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Pattern Recognition Techniques in Petroleum Geochemistry. L. Scott Ramos and Brian G. Rohrback Infometrix, Inc. Daniel M. Jarvie. Humble Instruments & Services, Inc. Computer-Assisted Geochemistry.
By overlaying chromatograms we can look both at the similarities and the differences in the crude oils. Software can use this underlying pattern to build quantitative and objective models.
Source rock typing can be done by using GC, GC/MS and stable isotopes on crude oils.
We employ a series of chemometric models to first separate the samples based on gross characteristics (I.e., lacustrine versus marine) and then use fine tuning models to further characterize samples.
Source Rock Type # of OilsMarine Shale 146Paralic/Deltaic Marine Shale 26Marine Carbonate/Marl 157Evaporite/Hypersaline Marls 11Coal/Resinitic Terrestrial Source 29Lacustrine, Fresh 35Lacustrine, Saline 20
The issue here is to assemble data on a sufficient number of oils to make the library valuable.
A data matrix is constructed based on geochemically significant ratios drawn from the GC, GC/MS and stable carbon isotopes (saturate and aromatic).
Chromatography allows us to determine if one reservoir is linked to another by looking at marker peaks that show between the normal alkanes. This process can be done either by choosing an appropriate set of marker peaks ahead of time or by evaluating the whole chromatographic pattern.
GC is usually the technique of choice due to the lower cost of analysis and faster turnaround time.
We can use chromatographic patterns to determine the relative yield from more than one reservoir even where there is no casing.
In this example, the field is undergoing water flood to drive the oil to producing wells. One of the producing zones is significantly more porous than the other. Because pumping water is the primary cost, knowing the relative yields from each reservoir is important.
Pattern recognition also can flag the unusual . . .
Production in the latest 30 production intervals (bbl/day)
After closing Well 696 in and pressurizing the reservoir system, an increase in production was noted.
Are the differences in hydrocarbon distribution significant?
Some other wells also seem to show Zone C input.
Yield by Zone in the latest 30 production intervals (bbl/day)
We have an implied interpretation based on the geochemical differences in the chromatograms.
Well 696, Region 4
Production 23 bbls/day
13% Zone A; 17% Zone B; 70% Zone C
Perhaps the best way to display the interpretation is by color-coding a map.
A Zone Dominates
C Zone Significant
B Zone Dominates