The Tuoku region in northern Tarim Basin of China is a key area for studying oil/gas reservoir rocks. The magnetic and mineralogical parameters of well cuttings from two wells, well S7, situated on oil/gas field, and well S6, at an oil/water interface, were measured. The two wells are located in the same structure with similar strata and types of lithology, but well S6 is a showing well of oil and gas 5km northwest of well S7. The purpose of this paper is to evaluate the possibility and distribution of secondary magnetic alteration that may have occurred due to hydrocarbon migration above an oil/gas accumulation. It is concluded that the magnetism of well cuttings from major strata in well S7, including source rocks, oil reservoir rocks and cap rocks, and in Quaternary (Q) soil is higher than that from well S6. The Cambrian oil2bearing strata and cap rocks have even higher magnetism in well S7. The shape and parameters of magnetic hysteresis loops indicate that soft (Hc＜20mT, Hs＜0.3T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S7, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S6 (for example, low coecivity Hc and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iron oxide (magnetite, maghemite and hematite) in well S7 are often higher than those in well S6. The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S7, and in strata Ekm and N1j are 1.215% and 1.498%, respectively. Typical spherical magnetite grains are found within the main source rocks and the soils in well S7. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is composed of aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background.

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.

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.

为探讨华北克拉通中部地壳的磁性结构与深部地质过程之间的相互关系，测量了五台一集宁地壳剖面44件岩石样品的磁性参量及磁滞回线。结合岩石学、地球化学及磁卫星资料的综合研究结果表明，整个断面具有明显的磁性分带结构特征。上地壳、中地壳与下地壳上部岩石饱和磁化强度（Js）的平均值分别为58.7A/m、681.2A/m与1068.0A/m，而饱和等温剩磁JSIRM为4.1A/m、77.9Am和138.4A/m。磁性与变质相及成分对应分析显示，断面内的磁性结构主要受变质作用控制（尤其是变酸性岩与变基性岩）。Js值的变异系数Vc，上地壳为62.2%、中地壳为62.5%、下地壳为143.7%，而JSIRM值的变异系数则分别为70.7%、86.1%、165.4%。中一下地壳之间vC值的差异远大于上一中地壳，显示了地壳深部磁性强度的非均一分布特征。本区中-下地壳岩石的磁化强度明显高于秦岭造山带北缘的登封群与太华群，这一差异可能与两区地壳深部热结构的明显差异相关。结合磁卫星长波长磁异常分析推测，地壳断面中基性麻粒岩的磁性代表了区域下地壳的磁化强度，如果地壳深部的剩余磁性以热粘滞剩磁（TVRT）为主，则可估算出本区下地壳磁化强度约为4.85A/m，与目前世界上许多地盾区下地壳的磁化强度值较为接近。