequences gives rise to a series of stacked, ductile-brittle couplets, each couplet bound by a marine flooding surface. At a still-finer, sub-parasequence scale, ductile and brittle couplets are often finely interbedded or interlaminated. It is possible to recognize or predict these different scales of couplets in outcrop, on logs and core, and sometimes on seismic, thus providing a means of predicting stratigraphic variability in geomechanical and other rock properties. Examples include: (1) Fracture Toughness, Youngs Modulus and Poisson’s Ratio vary at the sequence and parasequence scales, (2) Microseismic event-intensities vary at the parasequence scale, and (3), rock strength varies with amount of laminations/beds per stratigraphic interval at the sub-parasequence scale. Applications of these findings include (1) predicting the stratigraphic position of a horizontal wellbore for optimal artificial fracturing and penetration of gas/oil-rich horizons, (2) optimizing drilling orientation with respect to bedding and (3) predicting differential retention of fracture proppant.