There’s an elephant in the Fort Worth Basin of North Central Texas. Everyone in the oil business there has seen it many times, but hardly anyone acknowledges its presence. The big fellow is all dressed up with nowhere to go—yet. But that’s about to change.
The ponderous pachyderm goes by the name of the Forestburg Limestone, a sequence of very hard and brittle carbonates interbedded with smaller intervals of black petroliferous shales. The Forestburg is Chesterian (latest Mississippian) in age. Where present, it conformably overlies its immediate predecessor, the famous Barnett Shale, and its upper surface is everywhere truncated by the mid-continent-wide North American unconformity that separates the Mississippian and Pennsylvanian systems. In most areas of North Texas, the earliest overlying Pennsylvanian rocks belong to the Marble Falls Formation (Atokan) which was featured in an article by this author in the November 5, 2012, issue of the Oil & Gas Journal (pp. 74-79). At the very least, the Forestburg covers all or part of fifteen counties in North Texas. Refer to Figure 1 below.
Figure 1: Generalized orientation map of North Central Texas including the Fort Worth Basin and Bend Arch. The asymmerical area highlighted in blue indicates the location of the Forestburg Limestone in the subsurface. It is generaly thickest in the northern and eastern portions of the basin and thns southward and westward to a feather edge due to pre-Pennsylvanian erosion. The formation is nowhere present on the outcrop.
The Forestburg Formation first appeared in the literature a third of a century ago (Henry, 1982), and the name quickly came into popular use as the Barnett Shale boom erupted around the little hamlet of Forestburg in southeastern Montague County. Prior to that, this formation was commonly confused with the lower Marble Falls and often mistaken for the Comyn Formation and/or the Duffer Formation, two names that are, in fact, technically synonymous with the Marble Falls.
Surprisingly, oil and gas production came out of the Forestburg four decades before it did from the much more celebrated Barnett Shale. The Forestburg Field was discovered by the Sinclair Prairie #1 Jones about a mile north of the Forestburg post office just a few weeks after the US entered World War Two. It produced from a formation that had no proper name at the time, and it ultimately made about 341,000 BO and an unmetered volume of gas from eight wells that were hardly stimulated at a time when hydraulic fracturing was yet to be introduced. Later that same decade, ninety miles (145 km) away in Stephens County, the Texas Company #1 A. M. Moon well paused to drill stem test a gas show in an anonymous sub-Marble Falls limestone a few miles northeast of Breckenridge. What we now know to be the Forestburg flowed gas to surface in two minutes at the rate of 2.9 MMCFG per day. (Refer to sample log in Figure 2 below.)
The Moon was ultimately completed through perforations in that same zone for 11 MMCFG per day. Ultimate production (if that 1948 gas well was, in fact, ever connected to a pipeline) is not known.
Over the decades, a couple of dozen old vertical wells in southeastern Montague County were completed in the exact same stratigraphic horizon in the Bingham Creek Field, the South Dye Mound Field, and the Hundley Field, but the Forestburg boom never really took off.
In my own experience circa 2011, a client plugged back to the Forestburg after drilling a disappointing Barnett well in southwestern Jack County. The Forestburg was frac'd with 140,000 pounds of sand, and the operator reported testing 1100 MCFG per day (with little or no oil or water) before sand back flowed into the casing and necessitated a recompletion in a shallower zone.
It should be remembered that the Forestburg represents the latter half of a two-part sedimentary event that covered the Fort Worth Basin, the Bend Arch, and part of the Eastern Shelf of the Midland Basin in latest Mississippian time. Throughout the basin the Barnett Shale is easily identified on conventional porosity logs. It is notably more radioactive than any other shale in the basin, and its mean bulk density of about 2.50 grams/cubic centimeter makes it considerably lighter than any of the other Paleozoic shales. But mud loggers frequently have trouble calling the top of the Barnett while drilling, because the immediately overlying limey shales and shaley limestones of the Forestburg 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 rich petroliferous, bituminous depositional environment that produced the Barnett Shale was still largely present in the Fort Worth Basin while the Forestburg sediments were accumulating. Later, after lithification, the tectonic forces that uplifted the Muenster Arch and fractured the Barnett Shale were simultaneously available to fracture the Forestburg.
The Forestburg Formation consists primarily of massive limestones that appear to be extremely dense (circa 2.70 gm/cc) on the bulk density logs and extraordinarily clean on the gamma ray logs—cleaner, in fact, than any other rock unit in the Fort Worth Basin. These massive rock units are occasionally separated by less massive shale beds that do not closely resemble the Atoka shales slightly higher in the section. The Forestburg carbonates are very dark gray and non-fissile; the shales are invariably black. Chert is virtually unknown. It would appear that the Forestburg has experienced two episodes of fracturing. The first was tensional in nature and resulted in mostly vertical cracks that have been well cemented with what appears to be calcite. The basal Forestburg core that was cut in the EOG #1 House well in 2009 (API#42-237-39288) shows no oil staining along these vertical fractures. (The photograph immediately below was cut entirely in the Forestburg. Note yellow arrow pointing to vertical fracture.) A second and subsequent set of fractures are very largely horizontal, and they are extensively oil stained. We might surmise that the latter episode of fracturing resulted from the unloading of an undetermined thickness of overburden that occurred during the mid-Carboniferous errosion before the Atoka sediments were deposited. Mobilization of the hydrocarbons in the underlying Barnett source rock apparently occurred sometime after these fractures were created. In the core below, 23 oil-stained fractures in 15 feet of rock average about one oil-stained fracture every 8 inches. The Barnett has been identified as the source of the hydrocarbons in the even shallower Pennsylvanian oil and gas reservoirs, so the migration of the oil may have happened in the Permian or later. The API gravity of the Forestburg oil varies; it has been reported from 35 to 47 degrees; a value around 39-41 degrees is about average.
Unlike any other post-Ordovician formation in the Fort Worth Basin, deposition of the Forestburg was quite cyclical. Depending upon the degree to which pre-Atokan 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). How many more there may have been originally, we can not say, because a full, uneroded original section of the Forestburg is nowhere preserved. Refer to Figure 3 below. The formation can be seen to thicken gradually from west to east across most of the basin. The dip is about one degree (i.e., 100 feet/30 meters per mile) to the east, and there is little or no faulting involved.
Figure 3: Dual Induction log of the Taubert & Steed #9 Teague well, Sec. 2772 (A-714), Texas Emigration & Land Survey, northwestern Jack County. Three calcareous depositional cycles are preserved in the Forestburg Formation at this location. The third cycle has been partially truncated by pre-Atokan erosion and is unconformably overlain by the Pennsylvanian Marble Falls Formation.
Whatever the depositional environment factor was that made the Barnett so lightweight 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 without interruption in a relatively calm, shallow epicontinental sea. In parts of the Fort Worth Basin, the depositional environment of the classic Barnett lithology returned for a brief encore and left a couple of dozen feet of typically radioactive shale a hundred feet or so above the main mass of the Barnett beds. Industry geologists commonly refer to it as the “False Barnett” or “upper Barnett.”
Five hundred and four feet (153 meters) of post-Barnett, Forestburg rocks can be found in a vertical borehole in western Jack County about a mile northeast of the little hamlet of Jermyn. Thirty miles (50 km) away on the northeastern edge of Jack County that same formation reaches 502 feet in another well. In between those two locations lies an area where the Forestburg has been drastically thinned by pre-Atokan erosion, and in some places it is entirely gone, and the basal Pennsylvanian unconformity cuts directly into the Barnett Shale.
Together, the Marble Falls and the Forestburg occupy a vertical interval of very approximately 500 feet (150 meters) throughout most of Jack County. The two formations thicken and thin in an inverse manner with each other so as to suggest that the Marble Falls was deposited on a gentle, near-shore Forestburg terrain and preferentially filled in the paleotopographic lows. (Refer to “The Jacksboro River” elsewhere on this website.)
The greatest thickness of Forestburg yet penetrated by a vertical borehole is about 550 feet (168 meters) in the historic 1959 Magnolia #1 Trigg in the northwestern corner of Dallas County. A mile or two east of there, the Forestburg is laterally obliterated by the early Pennsylvanian Ouachita overthrust belt. To the north, all Mississippian strata are absent due to uplift and erosion on the southern flank of the Electra/Matador/Red River Arch. Like the Fort Worth Basin itself, the Mississippian rocks are abruptly terminated in east-northeast Montague County by the lofty Muenster Arch. In the Marietta Basin on the far side of that arch, Mississippian formations are again present, but they do not include any remnants of either the Barnett or the Forestburg. Nor do we find either of those rock units in the Ardmore or Anadarko Basins of southern Oklahoma.
Based upon geochemical analyses, Zumberge, et al. (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 contributed thereto. The shale beds of the Forestburg, especially in the lower part of the formation, are bituminous to a very substantial degree, and they very likely served, to some extent, as source rocks also. In any event, throughout the broad areas where the Forestburg is preserved, it is not possible for the Barnett-sourced hydrocarbons to have migrated into the shallower Strawn, Atokan, and Marble Falls reservoirs without first passing upwards through the entire thickness of the immediately overlying Forestburg Formation, almost certainly via vertical fractures in that exceedingly brittle formation. During that process, whatever primary and/or secondary porosity existed in the Forestburg was similarly charged with those same hydrocarbons, and the vast majority of that oil and gas is still in those rocks waiting for an economic climate that will justify its extraction. That day may be upon us sooner than later. In 2018 Andy Moon described the drilling strategy for a Forestburg completion:
"Generally, the operator will want to drill the horizontal well in the brittle rock [Foestburg Lime], but close to the plastic rock [Barnett Shale] which contains most of the oil and gas. The more brittle rock will drill faster, and it fractures (fracks) well (Jacobi, etal. 2008). Further, the proppant will hold the fractures open better in the harder more brittle rock. In the plastic rock, the rock can collapse the fractures by deforming around the proppant... [T]he Upper Barnett and the Lower Barnett are more plastic (lower Young’s Modulus) and the central Forestburg and lower Chappel Limestones are brittle (or stiffer, higher Young’s Modulus). These are often good places to put a horizonal well."
Leases and new boreholes are more affordable now than they have been in quite a while. Brittle fractured carbonates are fair game wherever they are found associated with source rocks, and there are few, if any, better source rocks in North America than the kerogen-rich, petroliferous Barnett Shale. When the Forestburg play arrives, it will likely generate more drilling activity in the Fort Worth Basin than the respectable Marble Falls has in recent years, and it has the potential to ultimately rival the Barnett Shale boom.
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REFERENCES
Henry, James D.,Stratigraphy of the Barnett Shale (Mississippian) and Associated Reefs in the Northern Fort Worth Basin,in C. A. Martin, ed., Petroleum Geology of the Fort Worth Basin and Bend Arch Area: Dallas Geological Society, (1982), pp. 157–178.
Henry, James D., Barnett-sourced Forestburg Provides Further Opportunity in Fort Worth Basin, in the Oil & Gas Journal (7 March 2016), pp. 36-39.
Jacobi, D., M. Gladkikh, B. LeCompte, G. Hursan, F. Mendez, J. Longo, S. Ong, M. Bratovish, G. Patton, and P. Shoemaker, Integrated Petrophysical Evaluation of Shale Gas Reservoirs, in CIPC/SPE Gas Technology Symposium. Calgary, (2008), https://www.onepetro.org/conference-paper/SPE-114925-MS.
Moon, Andy, Shale Reservoirs,Do They Work, Will They Spread?, in Watts Up With That, (Nov. 2018), https://www.beg.utexas.edu/-files/-content/beg/ext-aff/18-11/Shale%20Reservoirs,%20do%20they%20work,%20will%20they%20spread_Watts%20Up%20With%20That_.pdf
Zumberge, et al.,Oil and Gas Geochemistry and Petroleum Systems of the Fort Worth Basin,AAPG Bull., Vol. 91, No. 4, (April 2007), pp. 445-473.
