Page 91 - Fundamentals of Gas Shale Reservoirs

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SEQuENCE STraTIGraPHy OF uNCONVENTIONaL
rESOurCE SHaLES

Roger M. Slatt
Institute of Reservoir Characterization, School of Geology and Geophysics, Sarkeys Energy Center,
University of Oklahoma, Norman, OK, USA

Summary 4.1 INTrODuCTION
In this chapter, I have examined and interpreted the vertical There are vast quantities of shales worldwide, which are now in
stratigraphy, and sometimes lateral attributes, of the Barnett various stages of exploration and development because of their
Shale, Woodford Shale, New Albany Shale, Marcellus potential as oil and gas resources. Shales have for many years
Shale, Haynesville Shale, Eagle Ford Shale, LaLuna Shale, been considered the principal hydrocarbon source rock as well
and Brown Shale. It is interpreted that water depths during as an effective seal rock. But it was only after horizontal drilling
deposition of these shales varied from nearshore to basin; and hydraulic fracturing technologies were demonstrated to be
therefore, depositional processes also varied, as evidenced effective at releasing hydrocarbons from these tough, imperme­
by sedimentary features. able rocks that they became recognized as reservoir rocks—or
All of these shales exhibit stratigraphic zonation indi­ as they are often referred to—“unconventional resource shales.”
cating at least two scales of predictable relative sea‐level There are many factors that affect the storage and flow
cyclicity. On this basis, a general sequence stratigraphic capacity of these resource shales, including (i) mineralogic
model is established that consists of a basal erosion surface composition, which affects geomechanical properties and the
of underlying strata (sequence boundary, SB), which can be “artificial fracability” of the shales; (ii) regional to local
combined with a younger transgressive surface of erosion structure, which affects fracture patterns and distribution; (iii)
(TSE), generally overlain by an organic‐rich transgressive orientation of the modern stress field, which affects placement
systems tract (TST) capped by a condensed section/maximum and orientation of horizontal wells; (iv) organic matter type,
flooding surface (CS/mfs), which is overlain by downlap­ distribution, and degree of maturity (burial history,) which
ping highstand systems tract (HST) deposits. In the case affect the oil and gas content at a given location; and (v)
of Paleozoic shales, presumed third‐order cycles are origin and depositional history of the shales (summarized in
superimposed on longer frequency second‐order cycles; Abouelresh and Slatt, 2012a, b; Slatt, 2011; Slatt et al., 2012).
in Mesozoic, and probably Cenozoic, shales, biostrati­
graphically identifiable fourth‐order cycles are generally
superimposed on the third‐order cycles. 4.2 GENEraL SEQuENCE STraTIGraPHIC
A principal application of sequence stratigraphy of shales mODEL FOr uNCONVENTIONaL rESOurCE
is the ability to regionally correlate and map sequence SHaLES
stratigraphic intervals in a systematic manner. This ability
then allows for relating sequence stratigraphic characteris­ It is this latter property of the origin and depositional history
tics to geomechanical or geochemical characteristics for of the shales that is the primary consideration for this chapter,
sweet‐spot identification. especially within the context of a standard and well‐known

Fundamentals of Gas Shale Reservoirs, First Edition. Edited by Reza Rezaee.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.