320
19.5.2 Conventional Petroleum Fingerprinting
19.5.2.1 Terpane Signatures
All of the 2015 southern GoM sediment cores analyzed by Lincoln et al. (2017)
contained a complex hydrocarbon background, consistent with observations of
Boehm and Fiest (1980) and Scholz-Böttcher et al. (2008, 2009). Hopane was abundant and widespread; concentrations ranged from 21 to 711 ng/g sediment, with
highest levels near the Ixtoc 1 wellhead.
Acknowledging the high hydrocarbon background that likely contains petroleum
compounds derived from multiple oil sources, Lincoln et al. (2017) focused on biomarker parameters with the highest potential for differentiating Ixtoc 1 oil residues in
sediments from other oil families in the Bay of Campeche. Terpane signatures measured
in sediments near the Ixtoc 1 site were most consistent with those of southern GoM family 2b oils derived from Tithonian (late Jurassic) source rocks deposited in restricted
marine environments (Guzmán-Vega and Mello 1999). For fingerprinting purposes,
these terpane parameters were considered to have the highest diagnostic potential: C 23
tricyclic terpane, C 24 tetracyclic terpane, C 26 tricyclic terpanes (with R and S stereochemistry), 18R(H)-22,29,30-trisnorneohopane (Ts), 17R(H)-22,29,30-trisnorhopane (Tm),
C 29 norhopane, and the extended chain C 34 and C 35 hopanes (R and S).
The sediment core from site IXW-250 (583 m water depth, 80 km from wellhead; Fig. 19.1) contained the strongest evidence of Ixtoc 1 input based on the above
parameters. In particular, the “triplet ratio” of C 26 R + C 26 S tricyclic terpanes/C 24
tetracyclic terpane (Magoon 1985) and norhopane/hopane ratios showed significant
deviations from background values toward those detected near the Ixtoc 1 wellhead
(Fig. 19.2; Lincoln et al. 2017). The triplet ratio was lowest overall at the Ixtoc 1
site, ranging from 0.5 to 0.6; at other sites it was higher and more variable within
cores. At IXW-250, the triplet ratio declined from background values of ~2 to as low
as 1.2 at a depth of potential Ixtoc 1 input (based on geochronology). Similarly,
norhopane/hopane ratios were typically >1 at the Ixtoc 1 site, consistent with southern GoM family 2b oils (Guzmán-Vega and Mello 1999), and nearly always <1 at
the other sites. At IXW-250, however, this ratio reached a maximum of 1.03 at
7.5 cm, suggestive of Ixtoc 1 oil input (Fig. 19.2).
Principal component analysis focusing on data from analyses for which southern
GoM oil family data were available for comparison (Guzmán-Vega and Mello 1999)
enabled further investigation of potential Ixtoc 1 oil input to the IXW-250 sediment
core: samples from 5 to 9 cmbsf plotted most closely to the family 2b oils
(Bubenheim et al. 2020). The relative contributions of specific Ixtoc 1 biomarkers
to the pool detected in IXW-250 horizons containing evidence of putative impact
(3–9 cmbsf) were estimated using a simple mixing model requiring the assumption
of a compositionally consistent background signal at the site (Lincoln et al. 2017).
Using this approach with the triplet ratio, an estimated 63% of the C 24 tetracyclic
and C 26 tricyclic terpanes in the impacted horizon may be derived from Ixtoc 1 oil.
Applying the same model to norhopane/hopane, Ixtoc 1 input of norhopane and
hopane appears to have contributed 76% of these compounds present at IXW-250.
S. A. Lincoln et al.
19.5.2 Conventional Petroleum Fingerprinting
19.5.2.1 Terpane Signatures
All of the 2015 southern GoM sediment cores analyzed by Lincoln et al. (2017)
contained a complex hydrocarbon background, consistent with observations of
Boehm and Fiest (1980) and Scholz-Böttcher et al. (2008, 2009). Hopane was abundant and widespread; concentrations ranged from 21 to 711 ng/g sediment, with
highest levels near the Ixtoc 1 wellhead.
Acknowledging the high hydrocarbon background that likely contains petroleum
compounds derived from multiple oil sources, Lincoln et al. (2017) focused on biomarker parameters with the highest potential for differentiating Ixtoc 1 oil residues in
sediments from other oil families in the Bay of Campeche. Terpane signatures measured
in sediments near the Ixtoc 1 site were most consistent with those of southern GoM family 2b oils derived from Tithonian (late Jurassic) source rocks deposited in restricted
marine environments (Guzmán-Vega and Mello 1999). For fingerprinting purposes,
these terpane parameters were considered to have the highest diagnostic potential: C 23
tricyclic terpane, C 24 tetracyclic terpane, C 26 tricyclic terpanes (with R and S stereochemistry), 18R(H)-22,29,30-trisnorneohopane (Ts), 17R(H)-22,29,30-trisnorhopane (Tm),
C 29 norhopane, and the extended chain C 34 and C 35 hopanes (R and S).
The sediment core from site IXW-250 (583 m water depth, 80 km from wellhead; Fig. 19.1) contained the strongest evidence of Ixtoc 1 input based on the above
parameters. In particular, the “triplet ratio” of C 26 R + C 26 S tricyclic terpanes/C 24
tetracyclic terpane (Magoon 1985) and norhopane/hopane ratios showed significant
deviations from background values toward those detected near the Ixtoc 1 wellhead
(Fig. 19.2; Lincoln et al. 2017). The triplet ratio was lowest overall at the Ixtoc 1
site, ranging from 0.5 to 0.6; at other sites it was higher and more variable within
cores. At IXW-250, the triplet ratio declined from background values of ~2 to as low
as 1.2 at a depth of potential Ixtoc 1 input (based on geochronology). Similarly,
norhopane/hopane ratios were typically >1 at the Ixtoc 1 site, consistent with southern GoM family 2b oils (Guzmán-Vega and Mello 1999), and nearly always <1 at
the other sites. At IXW-250, however, this ratio reached a maximum of 1.03 at
7.5 cm, suggestive of Ixtoc 1 oil input (Fig. 19.2).
Principal component analysis focusing on data from analyses for which southern
GoM oil family data were available for comparison (Guzmán-Vega and Mello 1999)
enabled further investigation of potential Ixtoc 1 oil input to the IXW-250 sediment
core: samples from 5 to 9 cmbsf plotted most closely to the family 2b oils
(Bubenheim et al. 2020). The relative contributions of specific Ixtoc 1 biomarkers
to the pool detected in IXW-250 horizons containing evidence of putative impact
(3–9 cmbsf) were estimated using a simple mixing model requiring the assumption
of a compositionally consistent background signal at the site (Lincoln et al. 2017).
Using this approach with the triplet ratio, an estimated 63% of the C 24 tetracyclic
and C 26 tricyclic terpanes in the impacted horizon may be derived from Ixtoc 1 oil.
Applying the same model to norhopane/hopane, Ixtoc 1 input of norhopane and
hopane appears to have contributed 76% of these compounds present at IXW-250.
S. A. Lincoln et al.