At about the time in the mid-Twentieth Century that the Fort Worth Basin of North-Central Texas was being abandoned by the major oil companies as a worn-out backwater area, an entrepreneurial young Aggie by the name of George Mitchell began drilling there for Pennsylvanian-age Atokan (aka Boonesville Bend) sandstones and conglomerates. Mitchell Energy grew to be the largest independent oil and gas company in Texas, and George Mitchell became a billionaire.
By 1980 the Atoka had been largely exhausted, so Mitchell began drilling deeper gas wells in the late Mississippian Barnett Shale of the Fort Worth Basin and experimenting with massive frac jobs and new stimulation techniques. By the turn of the millennium, the Newark, East (Barnett) Field had become the largest gas field in Texas, both in terms of monthly production and areal extent. Before the end of the first decade, it was the largest onshore field in North America.
As the Barnett play is now reaching its mature stage in the second decade of the new century, another play has developed in the Fort Worth Basin. It involves the early Pennsylvanian-age Marble Falls Formation, and like the previous play in the Barnett, it consists of a vertically fractured reservoir rock that has rather poor primary porosity and is most productive from horizontal boreholes.
Unfortunately, the stratigraphy of the Marble Falls and all the rocks in the Fort Worth Basin that are sandwiched between the overlying Atoka and the underlying Barnett Shale is poorly understood and has rarely been studied or described in the literature. Many Texas researchers, beginning with Ferdinan Roemer in 1847, have described the Carboniferous rocks at their surface outcrops on the northern edge of the Llano Uplift, but there is a dearth of published information about those same—or near equivalent—rocks in the subsurface, and what little is available is regrettably inaccurate and repetitive almost to the point of plagiarism.
THE MARBLE FALLS (ATOKAN)
It has long been the practice for unwary geologists studying the Fort Worth Basin to lump all the rocks between the base of the Atoka shale section and the top of the petroliferous facies of the Barnett Shale into a single large heterogeneous lithologic taxon called "the Marble Falls." In fact, that rock sequence in the subsurface consists of multiple distinct lithologies deposited by three separate and discrete sedimentary events that are separated by at least one angular unconformity (in places two) which cuts out at least a dozen million years of time-rock. Very clearly we are dealing with two, probably three, formations between the Atokan/Bend Conglomerates and the Barnett Shale.
In 1946 Spivey and Roberts (AAPG Bull, V. 30, no. 2, pp. 181-186) determined that the presence of fusiform fusulinids in Marble Falls outcrops in Central Texas indicated the formation is post-Morrowan in age. That being the case, the formation has no choice to be anything other than Atokan, both at the surface and in the deepest areas of the Fort Worth Basin.
The Marble Falls of the Fort Worth Basin is a very impure limestone that contains a remarkably large percentage of sand and/or conglomerate in addition to shale. Chert, probably allochthonous, is not uncommon, especially in the upper few dozen feet of the formation. The lithology varies to a significant extent both vertically and laterally. One of the more common facies is an argillaceous limestone that exhibits a "salt and pepper" texture under the microscope. In some areas of the basin, it is more of a calcareous sandstone than a sandy limestone, and it is technically more correct to refer to it as the Marble Falls Formation rather than the Marble Falls Limestone. This fact has only come to light within the last few years with the availability of better quality wireline logs and the acquisition of whole cores in some key areas. The sand grains, which are virtually absent from the underlying Carboniferous sediments, are no doubt derived from the Ouachita fold belt that began rapidly advancing towards North Texas from the east during the relatively brief four-million-year time span of the Atokan Epoch of the early Pennsylvanian Period. Additional silica was supplied by spicules derived from shallow-water sponges.
In the northeastern quadrant of Jack County, there is a local area where the logs clearly show the sandy limestone beds of the upper Marble Falls grading laterally into the lowermost shales of the Atokan shales and conglomerates. Consequently, there can be no question that the upper Marble Falls and the lowermost Atokan/Boonesville Bend beds are all part of a single continuous sedimentary event that covered North Texas and the Fort Worth Basin after some unknown thickness of Chesterian and Morrowan rocks was removed by erosion. No regional unconformity higher in the Pennsylvanian and/or Permian section of North Texas has ever been documented.
The time-rock hiatus between the preserved Chesterian and Atokan strata in the Fort Worth Basin appears to include the entire six million years of the Morrowan Epoch and perhaps even more.
...Or does it?
THE MORROW SHALE . . . INTHE FORT WORTH BASIN?
Frederick Byron Plummer studied the fauna of the Carboniferous surface outcrops in Central Texas (Univ. of Texas Pub. No. 4329, issued posthumously in 1950) and concluded that the Barnett Shale was Chesterian, and on that there is general agreement. But that does not preclude the probability that true Morrowan strata were deposited unconformably upon the Chesterian sequence and that pre-Atokan erosion, which cut deeply into the Chester in many places, spared a few beds of Morrow shale before Pennsylvanian deposition recommenced.
To those who are willing to examine the evidence, that idea is not as radical as it might seem. Thick sections—multiple thousands of feet—of distinctive, low resistive, so-called “railroad track,” Morrowan shales were deposited in the Anadarko and Ardmore Basins a few dozen miles to the north in southwestern Oklahoma. Moreover, those same shales are clearly present, albeit as erosional remnants, in the somewhat shallower Marietta Basin immediately on the other side of the narrow but lofty Muenster Arch that separates it from the Fort Worth Basin. It will be remembered that the Muenster Arch did not rise above the waves until sometime in the Atokan. A Morrowan mariner could have sailed his boat from Ardmore, Oklahoma, to San Saba, Texas, without ever sighting land or scraping bottom.
The induction log immediately above illustrates a section of more than 200 feet (60 meters) out of a Morrow Shale section of more than a thouand in the deep part of the Anadarko Basin in southwest Oklahoma. From what we can tell from wireline logs, it is virtually identical to the Morrow we see in the northern Fort Worth Basin.
So what became of the Morrowan shales that were almost certainly deposited over much or all of the Fort Worth Basin? Were they obliterated by pre-Atokan erosion?
Almost . . . but not quite!
The area where the Morrowan strata are noticeably preserved is in almost half of Jack County, southwestern Clay County, and a few square miles in Archer County around the small town of Scotland. The Morrow is entirely absent along the eastern and western edges of Jack County where it abuts Wise and Young Counties respectively. Refer to the isopach map below.
The red contour above marks the very minimal limits of the Morrow Shale as I have identified them at this time. Occasional thin and isolated outliers of the shale may be encountered outside this boundary. The thickest section I have yet seen on a log is 124 feet (38 meters) in Clay County. It is no coincidence that the northwest-southeast orientation of the long axis of these preserved beds coincides closely with the deeper parts of the Paleozoic valley of the Jacksboro River (which see elsewhere on this website). The Morrow was the first formation deposited in the Fort Worth Basin when the shallow Pennsylvanian sea inundated the eroded surface of the Chesterian topography. It may have started out as an estuary of the Jacsboro River. We have no way of knowing how thick or widespread those Morrow sediments became before they were in turn subjected to pre-Marble Falls erosion. In the Anadarko Basin of southern Oklahoma the Morrow attains a thickness of at least 900 feet and covers several thousands of square miles. In the Fort Worth Basin only a few dozen miles to the south, it is preserved today only the deepest part of the buried Jacksboro River valley.
The Morrow shale is depicted at the center of this 1967 E-log of the Longhorn Production #1 Harlan well in the Dillard, S. E., Field of northernmost Jack County.
The Morrow Shale occurs on the E-log of the Longhorn Production Company #1 Harland well (API 42-237-02823) at a depth of 5896' to 5936'. The resistivity reading is not quite four ohm-meters, less than almost any other shale in the borehole and notably consistent with the Morrow Shale readings in the Marietta, Ardmore, and Anadarko Basins a short distance to the north. Below the Morrow Shale, the top of the Barnett Formation at 6050’ is overlain by about 114 feet (35 meters) of post-Barnett Chesterian beds that are assigned to the Forestburg Formation. Based upon stratigraphic relationships elsewhere in Jack County, I have used corrugated lines on this log to indicate both the upper and lower surfaces of the Morrow Shale as being unconformable.
In each case the Morrowan rocks are appropriately and predictably sandwiched in-between rocks of Chesterian age (i.e., Barnett or Forestburg Formations) immediately below and Atokan-age rocks (Marble Falls Formation) immediately above. The consistent chronostratigraphic position of these distinctive shale beds in the exact sequence we would expect to find them precludes mere coincidence; we would not expect to find Morrowan-facies rocks anywhere else in the stratigraphic column of the Fort Worth Basin, and indeed we do not.
The manner in which the unconformable contact at the base of the Atoka and/or the upper beds of the Chester appears to rise and fall in the Carboniferous section in the Fort Worth Basin, regardless of the presence or absence of intervening Morrowan strata, makes it seem likely that where the Morrow remnants do occur they are bounded by erosional surfaces both above and below. This supposition is supported by the precipitous manner in which the isopach values of the Morrow often change abruptly over relatively short lateral distances.
Where adequate electrical logs are available, the Morrow shale most often has a resistivity reading of four ohm-meters or less. Many factors affect logging tools, and readings of anywhere from two to four ohm-meters are not uncommon. On the majority of E-logs where these rocks are present, they will exhibit the lowest resistivity values of any shale in the borehole. The shale is often marginally more radioactive on the gamma ray curve than are the non-Morrow shales above it, although it is, of course, far less radioactive than the Barnett Shale below. Interestingly, where a sonic log is available, the Morrow shale will display a dramatically slower travel time than almost any other rock in the hole. Like many shales, this formation will show some evidence of washing out on the caliper, and it will often do so to a greater degree than most other shales above and below. This can, and often does, cause inaccurate readings on the density and neutron porosity tools, but with good pad contact the Morrow generally reads a bulk density of approximately 2.68 gm/cm3.
As always, the low resistive, Morrow-like shale is found sandwiched between the overlying Atokan Marble Falls Formation with contact circa 5860' on the above-left 1950 E-log of the Warren Oil #1 Cable-Smith in far northwestern Jack County (Putnam Survey, A-465)) and the underlying Chesterian Barnett or Forestburg Formations (5910'). The uppermost beds of the Barnett and all of the Forestburg Formation have been removed by pre-Morrow erosion in this depiction. The sample log above right (by R. T. Drake) documents the change in lithology versus the rocks immediately above and below.
THE FORESTBURG LIMESTONE (CHESTERIAN)
In most (but not quite all) of the Fort Worth Basin the Pennsylvanian strata, be they Morrow or Atoka, are immediately underlain by the Forestburg Formation. In parts of central Jack County the Forestburg is completely eroded away, and the Pennsylvania rests directly upon the Barnett, but that is the exception and not the rule. In most areas, the Atokan Marble Falls overlies the Chesterian Forestburg unconformably. The 2009 image below of the slabbed core from the EOG Resources #1 House well (API 42-237-39288) in south-central Jack County is the only known photograph ever made of that multi-million-year hiatus.
The green arrow points to the very basal few inches of the fragile and rubbly Marble Falls black shale beds; the red arrow indicates the precise location of the unconformity itself; the orange arrow points to the upper couple of inches of the well indurated Forestburg Limestone. For scale, the core is about 8 centimeters (3 inches) in diameter. The post-Barnett beds of the Chesterian Epoch do not outcrop at the surface anywhere in the Fort Worth Basin and have barely been mentioned in the literature; consequently the photograph above is, to the best of my knowledge, the only place anywhere the erosional surface between these two formations can be viewed in plain light. The amount of time-rock missing between the arrows represents a bare minimum of six million years and is probably closer to twice that.
In 1982 this writer proposed the name “Forestburg Limestone” for the 7190- to 7414-foot interval on the logs of the Resources Investment Corporation No 1 Donald Rector well (42-337-30279; McCown Survey, A-1311) about a mile northwest of the town of Forestburg in southeastern Montague County. Unfortunately the logs on that well are no longer readily available for public inspection, so another point of reference is needed. To that end, the dual induction log of the Taubert and Steed #9 Teague well that was drilled in 1983 in Section 2772 (A-714) of the Texas Emigration & Land Company Survey about a mile and a half northwest of Jermyn in northwestern Jack County will serve as the type log for this formation, and the interval from 4837 to 5280 feet therein is hereby designated the type subsurface section of the Forestburg Limestone.
Taubert & Steed #9 Teague (API 42-237-35307): The type log for the Forestburg Limestone in Section 2772 (A-714) of the Texas Emigration and Land Survey approximately 1.5 miles(2.5 km) northwest of the townsite of Jermyn. “B” denotes the top of the typically bituminous facies of the Barnett Shale; “F,” the top of the Forestburg; “MF,” the top of the Marble Falls Formation. The 443 feet (135 meters) of Forestburg logged in this borehole is one of the greatest thicknesses of that formation presently known in the Fort Worth Basin. The Morrow-like shale is not present in this borehole. The very greatest thickness of Forestburg yet encountered by the drill bit is 550 feet (168 meters) on the log of the 1959 Magnolia #1 Trigg dry hole in the northwestern corner of Dallas County.
Throughout most of the basin, the top of the Barnett Shale is readily identified on conventional wireline logs. It is substantially more radioactive than any other shale in the basin, and its mean bulk density of about 2.50 grams/cubic centimeter makes it noticeably lighter than any of the other Carboniferous shales. But mudloggers frequently have trouble calling the top of the Barnett while drilling, because the immediately overlying limey shales and shaley limestones, though easily distinguished from the Barnett on wireline logs, are lithologically extremely similar in color and texture. This similarity results from the fact that the typically greasy Barnett facies and the rocks that conformably overlie it were all part of a single depositional event that occurred during the Chesterian Epoch, the youngest of the four recognized North American epochs that make up the Mississippian (Lower Carboniferous) Period of the Paleozoic Era. Whatever the depositional environment factor was that made the lower part of the Barnett so light and radioactive, it ceased, or migrated laterally, rather abruptly at some point in the fifteen-million-year duration of Chesterian time, but the other lithologic constituents of the Barnett continued to be deposited in a relatively calm, shallow epicontinental sea. In parts of the Fort Worth Basin, the depositional environment of the bituminous Barnett returned for a brief encore and left a couple of dozen feet of typically petroliferous shale in the lower part of the post-Barnett interval. Industry geologists commonly refer to it as the “false Barnett” or “upper Barnett.”
Today, almost 450 feet (140 meters) of post-Barnett, Chesterian rocks can be found in the northwestern corner of the Fort Worth Basin near the Jack/Young county line between the hamlets of Jermyn and Loving. An equal or slightly greater thickness of those same rocks can be found in the northeastern corner of the same county between the small communities of Cundiff and Newport. Those strata are overlain unconformably by the Early Pennsylvanian Atokan Stage of the Marble Falls. In much of northern Jack County and parts of Clay and Archer Counties, the Forestburg is immediately overlain by as much as 124 feet (38 meters) of a low resistive shale that gives every evidence of being Morrowan. (See discussion of Morrow Shale above.)
The Forestburg Limestone consists mostly of massive, dark limestones sequences that appear to be extremely dense on the bulk density logs (circa 2.70 gm/cc) and extraordinarily clean on the gamma ray log—cleaner, in fact, than also any other rock unit in the Fort Worth Basin. These massive beds are occasionally separated by thinner shale beds that do not closely resemble either the petroliferous Barnett below or the Atoka shales above. The limestones are generally gray to very dark gray, and the shales are invariably black. Like the underlying Barnett, chert is virtually unknown in the Forestburg.Unlike any other formation in the Fort Worth Basin, deposition of the Forestburg Limestone was cyclical. At least one erosional event of sub-regional extent can be documented within the cyclical “benches” of the Forestburg. Depending upon the degree to which pre-Pennsylvanian (and/ or mid-Chesterian) erosion has breached the formation, the number of preserved shale/limestone cycles in any given borehole may vary from a mere fraction of the first one to almost all of the third (or fourth) one. How many more such cycles there may have been originally, we can not say, because a full original section of this unit is nowhere preserved.
Based upon geochemical analyses, Zumberge, et al. (Oil and Gas Geochemistry and Petroleum Systems of the Fort Worth Basin, AAPG Bull., Vol. 91, No. 4, pp. 445-473, April 2007) determined that the Barnett is the source rock for the large majority of producible hydrocarbons in the Fort Worth Basin but left open the possibility that other strata may have made minor contributions thereto. The shale beds of the Forestburg, especially in the lower part of the formation, are bituminous to a very substantial degree, and they may have served as source rocks also. It is at least possible that those shales, because of their close proximity to the underlying Barnett, were simply contaminated by hydrocarbons that originated in the latter formation. In either event, the massive and brittle Forestburg Limestone—several times more voluminous as the much more famous Barnett Shale—must be considered as a fractured hydrocarbon reservoir in the not-too-distant future.
THE DUFFER (ATOKAN)
A short distance west of the Fort Worth Basin, on the Bend Arch and the Eastern Shelf of the Midland Basin, the nomenclature changes, and rocks exactly equivalent to the Marble Falls are often referred to as “the Duffer.” From what we can see on the wireline logs, the lithology of the Duffer is not significantly different from the true Marble Falls of the Fort Worth Basin, and it is a very common, though historically unspectacular, reservoir rock.
THE COMYN (ATOKAN)
There is, regrettably, yet another name for the Marble Falls that is occasionally bandied about the Fort Worth Basin without regard for its original definition. The Comyn (“ko-`meen”; I called the local post office to confirm the correct pronunciation) Limestone first appeared in the literature when it was defined, but not described, by Marcus Cheney in 1940 (AAPG Bull., Vol. 1, January 1940, pp. 65-118) as being “that portion of the Marble Falls that occurs in the Ranger [Eastland County, Texas] area.” He named the unit for a small hamlet in northeastern Comanche County (J. Walker Survey, A-988) that was being used at the time by the Humble Pipeline Company as a storage and pumping facility for crude oil being transported from West Texas to refineries on the Gulf coast. The Comyn, of course, does not outcrop anywhere, so Cheney chose for a subsurface type section the interval between 4132 and 4320 feet in the Roxana Petroleum #1 Seaman well in the extreme northwest corner of the J. B. Hart Survey (A-1874) of west-central Palo Pinto County fifty miles away. Moreover, the Roxana well was drilled in 1918-1919, well before even the most primitive electric logs were available. Cheney published this new formation based solely upon a description of drill cuttings by Marcus Goldman of the USGS in 1921. Cheney himself never saw those cuttings, which he said were “no doubt on file with state and national geological surveys.” Other than that vague wave of his arm, he made no effort to document the cuttings’ location or accessibility. Fortunately, we are able to refer to that same depth interval on commonly available wireline logs of boreholes that have been drilled within a couple of miles of Cheney’s type section in more recent decades. In comparing these logs to the Comyn in the Roxana well, it is clear that Cheney did indeed give a new name to the Atokan-age Marble Falls that immediately overlies the pre-Pennsylvanian rocks in most of the Fort Worth Basin. What is not apparent is why the Marble Falls needed a new and different name in “the Ranger area,” whatever Cheney considered that to be. As a matter of taxonomic practicality, the Comyn should be deprecated in favor of the Marble Falls and/or the Duffer. The Comyn should absolutely not be confused with the post-Barnett Chesterian (i.e., Forestburg) beds, although that is a common error of industry geologists who are not familiar with the origin of the name.
