In-situ analyses of organic matter maturation heterogeneity of uranium-bearing carbonaceous debris within sandstones: A case study from the Ordos Basin in China

Abstract

Carbonaceous debris (CD) within uranium reservoir sandstones of the Zhiluo Formation has been studied in the Shuanglong uranium deposit of the southern Ordos Basin, northern China. Organic matter maturation (OMM) heterogeneity of uranium-bearing CD was analyzed by means of in-situ methods with optical microscope, laser Raman microprobe and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The results show that the contents of vitrinite, inertinite and minerals are 70.49%, 6.54%, and 22.97%, respectively, and the content of exinite is 0, indicating that vitrinite is the major maceral. Vitrinite reflectance (VR) of uranium-bearing CD ranges from 0.65 %Ro to 2.45 %Ro with the maximum difference of 1.80 %Ro and the mean value of 1.85 %Ro, and the distributions of VR frequencies are discrete, indicating that OMM of uranium-bearing CD might be heterogeneous. Uranium minerals in uranium-bearing CD are mostly coffinite and pitchblende (2–10 µm in diameter). For uranium-bearing vitrinite matrix in CD, VR, G peak shifts and uranium contents range from 1.14 %Ro to 1.84 %Ro, from 1586.5 cm−1 to 1600.1 cm−1 and from 0 to 6.0%, respectively, with the corresponding maximum difference of 0.71 %Ro, 13.6 cm−1 and 6.0%, respectively, indicating that uranium-bearing vitrinite matrix may be of heterogeneity. Shift differences of G peak (△G) and full width at half maxima of G peak (GFWHM) are unevenly distributed in uranium-bearing vitrinite matrix, and △G and GFWHM decrease with increased VR, which shows that OMM of uranium-bearing vitrinite matrix is heterogeneous, and its variation is associated with uranium enrichment. The uneven distributions of uranium minerals cause compositional contrast variations of vitrinite matrix. And compositional contrast is divided into three hierarchies by using backscattered electron images of SEM combined with the distributions of uranium contents, i.e., I (free uranium), II (uranium content less than 2%) and III (uranium content more than 2%). With uranium content increasing, VR shows an obvious rise while △G displays a significant decline. Moreover, when close to the uranium-rich area, VR exponentially increases, which implies that OMM increases with the increased difference in compositional contrast. And compared with VR in I hierarchy of compositional contrast, VR in II hierarchy and III hierarchy at least increases by 0.1 %Ro and 0.2 %Ro, respectively, which demonstrates that compositional contrast variation of organic matrix is relevant to OMM heterogeneity, and the increased difference in compositional contrast corresponds to higher OMM zone. Such findings are instructive in variation prospecting of OMM, and then OMM abnormality could act as a guide for uranium deposits prospecting.