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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Second-Order Accommodation Cycles and Points of
Stratigraphic Turnaround:
Implications for High-Resolution
Sequence
Stratigraphy and Facies Architecture of the
Haynesville and Cotton Valley Lime Pinnacle Reefs of the East Texas Salt Basin
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By
SONAT Exploration Co., Houston, Texas
Recent advances in high-resolution sequence
stratigraphy of carbonate
ramp systems have direct application to
enhancing our understanding of Late
Jurassic stratigraphy of the East Texas salt
basin. Currently, the East Texas salt basin
is enjoying a revival via the recent Cotton
Valley lime pinnacle reef play. This play
element complements the existing traditional Cotton Valley lime/Haynesville
oolite shoal play type. Consideration of
Gulf of Mexico regional Mesozoic
sequence
stratigraphy and paleoeography
aids in linking the two plays together
in an integrated chronostratigraphic
framework thus providing some predictive
capability for
reservoir distribution
and quality.
Although the pinnacle
play is currently
perceived as a 3-D
seismic play, regional
sequence
stratigraphic
analysis
assists greatly in
locating favorable
play trends and high-grading
existing
opportunities. In this
study, I present
a high-resolution
sequence
stratigraphic
analysis
from the
western shelf of the
East Texas salt basin derived from the
integration of 2-D and 3-D seismic, with
well
log
and facies information obtained
from cuttings.
The Middle Jurassic-Early Cretaceous stratigraphy in the East Texas salt basin consists of four major second-order supersequences of approximately 15 m.y. duration. These are defined as large, regionally correlative, retrogradational to aggradational/progradational accommodation packages. Each exhibits systematic vertical stacking patterns and associated lateral facies shifts within subordinate third-order sequences lasting between 1-3 m.y., with related facies and systems tracts. The four supersequences are: Supersequence 1(SS1)-Late Bathonian to Early Kimmeridgian (158.5-144 m.y.); SS2-Early Kimmeridgian to Berriasian (144-128.5 m.y.); SS3-Late Valanginian to Early Aptian (128.5- 112 m.y.); SW-Early Aptian to Late Albian (112-98 m.y.).
The Late Jurassic Smackover-Buckner- Cotton Valley lime-Haynesville-Bossier formational stratigraphy make up parts of two second-order supersequences, SS 1 and SS2 (Figure 1). The Smackover represents the second-order, late transgressive systems tract (TST) and highstand systems tract (HST) of SS1; the Buckner evaporite/red bed facies depicts latest HST condition of SS1 and lowstand systems tract (LST) development of SS2. The Haynesville/Cotton Valley lime paired ramp-shoal carbonate and offshore detached pinnacle reef facies marks the second-order TST of SS2, and the overlying Bossier equates to the second-order interval of maximum flooding.
Within the above framework, the second-order
HST of SS1 (Smackover-Buckner
carbonate-evaporite facies) consist of four
to five regionally correlative third-order
sequences, 250-350 ft thick and 1 m.y.
duration, which systematically stack in a
progradational fashion such that successive
ramp margins are progressively offset
downdip. In detail, each successive
sequence
is thinner than the underlying
one and each is progressively enriched in
blocky highstand carbonates and
proximal evaporite-red bed facies.
A typical sequence
contains an updip
anhydrite facies and
a ramp margin, high-energy
grainstone
belt composed of
a series of higher-frequency,
offlapping,
clinoforming shoal
packages beneath
each third-order
sequence
boundary.
There is little, if any,
pinnacle reef development
linked to these sequences.
The 144 m.y. supersequence
boundary
marks a zone of minimum second-order
accommodation (a point of stratigraphic
turnaround) and serves as a regional stratigraphic
datum useful for hanging well
log
cross-sections. This surface is recognized
in
well
logs by analyzing the vertical stacking
patterns of third-order sequences, as
recorded by overall thickness trends, and
the ratio of blocky highstand carbonates
Figure 1. Schematic regional cross-section for the Mesozoic of the The Gulf Coast.
Schematic
based on regional 2-D seismic, well
log
cross-sections, core/cuttings information from Texas
subsurface and outcrop data from Northeast Mexico.
End_Page 16---------------
low gamma ray
response) to spikey,
transgressive carbonates
(high
gamma ray
response). By tieing
the wells to
the 3-D seismic with
velocity suneys, the
true geometry of
the 144 m.y. terminal
progradational
ramp sequence
is
defined. Downdip
from the terminal
ramp margin of the
underlying second-order HST, 2-3
basinally restricted
reef cycles are recognized
within older,
larger downdip pinnacle
reefs which
were in a mid-slope
position. These basinally
restricted reef cycles record the initial floodback following
the 144 m.y. relative sea-level drop
and they have no equivalent ramp carbonate
on the shelf, which may have been
subaerially exposed.
Updip from the terminal ramp margin,
above the 144 m.y. horizon, the second-order
TST of SS2 (Haynesville/Cotton
Valley lime carbonate shoal-pinnacle reef
facies) consists of 4-5 regionally correlative
third-order ramp sequences and 4-5
pinnacle reef cycles, each 50-150 ft thick,
lasting 1 m.y. Pinnacle reef cycles are
detached in plan view from the ramp
cycles, yet linked in accommodation
space and time. Ramp sequences systematically
stack in a retrogradational or
aggradational fashion, whereas individual
pinnacle reefs progressively decrease in
diameter as they aggraded vertically. Each
ramp sequence
consisted of an updip,
proximal evaporite-red bed facies, a
ramp-margin oolite shoal belt (traditional
Haynesville reservoirs), and an outer
ramp slope composed of muddy, argillaceous
carbonate. During the second-order
regional transgression (TST of SS2) older
pinnacle reefs, over 1300 ft thick, grew in
progressively deeper water and were
eventually stranded downdip, passing
updip to younger pinnacles, typically less
than 300-500 ft thick, which grew in
successively more landward positions.
Younger pinnacles are missing the earlier
reef cycles, are not as tall, and are
enriched in shallower-water facies as
compared to their older, downdip counterparts.
Through high-resolution correlation of
ramp sequences with reef cycles, guided
by integrated seismic and well
log
control, updip oolite shoal regional porosity
can be correlated directly with time-equivalent
pinnacle reef reservoirs, casting
light on porosity distribution as
well
as mechanisms for porosity development
within the East Texas salt basin. The top
of the Cotton Valley lime/Haynesville carbonate
is a diachronous surface characterized
by appreciable depositional topography,
onlapped by the Bossier shale along
a
well
-documented submarine condensed
section. Little evidence exists for a
relative sea-level drop at this surface.
A high-resolution sequence
stratigraphic
model which summarizes the Smackover-
Buckner-Cotton Valley Lime/Haynesville
(Lou-Ark) stratigraphy is presented
in Figures 1 and 2. Figure 2 depicts the
accommodation history over the temporal
interval of concern. In this model,
beneath the
composite accommodation
changes are
produced by superimposing
high-frequency
4th-3rd-order relative
sea-level changes and
lower frequency 2ndorder
relative sea-level
changes on background,
regional tectonic, subsidence.
The horizontal
axis (Figure 2) represents
time moving forward
from left to right.
The vertical axis depicts
changes in sea level. The
timing of second-order
systems tracts are shown
at the top of the diagram.
Each high-frequency
eustatic cycle (eustatic
beat) is numbered from
0 to 12. As each beat
floods the ramp top,
sedimentation takes
place (light gray stipple
high - frequency sea-level
curve in Figure 2; "PWD" refers to paleowater
depth and delta X shows changes in
PWD). During high-frequency submergence,
the top of the sediment surface
climbs from lower left to upper right
in the diagram. When high-frequency sea
level falls beneath the ramp top (times
depicted by darker vertical shading),
marine sedimentation ceases.
Due to the effects of composite eustasy, the proportion of marine submergence and concomitant sedimentation to exposure and non-deposition per high-frequency beat varies systematically as the beats migrate through the lower-frequency 2nd-order eustatic cycles. These systematic and sequential changes in accommodation space during eustatic beats result in a predictable stacking architecture of high-frequency stratigraphic cycles. Eustatic beats 0-4 are within the 2nd-order highstand systems tract, and each eustatic beat is capable of generating one stratigraphic cycle.
During the 2nd-order HST, accommodation is progressively declining and submergence- prone eustatic beats pass into exposure- prone eustatic beats. Thus, ramp cycles 1-4 thin upward and prograde laterally into
Figure 2. Composite accommodation model proposed for Lou-Ark stratigraphy
End_Page 17---------------
the basin. Each ramp cycle has an updip evaporite facies (Buckner), a mid-ramp quiet-water facies, a ramp crest grainstone oolitic facies and a ramp slope facies. Small patches of biohermal or reefal facies are depicted by dark grey shading and these biohermal entities are located at the seaward margin of the ramp crest or slightly down the ramp slope. Biohermal masses within cycles 1-3 are spatially restricted and inhibited from becoming pinnacle buildups due to two factors: (1) the declining accommodation within 2nd-order HST, each bioherma1 entity is smothered in carbonate sand from above as the next cycle progrades out and over the bioherm (2) related to the same accommodation problem, "nasty" bank water of elevated salinities from the Buckner facies washes seaward over the bioherms adversely affecting their growth.
The 2nd-order HST passes into the 2nd-order LST between eustatic beats 4 and 5
where the rate of 2nd-order fall is at a
maximum (the inflection point on the 2ndorder
eustatic curve). This point marks the
2nd-order super-sequence
boundary and
equates hypothetically to the 144 m.y.
supersequence boundary in the Lou-Ark
framework presented previously. In this
position of stratigraphic reversal, the system
turns around from progradation related
to progressive accommodation loss,
to retrogradation caused by progressive
accommodation gain.
From here on, each high-frequency beat becomes progressively submergence prone and the ramp cycles display a retrogradational stacking architecture with increasing topographic relief as they march updip. Pinnacle buildup development is now promoted as problems (1) and (2) outlined previously are deviated. For example, between ramp cycles 4 and 5, biohermal growth which initiated during cycle 4 can continue because the ramp crest of cycle 5 (or rollover point) is now located slightly updip, or landward, of the ramp crest of cycle 4. Because of this relationship, it is hypothesized that the bioherma1 contribution from cycle 5 will stack vertically on the ready-made foundation of the healthy bioherm from cycle 4.
The 2nd-order TST occurs between eustatic beats 6- 12 as the rate of 2nd-order fall declines, and passes through its trough and back into a 2nd-order rise. The composite eustatic effect each of high-frequency beat becomes progressively submergence- prone and overall accommodation increases, promoting pinnacle development. In detail, above cycle 5, each reef cycle is broken into its high-frequency transgressive and regressive phases. The net result is that each pinnacle buildup is cyclic with contributions from 2 to 4 eustatic beats. The furthest downdip pinnacle reef consists of contributions from cycle 4 through the transgressive part of cycle 7. By contrast, the most updip pinnacle only contains contributions from cycle 8 and the transgressive phase of cycle 9. The most downdip pinnacles are therefore the oldest and were drowned during the overall regional 2nd-order transgression prior to the inception of the most updip pinnacle. A lack of appreciation of the true chronostratigraphic and dynamic relations summarized here has lead to the misperception by some workers that the downdip pinnacles are deep water and the updip pinnacles shallow water. With respect to internal facies composition and petrophysical parameters, each pinnacle is vertically heterogeneous.
End_Page 18---------------
Inspection of thin sections from cuttings
and rotary sidewall cores, coupled with
core descriptions from various operators,
indicates that the transgressive phase of
each pinnacle reef consists of slightly
argillaceous lime wackestones (increased
gamma ray count) composed of thrombolitic
facies or microbiolite facies marked
by an abundance of algal bin- and clotting.
These facies, with associated delicate
deeper-water skeletal allochems, suggest
moderate water depths related to high-frequency
rise in sea level. The maximum
flooding surface of each reef cycle is
approximated by the highest gamma ray
count. The regressive cap or highstand systems
tract of the reef cycles is composed of
in situ, apparently low-energy Late Jurassic
reef-builders, such as sponges and delicate
corns. The caps to some of the reef cycles
consist of high-energy grainstones with
oncolites and abraded, well
-washed, skeletal-
peloidal sand, indicative of shoaling to
very shallow water depths. On
well
logs,
the gamma ray within the highstand portion
of a reef cycle cleans upward, becoming
blocky to remarkably flat. A lack of
core data has hampered a complete understanding
of facies and diagenesis.
Carbonate systems in similar accommodation
settings, such as the younger Sligo formation
in south Texas, provide stratigraphic
analogues useful for driving well
log
correlations and seismic interpretation.
Analogous buildup or pinnacle reef facies
typically occur linked to the terminal phase
of carbonate deposition near the top of
regional, second-order TST's beneath deep
marine shales (second-order MFS) which
serve as source and seal facies. Pinnacle
geometries are promoted by increasing
accommodation within an overall retrogradational
stacking of carbonate
facies belts. Differential compaction of
shaly, onlapping facies around preexisting
rigid carbonate buildups enhances their
seismic recognition. Hydrocarbon-producing
examples include the Devonian of
Canada the Miocene of Southeast Asia,
the Mississippian Lodgepole of the
Williston basin, and the Upper Pennsylvanian
Horseshoe Atoll of the Midland
basin, among others. Integration of key
principles from the Late Jurassic of the
East Texas salt basin with these and other
examples should fuel the search for other,
as yet, unrecognized carbonate buildups
and pinnacle reefs within similar accommodation
windows in other areas.
REFERENCES
Goldhammer, R. K., Lehmann, P. J., Todd,
R. G., Wilson, J. L., and Wad, W. C.,
1991 , Sequence
stratigraphy and cyclostratigraphy of the Mesozoic of the
Sierra Madre Oriental, a field guidebook
Gulf Coast Section, Soc. Econ. Paleont
and Mineral. Found., 86 p.
End_of_Record - Last_Page 19---------------