2017 Vol. 28, No. 2
The geochemistry and paleogeography of the Doushantuo succession in the Shennongjia area have been insufficiently studied. Here, we report on the carbon and oxygen isotope compositions of Ediacaran Doushantuo cap carbonates from four sections (Longxi, Muyu, Yazikou and Songluo) in the Shennongjia area. A large C-isotopic gradient (~5‰) between the Longxi and Songluo sections, here identified for the first time, is inferred to have been dynamically maintained by photosynthesis in surface waters and anaerobic oxidation of dissolved organic carbon (DOC) in deep waters. Spatial variation in C-isotope chemostratigraphy among the four sections was related to Marinoan post-glacial sea-level elevation changes. At Longxi, a positive δ13Ccarb shift below the horizon containing tepee-like structure resulted from intense photosynthesis during early regression. A negative δ13Ccarb shift within the tepee-horizon was triggered by sulfate reduction and freshwater mixing with 13C-depleted dissolved inorganic carbon (DIC) during late regression. A positive δ13Ccarb excursion in the uppermost part of the cap carbonate was related to enhance primary productivity and organic matter burial during early transgression. At Muyu, the carbon isotopic variation tendency, which is similar to that at Longxi, may have been mildly influenced by the surface water environment. At Songluo, the positive δ13Ccarb excursion, up to-4‰ in the lower part of the cap carbonate, was probably associated with methanogenesis in deep waters during late transgression to early regression and subsequently disappeared due to decomposition of methane hydrate during late regression. At Yazikou, the consistently stable δ13Ccarb values around-4‰ indicate that the cap carbonate may have deposited at intermediate water depths. As evidenced by diagnostic sedimentary characteristics of the study sections, the palaeogeographic framework of the Shennongjia area exhibited deepening from SE to NW during the Early Ediacaran Period in the aftermath of Snowball Earth.
A synthesis involving the data for the Nalati mountain region (NMR) in west Tianshan with a dataset including zircon U-Pb ages, Hf isotopic composition, major and trace elements of Paleozoic intrusions are presented to improve the understanding of regional geodynamic evolution. Paleozoic intrusive rocks in the NMR could be classified into four categories based on chronological and geochemical data: 480±5 Ma, 445-410 Ma, 345-320 Ma, and 295 Ma, which correspond to (1) closure of the Terskey Ocean and the opening of the south Tianshan back-arc basin, which was followed with the opening of the south Tianshan Ocean, (2) initial subduction of the south Tianshan oceanic crust, (3) major subduction stage, and (4) collision to post-collisional stage, respectively. Following the closure of the Terskey Ocean, the south Tianshan Ocean opened at Early Silurian and subducted under Yili- central Tianshan by the end of Early Carboniferous Period. The following breakoff of the subducted slab triggered partial melting of continental crust and formed voluminous granitic rocks in the NMR.
Basaltic volcanic conglomerates near the Wulgai village in Balochistan occur in the undivided sedimentary rock unit of the Bagh complex which is the mélange zone beneath the Muslim Bagh ophiolite. The presence of Middle Triassic grey radiolarian chert within the upper and lower horizon of the conglomerates suggests that the lavas, from which these conglomerates were principally derived, were eroded and re-deposited in the Middle Triassic. The Wulgai conglomerate contains several textural and mineralogical varieties of volcanic rocks, such as porphyritic, glomerophyric, intersertal and vitrophyric basalts. The main minerals identified in these samples are augite, olivine, plagioclase (An35-78) leucite and nosean, with apatite ilmenite, magnetite and hematite occurring as accessory minerals. These rocks are mildly to strongly-alkaline with low Mg# and low Cr, Ni and Co contents suggesting that their parent magma had undergone considerable fractionation prior to eruption. Trace element-enriched mantle-normalized patterns with marked positive Nb anomalies are consistent with 10%-15% melting of an enriched mantle source in a within-plate tectonic setting. It is proposed that this Middle Triassic intra-plate volcanism may represent mantle plume-derived melts related to the Late Triassic rifting of micro-continental blocks (including Afghan, Iran, Karakorum and Lhasa) from the northern margin of Gondwana.
Basaltic pillow lavas near the Kazhaba village in Balochistan are found in the tectonic slivers of the Bagh complex: the melange zone beneath the Muslim Bagh ophiolite complex. These volcanics are mainly represented by alkali basalts. The petrography and chemistry suggest that these volcanic rocks belong to mildly to strongly alkaline, intra-plate volcanic rock series. Their low Mg# and low Cr, Ni and Co contents suggest that the parent magma of these volcanics was not directly derived from a partially melted mantle source, but resulted fractionation in an upper level magma chamber, en-route to eruption. Their LILE and HFSE, enriched primordial mantle-normalized patterns with marked positive Nb anomalies further confirm their within-plate geochemical signatures and are consistent with an enriched mantle source. Their highly enriched LREE patterns and high (La/Yb) N and (Ce/Yb) N ratios suggest a partially melted garnet-lherzolite parent magma source. The Zr versus Zr/Y studies suggest that these volcanics were derived from about 15% partially melted enriched mantle source. It is suggested that these Late Cretaceous intra-plate volcanics may represent the mantle plume activity of the Reunion hotspot, and were erupted during the passage of Ceno-Tethys Ocean floor prior to the passage of Indian Plate over it.
Blueschist accompanied by pelitic schist expose along the Xinlin-Xiguitu fault in the Toudaoqiao area, northeastern China. In this paper, the blueschist is systematically studied on the petrography and mineral chemistry. The amphiboles in the blueschist are zoned from winchite core to magnesioriebeckite/glaucophane rim to winchite outermost rim. The peak metamorphic conditions are defined by the mineral assemblage of magnesioriebeckite/glaucophane, epidote, high-Si phengite (Si<7.1) , chlorite, albite, hematite and quartz, indicating an epidote-blueschist facies metamorphism. The P-T conditions are estimated as T=350-400 ℃ and P=10-12 kbar. The occurrence of the blueschist along the Xinlin-Xiguitu fault strongly suggests the fault is the suture between the Ergun and the Xing' an blocks situated in the eastern portions of the Central Asia Orogenic Belt (CAOB) .
Early Carboniferous gabbros and diorites are distributed in West Ujimqin, Inner Mongolia. The LA-ICP-MS zircon U-Pb ages of the gabbro and diorite samples are 321±2 Ma (MSWD=0.65) and 319.4±1.5 Ma (MSWD=0.42) , respectively. In addition, new geochemistry data from three gabbro and three diorite samples are presented. All six samples show high Al2O3 contents but low-TiO2 contents, belonging to tholeiitic and calc-alkali basalt series. All of the samples have similar chondrite normalized REE patterns characterized by moderate depletion in LREE similar to normal middle oceanic ridge basalt (MORB) . The MORB and PM-normalized trace element patterns show the enrichment in large ion lithophile elements (LILE, e.g., Rb, Ba and Sr) , depletion in high field strength elements (HFSE) and distinctly negative Nb and Ta anomalies similar to volcanic arc basalt. Furthermore, as shown in the correlation plots of La/Ba vs. La/Nb, Ba/La vs. Ce/Pb, Th/La vs. Ce/Pb, Nb/La vs. Ba/Rb, and Nb/Y vs. La/Yb, the magma source has underwent contamination and metasomatism from the subduction fluid. According to the Zr/Nb, La/Nb, and La/Ta ratios and the Nb/Y vs. Zr/Y and Sm/Yb vs. La/Sm diagrams, the magma was derived from shallow depleted lithospheric mantle and formed by moderate (5%~20%) partial melting of spinel lherzolites. Tectonic setting discrimination diagrams reveal that the gabbros and diorites display both characteristics of MORB and volcanic arc basalt, which is consistent with their geochemical characteristics. On the basis of the geochemical features of these samples, combined with regional geological data and many previous researches in the study area, the Early Carboniferous gabbros and diorites of West Ujimqin are suggested to be formed in an intensely extensional rift setting, and a limited immature ocean basin probably formed after subsequent development.
Zircon U-Pb geochronology, Hf isotope and whole-rock geochemistry were performed on the tuffs of the Yeba Formation in the Gangdese magmatic belt, South Tibet. The results are used to detail the age, source nature and tectonic processes that led to the formation of the Gangdese belt. Dating results indicate that the rhyolitic-andesitic tuffs were formed at 174-170 Ma. Positive and variable zircon εHf (t) values of the rhyolitic tuffs reveal that the source was dominated by juvenile material, however, experienced crustal contamination. The basaltic tuffs have low HREEs, high contents of compatible elements (V and Cr) and no Eu anomaly. In contrast, the rhyolitic-andesitic tuffs show low compatible trace elements, depletion in Eu but enrichment in incompatible elements (Rb, Zr and Hf) . According to the discrimination diagrams of P2O5-SiO2 and Th-Rb, the rhyolitic-andesitic tuffs show a close affinity to I-type granitoids. Moreover, these tuffs are marked by significant depletion in Nb, Ta and Ti, plotted in calc-alkaline field, and with the andesitic-rhyolitic tuffs falling into an active continental margin setting. We suggest that these tuffs of the Yeba Formation were probably generated in an active continental margin above the northward subduction of the Neo-Tethyan oceanic lithosphere.
The Parkam exploration district represents an area of approximately 4 km2 located 50 km north of Shahr-E-Babak (Kerman Province, Iran) , and has several traces of old copper mining and smelting activities. This area lies in the Kerman Copper Belt which is part of the larger Sahand-Bazman igneous and metallogenic zone hosting numerous known porphyry copper deposits and systems. The geology of the Parkam exploration district demonstrates that the area contains a diorite-type porphyry copper system hosted by volcanic and pyroclastic rocks of predominantly andesitic composition. Based on field and microscopic investigation, it was determined that the dominant types of alteration were propylitic, phyllic, argillic, and potassic, and the alteration map of the study area was produced. Expect for the propylitic alteration which was observed mainly in the host rocks, the other types of alteration are associated mainly with the dioritic subvolcanic body. Accompanied by subordinate amounts of primary sulfides, fracture-filling malachite is widespread in the potassic and phyllic zones and comprises the dominant style of mineralization at the surface of the porphyry system. Lithogeoc hemical data resulting from 377 samples were analyzed, and the results of background and anomaly separation by means of conventional and the U-spatial statistic method were compared. The Cu and Mo mineralizations were subsequently delineated using the U-spatial statistic. The delineated Cu mineralization is closely associated with the defined zone of potassic alteration, which is also consistent with the field and microscopic observation of the Cu mineralization in this alteration zone. The Mo mineralization delineated by the U-statistic method is mostly associated with the phyllic alteration and is spatially conformable with the zone defined for it. The source code for a software program, which was developed in the MATLAB programming language in order to perform the calculations of the U-spatial statistic method, is additionally provided. This software is compatible with geochemical variates other than Cu and Mo and can be used in similar exploration projects.
The Eastern Guangxi area locates in the southwestern part of the transition zone between Yangtze and Cathaysia blocks, which is an important region because the boundary between two blocks probablycrosses there. We determined LA-ICPMS U-Pb ages for detrital zircons extracted from three sandstone samples in the Sinian-Cambrian strata in this region. The resulting ages are in the range of the Archeozoic and Neoproterozoic, with three notable concentrates at 991 Ma, 974 Ma, and 964 Ma, all of which are coeval to the Grenvillian magmatic activity. The new age distribution is similar to the data reported in the Precambrian strata of the adjacent southwestern Cathaysia Block, suggesting that most of our detrital zircons are likely derived from the Cathaysia Block. Combined with others' research, we are more inclined to accept the opinion that there was not an ocean basin between the two blocks during the Sinian-Cambrian period in Eastern Guangxi area if the timing of collision is the Early Neoproterzoic. But if the timing of collision is the Early Paleozoic, we conclude that Luzhai uplift (i.e., the uplift between Guilin-Yongfu faultand Lipu fault) beyond the west of Dayaoshan regoin might be one part of southwestern sedimentation boundary of Cathaysia Block and Yangtze Block. We also get a few of detrital zircons with ages of ~590 Ma which probably sourced from northeastern Gondwana and 13 detrital zircons with over 3 000 Ma U-Pb ages which record the early for mation of the earth.
The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum’s precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitiza-tion may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high- and low-frequency sea-level changes. This kind of dolomiti-zation can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity (normal, penesaline to hypersaline).
Calcite veins in carbonate fracture have been investigated by petrographic, fluid inclusion, geochemical analyses and coupled with basin modeling techniques to provide useful insights into fluid activity and deformation conditions of the Cambrian to Triassic Shizhu synclinorium from the western region of Mid-Yangtze, central China. The results of the fluid inclusion microthermometry show a wide range of homogenization temperatures (78.6-215.5 ℃) and salinities (0.18-23.11 wt.% NaCl equivalent), indicating the formation under diverse fluid conditions. All the calcite veins have negative Ce anomalies, which are the typical characteristic of marine carbonate sediments; it is therefore plausible that calcite veins were precipitated from the marine basin fluid. The stable carbon isotopic compositions of calcites (δ13C V-PDB=-2.5‰-4.26‰) and host limestones (δ13C V-PDB=-3.56‰-5.80 ‰) are very similar with a correlation coefficient of 0.86, however, four calcites from the Lower Permian and Lower Triassic show lower δ13C values relative to the host limestones, and they are depleted in total REE concentrations (∑REE ratio varies from 0.74 to 2.06), suggesting the derivation of dissolved carbon from marine carbonates hosting the calcite veins and, less commonly, from the degradation of organic matter. Calculated δ18O of the fluids-precipitating calcites (δ18O V-SMOW=-0.41‰-14.42‰), 87Sr/ 86Sr ratios varying in the range of coeval seawater and the distinct REE pattern simultaneously suggest calcite-forming fluids in each stratigraphic unit could have formed from the involvement of fluids that originated from coeval seawater and evolved through different degrees of water rock interaction. However, the presence of more radiogenic 87Sr/ 86Sr ratios than coeval seawater and pronounced positive Eu anomalies in calcites of Lower to Middle Ordovician rocks indicate that terrestrial input from upper strata mudstone and siliciclastic rocks could be involved in the precipitation of the Ordovician calcite. Fluid-inclusion data combined with burial and thermal history modeling indicate there was large-scale flow of evolved basinal fluids through the carbonate formation fractures spanning a time frame from 135 to 50 Ma (Early Cretaceous-Eocene). Therefore, the geochemical characteristics of calcite veins can provide the basis for deformation events in Late Yanshanian and Early Himalayan orogeny.
The Upper Permian Changxing dolomite reservoirs serves as one of the most important gas and oil reservoirs in the NE Sichuan Basin. Determining the dolomitizing fluid's pathway is regarded as the key to solve the "dolomite problem" and further petroleum exploration. Outcrop samples from Upper Permian Changhsingian Panlongdong Section were studied using oxygen isotopic analysis, cathodoluminescence (CL) and major element analysis, in an attempt to determine the migration path way and properties of the dolomitizing fluid. Of the Changxing dolomite, the δ18O values ranged from -3.494‰ to -5.481‰, which decreased from the top layer to the bottom in the section; the MgO contents varied from 9.24% to 21.43%, CaO contents from 28.65% to 39.87%, the CaO/MgO ratio from 1.40 to 4.31 and the MnO contents from 0.004% to 0.009 8%. The MgO contents showed a downwardly decreasing trend in the section, while the CaO/MgO showed an opposite rule. All of the dolomites looked dull or dark when they were exposed to the electron beam of the cathodoluminescence device. None of the fine- to medium grained dolomite showed a banded structure. Given that dolomitizing fluid's salinity decreased during the dolomitization process in its pathway, we concluded that the dolomitizing fluid migrated downwardly in Changxing Formation after excluding the possibility of deep burial or meteoric-marine mixing-water influences. As the dolomitizing fluid's pathway has always been difficult to be determined in highly dolomitized Formation, this study showed an important application of oxygen isotope values in resolving this problem.
The Yanchang gas field is located in the Ordos Basin of central China to the southeast of the Yishan Slope, covers an exploration area of 2.6×10 4 km 2 and has approximately 3.5×10 11 m 3 of gas reserves. The gas field is dominated by lithologic gas reservoirs but also has a few structural gas reservoirs. Sand bodies were deposited in the Carboniferous Benxi Formation around offshore barrier islands and in distributary channel fillings in the delta front of the P 1s2 and P 1s1 divisions of the Permian Shanxi Formation. The P 2h8 division of the Shihezi Formation contains the main reservoirs. The depths of the reservoirs are between 1 970 and 3 500 m. The Yanchang gas field can be classified as a typical tight sandstone gas reservoir filed because its porosity is lower than 10% and permeability lower than 1 mD. The discovery and development of the Yanchang gas field has led to a great increase in total natural gas reserves in the Ordos Basin. Its exploration has improved methods of locating large gas fields in deep-water depositional environments in the south part of the basin.
The deformation of coal is effected by thermal effect, pressures and tectonic stress, and the tectonic stress is the principal influence factor. However, the proposition of a useful quantitative index that responds to the degree of deformation of coals quantitatively or semi-quantitatively has been a long-debated issue. The vitrinite reflectance ellipsoid, that is, the reflectance indication surface (RIS) ellipsoid is considered to be a strain ellipsoid that reflects the sum of the strain increment caused by stress in the process of coalification. It has been used to describe the degree of deformation of the coal, but the effect of the anisotropy on the RIS ellipsoid has not yet been considered with regards to non-structural factors. In this paper, Wei's parameter (ε) is proposed to express the deformation degree of the strain ellipsoid based on considering the combined influence of thermal effect, pressure and tectonic stress. The equation is as follows:
The Ordovician of the Tazhong area in the Tarim Basin has suffered multi-cyclic hydrocarbon charging, making Tazhong a typical condensate gas district. In this paper, production and test data were gathered and a detailed comparison was conducted on the geology and the fluid distribution and characteristics between the eastern and western Tazhong area. Eastern and western regions exhibit significant differences in tectonic structure, fluid distribution, and physical-chemical properties of oil and gas. Compared with the eastern region, the western part has a greater development of discordogenic gas associated with strike-slip faults which, combined with the Tazhong No. 1 fault zone, control the fluid distribution. The eastern region is mainly controlled by the Tazhong No. 1 fault zone. Fluid have markedly homogeneous properties in the east, but are heterogeneous in the west. The origins of oil and gas are different between the east and the west. In the east, hydrocarbons are mainly from Ordovician source rocks and natural gas is mostly derived from kerogen pyrolysis. In the west, the hydrocarbons mainly originated from Cambrian source rocks, and the gas was mostly generated by crude oil cracking. In sum, the east region is dominated by primary condensate gas reservoirs, and the western region is dominated by secondary condensate gas reservoirs. Because of the different geological settings and fluid physical properties, differences in the condensate gas reservoirs in the eastern and the western Tazhong area have been analyzed, and appropriate formation mechanisms for condensate gas origins are established.
Understanding and modelling the wettability of tight rocks is essential for designing fracturing and treatment fluids. In this paper, we measure and analyze spontaneous imbibition of water and oil into five twin core plugs drilled from the cores of a well drilled in the Montney Formation, an unconventional oil and gas play in the Western Canadian Sedimentary Basin. We characterize the samples by measuring the mineralogy using XRD (x-ray diffraction), total organic carbon content, porosity, and permeability. Interestingly, the equilibrated water uptake of the five samples is similar, while, their oil uptake increases by increasing the core porosity and permeability. We define two wettability indices for the oil phase based on the slope and equilibrium values of water and oil imbibition curves. Both indices increase by increasing porosity and permeability, with the slope affinity index showing a stronger correlation. This observation suggests that part of the pore network has a stronger affinity to oil than to water. We also observe that the two indices decrease by increasing neutron porosity and gamma ray parameters measured by wireline logging tools. The samples with higher gamma ray and neutron porosity are expected to have greater clay content, and thus less effective porosity and permeability.
One of the potential risks associated with subsurface storage of CO2 is the seepage of CO2 through existing faults and fractures. There have been a number of studies devoted to this topic. Some of these studies show that geochemistry, especially mineralization, plays an important role in rendering the faults as conduits for CO2 movement while others show that mineralization due to CO2 injection can result in seep migration and flow diversion. Therefore, understanding the changes in reservoir properties due to pore alterations is important to ensure safe long term CO2 storage in the subsurface. We study the changes in the Representative Elementary Volume (REV) of a rock due to reactive kinetics over a time, using a statistical approach and pore-scale CO2-rock interactiondata.The goal of this study is to obtain the REV of a rock property that accounts for pore-scale changes over time due to reactive kinetics, and we call this as spatiotemporal REV. Scale-up results suggest that the REV changes with time when CO2-rock interaction is considered. It is hypothesized that the alteration in pore structure introduces more heterogeneity in the rock, and because of this the magnitude of REV increases. It is possible that these noticeable changes in REV at pore-scale may have an impact when analyzed at the reservoir scale.
On April 20, 2013, an Ms 7.0 earthquake struck Lushan County in Sichuan Province, China, and caused serious damage to the source region. We investigated the rupture process of the Ms7.0 Lushan earthquake by jointly inverting waveforms of teleseismic P waveforms and local strong motion records as well as static GPS observations. The inverted results indicate that the rupture of this earthquake was dominated by the failure of an asperity with a triangular shape and that the main shock was dominated by thrust slip. The earthquake released a total seismic moment of 1.01×10 19 Nm, with 92% of it being released during the first 11 s. The rupture had an average slip of 0.9 m and produced an average stress drop of 1.8 MPa. Compared with our previous work that was based mainly on a unique dataset, this joint inversion result is more consistent with field observations and the distribution of aftershock zones.