Hydrocarbon composition of authigenic inclusions: Application to elucidation of petroleum reservoir filling history

Abstract

Geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the Ula Formation in the North Sea Ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir. Using microthermometry and the burial history as a dating tools, it is concluded that the petroleum in the K-feldspar inclusions was present in the more porous and permeable parts of the Ula Formation as early as 45–75 My Bp when the field was at a depth of about 1.0–1.5 km, as compared with the current depth of 3.4 km. This early petroleum, which was trapped as inclusions in authigenic K-feldspar, shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the Ula Formation, which was derived from the Mandal Formation source rock in late Neogene time. Molecular parameters show that the oil in the K-feldspar inclusions is significantly less mature than the crude oil in the present reservoir. The approximate 90°C temperature increase occurring after entrapment of the early petroleum in Kfeldspar (the field is currently at 143°C) appears not to have reset the low maturity signature of the oil in the K-feldspar inclusions. This could suggest that the temperature in the inclusions is too low for isomerization/selective thermal degradation to occur (lack of catalysts?), or that there are other controls on the ratio of some of these parameters. Still, parameters like the ratio of C 21 to C 28 triaromatic steroids, and those based on dimethyl- and trimethyl-naphthalenes, are comparatively similar in both the inclusions and in the reservoir crude. The oil inclusions in authigenic quartz and albite, formed from about 10 My BP (burial depth ≈ 2.5 km) until the present (burial depth = 3.4 km), are interpreted as representing a palaeo-petroleum charge having a composition intermediate between the oil found in K-feldspar inclusions and the oil charge in the present reservoir. Our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil, now found only in K-feldspar inclusions, with oil from the Mandal Formation. Analysis of solution gas in these inclusions suggests that the first petroleum-associated gas to arrive at the trap from the progressively maturing Mandal Formation had a much wetter composition than the current reservoir solution gas. To explain the present comparatively drier and isotopically lighter reservoir solution gas composition, the inference is made that there has been a recent (<5 My Bp) influx of comparatively dry thermogenic gas to the field. This interpretation is independently supported by the noncogenetic δ13 C isotope signature of C 1 relative to δ 13 C 2−4 in the present reservoir solution gas. During formation of K-feldspar inclusions (45–75 My BP), maturation of the Upper Jurassic Mandal Formation, the main source rock in this region, was limited possibly to the Graben axis. This suggests that the petroleum found in the K-feldspar inclusions was most likely derived from a deeper post Caledonian Palaeozoic formation.