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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 759

Last Page: 760

Title: Thermal Contraction and Petroleum Maturation in Michigan Basin: ABSTRACT

Author(s): Jeffrey A. Nunn, Norman H. Sleep

Article Type: Meeting abstract

Abstract:

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Surface cooling of heated continental crust imposes a load on the lithosphere which causes subsidence as the basement rocks contract. Accumulation of sediments in the resulting depression forms a sedimentary basin. Studies of the geometry of sedimentary basins with horizontal dimensions of a few hundred kilometers suggest the lithosphere responds to loads by regional flexure of a strong elastic or viscoelastic upper lithosphere.

A three-dimensional model for flexure of the lithosphere due to thermal contraction is applied to the Michigan basin. Using the observed depth to basement and assuming a continuously filled basin, the magnitude of the thermal contraction load is calculated for each effective flexural rigidity of the lithosphere. For a sediment-mantle density contrast of 0.69 g/cu cm and a thermal decay constant for the lithosphere of 50 m.y., the subsidence record of the sediments and the observed free-air gravity anomaly are most satisfactorily explained by an elastic lithosphere with an effective flexural rigidity of 2 × 1028 dyne-cm or a viscoelastic lithosphere with an effective flexural rigidity of 1031 dyne-cm for a viscosity of 1025 poise.

The temperature history of stratigraphic horizons during basin development is determined from the excess temperature due to the thermal anomaly and the predicted burial history of the sediments. For an equilibrium temperature gradient of 22°C/km, a surface temperature of 10°C and an equilibrium surface heat flow of 1.1 HFU, the maximum paleotemperature and surface heat flow for both the elastic and viscoelastic models are 100°C and 2.5 HFU, respectively. These estimates are consistent with limits set by paleomagnetic studies. The low value for paleotemperature results from the concentration of the thermal anomaly below 15 km.

Once the thermal history of the sediments is specified, the oil potential of the basin can be determined from laboratory derived kinetic equations for the degradation of kerogens to petroleum. For the Michigan basin, temperature conditions sufficient for kerogen conversion have existed only in the Middle and Upper Ordovician and the Lower Silurian sedimentary rocks in the central section of the southern peninsula of Michigan. Published geochemical studies confirm this origin of the petroleum.

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