Isotachophoresis results indicate that the total concentrations of organic acids in Ordovician formation waters range from 9.17mg/L to 94.49mgPL and are obviously higher than those of Permo-Carboniferous formation waters. But both of them have the same compositional characteristics, i.e., acetic acid > > formic acid > propanoic acid > butanoic acid. The total alkyl phenol contents of Ordovician formation waters range from 0.57mg/L to 4.73mg/L, averaging 1.77mg/L. Among the samples for phenol detection, 47.06% samples contain less than 1.0mg/L, 35.29% samples 1.0-3.0mg/L, and only 17.65% samples more than 3.0mg/L alkyl phenol. The concentrations of organic acids and alkyl phenols in the Majiagou formation waters are of obviously heterogeneous distribution. There are several areas where the concentrations of organic acids and phenols are high in central Ordos Basin. These organic acid and phenol-high areas (organic acids >45mg/L and phenols > 2.0mg/L) are consistent with natural gas reservoir enriched areas in central Ordos Basin.

Gas sources of Ordovician weathered crust gas reservoirs in Central Gas Field in the Ordos basin are an important problem that has not been completely solved. On the basis of a large number of previous research, geochemical characteristics and indexes of mixed origin natural gases i~om Ordovician weathered crust gas reservoirs in Central Gas Field in the Ordos Basin are systematically studied. The gas sources from this gas field are also discussed through the comprehensive analyses of many indexes. The results demonstrate that natural gases from Ordovician weathered crust reservoirs in Central Gas Field in the Ordos Basin have the characteristics of gas components and isotope compositions of high mature and over-mature dl7 gases. In Central Gas Field in the Ordos Basin, there are both coal-ibrmed gas and oil-type gas, the ibrmer of which comes from Permo-Carborrfierous coal measures and the latter originates from lower Paleozoic marine carbonate. The coal-ibrmed gas is mainly located in the eastern part of Central Gas Field while oil-type gas lies in the northern, western and southern parts of Central Gas Field in the Ordos Basin.

In a thermal simulation experiment of closed-system gold tube-high pressure vessels under a given pressure (50M Pa) and calculation with the specific Kinetics and GOR2Isotope Kinetics softw are's, kinetic parameters of gas generation and methane carbon isotope from Triassic-Jurassic source rock sin the Kuqa dep ression of the Tarim Basin are obtained. The ranges of activation energies of methane paralyzed from Jurassic coal, Jurassic mudstone and Triassic mudstone in the Kuqa Dep ression are 197～268kJömol, 180～260kJ/mol, and 214～289kJ/mol, respectively. The frequency ratios are 5.265×1013/s, 9. 761×1011/s, and 2. 270×1014/s. Th is suggests that there are different activation energies and frequency ratios for methanes generated from Jurassic coal, coal-associated mudstone and T riassic mudstone in the Kuqa Dep ression, and reflect their differences of hydrocarbon generation behaviors. The kinetic parameters of methane carbon isotope are also different in Jurassic coal, Jurassic mudstone and Triassic mudstone. Their average activation energies were 228kJ/mol, 205kJ/mol, and 231kJ/mol, respectively. On the basis of the above research, combined with the geological background, the origin of natural gas in the Yinan-gas pool was further discussed using these kinetic parameters, and its gas accumulation model is thus established. The natural gas of the Yinan 2 gas poolwas primarily derived from Jurassic coal2bearing source rock s at the center of the Yangxia Sag and generally belong to the staged accumulative gas. The Yinan 2 gasw asmainly gathered at 5～0Ma with a Ro range of 1.25%～1. 95% and lost about 25%～ 30%.

Organic petrology is a marginal science that is quite practicable. At present, it has developed into a routine research tool that is widely applied in petroleum exploration and assess ment. Based on several years’research of the authors, this paper presents the advances in organic petrological studies on immature source rocks, including the classification and characteristics of macerals, the composition of macerals and types of organic matter, the abundance and evolution of organic matter, oil2prone macerals, hydrocarbon generation and expulsion. All these results show that organic petrology is of considerable value pertaining to its application in the assessment of immature oil and gas. The immature source rocks consist of various macerals with obvious heterogeneity, contain different hydrocarbon-generating macerals with different oil thresholds and oil peaks, and show a two-staged evolutionary pattern of organic matter.

The modelling of carbon isotope kinetics of natural gas is a leading research direction in oil and gas geochemistry at home and abroad1 Combined with basin sedimentary burial and geothermal histories, the modelling of carbon isotope kinetics provides a new and effective means for evaluating the origin and accumulation history of natural gas pools1 In this paper, we briefly introduce the modelling of carbon isotope kinetics of natural gas formation and its applications to the assessment of natural gas maturity, the determination of gas source, the history of gas migration and accumulation, and the oil-gas ratio1 We show that this approach is of great value for those applications1 The carbon isotopic characteristics of natural gas are not only affected by gas source and maturity, but also are related to accumulation conditions and the geothermal gradient in a basin1 There are obvious differences in the characteristics of carbon isotope ratios between instantaneous gas and cumulative gas1 The kinetic model of carbon isotope fractionation may be different for different basins, which depends on gas source condition, accumulation history and sedimentary-tectonic history1 Since the origin of natural gas pools in the superposed basins of our country is very complicated, and because natural gas pools have features of multiphase and variable gas-sources, this paper may provide new thought s for the study and evaluation of natural gas1