Characteristics and mechanism of hydrocarbon alteration of faded sandstone in the uranium-bearing Luohe Formation, Pengyang area, southwestern Ordos Basin

Abstract

Investigating hydrocarbon alteration characteristics and the alteration mechanism of faded sandstone in uranium-bearing red beds is important for understanding the relationship between hydrocarbon-bearing fluids and uranium mineralization. In this study, a series of experiments were conducted on the faded alteration sandstone of the Cretaceous Luohe Formation in Pengyang area of the southwestern Ordos Basin. The results show that the faded alteration of the Luohe Formation is consistent with temporal and spatial pyritization and uranium mineralization and that the alteration degree can be divided into three alteration levels: gray green to gray, yellow to gray white, and reddish brown to light red. Moreover, hydrocarbon alteration is observed in the faded sandstones with good permeability in the Luohe Formation, appearing as oil traces on fresh rock surface, diffuse fluorescence in the cement, hydrocarbon inclusions around fractures, and organic components in pores. In addition, the content of hydrocarbon increases with increasing faded alteration degree. The following results are obtained: (1) The most typical mineralogical response characteristics of hydrocarbon alteration are the decreasing change of trivalent iron oxide and the enrichment of pyrite and uranium minerals with a symbiotic relationship. (2) The hydrocarbon alteration intensity is generally weakened from bottom to bottom and enhanced from southwest to northeast. Except for in the northwest, which is controlled by fault migration, hydrocarbon micro leakage is the main mechanism. (3) Hydrocarbon alteration as a reduction mechanism is mainly affected by the sedimentary environment of the Luohe Formation and the oxidation conditions of the surface fluid, scale of fault, permeability of deep rock, and intensity of hydrocarbon generation and emission from deep source rocks.