Page 248 - Geology of Carbonate Reservoirs

DIAGENETIC RESERVOIRS 229

N

DB-1 DF-1

EC-1
EZ-1
SR-1
EO-2
TN-1 -11500
EO-1

EO-13
SMITH CO.
CHEROKEE CO. -11250
STRUCTURE MAP
Top of Cotton Valley Limestone RUSK CO. -11500
contour interval: 25 ft.
0 1 2 3 6 mi -11750
6 km
0 3 -12500 -12250 -12000

Figure 8.11 Present structure on top of the Cotton Valley Lime Formation at Overton Field.
Note that there are two shape trends in the structure: one is generally N – S oriented and the

other is NE – SW oriented. The north – south trend reflects the regional margin trend of the
underlying basement feature — the Ancestral Sabine Uplift. The NE – SW trends refl ect sub-
regional paleo - highs on the ancestral uplift. Circular trends in the western part of the map,
off the edge of the ancestral uplift, reflect younger salt deformation. There was not enough

salt on top of the old basement feature to have been involved in dome formation; therefore
salt structures are absent or unimportant on top of the ancestral uplift. Production in this
field is from oolite grainstones that were at precisely the right paleostructural elevation at

Jurassic time to have been formed as grainstones and subsequently modified by subtle dia-
genetic change to become microporous. Detecting the precise paleostructural elevation that
was necessary for these grainstones to have undergone porosity - enhancing diagenesis requires
interval isopach mapping of the overlying Bossier Shale.
level. As relative sea level advanced, the older oolites were cemented and ooid
microstructure remained well preserved. In contrast, the younger oolites on base-
ment highs underwent porosity - enhancing diagenesis early in their burial history.
Extensive intraparticle microporosity was formed by diagenetic (neomorphic) sta-
bilization of metastable oolites; no evidence of dissolution such as enlarged inter-
particle pores, molds, or vugs was found. Storage volume and productivity depend
instead on the microporosity, but because the micropores are so small, this is a gas
reservoir.