In this study, soil magnetic measurements (susceptibility and hysteretic parameters) and soil hydrocarbon analyses were conducted on samples from three profiles (profiles I and II run across, and profile III runs parallel to the trend of the Jingbian gas field in the Ordos basin, central China) to determine the relationship between themagnetic anomalies (e.g., volume-specific magnetic susceptibility k) and the hydrocarbon seepage environments. The results document a strong correlation between magnetic susceptibility and soil-gas hydrocarbon concentration. Furthermore, the spatial distribution of k and hydrocarbon anomalies correlate with those of the gas field. In addition, magnetic minerals in the soils with higher susceptibility are predominantly magnetite, with little or no substitution of titanium compared to that of samples with lower susceptibility (＜7×10-5 SI [International Unit of susceptibility]). These results provide strong evidences for the formation of highly magnetic minerals in close association with hydrocarbon seepage. Recognition of such seepageinduced magnetic anomalies can be used to facilitate the exploration for oil and gas in China and elsewhere.

In the central segment of the Xiangshui-Mandal Geoscience Transect Archean basement rocks of the North China craton are well exposed. The metamorphic grade of the tilted rock units, which are suggested to represent an oblique cross-section through the middle and lower crust, shows a northwestward progressive increase, from the subgreenschist facies to low amphibolite facies Wutai terrain, through the high amphibolite facies Henshan/Fuping terrains, to the granulite facies Jining terrain. We measured compressional wave (Vp) and shear wave (Vs) velocities and densities at confining pressures up to 600 MPa at room temperature and at temperatures up to 600℃ at 600 MPa on 12 representative rock samples from these terrains and determined the pressure and temperature derivatives of the P-and S-wave velocities and densities. The petrophysical data were correlated with chemical and mineralogical characteristics of the rocks. Based on a regional geotherm for a surface heat flow density of 40mW/mz, we calculated velocity-depth profiles for the various lithologies in order to provide clues for a lithological interpretation of the seismic refraction data reported by Maetal. [1]. The two-dimensional interpretation shows four mega-layers (upper crust, middle crust, upper lower crust and lowermost crust) exhibiting different seismic characteristics. The petrophysical and geochemical studies indicate that the middle crust and upper lower crust of the North. China craton are dominated by felsic and mafic rocks that are similar to the Archean amphibolite-granulite facies tonalitic-trondhjemitic-granodioritic gneisses, amphibolites and mafic granulites exposed in this area. The mafic granulites have lower velocities than usual, due to high contents of opaque minerals. The experimental and geochemical data give hints that the rocks representing the high-velocity part of the lowermost crust (Vp＞7kin/s) are not exposed at the surface. The bulk lower crust is suggested to have a relatively evolved, more felsic composition compared to recent estimates for shield/platform areas.

The Wutai-Jining crustal cross-section is located in the central North China Craton. The upper crustal cross-section is represented by the Wutai granite-greenstone terrain and overlying post-Archean sedimentary rocks, the middle crust by the comparable Henshan and Fuping amphibolite-to granulitefacies terrains, and the upper lower crust by the granulite-facies Jining terrain. Correlation of measured seismic velocities of rocks from the crustal cross-section with data on seismic refractions suggests that the section is likely to represent a ca 30km crust column with the 5 km thick lowermost crust being not exposed (Kern, H., Gao Shan, Liu Qingsheng, 1996. Earth Planet. Sci. Lett. 139, 439-455). Forty-four samples from the cross-section, some of which were used for measurements of seismic velocities and densities (Kern et al., 1996), are analyzed for saturation magnetization (Js) and saturation isothermal remanent magnetization (SIRM). Rock magnetism depends primarily on metamorphic grade and thus on different depth of mineral equilibrium. Lithology plays a less important role. Rocks of greenschist-amphibolite-and granulite-facies have average Js of 58.7, 686 and 1068A/m, respectively. SIRM corresponds to 4.1, 77.9 and 138A/m. Intermediate and felsic granulites from the Jining terrain show even higher magnetism than greenschist-facies metabasalts from the Wutai terrain. Variation coeffcient (Vc) of Js increases from the upper (62.2%) through the middle (64.3%) to the lower (144%) crust. Similarly, SIRM increases from 70.7 to 82.9 and to 165%. This documents considerably greater magnetic heterogeneity in the lower crust compared to the upper and middle crust. For the same lithology, magnetism (Js and SIRM) of rocks from the Jining terrain is remarkably higher than that of rocks from the Archean Taihua granulite terrain at the southern margin of the North China craton adjacent to the Qinling orogenic belt. This is attributed to lower heat flow (52-56mWm-2) in the central North China craton compared to the southern margin of the craton (62mWm-2). Combined with longwavelength magnetic anomalies from Magsat, it is inferred that the mafic ranulite in the cross-section is responsible for lower crustal magnetization in the region. They are comparable to magnetization of lower crust from shield areas in other parts of the world. The strong magnetism of the granulites under this investigation and pyroxenite xenoliths from the nearby Hannuoba alkaline basalt suggest that the magnetic bottom of the lithosphere in the central North China craton lies at the base of the crust or the uppermost mantle.

The alteration of iron-bearing minerals induced by hydrocarbon microseepage above oil/gas reservoirs has been evaluated using measurements of soil magnetic susceptibility κ, geochemical compositions (gas hydrocarbon and alteration carbonateΔC), and composition and concentration of iron-bearing minerals. The analyses were performed along two profiles across the Qiangtang basin in Tibet, China: the Nuoermahu-Xuehuanhu profile (C) and the Mugari-Huochetoushan profile (E). Results show that three strong magnetic anomalies (C1, E1 and E2 anomalies) are related to the distribution of Neogene volcanic rocks on the surface in the Gangmacuo-Xiyaergang uplift. Two other anomalies (C2 and E4 anomalies), characterized by both moderately amplitude magnetic susceptibility and elevated soil gas hydrocarbons, occur near fault zones in the Cuoni-Donghu synclinorium. These latter anomalies display characteristics of hydrocarbon microseepage anomalies commonly associated with oil and gas accumulations. Their presence in the Cuoni-Donghu synclinorium suggests that parts of the Qiangtang basin may have significant petroleum potential.