AAPG Bulletin, V. 84,
No. 7 (July 2000), P. 961-974.
Paleohydrology of the
Delaware Basin, Western Texas: Overpressure Development, Hydrocarbon Migration,
and Ore Genesis1
Ming-Kuo Lee and Daphne
D. Williams2
©Copyright 2000. The American
Association of Petroleum Geologists. All rights reserved.
1Manuscript received September 2,
1998; revised manuscript received October 15, 1999; final acceptance December
15, 1999.
2Department of Geology, Auburn University,
Auburn, Alabama 36849; e-mail: [email protected]
This research was supported by grants
from the AAPG Foundation (to Daphne Williams) and by the Petroleum Research
Fund, administered by the American Chemical Society under ACS-PRF 33111-GB8
(to Ming-Kuo Lee). We thank Craig Bethke (University of Illinois) for providing
the Basin2 computer software for basin hydrology simulations.
ABSTRACT
This study integrates quantitative
modeling techniques with field observations to establish a paleohydrologic
framework of the Delaware basin, western Texas. The reconstructed paleohydrologic
models allow for a better understanding of the development and maintenance
of anomalous overpressures, hydro carbon generation and migration, and
ore genesis in the basin. Results of numerical modeling show that disequilibrium
compaction and oil generation might generate excess fluid pressures during
the Late Permian in response to the rapid deposition of evaporite beds.
The preservation of this overpressure to the present, however, requires
the presence of an extremely low-permeability (<10-11 d)
top seal. Most shaly sediments, with permeability ranging from 10-4
to 10-8 d, thus may be too permeable, by several orders of magnitude,
to preserve overpressure for more than 250 m.y. The predicted present-day
gas window is located within the overpressure zone, suggesting that the
volume increase associated with the oil-to-gas conversion may be attributed
to present overpressures. The native sulfur deposits likely formed in a
fluid mixing zone resulting from the Laramide uplift of the western basin
during the Tertiary. In our model, meteoric water recharged along the basin's
uplifted western margin and discharged basinward. Hydrocarbons migrated
landward by pressure gradients and buoyancy and discharged upward along
faults in the western basin, where they mixed with meteoric water. Many
oil and mineral reservoirs may have formed in the fluid mixing zone, where
extensive chemical reactions take place. In the Culberson sulfur ore district,
for example, fluids including hydrocarbons and meteoric water migrated
upward through faults from underlying carrier beds, into the Permian Salado
limestone. There, the mixture of fluid drives biochemical reactions that
precipitate native sulfur.
