Karsting is the geological process by which shallow underground caverns are formed by meteoric waters flowing through and dissolving carbonate rocks. It is not rare, but neither is it especially common. Perhaps the most famous karst in the United States is Carlsbad Cavern in New Mexico, although Mammoth Cave in Kentucky is the largest known cavern complex in the world. In Texas, Inner Space Cavern (in Georgetown), Longhorn Cavern (Burnet), Natural Bridge Caverns (San Antonio), Cascade Caverns (Boerne), Kickapoo Caverns (Bracketville), Gorman Cave (Colorado Bend State Park), the Caverns of Sonora, and Devil’s Sinkhole (Rocksprings) are all tourist destinations that have been recently created, geologically speaking, in Cretaceous limestone surface formations.
Ancient karsting has long been hypothesized in the pre-Pennsylvanian rocks of North Texas, most notably in the limestone and dolomite strata of the Early Ordovician age Ellenburger Formation. Unquestionably, the Ellenburger was exposed to subaerial erosion in most of this area at some time between the late Ordovician Period and the early Pennsylvanian Period. But until recently, the evidence for karsting in the Ellenburger was largely circumstantial and was used anecdotally and conveniently to explain why the drill bit occasionally encountered the top of that eroded formation at an unexpectedly high or low subsurface elevation. The reality of the rocks turns out to be considerably more complicated than that.
In the summer of 2011, I was the consulting geologist for the operator of a horizontal Marble Falls test (API# 42-237-39443) drilled on the ranch of a Mr. Charles Birdwell in the A. D. Adams Survey (A-13) in far western Jack County about five miles south-southwest of the little hamlet of Bryson. The location was staked in the far-southeastern corner of the squarish little 153-acre (62 hectares) survey, and the borehole was designed to be kicked off just below 3700 feet (1130 meters) and directionally drilled almost due northwest for about 2000 feet (600 meters) horizontally through the Early Pennsylvanian Marble Falls Formation to a bottom hole location near the diagonally opposite corner of the survey. Everything was going swimmingly when we neared the top of our objective formation and kicked off of a cement plug at a depth of 3738 feet (1140 meters). Just before we got to the Marble Falls, somewhere in the lower part of the Atokan (aka Boonesville Bend) shale/sand/conglomerate section, the drill cuttings ceased to make sense. As we drilled ahead, we found that our proposed pay zone, the Marble Falls, which virtually blankets the entire county, was not to be found there. And if that was not bewildering enough, neither did we find the expected underlying Forestburg Formation nor the still-deeper Barnett Shale which together blanket almost the entire basin.
At a measured depth (MD) of about 4700 feet (4350' TVD) the drill bit landed horizontally as planned and continued boring northwestward across the lease in a straight line. At a more than adequate depth of 4825' MD (still circa 4350′ feet TVD), Tucker Wireline Services was called out to run electric and porosity logs to help us solve the mystery of what we were drilling in. Or…so we hoped.
Did I forget to mention that the #1H Birdwell was a seismic prospect? While Tucker’s logging tools were in the hole, the seismic crew showed up on the ranch and began setting out their geophones in preparation for collecting their very first bits of rough unprocessed data. Did I also forget to mention that the operator did not have extensive experience in the oil and gas business?
When we got to look at the Tucker logs the next day, they did not resolve many of our questions, but they were at least consistent with the enigma we had been seeing in the drill cuttings. All the shallower formations correlated nicely with those on the logs of other nearby wells until we got down into about the middle of the Atoka shale section at a depth of 4176' MD (4130' TVD) and then all our log correlations went to hell. Below that, there was no Marble Falls and no Forestburg (the bit did not go deep enough to give us a reading on the top of the Barnett). It soon appeared that we were just drilling in a very large pile of nameless, never-before-seen rubble. In fact, we were doing just exactly that, and we were stratigraphically lost somewhere on the dark side of the moon.
Faced with an over-budget apparent dry hole, a chaotic stratigraphic sequence from another planet, falling oil and gas prices, hole problems, and critically low on cold beer, we weighed our options somberly and then did what any baffled and discouraged oilies would do in the same circumstances—we put on a new bit and went back in the hole. The race was on to see which we would find first, a recognizable formation top or the remains of Amelia Earhart. The hands on the drilling rig had their money on Amelia.
We should have already been in easily identifiable Forestburg Limestone when we logged, but we were not, so we drilled another 315 feet (96 meters) sideways while hoping for a miracle that stubbornly refused to come up the hole. Two days later, dazed and confused, the operator opted to stop and run logs again, but this time he used a different logging company, Weatherford, just on the remote chance that there had been an undetected technical glitch in Tucker’s logging tools. The story told by those logs was disquieting to say the least.
But there was no glitch; Weatherford’s log suite was virtually identical down to the same depth of the Tucker logs. However, for this trip out of the hole, the operator elected to additionally run a dipmeter and borehole imaging tool which had not been available from Tucker.
The seismic crew finished their field work about then, and the raw data were promptly transmitted to a talented and experienced local geophysicist, Jim Davis, for final interpretation. A few weeks later, Mr. Davis came to the conclusion that the #1H Birdwell had, by the merest stroke of ill fortune, drilled into an ancient karst feature—a veritable cavern—in pre-Pennsylvanian carbonate rocks. I didn’t want to believe it at first because I had never seen anything even remotely analogous to this stratigraphic anomaly in the Fort Worth Basin, nor anywhere else, but the more I put the jigsaw pieces of my geological data together with those of Jim Davis’s seismic interpretation, the more everything he said began to make sense. It was exotic beyond description and geologically fascinating.
I advised the operator that his borehole contained no Marble Falls rocks of any kind, nonetheless, he set pipe to near bottom at 6980′ MD, perforated the cavern fill, and then put out the word that his well was a Marble Falls completion that had flowed oil at the rate of over 900 BOPD on a 24-hour test. As of December of 2016, more than four years later, the well had reported a total cumulative production figure of only 586 BO, less than a single full day’s touted production. Did I forget to mention that the operator was not terribly talented at arithmetic either.
After drilling a second well, the #2H Birdwell (42-237-39673), and a third well called the #1H Bright (42-237-39496), a few hundred yards to the east, the operator soon encountered some serious legal and financial problems and disappeared abruptly without ever putting his geological and geophysical data on record with the Texas Railroad Commission or any other facility or institution that would allow it to be preserved, discovered, or researched by future generations of Earth scientists. Nor were there any other industry partners who were entitled to the information involved in any of these wells, so the story of the Birdwell karst was never leaked to the geologic public. To the best of my knowledge, the hard copies and digital images of the data in my possession are the only ones extant.
Hence my story. Mark it well.
Okay, here’s what happened way back in the Paleozoic Era of North Texas—circa 550 million years before present (mybp) up until about 300 mybp,—and it all happened before there even was such a thing as the Early Pennsylvanian (Atokan Epoch) Fort Worth Basin.
Beginning very early in the Paleozoic Era, a large rift developed in the margin of what is now the south-central North American continent, and it immediately began to fill with shallow marine sediments ranging chronologically from at least the Early Ordovician Period up through the Early Permian. Today that rift basin is known as the Southern Oklahoma Aulacogen, or as it is even more commonly called, the Anadarko Basin. It occupies most of southwestern Oklahoma and the northern part of the Texas Panhandle. The Anadarko constitutes the deepest sedimentary basin in North America, and it is one of the very thickest anywhere on Earth. The geological region that is today North Central Texas lies on the relatively shallow southwestern flank of this northwest-southeast oriented feature.
For roughly 150 million years, predominately carbonate sediments accumulated in the deep part of the Anadarko and also on the adjacent shallower marine shelf that underlies North Texas. Sometime near the very end of the Mississippian Period (European geologists call it “Lower Carboniferous”) and/or the very early Pennsylvanian Period (“Upper Carboniferous”) worldwide sea level fell, and virtually all of the mid-North American continent was subjected to at least several millions of years of subaerial erosion. That erosional hiatus has been identified and traced on the surface and/or in the subsurface virtually everywhere between the Rockies and the Appalachians, and that includes North Texas. It was during this time of surficial erosion that the karsting occurred. Meteoric water—rain—absorbed carbon dioxide from the atmosphere to form carbonic acid, H2CO3. First the acid etched its way down into the most recently deposited Forestburg Limestone. Karsting may or may not have reached some distance down into the Barnett Shale; the depth of the Birdwell borehole does not allow us to say.
Unlike modern-day, near-surface karst features, the Birdwell karst is filled with rocks, not with tourists asking directions to the restrooms. Weatherford’s borehole imagery log tells the tale most eloquently. The part of the cavern that was penetrated by the drill bit is indeed filled with chaotically unsorted sand, silt, conglomerate, pebbles, cobbles, and small boulders. The lowermost levels of these exotica may well have begun falling off the cavern walls and ceiling, and almost certainly did, while karsting was still in progress.
The available evidence suggests that at some point before the shallow seas moved back in and put an end to the karsting, the roof of the cavern collapsed thus forming a sinkhole or cenote. One of the 2D seismic lines shot by the operator while drilling was already in progress contains an anomaly that seems to support this hypothesis.
FIGURE 1 (above): The Great Jack County Cenote probably started out like this small cenote in the Yucatan of Mexico when the roof of a shallow cavern could no longer support its own weight. In a low, moist, tropical environment, the cenote quickly becomes a botanical compost pile which, if buried promptly enough by subsequent sediments, results in a thin lens of low-rank coal. The Great Jack County Cenote is many times the size of this one.
No Marble Falls “ceiling rock” was encountered before drilling directly into the rubble fill. A cenote will often be at the center of a small, local semi-radial drainage system that brings in accelerated volumes of carbonic acid and small-grain dirt and rocks. Moreover, had the cavern roof not collapsed at some point, the subterranean void would still be filled mostly with water, not with the solid, rocky debris that the drill bit found.
FIGURE 2 (above): The borehole imagery log in what should have been the Marble Falls or Forestburg section of the #1H Birdwell shows an almost unstratified pile of unsorted rubble in the bottom of a collapsed cavern never before documented in the Fort Worth Basin.
The sinkhole still had a very substantial topographic expression when sea level again rose and the Atokan Epoch sea inundated the karsting. The oldest (i.e., lowest) external correlation point on the Birdwell logs is at 4176' MD (about 4135′ TVD) in the lower Atoka shales; the top of the carbonate wash in the filled-in cenote was found at about 4520' MD (4320' TVD). That suggests that approximately 185 feet (56 meters)—an eighteen-storey building-worth—of basal Atoka mud had to completely fill the cenote before the sea floor directly above it once again became a locally uniform, level surface.
FIGURE 3 (above): After the Birdwell Cenote was completely filled in by the encroachment of the shallow Atokan sea, it left no hint in the regular bedding of the overlying Caddo Limestone
But the transition from land to sea was not abrupt; an intermediate coastal, swampy mini-ecosystem existed, at least briefly, in the topographic depression above the partially filled cenote. At just below the mid-point of that lower Atoka anomalous mud column and at just about the subsea level we were expecting to drill the top of the Forestburg Limestone (4404' MD), the Birdwell borehole encountered an eleven-foot (3.4-meter) bed of coal. Karsting had ultimately resulted in a small peat bog that does not appear on the logs of any nearby wells. Burial of the concentrated organic material occurred at a rate faster than it could decay, and the ultimate result was a solid bed of coal. Occasional thin beds of coal are not unusual in the Fort Worth Basin, but neither are they common. When we do occasionally stumble upon one, they never appear to cover any great aerial extent, and they can rarely be correlated from one wellbore to the next. Verily, we must consider the possibility that coal beds in the lower Atoka Shale section may well be harbingers of karsting a very short distance directly below, and that there may be economic consequences—good or bad—associated therewith.
FIGURE 4 (above): A coal bed (light colored in the right hand column of the borehole imagery log) identified by Arrowhead mud logger Otis Johnson and also seen by me appears on the borehole imagery log at 4505' to 4516' somewhere in the lowermost Atoka section that was deposited in the Birdwell cenote after the original cavern ceiling collapsed. Extensive fracturing is seen in the brittle coal that is dipping into the deeper part of the cenote to the southeast at 30-someodd degrees. Note that the well-stratafied coal seam is immediately underlain by unstratified rubble and immediately overlain by well bedded Atokan shale.
The return of a marine environment in North Texas was the result of the North American plate colliding with the North African plate in Morrowan and/or very early Atokan time. The long-stable tectono-geometry of the Southern Oklahoma Aulacogen was catastrophically perturbed. North Texas ceased to be a mere shelf of the Anadarko and became instead an independent southeast-thickening clastic basin eventually named for Fort Worth.
So, how big is the Birdwell cenote? What are its dimensions? Unlike the physical sciences, all exact measurements in the natural sciences must start out as arbitrary and subjective estimations. The fact that the Birdwell’s drill bit remained nearly horizontal while transiting some subset of the karst’s longitudinal extent causes the actual internal depth of the cenote to remain a mystery, but the highest point where the drill bit entered the cavern fill was the aforementioned 4320' TVD (4520' MD), and thereafter the directional driller’s lowermost reading was at only 4382' TVD at the terminus of the hole, so that gives us a bare minimum height calculation of 62 feet (9 meters). In reality, the average heigth/depth of the original cavern was probably somewhere between that number and the aforementioned 185 feet of lowermost Atokan mud/shale infill, with the true figure probably being closer to the latter.
And how closely can we pin this karst event down in geological time? The youngest sediments beneath the Birdwell Ranch that we can reasonably be assured were damaged/removed by the karsting process are those of the Chesterian (Latest Mississippian) Forestburg Formation. Chrono-stratigraphers claim to have a pretty good number on the end of the Chester at 318 mybp. Of course, Mississippian sedimentation might have ended a few million years before that but not likely by much. Immediately above that, the very oldest Pennsylvanian Period rocks of the Morrowan Epoch are completely absent from most of Jack County, but we can see just enough of the Morrow a couple of dozen miles to the north and east to know that it once existed in all or part of North Texas. The evidence suggests that the base of the few Morrow strata that have been preserved is an unconformity, i.e., an erosional surface, and perhaps the very same widespread erosional surface that separates the Mississippian and Pennsylvanian Period rocks from each other all across most of middle North America. How convenient.
But wait. The manner in which the Morrowan beds are selectively preserved in only a few locations totaling perhaps 800 square miles is a strong indication that the Morrow itself was a victim of subaerial erosion. There seems to be general agreement among North American stratigraphers that the Morrowan Age lasted an uncommonly brief six million years (circa 318 to 312 mybp). Could it be that the widespread North American unconformity is actually the same one at the top of the North Texas Morrow and not the one at its base? The Birdwell karst event unquestionably happened during an erosional cycle, and the recent discovery of Morrowan rocks in this area gives us two unconformities, hence two different durations of time, from which to choose. If karsting occurred contemporaneously with the unconformity at the base of the Morrow, then an absolute time measurement would yield an answer of somewhere between 325 to 320 mybp. If the karsting occurred in conjunction with the unconformity at the top of the Morrow, we would expect the age of cavern formation to be closer to 312 to 310 mybp.
The borehole was almost horizontal when it entered the top of the cavern fill at 4520′ MD (4320′ TVD) and it remained very nearly flat and straight all the way to a final total depth of 6980' MD. We can say that, at the very least, the cavern/cenote is 2400 feet (730 meters) long in a one-dimensional northwest-southeast alignment. We have no way of knowing how close that entry point was to the nearest cavern wall. Nor do we know how far the bottom of the borehole was from exiting the opposite wall of the karst feature when drilling finally ceased.
By comparison, the Big Room in Carlsbad Caverns is not quite 4000 feet (1200 meters) long, 625 (190 meters) feet wide, and 255 feet (78 meters) high at its tallest point. But then, we’ve had all the detailed measurements on Carlsbad for decades; we have only a single borehole about eight inches in diameter in the entire Birdwell anomaly.
The same operator’s #2H Birdwell (42-237-39492) was drilled in March of the following year about 1800 feet (550 meters) almost due north (from surface location to surface location) of the #1H. I was the consulting geologist on that second well also. Those two horizontal holes do not cross or intersect, as they are essentially parallel to each other with the #2H on lying to the northeast of the #1H. The #2H was drilled laterally somewhere in the middle of the real Marble Falls Formation, and, for as deep as it went stratigraphically, it yielded no indication whatever of karsting. It too was completed as being a Marble Falls producer—and this time it really was—but it was substantially more uneconomical than the first well. Oh, and did I forget to mention that the mud logger reported no coal beds in the Atoka of this borehole?
From the same pad as the #2H, the operator drilled a third horizontal well and completed it beneath an adjacent property in the summer of 2012. It was called the #1H Bright (42-237-39496), and it was oriented closely parallel to the two Birdwell boreholes, i.e., northwest-southeast, but it was situated to the east and somewhat to the south of the #1H Birdwell. I was no longer involved in the fiasco by that time, but I have been advised by those who were that no karsting was encountered. So it would appear that the Birdwell cenote is not well developed to any great extent towards the north and east.
There are no other boreholes for thousands of feet in any other direction that are deep enough to confirm or delimit the true extent of the Birdwell cenote as seen in that first well. While we have not yet determined it to be larger than Carlsbad’s Big Room, neither have we drilled it up and found it to be smaller. At this time, it appears to be largest collapsed cenote known in the subsurface of Texas.