Identification and occurrence of novel C36–C54 3,4-dialkylthiophenes with an unusual carbon skeleton in immature sediments

A series of novel long-chain 3,4-dialkylthiophenes (C₃₆–C₅₄) was identified in a number of sediments ranging from Pleistocene to Cretaceous. The identifications were based on mass spectral characterisation, desulphurisation and mass spectral data of synthesised model compounds. These organic sulphur compounds are probably formed by sulphur incorporation into mid-chain dimethylalkadienes with two methylenic double bonds. These putative precursor lipids are unprecedented and may be considered rather unusual. The distribution of 3,4-dialkylthiophenes in sediments varies considerably with the depositional palaeoenvironment, indicating that these compounds have a potential as molecular markers reflecting changes in palaeoenvironment.     

Generation and hydrocarbon entrapment within Gondwanan sediments of the Mandapeta area, Krishna-Godavari Basin, India

The discovery of hydrocarbons (mainly gas) in commercial quantities from Gondwanan sediments in the Mandapeta field of Krishna-Godavari Basin, India, provided impetus for intensified exploration in Mandapeta and the adjoining Kommugudem, Draksharama and Endamuru fields. Both oil and gas have been found in the reservoirs of Mandapeta (Triassic) and Golapalli (Early Cretaceous) formations. Mature, localised, basal shales (1.0–1.1% Ro) in the Mandapeta formation have sourced the oils from the Mandapeta Sandstone reservoir (Triassic). The oils being produced from Golapalli Sandstone reservoir (Early Cretaceous) are relatively less mature and have been sourced by the underlying shales in the Mandapeta Formation at a maturity level of 0.80–0.85% Ro. The source and maturity data preclude liquid hydrocarbon sourcing from the Kommugudem (Permian) sequence. Permian coals and shales of the Kommugudem Formation are the major source rocks for gaseous hydrocarbons in this area. The hydrocarbon generation started in Early Cretaceous in the Kommugudem Formation, but the intermittent tectonic activity (with associated structural developments) has resulted in reorientation and redistribution of the then existing trap configurations. The present day maturity level of the Permian sediments in the Mandapeta field is 1.2% Ro or greater, capable of generating gas dominantly. The Raghavapuram shale in the Mandapeta area is adequately mature and has good hydrocarbon potential for oil generation. The probability of finding hydrocarbon reserves in the sands of Raghavapuram shales and other suitable traps is high. Modern seismic information together with geologic models can give new exploration leads.     

Acid catalysis of alkyl hydrogen exchange and configurational isomerisation reactions: acyclic isoprenoid acids

Acid-catalysed alkyl hydrogen exchange and configurational isomerisation has been studied in a series of acyclic isoprenoid acids when they were heated at 160°C in the presence of a montmorillonite. Hydrogen exchange occurred between the adsorbed water of the clay surface and the a position of the isoprenoid acids. In cases where this position was chiral, exchange was accompanied by configurational isomerisation. Configurational isomerisation occurred more slowly in experiments conducted without a clay matrix in the presence of water. These results have been rationalized in terms of a reaction mechanism involving protonation of the carbonyl oxygen causing enolization and consequent hydrogen exchange at the a position of the acids. This mechanism was used to account for the relatively fast rate of isomerisation of C-2 chiral centres in sedimentary acyclic isoprenoid acids during maturation.     

De-A-steroids in immature marine shales

Three series of de-A-diasterenes have been observed in a variety of immature marine shales. Two of them have been identified by comparison with authentic standards and the structures of the third assigned by extrapolation. The greater complexity in the number of series compared with diasterenes indicates that the ring A degradation occurred to differing extents. The rate of C-20 diagenetic isomerisation in the de-A-diasterenes proceeds at a similar rate to that in diasterenes. Two series of B-ring monoaromatic de-A-steroid hydrocarbons have also been found. Comparison with the B-ring monoaromatic anthrasteroids indicates that the de-A-compounds are 14α(H) and 14β(H) isomers. Diagenetic isomerisation of the 14α(H) isomers to the more stable 14β(H) isomers occurs at a rate similar to that in the B-ring monoaromatic anthrasteroid series.     

Sterane isomerisation and moretane/hopane ratios in crude oils derived from Tertiary source rocks

The extent of sterane isomerisation reactions and the moretane/hopane ratios of 234 crude oils, taken world wide, from a wide variety of source rocks of differing geological ages, have been measured.This data indicates that in 78 crude oils derived from Tertiary source rocks, sterane isomerisation reactions as determined by the 20S/(20S + 20R) ration of the C₂₉ 5α(H), 14α(H), 17α(H) normal-steranes and the C₂₉ iso/(iso + normal) ratio [iso = 5α (H), 14β(H), 17β(H)] are mainly incomplete and sometimes considerably so. In addition, the same crude oils have 17β(H), 21α(H)-moretane/17α(H), 21β(H)-hopane ratios which are significantly greater (predominantly in the range 0.10–0.30) than those of crude oils derived from older, mature source rocks (mainly less than 0.1).This data, for crude oils, lends support to the hypothesis, proposed by Mackenzie and McKenzie (1983) for source rock extracts, that the time/temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in Tertiary sediments is generally insufficient, despite generation of crude oil at relatively high temperatures.An alternative hypothesis is that the incomplete sterane isomerisation of Tertiary crude oils may be due to generation of these crude oils from their deltaic, land plant-containing source rocks under low heating conditions.A third hypothesis proposes that the Tertiary crude oils may have picked up the incompletely isomerised steranes from immature sediments during migration. Although possible in particular instances, such a mechanism does not appear to be generally applicable since, in that case, the phenomenon would then appear to be restricted to the Tertiary.The higher moretane/hopane ratios of the Tertiary crude oils could suggest that constraints, similar to those applying in sterane isomerisation, also operate in the conversion of moretane to 17α(H)-hopane.     

Strenes and triterpenes in brown coals

The method of gas chromatography-mass spectrometry was used to identify rearranged 24-methylcholestenes, 24-ethylcholestenes and isohopene in brown coals. Possible routes and sources of formation of these hydrocarbons in nature are discussed. Mass spectra both of original hydrocarbons and of their saturated analogues are given.