Cyclical sedimentation patterns are common in unconventional resources shales, whether they be carbonate- or siliciclastic-dominated. In many resource shales the cyclical patterns have been related to eustatic sea level fluctuations, even though these fluctuations may sometimes be obscured by tectonic overprint. The cyclicity is reflected in repetitive sedimentation patterns which represent alternating relatively shallow/oxic- and deeper/anoxic-water deposition. Typical cyclicity might occur in the form of alternating clay/organic-rich and quartz/calcite-rich strata. In more carbonate-rich sequences, organic-rich marls might alternate with organic-poor limestones. This cyclicity can occur at a variety of stratigraphic scales and provides a sequence stratigraphic framework for mapping, correlation, and interpretations. Incised valley fill may provide a localized, thicker, more organic-rich stratigraphic section (‘sweet spot’), than adjacent areas.

Within the context of geomechanics, these cyclical strata are termed ‘brittle-ductile couplets’. Using the Barnett and Woodford shales as examples, clay/organic-rich intervals tend to be relatively ductile (relatively low Young's Modulus and high Poisson's ratio) and cleaner quartz/calcite-rich intervals tend to be relatively brittle (relatively high Young's Modulus and low Poisson's ratio. In carbonate-rich deposits such as the Eagle Ford Formation, re-crystallized, TOC-poor limestones tend to be stronger and more brittle than TOC-rich marls.

Within the context of microseismic, microseisms may be vertically stratified or layered, with some horizons containing more events than other horizons. This stratification has been related to brittle-ductile couplets in at least one area, and placed within a sequence stratigraphic framework.

Within the context of geochemistry, ductile strata tend to contain more TOC, and are thus better potential HC source rocks, than brittle strata, which contain less TOC. Biomarkers (geochemical fingerprints) are useful for interpreting sources and environments of deposition of organic matter, and ultimately used for environmental zonation of shales.

Using these principles and observations, it is possible to build a sequence stratigraphic framework from multiple data sets to map and correlate brittle and ductile strata, organic-/hydrocarbon-rich zones, and more fracturable stratigraphic intervals. A suggested horizontal landing zone is the brittle strata within a brittle-ductile couplet. It is hypothesized that when hydraulically fractured, both brittle and ductile strata become fractured, and hydrocarbons move from the ductile to the brittle zone, whose fractures remain open after proppant emplacement. With time, ductile strata may close around the proppant and become sealed.

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