Page 249 - Geology of Carbonate Reservoirs

230 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

N DF-1
600
575
DB-1
550

SR-1
EC-1
EZ-1
EO-2
500
EO-1
TN-1
600
500
EC-13 625
650
SMITH CO.
CHEROKEE CO.
RUSK CO.
BOSSIER SHALE ISOPACH
contour interval: 25 ft.
0 1 23 6 mi
525
0 3 6 km

Figure 8.12 Interval isopach of the Bossier Shale Formation that overlies the Cotton Valley

Limestone reservoir at Overton Field. Note that thin intervals in the Bossier Shale corre-
spond to highs on the underlying Cotton Valley Limestone surface. The Bossier thins could
represent local thickening of the Cotton Valley Limestone or antecedent highs beneath the
entire stratigraphic section. In this case, the Bossier thins reflect high areas on the underlying

Ancestral Sabine Uplift and the Cotton Valley Limestone facies on these highs are the res-
ervoir rocks. Off the old highs, the Cotton Valley Limestone includes oolite grainstones on
some salt domes, but they were not at the right paleostructural elevation at the right time to
have been diagenetically altered and they are “ tight. ” An example of the altered ooids and
attendant microcrystalline microporosity are illustrated in Figure 6.7 .

The younger oolite bodies are interpreted to have formed at a higher stand of
relative sea level than the older, non - microporous oolites that were deposited off
the Ancestral Sabine Uplift. The combination of shallow water at time of deposition
and exposure to undersaturated waters during early burial is interpreted to be the
principal cause for microporosity formation, although the origin of this unusual type
of porosity is still debated by carbonate sedimentologists. The time of origin of the
microporosity at Overton Field has also been debated. Ahr ( 1989 ) presented evi-
dence that microporosity formed early in the burial history of the younger oolite

buildups, but Dravis ( 1989 ) argued for a late burial origin. Fretwell ( 1994 ) found
evidence that microporosity did form early in the oolite bodies on the basement