Page 230 - Geology of Carbonate Reservoirs

DEPOSITIONAL RESERVOIRS 211

interparticle pores are commonly enlarged by burial diagenesis, which enhances
both connectivity and pore volume. Of course, pores that have been enhanced by
diagenesis are hybrids rather than purely depositional pores. Appropriate caution
must be used in determining the spatial distribution of hybrid porosity because it
may not follow depositional facies boundaries or present structural contours.
Finding most depositional reservoirs in the subsurface involves knowing what
type of platform existed in the exploration target area and which sites on the plat-
form are most likely to have grainy or reefy rocks with preserved depositional
porosity. If porous facies extend over great distances to form “ trends, ” the trends
may be associated with regional structural or stratigraphic features. The Jurassic
Smackover of the ancestral Gulf of Mexico is an example of a regional ramp with
subregional basement and salt - generated structural anomalies that infl uenced depo-
sitional facies patterns. Many of the Smackover reservoirs are hybrid pore systems;
they are not purely depositional. Vast Smackover grainstone deposits blanket the
pinchout edge (the ancient Gulf strandplain) for hundreds of miles. If all those rocks

were in flow communication, they would create one gigantic reservoir extending
from eastern Texas to southeastern Alabama. That does not exist, of course, because
the required traps and seals are not equally widely distributed. Some structural or
stratigraphic trap has to be present to isolate portions of the regional trend and

confine individual reservoir units. For example, structural noses or embayments
along the pinchout edge are typically tested to fi nd traps.
Reservoirs are three - dimensional features in which width and length are deter-
mined by platform bathymetry and thickness is determined by time and rate of
deposition on different platform sectors. The spatial distribution of depositional
reservoirs across platforms can be determined with the aid of sequence - stratigraphic
methods such as those discussed in Chapter 4 . Grain - supported rocks and reefs are
usually associated with strandplains and shelf edges. They are also characteristically
located at or near the tops of shallowing - upward cycles, or parasequences. Shallow-
ing - upward successions in carbonate reservoirs are common over vast areas on the
Permian Central Basin Platform, Northwestern Shelf and Northern Shelf of Texas
and New Mexico. They are equally common in Jurassic rocks from the Persian Gulf
area, such as the Arab D limestones. Likely locations for grain - supported reservoir

rocks may be identified by structural and paleobathymetric mapping (interval
isopach mapping) used in conjunction with sequence - stratigraphic analysis to iden-
tify the zones on ramps and shelves that are most likely to have grain - rich or porous
reef facies by virtue of their paleobathymetric setting and the sea - level phase that
existed at the time. For example, beach and barrier island facies should be expected
to occur in updip positions on ramps during sea - level highstands. Debrites and
coarse, proximal turbidites would be expected at toes of slope on shelves during
sea - level lowstands.
Dunes, beaches, barrier islands, and grain - rich patch reefs are common along the
shoreline, or pinchout edge, of ramps and out to water depths of about 10 meters
except on platforms with extreme climates such as those off present - day Southwest
Australia described by James et al. ( 1992 ). In storm - swept platforms such as the
Lacepede Shelf, sediments may not be deposited in the extremely turbulent waters
shallower than 100 meters or more. That situation is extreme compared with most
sedimentary basins of the world. Typically, shelf - edge reefs and grainstone tidal bars
rarely exist in water deeper than 10 meters. Shallow settings within the fair - weather