Organic geochemistry of oil-impregnated cores from the Marraat-1 well, Nuussuaq, West Greenland - comparison with surface samples

Field work and drilling on the Nuussuaq peninsula, West Greenland in August 1993 revealed the existence of live oil seeping into vesicular Paleocene lavas. The oil plugs most available porosity in two fault-bounded zones that can be traced more than 1 km along strike and several hundreds of metres across strike and furthennore was confinned in the core of the uppennost 90 m of a 448 m deep borehole.

Based on organic geochemical data (extraction, class separation, GC, GC/MS, and carbon isotopes) from core and from surface samples collected in the vicinity of the drill site the depositional environment and thennal history of the inferred source rock, and the post-migration history of degradation have been interpreted.

From bulk compositions the degradation of the oil is interpreted as varying from very limited to moderate, with a composition varying from solid bitumen to heavy crude oil. Steranes, terpanes, and linear isoprenoids are unaffected by degradation in all samples and show remarkably constant ratios. N-alkanes are always present but are partly degraded in some of the samples. However, this pattern does not correlate with the degree of degradation interpreted from the bulk composition. Therefore active replenishment into the structure is suggested, either from an underlying reservoir or by long distance migration from an actively generating source rock.

The source rock seems to be in the early part of the oil window, judging from the high contents of NSO's, low Ts/fm values and high moretane contents and various biomarker ratios. Hopane isomerisation has reached equilibrium whereas 20S/20S+R sterane isomerisation ratios show values between 0.40 and 0.45. Calculated Methylphenanthrene Index (Average MPil = 0.40, average MPI2 = 0.31) further supports a low thennal maturity.

A number of characteristic fingerprints (high Sa/Ar, high wax, high CPI, high Pr/Ph (4-5), dominance of C29 steranes, low contents of tricyclics and extended hopanes, high contents of phenanthrenes and carbon isotope composition suggest that the oil-prone material in the source rock is terrestrial in origin but was deposited under marine conditions (C30 steranes are present). This is further substantiated by the high contents of bisnorlupane and oleanane which are typical for latest Cretaceous and Tertiary deltaic source rocks.

The implications of this significant discovery are very encouraging for future exploration offshore Labrador, West Greenland and in Melville Bugt since it for the first time demonstrates that a source rock capable of generating oil exists in the region and thereby challenges the 1970s' reputation of the area as being entirely gas-prone.