Page 266 - Geology of Carbonate Reservoirs

FRACTURED RESERVOIRS 247

meability greater than 1000 md is not consistent with average unfractured matrix
rock porosity of only 5%. This reservoir is interpreted to be a Type II fractured
reservoir because fractures provide essential permeability. There may be parts of
the lower, more intensely fractured mounds that behave as Type I fracture
systems.

Structural Setting The Dickinson Unit lies in a portion of the Williston Basin
where major north – south tectonic features overlie zones of vertical, basement fault-
ing. Major displacement along these zones took place during the Carboniferous
Ancestral Rocky Mountain Orogeny and again during the subsequent Laramide
Orogeny (Montgomery, 1996 ). Mapping on a Precambrian datum indicates that a
major tectonic boundary exists in this region of the Williston Basin, an interpreta-
tion supported by estimates of paleoheat flow determined from maturity measure-

ments in the Bakken Shale (Montgomery, 1996 ). DLU reservoirs are in a multimound
complex about one township in size and each field within the unit produces from

one or more of the mounds in the complex, although subregional and regional faults
with attendant fracture permeability may provide flow communication between

mounds. Mounds were initially identified from seismic data as structural anomalies,
not as Waulsortian mounds. Information from the early discoveries identifi ed the
anomalies as microbial mudstone and cementstone mounds and revealed that
the underlying Devonian – Mississippian Bakken Shale is thicker beneath and in the
near vicinity of the mounds than farther away from them. There is disagreement
about whether the thickened Bakken Shale served as a nucleation site on which the
mounds developed. The thickened Bakken Shale is interpreted to be the result of
dissolution of underlying salt combined with faulting (Young et al., 1998 ). Continued
or rejuvenated tectonism probably formed the large - scale fractures.

Depositional and Diagenetic Characteristics The Dickinson Lodgepole mounds

closely resemble the Early Carboniferous (Tournaisian to Vis é an) Waulsortian
mounds worldwide. The Dickinson mounds, like all typical Waulsortian mounds,
began to develop in an outer ramp environment near the toe of slope transition

between ramp and basin floor. Typically, Waulsortian mounds begin growth in the
early stages of transgressions — in the TST segments of stratigraphic sequences. The
presence of heterozoans and the absence of photozoans in the Dickinson area
buildups, along with other deep - water characteristics in the bedded, off - mound
strata, corroborate the distal ramp, deep - water environmental interpretation. The
DLU mounds range in size from about mile in diameter to 4 miles in longest axial
length. Mounds grew in complexes rather than as single domes and the complexes
vary in vertical thickness from 250 to 350 feet. The mounds consist primarily of
mudstones and wackestones with a macrofauna composed of crinoids, bryozoans,
brachiopods, bivalves, sponges, and scattered solitary corals. The lower portions of
the mounds are commonly bedded, while the upper parts are more massive. A
microbiota similar to other Waulsortian mounds around the globe is also present.
Taxonomic diversity in both macro - and microbiota tends to increase toward the
tops of mounds. Although the mounds are composed of mud - supported rocks, some
grainstones and packstones are present as pockets or pods of loose, skeletal allo-
chems that filled low areas between and around mound growth centers. Because the

detrital pockets are not laterally continuous over wide areas, the grainy rocks do