Source rocks kitchen

Stainforth presents new models for reservoir filling and mixing. He presents data to support the idea that, in many cases, petroleum does not mix at all during reservoir filling. As new petroleum enters the trap, it fills from the crest of the structure, forcing previously emplaced petroleum downwards. This is a result of the general decrease in fluid density with maturity. This model predicts that the shapes of saturation pressure versus depth curves are related to trap geometry (depth versus volume curves) as well as source rock kitchen parameters. Field data are presented to support this model. 

Fig. 15. API gravities and GORs of oil in a West African Tertiary delta with several stacked reservoirs. Note the excellent correlation between these for both the model predictions and the data, even though the modelled charge GORs are much greater than the solution GORs. This suggests that the solution GORs are controlled by the PVT evolution of the trap, whereas the APIs are controlled by the maturity evolution of the source rock kitchen.

The trap-filling model presented in this paper explains a common paradox of many Tertiary deltas. The oil charge is very gassy, because the source rocks are of mixed Type II/III, even where the source rock kitchens are entirely in the oil window, i.e. no parts have entered the gas window. The outcome of this gassiness is an abundance of reservoirs with gas caps, or ones entirely filled with gas, and abundant evidence of leaked gas in the form of gas chimneys etc. Yet the oils in the traps are often slightly under-saturated with gas regardless of whether they are capped by gas or not. 

As we have seen, the controls on the GOR of the oil in the trap may be different from those of the API gravity. If the oil had a gas cap during part of the trap-filling period, the evolving PT conditions in the trap control the GOR, rather than the evolving maturity of the source rock kitchen. Thus, the shape of a GOR trend versus depth in a trap results mainly from the interaction between trap geometry ((2) above), and... 

The range of API gravity of oil in a trap reflects the maturity change of the source rock kitchen during the trap-filling, constrained by the capacity of the trap. 

Spatial fluid density variations are frequently inherited from the filling history of the reservoir. The initial fluids expelled from a source rock are relatively dense liquids. As a source rock becomes more thermally mature, it expels progressively lighter fluids and eventually gases. When such fluids fill a reservoir, and fill and spill from compartment to compartment within a reservoir, each part of the reservoir can end up with different proportions of fluids of different maturity and density. Field observations show that the segment of the reservoir closest to the source kitchen has often received the latest, lowest density charge. Those areas farthest away from the source kitchen may contain earlier denser fluids that have filled and spilled to their current location. 

Modelling of petroleum generation was performed using asphaltene kinetics determined on petroleum asphaltenes from Snorre oils. This approach was chosen in order to avoid problems associated with the kinetic variability encountered in the Draupne formation. The petroleum asphaltene kinetics was used to delineate the extent of the kitchen area, which reached the time/temperature conditions necessary for the generation of the analysed oil phase. The results thus differ from conventional oil window approximations as we utilize kinetic source rock parameters in the migrated oil for tracing out the generative basin. 

The concept of asphaltene as a tracer of maturity brings us to the concept of active kitchen determination . The asphaltene kinetics delineates the P T conditions which have been subjected to the source rock and contributed to the accumulation. From numerical modelling these areas are identified, compared to traditional SR kinetics and evaluated from the migration modelling. 

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... 

Fig. 7. Maturity development in the deepest are in the 34/5 kitchen area, indicating that the source rock is matching (and in the deepest part partly exceeded) the maturity required to match the asphaltene maturity in the reservoired Snorre oil.

The Snorre Field is located in the northern part of the Tampen Spur (Fig. 7). Major reservoir units are the Triassic Lunde Formation (northern part of the field) and the lower Jurassic Statljord Formation (southern part of the field). The reservoir contains an undersaturated black oil and has a generally low GOR (62-160 Sm /Sm ), typically increasing towards the north. The Snorre Field represents a rotated fault block cut by NNE-SSW and NE-SW faults during Upper Jurassic rifting (Horstad et al. 1995). The main source rock in the area is the Upper Jurassic Draupne Formation, a prolific Type II marine source rock. The underlying Heather Formation is also locally developed as a Type II, oil prone source rock. The main kitchen area of the oil encountered in the Snorre Field is situated in the basin directly east and south of the field (Horstad et al. 1995 Skeie et al. this volume). The Snorre... 

Generation and expulsion. In the western source kitchen, petroleum generation from Upper Jurassic source rocks commenced towards the end of the Cretaceous (56.5 Ma bp) and was almost complete by mid Miocene times (10.4 Ma bp) when transformation ratios of 0.9 were reached. In the easterly kitchen, generation from the Kimmeridge Clay Formation (KCF) source rock was only just starting at this time (TR<0.3). At 10.4 Ma bp, no generation has occurred from Upper Jurassic... 

At present, the westerly kitchen is totally cooked out TR> 0.9), whilst, in the east, the KCF still retains some potential for petroleum generation, as do source rocks on the crest of the structure. 

The black oils tested from 30/7a-3 and 30/7a-6 appear to have been isolated from the deep source kitchen to the west by the major bounding fault to the Joanne structure. It is likely that these oils were sourced from locally mature Upper Jurassic source rocks immediately over-lying the reservoir. 

Figure 9. Oxygen isotope compositions of nominally fresh MORE glasses and whole-rocks. Unfilled boxes are data collected using conventional (resistance heated) fluorination methods between 1966 and 1993 filled boxes are data collected only on glass using laser-based methods. Where these two data types overlap, conventional fluorination data are shown as white-outlined boxes. Data sources Taylor (1968), Muehlenbachs and Clayton (1972), Pineau et al. (1976), Kyser et al. (1982), Muehlenbachs and Byerly (1982), Ito et al. (1987), Barrat et al. (1993), Harmon and Hoefs (1995) and references therein, Eiler et al. (2000b), and Eiler and Kitchen (unpublished data).

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