Page 267 - Geology of Carbonate Reservoirs

248 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

not significantly affect reservoir volume or performance. Locally, however, leached
grainstones and packstones are among the most porous mound facies. Stromatactis -
like cavities are common and volumetrically abundant in the lower and middle parts
of the mounds. These vuggy pores are commonly formed around shelter cavities
beneath fenestrate bryozoan fronds or where sponge bodies existed. Radiaxial
calcite usually lines the cavities. Although variations in microfacies character are
present in the Dickinson mounds, the microfacies are not organized in systematic
patterns that would simplify exploration and development in the fi elds. Instead, the
Dickinson mounds make reservoirs because of the three main types of fractures
that connect diagenetically enhanced depositional porosity — particularly the
solution - enlarged, stromatactoid vugs.
The DLU mounds were deposited as microbially mediated or microbially gener-
ated peloidal mud, skeletal meshworks (especially fenestrate bryozoan meshworks),
and submarine cementstones. Early marine diagenetic radiaxial calcite cements
around fenestrate bryozoans, along with syntaxial overgrowth cements on crinoids,
created a bindstone to cementstone fabric. Radiaxial calcite commonly coats fenes-
trate bryozoan sheets and polyform surfaces to form ornate, lined cavities similar
to classical Stromatactis vugs (Figure 8.18 ). Many of these vugs were enlarged by
early burial dissolution that resulted in enhanced storage capacity for DLU hydro-
carbons. After the early dissolution, at least two generations of calcite cements were
deposited: a blocky calcite pore - filling spar and a dogtooth (scalenohedral) calcite

pore - lining druse. Some of these burial cements were partly to completely dissolved
by a second invasion of undersaturated diagenetic waters. Both vuggy porosity and
fracture porosity were enhanced by this late stage leaching. Etched vug and fracture
surfaces were overlain or partly replaced by saddle dolomite (Figure 8.18 ), some of
which exhibits post - saddle - dolomite leaching and flooding by migrating hydrocar-

bons still present as oil stains. Some saddle dolomites are partly replaced and over-
lain by late - diagenetic anhydrite and some saddle dolomites that partially fi ll
fractures are overlain by small, equant, subhedral dolomite crystals that appear to
have sifted into fractures as internal sediment.

Geological Concept The discovery well at Dickinson Field was drilled on a seismi-

cally defined prospect in the deeper, Ordovician – Silurian Interlake Formation. Dis-

appointing results led to testing in the lower Mississippian Lodgepole Formation.
This decision was based on gas shows in the mud log and oil stains in cuttings, log
analyses of the zone, and problems with lost mud circulation at that stratigraphic
level (Young et al., 1998 ). Prior to the discovery at Dickinson Field, according to
Montgomery ( 1996 ), the Lodgepole was known as a low - volume, sporadic reservoir
along the Nesson anticline, where fracturing was evident. The formation tested tight
with only occasional oil shows elsewhere in the Williston Basin. After the Lodgepole
mounds were recognized as the reservoir at Dickinson Field, 3D seismic surveys
were acquired. The new seismic data along with existing well data enabled the
operators to define the general outlines of the mounds that comprise the Dickinson

complex, but the resolution of the data was not sufficient to outline separate micro-

facies or fractured zones. It was after pressure transient testing and study of whole

cores and image logs that the significance of fractures on reservoir performance was
confirmed (Young et al., 1998 ). As in many other cases, this fractured reservoir was

discovered by accident. It was not recognized as an exploration target and was tested