The Drill Stem test on the discovery well 15/9-19SR indicated a low API gravity and a very high sulphur content, when compared to other North sea crudes. Further, geochemical analysis was performed to understand the composition, origin and maturity.
| Formation | Sample Type | Geochem. sample. no | Perforated Interval (mRKB) | Gas/oil ratio (SCM/SCM) | API gravity | H2S (ppm) |
|---|---|---|---|---|---|---|
| Hugin | DST#1 | S6891 | 4316-4338 | 98-141 | 30.0-31.1 | 1-6 |
| Values taken from rigsite report | ||||||
| Uncorrected Perforation Depth | ||||||
General information of the crude oil from 15/9-19SR
Geochemistry
Composition of Gas
The gas is found to be relatively similar to the composition found in the North Sea province, expect that this gas has a high carbon dioxide concentration.
The gas isotope signature from well 15/9-19 SR shows a zig zag pattern, and with unusually light, isotope values on going from methane to ethane, propane to butanes.
Isotopic composition of gas from DST#1, well 15/9-19SR
Steadily increasing heavier isotope indicates that the amount of 13C should keep increasing with the carbon number.
δ13C (pronounced “delta C thirteen”) is an isotopic signature, a measure of the ratio of stable isotopes 13C : 12C, reported in parts per thousand (per mil, ‰).
Composition of Oil
The low API gravity (30o) is reflected in the high C15+ fraction (79%) and very low
saturated hydrocarbon content (30%). The aromatic hydrocarbon content is
exceptionally high (57%), whilst asphaltenes are somewhat elevated (3%).
Saturates
Aromatics
They are also cyclic but are derivatives of benzene. Once released into the environment, their mobility and toxicity make them a significant environmental threat. The single ring molecules occur mostly in the lighter fractions, while the more extensive ring systems can be found in the heavier fractions.
Polars
These are polar organic compounds that contain N, S, and O atoms in various functional groups. This classification for the samples seems to generally indicate resins. Resin adds to the stickiness and viscosity of heavy oil. From <>
Asphaltenes
It is a group of high molecular weight components, with approx. 85% carbon. Due to their high amount of polar components and large, complex molecular structure, asphaltenes are considered the cause of numerous problems with production, transportation and processing.
The carbon isotopic composition (δ13C) of the oil (-27.8 ‰) is slightly heavier than is usually observed in the North Sea, and individual fractions surprisingly show almost no variation.
Molecular Composition
Thompson indices show that the light hydrocarbons are depleted in branched and cyclic alkanes relative to n-alkanes and aromatics (e.g. high W, H, S).
Significance of Thompson indices
Paraffinicity
It is defined based on n-alkane (paraffin) versus naphthene ratios. It is associated with increasing levels of maturity. Decrease in paraffinicity in oils is diagnostic of alteration, particularly of biodegradation.
Aromaticity
It is defined on the basis of the relative concentrations of aromatic hydrocarbons and n-alkanes of matching carbon number. These ratios are of primary value in the recognition of Evaporative Fractionation (EF).
Evaporative fractionation was introduced to describe a major process in the alteration of petroleum (Thompson, 1987). Three steps are visualized: (a) the injection of gas into an oil reservoir; (b) development of near or complete saturation at higher pressure; (c) pressure release through faulting resulting in rapid degassing and migration of only a gas phase.
Biomarkers
There are some compounds in crude oils, that are more degradation-resistant in the environment as for example: Pristane, phytane, steranes, triterpanes and porphyrins. These undegradable compounds are known as Biomarkers .
Pr/Ph (Pristane and Pytane) ratio is very low from GC analysis of both the whole oil and saturated hydrocarbon fraction (0.64 and 0.65 respectively).
The chromatogram from analysis of the latter fraction reveals also relatively large amounts of acyclic isoprenoid alkanes and long chain n-alkanes compared to most North Sea oils. These features are reflected in a high Ph/nC18 and low nC17/(nC17+nC27) ratio.
Pr/Ph ratios substantially below unity could be taken as an indicator of petroleum origin and/or highly reducing depositional environments. Very high Pr/Ph ratios (more than 3) are associated with terrestrial sediments. Pr/Ph ratios ranging between 1 and 3 reflect oxidizing depositional environments (Hunt, 1996).
Understanding the importance of Isopreniods/n-alkanes ratio
Isopreniods/n-alkanes (Pr/n-C17 and Ph/n-C18) ratios provide valuable information on biodegradation, maturation and diagenetic conditions.
Isoprenoid hydrocarbons are generally more resistant to biodegradation than normal alkanes. Thus, the ratio of pristane to its neighboring n-alkane C17 is provided as a rough indication to the relative state of biodegradation.
This ratio decreases as weathering proceeds. relatively higher concentrations of C27 steranes indicate more input of marine organic source (Wales, 1985).
The GC trace of the aromatic hydrocarbon fraction contains abundant phenanthrenes which resulted in calculation of relatively low F1, F2 and MPI-1 parameters.
This article quotes the Discovery Well report in the images and analysis.
