Yongchang Xu

Tectonic and geochemical characteristics and reserved conditions of a mantle source gas accumulation zone in eastern China

Along both sides of the Tancheng-Lujiang Fracture Zone in eastern China, a series of mantle source gas pools constitute a massive-scale tectonic accumulation zone in NNE direction, with the mantle geochemical characteristics of high concentrations of C02 and He, high3He/4He-40Ar/36Ar ratio system and high δ13Coo2 ratios (the main frequency, -3.4%— 4.6%), showing no difference from the tectonic framework of the area. In the area, the tectonic environment is a rift formed as a result of diapiric mantle injection and crust thinning to form graben-type basins and lithospheric fractures. The mantle-derived volcanic rocks and inclusions are well-developed and a high geothermal zone (mantlesource) exists in the area. The characteristics of the three components (solid, liquid and gas) of mantle, concentrated all over the same tectonic space zone, show that the rift system is of a good tectonic environment or passage for mantle degassing and gas migration. The main types of the gas pools are volcano, fault-block, anticline, buried hill and so on, but most of them are combination traps closely related with fracture. For the mantle source gas pools, rift is an optimum tectonic region, and nearby lithospheric fracture, mantle source volcanic rocks or basement uplifts are a favourable structural location when reservoir-caprock association develops.

Geochemistry on mantle-derived volatiles in natural gases from eastern China oil/gas provinces (I)——A novel helium resource——commercial accumulation of mantle-derived helium in the sedimentary crust

Commercial accumulation of mantle-derived helium in the sedimentary shell is discussed. Generally speaking, a commercial helium pool is formed by accumulated4He that comes from uranium and thorium via α-decay; therefore, it has a very low3He/4He value in the magnitude of 10-8. The helium concentration in some gas wells of eastern China oil/gas provinces is about or over 0.05% —0. 1%, consequently forming commercial helium wells (pools), such as the Wangjinta Gas Pool in Songliao Basin, Huangqiao Gas Pool in North Jiangsu Basin and some gas wells in Sanshui Basin. Studies have proved that when the3He/4He value of a helium gas pool is about 3.7 × 10-6 -7.2×10-6 namely mantle-derived helium in its total helium concentration accounts for 33.5%—65.4%, it is a crust-mantle dual-source or dominantly mantle-derived helium gas pool, which is a novel helium resource and its formation is mainly related to the distribution of megafractures.

Isotopic composition characteristics and identification of immature and lowmature oils

Isotopic composition characteristics and the significance of immature and low-mature oils are first systematically discussed. The carbon isotopes of the whole oil can be divided into two groups, one has δ13C main peak values ranging from -30‰ to -29 and the other from -27‰ to-25‰, they are related to lacustrine and salt-lake facies or swamp facies, respectively. The carbon isotopic fractionation among different group components is relatively small, usually less than 2‰ and the biggest difference in fractionation often occurs between saturated and aromatic fractions. Their δD values vary between -180‰ and -130‰ The main peak of their δD values concentrates between -170‰ and -150‰, suggesting a domination of lacustrine fades. However, the secondary peak ranges from -160‰ to -150‰, showing a frequent salinization of paleo-water bodies. The average δ13C values of the methane vary between -50‰ and -52‰, about 10‰ lighter than those of mature oils. There is a relatively good correlationship between immature and low-mature oils and their source rocks in carbon isotopic compositions of group fractions and monohydrocarbons; moreover, compared with the source rocks of mature oils, that of immature oils is often relatively depleted in13C, which is one of the characteristics of immature oils, differing from those of mature oils.

Geochemistry on mantle-derived volatiles in natural gases from eastern China oil/gas provinces (I )——Helium, argon and hydrocarbons in mantle volatiles

Rescarches on helium, argon, carbon dioxide and methane are very significant in studies of mantle substance characteristics and mantle evolution. A < -shaped pattern of the isotope composition distribution of helium and argon sourced from the mantle and the crust, abundance distribution, isotopic composition and reservoir formation of carbon dioxide, and mantle-sourced methane are discussed.Rescarches on helium, argon, carbon dioxide and methane are very significant in studies of mantle substance characteristics and mantle evolution. A < -shaped pattern of the isotope composition distribution of helium and argon sourced from the mantle and the crust, abundance distribution, isotopic composition and reservoir formation of carbon dioxide, and mantle-sourced methane are discussed.

Natural gas geochemistry of low evolutionary stage of medium- and small-sized basins in Yunnan Province, southwestern China

AbstractThe biogenic gas and premature-low mature associated gases in some medium and small-sized basins of Yannan Province, such as Luliang, Yanglin, Baoshan and Jinggu basins, have beed researched. The results show that the biogenic-gas consists mainly of methane which is more than 99% in gasous hydrocarbons and of lighter carbon isotopic composition with δ13C1 values from −60.0‰ to −75.4‰ The methane carbon isotopic compositions in the Baoshan Basin is relatively heavy (δ13C1, −60‰ – −65‰), but those in Luliang and Yanglin basins are lighter (δ13C1, less than −70‰), which implies that the gas field of the Baoshan Basin formed earlier than the others. In the Jinggu Basin, where crude oil is premature-lower mature, the natural gas of associated oil relatively wet and the relative content of methane about 58%–95% in gasous hydrocarbons. Constituently the gas composition is much similar to associated one, but the methane carbon isotopic compositions from −53.8‰ to −57.8‰ are obviously richer in12C than those of general oil fields and similarly characterize the biothermocatalytic transitional zone gas. Their ethane carbon isotopic compositions from −34.6‰ to −29.0% show that they may be derived from type I or II source rock. But for the associated gas from lower evolutionary stage, the heavier ethane carbon isotopic composition as well as the reversed order among the carbon isotopic composition of ethane, propylane and butane also implies that some gases from the type III organic matter are mixed. The

Evolution model and formation mechanism of bio-thermocatalytic transitional zone gas

\delta ^{13} C_{CO_2 }

Hydrogen isotope characteristics of thermogenic methane in Chinese sedimentary basins

of the samples essentially less than −10‰ may be generated from organic matter.

Comprehensive geochemical identification of highly evolved marine carbonate rocks as hydrocarbon-source rocks as exemplified by the Ordos Basin

GC-C-MS on linear isotope analysis equipment makes it possible to measure the hydrocarbon gases at the level of 10−3–10−2 μL. By applying this technique the carbon isotopes of C1-C3 of the adsorbed gas from the Triassic oil sand bed of the Aican-l Well in the Turpan-Hami basin were analysed. The δ13C values of C1-C3 are −55.1‰, −38.6‰ and −35.0‰ respectively. In terms of geochemical characteristics of natural gases and crude oils, in combination with basinal geological backgrounds, it is considered that the reservoir adsorbed gas was formed by crude-oil biodegradation, absorbed by reservoir rocks and its oil-gas source is related with the Permain (perhaps including the Carboniferous). The adsorbed gas is obviously different from the Jurassic coal-generated oil and gas.