Xu Yongchang

Geochemical Characteristics and Origin of Natural Gas in Tarim Basin, China

The Tarim basin, located in northwest China, is one of the largest basins in the world that has not been well explored. In recent years the large scale of petroleum exploration in the basin has led to the discovery of a series of oil and gas fields with an approximate 1:1 ratio of oil to gas resource. This means that the natural gas resource is very important for the hydrocarbon resource in the Tarim basin. In this basin most of the discovered gases are condensate-associated gas and oil-associated gas. Research on chemical components and isotope composition of carbon and hydrogen of natural gases from the Tarim basin shows that all discovered gases in this basin are thermogenic. With respect to source organic matter, there are two types of gases: (1) oil-type gas generated from Sinian to lower Paleozoic marine source rocks with the sapropelic organic matter, with -44.5 to -33.8o/oo of d13C1, -42.0 to -28.1o/oo of d13C2, -35.4 to -28.4o/oo of d13C3, and heavier than -200o/oo of dD1 and (2) coal-type gas generated from Mesozoic terrestrial source rocks with the humic organic matter, with -40.5 to -33.1o/oo of d13C1, -29.7 to -21.3o/oo of d13C2, -26.3 to -20.3o/oo of d13C3, and lighter than -200o/oo of dD1.

Low-mature gases and typical low-mature gas fields in China

No natural gas pool of industrial importance could be formed at the low-evolution stage of organic matter. In the 1980s, on the basis of the development in exploration practice, the hypotheses of bio-thermo-catalytic transitional zone gases and early thermogenic gases were proposed. The lower-limit Ro values for the formation and accumulation of natural gases of industrial importance have been expanded to 0.3%–0.4%. In the light of the two-stage model established on the basis of carbon isotope fractionation in coal-type natural gases, the upper-limit Ro values have been set at 0.8%–1.0%. In terms of the geological practice in the low-mature gas zones and China’s main coal-type gas fields, it is feasible and proper to set the upper-limit Ro value of low-mature gases at 0.8%. Supper-large gas fields such as the Urengoy gas field in western Siberian Basin should belong to low-mature gas fields, of which the natural gas reserves account for more than 20% of the global proven reserves, providing strong evidence for the significance of such a type of resources. The proven natural gas reserves in the Turpan-Hami Basin of China have almost reached 1000 ×108 m3. The main source rocks in this area are the Jurassic Xishanyao Formation, which occurs as a suite of coal series strata. The corresponding thermal evolution indices (Ro) are mainly within the range of about 0.4%–0.8%, the δ13C1 values of methane vary between −44‰ and −39‰ (correspondingly Ro =0.6%–0.8%), and those of ethane are within the range of −29‰–−26‰, indicating that natural gases in the Turpan-Hami Basin should be designated to coal-type low-mature gases. The light hydrocarbon evolution indices of natural gases also fall within the area of low evolution while the precursor type of light hydrocarbons also shows the characteristics of the coal-type. The geological background, carbon isotopic composition and light hydrocarbon index all provide strong evidence suggesting that the proven natural gases in the Turpan-Hami Basin are low-mature gases. In China a gas field with the gas reserves reaching 300 ×108 m3 can be defined as a large gas field, and thus the proven low-mature gases in the Turpan-Hami Basin are equivalent to the reserves of three large gas fields. Its existence is of great significance in research on and exploration of low-mature gases in China.

Genetic identification of natural gases from shallow reservoirs in some oil- and gas-bearing basins of China

Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from −50‰ to −60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ13C1 ranging from −50‰ to −60‰, δ13C2>−35‰, Δ values (δ13C3 - δ13C2)<5‰ and C1/ΣC2+ ratios <40. Immature to low-mature gases display the characteristics of δ13C1 ranging from −50‰ to −60‰, δ13C2<−40‰, Δ values (δ13C3 - δ13C2)>7‰, and C1/ΣC2+ ratios >60.

Influences of water media on the hydrogen isotopic composition of natural gas/methane in the processes of gaseous hydrocarbon generation and evolution

The influences of water media on the hydrogen isotopic composition of organic-thermogenic natural gas were tested in three series of experiments on coal pyrolysis, with no water, deionized water (

The evolution characteristics and fractionation mechanism of carbon isotopes in the process of “multi-stage hydrocarbon generation”

\delta D_{H_2 O}

A Study of Natural Gas Origins in China

=−58‰), and seawater (

CORRELATIVE STUDY ON PARAMETERS OF INORGANIC GEOCHEMISTRY AND HYDROCARBON SOURCE ROCKS FORMATIVE ENVIRONMENT

\delta D_{H_2 O}

Discovery and its geological significance of the mantle-derived helium in the inclusions of the Ordovician oil-bearing reservoir rocks in the Huanghua depression, China

=−4.8‰) added, respectively. The experimental results show that the productivities of H2 and CO2 obviously increased under hydrous conditions and that the productivity of CH4 also remarkably increased in the high-evolution phase of hydrous experiments. Water was involved in the chemical reaction of hydrocarbon generation, and then the hydrogen isotopic composition of methane was affected. There is a linear correlation between the hydrogen isotopic composition of methane and its productivity, as reflected in the three series of experiments. In the case of the same CH4 productivity, the hydrogen isotopic composition of the methane produced in anhydrous experiments was the heaviest, that of the methane produced in seawater-adding experiments came second, and that of the methane produced in deionized water-adding experiments was the lightest. The hydrogen isotopic composition of natural gas/methane is affected by the following factors: 1) the characteristics of hydrogen isotopic composition of organic matter in source rocks, 2) the thermal evolution extent of organic matter, and 3) fossil-water media in the natural gas-generation period. The experimental results show that the influence of the fossil-water medium in the natural gas-generation period was lower than that of the other factors.

Comprehensive geochemical identification of highly evolved marine hydrocarbon source rocks: Organic matter, paleoenvironment and development of effective hydrocarbon source rocks

The Jiyang Sag and the Liaohe Basin are the two important areas where immature oil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, Ro, for source rocks varies from 0.25% (immature) to 0.65% (mature). Studies have shown that within this range of Ro values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e., light→heavy→light→heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bond-breaking ratio of12C-13C also increases, making the relatively12C-rich materials at the low mature stage evolve again towards13C enrichment.The Jiyang Sag and the Liaohe Basin are the two important areas where immature oil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, Ro, for source rocks varies from 0.25% (immature) to 0.65% (mature). Studies have shown that within this range of Ro values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e., light→heavy→light→heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bond-breaking ratio of12C-13C also increases, making the relatively12C-rich materials at the low mature stage evolve again towards13C enrichment.