Page 265 - Geology of Carbonate Reservoirs

246 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Figure 8.18 Photograph of a core segment from the Kadrmas 35 - 2 well in the Dickinson

area showing depositional (stromatactis), diagenetic (solution - enlarged vugs), and fracture
porosity that characterizes the Dickinson Field reservoirs. Note the light colored crystals of
fracture - and - vug lining saddle dolomite that were emplaced as late burial diagenetic features.
The core is approximately 4 inches in diameter.

facies consists of a series of platy layers a few centimeters to 15 cm thick. The upper
mound zones are typically massive mudstone and cementstone beds up to a meter
or more in thickness and lacking the well - defined bedding that is associated with

compaction fractures in basal mound strata. Compaction fractures are interpreted
to have formed by overburden loading as the thin, platy layers were continually
fractured by overburden load from the massive, upper buildup accumulation. Com-
paction fractures formed a network of small, brittle fractures that occur only in the
platy, lower portions of the buildups. Larger (multicentimeter - and meter - scale)
fractures are interpreted to have formed by tectonism. A third, even larger scale set
of fractures at multimeter scale are interpreted to have been formed during regional
or subregional faulting. This interpretation was confirmed by monitoring pressure

communication across the field. Reservoir permeability determined by pressure
transient tests varies from 200 to 2000 md and wells 2 – 3 miles apart in a single fi eld
showed pressure responses to tests in minutes to no longer than a few hours (Young
et al., 1998 ). This pressure communication exists between widely separated wells

that are not connected by porous or permeable depositional facies. Moreover, per-