RED-BED BLEACHING IN FLUID-ROCK INTERACTIONS：INSIGHTS FROM THE TRIASSIC OF THE LUXI BLOCK, NORTH CHINA CRATON

Red sandstone bleaching is an important process to reveal fluid-rock interaction, playing an important role in indicating the direction, distance of fluid migration. Red-bed bleaching is found in the Triassic and Jurassic of the Luxi Block, North China Craton, but its genesis is less studied. In this paper, red sandstone bleaching in Middle Triassic Liujiagou Formation(T₂l) in north of the Luxi Block is systematically studied by petrological methods, diffraction of x-rays, carbon and oxygen isotopes and raman spectroscopy analysis. The bleaching occurred along bedding and rock fractures in outcrop. The white sandstone section shows the iron oxide grain coating of the mineral edge dissolved under the microscope. Noticing the content of hematite in the bleached sandstone is 0.69%, implying the bleaching did not a complete fluid-rock interaction. Furthermore, the calcite content of bleached sandstone is higher than that of red sandstone, which is evidence of fluid-rock interaction, indicating that there is dissolution of primary calcite and formation of secondary calcite during the reduction fluid migration. Carbon and oxygen isotopes show that δ¹³C vales of the T₂l bleached sandstone are -3.65‰ to -1.75‰ and δ¹⁸O vales of the T₂l bleached sandstone are -14.45‰ to -12.83‰, which representing a hydrocarbon fluid bleaching. In addition, we found that the inclusions in the white sandstone mainly showed fluorescence-type raman spectrogram, indicating that the inclusions contained more aromatic hydrocarbons. All these evidences stated above demonstrating that the red-bed bleaching of T₂l can be attributed to the interaction between petroleum and hematite. However, low solubility of hematite in oil, implies that the bleaching is more likely to be caused by the interaction of rocks with hydrocarbon reducing aqueous fluids such as organic acids. Organic acids can be derived from the epigenesis of organic matter or the water-oil phase. In conclusion, the red-bed bleaching of T₂l is the result of fluid-rock interactions between organic acids and hematite. The oil most probably migrated from the Dongying Sag south to the Luxi Block, for the bleaching occurred adjacent a SN trending fault in the Luxi Block connected with the fault in the Jiyang depression. The estimated oil migration distance is at least eighty-six kilometers.