MAGNETIC AND MINERALOGICAL CHARACTERISTICS OF HYDROCARBON MICROSEEPAGE ABOVE OIL GAS RESERVOIRS OF TUOKU REGION, NORTHERN TARIM BASIN, CHINA/

The Tuoku region in northern Tarim Basin of China is a key area for st udying oil/gas reservoir rocks. The magnetic and mineralogical paramet ers of well cuttings from two wells, well S-7, situated on oil/gas fie ld, and well S-6, at an oil/water interface, were measured. The two we lls are located in the same structure with similar strata and types of lithology, but well S-6 is a showing well of oil and gas 5 km northwe st of well S-7. The purpose of this paper is to evaluate the possibili ty and distribution of secondary magnetic alteration that may have occ urred due to hydrocarbon migration above an oil/gas accumulation. It i s concluded that the magnetism of well cuttings from major strata in w ell S-7, including source rocks, oil reservoir rocks and cap rocks, an d in Quaternary (Q) soil is higher than that from well S-6. The Cambri an oil-bearing strata and cap rocks have even higher magnetism in well S-7. The shape and parameters of magnetic hysteresis loops indicate t hat soft (H-c < 20 mT, H-s < 0.3 T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S-7, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S-6 (for example, low coecivity H-c and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iro n oxide (magnetite, maghemite and hematite) in well S-7 are often high er than those in well S-6 The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S-7, and in strata Ekm and N(2)j are 1.215% and 1.498%, respectively. Typi cal spherical magnetite grains are found within the main source rocks and the soils in well S-7. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is compo sed of aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background.