Page 239 - Geology of Carbonate Reservoirs

220 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Figure 8.6 Photomicrograph of the principal reservoir rock at Conley Field: intragranular

porosity in bioclastic grainstones and packstones composed primarily of fenestrate bryozoan
fragments and lesser amounts of crinoidal debris. Most of the reservoir porosity occurs as
intraskeletal pores within fenestrate bryozoan fragments that accumulated with criniodal
debris on the crest of an antecedent high in the Mississippian Hardeman Basin of North
Texas. The nonreservoir zones, although structurally high in part, have diagenetically reduced
porosity caused by abundant syntaxial overgrowths on crinoidal grains. The width of the

photo is 500 μ m.
skeletal grains (Figure 8.6 ) that has undergone minor diagenetic alteration, making
it a hybrid reservoir with depositional attributes dominant.

Structural Setting As at the North Haynesville Field, the development of a geo-
logical concept to explain Conley Field began with constructing a present - structure
map on the top of the Chappel Formation (Figure 8.7 ). A structural cross section

(Figure 8.8 ) through the field reveals that the Chappel Formation is thicker in the

field area than away from the field. It also reveals that the underlying Osage Forma-

tion (an informal rock unit name) is thinner in the field area than away from it.

Beneath the Osage Formation is an unconformity on top of the Ordovician Ellen-
burger Formation. The thin Osage interval is interpreted to represent a topographic

high in the field area at the time of Osage deposition. The high area was apparently
too high for thick Osage sediments to accumulate on and around it. Later, the envi-
ronment on the paleo - high evolved to become favorable for skeletal carbonates to
accumulate, resulting in a thickened Chappel section in the field area. This localized

thick shows up as a thin in the interval isopach map of the overlying St. Louis
Limestone, indicating that the Chappel carbonates had accumulated synoptic relief
and had become an in situ loose skeletal grainstone buildup. The next step in devel-
oping the geological concept involved determining why the Chappel section is
thicker in the field area than away from it. Before cores were made available for

study, our initial hypothesis was that the thick section must consist of a localized