The growth of Mississippian pinnacle reefs was initiated with the inundation of North-Central Texas by the shallow Barnett seas in late Meramecian time continuing through most of the Chesterian. 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 County, where they reach their maximum vertical development and constitute true reef complexes. 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 an unremarkable calcareous facies of the lower part of the Barnett Shale. See Figure 7 below.
FACIES MAP OF THE MISSISSIPPIAN REEF TERRAIN: Map after Henry, 1982.
It is tempting to want to correlate these Barnett-enclosed reefs with outcrops of the Chappel Limestone in Central Texas, but that concept is not valid. In San Saba County, researchers have documented an unconformity between the Barnett and the type Chappel. Conodonts have confirmed the true Chappel to be early Mississippian (Kinderhookian and perhaps early Osagean) and the overlying Barnett to be Late Mississippian (Meramecian and Chesterian). In popular useage the reefs are informally known as "Mississippi(an) reefs," and that practice is not likely to change anytime soon.
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 Ordovician surface. 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 found 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 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 reef crests undoubtedly were elevated somewhat above the shallow 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. Stratified 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 Barnett reefs 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.
Volumetrically the Barnett 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, presumably 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.
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.
As the shallow Late Mississippian (Meramecian) 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 Barnett 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 cause of death of the reefs is not known. One possibility is that the rate of Barnett sedimentation overtook the reefs' ability to stay ahead of it, and so the reefs were smothered beneath the rapidly accumulating organic shale. In a few wells (e.g., the Bayview Oil Corp. No. 1-A Gilmore, G. Dedrick Survey, A-176), both the Barnett's basal shale member and 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.
An understanding of the reefs is important primarily because of the substantial 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 Barnett 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 classified as gas wells. Some percentage of saltwater commonly accompanies oil production. 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 Barnett reef if the overlying Barnett Shale is thinned to approximately one-half of its expected regional thickness.
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.
A detailed isopach map of the Barnett Shale is the primary exploration technique for the Barnett 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.