Page 229 - Geology of Carbonate Reservoirs

210 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

reef reservoir should be investigated individually to determine the amount of skel-

etal framework, detrital infill, mud, or cementstone components, and how those
elements are related to porosity and genetic pore types.

8.3.1 Finding and Interpreting Depositional Reservoirs

There are only about five different pore types in depositional reservoirs. Finding
them means identifying locations where one or more of those pore types exist and
where pores are connected through a large enough volume to make a commercial
hydrocarbon reservoir. Three of the basic pore types exist only in detrital, grain - rich
rocks such as grainstones and packstones; they are interparticle, intraparticle, and
keystone/shelter pores. The most common pore type in depositional reservoirs is
probably interparticle porosity — space between grains. Intraparticle porosity, pores
within grains, is more important as a type of reservoir porosity in grain - rich rocks.
Keystone pores form on beaches when air is compressed and expelled through
grain - rich beach sediment when breaking waves fall on the sediment surface. Key-
stone pores are rare and are not important in carbonate reservoirs. Shelter pores
are formed by accidental preservation of open space beneath some fragment large

enough to act as an “ umbrella ” to block sediment from filling all available spaces
beneath it. Shelter pores are insignificant in carbonate reservoirs. Reef pore types

include fl oatstones and rudstones, both of which are grain - rich detrital rocks. Skel-
etal reefs typically have grain - rich, carbonate sand or gravel infi ll between the
skeletal elements. In short, depositional porosity is virtually limited to rocks with
high grain - to - mud ratios. Exceptions are fenestral porosity that forms in mud - rich
rocks usually associated with tidal flats and constructional vugs formed during the

growth of mudstone and cementstone “ reefs ” and mounds. Some of these vugs are
called stromatactis cavities and they may be especially abundant in deep - water
buildups, particularly those of Carboniferous age, where they may contribute most
of the hydrocarbon reservoir storage volume (Ahr, 1998 , 2000 ). Most depositional
pore types, except stromatactis cavities and constructed vugs in mud or cementstone
buildups, occur in grain - rich rocks that required vigorous water motion to concen-
trate grains and winnow mud. Environments with sustained, vigorous wave and
current action occur at shorelines and in shallow subtidal zones where typical waves
and currents — those related to average yearly climate — sweep the seafl oor almost
constantly. Some writers call this the “ fair - weather wave base ” or the idealized,

average depth at which waves influence sediment movement on the seabed. It is an
idealized average depth because wave climates vary greatly with weather changes —
changes in atmospheric pressure that make waves larger or smaller, depending on
wind velocity, duration, and fetch.
Of the seven standard depositional successions, those with the best potential to
be reservoirs are the shallow - water, high - energy successions such as beaches with
or without dunes, barrier islands, large patch reefs, shelf - edge reefs, and tidal bars.
Deep - water deposits such as mudstone — cementstone buildups (mud - mounds), car-
bonate debrites, and turbidites may also be good depositional reservoir rocks. Stro-
matactis cavities may store large volumes of hydrocarbons and interparticle porosity
in debrites and turbidites may be abundant enough for those deep - water deposits
to be classified as reservoirs. Usually hydrocarbon production from stromatactis

cavities requires natural fractures to provide permeability. Debris flow and turbidite