Download PDF - Speleogenesis

IMPLICATIONS OF HYPOGENIC TRANSVERSE SPELEOGENESIS
abundant evidence of intense karstification such as bit
drops, sudden rushes of oil during drilling, extremely high
flow rates, etc. In 92 of 400 wells from this field, used for
the analysis by Craig (1988), unfilled cavities were
documented. This gives a 23% probability of hitting
cavities, which can be compared with an average 29.7% of
areal coverage shown by typical hypogenic maze caves
(see Tables 1 and 2). The above value of areal coverage,
however, is calculated from the “cave fields,” whereas the
probability of wells hitting cavities for the Yates field is
derived from the sample of wells not necessarily located
within cave clusters. Distribution of wells that hit caves in
the field (Figure 16.4 in Craig, 1988) clearly shows a
clustered pattern characteristic of hypogenic transverse
speleogenesis.
Caves encountered in the Yates field average 0.9 m in
height and range from 0.3 m to 6.4 m, which are typical of
confined maze caves such as the presently relict Amazing
Maze Cave, located in an adjacent area above the
production horizon of the White and Backed oil fields.
They are concentrated in the upper 15 m of the San Andres
Formation, but also occur in several other stratigraphic
intervals.
In accordance with established views, karst features in
the Yates field were interpreted as late Permian paleokarst,
and their spatial distribution was speculatively fitted to a
model of a freshwater/saltwater mixing zone beneath a
cluster of small limestone islands, which were
hypothetically emergent in the Permian seas. It is the
present author's contention that the model of ascending
hypogenic speleogenesis is a more feasible alternative not
only to the Yates field but for the majority of carbonatehosted
petroleum reservoirs in the Permian Basin region.
5.4. Implications for sinkhole hazard and site
assessments
The sinkhole hazard problem is commonly approached
from the perspective of surface investigations and studies
of subsurface structures by oblique methods, e.g.
geophysics, drilling, etc. Caves inherently lie at the core of
the problem, but the potential for gaining deeper and more
adequate understanding of sinkhole-forming processes
from a speleological perspective remained largely
unexploited, prompting Klimchouk and Lowe (2002) to
draw attention to this possibility.
The speleogenetic approach to the problem is very
promising. Clearly, the difference in cave porosity
structures created by epigenic and hypogenic speleogenetic
processes and respective groundwater flow systems points
to potential peculiarities of sinkhole formation processes.
Klimchouk and Andrejchuk (2005) provided an
instructive case study of sinkhole formation processes in
intrastratal (entrenched, subjacent and deep-seated)
gypsum karst of the western Ukraine, using extensive
hypogenic maze caves to map and investigate breakdown
structures at the cave level (Figure 62). Extrapolated
density of breakdown structures (localities where cavities
had collapsed and the caprock sediments penetrated into
the cave unit) varies from a few hundred to over 5,000
features per square kilometer between different caves and
morphologically distinct areas of large caves, although
only a small proportion of these structures propagate
through the overburden to cause sinkhole expression at the
surface. It was found that, in contrast to the conventional
wisdom, distribution of breakdown structures does not
appreciably correlate with the size of the passages and
rooms. The study has shown that breakdown is initiated
mainly at specific speleogenetically or geologically
“weakened” localities, which fall into a few distinct types.
Most breakdowns that are potent enough to propagate
through the overburden relate with the outlet
cupolas/domepits that represent places where water had
discharged out of a cave to the upper aquifer during the
period of hypogenic transverse speleogenesis. This is
because, by virtue of their origin and hydrogeological
function within a hypogenic transverse system, such
features had exploited the points of lowest integrity within
the main bridging unit of the upper aquifer and the entire
overburden. The study also gave an important insight into
the mechanisms of breakdown propagation to the surface
and demonstrated numerous potential implications of the
speleogenetic approach for more adequate and efficient
sinkhole hazard assessment in areas of hypogenic karst in
stratified sequences.
Vertically extensive breccia pipes (or vertical
breakdown structures) known from many regions of the
world are related to yet another type of hypogenic
transverse speleogenesis, where large cavities are formed
due to buoyancy-driven upward dissolution at the base of
evaporite formations, with fresh water rising from the
basal aquifer and dense brine sinking and outflowing
through the aquifer (Anderson and Kirkland, 1980;
Kempe, 1996). Subsequent collapsing of cavities gives
rise to the formation of breakdown structures (Figure 59).
These propagate through upward-stoping maintained by
active groundwater circulation, accompanied by
dissolution and suffosion (Huntoon, 1996; Klimchouk and
Andrejchuk, 1996). The developing structures drain any
intercepted aquifers and serve as pathways facilitating and
focusing vertical hydraulic communication across thick
sequences. Outstanding examples are breccia pipes within
the Phanerozoic sedimentary succession of the Grand
Canyon region, Arizona, USA. Recent mapping of
fossilized and epigenetically calcified breccia pipes
(“castiles”), performed in the Gypsum Plain region of the
Delaware Basin, USA, identified 1,020 features over an
area of 1,800 km 2 , which suggests an average density of
0.57 features/km 2 (K .Stafford, personal communication).
95