The growth of Mississippian pinnacle reefs was initiated with the inundation of North-Central Texas by the shallow Barnett seas. The reef complexes are subdivisible into three constituent facies: the reef core, the reef flanks, and the inter-reef area. The reef cores are porous enough to serve as stratigraphic traps for oil and gas, and they have yielded excellent production in the northern part of the Fort Worth Basin for two-thirds of a century. The Barnett Shale is believed to be the hydrocarbon source for the reef production.
Mississippian-aged bioherms associated with the Barnett Shale are known from several counties in the Fort Worth Basin. These bioherms are perhaps best studied in Jack and Montague Counties, where they reach their maximum vertical development and constitute true reef complexes (Krumbein and Sloss, 1963, p. 575). A typical reef complex in Jack County is subdivisible into three constituent facies. (1) the reef core, which was the living and actively growing part of the complex, (2) the reef flank, characterized by bedded reef debris and fine calcareous sediment, and (3) the inter-reef areas where deposition was essentially a calcareous facies of the lower part of the Barnett Shale. See Figure 7 below.
Figure 7: FACIES MAP OF THE MISSISSIPPIAN REEF TERRAIN: Map after Henry, 1982.
Distribution of these reefs or, more properly, reef cores, appears to be fairly random, suggesting that they developed upon a relatively featureless and shallow shelf. Some, if not most, of the reefs probably began under shoaling conditions caused by the slightly uneven topography of the eroded Lower Paleozoic surface. Very few of the reef cores exceed one-half square mile in areal extent, and they may occur either individually or, less frequently, as two or three discrete reef pinnacles clustered together.
Drill cuttings and well logs show the reef cores to be massive, unbedded biomicrites, ranging in color from white to buff to gray. Lithology cross-plots from density-neutron logs reveal both the reef cores and the reef flanks to be almost entirely limestone; little or no dolomite has ever been logged in these rocks.
If the crests of the living reefs were elevated into the zone of breaking waves, the available wave energy must have been relatively low, because sparry calcite is extremely rare in the drill cuttings. It is also possible that the reefs grew only up into the photic zone, which was unlikely to have been very deep in the silt-laden and organic-rich waters of the Barnett shelf. Whichever the case may have been, the reefs undoubtedly were elevated somewhat above the sea floor, as evidenced by their well developed flank deposits and the complete absence of approximately the lower half of the Barnett Shale over their crests.
Modern-day reefs are often observed to grow laterally in a windward direction out over their lower flank deposits, and this appears to have been the case with many, if not most, of these Mississippian reefs. Bedded reef flank rocks often underlie the reef cores, but the cores are never found to be overlain by flank beds. The dipmeter log is particularly useful in identifying the abrupt contact between the reef flanks and the unstratified rocks of the reef core.
The maximum development of any given reef appears to be, to some degree, a function of its position on the subsiding Barnett shelf. The more northeasterly reefs, presumably the older ones, tend to have thicker cores and thicker overall sections than those found in the southwestern part of the study area. The known reefs in Montague County, for example, are as much as 350 feet (107 meters) thick, whereas the reefs along the eastern flank of the Bend Arch in western Jack County are typically 100 to 150 feet, (30 to 45 meters) thick. Contrary to some published accounts (e. g., Turner, 1957, p. 61), thicknesses in the range of 500 feet (150 meters) cannot be substantiated.
The Chappel buildups are often referred to as “pinnacle reefs,” but that is a misnomer. They may appear as pinnacles on a cross section with an exaggerated vertical scale (see cross section A-A′ above), but in reality they have almost exactly the same height/width aspect ratio as a fried egg sunny side up. The reef core, of course, is represented by the egg yolk, and the reef flank debris are represented by the egg white.
(Author’s Note: The average yolk of a fried egg is about eight-tenths of an inch thick; a stack of fifteen such eggs would be about a foot tall; a fairly good-size productive reef might be, say, 200 feet [i.e., 3000 eggs] tall. Reefs, like eggs, are three-dimensional creatures, so our hypothetical reef would have a volume of about 3000 eggs cubed, i.e., 27 billion fried eggs.)
The reef core rocks do not appear to be unusually fossiliferous. The most common fossil material identified in drill cuttings consists of brachiopod tests, ostracods, and small ramose bryozoans. Pelmatozoan columnals, which are occasionally observed in Barnett Shale beds, are generally absent in the reef cores. However, the organisms most responsible for the occurrence and growth of the reefs were probably calcareous blue-green algae. In modern reefs these algae are observed to grow in mats, trapping and binding minute particles of carbonate sediment and the tests of other small creatures into an amorphous, wave resistant structure (Krumbein and Sloss, 1963, p. 576).
Volumetrically the Chappel reef flanks are much larger than the cores. The argillaceous flank beds dip quaquaversally away from the core and are largely composed of debris dislodged therefrom, perhaps by wave action. Some calcareous material was no doubt contributed by reef flank biota, including algae and other frame builders, so that there is a narrow transition zone from unbedded core material to well-bedded flank material. There are at least half a dozen wells in Jack County in which a prominent talus zone can be identified immediately adjacent to a reef core. These beds range in thickness from roughly 100 feet (30 meters) near the reef cores to perhaps 30 feet (9 meters) at the point where they grade laterally into the inter-reef beds. In those areas where reef flank beds are present, the basal shale member of the Barnett cannot be identified. From approximately the Jack/Young County line westward cross the Bend Arch, this basal member grades into a reefless limestone formation that is commonly referred to as the “Chappel Lime” in North-Central Texas. The Chappel Limestone is occasionally oil productive, but it does not constitute a major regional reservoir.
The inter-reef facies is represented by a black, calcareous, bituminous shale. Where it occurs in Jack County it is typically 30 to 40 feet (9 to 12 meters) thick, and it is synonymous with the calcareous basal shale member of the Barnett. Consequently, the proximity of a given borehole to a nearby reef complex can be qualitatively estimated by the degree to which this lower member of the Barnett has been impregnated with calcite. The lateral limits of the reef flank and inter-reef facies are gradational, and any attempt to represent these on a map must necessarily be somewhat arbitrary.
The so-called Chappel Lime in the subsurface of North Texas may or may not be correlative with the type Chappel of the San Saba County outcrops. There are two primary objections to such a correlation: (1) The two limestone units represent very different depositional environments; the reef complex facies of the subsurface is unknown on the outcrop, where the formation rarely exceeds 45 feet (14 meters) and is most often found to be less than a meter. Also, the pinkish color and the abundance of crinoid fragments that characterize the central Texas outcrops are not found in the Fort Worth Basin reef beds. (2) Paleontological studies over the years have reached different conclusions concerning the exact dates for the exposed Chappel in San Saba County (early Kinderhookian to Osagean) and Barnett (late Osagean to late Chesterian) beds, so it would appear likely that these two formations are not exactly the same age. Yet the manner in which the Barnett beds were draped over and around the reefs leaves no doubt that the reefs are definitely time correlative with approximately the lower half of the Barnett Formation (Chesterian). In fairness, it must be pointed out that the longstanding diversity of opinion among conodont biostratigraphers concerning the exact age of the Chappel at its various outcroppings has not been resolved. Until a definitive date can be assigned to the exposed Chappel, its relationship to the buried reefs will remain uncertain, and in my opinion, highly questionable.
Whatever the true relationship between these two carbonate units, the Mississippian-age limestone beneath the Barnett Shale in North Texas will no doubt continue to be referred to as “the Chappel.” Mississippian bioherms remarkably similar to the Chappel reefs of North Texas have been described from the Lake Valley Formation of the Sacramento Mountains in New Mexico. Laudon and Bowsher (1941, p. 2126) believed them to be Osagean (Middle Mississippian).
As the shallow Late Mississippian seas spread southward and westward from the subsiding Southern Oklahoma Aulacogen, they inundated an uneven Lower Paleozoic surface and almost immediately initiated the growth of reef-forming organic communities. All of the Mississippian-age reef complexes whose bases have been penetrated by boreholes have been found, without exception, to be resting directly upon the underlying Ordovician rocks. But although reef growth began at the same time as Barnett Shale deposition, the reefs did not survive to the end of Barnett time; all known Chappel reefs are immediately overlain by the typical Barnett Shale facies except for a very few in central Clay County that have been very deeply breached by pre-Atokan erosion.
The depositional time span of an ancient sedimentary rock unit is almost impossible to determine quantitatively with any reasonable degree of certainty, but in the case of fossil reefs we have an unusual opportunity to make at least a rough estimate of their life spans based upon the known rates of vertical accumulation of their modern analogues. Using radiometric age dating. Thurber, et al. (1965, p. 57) have estimated coral reef growth on Enewetak Atoll in the northwestern Marshall Islands at about 6 feet (1.8 meters) per thousand years. E. G. Purdy (in Macintyre, 1972) reported a maximum mean rate of about 5 meters (16.4 feet) per thousand years. Lalou, et al. (in Braithwaite, 1973, p. 1104) came up with figures of about 5 feet (1.5 meters) and 10 feet (3 meters) per millennium for two measurements on Moruroa in French Polynesia. If we use a conservative figure of 5 feet (1.5 meters) per thousand years, one of the larger Chappel reefs—let us consider one that is about 300 feet (90 meters) thick—would have had a life span of about 60,000 years. Assuming that the reefs are correlative with approximately the lower half of the Barnett Shale in this area, the depositional duration of the Barnett would be about twice that—120,000 years. By using an accumulation rate closer to Purdy's figures, say 15 feet (4.6 meters) per thousand years, the life span of the larger reefs would have been about 20,000 years, and the entire Barnett section could have been deposited in as little as 40,000 years. Clearly there may be ecological and sedimentary factors unknown to us that caused the Chappel reefs to grow at a different rate than do the modern ones, but the point to be made here is that regardless of which rate of vertical growth we assume, the length of time required for the life span of the reefs and the deposition of the Barnett might have been only a fraction of one percent of the 41,000,000-year duration of the Mississippian Period and the 15,000,000-year term of the Chesterian Epoch.
The cause of death of the reefs is not known, though it may well have been related to rapid fluctuations in sea level and changes in circulation patterns resulting from the catastrophic tectonic movements which were beginning to occur at about this time in North Texas and also the multiple advances and retreats of glaciation in the southern hemisphere. In a few wells (e. g., the Bayview Oil Corp. No. 1-A Gilmore, G. Dedrick Survey, A-176), both the basal shale member and the major shale member are absent over the crest of a large reef core, but the minor shale member, though often thinned, is invariably present in its entirety. It is tempting to speculate that the geological event that caused the distinctive low-resistivity break between the major and the minor shale members was also responsible for the death of the last remaining reefs.
An understanding of the Chappel is important primarily because of the relatively large amounts of oil and gas that are often contained in the reef cores. Initial potentials of several hundred barrels per day and reserves in excess of 100,000 barrels per well are not uncommon, thus making the reef cores among the most attractive objectives in the Fort Worth Basin.
Porosity development within a reef core occurs in an apparently random and unpredictable fashion. Good porosity (i. e., 5 to 10 percent) is sometimes encountered in the upper few feet of the reef. Other times it may be necessary to drill 50 feet (15 meters), 100 feet (30 meters), or even more into the reef before it is reached. A good example of this is the Four-B Trust No. 1 Lindsey dry hole (A. Brumbelow Survey, A-109), which, in 1966, was drilled into the crest of a large reef core in north-central Jack County. The entire reef section was penetrated; the well logs and four drill stem tests indicated low permeability throughout. Twelve years later the Southwestern Gas Pipeline No. 4 Lindsey Ranch was drilled into the same reef core a few hundred feet away, and profitable oil production was established in the upper part. From experiences such as this, it is apparent that zones of porosity exist somewhere within virtually all Chappel reef cores, and a single well may not be sufficient to definitively evaluate even a small reef.
Oil production with a relatively low gas-oil ratio is most common, although a few reefs have been gas productive. At least two reefs in western Jack County have been adequately tested and found to contain only salt water.
The source of the hydrocarbons is certainly the petroliferous Barnett shale beds that encase all but the basal surfaces of the reefs. As a general rule, production may be expected from a Chappel reef if the overlying Barnett is thinned to approximately one-half of its expected regional thickness.
Different types of reservoir drives are found in the various reef oil fields. A few miles west of Jacksboro, the South Berwick field has suffered a pressure drop of only a few psi since its discovery in 1974, thus giving evidence of an active water drive. Water drives such as this are the result of wet, dolomitic beds in the upper part of the Ellenburger being in vertical communication, possibly through fractures, with the overlying reef, Other reef fields have experienced less gradual pressure declines that are indicative of gas cap or solution gas drives.
Adequate data concerning the relationship of the reefs to the underlying Lower Paleozoic structure and/or paleotopography are lacking. However, at least two or three reefs appear to be located on Ellenburger paleotopographic highs. Although the Ellenburger is generally nonproductive throughout most of the reef trend, it will occasionally yield favorable indications from the drill cuttings or well logs where it is overlain by a reef.
As previously mentioned, differential compaction of post-Chappel beds over the reef crests often tend to create structural noses and closures in sediments at least as young as the Canyon Group (Makins, 1962, p. 20). Dipmeter readings at least as high as 21 degrees have been measured on Barnett beds immediately above the edge of a large reef core. Consequently a reef prospect will also have as its objectives all reservoir-quality beds from the Chester up through the Upper Pennsylvanian. A detailed isopach map of the Barnett Shale is the primary exploration technique for the Chappel reefs. Prospects may then be further defined by structural contouring on the top of the Marble Falls Formation in areas where the Barnett is anomalously thin. Final selection of a drill site is best determined with the aid of seismic data.
Henry, J. D., 1982,Stratigraphy of the Barnett Shale (Mississippan) and Associated Reefs in the Northern Fort Worth Basin,in Petroleum Geology of the Fort Worth Basin and Bend Arch Area: Dallas Geological Society.
Laudon, L. R. and Bowsher, A. L., 1941,Mississippian formations of Sacramento Mountains, New Mexico: Asssoc. Petroleum Geologists Bull.,v. 25, no. 12, pp. 2126-2129.
Macintyre, K. G., 1972,Submerged reefs of eastern Caribbean:Am. Assoc. Petroleum Geologists, Bull.,v. 56, no. 4, pp. 720-38.
Turner, Gregory L., 1957,Paleozoic Stratigraphy of the Fort Worth Basin,in 1975 Joint Field Trip Guidebook: Abilene and Fort Worth Geol. Socs., p. 77.