PSThe
Upper Devonian Rhinestreet Shale, Western New York State: from Seal
to Fractured
Reservoir*
By
Gary G. Lash1
Search and Discovery Article #10112 (2006)
Posted September 27, 2006
*Adapted from poster presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006. A closely related article by the author is “The Upper Devonian Rhinestreet Shale: An Unconventional Fractured Reservoir in Western New York State, Search and Discovery Article #10108 (2006).
Click to view posters in PDF format; right click to download.
Poster 1 (~0.8 mb) Poster 2 (~0.9 mb) Poster 3 (~0.8 mb)
1Dept. of Geosciences, SUNY-Fredonia, Fredonia, NY 14063 ( [email protected] )
Abstract
The Upper Devonian Rhinestreet black
shale of the Catskill Delta complex, western New York State, reflects a complex
burial and overpressure history that resulted in the generation of multiple sets
of vertical joints interpreted to be natural hydraulic fractures (NHFs). The
EASY%Ro chemical kinetic model was used in this study to model an
average vitrinite reflectance value of 0.74% measured on samples collected from
along the Lake Erie shoreline. The earliest set of joints, a N-S-trending set,
is found almost exclusively at the contact of the Rhinestreet shale and
underlying Cashaqua gray shale, indicating that the former served as a hydraulic
top seal
at the modeled burial depth of ~2.1 km and prior to the onset of
catagenesis (modeled Ro=0.50%). The pre-catagenic NHFs that
propagated from the top of the Cashaqua gray shale into the base of the
Rhinestreet shale may have initiated during uplift of the basin caused by the
Morrowan docking of the Goochland terrane in the southern Appalachians. A
renewal of subsidence during the Atokan carried the Rhinestreet into the oil
window by the Middle Permian (modeled Ro=0.60%) when bitumen-filled
horizontal μm-scale
microcracks propagated through these laminated, low-permeability deposits now
pressurized by catagenesis. Soon after this, NW-trending vertical NHFs formed
within the Rhinestreet, especially its organic-rich basal interval. Thus, those
characteristics of the Rhinestreet that enabled it to serve as an efficient top
seal
favored its hydraulic fracturing during catagenesis. The final phase of NHF
generation, an ENE-trending set, probably occurred near the end of the Permian
in response to Alleghanian dextral tectonics and at the modeled maximum burial
depth of ~ 3.2 km.
Selected Figures
Generalized geologic map, western to west-central New York, showing outcrop patterns of upper Devonian units and location of Eighteenmile Creek section. |
|
Upper Devonian – Upper Carboniferous stratigraphic column, western New York to west-central New York (modified from Lindberg, 1985). |
|
Rose diagram of three regional sets of joints (N-S, NW, and ENE) in study area. |
|
Paleogeography, Late Carboniferous – Early Permian, showing joint patterns resulting from the oblique approach of Gondwana (after Hatcher, 2002). |
Conclusions
-
The Rhinestreet shale, like other Upper Devonian organic-rich shale units of western New York State, served as a top
seal
early (pre-catagenesis) in its burial history
-
Compromise of the top
seal
by a combination of natural hydraulic fracturing elastic contraction prior to entering the oil window may have resulted from widespread uplift and consequent reduction of confining pressure (Sh) of the basin during Morrowan time.
-
Passage of the Rhinstreet Shale into the oil window is first recorded by formation of horizontal bitumen-filled microcracks, followed by NW-trending joints that record the Late Carboniferous-Early Permian collision of Africa and Laurentia related to the clockwise rotation of Gondwana.
-
ENE-trending joints propagated soon after the NW joints, probably as a consequence of further oblique convergence of Gondwana and Laurentia.
-
Those features of the Rhinestreet shale that made it such a strong
seal
prior to entry into the oil window, the strongly oriented clay grain microfabric and flattened ductile organic particles, resulted in pressurization of these rocks during catagenesis and consequent propagation of natural hydraulic fractures (NW– and ENE-trending joints).
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