A trace element is a chemical element whose concentration (or other measure of amount) is very low (a "trace amount").
Application in Source Rock Correlation and Hydrocarbon Migration
Trace elements are widely used in mineralogy and petrology, and the development of rare earth elements geochemistry is especially prominent. By studying the distribution characteristics of trace elements in different rocks, minerals and geological bodies, trace elements as indicator and tracer were used to study complex diagenesis and mineralization, and a quantitative theoretical model was built (Saprykin et al., 1973; Gorham and Janssens, 2005; Zhao, 2010). Now the application of trace elements in petroleum geology mainly includes sedimentary environment discrimination and geochemical exploration (Wang et al., 2005; Peng et al., 2006; Liu and Liu., 2007; Tang et al., 2008). Only in the pre or post 1990’s in the 20th century rare earth elements were used to find oil source in China (Zhao et al., 1996). During the time of rapid development of organic geochemistry, the application of trace elements to hydrocarbon accumulation study suffered stagnation. Now facing multiple solutions of organic geochemical methods under the complicated condition of hydrocarbon accumulation, it is necessary to reconsider the application of trace elements. This paper focuses on the fingerprint feature of trace elements and its application to oil-source correlation and hydrocarbon migration, and looks for a new way to study hydrocarbon accumulation.
The strict definition of trace elements in geochemistry is that if the contents of elements in the object are so low that their activities could be described approximately by using the Henri law, such elements are called trace elements (Zhao, 2010). More than forty trace elements can be identified in crude oil, with the predominant ones as V and Ni (50%~70% of the total) (Li et al., 2008). Because of the action of organic acid, trace elements in source rock were activated and transported to crude oil. Trace elements mainly exist in resin or asphaltene in crude oil as transition metal complex or porphyrin chelate . Accompanying hydrocarbon, trace elements can travel a long distance.
In the samples studied, trace elements obeyed Henri law. In dilute solution, solute distribution between solution and solid was affected by temperature, pressure and components, but independent of the concentration of trace elements. Trace elements are present in crude oil in low contents, but played a significant role, similar with stable isotopes. So they are known as geochemical indicator, tracer, migration path finder, fingerprint, etc. (Sun et al., 2006; Mezhibor et al., 2009). During sedimentation of organic matter, many kinds of trace elements were enriched, and were inherited and saved by hydrocarbons generated from source rocks. The trace elements record the genetic information of source rock like fingerprint, which does not change with destruction, oxidation and biodegradation of oil reservoir (Jin et al., 2001). Abundance and kinds of trace elements in crude oil are related to the species of organisms firstly, and secondly are mainly affected by sedimentary environment of source rock and migration of hydrocarbon (Sun et al., 2009b). During migration the absolute amount of trace elements may change, but the correlation among trace elements remains consistent (Tang et al., 2009). So this change and nochange feature of trace elements during hydrocarbon generation and accumulation can be used to study oil-source correlation and hydrocarbon migration
(1)REE have special geochemical behavior. Their original abundance, distribution form, dispersion or concentration in geological process and migration are all determined by their atomic structure and properties, which make all the REE coexist symbiotically in nature, and have the same geochemical behavior (Curiale, 1983). During migration crude oil may absorb trace elements in surrounding rock and oilfield water, which changes the absolute amount of REE. Because of the special correlation, REE distribution remains consistent. So the indicator feature of REE can be used for oil-source correlation
(2) Transition metals Transition metals mainly exist in asphaltene and non-hydrocarbon components in crude oil (Ding et al., 1992). Because transition metals have non-occupied electron orbits “d”, which are easy to be hybrid and form combination bond with coordinate bond. Heterocyclic or aromatic compounds have plenty of unpaired or non-bonding electrons and all kinds of functional groups, which have strong absorption capacity to cationis. So they have strong complexation ability, and can form stable macromolecule (Wu, 1981; Ding, 1989). (3) Ni and V Ni and V are the main trace elements in crude oil, which exist in resin or asphaltene as nickel/vanadium porphyrin. During migration Ni and V will exchange with surrounding rock, because of the effect of surrounding rock and underground fluid and the sealing ability of cap rock, the change of absolute amount of Ni and V is difficult to determine. It is proved that V/Ni ratio shows a specific trend of variation with hydrocarbon migration, which could be used to do oil-source correlation. V and Cu usually accumulate in asphaltene, and Ni mainly exists in resin. The polarity of asphaltene is stronger than that of resin, so when hydrocarbon migrate, compared with resin, asphaltene is easier to be absorbed by rock grain. Along the migration direction V/Ni ratio shows a decreasing trend (Zhou, 1983). So V/Ni ratio can be an important support for tracing hydrocarbon migration pathway.