Cambrian-Ordovician Carbonate Research Articles

Types, structural evolution difference and petroleum geological significance of Cambrian–Ordovician carbonate platforms in Gucheng–Xiaotang area, Tarim Basin, NW China

Based on the latest drilling cores, thin sections, and 3D seismic data, types, features, and evolution processes of Cambrian–Ordovician carbonate platforms in the Gucheng and Xiaotang areas are studied, and favorable exploration zones in this area are also discussed. There are two types of carbonate platforms developed in the Cambrian–Ordovician in the Gucheng–Xiaotang area, namely, carbonate ramp and rimmed platforms, and the evolution process of the platform in the Gucheng area is different from that in Xiaotang area. In the Early Cambrian, the study area was a homoclinal carbonate ramp. During the Middle to Late Cambrian, it evolved into a rimmed platform, with 5 phases of mound shoals developed. In the southern area, mound shoals were in progradational arrangement and the third and fourth stages of mound shoals suffering exposure and further developed abundant karst vugs. In the northern area, the mound shoals appeared in a superposition of aggradation–weak progradation, the third, fourth, and fifth stages of mound shoals suffered exposure and dissolution, and the platform slope developed gravity flow deposits. In the Early to Middle Ordovician, the southern area gradually evolved into a distally steepened carbonate ramp, where retrogradational dolomitic shoal developed; while the northern part experienced an evolution process from a weakly rimmed platform to a distally steepened carbonate ramp, and developed two or three stages of retrogradational mound shoals. The high-frequency oscillation of sea level and local exposure and dissolution were beneficial to the formation of mound or shoal reservoirs in platform margin and ramp, and the configuration of these reservoirs with low energy slope–basin facies source rocks could form good oil-gas enrichment zones. The dolomitic shoal in the Ordovician platform ramp is the practical exploration field for increasing reserve and production in the Gucheng area. The mound shoal at the Cambrian rimmed platform margin is the key exploration object in the Xiaotang area. In addition, the Cambrian slope gravity flow deposits can be taken as the favorable exploration fields in the study area.

Multi-scale evaluation of fractured carbonate reservoir and its implication to sweet-spot optimization: A case study of Tazhong oilfield, Central Tarim Basin, China

Ordovician limestone and Cambrian dolomite are the main oil and gas producing layers in eastern Tazhong field, Tarim basin. Industrial oil flow has been obtained from drilling in this zone, proving that the Lower Paleozoic strata is important for oil and gas exploration and development. However, the main controlling factors and distribution of high-quality reservoirs remain unclear. Based on the logging constrained seismic inversion , FMI (Fullbore Formation MicroImager) log interpretation and core analysis of six wells, the main controlling factors of Paleozoic carbonate reservoirs in the study area were investigated. The results reveal that the two buried cuestas control the distribution of the Cambrian and Ordovician strata, the ratio of mudstone in north buried cuesta (NBC) is more than that in south buried cuesta (SBC), and the average porosity of ▪3xq dolomite formation is more than that of O1−2y limestone formation. FMI logging interpretation results show that fracture density of well TZ-1 is significantly higher than that of well TZ-2, combined with core observation and microscopic analysis we can believe that reconstructed dolomite reservoir has developed dissolution pores and vugs. By the means of seismic impedance reconstruction, the most favorable production areas, so-called sweet-spots, are karst residual mounds that formed independent small anticlinal high points and have been depicted. Finally, total 28 traps in the Cambrian–Ordovician​ carbonate buried hill are characterized, covering an area of 66.4 km2. 17 anticline traps with an area of 26.9 km2 in dolomite reservoirs are optimized to be the next exploration targets.

REE and Sr–Nd Isotope Characteristics of Cambrian–Ordovician Carbonate in Taebaek and Jeongseon Area, Korea

Carbonate formations of the Cambro-Ordovician Period occur in the Taebaek and Jeongseon areas, located in the central–eastern part of the Korean Peninsula. This study analyzed the rare earth element (REE) contents and Sr–Nd isotope ratios in these carbonates to elucidate their depositional environment and diagenetic history. The CI chondrite-normalized REE patterns of the carbonates showed negative Eu anomalies (EuN/(SmN × GdN)1/2 = 0.50 to 0.81), but no Ce anomaly (Ce/Ce* = CeN/(LaN2 × NdN)1/3 = 1.01 ± 0.06). The plot of log (Ce/Ce*) against sea water depth indicates that the carbonates were deposited in a shallow-marine environment such as a platform margin. The 87Sr/86Sr ratios in both Taebaek and Jeongseon carbonates were higher than those in the seawater at the corresponding geological time. The 87Sr/86Sr ratios and the values of (La/Yb)N and (La/Sm)N suggest that the carbonates in the areas experienced diagenetic processes several times. Their 143Nd/144Nd ratios varied from 0.511841 to 0.511980. The low εNd values and high 87Sr/86Sr ratios in the carbonates may have resulted from the interaction with the hydrothermal fluid derived from the intrusive granite during the Cretaceous Period.

The origins of carbonate minerals of a source-controlled lacustrine carbonate succession in the Shulu sag, Bohai Bay Basin: Implications for porosity development and paleoenvironment

Lacustrine carbonates are widely distributed in the third member of the Eocene Shahejie Formation (Es3L) of the Shulu sag and other rift basins in the Bohai Bay Basin, which can act as reservoirs for hydrocarbon and provide insights into paleoenvironment. However, the origins of lacustrine carbonate deposition and their implications for porosity development and paleoenvironment are not well understood. Based on micropetrological characteristics, X-ray diffraction (XRD), stable carbon and oxygen isotope analyses, and organic geochemistry data, four types of calcite (calcite debris; calcareous biological detritus; micrite in fine-grained marlstone; fibrous sparry calcite) and two types of dolomite (dolomite debris; rhombic euhedral dolomite) with distinctly different characteristics and origins are identified. The δ13C and δ18O values of calcite and dolomite debris are consistent with that of the Cambrian-Ordovician carbonate rock in the provenance area, suggesting that the carbonate debris was sourced from the surrounding Cambrian-Ordovician carbonate bedrock. Microcrystalline calcites are commonly developed in organic-rich marlstones with a large number of algae. The δ13C values of the microcrystalline calcites are higher than that of carbonate debris and the positive excursion of the δ13C value is along with the increase of TOC content. These indicate that origin of the microcrystalline calcite was authigenic calcite and was formed by biochemical process. Bedding-parallel fibrous sparry calcite occurs in intervals with high TOC contents and within the thermally mature zone of hydrocarbon generation, indicating that oil generation should be responsible for the sparry calcite formation. Rhombic euhedral dolomite commonly occurs as isolated micro-crystals with euhedral to subhedral rhombs shape. Micro-sized spherule-like dolomite in the early stage of crystal growth were also present. Rhombic euhedral dolomite are commonly associated with organic matter and developed in organic-matter rich intervals with plenty of fossilized bacterial filaments and framboidal pyrite. These suggest that rhombic euhedral dolomite were probably microbially mediated dolomite. Mineral components can affect reservoirs quality. The porosities of samples show a positive correlation with dolomite content and the porosities of fibrous sparry calcite-bearing intervals are higher than non-parry calcite-bearing intervals. The increase in authigenic microcrystalline calcite content reflects a rise in lake level and an increase in water depth and primitive productivity.

Researching Candidate Sites for a Carbon Storage Complex in the Central Appalachian Basin, USA

The Central Appalachian Basin-Carbon Storage Assurance Facility Enterprise (CarbonSAFE) (CAB-CS) Integrated Prefeasibility Project identified several selected areas in eastern Ohio where the Cambrian-Ordovician age sandstones and carbonates show the most promising reservoir potential for carbon capture and storage (CCS). These selected areas are collocated near depleted oil and gas fields where oil recovery could be improved with enhanced oil recovery. This region has many large carbon dioxide (CO2) point sources that represent a diverse array of industries, including coal-fired power plants, natural gas processing, refineries, chemical plants, and natural gas power. In a carbon-constrained future, one of the most promising ways to make coal a viable fuel option for power generation or industrial processes is to capture and geologically store CO2 emissions. The project is a pre-feasibility study to mitigate the emissions from 1.67 million metric tons of CO2 per year by developing a storage complex capable of storing 50 million metric tons. The project team analyzed six source-sink scenarios in the CAB-CS study area with analysis of capture and transport; defined infrastructure needs for a CCS project; and conducted an economic, liability, and social assessment. Major accomplishments include experience in investigating CCS projects in the area (learning by doing), adding to National Energy Technology Laboratory best practices tools and tool sets, and building the elements of the CCS road map for the region.

Lithofacies and reservoir characterization of a source-controlled carbonate succession in a lacustrine rift basin, the Shulu Sag of Bohai Bay Basin, East China

The lower part of the third member of the Eocene Shahejie Formation (Es3L) is one of the most important petroleum exploration targets in the Bohai Bay Basin. Source-controlled carbonate deposits are widely distributed in the Es3L unit of the Shulu Sag and display strong vertical and horizontal variations in lithofacies, geochemical, and reservoir characteristics. Based on core data, thin-section petrography, stable δ13C isotope analyses, and organic geochemistry data, eight lithofacies with distinctly different characteristics and origins are identified. Clast-supported terrigenous carbonate rudstone and matrix-supported terrigenous carbonate rudstone were sourced from the surrounding Cambrian-Ordovician carbonate bedrocks and are widely distributed at the edge of the sag in fan delta environments. Some of the rudstones were transported from the littoral zone to profundal zone by slide and slump processes. Mixed-source rudstone was mainly deposited by sandy debris flow. Carbonate lithic sandstone and carbonate lithic siltstone were mainly formed by turbidity current deposition in a sublacustrine setting. Irregular-laminated marlstone was mainly formed by muddy turbidity currents, and massive marlstone was formed by turbid clouds in a relatively turbulent deep water environment. Regular-laminated marlstone is thought formed by seasonal laminae due to seasonal variations by suspension settling in the anoxic deep, quiet lake environment. Calcareous mudstone possibly formed by shallow lake gravity flow in a shallow lake environment. Various of pores have been identified in the Es3L rocks: pores and cracks inside gravels, seams around gravels, intergranular pores, intercrystalline pores, intragranular pores, dissolution pores, organic matter pores, fractures and fracture-related pores. Different lithofacies have different space types, porosities and permeability. Clast-supported terrigenous carbonate rudstone has the best reservoir qualities, with the highest porosity and permeability. Laminated marlstone is the second best reservoir type. These two lithofacies can be the target for tight oil exploration in the study area.

A study of igneous rocks related to Zn–Pb mineralization in the Shinyemi and Gagok deposits of the Taebaeksan Basin, South Korea

AbstractThe Shinyemi and Gagok deposits, located in the Taebaeksan Basin, South Korea, display Zn–Pb mineralization along a contact between Cretaceous granitoids and Cambrian–Ordovician carbonates of the Joseon Supergroup. The Shinyemi mine is one of the largest polymetallic skarn‐type magnetite deposits in South Korea and comprises Fe and Fe–Mo–Zn skarns, and Zn–Cu–Pb replacement deposits. Both deposits yield similar Cretaceous mineralization ages, and granitoids associated with the two deposits displaying similar mineral textures and compositions, are highly evolved, and were emplaced at a shallow depth. They are classified as calc‐alkaline, I‐type granites (magnetite series) and were formed in a volcanic arc. Compositional variation is less in the Shinyemi granites and aplites (e.g., SiO2 = 74.4–76.6 wt% and 74.4–75.1 wt%, respectively) than in the Gagok granites and aplites (e.g., SiO2 = 65.6–68.0 wt% and 74.9–76.5 wt%, respectively). Furthermore, SiO2 vs K/Rb and SiO2 vs Rb/Sr diagrams indicate that the Shinyemi granitoids are more evolved than the Gagok granitoids. Shinyemi granitoids had been already differentiated highly in deep depth and then intruded into shallow depth, so both granite and aplite show the highly evolved similar chemical compositions. Whereas, less differentiated Gagok granitoids were separated into two phases in the last stage at shallow depth, so granite and aplite show different compositions. The amounts of granites and aplite are similar in the Shinyemi deposit, whereas the aplite appears in an amount less than the granite in the Gagok deposit. For this reason, the Shinyemi granitoids caused not only Fe enrichment during formation of the dolomite‐hosted magnesian skarn but also was associated with Mo mineralization in the Shinyemi deposit. Zn mineralization of the Gagok deposit was mainly caused by granite rather than aplite. Our data suggest that the variation in mineralization displayed by the two deposits resulted from differences in the compositions of their associated igneous intrusions.

Pore system characterization of Cambrian‐Ordovician carbonates using a new mercury porosimetry‐based petrofacies classification system: application to carbon sequestration reservoirs

AbstractTo better understand injection and post‐injection flow processes and the entrapment of supercritical CO2 during geological carbon sequestration in a carbonate reservoir, the pore system was analyzed in 66 Cambrian‐Ordovician carbonate samples from several locations in the midwestern USA. This work employed standard microphotography from thin sections, helium porosimetry for porosity and permeability, and mercury injection capillary pressure analysis, aiming to understand which elements of the pore system dominantly control the overall flow and CO2 storage potential in the subsurface. In particular, mercury injection capillary pressure analysis has been fundamental in understanding several petrophysical properties of rocks, including porosity, permeability, and the pore‐size distribution of the samples under study. This work analyzes mercury injection capillary pressure data and proposes a petrophysical subdivision of the samples into four petrofacies, based on values of porosity, permeability, and capillary entry pressure. This system aims to predict the portions of the studied carbonate sequence that are more likely to have a higher potential for injectivity and storage, and to better understand how porosity, permeability, capillary entry pressure, and pore size all play a role in ensuring both buoyant and capillary trapping mechanisms to secure the injected supercritical CO2.Results from this investigation suggest that in these Cambrian‐Ordovician carbonate reservoirs, pore size inversely correlates with capillary entry pressure, and that permeability does not always have a direct relationship with pore size but rather relates to the overall interconnectivity of the complex pore system. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.

Source to sink studies between the Shaleitian uplift and surrounding sags: Perspectives on the importance of hinterland relief and catchment area for sediment budget, Western Bohai Bay Basin, China

Source-to-sink system analysis, a cutting-edge topic in the field of earth science, encompasses the whole system, from erosion and transportation to sediment accumulation on the earth’s surface, and involves multidisciplinary collaboration. This current analysis, based on high-precision 3D seismic data, well logs, and other drilling data, using quantitative characterization of the source-to-sink elements, documents that Archean-Proterozoic migmatitic granite, Cambrian-Ordovician carbonate and clastic rocks, and Mesozoic volcaniclastic rocks are developed in the Shaleitian uplift from south to north across large relief differences (up to 2300 m). The relief and size of the catchment in the source area were calculated by denudation recovery, time-depth conversion and high-resolution interpretation on seismic data. Three types of sediment-transporting channel system and 20 catchment areas ([Formula: see text]) were documented around the edges of the uplift: paleovalley channels, fault-controlled channels, and fault-transfer channels. The Paleogene sink is dominated by near-source coarse-grained depositional systems, with the lithofacies characteristics of low lake level (sand rich), lake transgressive (mud rich), and uplift period (sand rich). Three types of boundary conditions developed in the region of the Shaleitian uplift: fault-related steep-slopes (single or multiple), fault ramps, and slope patterns. The bedrock composition, catchment area, channel systems, and fault-border patterns in the Shaleitian uplift jointly controlled the types and scales of sedimentary sandbodies. The south Shaleitian tectonic zone functioned as a high-efficiency coupling system in which reservoir sandbodies were developed (extensive length distance, with well-sorted and round-grained sediments, but weak physical properties). The coupling system for the southwest and west Shaleitian tectonic zones is subordinate (near source and sand rich, sand and mud interbedded, and weak physical properties). The coupling system of the northeast Shaleitian tectonic zone is lowest in efficiency (relatively mud rich).

The Sanrafaelic remagnetization revisited: Magnetic properties and magnetofabrics of Cambrian-Ordovician carbonates of the Eastern Precordillera of San Juan, Argentina

Systematic rock-magnetic and magnetofabric studies were carried out on samples from twenty-three palaeomagnetic sites distributed on the La Laja, Zonda, La Flecha, La Silla and San Juan Formations, which constitute a thick middle Cambrian to early Ordovician carbonate sequence exposed in the Eastern Precordillera of Argentina. Previous palaeomagnetic studies on these rocks showed that this succession is characterized by a recent full overprint in the lower levels and a post-tectonic Permian remagnetization associated to the widespread Sanrafaelic event in the upper part. Our investigation revealed that the fluctuations of the magnetic properties are stratigraphically (lithologically) controlled. Anisotropy of magnetic and anhysteretic susceptibility measurements defined consistent fabrics along the entire section that switch progressively from “inverse”, at the bottom, to “normal”, at the top, with “intermediate” fabrics occurring mainly at medium levels. Degree of dolomitization significantly affects many rock-magnetic parameters, but appears unrelated to the presence of the Permian remagnetization, which is determined to reside in magnetite despite the complex magnetic mineralogy shown by the studied carbonates. Hysteresis cycles of rocks affected by the Sanrafaelic remagnetization are governed by ferromagnetic fractions showing a clear difference respect to those not affected and characterized by the dominance of paramagnetic or diamagnetic signals. The magnetic fabrics and mineralogical characterization rule out a thermoviscous origin and suggest a chemical remagnetization originated in the authigenic formation of magnetite for the Sanrafaelic overprint. X-ray diffraction analyses indicate that clay minerals are virtually absent in the whole succession with no traces of illite in any sample, discarding burial diagenesis of clay minerals for the origin of the remagnetization. Lack of late Palaeozoic magmatic rocks near the study area difficults correlation of this event with hydrothermal brines as well as casts serious doubts on any effect produced by an assumed geothermal anomaly associated with the Permo-Triassic Choiyoi magmatic province. The original model of remagnetization associated to chemically active fluids expelled from the San Rafael Orogen towards the foreland still holds as a viable mechanism.

Positive carbon isotope excursions: global correlation and genesis in the Middle–Upper Ordovician in the northern Tarim Basin, Northwest China

Stable carbon isotope ratio (δ13Ccarb) analysis has been widely applied to the study of the inter-continental or global marine carbonate correlation. Large-scale Cambrian–Ordovician carbonate platforms were developed in the Tarim Basin. But research on fluctuation characteristics and global correlation of δ13Ccarb is still weak. Based on conodont biostratigraphy and whole-rock δ13Ccarb data in the Tahe oil–gas field of the northern Tarim Basin, the global correlation and genesis of positive carbon isotope excursions in the Darriwilian—Early Katian was examined. Three positive excursions were identified in the Tahe oil–gas field including the middle Darriwilian carbon isotope excursion (MDICE), the Guttenberg carbon isotope excursion (GICE), and a positive excursion within the Pygodus anserinus conodont zone which is named the Early Sandbian carbon isotope excursion (ESICE) in this paper. Furthermore, these positive excursions had no direct relation with sea level fluctuations. MDICE and GICE could be globally correlated. The Middle–Upper Ordovician Saergan Formation source rocks of the Kalpin outcrops were in accordance with the geological time of MDICE and ESICE. GICE had close relationship with the source rock of the Lianglitag Formation in the basin. Massive organic carbon burial was an important factor controlling the genesis of these positive excursions.

Fracture filling mechanisms in the carbonate buried-hill of Futai Oilfield in Bohai Bay Basin, East China

Abstract Genesis and distribution of fracture fillings of the Cambrian-Ordovician carbonate buried-hill in Futai Oilfield of Bohai Bay Basin were investigated based on observation of core samples and thin sections, analysis of inclusions in fracture fillings, stable isotopic compositions of carbon and oxygen as well as trace element compositions in the fillings and surrounding rocks. There are many kinds of fractures developed in Futai buried-hill structure. Generally, the fracture spaces are filled over 60% by calcite, mostly sparry calcite and formed once time in the fractures. The homogenization temperatures of inclusions in calcite fillings in the fractures are higher than those in the buried-hill reservoirs. Oil-bearing fluid inclusions were found in the calcite fillings in the fractures, which means that the fracture filling is at the same time as oil-pool formation. Analysis of compositions of stable carbon and oxygen isotopes and rare earth elements showed that calcite mother fluids (including oil-pool forming fluid) were hydrothermal fluids coming from deep strata in the basin, and the hydrothermal fluids dissolved the limestone and dolomite and then deposited calcite in the fracture networks in the shallow part of the buried-hill structure. The hydrothermal genesis of calcite fillings in the fractures is useful in understanding the distribution of the calcite filling contents in the buried-hill structure, that is, the fractures near Chengnan and Chengnan Ertaijie faults have higher filling contents, while the fractures far from these faults have fewer fillings.

The Source of Ore-Forming Material in Barite-Fluorite Deposits, Southeast Sichuan in China: Sr Isotope Evidence

Barite-fluorite metallogenic belt of southeast Sichuan is one of the important metallogenic belt in China. The initial ratio of 87Sr/86Sr is the good tracer in study of diagenetic mineralization process and sources of ore-forming materials. The 87Sr/86Sr ratio of fluorite and barite from barite-fluorite deposits in southeast Sichuan is 0.708800~0.712999, and it similar to The 87Sr/86Sr ratio of Cambrian carbonate rocks and ordovician limestone, but expect the lower Cambrian Niutitang group stratum, the Ba content of other layers are generally low, Therefore, the Ca source of ore-forming materials should be multisource, mainly from Cambrian-Ordovician carbonate strata , and the main Ba source of layer should be the lower Cambrian Niutitang group stratum.

The Parautochthonous Gondwanan origin of the Cuyania (greater Precordillera) terrane of Argentina : a re-evaluation of evidence used to support an allochthonous Laurentian origin

A substantial, diverse body of evidence has been interpreted as suggesting that the Cuyania terrane of northwestern Argentina, which includes the Argentine Precordillera, rifted from the Ouachita embayment of Laurentia in the Early Cambrian, drifted across the Iapetus ocean as a microcontinent, and docked with the proto-Andean margin of Gondwana in the Mid to Late Ordovician. This is the so-called Laurentian microcontinent model. However, several lines of evidence (basement age and affinity, stratigraphic, paleomagnetic and paleobiogeographical records) also point to a parautochthonous origin of this terrane. In this parautochthonous model, Cuyania migrated along a transform fault from a position on the southern margin of West Gondwana (present coordinates) in the Mid Ordovician to its modern position outboard of the Famatina magmatic belt in Devonian time. With regard to basement age and characteristics, recently acquired U-Pb geochronology of detrital zircons from Cambrian and Ordovician sandstones and of zircons from igneous clasts in an Ordovician conglomerate are difficult to explain with the Laurentian model and indicate, instead, a Gondwanan origin of the Cuyania terrane. Furthermore, potential basement rocks of Cuyania of Neoproterozoic to Early Cambrian age and Early Mesoproterozoic age are characteristic of Gondwana, rather than Laurentia. Pb isotopic ratios of Grenvillian-age basement rocks are not only similar to those of Grenvillian basement in Laurentia but also to those in other areas of West Gondwana. In terms of the stratigraphic record, the similarity of the Cambrian-Ordovician carbonate platform succession of Cuyania to that of Laurentia reflects similar paleolatitude and eustatic histories but not a direct connection. Moreover, the Middle-Upper Ordovician siliciclastic successions of Cuyania do not represent a peripheral foreland basin, but instead were deposited in strike-slip related basins in a transform fault zone. Middle Ordovician K-bentonites do not indicate that Cuyania was approaching the Famatina Magmatic arc from the west (modern coordinates), but instead that it was located to the southeast. In light of paleomagnetic data, the Cambrian paleolatitude of Cuyania is consistent not only with the location of the Ouachita embayment of Laurentia but also with the southern margin of West Gondwana. Finally, most of the paleobiogeographic criteria used to support the Laurentian model must be reconsidered. Brachiopod and conodont faunas in lower Middle Ordovician strata of the Precordillera have many more genera in common with Laurentia than those in Lower Ordovician strata. Cambrian trilobites faunas of Cuyania are of very limited abundance and diversity in comparison to correlative faunas of southeastern Laurentia; many species are endemic to Cuyania; olenellid trilobites considered to be restricted to Laurentia probably had the ability to disperse between paleoplates with similar environments. Mid Ordovician graptolites of the Precordillera on the one hand and of the Famatinian belt and Cordillera Oriental on the other belong to different oceanic provinces and likely did not live in close proximity.

Bidimensional empirical mode decomposition (BEMD) for extraction of gravity anomalies associated with gold mineralization in the Tongshi gold field, Western Shandong Uplifted Block, Eastern China

A bidimensional empirical mode decomposition (BEMD) program on a MATLAB platform was effectively used to handle gravity signals for the Tongshi gold field. This yielded a two-dimensional intrinsic mode function (IMF 3) image that meticulously depicts the spatial distribution relationship between various gold deposits and the different geological units of the gold field. By combining the IMF 3 image with geological features, the Tongshi gold field can be divided into three geological units. Unit I is made up of both Archean greenstone and Cambrian–Ordovician carbonate rocks and is defined as a concealed basement that exhibits weak negative and positive gravity anomalies. Unit II is defined as a Mesozoic volcanic sediment basin consisting of Jurassic and Cretaceous volcanic-sedimentary rocks, which developed on the concealed basement and exhibits an obvious negative gravity anomaly. Unit III, located on the concealed basement on the SW side of the Mesozoic volcanic-sedimentary basin, can be subdivided into subunits IIIa and IIIb. The former, with a negative circular gravity anomaly, indicates that the Tongshi complex consists of both Yanshan diorite-porphyrite and syenite-porphyry. Unit IIIb, with a positive ring gravity anomaly distributed around a negative circular gravity anomaly, indicates a possible contact metasomatic mineralization zone around the Tongshi complex. Almost all types of gold deposits are located within this contact metasomatic mineralization zone and thus the zone is a prospective area for gold deposits.

Studies on geological background and source of fluorine in drinking water in the North China Plate fluorosis areas

Endemic fluorosis in northern China is usually produced by high fluorine (F) content in drinking water. Thirty-one samples of drinking waters, mainly well waters and nearly 200 samples of rocks, loess, and coal were analyzed for F content using the combustion hydrolysis–fluoride-ion selective electrode (ISE) method. The geologic cross sections of two well-known fluorosis basins were studied. The solubility of F in different rock types collected from fluorosis areas was determined. Results showed that areas of endemic fluorosis in northern China are located in coal-bearing basins which are comprised of three stratagraphic portions. The lowest portion is Precambrian granitic rocks or Cambrian–Ordovician carbonates. The middle portion consists of Permo-Carboniferous or Jurassic coal-bearing sequences. The upper portion is 0–400 m Pleistocene loess. Flourine content in the Precambrian granite-gneiss contained (a) 1090–1460 ppm, in the Cambrian-Ordovician limestone and dolomite, (b) 52–133 ppm, in black shales and coal gob of Permo-Carboniferous coal-bearing strata, (c) 200–700 ppm, and (d) Pleistocene loess 454–542 ppm. The solubility of F in black shales of coal-bearing sequences was higher than in Precambrian granitic rocks, and both were more soluble than loess. F solubility from Precambrian granitic rocks was moderate, but Precambrian granitic rocks have high F content and thus contribute an appreciable amount of ion to the shallow groundwater (well water). Varying F content in shallow groundwater is controlled by geological conditions. The sources of F in the shallow groundwater from fluorosis areas in northern China are mainly derived from black shales of coal-bearing sequences and Precambrian granitic basement in the basins of northern China.

Strontium Isotope Composition and Characteristic Analysis of Cambrian-Ordovician Carbonate in the Region of Tazhong, Tarim Basin

The research on the trace elements of Ordovician carbonates plays an important part in the whole work on reservoir in Tazhong ( ) area. This paper systematically studies the characteristics and sedimentary settings of Ordovician dolomites in Tazhong area, Tarim basin, and debates their enrichment of mechanisms and different element existing patterns. The study makes use of ICP-MS analysis technology to test the strontium and manganese content of 109 samples from four wells in the Tazhong area, Tarim basin and strontium isotope composition tests have also been done on 25 samples from wells Zhong-1 and Zhong-4 on VG354 solid isotope mass spectrograph. By means of analyzing contents of strontium and manganese elements, doing research on the strontium isotope composition characteristics from wells Zhong-1 and Zhong-4 and comparing the results of strontium analysis to the global Ordovician marine carbonate and its evolution trend, in the combination of sedimentary facies characteristics of isolated wells in this area, we can come to the following cognitions: (1) The marine carbonate strontium isotope curve in the Tazhong area of Tarim basin is consistent to the global evolution trend which is overall descending with time, the direct reason of which is the evolution of paleogeographic environment. The Ordovician paleogeographic environment goes through restricted platform to open platform and then shallow marine shelf in Tazhong area, Tarim basin. Dolomitization is another subordinate reason and the inversion of fluid with high manganese can lead to heavy strontium; (2) The fact that the 87 Sr/ 86 Sr ratios of the Upper Ordovician in Tazhong area have an apparent mono-decline trend with the time going by, which is similar to the global strontium isotope ratio, is suggesting that the variation of the Upper Ordovician sea level is starting, the overall trend of which is the rising of the sea level; (3) Compared to the global seawater strontium ratios, the 87 Sr/ 86 Sr ratio of the Lower Ordovician is much high, and 87 Sr/ 86 Sr ratio of the Lower Ordovician is overall increasing with its maximum value reaching 0.709 727, the reasons of which are dolomitization, evaporation of the supratidal and the reflux of high manganese haline, those reasons can lead to the increasing amount of the strontium and the rising of the 87 Sr/ 86 Sr ratio; (4) The manganese element content changes little. Dolomitization has little impact on it; (5) Big variation occurs in the strontium values of Ordovician carbonate in the region of Tazhong. Apart from paleogeographic environment and petrography, the factors which can affect the values of strontium are the content of the terrestrial elastics and the diagenesis modification which may result in the further changing of the strontium values.

Carbon isotope record of Late Cambrian to Early Ordovician carbonates of the Argentine Precordillera

A more than 2000-m-thick Cambrian–Ordovician carbonate platform succession developed on the exotic Argentine Precordillera terrane during rifting from Laurentia and drifting towards Gondwana. On base of these carbonates, a carbon isotope curve could be developed for the Cambrian–Ordovician. We measured δ 13C carb and δ 13C org values on bulk rocks, selected components and diagenetic cements. Whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis, most subtidal carbonates exhibit primary marine values. We can report reliable curves for the Middle–Late Cambrian transition and for the latest Cambrian to earliest Middle Ordovician. The δ 13C curve matches well with the published data from global Cambrian–Ordovician boundary sections. Excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy. This indicates the interdependence on sea level which rules the productivity and/or preservation of organic carbon and therefore the partition between C org and C carb burial.

Cambrian-Ordovician sequence stratigraphy on the

The Cambrian and Ordovician on the northern Tarim Platform are mainly composed of carbonates. On the basis of detailed outcrop analysis, the sequence stratigraphic system of the Cambrian-Ordovician in the northern Tarim Platform is outlined in this paper. Altogether 35 third-order sequences, 12 supersequences, 4 supersequence sets and 2 megasequences are recognized. The characteristics of the major sequence boundaries have been documented with an integrated examination of outcrop, seismic and borehole data, and the ages of these sequence boundaries have been calibrated through the combination of sequence stratigraphy with biostratigraphy. It is discovered that there is good correlation of the sequence stratigraphy of the Cambrian-Ordovician among Tarim, Yangtze and North China platforms. This may illustrate that the development of the Cambrian-Ordovician carbonate sequences in these three platforms is mainly controlled by regional or global sea level changes. This forms the theoretical basis for the construction of high-resolution chronostratigraphic system of the Cambrian-Ordovician in the three platforms in China.

Basin evolution in western Newfoundland: New insights from hydrocarbon exploration

The Humber zone is the most external zone of the Appalachian orogen in western Newfoundland. It records multiphase deformation of the Cambrian-Ordovician passive margin and of the Ordovician to Devonian foreland basins by the Taconian, Salinian, and Acadian orogenic events. The recent phase of exploration drilling has provided new evidence for structural, stratigraphic, reservoir, and source rock maturation models of western Newfoundland. The first well, Port au Port 1, supported the hypothesis that the Round Head thrust had an earlier extensional history prior to the Acadian compressional inversion that created the present-day structural high of the Port au Port Peninsula. The well tested a small anticline formed in a footwall shortcut fault of the Round Head thrust. The second well, Long Point M-16, was drilled at the northern tip of Long Point to test a triangle zone identified by previous workers. This well demonstrates that the frontal monocline at the western edge of the triangle zone is elevated by a stack of imbricate thrusts composed of rocks of the Taconian allochthon and compressional basement-involved faults that have uplifted the Cambrian-Ordovician carbonate platform. The structural model developed in the Port au Port area with the aid of these wells has been extended throughout the Humber zone in western Newfoundland. Changes in structural style illustrated by regional cross sections suggest that prospective trap geometries are only developed in the southern and central parts of the region. The reservoir model proposed invokes exposure and karsting of the footwalls of extensional faults formed as the carbonate platform collapsed during a Middle Ordovician hiatus, the St. George unconformity. Structural relief became more pronounced as extensional collapse continued through the Middle Ordovician. (Begin page 394) These structurally high fault footwalls became the foci for dolomitizing and mineralizing fluids that used major faults as fluid conduits during the Devonian. Fluids deposited sulphide ores and created zebra and sparry dolomite and some sucrosic hydrothermal dolomites in the St. George Group and the Table Point Formation. The reservoir model, maturity and source rock data, and the structural models have been combined with seismic and onshore surface geology. This enables the prospectivity of the western Newfoundland Cambrian-Ordovician play trend to be evaluated for further exploration.