The Lower Pennsylvanian formations, which produce oil and gas along the Bend flexure and in the Fort Worth basin, crop out in Llano uplift area. The outcrops have been studied extensively from both paleontological and lithological viewpoints from which several conflicting classifications have evolved. With the addition of subsurface terminology, stratigraphic classifications vary with individuals and companies.
Sample Shows and Mud Log Characteristics of the
Mississippian of North Central Texas.
Author: David E. Noller
The Mississippian sample cuttings observed in north central Texas are from the
Barnett Shale, the Chester detrital limestone, and the Chappel reef. The Barnett is a
black, very carbonaceous and methane-rich shale. The Chester is a brown, oolitic,
chert-rich limestone that represents the inner-reef facies. The Chappel reef is
composed of white, fine to microcrystalline, chalky limestone, with porosity
occurring primarily in fractures. Chappel sample hydrocarbon shows are subtle,
with a trace to forty percent (40%) of the sample cuttings having bright
fluorescence, no stain, no to fair odor, and a subtle solvent-crush cut. Hot wire gas
detectors usually record small gas measurements while Chappel reefs are being
drilled, due to Barnett Shale gas contamination, and because of the fractured nature
of the reservoir. Hydrocarbons in the Chappel reef are indicated by increases in
Propane (C3), Isobutane (IC4 ), and Normal Butane (NC4) on the chromatagraph.
This paper will describe the sample cuttings, sample shows, and mud log
characteristics of the commonly recognized Mississippian subsurface units of the
Bend arch and the Forth Worth basin. In order of increasing depth the units are the
Barnett Shale, the Chester limestone, and the Chappel Limestone. Two recent cores
from Archer County provide more detailed information of the Chappel and the
The youngest of the Mississippian units in the area is the Barnett Shale of
Chesterian equivalence. It is dark gray to black but shows a brownish tint in direct
sunlight. It is very well indurated (hard), smooth to waxy to touch, and both uneven
and conchoidal in fracture.
The Barnett is very carbonaceous and distinct from the overlying Atokan and
Strawn shales. The cuttings stain sample bags, leave a brown streak when rubbed on
white paper, and leave a greasy brown stain when squeezed between the hands.
Figures 1,2, and 3 illustrate the drilling and mud log characteristics of the Barnett.
Note that the drilling (penetration) rate is higher for the Barnett than for the
overlying Marble Falls, particularly in the lower 20 to 15 feet of the shale. The shale
gas recorded by the chromatograph, predominantly C1 (methane), increases to
several hundreds of units, particularly in the lower third of the section.
As used here, the term Chester Limestone refers to a typically dirty white to dark
gray, dense, wackestone-grainstone with varied, detrital grains, which include
oolites, chert and quartz grains. The textural contrast between the microcrystalline
matrix and the grains may produce a mottled aspect. There is little or no observable
porosity. The lithology is distinctive and often designated, incorrectly but
understandably, the Chester facies. It is considered to be material that was shed by
the Chappel reefs and reworked into related deposits, occurring typically on reef
margins and inter-reef areas. The thickness of the Chester limestone is inversely
proportional to proximity to the reef core. Reef crest wells typically have no Chester
limestone (Fig. 2-3). Reef flank wells should have Chester and Chappel (Fig. 1).
Some inter-reef wells have cut only Chester between the Barnett and the
As shown by the example of the Grand Energy, McComas No. 1 (Fig. 1), the
drilling (penetration) rate falls notably in the Chester Limestone, but it varies in
detail, testifying to the presence of argillaceous interbeds. Subtle hydrocarbon
shows and gas kicks have been encountered while drilling the Chester, but density
logs have usually shown low porosities. Production from the Chester is very rare
and it is the author’s opinion that one should be hesitant to drillstem test this
As employed here, the term Chappel is used for the basal Mississippian unit as at is
encountered in reefs. It is a white to ivory , microcrystalline to very finely
crystalline, variably but typically sparsely fossiliferous limestone (wackestonepackstone).
The most common fossils are fragmental grains of brachiopods,
ostracods, ramose bryozoans, and crinoid stems. The latter may be very abundant.
Cuttings vary from soft and poorly indurated (chalky) to well indurated and variably
granular. Some chert may occur, particularly in the lower third of a reef.
The drilling rate (penetration) is high in the Chappel, due to high porosity, which
appears to be mainly fractured porosity. This is indicated indirectly by erratic
drilling rates, bit torquing, and
sudden loss of circulation. The cuttings reveal fracturing by smooth-sided
fragments, representing part of a fracture surface, and rarely by a fracture segment
in a cutting, which may show thin linear lines of fluorescence. Fluorescence on only
one side of a cutting indicates oil on a fracture surface. Microvugular porosity may
be inferred from patches of pinpoint fluorescence.
Shows in the Chappel are infrequent or subtle for various reasons, notably because
up to 99 percent of the penetration may be hydrocarbon barren (Thomas, 1986) and
because oil may be flushed from the fractures and into the reservoir, with the result
that only a relatively small amount enters the mud and it is further diminished by
dispersion in the course of circulation to the surface. Typical cuttings shows include
10 to 40 percent fluorescent grains. The fluorescence ranges from bright white, to
bright yellow-white, to bright bluish white and tends to be intense, in contrast to the
dull, uniform mineral fluorescence.
Presumably because of the flushing, odor and stain may be absent. Also immersion
in chlorthane or carbon tetrachloride rarely produces a streaming cut, although
crushed cuttings may yield a cut.
Even the mud-logging hot wire recorder sometimes will not register a gas kick in
the Chappel for any of the following reasons:
1) high mud weight will suppress the amount of oil/gas liberated into the drilling
2) relatively small volume of oil and gas actually present in fractures;
3) masking effect from large volumes of shale gas liberated into the mud from the
However, the effect of the last can be countered by consideration of the ratio of the
heavier gases, Ethane (C2), Propane (C3), Isobutane (IC4), and Normal Butane
(NC4) to the lighter gas, Methane (C1). The shale gas of the Barnett is
predominantly Methane. For a hydrocarbon show in the Chappel, the amount of
heavy gases and consequently the C3+IC4+NC4/C1 ratio increases, although the
total gas recording may increase, remain the same, or decrease. Notice in the mud
log examples of Figures 2 and 3, there is only a small increase in total gas (Fig. 2)
and a decrease in total gas (Fig. 3), yet the oil indicator line, which is the ratio of
C3+IC4+NC4/C1, increases for both holes.
ARCHER COUNTY CORES
Recently two wells were drilled in Archer County, in which cores were taken from
the Chappel. In one, 13 feet of core were recovered, beginning at 8 feet below the
top of the Chappel. The uppermost several inches of the core are shown in Figure 4.
The limestone is white to light gray, with predominantly vertical fractures that are
oil stained and range in thickness from hairline to two inches (secondary enlarged).
The fracture surfaces are veneered with a thin layer of pyrite, overgrown by calcite
and quartz. Many of the quartz crystals are euhedral. Very little definite primary
porosity was seen. The few, randomly oriented vugs observed could have formed
from the leaching of calcite or fossil fragments. The section of core has a some what
mottled aspect because of the contrast between matrix and bryozoan and crinoid
grains. When examined, it had an oil odor and stain.
The lower part of the core is very dense, and open fractures are absent, although
some, calcite filled, vertical fractures are present. A few of the fracture fillings
display limestone clasts surrounded by microcrystalline limestone.
In the second well 21 feet were recovered, beginning in the basal one foot of the
Barnett and continuing 20 feet into the Chappel (Fig. 7-8). As in the first well,
vertical fractures are the dominant observable porosity, and pyrite veneers many of
the open fracture surfaces (Fig. 7-8). Some randomly oriented vugs are also present.
All 20 feet of the Chappel had a good oil odor. The open vugs were coated with a
heavy oil residue, indicating that they had been filled with oil that had escaped
during core recovery.
OBSERVATIONS ON POTENTIALLY
PRODUCTIVE CHAPPEL TRAPS
Typically the Barnett Shale thins over Chappel reefs and little or no Chester is
present. The top few feet of the Chappel drills very slowly, after which there is a
subtle drilling break accompanied by erratic penetration and torquing. This interval
should be carried 15 to 20 feet, depending on the completion techniques and the part
of north Texas involved.
At 15 to 20 feet below the top of the Chappel, drilling should be halted and samples
circulated to the surface. The cuttings should be ivory-white, microcrystalline,
chalky, variably crinoidal limestone with good visual evidence of fracture porosity.
Thirty to forty percent of a sample should have bright white fluorescence, although
much less can be significant. The author has found that successive circulated
samples showing increasing fluorescence provide the most positive aspect in
evaluation of a show, even if the range in fluorescence is only a trace to 10 percent.
The samples from a good Chappel show should have odor and a fair to good solvent
cut from crushed grains, but these properties are not as important as observable
Ideally, the gas detector should record an increase in total gas and particularly in the
heavy - light gas ratio, but if neither occurs, the show can still be regarded
favorably, if visual fluorescence is present.
Because of the uncertainties involved in selecting production casing seats for
fractured reservoirs, it is highly recommended that drillstem tests be taken through
intervals of show in the Chappel.
I would like to thank Danny Stivers for his support and review of this manuscript.
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lithology, porosity and shows in drill cutting samples. Amer. Assoc. Petrol. Geol.,
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Haworth, J.H., Sellens, M., and Whittaker, A., 1985, Interpretation of
hydrocarbon shows using light (C1-C5s) hydrocarbon gases from mud-log data.
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Methods in Explor. Series.
Thomas, E.P., 1986, Understanding fractured oil reservoirs. Oil and Gas Jour., v.
84, p. 75-79.
Washburn, J.R.H., 1987, Deposition, diagenesis, and porosity relationships of the
Mississippian Chapel limestone, Shackelford County, Texas. M.S. Thesis, Texas