ORIGIN OF EARLY OVERPRESSURE IN
THE UPPER
DEVONIAN CATSKILL DELTA COMPLEX,
Gary G. Lash1 and David R. Blood2
1Dept. of Geosciences,
SUNY Fredonia, Fredonia, NY 14063, USA, [email protected]
2Chesapeake Appalachia, Charleston, WV 25302 USA
The Upper Devonian Rhinestreet black shale of the
western
Appalachian Basin has experienced multiple
episodes of overpressure generation manifested by at least two sets of natural
hydraulic fractures. These overpressure
events were thermal in origin and induced by the generation of hydrocarbons
during the Alleghanian orogeny close to or at the Rhinestreet’s ~3.1 km maximum
burial depth. Analysis of differential
gravitational compaction strain of the organic-rich shale around embedded
carbonate concretions that formed within a meter or so of the seafloor
indicates that the Rhinestreet shale was compacted ~58%. Compaction strain was recalculated to a
paleoporosity of 37.8%, a value well in excess of that expected for burial >
3 km. The paleoporosity of the
Rhinestreet shale suggests that porosity reduction caused by normal
gravitational compaction of the low-permeability carbonaceous sediment was
arrested at some depth shy of its maximum burial depth by pore pressure in
excess of hydrostatic. The depth at
which the Rhinestreet shale became overpressured, the paleo-fluid retention
depth, was estimated by use of published normal compaction curves and empirical
porosity-depth algorithms to fall between 850 and 1,380 m. Early and relatively shallow overpressuring
of the Rhinestreet shale likely originated by disequilibrium compaction induced
by a marked increase in sedimentation rate in the latter half of the Famennian
stage (Late Devonian) as the Catskill Delta Complex prograded westward across
the Appalachian Basin in response to Acadian tectonics. The regional Upper Devonian stratigraphy of
western
INTRODUCTION
Burial-induced mechanical compaction of
argillaceous sediment is accomplished by the loss of porosity as sediment
particles respond to increasing effective stress by reorienting into more
mechanically stable arrangements and pore fluid is expulsed (Hedberg, 1936;
Hamilton, 1976; Magara, 1978; Goulty, 2004).
This elasto-plastic reduction in porosity of clayey sediment under hydrostatic
conditions generally is expressed as some form of the exponential decay
function first proposed by Athy (1930),
f=foe-cz
where z is depth in meters, fo is the initial porosity
at z = 0, and c is the compaction coefficient. Indeed, the majority of porosity-depth algorithms
created from empirical data (e.g., Sclater and Christie, 1980; Huang and
Gradstein, 1990; Hansen, 1996; among others) define by a rapid reduction in
porosity at shallow depth, followed by a reduced rate of porosity occlusion in
progressively older and more deeply buried sediment.
Under certain conditions, notably when fluid expulsion during burial is
restricted due to low permeability and/or rapid sedimentation, mechanical
compaction fails to keep pace with increasing vertical effective stress such
that the pore pressure is greater than hydrostatic (Swarbrick et al., 2002). This phenomenon, termed disequilibrium
compaction.
This paper seeks to demonstrate that Upper Devonian rocks of the Catskill
Delta Complex of western New York state were overpressured by disequilibrium
compaction relatively early in their burial history. We will use compaction strain measurements
from around early formed carbonate concretions in the Upper Devonian
Rhinestreet black shale to calculate the final porosity achieved by
gravitational mechanical compaction.
These results, interpreted in the context of the Upper
Devonian-Mississippian stratigraphy of this region of the Appalachian Basin,
will be used to estimate the depth at which the Rhinestreet shale became
overpressured, its fluid retention depth.
The approach documented in this paper may find application in studies of
other shale-rich basinal sequences.
Figure 1-1. Relationship of (A) porosity and depth
and (B) pressure and depth for a shale that becomes overpressured by
disequilibrium compaction at the fluid retention depth (modified after Harrold
et al., 2000). (C) porosity-depth profile for Brunei Darussalam (modified after
Tingay et al., 2000).
Figure 1-2.
Stratigraphic section, showing Rhinestreet Shale, along with overlying
Figure 1-3. Geologic map of part of western
#2
Figure 2-1. Rhinestreet shale exposed along
Eighteenmile Creek
The Upper Devonian
succession of western New York Sate, which includes the Rhinestreet shale,
grades upward from a base of marine shales and scattered turbidite siltstones
into shallow marine or brackish-water deposits thus recording progradation of
the Catskill Delta across the Acadian foreland basin (Faill, 1985; Ettensohn,
1992). Marine deposits of the Catskill
Delta Complex in the northern Appalachian Plateau are arranged in several cycles,
each one defined by a basal unit of uniformly laminated fissile black shale
that passes upward through a transition zone of alternating black and gray
shale beds into strata dominated by poorly bedded (poorly fissile) gray shale
and occasional turbidite siltstone and thin black shale beds.
The Rhinestreet shale, the thickest of the black
shale units of the Lake Erie District, western
fissile and thinly laminated pyritic shale, thin
gray shale intervals, sparse thin siltstone beds and several intervals of
carbonate concretions (Buehler and Tesmer, 1963; Lash and Blood, 2006). The Rhinestreet shale is underlain by the
Cashaqua gray shale, the contact being sharp and easily recognized in the
field, and passes upward through a zone of interbedded black and gray shale
into the
-------
Figure 2-2. Cashaqua shale-Rhinestreet shale
contact along Eighteenmile Creek (note heavily jointed nature of the Rhinstreet
shale and carbonate nodule horizons in the Cashaqua shale).
Figure 2-3. Middle concretion horizon in the Rhinestreet
shale along Eighteenmile Creek.
Figure 2-4. Close-up of middle concretion horizon
in the Rhinestreet shale; note differential compaction of host shale around the
concretions (stick = 1.9 m).
Figure 2-5. Small carbonate concretion in the lower concretion horizon
showing differential compaction of host black shale around the concretion and
laminated internal structure of the concretion.
Figure 2-6. A small, tilted concretion in the lower concretion horizon
(scale = 1 m).
Figure 2-7. Differential compaction of host black shale around a
concretion; note laminated internal structure of the concretion (scale = 13 cm).
The majority of carbonate concretions of the Rhinestreet shale are found in
three stratigraphically confined but laterally persistent horizons (see
lithologic log). Most concretions are
oblate ellipsoids with maximum diameters and thicknesses ranging up to 2.7 m
and 1.1 m, respectively. Field
observations, including randomly tilted concretions and differential compaction
of host sediment laminae around concretions, are consistent with early
diagenetic growth in unconsolidated sediment.
Further, estimates of pre-cementation host sediment porosity based on
the volume percentage of calcium carbonate cement (74 to 93%) and, perhaps most
importantly, the preservation of a cardhouse clay fabric observed within
concretion samples studied with the scanning electron microscope, suggest that
concretionary growth occurred rapidly within perhaps a meter of the seafloor
(Lash and Blood, 2004a,b).
---
Concretions offer a
unique opportunity to quantify the effects of gravitational compaction of the Rhinestreet shale. However, to ensure that our
calculations yield finite compaction strain of the host shale, we must be
certain that the Rhinestreet concretions formed rapidly and, most importantly,
close to the sediment-water interface.
Field observations, including the wrapping of shale around concretions
and the lack of center-to-edge deviation in laminae thickness, demonstrate that
concretions formed rapidly at shallow depth, perhaps a meter or so below the
seafloor (Lash and Blood, 2004a, b).
Lash and Blood (2004a) maintain that Rhinestreet concretions formed by
the passive infilling of host sediment porosity by carbonate cement (e.g.,
Raiswell and Fisher, 2000). Accordingly,
the volume percent of carbonate cement in the concretion matrix is a proxy for
the porosity of the host sediment at the time of concretion growth (Raiswell
1971; Gautier, 1982). Volume percent of
21 Rhinestreet concretion samples collected from four concretions varies from
74 to 93% (mean = 83%), a range that encompasses the high end of porosity of
modern marine clay deposits close to or at the water-sediment interface (e.g.,
Müller, 1967; Velde, 1996) further suggesting a very shallow depth of origin.
-----
Scanning electron
microscopic analysis of concretion and host shale samples also provides
evidence for shallow concretionary growth (Lash and Blood, 2004a, b). Specifically, SEM images of mudstone samples
collected from concretion strain shadows reveal a porous fabric of randomly
oriented platy particles, which higher magnification proves to be face-to-face
clay flake stacks or domains. Domains
typically are arranged in a low-density network or cardhouse fabric of
edge-to-edge and edge-to-face contacts marked by large voids relative to the
thickness of clay flakes and domains (Lash and Blood, 2004b). Secondary electron images of shale samples collected
only 20 to 30 cm from strain shadows, however, show a generally low-porosity
microfabric defined by a moderately to strongly preferred orientation of clay
flake domains (Lash and Blood, 2004b).
The almost negligible degree of compaction observed in strain shadow
samples demonstrates that gravitational compaction of the Rhinestreet
shale was minimal before carbonate concretions had become
incompressible, pointing to a shallow diagenetic origin of the concretions
(Lash and Blood, 2004b). Moreover, SEM
observations of concretion samples evince a generally open arrangement of
detrital clay grains typical of newly deposited flocculated clayey sediment
preserved by diagenetic carbonate precipitation (Lash and Blood, 2004a). However, the locally moderate planar clay
grain microfabric observed in some concretion samples suggests that the
sediment had started to compact, at least locally, as concretions began to
form.
-------- ---
Figure 2-8. Secondary
electron image of a Rhinestreet shale collected from a concretion strain shadow
showing relatively open or porous microfabric composed of randomly arranged
clay flake domains in edge-to-edge and edge-to-face domain contacts.
------- ---
Figure 2-9. Secondary
electron image showing the strongly oriented microfabric in sample recovered 24
cm from a concretion strain shadow.
Figure 2-10. Secondary
electron image showing the open framework of an etched Rhinestreet carbonate
concretion sample showing a framboid (F) and randomly oriented clay grain
domains (c), and microscaprite (ms).
Figure 2-11. Secondary
electron image showing a moderately planar clay-grain microfabric in an etched
carbonate concretion sample. Many of the
clay grains are
oriented
subparallel to the page. A more open microfabric can be seen in the center of
the image.
Figure 2-12.
??? The most obvious measure of
gravitational compaction strain sustained by a volume of sediment
following accumulation on the seafloor is the
change in layer thickness from the concretion into
correlative layers of the encapsulating
shale. We measured the thickness of
bedding or a bedding
interval inside concretions (Ti)
and the thickness of that same interval in the shale (To), a
presumed
proxy for the original seafloor thickness of the
host sediment. Gravitational compaction
strain of
the shale outside the strain shadow of the
concretion, ε3, is calculated by the following expression,
e3=(Ti-To)/Ti
The mean e3 of
the Rhinestreet black shale based on the analysis of 118 concretions and
encapsulating shale throughout the unit, expressed
as a negative value, is –0.518 or 51.8%
(± 4.9%).
This value is noteworthy because normally compacted marine shales
typically compact more than 65% upon burial to depths comparable to the maximum
burial depth of the Rhinestreet shale
(Burland, 1990).
Compaction strain of the
Rhinestreet shale can be used as a measure of the porosity achieved at the
termination of gravitational mechanical compaction if we assume that all volume
loss was caused by vertical shortening, a reasonable assumption based on the
lack of layer-parallel shortening caused by Alleghanian tectonics in rocks of
the Lake Erie District. Compaction
strain is converted to paleoporosity, fp, by the following equation
derived by Jacob (1949)
fp =(fo +100e3)/(e3
+1)
in which fp is expressed as volume
percent.
However, in order to calculate the fp of the Rhinestreet shale, we first must obtain a
reasonable value for the porosity of the sediment at the onset of normal
compaction, fo. Textural evidence described in more detail by Lash and
Blood (2004a) relates the formation of Rhinestreet concretions to the passive
precipitation of diagenetic carbonate in void spaces of the organic-rich host
sediment. Thus, the estimated
Rhinestreet sediment porosity at the time of concretion growth, based on the
volume percent carbonate cement (e.g., Raiswell, 1976; Gautier, 1982), varied
from 74 to 93%. However, some authors
have suggested that the porosity of newly deposited clayey sediment decreases
from as much as 90% to perhaps 60-65% within a decimeter or so of the seafloor
(Weller, 1959; Von Engelhardt, 1977; Magara, 1978; Luo et al.,
1993). Moreover, Kawamura and Ogawa
(2004) demonstrated an especially rapid reduction in void ratio of pelagic clay
equal to a 5% drop in porosity down to a depth of 10 cm below the
seafloor. Luo et al. (1993)
postulated that such marked losses of porosity within several meters to a few
tens of meters of the seafloor should be considered a continuation of the
depositional process rather than the initial phase of normal load-induced
mechanical compaction. Thus, we
interpret the range in CaCO3 volume in analyzed Rhinestreet
concretions (74 – 93%) to reflect the rapid occlusion of porosity as the
water-rich carbonaceous clay passed into and through the zone of anaerobic
methane oxidation where concretionary growth occurred (Lash and Blood,
2004a). Indeed, the strong tendancy of
organic matter to absorb water thereby favoring a very open depositional
microfabric results the rapid collapse of the clay grains into a preferred
orientation very early (and at very shallow depth) in the diagenetic history of
these types of deposits (e.g, Meade, 1966; Keller, 1982). Thus, we arbitrarily set fo = 70% for our calculation of the Rhinestreet shale fp. Our calculated Rhinestreet shale fp using Jacob’s equation is 37.8% (±7.1%), a value
markedly higher than that expected for shale normally compacted to the modeled
3.1 km maximum burial depth of the Rhinestreet shale (Lash and Blood, 2006).
Figure 2-13. Porosity
versus depth of burial relationships for shale and clayey sediments (after
Rieke and Chilingarian, 1974) showing the estimated maximum PFRD depth range
based on the mean and the upper and lower standard deviation ranges for the
calculated paleoporosity of the Rhinestreet shale. The dashed normal compaction trend, which was
used to define the maximum depth for the Rhinestreet shale PFRD, is from Ham
(1966); all other trends predict a
shallower
PFRD. MBD = modeled maximum burial depth
of the Rhinestreet shale.
-----
#3
Figure 3-1. Calculated
PFRD values for the Rhinestreet shale (fp =
37.8%) using empirical algorithms.
Shaded interval defines the PFRD range based on comparison of the
Rhinestreet fp with published normal compaction
curves for shale. BB = Baldwin and
Butler (1985); FD = Falvey and Deighton
(1982); GL = Gallagher and Lambeck (1989); HA = Hansen (1996); HGY = Hegarty et
al. (1988); H = Hermanrud et al. (1998); HG = Huang and Gradstein
(1990); LR = Liu and Roaldset (1994); SC = Sclater and Christie (1980); T =
Tingay et al. (2000); V = Velde (1996); MBD = modeled maximum burial
depth of the Rhinestreet shale. The majority
of calculated PFRDs fall within the depth range estimated from comparison with
published compaction trends.
Figure 3-2. Upper
Devonian-Mississippian stratigraphic relations of western
CONCLUSIONS
Strain analysis of
overburden-induced differential mechanical compaction of shale around early
(and shallow) formed carbonate concretions in the Rhinestreet shale indicates
that the host shale was mechanically compacted ~ 58%, less than that expected
for shale buried to 3.1 km, the modeled maximum burial depth of the Rhinestreet
shale. Using a reasonable assumption
regarding the porosity of the organic-rich sediment at the onset of normal
compaction, the calculated compaction strain was translated to a paleoporosity
of 37.8%, well in excess of the actual porosity of the Rhinestreet shale as
determined by mercury capillary injection pressure measurements. Normal compaction of the Rhinestreet shale was
halted well before it entered the oil window as a consequence of the
pre-catagenic elevation of pore pressure above hydrostatic. The depth at which the Rhinestreet shale was
overpressured, the paleo-fluid retention depth, was estimated by (1) comparison
of the paleoporosity with published normal compaction curves and (2) use of
several empirically derived porosity-depth algorithms describing the normal
compaction of shale. The onset of
overpressuring of the Rhinestreet shale appears to have taken place between 850
and 1,380 m below the seafloor, not even half way to its maximum burial
depth. The most likely explanation for
the early and relatively shallow onset of overpressure in the Rhinestreet shale
is disequilibrium compaction. The marked
increase in sedimentation rate from the Frasnian and early Famennian (30 m Ma-1)
to the latter half of the Famennian (118 m Ma-1) followed by a sharp
decrease in sedimentation rate in the Mississppian suggests that disequilibrium
compaction was induced toward the end of the Famennian in response to an
acceleration of the rate of progradation of the Catskill Delta Complex perhaps
induced by a late Famennian glacio-eustatic event. The presence of ~1.1 km of Devonian strata on
the base of the Rhinestreet shale suggests that the PFRD must have been at a
depth of ~ 1,100 m, well within the estimated range of the PFRD based on
published compaction curves and porosity-depth algorithms.
References
Athy, L.F.,
1930, Density, porosity and compaction of sedimentary rocks. American
Association of Petroleum Geologists Bulletin, v. 14, p. 1-24.
Baldwin, B.,
and
Buehler, E.J.
and Tesmer, I.H., 1963,Geology of
Burland,
J.B., 1990, On the compressibility and shear strength of natural clays: Geotechnique,
v. 40, p. 329-378.
Ettensohn,
F.R., 1992, Controls on the origin of the Devonian-Mississippian oil and gas shales, east-central
Falvey, D.A.,
and Deighton,
Faill, R.T.,
1985, The Acadian orogeny and the Catskill Delta, in Woodrow, D.L., and Sevon,
W.D., editors, The Catskill Delta: Geological Society of America
Special Paper 201, p. 15-37.
Gallagher,
K., and Lambeck, K., 1989,
Subsidence, sedimentation and sea-level changes in the Eromanga Basin,
Gautier, D.L., 1982, Siderite concretions:
indicators of early diagenesis in the Gammon shale (Cretaceous): Journal of
Sedimentary Research, v. 52, p. 859-871.
Goulty, N.R., 2004, Mechanical compaction
behaviour of natural clays and implications for pore pressure calculation: Petroleum
Geoscience, v. 10, p. 73-79.
Gradstein,
F.M., OGG, J.G. and Smith, A.G., 2004, A geologic time scale 2004:
Cambridge University Press,
Ham, H.H.,
1966, New charts help estimate formation pressures: Oil and Gas Journal,
v. 64, p. 58-63.
Hansen, S., 1996,
A compaction trend for Cretaceous and Tertiary shales on Norwegian Shelf based
on sonic transit times: Petroleum Geoscience, v. 2, p. 159-166.
Harrold,
T.W.D., Swarbrick, R.E., and Goulty, N.R., 2000, Pore pressure estimation from
mudrock porosities in Tertiary basins, southeast Asia, in Swarbrick,
R.E., ed., Overpressure 2000 – workshop proceedings: CD volume, paper
OP2000_9, 6 p.
Hedberg, H.D., 1936, Gravitational compaction of
clays and shales: American Journal of Science, v. 31, p. 241-287.
Hegarty,
K.A., Weissel, J.K., and Mutter, J.C., 1988, Subsidence history of
Hermanrud,
C., Wensaas, L., Teige, G.M.G., Vik, E., Nordgard Bolas, H.M., and Hansen, S.,
1998, Shale porosities from well logs on Haltenbanken (offshore mid-Norway)
show no influence of overpressuring, in Law, B.E.,
Ulmishek, G.F., and Slavin, V.I., eds., Abnormal pressures in hydrocarbon
environments: American Association of Petroleum Geologists, Memoir 70, p.
65-85.
Huang, Z.,
and Gradstein, F., 1990,
Depth-porosity relationship from deep sea sediments: Scientific Drilling,
v. 1, p. 157-162.
Jacob, C.E.,
1949, Flow of Ground Water, in Rouse, H., ed., Engineering hydraulics:
John Wiley and Sons, Inc.,
Kaufmann, B.,
2006, Calibrating the Devonian time scale: a synthesis of U-Pb ID-TIMS ages and
conodont stratigraphy: Earth-Science Reviews, v. 75, p. 175-190.
Kawamura, K., and Ogawa, Y., 2004, Progressive
change of pelagic clay microstructure during burial process: examples from
piston cores and ODP cores: Marine Geology, v. 207, p. 131-144.
Keller, G.H., 1982, Organic matter and the
geotechnical properties of submarine sediments: Geo-Marine Letters, v.
2, p. 191-198.
Lash, G.G.,
and Blood, D.R., 2004a, Geochemical and textural evidence
for early diagenetic growth of stratigraphically confined carbonate
concretions, Upper Devonian Rhinestreet black shale, western
Lash, G.G.,
and Blood, D.R., 2004b, Depositional clay fabric preserved in early diagenetic
carbonate concretion pressure shadows, Upper Devonian (Frasnian) Rhinestreet
shale, western
Lash, G.G.,
and Blood, D.R., 2006, The Upper Devonian Rhinestreet black shale of western
Lindberg,
F.A., 1985, Northern Appalachian Region:
COSUNA Project: AAPG Bookstore,
Liu, G., and
Roaldset, E., 1994, A new decompaction model and its application to the
northern
Luo, X.,
Brigaud, F., and Vasseur, G., 1993, Compaction coefficients of argillaceous
sediments: their implications, significance and determination, in Dore,
A.G., et al., eds., Basin modeling: advances and applications: Norwegian Petroleum Society Memoir
3, p. 321-332.
Magara, K., 1978, Compaction and fluid
migration, practical petroleum geology: Elsevier,
Meade, R.H., 1966, Factors influencing the
early stages of compaction of clays and sands – review: Journal of
Sedimentary Petrology, v. 36, p. 1085-1101.
Müller, G.,
1967, Diagenesis in argillaceous sediments. In: Diagenesis in
Sediments, in Larson, G., and Chilinger, G.V.,
eds., Developments in Sedimentology 8, p. 127-177.
Raiswell, R., 1971, The growth of Cambrian and
Liassic concretions: Sedimentology, v. 17, p. 147-171.
Raiswell, R., 1976, The microbiological formation
of carbonate concretions in the Upper Lias of
Raiswell, R.,
and Fisher, Q., 2000, Mudrock-hosted carbonate concretions: a review of growth
mechanisms and their influence on chemical and isotopic composition: Geological
Society of London Journal, v. 157, p. 239-251.
Rieke, H.H.,
III, and Chilingarian, C.V., 1974, Compaction of Argillaceous Sediments:
Sclater, J.G., and Christie, P.A.F., 1980, Continental
stretching: an explanation of the post-mid-Cretaceous subsidence of the central
Swarbrick,
R.E., Osborne, M.J., and Yardley, G.S., 2002, Comparison of overpressure
magnitude resulting from the main generating mechanisms, in Huffman,
A.R., and Bowers, G.L., eds., Pressure regimes in sedimentary basins and
their prediction: American Association of Petroleum Geologists, Memoir 76, p. 1-12.
Tingay,
M.R.P., Hillis, R.R., Swarbrick, R.E., Mildren, S.D., Morley, C.K., and Okpere,
E.C., 2000, The sonic and density log expression of overpressure in Brunei
Darussalam, in Swarbrick, R.E., ed., Overpressure 2000 – workshop
proceedings: CD volume, paper OP2000_21, 8 p.
Veevers, J.J.
and Powell, C. McA., 1987, Late Paleozoic glacial episodes in Gondwanaland
reflected in transgressive-regressive depositional sequences in Euramerica: Geological
Society of America Bulletin, v. 98, p. 475-487.
Velde, B.,
1996, Compaction trends of clay-rich deep sea sediments: Marine Geology,
v. 133, p. 193-201.
Von
Engelhardt, W., 1977, The
origin of sediments and sedimentary rocks: John
Weller, J.M., 1959, Compaction of sediments: American
Association of Petroleum Geologists, v. 43, p. 273-310.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California