The Distribution and Continued Existence of Overpressure in the Delaware Basin?
T. Sinclair, R.E. Swarbrick, and S. Jones
Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK
The concept of overpressure within clastic dominated basins has been known about for several decades. Primarily because in oil and gas exploration, predicting the depth and magnitude of an overpressured reservoir before and while drilling could potentially save the costs of a blow out, kicks, stuck pipe, and other problems associated with overpressured rocks. Overpressure is more commonly associated with young Cenozoic basins. Where the deposition of sediment is rapid enough that the sediments do not have time to de-water, and hence they become overpressured.
Overpressure is also seen in Palaeozoic Basins, such as the Delaware Basin in Texas US, where:
- Deposition of sediment began in the Cambro-Ordovician. The majority of basin subsidence and associated deposition occurred during the Permian.
- The overpressured units are found within shales of Mississippian, Pennsylvanian and Early Permian age.
- The overpressured section is isolated; it is underlain by 1km of normally pressured dolomitic material, and overlain by up to 3 km of normally pressured sedimentary rocks.
If the Delaware Basin became overpressured due to the deposition of 3km of Permian sediment 250 ma, it might be expected that the excess pressure would have dissipated off through geological time. Therefore understanding why Palaeozoic Basins can remain overpressured, could add a new twist to the already extensive knowledge on the behaviour of sediments as they become buried.
By re-assessing the basin’s depositional and tectonic history, an idea of how the basin became overpressured can be obtained, and re-assessing the characteristics of buried sediments, will answer the question as to how the basin is still overpressured through time.
Research using wireline logs and core has already shown the importance that the sediments and their style of deposition have had on the basin. The acoustic response of a shale compacting with depth should follow the set ‘shale compaction curve’ for that basin, as pore space decreases with burial. However the wells in the Delaware Basin are showing two contrasting styles of compaction (Figure 1).
- Those that show very low travel time sonic values indicating either the sediments are very tight, or that they have undergone uplift. These sediments are seen more in the younger Late Permian sequences.
- Those that show high travel time sonic values. These horizons seem correlate to the overpressured zone, where the travel times of the shales shows a marked increase indicating preserved pore space.
By looking at wireline logs, well analysis, petrology of core samples, numerical models of basin evolution (Temis 2D TM ), it is seen that tight horizons throughout the well are acting as good seals that have had an influence on the distribution of overpressure throughout the basin.
Acknowledgements:J. ILIFFE, BP - Sunbury
P. HEPPARD, BP - Houston
R. WEBSTER, - Anadarko – Houston
Figure 1. A sonic compaction plot for shales in the Delaware Basin.