Lower Ordovician Carbonate Rocks Research Articles

Critical Condition of the Depth Limit of Oil Accumulation of Carbonate Reservoirs and Its Exploration Significance in the Lower Ordovician of the Tazhong Area in the Tarim Basin

Critical Condition of the Depth Limit of Oil Accumulation of Carbonate Reservoirs and Its Exploration Significance in the Lower Ordovician of the Tazhong Area in the Tarim Basin

Dolomitization and its impact on porosity development and preservation in the deeply burial Lower Ordovician carbonate rocks of Tarim Basin, NW China

The deeply buried dolostones (>6000 m) of the Lower Ordovician Penglaiba Formation are currently the key targets of hydrocarbon exploration in the Tarim Basin. Petrographic, isotopic, and fluid inclusion microthermometric data were obtained in an attempt to determine the origin of dolostones and its effect on porosity development. Based on the petrography examination, three matrix dolomites and two dolomite cements were identified: very finely to finely crystalline, nonplanar-a to planar-s (D1) dolomite; finely to medium crystalline, planar-s to planar-e (D2) dolomite; medium to coarse crystalline, nonplanar-a (D3) dolomite; medium to coarse crystalline dolomite cements (Cd and Sd). D1 dolomite, which commonly occurs widely distributed throughout the study area, is characterized with δ13CVPDB values of −2.4 to −0.1‰, δ18OVPDB values of −8.1 to −4.1‰, and 87Sr/86Sr ratios of 0.70899–0.70979, inferring the seawater origin. D2 dolomite, merely confined to one side of the stylolite, has δ13CVPDB values of −1.8 to −0.4‰, δ18OVPDB values of −8.3 to −3.7‰, and 87Sr/86Sr ratios of 0.70871–0.70955, suggesting that D2 is precipitated in shallow-middle burial conditions and the dolomitizing fluids is derived from the residual primitive seawater in the formation. D3 dolomite is considered to be formed at higher temperatures under deep burial conditions considering its crystal textures and the δ18OVPDB values of −9.0 to −6.8‰. The two dolomite cements, Cd and Sd dolomites, are characterized with the inclusion Th values of 119–163 °C and 87Sr/86Sr ratios of 0.70941–0.70993, inferring the hydrothermal origin. Based on the distribution, petrography and geochemistry data of D1, the mainly hydrological system for dolomitization in the Penglaiba Formation is interpreted to be the seepage-reflux model. Intercrystalline pores and vugs are two types of pores associated with dolomitization, the latter being the most dominant type that is related to hydrothermal dolomitization.

The lithology and reservoir properties of the Lower Ordovician Majiagou formation at the Tabamiao gas field (Ordos Basin, China)

The Ordos Basin is known for rich natural gas, coal, and oil fields; it is an important mineral raw material and electric power base of China. Among the recently discovered gas fields within the Yimeng Monocline, the Sulige and Tabamiao gas fields, where gas reserves are found in Lower Ordovician carbonate rocks, are best known. We have identified five types of gas-reservoir rocks, which are confined to the ancient weathering crust of Ordovician carbonate deposits.

Hydrothermal Fluid Sources of the Fengjia Barite-fluorite Deposit in Southeast Sichuan, China: Evidence from Fluid Inclusions and Hydrogen and Oxygen Isotopes

The Fengjia barite–fluorite deposit in southeast Sichuan is a stratabound ore deposit which occurs mainly in Lower Ordovician carbonate rocks. Here we present results from fluid inclusion and oxygen and hydrogen isotope studies to determine the nature and origin of the hydrothermal fluids that generated the deposit. The temperature of the ore-forming fluid shows a range of 86 to 302 °C. Our detailed microthermometric data show that the temperature during mineralization of the fluorite and barite in the early ore-forming stage was higher than that during the formation of the calcite in the late ore-forming stage. The salinity varied substantially from 0.18% to 21.19% NaCl eqv., whereas the density was around 1.00 g/cm3. The fluid composition was mainly H2O (>91.33%), followed by CO2, CH4 and traces of C2H6, CO, Ar, and H2S. The dominant cation was Na+ and the dominant anion Cl-, followed by Ca2+, SO42-, K+, and Mg2+, indicating a mid–low-temperature, mid-low-salinity, low-density NaCl–H2O system. Our results demonstrate that the temperature decreased during the ore-forming process and the fluid system changed from a closed reducing environment to an open oxidizing environment. The hydrogen and oxygen isotope data demonstrate that the hydrothermal fluids in the study area had multiple sources, primarily formation water, as well as meteoric water and metamorphic water. Combined with the geological setting and mineralization features we infer that the stratabound barite–fluorite deposits originated from mid–low-temperature hydrothermal fluids and formed vein filling in the fault zone.

Fractured - Vuggy Genesis Model of Middle - Lower Ordovician Carbonate Rocks in Tahe Oilfield of Trim Basin, Western China

The fracturedvuggy characteristics of carbonate rocks of Middle Lower Ordovician limestone in Tahe Oilfield was studied in the article. By analysis of the affecting factors of reservoir space include sedimentary diagenesis, tectonism and karstification, we found deposition determined the Formation and the number of primary porosity, diagenesis affected the pore destruction and construction, tectonism controled the reservoir burial history and the Formation of structure cracks and weathering fissure, karstification affected the latter change of the previous diagenetic porosity and remodeled the large hole-cave reservoir in Tahe Oilfield. Then the fissure-cave porosity evolution is studied. And we found that the Lower or dovician carbonate rocks in Tahe area experienced shallow marine diagenetic environment, shallow surface meteoric diagenetic environment, early shallow burial diagenetic environment, uplift exposed supergene meteoric diagenetic environment, shallow deep burial diagenetic environment. As a result, the Tahe Oilfield Ordovician fissure-cave porosity evolution model is established.

ABSTRACT: Characteristics and evolutional models of karst reservoirs of Lower Ordovician carbonate rocks in Lunnan area of Tarim Basin China

ABSTRACT: Characteristics and evolutional models of karst reservoirs of Lower Ordovician carbonate rocks in Lunnan area of Tarim Basin China

Lead isotope geochemistry of mississippi valley-type deposits of the Southern Appalachians

Lead isotope compositions of sphalerite, fluorite, barite and sparry dolomite in Mississippi Valley-type deposits of the southern Appalachians fall into four different isotopic groups. Most sphalerite forms two small clusters, whereas barite, fluorite, and sparry dolomite compositions are more scattered. These lead isotope compositions reflect the distribution of the deposits, which are hosted by two main palcoaquifers. Lower Cambrian clastic and carbonate rocks host the Austinville (Zn-Pb), Cartersville (Ba), Embreeville (Zn), and Doughtery (Zn) deposits, all of which fall in a cluster of lead isotope compositions near 207 Pb/ 204 Pb = 15.73 and 206 Pb/ 204 Pb = 19.45. Lower Ordovician carbonate rocks of the Knox Group host the Copper Ridge (Zn), Eve Mills (Zn), Greeneville-Fall Branch (Ba-F), Mascot-Jefferson City (Zn), Powell River (Zn-Pb), Russell-Tazewell (Ba-F), and Sweetwater (Ba-Zn-F) deposits, which have more complex compositions. Sphalerite from Mascot-Jefferson City, Eve Mills, Sweetwater, and Russell-Tazewell plots in a cluster around 207 Pb/ 204 Pb = 15.67 and 206 Pb/ 204 Pb = 19.0, whereas that from Copper Ridge plots in both clusters. Fluorite from Sweetwater falls on a long array in 207 Pb/ 204 Pb- 206 Pb/ 204 Pb space that has a two-stage age of 1.55 to 1.9 Ga for t 2 = 360 Ma, the previously published 87 Sr/ 86 Sr isochron age obtained for the Mascot-Jefferson City district. Barite and sparry dolomite compositions are relatively similar and have lower 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios than most sphalerite and fluorite. The spatial distribution of lead isotope compositions indicates that there were at least two mineralizing solutions, represented by the two lead clusters; the upper cluster solution dominated the Lower Ordovician palcoaquifer whereas the lower cluster dominated the Lower Cambrian palcoaquifer; and that the two solutions mixed in most areas except the Mascot-Jefferson City district, where zinc mineralization is strongest. Although the specific sources of these leads cannot be determined from available data on possible source rocks, the two lead clusters probably came from relatively young, well-mixed sedimentary basins and the lead in the fluorite must have come from a source with an unusally high U/Th ratio such as phosphoritc. This study confirms that the isotopic composition of lead in southern Appalachian Mississippi Valley-type deposits differs significantly from that found in mid-continent deposits.

Timing of Mississippi Valley-type mineralization: Relation to Appalachian orogenic events

Although Mississippi Valley-type deposits in Lower Ordovician carbonate rocks of the Appalachian orogen are commonly interpreted to have been precipitated by basinal brines, the timing of brine migration remains poorly known. Late Paleozoic K-Ar isotopic ages on authigenic K-feldspar, which is widespread in Appalachian carbonate rocks, as well as evidence of paleomagnetic overprints of similar age, have focused attention on the possibility that these Mississippi Valley-type deposits formed as a result of late Paleozoic deformation. Geologic and geochemical similarities among most of these deposits, from Georgia to Newfoundland, including unusually high sphalerite/galena ratios, isotopically heavy sulfur, and relatively nonradiogenic lead, suggest that they are coeval. Sphalerite sand that parallels host-rock layering in many of the deposits indicates that mineralization occurred before regional deformation. Although the late Paleozoic age of deformation in the southern Appalachians provides little constraint on the age of Mississippi Valley-type mineralization, deformation of these deposits in the Newfoundland Appalachians is early to middle Paleozoic in age. Thus, if Ordovician-hosted, Appalachian Mississippi Valley-type deposits are coeval, they must have formed by middle Paleozoic time and cannot be the product of a late Paleozoic fluid-expulsion event. This hypothesis has important implications for basin evolution, fluid events, and remagnetization inmore » the Appalachians.« less

Lower Paleozoic Carbonate Rocks of Baird Mountains Quadrangle, Alaska: ABSTRACT

Lower Paleozoic carbonate rocks in the Baird Mountains quadrangle form a relatively thin (about 550 m), chiefly shallow-water succession that has been imbricately thrust and metamorphosed to lower greenschist facies. Middle and Upper Cambrian rocks--the first reported from the western Brooks Range--occur in the northeastern quarter of the quadrangle, south of Angayukaqsraq (formerly Hub) Mountain. They consist of marble grading upward into thin-bedded marble/dolostone couplets and contain pelagiellid mollusks, acotretid brachiopods, and agnostid trilobites. Sedimentologic features and the Pelagiellas indicate a shallow-water depositional environment. Overlying these rocks are Lower and Middle Ordovician marble and phyllite containing graptolites and conodonts of midshelf o basinal aspect. Upper Ordovician rocks in this area are bioturbated to laminated dolostone containing warm, shallow-water conodonts. In the Omar and Squirrel Rivers areas to the west, the Lower Ordovician carbonate rocks show striking differences in lithofacies, biofacies, and thickness. Here they are mainly dolostone with locally well-developed fenestral fabric and evaporite molds, and bioturbated to laminated orange- and gray-weathering dolomitic marble. Upper Silurian dolostone, found near Angayukaqsraq Mountain and on the central Squirrel River, contains locally abundant corals and stromatoporoids. Devonian carbonate rocks are widely distributed in the Baird Mountains quadrangle; at least two distinct sequences have been identified. In the Omar area, Lower and Middle Devonian dolostone and marble are locally cherty and rich in megafossils. In the north-central (Nakolik River) area, Middle and Upper Devonian marble is interlayered End_Page 662------------------------------ with planar to cross-laminated quartz-carbonate metasandstone and phyllite. End_of_Article - Last_Page 663------------

ORDOVICIAN LIMESTONES IN THE WESTERN ANTICLINES OF THE APPALACHIANS IN WEST VIRGINIA AND VIRGINIA NORTHEAST OF THE NEW RIVER

Middle Ordovician limestones of the Chazyan, Bolarian, and Trentonian series are exposed in the western anticlines in the Appalacian Mountains of West Virginia and in Virginia northeast of the New River. The term Western Anticlines is applied to those cove-breeched structures which contrast with the dominantly homoclinal, fault-bounded slices west of the New River. The Chazyan comprises the Lurich and Lincolnshire formations. The Lurich has a persisting calcilutite, the Five Oaks, at its top; in the southwest it has a middle member, the Elway, a cherty limestone, overlying a lower, Blackford, member containing chert-pebble conglomerate. The Lincolnshire limestone has disconformities above and below, thins to local extinction, and is overlapped by younger beds northward in Pennsylvania. The Bolarian rocks are in two groups. The lower has three limestone formations, Ward Cove, Peery, and Benbolt; the Peery persists as a cherty datum. The Ward Cove varies in lithic sequence and thins toward the northwest, probably by overlap and convergence. The Benbolt also has changing facies but is of more constant thickness. The upper Bolarian is of two newly named units, McGlone and McGraw limestones; the latter is particularly distinctive and disappears southeastward, probably beneath a regional unconformity. Trentonian rocks lie with disconformity on the Bolarian. In the southwest the lower Trentonian is dominantly Moccasin reddish argillite; northeastward it changes into fossiliferous Nealmont limestone of a facies like that in Pennsylvania. In the southwest the Eggleston argillaceous shale has a basal quartz sandstone, and in the northwest it passes into the Onego argillaceous limestone which resembles the Oranda limestone of the Appalachian Valley. Volcanic ash beds persist in all three. The younger Trentonian has not been successfully divided and traced and is called the “Martinsburg,” though in the northeastern exposures it is essentially like the Salona limestone of Pennsylvania. An erosion surface of low relief bevelled the lower Ordovician carbonate rocks. The Chazyan Lurich carbonate rocks blanketed the Western Anticlines and areas to the southeast, and the Lincolnshire covered most of the outcrop area in the Virginias. Differential deformation increased in Bolarian, for lower units thicken southeastward and become argillaceous and quartz-arenaceous. During late Bolarian there was gentle northwest tilting. In the south sources to the southeast were rejuvenated in early Trentonian, producing reddish clays and silts that filled the margins of an adjoining trench. In later Trentonian diastrophism was moderate, but calcareous facies in the Western Anticlines grade southeastward into argillaceous sediments. Structural trends are similar to those in Pennsylvania and New York, although the tectonic events that most strongly influenced sedimentation were not synchronous along the whole Appalachian belt. Stable periods were interrupted by deformation of belts to the southeast.