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Shuai Y.H.,Petrochina | Zhang S.C.,Petrochina | Gao Y.,Research Institute of Geology | Lu H.,CAS Guangzhou Institute of Geochemistry | And 4 more authors.
Science China Earth Sciences | Year: 2013

Tight reservoirs are widely distributed, especially in coal measure strata. Identification of the densification mechanism of the tight sandstone reservoirs is critical in effectively exploring and exploiting tight gas/oil resources. In this study, the gas formation from type III organic matter in coal was kinetically modeled for the whole diagenetic stage, from the shallow buried biogas generation stage to the deep buried thermal gas generation stage. The results demonstrated that during hydrocarbon formation, quantities of non-hydrocarbon gases, such as CO2, were generated. The proportion of CO2 is about 50%-70% of that of the C1-5, which far exceeds the CO2 content (0-5%) in the natural gas in the sedimentary basins. Geological case study analysis showed that a considerable part of the "lost" gaseous CO2 was converted into carbonate cement under favorable environments. Under the ideal conditions, the volume of the carbonate cement transformed from total CO2 generated by 1 m3 coal (Junggar Basin Jurassic, TOC 67%) can amount to 0. 32 m3. Obviously, this process plays a very important role in the formation of tight sandstone reservoirs in the coal measures. Our results also show that the kinetic generation processes of C1-5 and CO2 are asynchronous. There are two main stages of CO2 generation, one at the weak diagenetic stage and the other at the over-mature stage, which are different from large-scale multi-stage hydrocarbon gas generation. Therefore, we can understand the mechanism of tight gas charging by determining the filling time for a tight gas reservoir and the key period of CO2 generation. Further analysis and correlation studies of a specific region are of great significance for determining the mechanism and modeling gas charging in tight reservoirs. It should be noted that the formation of tight sandstone reservoirs is the combined result of complex organic-inorganic and water-rock-hydrocarbon interactions. The details of spatial and temporal distributions of the carbonate cement derived from the organic CO2, which combines with metal ions (Ca/Mg/Fe) in the formation water, should be further investigated. © 2013 Science China Press and Springer-Verlag Berlin Heidelberg.

Xia X.-H.,Research Institute of Geology | Tan Y.-J.,General Institute of Geological Investigation | Yang H.-Y.,Research Institute of Geology | Yuan J.-Z.,Research Institute of Geology | And 2 more authors.
Geology in China | Year: 2012

This paper deals with the mineral-bearing characteristics of magmatite complex rocks in the Tianshan metallogenic belt. 4 mineral-bearing complex rock belts and 5 deposit genetic types have been determined. The research on metallogenic specialization shows the following characteristics: 1) Magmatite iron phosphate is controlled by the deep fracture at the edge of the platform; 2) The host rock is mainly of the alkaline ferruginous-mafic rocks suite; 3) The major and trace elements in host rack are obviously inconsistent with each other; 4) The pangenesis of rock types mainly shows the association relationships of biotite diopsidite, pyroxenite, alkaline gabbro, plagioclase and carbonate rock; 5) Mineral pangenesis mainly shows the characteristics of diopside + apatite + magnetite + biotite, plagioclase + apatite + magnetite + soda-zoisite and pyroxene + apatite + magnetite. The elements paregenesis shows notable correlation of P, Fe, Ti, V and REE. Based on prognostic research, the authors delineated 9 ore perspective areas with ore-forming conditions.

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