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Phylloid algal banks form reservoir rocks in Upper Pennsylvanian shelf carbonates in many oil provinces of the United States. They are of special exploration interest in the Strawn (Desmoinesian) of West Texas and eastern New Mexico. Furthermore, the quantity of hydrocarbons in major fields which produce from these stratigraphic traps compares favorably with that produced from structural traps.
Phylloid algal banks were studied by the writer. Data were derived from studies of surface and subsurface occurrences of these carbonate buildups. The stratigraphic and regional distribution of these algal banks, their mode of formation, their environmental dependencies, and their synecological associations with other fossil assemblages were studied together with the evaluation of reservoir properties, such as formation and destruction of porosity, log characteristics, production data, and statistics on primary and secondary reserve estimates of major representative fields.
Algal mounds are formed by the sediment-baffling action of leaf-like (i.e., "phylloid") algae of the Ivanovia group, a branch of CaCO3-secreting green algae of the family Codiaceae. The dense, pitchy growths of these algae on local shoals on the sea floor form an efficient sediment baffle. Fine-grained carbonate sediment accumulates between the algal blades where it is sheltered from winnowing by wave and current action. This results in the gradual building of a mound-like accumulation of sediment in those places where dense growths of these algae occurred. Thus, these algal mounds are biogenic banks, which, if preserved in the geologic record, would be bioherms and biostromes.
Lithologic and paleontologic evidence indicates that these algal banks preferred shallow-water, wave-sheltered shelf environments in areas of clean carbonate deposition, distant from sources of land-derived clastics. Changes of water depth during transgressive and regressive cycles apparently exercised a sensitive control on the growth of these algae. The most luxuriant growth of these algae is obviously confined to an energy level below wave base, although these algae probably could endure intermittent higher wave action. Whenever the water became too shallow and the algal growths were above wave base, the algal mound development was interrupted. In many places, algal mounds are interbedded with layers of cleanly winnowed, well-sorted calcarenite or oolite.
Phylloid algae have been reported in the United States from areas in southeast Kansas, the Panhandle of Oklahoma, north-central Texas, the eastern shelf of the Midland basin, the northwestern shelf of the Delaware basin, Hueco Mountains, Franklin Mountains, Sacramento Mountains, Robledo Mountains, and the Four Corners area. These phylloid algae range in age from Morrowan to Wolfcampian in the United States, and to early Middle Permian in Europe. The major occurrences of these algal banks in the Permian basin area are in strata of Desmoinesian, Virgilian, and Wolfcampian ages.
In general, algal banks show evidence of a high primary porosity which formed when the highly warped algal blades were piled into a mound having a loose, or open fabric. The presence of such high primary porosity and permeability commonly leads to the development of secondary leaching porosity. Most commonly, the CaCO3 mud matrix between the algal blades is leached. Selective leaching of the algal blades
is less common. Recrystallization of the CaCO3 mud matrix also is a common source for secondary porosity development. The combined amounts of primary and secondary porosity and the resulting permeability values may be large. In Greater Aneth field, Four Corners area, porosity values range from 3.5 to 26.2 percent, with an average of 10 percent. Permeability values reach a maximum of 932 md, with an average of 25 md. Estimated primary and secondary petroleum reserves may amount to 500 million bbl.
Porosity destruction is caused primarily by secondary sparry calcite vug filling. Extensive leaching in the upper zones of an algal bank forms solutions which are oversaturated in CaCO3. When these supersaturated solutions percolate downward into the lower zones of the mound, precipitation of sparry calcite commonly begins. A rarer type of porosity destruction is that which results from a total collapse of the algal fabric. A relatively rapid diagenetic hardening of the CaCO3 mud matrix apparently is required to prevent collapse of the algal fabric under the weight of overlying sediment. In some places, anhydrite caused porosity occlusion. In one example, the porosity in a core had been destroyed completely by vug fillings composed of isolated small dolomite rhom ohedra.
Synecological fossil assemblages associated with algal mounds or mound-associated facies have different compositions in mounds of different stratigraphic and regional settings. The following groups of fossils were recorded in algal banks: Foraminifera, including ophthalmid and encrusting Foraminifera, and fusulinids, ostracods, fenestellid and fistuliporoid Bryozoa, crinoids, echinoids, gastropods, tetracorals, brachiopods, sponges, Chaetetes (tabulate corals), Komia (questionable stromatoporoid), Girvanella (blue-green algae), and Ungdarella (red algae).
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