Download Palo Duro Basin Lower Penn

Palo Duro Basin Lower Penn
Gas Shale Project
Motley County & Floyd County, Texas
Summary
The Palo Duro Basin Lower Penn Gas Shale Development Project is located
approximately 70 miles Northeast of Lubbock, Texas and covers approximately
1,150,000 acres. Geologically, the project is located in the east central portion of the Palo
Duro Basin. The Penn Shales were deposited in an area composed on numerous down
thrown Graben Blocks. This resulted in thick gas charged Lower Penn Shale interval
with gas charged Atoka Sandstone/Granite Wash intervals. (see log type curve) The
shale reservoir was self sourced. The shale gas is trapped in fractures and in a low
permeability shale matrix. Currently there is one shut-in gas well completed in the
Lower Penn Shale. A small number of wells have penetrated the Lower Penn Shales in
the Motley and Floyd Counties. Well control and geo-chemical coverage delineate the
productive reservoir.
Introduction
In 1950, the Amerada Birnie #1, a deep test in Motley County, Texas, discovered an
unusually thick, gas-charged Pennsylvanian (Lower Bend Group) clastic section. The
Birnie #1 was re-entered in 1958 and tested gas and condensate from a thick Morrow
Granite Wash/Sandstone zone and gas and condensate from an Atoka Granite
Wash/Sandstone zone. The well was never produced due to the nearest pipeline being
over 40 miles away at the time.
Several additional wells were drilled in the area
apparently attempting to find an oil column. These wells helped to delineate the Birnie
Field. The wells were plugged when they were determined to be gas wells based on dst’s
and cores. The Birnie #1 was eventually plugged. The Birnie #2 was drilled and
completed in the field in the 1970’s. Legacy Exploration acquired the well and has done
some testing on the well. The Gas was 1250 BTU and the well is expected to produce
about 10 bbls of free condensate per 1 mmcf of gas produced. If the gas is stripped an
additional 60 bbls of liquids per 1 mmcf of gas can be produced. A 24 inch high-pressure
pipeline was built from Waha to Wheeler in the mid 1980’s. The line is located 18 miles
Northwest of the Birnie #2 well.
The Birnie Field was deposited in a down thrown Graben Block. This resulted in gas
charged Atoka Granite Wash/Sandstone intervals. The pay zones were sourced from
Lower Penn Shales. The gas is trapped in stratigraphically and structurally segregated,
low-permeability Atoka reservoirs. Based on the Birnie #2 a new well was drilled in
2003. The Legacy Exploration – D. M. Cogdell Jr, Est. #1-01 was drilled to 9,200 feet.
The well found the gas charged Granite Wash Zones. More importantly the well cut 657
feet of fractured organic rich black shale in the Lower Penn. Perforations in the Lower
Penn Shale flowed at rates of up to 5 mmcfpd. The excellent test results in the Lower
Penn Shale have ignited a great deal of interest in the shale gas potential of the Lower
Penn Shale in the Palo Duro Basin.
Geological Discussion
TECTONIC SETTING
The Lower Penn Shale productive hydrocarbon system lies in a pull-apart basin in
Motley, Floyd, Hale, Swisher, Briscoe and Hall Counties, Texas. The basin is a
component of the Red River-Matador structural trend of the greater Ancestral Rocky
Mountains. Arkosic detritus originating from the Amarillo-Wichita uplift was transported
southward, over the region. Episodic localized subsidence accumulated a part of this
sediment load as syntectonic, cyclically stacked Bend Group (Atokan, lower
Pennsylvanian) fluvial-deltaic to marine deposits. Organic facies within the fill are
predominantly terrestrially derived (gas prone) and present in sufficient quantity for
significant hydrocarbon generation. Lopatin method basin modeling, vitrinite reflectance
(Ro) measurements, and Ro calibrated pyrolysis derived maturity measures demonstrate
that the Lower Penn organic facies in the subject area have approached peak gasgenerating maturation levels. Generated gas migrated within and outside of the basin
following non-sealing faults and channelized fluvial pathways into several reservoir rock
types in combination structural and stratigraphic traps. However, substantial amounts of
gas are still trapped in the source/reservoir rock of the Lower Penn Shale.
The nearest significant non-associated gas production from Atokan and Morrow
reservoirs is 30 miles to the east in the Cottle-King gas fields of the Hardeman basin
(eastern extension of the Palo Duro basin). More than 125 billion cubic feet of gas
equivalent (bcf) have been produced from the Cottle-King gas fields through December
1999, and ultimate recovery from existing wells is projected to exceed 150 bcf. Two new
fields were discovered in the past decade in Cottle County.
This discussion will cover the tectonic setting, deposition, hydrocarbon system source
rocks and source rock maturity, migration, and location of the potentially productive
Lower Penn Shale Development Project area.
Figure 1 is a composite of regional tectonic elements that evolved throughout the
Pennsylvanian in the southern mid-continent region.
Figure 2 shows a detailed
composite of the tectonic elements in Motley and eastern Floyd counties, Texas. Cross
sections 1 and 2 are across the Birnie Field. The subject area is located in the eastern
portion of the Palo Duro basin near the juncture of the Red River uplift/arch [rhornpson,
1988; Ewing, 1991) and the Matador uplift/arch (Hills, 1963, 1985; Budnik, 1989;
Ewing, 1991). The east end of the Palo Duro basin is separated from the Hardeman basin
by the Narcisso uplift and Narcisso saddle. The Matador Uplift and the Amarillo Uplift
comprise an elongate structural trend that extends more than 310 miles from Montague
County in north central Texas to Roosevelt County in east central New Mexico. The
Matador Uplift is marked by a series of basement-cored pop-up horsts, many of which
trap oil accumulations, separated by poorly documented depressions. This trend is a
component of the late Paleozoic Ancestral Rocky Mountains (Kluth and Coney, 1981;
Budnik, 1986; Kluth, 1986; Ye et al., 1996) formed by foreland deformation inboard of
the Ouachita-Marathon over-thrust belt (Walper, 1977; Kluth and Coney, 1981; Kluth.
1986).
The structurally complex zone located between the Red River Uplift and Matador Uplift
features is referred to as the Red River-Matador tectonic zone or Palo Duro Basin.
Structures within this area are interpreted from surface expressions of faults, geochem,
seismic and subsurface data. This zone is significantly faulted. Faulting was most intense
during deposition of the Bend Group (Atokan), which is consistent with broad-scale
observations of Ham and Wilson (1967). Locally, post-Atokan tectonic activity continued
until as late as the Early Permian, but primarily as minor continued motion of established
fault zones.
The orientation and distribution of structures along the length of the Red River-Matador
trend reflect left-lateral strike slip. (Budnik, 1986) The over- lapping left step from the
Red River to the Matador feature created complex lesser order, en echelon pop-up horsts
and pull-apart grabens, the orientations of which are consistent with left-lateral strike slip.
Evidence for strike-slip and oblique-slip (wrench) motion within the zone, interpreted
from a combination of seismic and subsurface data include (1) the presence of enechelon,
laterally extensive, basement-offsetting faults, (2) faults bounding basement-involved
structural blocks that tend to be high-angle normal or reverse with the two types
coexisting locally, (3) along-strike change in amount of vertical displacement of faults
and reversal in dip of some fault planes, (4) splaying of fault tips, apparent braiding of
faults, or multiple parallel faults in narrow fault zones, (5) presence of flower structures,
and (6) existence of localized, thick, rapidly deposited syntectonic Atokan sediments in
graben blocks adjacent to horst blocks with coeval unconformities. Literature supporting
these observations as typical of wrench- fault conditions includes Moody and Hill (1956),
Wilcox et al. (1973), Reading (1980), Christie-Blick and Biddle (1985), Sylvester (1988),
and Harding (1990).
No Pennsylvanian or older outcrops exist in the region of this study. Therefore, no direct
kinematic evidence ( e.g., piercing point data) is available that would quantify the amount
of lateral slip along the Red River-Matador tectonic zone or for component faults.
Subjective palinspastic reconstruction suggests strike-slip displacement on the order of
kilometers for some of the more significant individual faults with cumulative
displacement of the tectonic zone perhaps an order of magnitude higher. For comparison,
displacements in the range of 12-26 km have been inferred for oblique left-reverse-slip
faults on the northern margin of the Wichita uplift in Oklahoma (McConnell, 1989). PreAtokan sedimentary units are relatively consistent in thickness and lithofacies
distributions regionally and were flat lying at the time of initiation of deformation, thus
no sedimentary-related tectonic piercing lines are known.
The Red River-Matador trend reactivated a major Precambrian terrain boundary, possibly
marking the southern edge of the Middle Proterozoic Oklahoma aulacogen (Ham et al.,
1964; Brewer et al., 1981; Denison et al., 1984).
STRUCTURE
The subject area appears to contain a series of elongate, rhomboid depression grabens
bounded on all sides by fault zones with maximum structural relief on the base of the
Pennsylvanian sometimes exceeding 2.7 km from the graben floor to the top of the
adjacent horst block. The structural characteristics of the grabens suggest that some of
them are rhombochasms (Crowell 1974) formed through left-step overstepping of leftlateral strike-slip fault zones in the Red River-Matador tectonic zone. Literature
illustrating how such basins are created includes studies by Rodgers (1980), Mann et al.
(1983), and Aydin and Nur (1985).
The structural trends run east-west near the east-
west running Matador Arch and northwest-southeast near the northwest-southeast
trending Amarillo Uplift.
Deposition
Atokan and Morrow stratigraphy preserved within the grabens is unique in that it
represents a depositional record largely absent on the uplifted structures due to
nondeposition.
During the major Atokan period of subsidence, the grabens formed
within a larger piedmont region between the Amarillo-Wichita mountain chain to the
north and the Midland Basin to the south (Figure 1).
No widespread fault-derived cataclastic breccia deposits exist in the graben fill as
interpreted from existing well control. This observation suggests that topographic relief
across the basin margins at any given time was minimal and that sufficient sediment was
being transported to the graben to quickly fill the depression as it formed. Therefore,
episodic graben subsidence periodically accommodated a part of an abundant sediment
supply. Cycles in the basin fill are attributed to sedimentary response to local basin
subsidence rather than to more regional sea level fluctuations or extrabasinal source
composition or tectonic fluctuations. Cyclically stacked sheets of quartz conglomerate,
poorly sorted sandstone and mudstone units, alternating with winnowed sand-rich units,
were deposited in alternating deltaic and braid-plain environments depending upon base
level.
Conceptually, during and immediately following a fault motion-related subsidence
episode, the basin accepted all detritus until the depression was filled. Characteristic
associated facies are debris fan (pebbly quartz conglomerate), overbank (coaly
mudstone), and discrete laterally limited distributary channel (sandstone) depositional
environments.
During tectonically quiescent periods and after reestablishment of a higher base level,
stream systems were re-established, and fine materials were winnowed and carried
southward toward the Midland basin, whereas the coarser material dropped out of the
system over the graben area. This resulted in creation of stratigraphically
compartmentalized, reservoir-quality , braided channels that amalgamated laterally into
sheet like deposits.
Within the grabens, the Bend Group is an unconformity-bounded stratigraphic sequence.
The basal unconformity is erosional, marked by the contact of the Lower Bend Group
and Morrow clastic lithofacies over the shallow-marine Mississippian limestone. The
lower two thirds of the Bend Group are primarily nonmarine. The beginning of regional
transgression is recorded in the upper one third of the lower Bend Group, which contains
thin limestone units interbedded with nonmarine clastic deposits. The lower Bend Group
can be divided into numerous parasequences by correlating radioactive, coaly-mudstone
beds in adjacent wells. Subdivisions at all levels of detail tend to expand in thickness
toward the structurally deep parts of the basin. In a regional sense, continued marine
transgression and eventual subsidence of the Palo Duro basin, north of the graben, caused
the regional depocenter to shift to a position nearer the Wichita-Amarillo mountain front,
shutting off the source of coarse clastic supply to the graben area. Therefore the upper
Bend Group is entirely marine and composed of calcareous shale and argillaceous basinal
carbonate units that correlate to basin-rimming carbonate buildups. Discrete sandstone
beds in the upper Bend in a few wells are interpreted to be offshore sandbars deposited
locally along the basin rim.
GRABEN RELATED HYDROCARBON SYSTEMS
Petroleum-hydrocarbon systems encompass source rocks, the processes of generation and
migration of the hydrocarbons, and the geologic elements of traps, seals, and reservoirs
that are essential for a hydrocarbon accumulation to exist (Magoon and Dow, 1994).
There are three separate systems in the subject area, each related to distinct source-quality
shale units. Two systems are of relatively minor importance because of minimal
petroleum generation: (I) Canyon Group (Missourian) oil-prone black shale-sourced
Canyon reef and Cisco sandstone oil accumulations (e,g. Wolf Flat oil field), and (2)
Strawn Group (Desmoinesian) oil-prone shale, the probable source for several Strawn oil
and gas shows. The third system, which is the most important economically, is the gas
system charged by Bend Group (Atokan) and lower Penn gas-prone shales. Molecular
composition of gas from selected fields is similar throughout the Bend hydrocarbon
system in the east end of the Palo Duro basin and the Hardeman basin, suggesting that the
gas in the system was generated from a common source rock type. The deepest portion of
the Palo Duro Basin (Motley, Floyd, Briscoe, Hale and Swisher Counties) is the so-called
hydrocarbon kitchen (Demaison, 1984) in which the source rocks were concentrated and
then cooked. Some of the gas then migrated to the various traps that have been
discovered as undeveloped gas fields in Motley and Floyd counties while substantial
amounts of gas remain in the source/reservoir rocks of the Lower Penn Shale.
SOURCE ROCKS
The muddy deposits of the Bend Group preserved in deepest portion of the Palo Duro
Basin are obvious candidates for gas source rocks. The deepest portion of the basin filled
rapidly with clastic and organic debris during tectonically induced subsidence events.
Rapid burial favored preservation of organic matter from oxidation and biodegradation.
Examination of lower Bend Group drill cuttings from the center of the basin reveals
abundant terrestrially derived woody kerogen in a variety of rock types, but particularly
in dark gray to black mudstone and pebbly mudstone units. Disseminated bituminous
coal is common in drill cuttings from fine-grained intervals, and thin coal seams exist.
Nitrogen content of gas in the systems ranges from 3.35% in western Motley county to
3.8% in Cottle and King counties. This is consistent with derivation from coaly (humic)
organic matter (Whiticar, 1994).
Kerogen microscopy tests and TOC studies have been performed in Motley, Cottle and
King Counties. Results from these studies should be representative of the subject area.
The following analysis is taken from work published by Brister, Stephens and Norman.
Kerogen microscopy of upper Bend Group samples yields primarily unstructured lipid
organic matter with relatively small percentages of terrigenous vitrinite and inertiniteo
Lower Bend Group samples yielded abundant terrigenous organic matter including a high
percentage of vitrinite. Rock-Eval pyrolysis data from Bend Group shale samples from
five wells in the deepest portion of the basin support the terrestrial (type III) to mixed
terrestrial/marine (types II and III) origin of the Bend organic facies within the graben as
plotted on a modified Van Krevelen diagram. (Figure 3 ) The presence of gas-prone coal,
which tends to give anomalously high hydrogen index values and plots toward the type II
area of the diagram (Peters and Cassa, 1994), over represents the true contribution of type
II kerogen to total organic carbon in the Bend Group.
Measured total organic carbon (TOC) from 20 Bend shale samples collected from three
deep basinal wells ranges from 1.05 to 20.01 %, with a mean value of 4.98%. The level
of maturity determined from pyrolysis suggests that the original organic carbon values
were at least twice the measured values and have been reduced by hydrocarbon
generation and expulsion. These samples were collected from shale units with well-log
characteristics of high gamma-ray values, high resistivity, and low density, which are
typical of organic shales (Schmoker, 1979, 1981; Passey et al., 1990; Herron, 1991).
Approximately 20% of the bulk volume of the Bend Group in the Broken Bone graben is
estimated to be similarly organic rich. Therefore, several hundred meters of source rockquality Bend Group shale resides in the graben.
SOURCE ROCK MATURITY
In the late 1980’s, only relatively immature source rocks were known to exist in the
region (Dutton, 1980, 1986; Dutton et al., 1982; Ruppel, 1985), thus the source of gas in
the Motley, Cottle and King counties gas fields was unidentified. As wells were drilled
in the deeper portions of the Palo Duro and Hardman Basins analysis of the shales
indicate that the source rocks have achieved peak gas-generation levels (lower oil
window/upper gas window). For example Vitrinite Reflactance tests on samples from the
D. M. Cogdell Jr. Estate #1-01 in Motley County show values in the Lower Penn Shale
ranging from 0.92% to 1.2%. This is in the wet gas window.
Well-based observations that support thermal maturity of the Lower Penn Shales in the
Palo Duro Basin include gas-bleeding shale cuttings, bituminous-rank coal, and gas
production from wells deep in the basin that have no other potential source.
RESERVOIRS AND TRAPS
The Atoka Granite Wash/Sandstone reservoirs and the Morrow Granite Wash/Sandstone
reservoir appear to be productive in the grabens based on the Birnie #2 which is an
existing shut-in gas well, well test from the Birnie #1 that was completed, tested and then
eventually plugged, DSTs, mud logs, and electric well logs.
The Lower Penn Shale tested gas at rates of up to 5mmcfpd in the D. M. Cogdell Jr,
Estate #1-01. The Lower Penn Shale is a highly fractured organic shale which is both the
source rock for the gas and the reservoir rock for the gas.
CONCLUSIONS
The subject area, in western Motley County and eastern Floyd County, is a complex
wrench-fault related pull-apart basin that filled with sediments as it subsided. A
combination of abundant terrestrial organic material, depositional environments favorable
for organic material deposition, and rapid burial preserved significant quantities of
source-quality organic facies. Early deep burial and relative tectonic quiescence since the
Permian has allowed sufficient time and temperature for maturation of organic matter.
The gas-prone organic matter has expelled gas that has migrated within and outside of the
basin. The faults that formed during basin construction apparently range from excellent
seals (reverse faults) to open conduits for vertical migration (normal faults). The complex
regional structure and depositional facies of the Bend Group combine to create
compartmentalized reservoirs. In addition to the traditional gas reservoirs, this area
contains thick fractured Lower Penn Shales that serve as both gas source rock and
reservoir rock. Together, the basin, source rocks, maturation history, migration, trapping
mechanisms, and reservoir rocks define a discrete hydrocarbon system.