PRELIMINARY ORGANIC GEOCHEMICAL STUDIES OF SAMPLES FROM THE NIGER DELTA (NIGERIA) .2. ANALYSES OF SHALE FOR TRITERPENOID DERIVATIVES

Immature shale of the Agbada Formation of the Tertiary Niger delta, Nigeria, cored at various depths < 2400 m in two eastern oilfields, was analysed for triterpenoid derivatives by gas chromatography and computerised gas chromatography-mass spectrometry. The branched and cyclic alkane fraction of the soluble organic matter contained a relatively low concentration of hopanes, namely 22, 29, 30-trisnorhopane; 17 αH, 21βH-hopane; 17αH, 21βH-homohopane (22R or 22S) and homohopane. The dominant triterpenoid hydrocarbons were olean-13(18)-ene and a pair of C28 isomeric pentacyclic triterpanes of unknown structure. Another geochemically significant component, 18αH-oleanane, which was also present in relatively low concentration had so far not been reported in sedimentary rocks. A simulation experiment carried out by heating the immature shale at 300°C in vacuo (0.4 Torr) for ten days resulted in a triterpane composition similar to that of the delta oils. The disappearance of olean-13 (18)-ene and the generation of oleananes had apparently occurred simultaneously with reported structural changes in the hopane components. The presence of the triterpene in immature shale and its apparent transformation to 18αH-oleanane by in vitro thermal alteration was consistent with the current hypothesis of the origin of oleananes in Nigerian petroleum. © 1979.