Paleozoic Zone Research Articles

Fluid migration paths in the marine strata of typical structures in the western Hubei-eastern Chongqing area, China

The western Hubei-eastern Chongqing area is an important prospective zone for oil and gas exploration in the central Yangtze area. Three representative structures, the Xinchang structure, Longjuba gas-bearing structure and the Jiannan gas field, were selected to analyze biomarker parameters in marine strata and to examine various types of natural gas and hydrocarbon sources. Fluid inclusions; carbon, oxygen, and strontium isotopic characteristics; organic geochemical analysis and simulation of hydrocarbon generation and expulsion history of source rocks were used for tracing fluid migration paths in marine strata of the study area. The Carboniferous-Triassic reservoirs in three typical structures all experienced at least two stages of fluid accumulation. All marine strata above the early Permian were shown to have fluids originating in the Permian rocks, which differed from the late stage fluids. The fluids accumulated in the late Permian reservoirs of the Xinchang structure were Cambrian fluids, while those in the late Carboniferous reservoirs were sourced from a combination of Silurian and Cambrian fluids. A long-distance and large-scale cross-formational flow of fluids destroyed the preservation conditions of earlier accumulated hydrocarbons. A short-distance cross-formational accumulation of Silurian fluids was shown in the late Permian reservoirs of the Longjuba structure with favorable hydrocarbon preservation conditions. The fluid accumulation in the Carboniferous reservoirs of the Jiannan structure mainly originated from neighboring Silurian strata with a small amount from the Cambrian strata. As a result, the Jiannan structure was determined to have the best preservation conditions of the three. Comparative analysis of fluid migration paths in the three structures revealed that the zone with a weaker late tectonism and no superimposition and modification of the Upper and Lower Paleozoic fluids or the Upper Paleozoic zone with the fluid charging from the Lower Paleozoic in the western Hubei-eastern Chongqing area are important target areas for future exploration.

A Paleozoic subduction complex in Korea: SHRIMP zircon U–Pb ages and tectonic implications

The Wolhyeonri complex in the southwestern margin of the Korean Peninsula is divided into three lithotectonic units: Late Paleozoic Zone I to the west, Middle Paleozoic Zone II in the middle and Early Paleozoic Zone III to the east. Zones II and III display characteristics of continental arc magmatic sequence. Zone II is dominated by mafic metavolcanics, whereas zone III is characterized by the presence of dismembered serpentinite bodies including chaotic mélange. These zones are proposed to have been formed in a convergent margin setting associated with subduction. Here we present zircon SHRIMP U–Pb ages from the various units within the Wolhyeonri complex which reveal the Paleozoic tectonic history of the region. The Late Carboniferous ages obtained from the main shear zone between the Wolhyeonri complex and the Paleoproterozoic Gyeonggi massif are thought to mark the timing of continental arc magmatism associated with the subduction process. In contrast, Zone I with Neoproterozoic arc magmatic remnants might indicate deposition in a forearc basin. The Wolhyeonri complex also preserves strong imprints of the Triassic collisional event, including the presence of Middle Triassic high-pressure metabasites and eclogites near the eastern boundary of the Zone III. These range of radiogenic ages derived from the Wolhyeonri complex correlate well with subduction and accretion history between the North and South China cratons. Similar geochronological features have also been indentified from the Qinling, Tongbai–Xinxian, and northern Dabie areas in east-central China. The existence of Paleozoic coeval subduction in East Asia prior to the Triassic collision is broadly consistent with a regional tectonic linkage to Gondwana.

Late Paleozoic faults of the Altai region, Central Asia: tectonic pattern and model of formation

The present kinematic and dynamic analysis of large-scale strike-slip faults, which enabled the formation of a collage of Altai terranes as a result of two collisional events. The Late Devonian–Early Carboniferous collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent resulted in the formation of the Charysh–Terekta system of dextral strike-slip faults and later the Kurai and Kuznetsk–Teletsk–Bashkauss sinistral strike-slip faults. The Late Carboniferous–Permian collision of the Siberian and Kazakhstan continents resulted in the formation of the Chara, Irtysh and North-East sinistral strike-slip zones. The age of deformation of both collisional events becomes younger toward the inner areas of the Siberian continent. In the same direction the amount of displacement of strike-slip faulting decreases from several thousand to several hundred kilometers. The width of the Late Paleozoic zone of deformation reaches 1500 km. These events deformed the accretion-collision continental margins and their primary paleogeographic pattern.

The Gondwanan Ordovician-Silurian Terranes in the Structure of the Altai-Sayan Folded Area and Timing of Their Accretion Towards the Margin of the Siberia Continent

The present kinematic and dynamic analysis of large-scale strike-slip faults, which enabled the formation of a collage of Altai terranes as a result of two collisional events. The Late Devonian–Early Carboniferous collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent resulted in the formation of the Charysh–Terekta system of dextral strike-slip faults and later the Kurai and Kuznetsk–Teletsk–Bashkauss sinistral strike-slip faults. The Late Carboniferous–Permian collision of the Siberian and Kazakhstan continents resulted in the formation of the Chara, Irtysh and North-East sinistral strike-slip zones. The age of deformation of both collisional events becomes younger toward the inner areas of the Siberian continent. In the same direction the amount of displacement of strike-slip faulting decreases from several thousand to several hundred kilometers. The width of the Late Paleozoic zone of deformation reaches 1500 km. These events deformed the accretion-collision continental margins and their primary paleogeographic pattern.

Comparison of Resource Assessment Methods and Geologic Controls-- Deep Natural Gas Plays and Zones, United States and Russia: ABSTRACT

Deep (greater than 4.5 km--15,000 ft) conventional natural gas resources will play an important role in the future energy needs of the United States and Russia. Deep sedimentary basins are widespread in these countries and have formed in a variety of depositional and tectonic settings. Significant volumes of undiscovered deep natural gas are in the Gulf Coast, Anadarko, Permian, and Rocky Mountain basins of the U.S., and in the Timan-Pechora, West Siberia, East Siberia, and North and South Caspian basins of the former Soviet Union. Deep natural gas resources are regularly assessed by the All-Russia Petroleum Research Exploration Institute (VNIGRI) and the U.S. Geological Survey (USGS) as part of their normal research activities. Both VNIGRI and the USGS employ similar assessment methods involving play (or zone) analysis using geological data and based on an analysis of confirmed and hypothetical plays using field-size distributions, discovery-process models, and statistical estimation procedures that yield probabilistic estimates of undiscovered accumulations. Resource estimates for the deep structural and statigraphic plays of the Anadarko basin and deep Paleozoic zones in the Timan-Pechora basin are compared and contrasted using both methods. Differences in results of assessments between VNIGRI and USGS arise due to (1) the way inmore » which plays/zones are defined, (2) different geochemical models for hydrocarbon generation as applied to hypothetical plays, (3) variations in the ways in which statistical estimation procedures are applied to plays and regions, and (4) differences in economic and technologic assumptions, reserve growth calculations, and accumulation size limits and ranges.« less

Tectonic evolution of the West Junggar Region, Xinjiang, China

The West Junggar region of northwestern China consists of a Hercynian‐age fold belt occupying a late Paleozoic zone of convergence between three major Eurasian plates: Siberian, Tarim, and Kazakhstan. The northern part of the West Junggar area includes part of the Irtysh‐Zaysan fold belt that extends to the northwest into the USSR, where it marks a broad boundary between the Siberian and Kazakhstan plates. The central part of the West Junggar area includes the southeastern extension of the Chingiz‐Tarbgatay early Paleozoic island arc sequence. The southern part of the West Junggar area is truncated by the deep fundamental Junggar‐Alakol fault that has brought the Yili microplate into juxtaposiition with the West Junggar region in the vicinity of Ebin Nur Lake. To the east the Junggar area is covered by sediments of the Junggar Basin. Paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the Middle Carboniferous, and by Late Carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates. Paleomagnetic data reveals that the plates continued to move along strike‐slip faults, bringing the Tarim plate and Yili microplate into their present positions sometime in the early Mesozoic. Six separate ophiolite occurrences in the West Junggar region are strongly tectonized and dismembered. The Tangbale ophiolite is considered to be Late Cambrian (508 ± 20 Ma.) and represents the earliest known oceanic crust in Western China. Petrologic and chemical data indicate that the Tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc. The Darbut ophiolite has been tectonized to a melange and is overlain by Late Devonian flysch. Ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the Middle Devonian. Other occurrences of ophiolite melange in the northern part of the West Junggar region may be as young as Late Devonian. These ophiolites record ocean crust formation in the Early to Middle Paleozoic within the Junggarian ocean, and their present tectonic setting indicates continental accretion and imbrication shortly after their formation. Associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by B‐type subduction and associated obduction. Postcollisional alkali granites intrude the oceanic sequences of the West Junggar Region and are considered to represent melted lower crustal material. Lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned. The implications of this finding is that the Junggar Basin basement rocks in the north and west are probably oceanic. Evidence presented in this study shows a complex history for the development of the West Junggar region. Major movements of consolidating plates in the Late Paleozoic followed by rejuvenation of these same zones of closure during the Himalayan tectonic event of the Late Tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity.

Background radioactivity of soil in Wakasa Bay area (Southern Fukui Prefecture)

Soil samples of surface layer were collected in Wakasa Bay area which is composed of various geologic properties. Concentrations of background radionuclides in soil samples were determined by γ-ray spectrometry with a coaxial type and a planer type of Ge(Li) detectors. The spectra of soil samples obtained by X-ray diffraction analysis show the characteristic composition of minerals based on the geologic properties of their mother's rocks. The concentrations of 238U, 226Ra, 228Ra and 40K in soil roughly correspond to the geologic properties. The highest concentrations of these four nuclides are the soil collected from granitic zone and next comes the soil from paleozoic zone. For homologous soil from granite and paleozoic the 40K contents decrease with increasing surface area, respectively. A good correlation between the 137Cs contents and the unsupported 210Pb contents was found in the soil samples of surface layer.

Williston Basin--Reawakening of Giant Petroleum Province: ABSTRACT

The Williston basin, covering parts of Montana, North Dakota, and South Dakota, is one of the largest and most active petroleum provinces in the Rocky Mountains. Since the discovery of gas on Cedar Creek anticline in 1913 and oil in 1950, the basin has been the scene of several periods of intense drilling activity, the most recent beginning in the mid-1970s and continuing to the present. Early exploration in the basin centered around structural features with surface expression, such as Cedar Creek anticline, and shallow objective zones. As deeper Paleozoic zones were proven productive on these structures, seismic techniques became essential for defining structural leads with no surface expression. More detailed seismic data and more sophisticated processing techniques wer necessary to define deeper and more subtle structures in remote parts of the basin. Existing technology was rapidly adapted by the petroleum industry to explore for these deeper traps. Although a mature basin by some standards, the Williston basin still contains vast areas that are virtually unexplored. Intense competition by the industry and escalating land acquisition costs indicate that these parts of the basin will be the center of exploration activity in the future. The reserve potential of these areas should be at least as large as the proven reserves in the basin to date. Of equal importance is the exploitation of the more mature parts of the basin, including new pay zones. Future exploration in the Williston basin must incorporate new exploration techniques and new exploration philosophies in the evaluation of new pay zones and unexplored parts of the basin. End_of_Article - Last_Page 723------------