Trace and Rare Earth Element Partitions in Organic Fractions of Mudstones During the Oil Formation

Abstract

In order to investigate trace and rare earth element partitions in organic fractions of mudstones during the oil formation, the paper isolated organic fractions in mudstones, including insoluble organic fraction (kerogen), soluble organic fraction (extract) or expelled hydrocarbon (reservoir bitumen), and analyzed the isolated organic fractions and their corresponding whole rocks for trace and rare earth element compositions by ICP-MS. Based on the analysis of trace and rare earth element compositions in organic fractions of lacustrine mudstone from the lower Cretaceous in Aer Sag, Erlian Basin and marine mudstone from the lower Cambrian, as well as the upper Ediacaran reservoir bitumens in the Central Sichuan Basin, the following conclusions can be drawn. (a) REE pattern of mudstone extracts is similar to that of kerogens, indicating that REE in soluble hydrocarbons may originate directly from kerogens. During the oil generation of kerogen, middle rare earth elements (Sm-Ho), especially Eu, migrated from kerogen to extract more easily than the rest REEs. (b) REE and redox-sensitive trace elements (e.g., U, Mo and Ni) in mudstone kerogen are enriched relative to whole rock. During the oil generation of kerogen, transition metal elements (e.g., Mo, V, Cr, Co, Ni, Cu and Zn) more easily released from kerogen to extract than the rest elements, especially V and Ni. (c) The enrichment and mobilization of trace elements in organic fractions of mudstones are closely related with their geochemical behaviors. For example, V occurs in not only organic phases of mudstone but also mineral phases. V porphyrin complex has the greater stability than Ni porphyrin complex and V can be bound to sulfur-containing compounds, resulting that V is much more enriched in marine oils than Ni. Compared with V, Ni is more concentrated in organic phases of mudstone. Under the absence of sulfur-containing compounds, Ni is more enriched in lacustrine oils relative to marine oils. (d) The similar trace and rare earth element patterns in reservoir bitumens indicate that trace and rare earth elements can be employed in oil-oil correlation. The enrichment and mobilization of trace elements in reservoir bitumens (e.g., Mo, U, V, Ni, Ba, etc) can provide potential information for predicting the distribution characteristics of trace elements in the expelled hydrocarbons of mudstones (e.g., crude oil, bitumen, etc) and fingerprinting of oil to source.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015