Occurrence and concentration of uranium in the hydrocarbon source rocks of Chang 7 member of Yanchang Formation, Ordos basin.

Abstract

The Seventh Member (abbreviated to Chang-7) of the Yanchang Formation of Late Triassic in the Ordos Basin is a lacustrine suite of U-rich hydrocarbon source rocks. These rocks within the Chang-7 Member are rich in frambodial pyrite, organic matter laminas and collophane. In this paper the uranium occurrence in the rocks of the Chang-7 Member were studied using electron microscopy, LA-ICPMS, sequential extraction procedure and a-track etching. The results show that in the source rocks more than 50% uranium occurs in collophane, and more than 20% uranium is absorbed in organic matter. Collophane plays an important role in the enrichment of uranium. The petrochemical parameters show that the rocks of the Chang-7 Member were formed in anoxic sedimentary environment. Frequent volcanic eruption in the early deposition of the Chang-7 Member provided abundant uranium for the U-rich hydrocarbon source rocks. In the anoxic sedimentary environment during the early diagenetic stage, uranium migrated from tuffs to black shale of the Chang-7 Member and enriched in coUophanes and organic matter laminas as a result of exchange between bottom and interstitial waters. In a word, the anoxic deposition environment, the U-abundant tuffs, and the high contents of organic matter and collophane led to the enrichment of uranium in the U-rich hydrocarbon source rocks of the Chang-7 Member in the Ordos basin.