Tar-Mat Samples as Timing Indicators of Episodic Fill-and-Spill Oil Migration History in from Albian Channel-Splay Complex from the Outeniqua(!) Petroleum System, South Africa
Christopher P. Davies
Business Development, PetroSA, Parow, South Africa
Biomarker analyses of a series of oil and tar-bearing sandstone samples from an Albian shelf-margin channel-splay complex in the Bredasdorp Basin and source rock samples from the immediately subjacent Aptian deep marine shales show a time-related sequence of fill-and-spill along the channel. Optical and pyrolysis analyses from many samples of the source rock both locally and regionally show a maturation history that fits the known multiple-event basin heat flow and is similar to the calculated oil maturities.
As oil was generated from the kerogen, the shale porosity gradually became charged with oil but this oil was only actively expelled from the shale once the threshold oil saturation was exceeded; this threshold oil volume can be estimated from the available source rock analyses. Comparing the known rates of maturation with the calculated oil saturation potential of the shale suggests a punctuated oil expulsion and migration history that broadly matches the fill increments evident from multiple tar-mats in the field.
This compelling synchroneity suggests a complete ‘source-to-reservoir fill’ sequence passing into and through the channel system in which tarmats represent previous oil fills and live oil represents the latest fill(s). The volume of oil shown to be lost from the studied reservoir before the present fill is probably of similar volume to that currently trapped, namely a few tens of MMbbl, constituting an updip target for future drilling.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery