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TESTING THE SOURCE ROCK MODEL OF-MOD IN A MATURE PETROLEUM PROVINCE

M. Inthorn1, B. Badics2, M. Daszinnies1, O. Lauvrak2, U. Mann1, J.K. Nielsen1, C.Vandré2, and J. Zweigel1, 3

1SINTEF Petroleumsforskning AS, S. P. Andersens vei 15B, NO-7031, Trondheim, Norway, maik.inthorn@iku.sintef.no

2 StatoilHydro ASA, Research Centre, Postboks 7200, NO-5020, Bergen, Norway

3 now with: StatoilHydro ASA, Strandveien 4, 7501 Stjørdal, Norway

This modelling study investigates the source rock potential of Top Pliensbachian to Top

Hauterivian sediments of the Northern Carnarvon Basin offshore north-west Australia

(Figure 1) spanning the eastern Exmouth Sub-basin and the entire Barrow and Dampier

sub-basins (Figure 2). The study area was large (386 x 140 km) and the long modelled

time interval (67 Ma) covered a rather complex tectonic setting. The study was a test of the

capabilities of SINTEF’s Organic Facies Modelling software OF-Mod to predict source

rock quantity and quality away from well control. For this purpose, only half of the wells

with geochemical data were available to SINTEF during modelling, while the remaining

wells were held back by the client (StatoilHydro ASA) until the end of the project. The OF-

Mod model achieved a good fit with the majority of the geochemical data from the hidden

wells confirming the capabilities of the software.

The Carnarvon Basin is Australia’s leading production region for both liquid hydrocarbons

and gas (Baillie and Jacobson, 1997). The region provides an advantageous combination

of good quality, mature Mesozoic source rocks and reservoir rocks of excellent porosity

and permeability. The reservoirs are sealed effectively by several thick shale sequences.

The Carnarvon Basin is an elongate arcuated basin covering an area of 650 000 km2

along the coastline of north-western Australia with 115 000 km2 lying offshore (Baillie and

Jacobson, 1997).The offshore part of the Northern Carnarvon Basin is subdivided into four

major sub-basins (Figure 2). Aligned from SW to NE these are the Exmouth, Barrow,

Dampier and Beagle sub-basins. Although the Carnarvon Basin is a mature petroleum

exploration area, relatively little specific information on the source rocks in this area has

been published so far. The principal effective oil source rock of the Dampier, Barrow and

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Exmouth sub-basins are syn-rift anoxic marine shales of the Dingo Claystone (Oxfordian

to Tithonian, Longley et al., 2002).

The good fit of the OF-Mod model with the majority of the geochemical data from the

hidden wells was achieved by combining the input data with the principle algorithms

accounted for in OF-Mod and general geological understanding based on literature

information. A 3D grid of 2.8 million cells with organic carbon content and quality were

calculated. Maps of source rock thickness, average organic carbon content and hydrogen

index were produced for the most important source rock intervals for later usage in

petroleum system models.

The geological setting with rift basins in a shallow water shelf environment, low distance to

the coastline, and several large river deltas within or in the vicinity of the study area leads

to a high content of terrigenous organic matter in the sediments (e.g. Bishop, 1999;

Longley et al., 2002). This was confirmed by geochemical interpretation of the available

well data and the modelling results. Accordingly, the dominant part of the modelled

Jurassic to Lower Cretaceous section only has poor or fair source rock potential in the

study area.

While the fit of the modelling results with the well data is generally very good for the Lower

Jurassic Athol Fm. and the Early Cretaceous section (Figure 1), the results for the Dingo

Claystone are more ambiguous. One main advantage of the OF-Mod software is that

several conceptual settings can be easily tested within short time. Three settings are

presented differing in the way deposition during the formation of the lower Dingo Claystone

(Oxfordian to Kimmeridgian) was handled. Setting 1, modelling a conservatively low

production of marine organic matter and a completely oxic water column, is the best-fit

model if the entire set of well data is considered. However, this setting results in

dominantly fair and only locally good source rock potential of the lower Dingo Claystone

sediments. In Setting 2 the productivity was increased, while in Setting 3 anoxia was

modelled below 110 m water depth during deposition of the Dingo Claystone. Settings 2

and 3 predict several hundred meters accumulated thickness of sediments with very good

and good source rock potential but overestimate the well data at some well positions. The

XI CONGRESO LATINOAMERICANO DE GEOQUÍMICA ORGÁNICA 2-6 de Noviembre de 2008 Isla de Margarita, Venezuela

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results indicate that anoxia may only have occurred temporally and in parts of the study

area, where the deepest parts of the rift-basins were protected from water exchange by

currents.

References

Australian-Government, (2006) Areas W06-13, W06-14 and W06-15 Barrow Sub-basin,

Carnarvon Basin Western Australia. Release of offshore petroleum

exploration areas Australia (Ed. by Department of Industry, Tourism and

Resources), pp. 33.

Baillie, P.W., Jacobson, E.P., (1997) Prospectivity and exploration history of the Barrow

Sub-basin, Western Australia. APPEA Journal, 37(1), 117-135.

Bishop, M.G., (1999) Total Petroleum Systems of the Northwest Shelf, Australia: The

Dingo-Mungaroo/Barrow and the Locker-Mungaroo/Barrow, Open-File Report

99-50-E (Ed. by U. S. Department of the Interior, U. S. Geological Survey),

pp. 49.

Longley, I.M., Büssenschütt, C., Clydsdale, L., Cubitt, C.J., Davis, R.C., Johnson, M.K.,

Marshall, N.M., Murray, A.P., Somerville, R., Spry, T.B., Thompson, N.B.,

(2002) The North West Shelf of Australia – a Woodside perspective. In: M.

Keep, S.J. Moss (Eds.), The Sedimentary Basins of Western Australia 3:

Proceedings of the Petroleum Exploration Society of Australia Symposium,

Perth, WA, 2002 (Ed. by M. Keep, S.J. Moss), pp. 27-88.

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Figure 1. Regional stratigraphy of the Carnarvon Basin (modified after Australian-Government, 2006). Red box outlines time interval of particular interest.

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Figure 2. Structural elements of the northern Carnarvon Basin (modified after Australian-Government, 2006).