Basin Subsidence Decompaction and Burial History Modeling Techniques.

Abstract

A discussion of somc burial and thermal history modelling techniques is given, together with

the applicability of these to some sedimentary basins in Nigeria. The rilcchanisms of basin formation

discussed follow the basic concept that basin subsidence is an isostatic response to thinning of

the crust and cooling of a thermal anomaly. The rate of subsidence! is mainly a function of thc

cooling rate, further modificd by additional load related to replaccrnent of sea water by accumulating

sediments. Compaction effects during sediment diagenesis are discussed. As a result of diff'erential

compaction, the gcometry of strata in a sedimentary basill changcs continually during progressive

burial. Thus, a stratigraphic cross section constructed from present bcd thickness may

differ substantially from the cross section at the time of deposition. Decompaction studies arc

also important in basin analysis because of the thernial effects assuciatcd w~ t hth e con~pactionp rocess.

The isostatic subsidence model has been applied to deterniiric burial history curves, temperature

- tirnc, arid temperature - depth relations for s c d i~~~eonil tsh e Cretaceous h amb r a basin

of Nigeria. With subsidence assumed to havc started about 80 million years ago (LC. in the Santonian),

the model study predicts maturation at fairly shallow depths of 1.5-2.01:m! and overmaturation

at depths below 4km, a disti~lct contrast with the casc of the Nigcr Della Basin. The

predicted geotherma1 gadients, level of organic maturation, and subsidence data are quiic conipatible

with information so far available from the hambra basin.