Main Factors Controlling the Compositional Variability of Seepage Oils From Trujillo State, Western Venezuela

Crude oil samples from surface seeps in the Douala Basin (southern Cameroon) and from producing fields in the nearby Rio del Rey and Kribi‐Campo sub‐basins were analysed for bulk and molecular geochemical parameters by inductively coupled plasma – mass spectrometry (ICP‐MS), gas chromatography – mass spectrometry (GC‐MS) and isotope ratio mass spectrometry (IRMS). The aims of the study were to assess the composition of the oils, to evaluate the relationship between the seep oils and the oils from producing fields, and to highlight the significance of the data for oil exploration in the region. Chromatograms of the saturate fractions of the oils exhibit biodegradation ranging from very light (PM1 on the scale of Peters and Moldowan, 1993) in oil from the offshore Lokele field in the Rio del Rey sub‐basin, to severe (PM 6+) for seep oils from the Douala Basin. A plot of Pr/n‐C17 (1.3– 5.0) versus Ph/n‐C18 (0.8–2.6) for the samples further supports mild biodegradation in some samples (Lokele, Kole, Ebome), and demonstrates that the oils from the Lokele and Kole fields (Rio Del Rey sub‐basin) and from Ebome field (Kribi‐Campo sub‐basin) originated from mixed organic matter with a dominant marine contribution. The Pr/Ph ratio (1.8–2.3) for the Lokele, Kole and Ebome oil samples, and the V/(V+Ni) ratios (< 0.5) for the seep oils (Douala Basin) and the oils from the Lokele, Kole and Ebome fields, indicate derivation from source rocks deposited in oxic – dysoxic environments. The CPI (1.0–1.1) demonstrates that the Lokele and Ebome oils originated from mature source rocks, with the ratios of C31 22S/(S+R) (0.57 to 0.63) and C30‐βαH/C30‐αβH (0.18–0.23) for the Lokele, Kole and Moudi samples indicating early oil window maturity. Both V/(V+Ni) ratios (0.06–0.22) and δ13C (‐26.96 to ‐24.89 ‰) were used for correlation of the oils, with the seep oils from the Douala Basin showing the closest relationship to the oil from the Lokele field. The presence of mature Type II / III source rocks in different basins in southern Cameroon suggests significant potential for oil exploration in the region.

Section: Oil Qualitymentioning

confidence: 99%

Section: Oil Qualitymentioning

confidence: 99%

Geochemistry of Seep and Produced Oil Samples From Southern Cameroon

Agyingi

Abanda

Fozao

et al. 2019

“…For the Une and Gachetá oils, the apparently abnormal MPI-1 and DNR-1 values may be explained by the presence of two oil charges of differing maturity but originating from a single source rock. However, these values may be due to other effects such as waterwashing (Galarraga et al, 2010).…”

Section: Thermal Maturity Determinationmentioning

confidence: 99%

Geochemical Evaluation of Crude Oils From the Caracara and Tiple Areas, Eastern Llanos Basin, Colombia: Palaeo Biodegradation and Oil Mixing

Esquinas

Márquez

Permanyer

et al. 2018

Self Cite

This study presents an organic geochemical characterization of heavy and liquid oils from Cretaceous and Cenozoic reservoir rocks in the Tiple and Caracara blocks in the eastern Llanos Basin, Colombia. Samples of heavy oil were recovered from the Upper Eocene Mirador Formation and the C7 interval of the Oligocene – Miocene Carbonera Formation; the liquid oils came from these intervals and from the Cretaceous Guadalupe, Une and Gachetá Formations. The heavy oil and most of the liquid oils probably originated from multiple source rocks or source facies, and showed evidence of biodegradation as suggested by the coexistence of n‐alkanes and 25‐norhopanes. The results indicate a close genetic relationship between the samples in the Carbonera (C7 interval), Mirador and Guadalupe Formation reservoirs. These petroleums are interpreted to result from at least two separate oil charges. An early charge (Oligocene to Early Miocene) was derived from marine carbonate and transitional siliciclastic Cretaceous source rocks as indicated by biomarker analysis using GC/MS. This initial oil charge was biodegraded in the reservoir, and was mixed with a later charge (or charges) of fresh oil during the Late Miocene to Pliocene. A relatively high proportion of the unaltered oil charge was recorded for heavy oil samples from the Melero‐1 well in the Tiple block, and is inferred to originate from Cenozoic carbonaceous shale or coaly source rocks. Geochemical parameters suggest that oils from the Gachetá and Une Formations are similar and that they originated from a source different to that of the other oil samples. These two oils do not correlate well with extracts from transitional siliciclastic source rock from the Upper Cretaceous Gachetá Formation in the Ramiriqui‐1 well, located in the LLA 22 block to the north. By contrast, one or more organofacies of the Gachetá Formation may have generated the heavy oil and most of the liquid oil samples. The results suggest that the heavy oils may have formed as a result of biodegradation at the palaeo oil‐water contact, although deasphalting cannot entirely be dismissed.

“…(a, above) m/z 191 and (b, below) generated from a marine source rock (Budzinski et al,1995). However, water washing and biodegradation may have affected phenanthrene and dibenzothiophene homologues and their patterns must be interpreted with care (Galarraga et al, 2010). The average methylphenanthrene/chrysene ratio was 3.6 (± 0.2).…”

Section: Organic Matter Source and Depositional Environmentmentioning

confidence: 99%

“…Nevertheless, these observations could be explained firstly by the modification of the Pr/Ph ratio as a result of biodegradation; but perhaps more significantly by the influence of biodesulphurization and/or water washing on the low contents of dibenzothiophene and other organic sulphur compounds present in crude oils and seepage oils (Lee et al, 1995;Galarraga et al, 2010). In this context, focussing on aromatics, an anomalous methylphenanthrene index value of 0.98 (MPI-1; Radke and Welte, 1983) was recorded, and was possibly a result of water washing (Galarraga et al, 2010) and/or the fact that MPI-1 is not a reliable maturity indicator in organic matter derived from carbonate source rocks (Cassani et al, 1988).…”

Section: Tar Weatheringmentioning

confidence: 99%

Geochemical Composition of Beach Tar From the Se Coast of the Paria Peninsula, Ne Venezuela: Derivation From Natural Seepages

Márquez

Galarraga

Fernández

et al. 2013

Self Cite

Tar residues (“tarballs”) occur frequently on the SE coast of the Paria Peninsula, NE Venezuela. This paper reports on tarballs recovered from approximately 14 km of shoreline during monthly sampling over a two‐year period ending in April, 2011. The tarballs were analysed geochemically and results show that more than 70% of them could be included within a single compositional group on the basis of their physical and organoleptic properties. The tarballs were fingerprinted using biomarkers (hopanes, steranes, alkanes, aromatic steroids, phenanthrenes and dibenzothiophenes) by gas chromatography and gas chromatography – mass spectrometry. Sulphur and trace element contents were also determined. These analyses indicate that the tarballs do not have an anthropogenic origin, but that they probably originated from petroleum generated by argillaceous limestones in the Turonian – Campanian Naparima Hill Formation. This formation includes marls and organic‐rich shales and limestones, and is an important source rock at oilfields in Trinidad and the southern Gulf of Paria. In the southern part of the Gulf, petroleum escapes from Neogene reservoirs to the seafloor via natural seepages associated with the Los Bajos and other fault systems. It is inferred that the petroleum is then transported by wind and tidal currents to the coast of the SE Paria Peninsula where it strands as tarballs. The geochemistry of the tarballs collected is discussed to investigate their source.