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In recent years, the typical crude oil from the Cambrian Lower Ordovician source rocks has been found in the Tarim Basin, and it is characterized by 13C enrichment. The stable carbon isotope of the oil is concentrated at approximately −28‰, which is 3‰–6‰ heavier than that of the crude oil from the Middle and Upper Ordovician source rocks. The stable carbon isotope between crude oil and its kerogen is characterized by inversion, with an amplitude of approximately 3‰–4‰. However, the origin of this phenomenon remains controversial, which restricts the next exploration of crude oil in the Cambrian–Lower Ordovician in the Tarim Basin. Samples were collected from seven outcrop profiles in the Kuruketage region. The basic geochemical features of the Cambrian strata, including total organic carbon abundance, δ13Corg, δ13Ccarb, and δ18Ocarb, were studied and compared with those of well TD2. The results show that the lithologies, isotope stratigraphic evolutional curves, and correlations between the organic carbon and inorganic carbon of the profiles from the northern Kuruketage region are successively comparable with those of the profiles from the southern Kuruketage region and the well TD2. In this study, the carbon isotope records are supposed to indicate a vertical gradient Cambrian oceanic mode, including the shallow decoupled isotope zone, middle gradient‐coupled isotope zone, and deep isotope‐decoupled zone. The discovery of this isotope record reflects the formation of stable carbon and oxygen isotope stratified oceans during the Cambrian period. Additionally, it provides important insights into the origin of the 13C enrichment observed in some Lower Cambrian crude oils in the Tarim Basin. [ABSTRACT FROM AUTHOR]

The natural gas resources in the Sichuan Basin are abundant and exhibit various phenomena, for example, mixed sources, mixed maturities and overlapping combinations, resulting in great difficulties in identifying natural gas sources. In this study, several groups of thermal simulation experiments were conducted on various types of kerogens using closed and/or semi‐open systems to investigate the characteristics of the carbon isotope compositions of light hydrocarbon gases derived from different typical kerogens. The results showed that the carbon isotope compositions and variation trends of methane and ethane are controlled by the kerogen type, thermal simulation system and thermal evolution degree. The carbon isotope compositions of light hydrocarbon gases derived from different types of kerogens exhibit some differences with increased thermal evolution degree. When the thermal simulation temperature increases from 320 to 550°C, the methane carbon isotope of typically marine source rocks, mainly type I and type II1 kerogens, is slightly higher than or equivalent to that of ethane. However, the ethane carbon isotope of terrestrial source rocks, primarily types II2 and III kerogens, is significantly heavier than methane. By comparison, the evolution trend of carbon isotope compositions of actual natural gas is well consistent with that of light hydrocarbon gases cracked from thermal simulation experiments with increased thermal evolution degree. These findings are of great importance to identify the source rock of actual natural gas with high thermal maturities, as well as to determine the sources of deep shale gas in the southern Sichuan Basin, the Yuanba gas field and the Puguang gas field. Finally, a fitting method of carbon isotope values of methane and ethane of light hydrocarbon gases can be used to more finely differentiate the sources of natural gas in complex areas with mixed sources, mixed maturities and overlapping combinations. [ABSTRACT FROM AUTHOR]

The probable N055°-oriented extinct spreading ridge of the East Sub-basin is cut across by the post-spreading volcanic ridge oriented approximately E-W in its western part and called the Zhenbei-Huangyan seamounts chain. A three-dimensional ocean bottom seismometer survey was conducted in 2011 and covered both the extinct spreading ridge and the Zhenbei-Huangyan seamounts chain in the East Sub-basin. The comprehensive seismic record sections of 39 ocean bottom seismometers underline the high-quality data with clear and reliable P-wave seismic first arrivals. The three-dimensional tomographic results show that the crust is oceanic on each side of the Zhenbei-Huangyan seamounts chain. The seafloor spreading lineaments and associated fracture zones pattern provide channels for magma migration and eruption. Based on the ratio of volcanic extrusive and intrusive materials and the shape of the upward convex 6.4 km/s iso-velocity curve beneath the Zhenbei and Huangyan seamounts, we conclude that the Zhenbei and Huangyan seamounts appear to be dominantly formed by intra-crustal intrusion. Combining previous geochemical studies of post-spreading volcanic ridge outcropping samples, the formation mechanism of the seamounts chain might be explained by a buoyancy-driven decompression melting mechanism occurring as separately evolving partial melting bodies beneath each seamount. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A group of Rb-rich granitic pegmatites near Ziyugou, Danfeng, in the North Qinling Orogen lack typical lithological zoning and have a simple mineralogy dominated by quartz + microcline + albite with garnet in minor quantities. Biotite is rare, and muscovite was mostly formed during greisenization. Graphic texture can be observed within the pegmatites near the contact with the country rock. The pegmatites have localized garnet-rich zones that contain 5-15 vol.% garnet and accessory zircon, columbite, xenotime, and thorite. The pegmatites are peraluminous (A/CNK = ~1.02-1.23) and contain low Ca but high alkali and elevated Rb (~600-1300 ppm). In REE patterns and spider diagrams, the pegmatites exhibit a pronounced negative Eu anomaly, strong Ti, P, Sr, and Ba depletion, and enrichment of Rb, HFSE, and HREE. In addition, the garnet-rich zones contain lower K and Rb but higher Na, Mn, Fe, Zr, Hf, Nb, Y, Th, and U concentrations than the part with graphic texture and accessory garnet. Two types of garnets occur in the pegmatites. Type 1 garnet is well-zoned and closely associated with the above-mentioned HFSE minerals in the garnet-rich zones, whereas Type 2 garnet included in either feldspar or quartz defining graphic texture has no associated HFSE minerals. Both types of garnets are almandine-spessartine solid solution and contain trace to minor amounts of pyrope, grossular, and uvarolite. Based on garnet chemistry, Type 1 garnet can be subdivided into two subtypes, namely, Type 1a with higher Alm content and Type 1b with higher Sps and Prp contents. Unlike garnets from LCT-type pegmatites but similar to those from NYF-type pegmatites, the garnets from the Ziyugou pegmatites contain up to 1.12 wt.% Y2O3 and 1.16 wt.% Yb2O3, which makes garnet the main carrier of Y and HREE. This explains why the garnet-rich part contains higher Y and HREE. Type 1 garnet has higher Mn/Fe ratio than Type 2 garnet, indicating that the garnet-rich part of the pegmatites likely represents a more evolved melt. Although the pegmatites somehow show S-type granite affinity (i.e., elevated Rb concentration and strong depletion in Sr and Ba), the HFSE and HREE enrichment of the pegmatites and garnet chemistry suggest that the pegmatites may have had a hybridized source. [ABSTRACT FROM AUTHOR]