Search

The North Qilian orogenic belt is the key in the exploration of the formation and assembly of Asia. The Early–Middle Devonian Xueshan Formation, which is deposited in the north area of North Qilian Orogenic Belt, is the most important for revealing the evolution basin–mountain transition of the North Qilian Belt. In order to reveal the tectonic evolution of the central orogenic belt in China and Paleozoic global plate reconstruction, it is necessary to perform provenance analysis of the Xueshan Formation. Eighteen samples were collected from Yumen and Yongchang profiles in the west of North Qilian Belt (WN), and 16 samples were obtained from Gulang and Jingtai profiles in the east of North Qilian Belt (EN). The compositions of clastic conglomerates and the sandstones suggested that felsic volcanic was the main source of the WN Xueshan Formation deposits and that the sedimentary and metamorphic rocks were the main source of the EN sediments. According to major geochemistry indexes, such as Al2O3/SiO2 ratio ranging from 0.08 to 0.37 and the chemical index of alteration less than 80, the source rocks were moderately weathered and of moderately matured. The samples from the EN were relatively enriched in Cr, Ni, Sc, and V elements and depleted in Hf element. However, those from the WN were enriched in Hf without the enrichment of Cr, Ni, Sc, or V. The samples from WN showed a strong negative Eu anomaly, but the samples from EN showed a weak negative Eu anomaly or a positive Eu anomaly. All the above geochemical characteristics suggested the prominent input of felsic clasts with granitic rocks into the WN. Meanwhile, the discrimination diagrams of trace elements implied that WN rocks were derived from continental island arc and that the EN rocks were derived from orogenic belt and the passive continental margin. The North China Plate subducted southwards and formed subduction‐related arc magmatism along the southern margin of the North Qilian Terrane during the Early–Middle Devonian. In conclusion, the WN is a fore‐arc basin, but EN is a foreland basin. [ABSTRACT FROM AUTHOR]

The carbonates in the Middle Ordovician Ma submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized. Two types of replacive dolomite are distinguished: (1) type 1 dolomite, which is primarily characterized by microcrystalline (<30 µm), euhedral to subhedral dolomite crystals, and is generally laminated and associated with gypsum-bearing microcrystalline dolomite, and (2) type 2 dolomite, which is composed primarily of finely crystalline (30-100 μm), regular crystal plane, euhedral to subhedral dolomite. The type 2 dolomite crystals are truncated by stylolites, indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites. Stratigraphic, petrographic, and geochemical data indicate that the type 1 dolomite formed from near-surface, low-temperature, and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head. The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater. The δO values (−7.5 to −6.1 ‰) of type 1 dolomite are slightly lower than those of seawater-derived dolomite, suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial. The type 2 dolomite has lower δO values (−8.5 to −6.7 ‰) and Sr concentration and slightly higher Na, Fe, and Mn concentrations and Sr/Sr ratios (0.709188-0.709485) than type 1 dolomite, suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial. [ABSTRACT FROM AUTHOR]

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen, North China Craton, and is mainly composed of greenschist-facies metabasalt, meta-sandstone and dolomitic marble. We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group. SHRIMP dating yielded a weighted mean Pb/Pb age of 2087±16 Ma (MSWD=1.3) for magmatic zircons, but there are also abundant ca. 2.5 Ga inherited zircon xenocrysts. The magmatic zircons shows a large ɛ( t) variation in ɛ( t) from −7.17 to +0.45, suggesting an isotopically highly heterogeneous source for the metabasalt. Chemically all samples show no distinct Zr or Hf anomalies, and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram, and their whole-rock ɛ( t) values range from 4.0 to −0.8. We suggest that the basalt is formed by partial melting of a depleted mantle source, followed by significant crustal contamination. Field observations, the presence of abundant inherited zircon, as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement. These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean. [ABSTRACT FROM AUTHOR]

Abstract: Based on the analysis of geochemical characteristics and distribution of different formation waters, a new origin identification standard was established for the formation water in the Paleogene Shahejie Formation in the Dongying Sag. The migration paths of the connate water expelled from source rocks are similar to those of the hydrocarbons coming from the same rocks, so the geochemical characteristics of the connate water coming from the source rocks can be used as auxiliary indexes to study hydrocarbon migration. Considering fluid pressure and formation water distribution, the Paleogene fluids are divided into three fluid systems: normal-pressure open fluid system, overpressure confined fluid system, and transition fluid system. The differences in hydrocarbon-bearing fluid characteristics, hydrocarbon migration dynamic, hydrocarbon-bearing fluid flow type between these fluid systems were studied. According to characteristics of the hydrocarbon-bearing fluid charging into traps, and formation water drainage pattern, three typical oil accumulation models were classified. In the overpressure confined fluid system, the main oil accumulation mode is high oil saturation fluid entering traps and displacing formation water, and “continuous” reservoirs are prone to form, and fluid oil saturation, fluid charging pressure and fluid seeping conditions affect reservoirs’ oil accumulation extent. In the normal-pressure open fluid system, the main oil accumulation modes include low oil saturation fluids enter a trap while formation water is overflowing out of the trap, and low oil saturation fluids enter a trap while formation water is seeping out of the trap. The amount of hydrocarbon-bearing fluid, fluid oil saturation, trap height, and caprock sealing ability affect reservoir''s oil accumulation extent. [Copyright &y& Elsevier]

High-resolution grain size and major element geochemical measurements were performed on the marine sediments of Core MD05-2895 to help understand the formation of turbidite sequences. Grain-size results show that these turbidite sediments contain more coarse sediment grains than normal marls. The coarse sediment grains are mostly derived from relict sediments on the Sunda Shelf. Relict sediments are composed mainly of quartz, feldspar, tephra and a few titaniferous or ferruginous heavy minerals. Corresponding to the concentration of these minerals, increases in Si/Al, K/Al, Ti/Al and Fe/Al ratios are observed in the turbidite layers. As all the observed turbidite sequences were deposited during the last glacial, the occurrence of these turbidity events is implied to be closely related to instabilities in sea-level-induced sediment supply. We suggest that deposition of sediment particles from a single turbidity current is usually controlled by a counterbalance between gravity and buoyancy, with the interaction of individual grains being of minor importance. [ABSTRACT FROM AUTHOR]

Abstract: Deep energy fields in the Tahe Oilfield are studied and the relationship between energy fields and hydrocarbon migration and accumulation is discussed through the comprehensive analyses of energy field characteristics. Multi-phase tectonic stresses are the main controlling factor of the developmental morphologies of the present fractures. The characteristics of the present regional tectonic stress indicate that the Tahe Oilfield is a favorable area for hydrocarbon collection. The present pressure field of the Ordovician is a slightly low abnormal pressure system. The fluid potential characteristics show that there are two hydrocarbon migration trends: one is from east to west in part of the eastern area, and the other is from south to north in the whole southern area. The present formation water is of typical CaCl2 type. According to the formation water geochemistry, the Tahe Oilfield has a good seal capability, favorable for hydrocarbon accumulation and preservation. Aqueous inclusions indicate that homogenization temperatures are mainly 80–100°C, 120–130°C, and 150–170°C, therein, 80–100°C and 120–130°C represent two large scale hydrocarbon accumulation phases, Early Hercynian and Himalayan, and 150–170°C is controlled by tectonic event and Permian magmatic activities, representing a partial hydrocarbon accumulation event. [ABSTRACT FROM AUTHOR]