Mature Crude Oil Research Articles

The effect of maturation on the distribution of monoethylalkanes in Late Cretaceous sedimentary rocks and crude oils from the Gippsland Basin, Australia

Rock samples from a sedimentary sequence of Late Cretaceous age were analysed for ethylalkanes using GC MS techniques. In the less mature samples, 3- and 5-ethylalkanes were in higher abundance relative to the other isomers at odd carbon numbers from C 17 to C 23. In the more mature samples, this odd preference was no longer apparent. Several other low maturity samples were analysed and found to have a similar ethylalkane distribution to the shallow sample from the sedimentary sequence. A mature crude oil which was also analysed exhibited a distribution similar to the deeper sample from the sedimentary sequence. It is suggested that the initial odd preference is the result of defunctionalisation of specific natural product precursors, and that with increasing maturity this preference is diluted by input of ethylalkanes without preference. A mechanism involving acid-catalysed rearrangement of n-alkenes to form T-branched alkanes is proposed to account for the change in relative ethylalkane abundances.

Biomarkers in fluid inclusions of polymetallic deposit of Qixiashan, Nanjing

Organic fluid involved in Pb-Zn mineralization in the polymetallic deposit of Qixiashan probably derives from bacteria and algae, and is similar to that of the mature crude oil system. Part of lead and zinc in fluids can be transported by organic complexes, while their precipitation is caused by organic reduction. Biomarkers in ores can indicate the source of organic matter in fluids, but organic matter in inclusions must be studied to ascertain organic metallogenesis because of the EOM difference between ores and inclusions. The host strata of Upper Carboniferous (C2) and Lower Permian (PI) in Qixiashan deposit are both oil-bearing in the region. Identification of organic fluid and its biomarkers in the deposit provides significant information for further studies on the relationship between ore deposits and oil pools in the region.

Organic geochemical characterization of bitumens, seeps, rock extracts and condensates from Tanzania

Several bitumens, oil seeps, and rock extracts from several areas of Tanzania have been characterized by a variety of geochemical techniques. Samples analysed included liquid seeps from Pemba, Ruvuma, bitumen extracts from sandstone outcrops from the Wingayongo area, cores and cuttings from Kimbiji East-1, Mnazi Bay-1, Pemba-5 wells and a condensate from Songo Songo-1 well. Samples from two areas, Wingayongo and Msimbati (Ruvuma) were particularly noteworthy and are discussed in detail in this paper. The extracts of the Wingayongo sandstone outcrop are virtually devoid of n-alkanes and steranes; hopanes are dominated by isomers with the ßß-stereochemistry suggesting an unusual source material or maturity history for this sample. The Msimbati seep samples are also characterized by high concentrations of immature hopanoids present as both hydrocarbons and fatty acids. The carbon isotopic compositions for the saturate and aromatic fractions are exceedingly light with δ 13C values of −47 and −59%, respectively. From the results of this study it is proposed that these seeps do not represent the biodegraded residues of mature crude oil but rather are a type of paraffin dirt where bacteria have utilized thermal methane as a substrate and biosynthesized the various compounds found in these samples. Evaluation of the biomarkers in a condensate from Songo Songo suggests the source rocks contained Type II/III kerogen. The only successful correlation that could be made was between a seep on Pemba Island and the Campanian/Maastrichtian formation of the Kimbiji East-1 well. This same seep did not correlate with extracts from the nearby Pemba-5 well and similarly the Ruvuma seeps did not correlate with extracts from the nearby Manzi Bay-1 well.

Thermodynamic stability of various alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers using molecular mechanics calculations

The thermodynamic stability of selected alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers in the C27, C28, C29, C30 and C31 families were calculated using molecular mechanics (MM2) methods and, where possible, calculated equilibrium ratios of certain isomers were compared with observed ratios of isomers in thermally mature crude oil samples. Those calculated and observed ratios having similar values include: (1) the relative distributions among 17β(H)/17α(H) and 21β(H)/21α(H)-hopanes including the absence of the 17β(H),21β(H)- and 17α(H),21α(H)-hopanes; (2) the 22R/22S ratios in 30-methyl-17α-hopane and 30-methyl-17β-moretane; (3) the relative distributions among 17α(H)/17β(H)- and 21α(H)/21β(H)-28,30-bisnorhopanes and among 25,28,30-trisnorhopanes, including the relatively greater stability of 17β(H) isomers in contrast to the regular hopane series; and (4) the ratios of 28(18−17S)abeo hopanes with respect to their unrearranged counterparts including the C27 compounds, Ts/Tm.