Page 121 - Geology of Carbonate Reservoirs

102 STRATIGRAPHIC PRINCIPLES

successions such as those described earlier in this chapter. Flow units, on the other
hand, may be less than 1 m thick, may extend only over a few acres, and may occupy
all or only part of one ideal depositional succession. Flow units may occupy all or

part of parasequences that, in turn, could occur as field - scale (or smaller), shallow-
ing - upward cycles. On platforms with complex bathymetry, shallowing - upward
cycles may not cover large areas. Grainstone buildups on small antecedent highs are

a good example. On the other hand, monotonously flat platforms may have shal-
lowing - upward successions that extend over many square kilometers. An example

of such a large, relatively flat platform existed during Permian times from northern
Texas through Kansas. During this time, the Permian Chase Group that contains

the reservoir for the Guymon – Hugoton gas field was deposited (Siemers and Ahr,

1990 ; Olson et al., 1997 ; Mazzullo, 1994 ) Parasequences can sometimes be grouped
into parasequence sets , where vertical trends in parasequence thicknesses and lithol-
ogy display stacking patterns , in the manner discussed by Van Wagoner et al. (1990) .
These diagnostic trends in thickness and lithology are used to interpret the history
of relative sea - level change and the relationship between parasequences and

sequences at field scale or larger.
4.5.2 Sequence Stratigraphy in Carbonate Reservoirs

Carbonate sequence stratigraphy differs from siliciclastic sequence stratigraphy
mainly because carbonates are intrabasinal in origin and siliciclastics are extraba-
sinal in origin. Early work on seismic and sequence stratigraphy focused on terrig-
enous clastic depositional systems in which the sediment source is extrabasinal and

continental sedimentary processes such as fluvio - deltaic sedimentation and coastal
progradation play dominant roles in the formation of stratigraphic architecture.
First attempts to force the models for siliciclastic sequences on carbonates resulted
in confusion and misinterpretations. Carbonates are intrabasinal in origin and are

largely independent of continental influence. Instead, carbonate sequences consist
mainly of in situ marine biogenic and chemical sediments created by what is infor-
mally called the “ carbonate factory. ” Carbonate sedimentation reacts differently to
changes in relative sea level than do siliciclastics, sites of maximum carbonate pro-
duction are different on ramps and rimmed shelves, and carbonate production
occurs at different depths and in different volumes on temperate and tropical plat-
forms. Ramps and shelves exhibit different sequence architecture. A major step
forward in carbonate sequence stratigraphy was made by Sarg (1988) , who recog-
nized the essential differences in sedimentation and sequence architecture of silici-
clastics and carbonates. Later, Handford and Loucks (1993) illustrated the major
differences between carbonate sequences on ramps, shelves, and detached (isolated)
platforms. Their sequence - stratigraphic model for a rimmed shelf is illustrated in
Figure 4.10 .

4.5.3 Sequence Stratigraphy in Exploration and Development

Carbonate reservoirs are porous and permeable bodies of rock that contain com-
mercial quantities of hydrocarbons. Pore systems may be depositional, diagenetic,
fractured, or combinations of all three. Recognition and mapping of reservoir
boundaries and internal flow units and flow barriers requires examination of cores