Most of the oils that have accumulated in the reservoirs, may be generated from many source rocks.
Do you mean the oils that generated from different source rocks and then accumulated and mixed in one reservoir under specific pressure and temperature?
Usually different source rocks will give different oils ( at least different contents). Of course different source rocks may give same type of oil but still using biomarkers may give indicator for the type of source rocks!
The best way to distinguish between mixed oils and those from different source rocks is to use biomarkers and isotopes. These technics are with high efficiency in hydrocarbon correlations and definition of different petroleum systems.
To add to Nikola's response; ideally you will need to secure end-members, specifically samples collected from each individual source without any comingling. Then have those individual end-members analyzed for GC fingerprinting, biomarkers, and isotopes. Once each of the end-member has been isolated, identified and categorized, you can then examine your comingled produced sample and determine which zones are contributing the most to the comingled production.
https://labs.weatherford.com/oiltracers
In general I agreewih Nikola and James' answers however it is important to understand not just the composition of end member molecular and isotopic compositions, but it is also important to understand the impacts of thermal maturity, migration and alteration, especially biodegradation on the composition of the oils. While both polycyclic biomarkers and bulk, fractional or compound specific isotopes my be quite useful in distinguishing contributions it is useful to consider the more common compounds to understand mixing proportions more accurately. A pubilished example of the characterization of mixing of oils from two different and distinctive sources is illustrated in the attached paper.
It may be possible if you have the chance to have contrasted contribution for instance a Tertiary source rock with oleanane and Jurassic source rocks without But to really reach the task to have convincing proofs of a mixture is not really easy for you don't know what has been the contribution of each source and at what level of maturity.
Best
As it is mentioned above applying biomarkers is the best way for identifying the mixed oils. Statistical methods such as principal component analysis (PCA) is very usefull. Also there is a method alternating least squares to concentration (ALS-C) that is fully explained by Peters et al. (2008). However, you need to have biomarker concentrations rather than biomarker ratios then you could even determine the ratios of contributions of different source rocks.
The aforementioned paper is
Organic Geochemistry 39(6):623-645 · June 2008
DOI: 10.1016/j.orggeochem.2008.03.001
Best regards
http://www.sciencedirect.com/science/article/pii/S0146638008000740
Dear Dr Al-Khafji,
The biomarker occurrence and their concentration may give a clue to recognize mixed oils from non-mixed oils. For example the occurrence of gammacerane and oleanane together within the same crude oil sample such as in case of Kirkuk-247 and Kirkuk-331. For further information please find the attached paper and look to page 18 (table) and page 21.
Kind regards
