Balance of Rare-earth Elements in Sand Rocks of Western Siberia Oil-bearing Zones

Abstract

Distribution of REEs upon organic substance (OS) fractions of rocks, their clay component, soluble weak 1.8% HCl, and the insoluble reside (IR) is studied. Studies were performed in sand rocks of oil bearing reservoir beds and embedding sedimentary formations of the Western Siberia lowland Jurassic deposit. As a rule, contents of individual lanthanides in OS is lower than that in initial sand rocks, and REEs in OS of oil bearing rocks are distinguished by high enrichment with Eu and heavier lanthanides, especially Yb, and for OS of embedding sand rocks-with REEs, especially La. AE fractions of oil bearing sand rocks are enriched with light and medium REEs, rarely with Yb; AE embedding sand rocks are distinguished by predominant accumulation of medium, and to a lesser extent, light REEs as compared to the initial rock. The composition of mineral constituent rocks (CR) in all sand rocks in relation to other lanthanides is described by the presence of heavier REEs. The degree of abnormal accumulation of Eu and Yb in oil bearing reservoir bed OSs is maximal for the roof, and in the bottom of beds, a significant excess of Eu only is revealed; minimal similar abnormalities in REE redistribution are expressed in sand rocks of the medium part of reservoir beds. No abnormalities on Eu and Yb in embedding sand rock OSs and their presence in reservoir bed OSs is a geochemical criterion for identification of oil bearing rocks and adjustment of geophysical data in studying oil bearing sections of sedimentary rocks.