Seismic Evidence of the Effects of Carbonate Karst Collapse on Overlying
Clastic Stratigraphy and Reservoir Compartmentalization1
A. Hardage,2 D. L. Carr, D. E. Lancaster,
J. L. Simmons, Jr.,
Y. Elphick, V. M. Pendleton, and R. A. Johns1
team, composed of stratigraphers, petrophysicists, reservoir engineers,
and geophysicists, studied a portion of Boonsville gas field in
the Fort Worth Basin of north-central Texas to determine how modern
geophysical, geological, and engineering techniques can be combined
to understand the mechanisms by which fluvio-deltaic depositional
processes create reservoir compartmentalization in a low- to moderate-accommodation
basin. An extensive database involving well logs, cores, production,
and pressure data from more than 200 wells, 26 mi2
(67 km2) of 3-D seismic data, vertical
seismic profiles (VSPs), and checkshots was assembled to support
this investigation. We found the most important geologic influence
on stratigraphy and reservoir compartmentalization in this basin
to be the existence of numerous karst collapse chimneys over the
26-mi2 (67 km2)
area covered by the 3-D seismic grid. These near-vertical karst
collapses originated in, or near, the deep Ordovician-age Ellenburger
carbonate section and created vertical chimneys extending as high
as 2500 ft (610 m) above their point of origin, causing significant
disruptions in the overlying clastic strata.
disruptions tend to be circular in map view, having diameters ranging
from approximately 500 ft (150 m) to as much as 3000 ft (915 m)
in some cases. Within our study area, these karst features were
spaced 2000 ft (610 m) to 6000 ft (1830 m) apart, on average. The
tallest karst collapse zones reached into the Middle Pennsylvanian
Strawn section, which is some 2500 ft (760 m) above the Ellenburger
carbonate where the karst generation began.
We used 3-D
seismic imaging to show how these karst features affected the strata
above the Ellenburger and how they have created a well-documented
reservoir compartment in the Upper Caddo, an upper Atoka valley-fill
sandstone that typically occurs 2000 ft (610 m) above the Ellenburger.
By correlating these 3-D seismic images with outcrops of Ellenburger
karst collapses, we document that the physical dimensions (height,
diameter, cross-sectional area) of the seismic disruptions observed
in the 3-D data equate to the karst dimensions seen in outcrops.
We also document that this Ellenburger carbonate dissolution phenomenon
extends over at least 500 mi (800 km), and by inference we suggest
karst models like we describe here may occur in any basin that has
a deep, relatively thick section of Paleozoic carbonates that underlie
Vol. 61, No. 5, pp. 13361350, 11 figs., 1 table.
of Economic Geology, The University of Texas at Austin, University
Station Box X, Austin, Texas 78713; e-mail: email@example.com.