Trenton Limestone Research Articles

Depositional Setting of Ordovician and Cambrian Rocks in Central Appalachian Basin along a Section from Morrow County, Ohio, to Calhoun County, West Virginia: ABSTRACT

A 200-mi (320 km) long restored stratigraphic section from Morrow County, Ohio, to Calhoun County, West Virginia, contrasts Ordovician and Cambrian rocks deposited on a relatively stable shelf with those deposited in rift and postrift basins. Lithologic data are from commercial logs and from detailed descriptions of cores in five of the nine drill holes used to construct the section. Particularly instructive was the 2,352 ft (717 m) of core from the Hope Natural Gas 9634 Power Oil basement test in Wood County, West Virginia. Rift basin deposits are dominated by medium to dark-gray argillaceous limestone, argillaceous siltstone, and by green-gray to black shale of probable subtidal origin. Dolomite is the dominant rock type in the postrift basin and adjacent stable shelf deposits. The upper part of the postrift sequence, composed of the Middle Ordovician Black River Limestone, the Middle Ordovician Trenton Limestone, and Middle and Upper Ordovician Antes (Utica) Shale with a high organic content, represents deposition in gradually deepening water on an open shelf.

Geology of Middle to Upper Ordovician Sebree Trough Northwest of Cincinnati Arch, Ohio, Indiana, and Kentucky: ABSTRACT

The Sebree trough, which has attracted interest in hydrocarbon exploration, is a broadly linear, northeast-oriented, troughlike structure northwest of the Cincinnati arch that can be traced from western Ohio through southeastern Indiana to western Kentucky. The post-Black River, pre-upper Kope succession of the Sebree trough differs markedly in lithology from that of the Cincinnati arch, in which limestones dominate (Trenton and Lexington Limestones), in that it consists mainly of dark-brown to grayish shales (Utica of drillers). The Sebree trough is not displayed by the isopach pattern of the Black River, nor by that of post-Kope strata. Apparently, this feature is restricted to the upper Middle and lower Upper Ordovician. Absence of biostratigraphic control in the Utica, as well as poor well control in the area bordering the Cincinnati arch carbonate region, made previous interpretations of the stratigraphic relations between the Utica and the Lexington-Trenton hypothetical. Recent detailed study of two cores from eastern Indiana and one from western Ohio led to the discovery of a succession of biostratigraphically diagnostic graptolites in the Utica that can be tied into that of the C. spiniferus and C.pygmaeus zones in the Utica Shale of New York. The new evidence shows that at leastmore » the major parts of the Utica and the Lexington-Trenton are correlatives, although these units are developed in quite different facies.« less

Seismic Stratigraphy of a Fracture Reservoir: Case Study of Albion-Scipio Trend, South-Central Michigan: ABSTRACT

The Albion-Scipio trend is a narrow, divergent left-lateral wrench system in the south-central flank of the Michigan basin. The initial discovery well, Perry 1 Houseknecht, was drilled in 1956 to the Trenton Limestone, allegedly on the advice of a fortune-teller. In 1957, the well was deepened and completed in the Black River, flowing 150 BOPD. Throughout the late 1950s and early 1960s, all subsequent field extensions were controlled by random drilling along a straight-line, northwest-southeast projection through the Scipio and Albion fields. By the early 1960s, 320 wells were on production, and development of the Albion-Scipio trend was nearly completed. The field has produced approximately 126 million bbl of oil and 230 bcf of gas since 1957. Geophysical techniques were not employed in the discovery, extension, or development work in the field. Ladd Petroleum conducted a modeling study to determine if there were attributes that could be used to identify the Albion-Scipio field seismically. Interpolative modeling and field testing indicated that where frequencies above 70 Hz are recovered in the field, several seismic attributes associated with the productive fracture portions of the Trenton-Black river reservoirs could be identified. These attributes included (1) direct observation of faulting, (2) sagging of themore » Trenton reflector, (3) disruption of the Black River shale reflector due to destructive or constructive interference, and (4) the seismic color attributes of frequency and amplitude. Because Trenton-Black River stratigraphy varies little in southern Michigan, these seismic attributes should remain constant regionally and aid in finding other fractured Trenton-Black River reservoirs in southern Michigan.« less

Reservoirs resulting from facies-independent dolomitization: Case histories from the Trenton and Black River carbonate rocks of the Great Lakes area

Reservoirs in the Trenton Limestone and the Black River Group (Limestone) both Upper Ordovician) in Indiana, Michigan, and Ohio formed a major oil province that increased rapidly in importance beginning in 1884. This province became America’s first giant oilfield and has yielded more than 600 MBO, including that produced from the giant Albion-Scipio Trend (120 MBO), which was discovered in 1956. Trenton and Black River carbonates have always excited exploration interest, but very little was known about the nature and distribution of their reservoirs. Recent research on the nature of reservoir development in these units indicates that they do not contain any depositional porosity in the Great Lakes area. Reservoirs are found only where there has been dolomitization or fracturing or some combination of the two.

Petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the Trenton Formation, Ordovician, Michigan Basin, U.S.A.

ABSTRACTThree major types of dolomite occur in the Trenton Formation (Mid‐Ordovician) of the Michigan Basin. These are: (1) ‘regional dolomite’ which is confined to the extreme western edge of the basin; (2) ‘cap dolomite’ which occurs in the upper portion of the Trenton and is confined to the basin's southern margin; and (3) ‘fracture‐related’ dolomite which occurs in association with both large‐ and small‐scale faults and fractures. These three dolomite types can be distinguished from one another by their major element chemistry, oxygen isotope ratios and rock texture.The regional dolomite is fine‐grained, has <0.34 mol% FeCO3, and mean δ18O of −6·8‰OPBD. The cap dolomite is texturally similar to regional dolomite but contains 3–13·0 mol% FeCO3 and has a mean δ18O of −7·7‰. Fracture‐related dolomites are coarse‐grained, low in iron, and have the most depleted δ18O ratios (x̄=–9·0%PDB).Petrographic relationships imply that the regional dolomite, formed prior to the cap dolomite probably during early diagenesis. The cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying Utica Shale and the Trenton Limestone. Fracture‐related dolomites post‐date the cap dolomite and formed during deeper burial. A temperature of precipitation of approximately 80°C was calculated for fracture‐related dolomites using oxygen isotope data.The distribution of the cap dolomite was controlled by the availability of Fe2− which was in turn controlled by the availability of S2−. In the centre of the basin Trenton‐Utica deposition was continuous. The upper Trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce H2S from seawater sulphate. The precipitation of iron sulphides (pyrite + iron monosulphide) followed and used up most of the available Fe2−. As a result only small amounts of ferroan dolomite formed. On the periphery of the basin, subaerial exposure resulted in the oxidation of most of the available organic matter. Sulphate reducing bacteria were therefore limited and produced limited amounts of H2S. As a result only a minor amount of iron sulphide (iron monosulphide) formed. The remaining Fe2‐ was then available for the formation of the ferroan cap dolomite. This model is supported by the following: (1) In the southern margin of the basin, the contact between Trenton cap dolomite and the overlying Utica Shale is sharp and probably unconformable. In the centre of the basin the contact is gradational. (2) In the centre of the basin, the total organic carbon content in the upper Trenton is an order of magnitude higher than in the cap dolomite. (3) The whole‐rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre. (4) Iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite. In the cap dolomite, iron sulphide is minor and primarily in the form of iron monosulphide.

La viabilité des bétons du Québec: le rôle des granulats

The petrographic study of numerous concrete structures in Quebec has revealed the extent of postconstruction chemical reactions. Those reactions between the aggregates, which form 70% of the volume of the concrete, and the cement paste result in early deterioration and undesirable mechanical deformation of the structures. From this study, it appears that the coarse aggregates can react in three different ways: (a) by peripheral reaction, which is the case for massive granitic rocks, (b) by bulk swelling, as illustrated by the Potsdam orthoquartzite, and (c) by formation of silica gel veinules within the aggregate, as shown by some Trenton limestone. Considering these three mechanisms, it is easy to understand why certain aggregates react rapidly, that is within a few months, whereas others show signs of distress only after scores of years. Using the three rock types cited above, we have also tried to evaluate existing concrete standards and their capability to detect the most important pathologies encountered in concrete. Key words: concrete, aggregate, alkalis, disintegration, expansion, testing, durability.

Diagenetic Features of Trenton Limestone in Northern Indiana: Petrographic Evidence for Late (Mesogenetic) Dolostone: ABSTRACT

Three conventional cores of the entire Trenton section were examined in detail by in-depth visual description, analysis of more than 250 thin sections, scanning electron microscopy, and x-ray diffraction. The cores are located in the northern half of Indiana where they span the major dolostone pinch-out that is the trap for the prolific Trenton oil and gas field. The Trenton Limestone is completely dolomitized in northern Indiana. Dolostone abundance decreases to the south where the dolostone is restricted to the upper few feet of the formation. Two major types of dolostone are recognized. The top 5-20 ft of the Trenton cores consists of medium crystalline nonporous xenotopic ferroan dolostone. Mesogenetic dewatering of the overlying Maquoketa shale is the proposed dolomitizing mechanisms for this ferroan dolostone cap. Below the ferroan dolostone cap in northern Indiana is coarsely crystalline dolostone, which consists of thin intercalated subfacies of porous idiotopic and nonporous xenotopic dolostone. This is the dominant dolostone type and is the reservoir in the Trenton field. The coarsely crystalline dolostone postdates the ferroan dolostone cap, chert nodule formation, and initial pressure solution. Therefore, this dolostone is considered to have formed relatively late in the diagenetic history of the Trenton under mesogeneticmore » conditions. In the northernmost core, nearly all of the secondary dolomitic porosity is plugged by poikilotopic gypsum and minor amounts of calcite and celestite. Other diagenetic features observed in Trenton are also discussed, including silicification, ferroan calcite cement, upper Trenton contact formation, hardgrounds, and pressure solution.« less

Mississippi Valley-Type Mineralization and Dolomitization in Trenton Formation, Wyandot County, Ohio: ABSTRACT

Mississippi Valley-Type Mineralization and Dolomitization in Trenton Formation, Wyandot County, Ohio: ABSTRACT

Depositional Patterns and Diagenetic History of Trenton-Black River Formations in Michigan Basin: ABSTRACT

The Trenton-Black River formations (Middle Ordovician) exhibit depositional facies and regional thickness variations indicative of normal marine, carbonate platform accumulation adjacent to a deeper water, basinal environment. Thickening of both formations occurs in the southeast part of the Michigan basin, possibly in response to basement movement accompanied by a shift in the basin depocenter. Early mixed-water diagenesis introduced the first generation of replacive dolomite, which varied in minor element composition and abundance depending on proximity to structural highlands southwest and southeast of the Michigan basin. A ferroan cap dolomite is interpreted to be partly representative of this early dolomite stage. Later stage replacive dolomite and dolomite cements are commonly associated with large fracture systems active during the Paleozoic. Although ferroan dolomitization is common throughout the Trenton and Black River formations, it is most extensive within these fracture zones, which also host significant oil and gas deposits in several locations. In addition, several accessory minerals are associated with late-stage dolomite cements, including pyrite, calcite, anhydrite, barite, celestite, sphalerite, and fluorite. The trade occurrence of these minerals in association with late-stage ferroan dolomite cements indicates that the Trenton-Black River formations in southern Michigan have undergone hydrothermal alteration analogous to a typical Mississippianmore » Valley-type deposit. Evidence based on the stratigraphic distribution of this mineralization indicates that the lower Paleozoic and Precambrian basement rocks were important sources for iron, sulfur, and other trace metals deposited in the Trenton-Black River during the late Paleozoic.« less

Oil generation in the michigan basin: A biological marker and carbon isotope approach

Palaeozoic strata in the Michigan Basin produced crude oils which, despite their rather small volume and economic value, are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting. Based on n-alkane profiles, biological marker distributions and carbon isotope ratios, the oils belong to three main families of different genetic origin and a few less important mixed types. Silurian oils from Salina and Niagara Limestones have broad n-alkane distributions and abundant isoprenoid hydrocarbons. A strong phytane-over-pristane predominance and the lack of diasteranes indicate a carbonate source for these oils. Chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n-alkanes. Oils found in the calcareous Ordovician Trenton formation contain n-alkane, cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils. Many oils from the Devonian Dundee reservoirs are very similar in overall composition to the Trenton oils. Devonian Traverse oils are considered to be mainly from a Devonian source with some contribution of Ordovician-type oil and are of intermediate maturity. Based on calculations using kinetic parameters of biological marker reactions, considerably deeper subsidence of the Devonian source rock in the past is implied.

Magnetic anisotropy in the Trenton Limestone: Results of a new technique, anisotropy of anhysteretic susceptibility

A new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described. The magnetic fabric of a collection of Trenton Limestone specimens has been determined using this method, as well as by conventional anisotropy of magnetic susceptibility. The results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the Trenton in which the bulk magnetic susceptibilities are low. A model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses. The original fabric is inferred to have been isotropic; the anisotropy resides in secondary magnetite of Late Paleozoic age. It is argued that the observed magnetic fabric must therefore be Alleghenian or younger in age. Our method has the potential to determine paleostress directions in carbonates elsewhere, provided our assumptions are correct.

Sedimentary Basin Analysis of Middle Ordovician Limestones in Central Appalachians: ABSTRACT

Seven of 28 wells penetrating the Trenton Limestone in West Virginia have reported shows of natural gas, enough to continue industry's interest in this potential reservoir. The formation consists of thin limestones interbedded with shales and bentonites. Sediments were deposited on a ramp that sloped eastward from a shallow platform in northwest Ohio into the foreland basin of Virginia, and the unit forms a wedge-shaped mass that thickens into the basin. Limestones of the upper ramp were deposited on sand shoals (skeletal grainstones) and restricted flats (lime mudstones), which passed downslope into skeletal patches of a deep, muddy environment (packstones and wackestones). Rapid downwarping of the carbonate ramp produced a major transgression, and deeper limestone facie migrated westward during time. Bentonites spread across the region from distant volcanic islands. In West Virginia, a lower bentonite package is present in the Black River Limestone to the southwest, whereas an upper package occurs in the Trenton to the east and north. This distribution End_Page 1444------------------------------ indicates that the site of volcanic activity shifted during time. A growing orogenic source area shed terrigenous sediment into the basin and onto the Trenton ramp. Initially, these muddy influxes came from the north and from the south, but terrigenous mud eventually swamped the entire ramp and carbonate sedimentation then came to an end. Trenton Limestones have only minor intergranular porosity because of abundant mud matrix and cementation. The only significant contribution to porosity is in fractures. Thus, wells with Trenton shows are characterized by high initial potential that decreases rapidly. End_of_Article - Last_Page 1445------------

Trenton Formation in New York, a Computer-Aided Study: ABSTRACT

The Trenton Formation is an Upper Ordovician sequence of interbedded limestones and limy shales that extends from the Hudson River valley in the east throughout the western part of New York state. It was one of the first gas-producing reservoirs in the area with most of the more than 300 test wells producing or indicating natural gas. Subsurface information on the Trenton includes century-old reports, operators tops, sample logs, and geophysical logs. The logs, run over more than a 40-yr span, vary widely in type, lithology response, and depth scale. To aid interpretation, all available data were added to the New York state computer data base for map processing, and the geophysical logs were digitized for uniform evaluation and presentation. Individual log curves were corrected to a standard lithology response, combined, and played out as a series of standard control cross sections. Computer posted and contoured maps were constructed and compared to log sections for correction of errors. Finally, geologic interpretation was added to the maps and cross sections to produce a new evaluation of the Trenton Formation. > The Trenton in the subsurface can be divided into three distinct members with a disconformity between the lower two. Depositional centers shifted from central New York for the lower member to the northern part of the state for the upper member. The formation is a fractured porosity reservoir with the main fracture zones located along basement-controlled tectonic structures. The main features trend northeast-southwest in the northern part of the state, whereas orthogonal, northwest-southeast trends dominate in the south. End_of_Article - Last_Page 301------------

Oil and Gas Developments in Mid-Eastern States in 1984

The mid-eastern states are in the southern half of the northeastern United States and include Maryland, Ohio, Pennsylvania, Virginia, and West Virginia. Once again, drilling in this, the oldest petroleum province in the United States, was relatively shallow and quite successful. Mean well depths ranged from approximately 2,300 ft/well in Pennsylvania to 5,300 ft/well in Virginia, and success rates ranged from 94% in Ohio to 98% in Virginia. In Ohio and northwestern Pennsylvania, oil and gas drilling to Lower Silurian sandstones was of prime importance. Farther east, in Pennsylvania, West Virginia, and Virginia, the main targets ranged from middle Mississippian to Upper Devonian, and gas plays were the norm. Due to the great number of wells that have been drilled in this a ea (nearly 600,000 wells; 14,350,000 productive ac) most exploratory wells are either outposts or deeper pool tests. Scattered wildcats continue to be drilled, however, in the plateau portion of the basin, and deep wildcats to Lower Devonian Oriskany Sandstone or older targets were drilled in the Eastern Overthrust belt. Drilling activity, as measured by completions received by state agencies during 1985, increased in 3 of the 5 states, led by Pennsylvania and Ohio. Overall, activity increased 29.1% with 12,927 wells reported. This increase in activity was reflected in footage drilled in the region, up 24.3% from 1984. The number of new gas wells increased by 20.5% as both Ohio and Pennsylvania surpassed West Virginia in this statistic. However, the biggest increase in activity was in oi well drilling, as oil plus combination completions (again led by Ohio and Pennsylvania) were up 44.9%. Exploratory activity, however, failed to increase significantly. An increase in Pennsylvania was nearly offset by declines in Ohio and West Virginia. Thus, overall, the number of exploratory completions increased only 2.6%. Production of oil and gas failed to increase at all, as production of gas was down 2.5% and oil production declined 0.9%. Only Virginia showed a significant increase in gas production. For the third time in the last 4 years, no wells were drilled in Maryland. Gas production from the state's 2 fields decreased 35% due to normal decline. Seismic activity and leasing of suspected buried Triassic basins continued in the coastal plain of southern Maryland. Activity in Ohio reached a record high, surpassing that of 1983. Completions increased 24.7% from 1984, and footage drilled increased 28.4%. New completions were predominantly combination oil and gas wells (63.7%), mostly to the Silurian "Clinton-Medina" sandstone. Exploratory activity decreased 7.4%, and only 2.4% of all completions were classified as exploratory. These completions were dominated by Clinton-Medina tests with a high (81.5%) success rate. However, scattered deeper tests to the Ordovician Trenton Limestone and Rose Run Sandstone and various Cambrian formations resulted in only 4 successes in 41 attempts. Shallow exploration was most successful in the Devonian Ohio Shale and Mississippian Berea Sandstone. Oil production decreased 1.9%, and gas production decreased 2.3%. Drilling activity in Pennsylvania during 1985 increased by 79.3%, and footage drilled increased 70.7%. Exploratory activity also increased, as completions were up 27.9%, footage increased 30.0%, and seismic activity increased 32.6%. The exploratory success rate dropped from 1984, but was still excellent at 76.6%. Deep drilling increased 70.1% and again was concentrated on extending and developing Lower Silurian Medina fields in northwestern counties. Shallow drilling increased 75.5% and was dominated by continued extension and development of Upper Devonian Venango and Bradford oil and gas fields. Farther east, deep exploration programs included scattered tests of the Lower Devonian to Upper Silurian carbonates, and Lower Ordovician to Middle Cambrian formations. Oil production increas d less than 1%, and gas production decreased 9.5%. Virginia had another increase in drilling activity (39.7%) and footage drilled (48.3%). Exploratory activity remained fairly stable with a decrease of 1 well from 1984. This drilling was successful, however, with 6 of 8 wells producing gas in the Mississippian Big Lime, Weir, Price, and Berea, and the Devonian shale. All of the new development wells were completed in this same stratigraphic interval, and all produced gas. Gas production increased 78.5%, reaching a record high. Oil production, however, decreased 22.7%. End_Page 1259------------------------------ Drilling in West Virginia decreased 10.1%, and total footage decreased 26.5%. Exploratory activity also decreased, with completions down 6.0%, footage down 21.0%, and seismic activity down 72.5%. Exploration programs of interest included deep Oriskany tests on major structural features in eastern counties, Tuscarora tests in several southern counties, and continued drilling to Upper Devonian shales, siltstones, and sandstones in northern West Virginia. Deep drilling decreased 16.9%, and the success rate (44.6%) was far below the state's average (96.1%). Although the number of new gas well completions decreased from 1984, the state is still predominantly a gas province. Production of gas, however, increased only 1.0%, and oil production increased only 1.4%.

Late Paleozoic remagnetization of the Trenton Limestone

Samples of the Middle Ordovician Trenton Limestone were collected from 8 sites in Quebec, Ontario, and New York State for paleomagnetic study. Stepwise thermal demagnetization reveals a very well defined characteristic magnetization that has a corresponding paleomagnetic pole at 53.0° N, 126.9° E (K=97.7, A‐95=2.7°). This pole is not at all close to other Ordovician poles from cratonic North America but is near Late Carboniferous and Early Permian ones, indicating that the Trenton was remagnetized in the Late Paleozoic. An earlier paleomagnetic investigation of the Trenton gave a pole that did not appear to be Late Paleozoic in age. However, we demonstrate that this earlier study failed to isolate the ancient characteristic magnetization.Rock magnetic experiments and analyses of magnetic extracts show that the carrier of magnetization is Fe3O4 that does not contain Ti or other elements in solid solution that would indicate a detrital source. The magnetite and its associated remanence appear to have a diagenetic origin.

841232 Slope stability in teh trenton limestone of a Montreal area quarry : Edd, J E; Betourney, M C CIM BullV76, N856, Aug 1983, P72–78

841232 Slope stability in teh trenton limestone of a Montreal area quarry : Edd, J E; Betourney, M C CIM BullV76, N856, Aug 1983, P72–78

Six Types of Trap or Reservoir-Producing Configurations in Trenton Limestone Reservoir, Northwestern Ohio: ABSTRACT

Six types of petroleum-producing configurations are recognizable in the oil and gas fields of the Lima-Indiana trend whose reservoirs are the Trenton Limestone of Ordovician age. The data that support recognition of these six structural, structural-stratigraphic, and possibly stratigraphic-permeability trapping configurations are mixed, but involve consideration of the pattern of 34 fields or pools on the main anticlinal trend of the Findlay arch as well as 12 smaller fields or pools to the northwest in the Michigan basin and 20 fields to the southeast at the updip edge of the Appalachian basin. The reservoir is mainly dolomite and the producing portion of the reservoir generally occurs near the top of the Trenton Limestone. The more porous dolomite has been analyzed chemically for Ca/Mg ratios, Na, Sr, Fe, and other elements in cores to supplement petrographic studies off the main oil field trend in Wyandot County. The six play configurations are the following. (1) An anticlinal trap along the crest of the Findlay arch. Here, as elsewhere, the seal and presumably the source are the overlying Utica Shale. (2) A faulted anticlinal trap on the western side of the Findlay arch. The fault, the Bowling Green fault, generally limits production to the upthrown eastern side. (3) An updip facies change from the Trenton Limestone into the overlying Utica End_Page 1917------------------------------ Shale to the south. This change, together with draping of the Utica over the underlying competent Trenton, traps petroleum along the southwestern extension of the Lima-Indiana trend. (4) Fracture systems related to regional fracturing of the early Paleozoic rocks. Apparently these systems have provided fracture-enhanced reservoirs. Similar configurations are well known in the Scipio-Albion trend in southern Michigan. Secondary dolomitization and sulfide mineralization are common in association with these fractured features. (5) Possible porosity-permeability traps, probably on structure, where dolomite is replaced laterally by dense limestone. (6) Small anticlinal terraces off the main arch system, related to minor production. These terraces appear as down-to-the-basin noses on struct ral maps. Considered together, the Lima-Indiana fields are a giant field extending 120 mi (193 km) in Ohio and another 50 mi (80 km) in Indiana. Before 1900, thousands of wells were drilled, as shallow as 1,100 ft (334 m), producing over 220 million bbl of oil at initial rock pressures between 100 and 450 psi (690 and 3,100 kPa). Exploration continues at a modest pace. End_of_Article - Last_Page 1918------------

Basin Analysis and Petroleum Potential of Michigan Basin: Deposition and Subsidence History from Middle Ordovician (Trenton Formation) to Early Devonian: ABSTRACT

The history of the Michigan basin (Early Ordovician to Early Devonian) is that of a nonuniformly subsiding basin, with the Michigan basin, at times, nearly disappearing as either a topographic feature or a depositional center. This history is interpreted from the analysis of lithostratigraphic units, time stratigraphic features, and log formats (term by J. Forgotson). These units are defined for wells throughout the Michigan basin, and they extended eastward into the Appalachian basin. The definition and thickness mapping of these lithostratigraphic units and formats are accomplished using well cuttings, cores, and wire-line geophysical well logs. Furthermore, these units are correlated to both outcrops and stratigraphic type sections for areas adjacent to the Michigan ba in. From these data, it is possible to interpret the major aspects of both the subsidence and depositional history of the basin. During deposition of both the Trenton limestones and Early Silurian carbonates and shales, the Michigan basin behaved as if it were part of the greater Appalachian basin, whereas prior to the deposition of the Trenton (Middle Ordovician) and during Middle and Late Silurian, the Michigan basin was an entity separate from, and with an apparent structural independence of, the greater Appalachian basin. However, even during times of Michigan basin structural independence, the stratigraphic and sedimentologic characteristics of these two basins were closely related. The structure and topography of the Trenton prior to the deposition of the Utica Shale was mapped throughout Michigan to provide insight into the nature of petroleum entrapment in the Trenton formation. The structural entrapment of petroleum in southeast Michigan contrasts with the combination diagenetic to structural Albio-Scipio trend of south-central Michigan. Evidence is available that more of these two types of traps occur in unproducing areas of the Michigan basin. End_of_Article - Last_Page 1925------------

Epigenetic Dolomitization and the Origin of Xenotopic Dolomite Texture

ABSTRACT Xenotopic texture, which is commonly observed in pre-Cenozoic rocks, is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and, usually, undulatory extinction. Xenotopic dolomite texture is similar in appearance to neomorphic limestone textures. Idiotopic dolomite texture (euhedral to subhedral crystals with straight, intercrystalline boundaries) contrasts with xenotopic texture and is commonly observed in both Cenozoic and more ancient dolomites. Texture may be controlled by the at which crystals grow. Crystal growth theory predicts that at low a smooth crystal surface is energetically favored, and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites. This results in faceted crystals and euhedral to subhedral crystal mosaics. Above a critical roughening temperature (CRT), a rough surface is energetically favored, surface nucleation does not require dislocations, and atoms are randomly added to the crystal surface, resulting in nonfaceted growth and an anhedral crystal mosaic. It is hypothesized that a critical roughening temperature exists for dolomite above 50°C. Xenotopic dolomites are produced by dolomitization of limestone and/or neomorphic recrystallization of dolomite at elevated (above the CRT) after burial. Idiotopic dolomites are produced below the CRT by near-surface processes and are occasionally produced above CRT if growth-inhibiting effects of the solution or impurities stabilize crystal faces. Calcite has a CRT below 25°C and, therefore, develops anhedral crystal mosaics (characteristic of neomorphic texture) both at near-surface and at elevated temperatures. Synthetic xenotopic dolomite was produced in the laboratory at 250°C and 300°C by dolomitization of aragonite and calcite skeletal fragments and by recrystallization (neomorphism) of nonstoichiometric Cenozoic dolomites. Xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic Hueco dolomite (Permian, Texas) near the Marble Canyon intrusion, at temperatures between 350°C and 600°C. Hydrothermal dolomitization of periclase-calcite marble near the intrusion also resulted in a xenotopic texture. Xenotopic dolomite in the Galena Group (Ordovician, Wisconsin) was produced by neomorphism of a preexisting dolomite during the emplacement of lead-zinc sulfides at temperatures between 80°C and 227°C. In the Trenton Limestone (Ordovician, Michig n), xenotopic dolomite replaced limestone during the migration of hot (> 50°C) fluids along fracture systems. Xenotopic dolomite was not observed in Cenozoic dolomites that were subjected to temperatures near surface values only.

Trenton Limestone Fracture Reservoirs in Lee County, Southwestern Virginia: ABSTRACT

Exploration for fractured reservoirs in the non-porous biomicrite of the Trenton Limestone has taken place since 1910. Increased drilling since 1981 has doubled production from this structurally complex area of the Eastern Overthrust.