Page 181 - Geology of Carbonate Reservoirs

162 DIAGENETIC CARBONATE RESERVOIRS

dramatic changes in shape or volume. By comparison, karst features such as caves,
caverns, sinkholes, towers, and pinnacles represent massive dissolution of carbonate
strata that produces landforms so large and distinctive that they can be recognized

on seismic profiles (Loucks, 1999 ; Purdy and Waltham, 1999 ). Towers, pinnacles, and
cones, the striking landforms seen in South China, Sarawak, and Java, respectively,
are interpreted to be the result of karst development following regional uplift
attended by lowering of the base level of erosion (Purdy and Waltham, 1999 ). On
the other hand, sinkholes, caves, and caverns are in general more commonly associ-
ated with passive sea - level lowering without tectonic uplift.
Loucks (1999) focused attention on karst - related paleocave systems as hydrocar-

bon reservoirs. He listed as examples of giant oilfields in karst reservoirs the Lower
Ordovician Puckett Field in West Texas, the Permian Yates Field in West Texas, and
the Lower Cretaceous Golden Lane Field in eastern Mexico. Loucks (1999) also
formulated an idealized developmental history of pore types in paleocave reservoirs.
According to his scheme, cave - related pore networks change character with increas-
ing depth of burial. He points out that large dissolution pores may persist with burial
depth to several thousand meters, “ but eventually (the pores) collapse forming
smaller interbreccia pores and fractures associated with crackle and mosaic breccias.
Coarse - interbreccia pores between large clasts are reduced by rotation of clasts to
more stable positions and by rebrecciation of clasts to smaller fragments. As pas-
sages and large interbreccia pores in the cave system collapse, fine - interbreccia

porosity first increases and then decreases, whereas fracture pore types become

more abundant. ” More recently, it has been argued that coastal - zone caves like those
in northeastern Yucat á n, Mexico may be more realistic than continental caves as
modern analogs for paleocave reservoirs (Smart and Whitaker, 2003 ).
Purdy and Waltham (1999) argue that positive karst features such as towers and
cones are associated with uplift, which is in turn commonly associated with tectonic
activity and attendant fracturing. Passive sea - level lowering, in contrast, is typically
associated with more prolonged runoff and sustained dissolution that produces con-
nected vug and channel systems with a high degree of connectivity. In their words
with minor omissions: “ Uplifted grain - dominated carbonates seem to be particularly

well lithified and prone to develop fault and fracture avenues of solution that leave
tower and cone karst as intervening residuals. The net effect is to minimize matrix
porosity in the grain - dominated lithologies. Carbonates with this history essentially
behave as fractured reservoirs. In the case of nonfractured karst residuals, the slope
of the uplifted carbonate surface controls runoff direction and resulting karst mor-
phology. In this case, solution conduits and caves provide large - scale permeability
avenues. In both cases, however, recovery efficiency is geared to matrix porosity: the

higher the matrix porosity, the greater the rate of deliverability of matrix oil to that
produced from fracture and karst conduits. Recovery efficiency is generally higher

in these reservoirs than in their uplifted counterparts. In contrast, a passive drop in
sea level militates against development of positive karst relief. The slopes of the
subaerially exposed surfaces generally are the lesser ones of deposition rather than
uplift and, therefore, the probability of developing runoff directions with more than
a few degrees of slope and resulting karst relief is correspondingly reduced. Addi-
tionally, these subaerially exposed surfaces have reduced potential for the occur-
rence of coeval fracturing and faulting. Extensive moldic and vuggy porosity