A typical oil contains various amounts of tens of thousands of different compounds, and the relative abundances of those compounds form a "fingerprint" of that oil. This natural fingerprint can be used to answer some of the most important field-development and production-optimization questions that arise during development of unconventional reservoirs. Applications of this natural fingerprint include:
Assessment as to whether or not induced fractures have propagated out of the formation containing a lateral and into either an overlying or underlying zone, causing the commingling of production from multiple intervals.
Quantitative allocation of the contribution of 2–6 individual pay zones to commingled oil or gas production.
Because there are so many different compounds in an oil, even if two oils which occur in adjacent formations are 99% similar in composition, those two oils would still have more than 50 discrete geochemical differences. Any of those geochemical differences between oils from different formations could be used as natural tracers to distinguish the contribution of each reservoir to a commingled production stream.
To construct the oil fingerprint, dead oil samples are analyzed by a specialized type of high-resolution gas chromatography. In the average project, 175–250 different natural tracers are quantified in each sample, and the contribution of individual oils to a commingled sample is calculated by a linear-algebra solution of simultaneous equations, where the number of equations is equal to the number of natural tracers.
This paper illustrates these concepts using oils from 6 wells in Mitchell County, Texas in the Eastern Self of the Permian Basin.
More than 50 years ago, Jones and Smith (1965) reported compositional groupings within a set of more than 310 Permian Basin oils collected from reservoirs ranging in age from Cambrian to Cretaceous. Several years later, Kvenvolden and Squires (1967), Chuber and Rodgers (1968), Frenzel (1968), and Holmquest et al. (1968) demonstrated compositional differences among oils from various reservoirs of the Permian Basin; the differences that they noted were differences in whole oil stable carbon isotope values and molecular distributions. The technological constraints of the day prevented those authors from constructing detailed molecular fingerprints of the oils they studied. However, the whole oil isotopic data reported by Kvenvolden and Squires (1967), Chuber and Rodgers (1968), Frenzel (1968) and Holmquest et al. (1968) are still relevant to field development applications of geochemistry in the Permian Basin – even 50 years after the data were originally published.