Page 122 - Geology of Carbonate Reservoirs

SEQUENCE STRATIGRAPHY 103

Type 1 Sb
Ls Shelf Edge

Caves Type 1 Sb
and Karst Ls
Shelf Edge
Lsw
4th Order
Hst Lsf
3rd Order
System Tracts
Eustasy Lst Tst Lst Highstand
Time Transgressive
Lowstand
Subsidence Sequence Boundary
Thickness Relative Sea Level Maximum Flooding Surface
Time
Figure 4.10 An illustration of the sequence - stratigraphic evolution of a tropical rimmed
shelf in a humid climate as illustrated by Handford and Loucks (1993) . As relative sea level
and subsidence progress, depocenters migrate seaward or landward, depending on the direc-
tion of relative sea - level change. Sequence - stratigraphic terminology for depositional succes-
sions formed at low sea level stands is LST, or lowstand systems tracts, those at highstands
of sea level are HST, and those deposited during relative sea - level rise are TST, or transgres-
sive systems tracts. LS indicates lowstand. LSW and LSF represent lowstand wedge and
lowstand fan, respectively. (Adapted from an illustration in Handford and Loucks (1993) .)

or cuttings, borehole logs, and geophysical data. It also requires an ability to recog-
nize facies patterns, to synthesize depositional models, and to formulate geological
concepts to incorporate the information into a working plan for exploration and
development. Small reservoirs may become only one - well fields. Others, such as the

giant Ghawar Field in Saudi Arabia, may extend over thousands of square kilome-
ters. In the end, it is the individual flow units or the intervals that exhibit optimum

combinations of porosity and permeability with the highest recovery effi ciency that

have the greatest influence on the economic value of a field. Flow units are rarely
larger than parasequence scale and may, in fact, be contained within a single para-
sequence. An entire reservoir may not incorporate much more than a few stacked
parasequences or parasequence sets. In view of this range in the size of productive
segments within carbonate sequences, it is important to keep in mind how sequence -

stratigraphic analyses can be used to find and develop reservoirs. One of the most
important bits of information needed to determine the size and shape of a deposi-
tional body is the shape (bathymetry) of the antecedent platform. If it was monoto-
nous, flat, and extensive, then depositional successions will probably be rather thin,

extensive, and relatively monotonous. If bathymetry varied greatly over small areas,
depositional successions, parasequences, and flow units will vary in size and

complexity accordingly.
For the explorationist, the fundamental problem is locating potential reservoir
facies within sedimentary basins. Reservoirs with the greatest amount of deposi-
tional porosity normally are present in rocks with high grain/mud ratios and in some
reefs and mounds. The most extensive grainstone deposits are usually present near
the shoreline on ramps and at the slope break on rimmed shelves; therefore the
explorationist can focus on those zones in ramp and shelf sequences. Some