Ordos Basin

The Ordos Basin is located about 300 km to the north of Xi an, central China. With an area of 260,000 km it is the second largest sedimentary basin in China. Its central part is characterised by nearly horizontal strata some 4,000 m thick. The sedimentary column can be divided into three major units Lower Palaeozoic marine carbonates Upper Palaeozoic coal-bearing series and Mesozoic terrestrial sediments. Oil and gas fields occur throughout these sediments. The gas is believed to originate from over-matured Ordovician carbonates and coal beds, whilst the source of the oil is thought to be lacustrine sediments of Triassic age. The region is semi-arid and the soils are thin and poorly developed. 

Jingbian is located in the central part of the Ordos Basin, where the oil and gas potential has long been considered to be poor because favourable structures are not evident in the seismic data. In 1988, however, the Changqing Oil Company proposed a scientific research well. This was preceded by a surface gas geochemical survey along several traverses, one of which passed through the site of the proposed well. The surface material, which is mainly aeolian sand, dry river-bank sediments and loess, was sampled at depths of 2-3 m at intervals of one km along traverse lines. At each sample site, Hg in 

Still other factors point to the significance of the anomalies. The hydrocarbon anomalies in the river bank sediments include the complete range from C to C5, which excludes the possibility of the hydrocarbon anomaly being of surface biological origin. The aeolian sand is a comparatively poor medium for the development and detection of hydrocarbon anomalies in soil, but is highly suitable for the detection of Hg in soil air, for which there is no obvious source other than natural gas at depth. 

Subsequently the scientific research well was drilled as planned. When it penetrated Lower Ordovician carbonates it produced 16.3 x lO m per day of natural gas. The scientific research well thus became the discovery well in this region. Since the well was drilled directly on the surface hydrocarbon gas anomaly, the soil gas survey traverse was thought to cross an apical or linear anomaly. Further work, however, was to reveal that the gas field has an annular anomaly. 

The resulting hydrocarbon gas patterns and Hg patterns are presented in Figs. 6-8 and 6-9, respectively (which also show the location of the traverse discussed above and shown in Fig. 6-7). The regional patterns comprise an arc of anomalous values extending from the southwest to the northeast. This arc is interpreted as part of an annular anomaly that is still open to the north and west. The exploration implication is that a gas field underlies the low values in the northwest of the survey region. In fact the earlier gas survey traverse and the scientific research well that became the discovery well were already located in this part of the region. 

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Fig. 6-7. Geochemical traverse of C, C2 in soils and Hg in soil air, Jingbian gas field, Ordos Basin.

Fig. 6-10. Contour map of Ci in soils, Pinqiao oil field, Ordos Basin.

Gas geochemical surveys of light molecular-weight hydrocarbons (C1-C5) and Hg have proved to be useful methods of detecting oil and gas fields in the Ordos Basin of northern China and of indicating potential oil or gas fields in the Lixian Depression of southern China. The annular anomaly is the type most usually found in these regions, although apical and linear anomalies occur under particular conditions. 

Zhao M, Behr HJ, Ahrendt H, Wemmer K, Ren Z, Zhao Z (1996) Thermal and tectonic history of the Ordos Basin, China evidence from apatite fission track analysis, vitrinite reflectance, and K-Ar dating. Bull Am Assoc Petrol Geol 80 1110-1134... 

Tong Hongshu Hu Baolin 2004. Research on the fractal characteristics of pore of coal reservoirs tested with cryogenic nitrogen adsorption in the ordos basin. Coal Technology, 23(7) 1-3 (in Chinese). 

The study area is located in SE of Ordos Basin, the joint between Jinxi ply belt and Shanbei slope (Fig. 1). It spans within a monoclinal structure, with strata flattish and trend NS. The two parallel faults, which are reverse faults, runs 20 km long, trends NNE and dips ES, with the angle of dip 60° and distance of displacement 25-60 km and depth of the target 2 coal, within Shanxi Formation, runs ranging from 600 m to 1500 m, which indicates a favourable exploration area for coalbed methane (CBM). 

Figure 1. (a) Location of Ordos Basin in China, (b) Figure 2. (a) Coal cores of well A1, (b)Coal cores of well... 

Yang YC, et al. 2004. The age and origin of ground-water in the Ordos basin. Northwestern Geology 36(1) 97-100. 

Gong, S., Peng, P.A., Shuai, Y.H., Dai, J.X., Zhang, W.Z. (2008) Primary migration and secondary alteration of the Upper Paleozoic gas reservoir in Ordos Basin, China— application of fluid inclusion gases. Science in China Series D-Earth Sciences, 51,165-173. 

The carbon dioxide produced during decarboxylation of the organic matter is not the only factor responsible for the formation of secondary porosity, although on a regional scale in the typical sedimentary basins it may explain a certain portion of the secondary porosity volume. The representative petrographic data, e.g. in the Cambro-Ordo-vician sandstones of the Hassi Messaoud field, show that the dissolved feldspars here account at least for 5% in volume. The amount of acid necessary to dissolve the feldspars depends on their mineralogical composition and on the type of dissolved aluminium. For this purpose each mole of mineral dissolved requires at least 1 mole of protons. In the sandstones, carbonate cementation took place probably because of the low solubility of CO in the formation waters migrating upwards. 

Two anchor supports on a monorail row must fiiil in ordo to cause sufiSdent damage to the monorail row to ifislodge assemblies from thdr haiigers. Due to the good condition of the hangers and routine inspections, these fiiilures are assumed to be independent These inq>ections are assumed to be conducted on a quarterty bads concurrent vnth the basin inspection. 

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