Immature, source rocks

One main application of basin modelling has been the assessment of timing and extent of petroleum generation in sedimentary basins using chemical kinetics of kerogen degradation determined experimentally on immature source rock samples (Vandenbroucke et al. 1999). However, in view of the kinetic variability observed in source rocks (Jarvie 1991 Sundaraman 1995) it is questionable whether such predictions are valid. 

Determination of maturity levels is critical to the success of any exploration program. Recovery of immature, but organic-rich, source rocks would indicate good source potential for such rocks if buried more deeply in other parts of the basin. At the other extreme, an overmature source rock would indicate a mature part of the basin not capable of generating additional liquid hydrocarbons, but possibly gas. There are several indicators available that can be used to estimate the relative maturity of a source rock. The traditional method is measuring the maturity of vitrinite. The chemical composition of the maceral... 

Hydrothermal flnids at immatnre back-arc rifts, which occnr behind island-arcs in areas where oceanic crust is being snbdncted (Fig. 13.13), clearly reflect the compositional differences of the source rocks (fractionated, calc-alkaline snites versus MORE). The fluids at immature back-arcs are typically enriched in Zn, Pb, As and Ba (and depleted in Fe) compared to mid-ocean ridge flnids, which is related to the higher concentrations of these elements in the associated lavas (Herzig and Haimington 1995 Table 13.5). 

These results indicated that the Draupne formation in Kitchen 1 west and northwest of Snorre has not reached the level of transformation required by the asphaltene kinetic data from the Snorre field. Kitchen 1 was modelled to only have reached a transformation ratio of less than 5% (Fig. 6) at present. In addition the limited source rock volume in this area cannot account for the petroleum volumes in the Snorre field. Thus, we would suggest on the basis of our maturity modelling, that the proposed kitchen area west and northwest of the Snorre (Kitchen 1) is immature. Shows in the 33/6-2 well, classified as in situ generated, not representative of significant petroleum migration, support this hypothesis. Additionally the fact that the main intra-field maturity trend known in Snorre (Horstad et al. 1995) is the increase in biomarker and aromatic maturity parameters from south to north, suggests clearly that Kitchen 1 did not contribute to Snorre as any influx of lower maturity petroleum from... 

Turbidite samples from Sites 950, 951 and 952 (ODP Leg 157) on the MAP have between 0.75 and 1.68 wt% TOC and C/N ratios near 10. Rock Eval pyrolysis data indicate a fair source rock and immature sediments (Schmincke et al, 1995). The overall organic geochemical shipboard data indicate a predominantly marine origin of the OM (Schmincke et al, 1995), but including a terrigenous component. 

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