|
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
uIntroduction
uFigure
Captions
uSubaerial
meteoric model
u Porosity
in limestones
u Porosity
in dolomites
uCavernous
porosity
uMesogenetic
model
uMicroporosity
uConclusions
uReferences
|
Figures Captions
|
 |
Figure 1. Processes by which porosity is
reduced in carbonate rocks. Syndepositional marine cementation
occurs only in the eogenetic zone, and mechanical compaction is
unlikely to affect telogenetically-exposed older carbonate rocks.
|
|
 |
Figure 2. Meteoric subaerial exposure of
newly-deposited sediments in the eogenetic zone and of older
carbonate rocks in the telogenetic zone. Note that the freshwater
lens in the eogenetic zone can extend some distance below
sea level ("B"), and that freshwater may
extend down -dip for a considerable distance in the telogenetic
zone ("A"). |
|
 |
Figure 3. Typical secondary dissolution
pore types in carbonate rocks that are readily identifiable in
cuttings and core samples. A - interparticle pores in a grainstone
(cuttings sample) and A' core showing interparticle porosity in a
carbonate grainstone. B - thin-section
photomicrograph of intraparticle porosity (arrows) within a
fusulinid and B' core showing intraparticle porosity within a
coral. C- Fenestral porosity in a tidal-flat dolomite. Tilted
arrows point to planar (laminar)
fenestral pores, and horizontal arrows point to smaller "birdseye"
pores. D - Cuttings samples with oomoldic pores (arrows) in an
oolite grainstone. E - Carbonate grainstone with identifiable
skeletal particles (circled) and
larger vug (arrow) that formed from the initial dissolution of a
particle and then further dissolution of the matrix around it
(cuttings sample). F - Thin-section photomicrograph of
interparticle porosity in a carbonate
sand wherein remnant cement (arrow) restricts pore throats and
reduces permeability .
|
|
 |
Figure 4. Core slab with secondary vugs
(arrows) resulting from the partial dissolution of earlier
dolomite cement (white). |
|
 |
Figure 5. Thin-section photomicrograph of calcite cement
overgrowths on crinoid fragments that occlude interparticle
porosity . |
|
 |
Figure 6. Relationship of porosity to
percent dolomite in carbonate rocks (after Murray, 1960).
|
|
 |
Figure 7. Origin of secondary dissolution porosity in dolomites.
|
|
 |
Figure 8. Intercrystalline pores and pore
throats in dolomites. Relative size of pores and pore throats is
not necessarily correlative to dolomite crystal size because
porosity is a percentage of total rock volume. Pore throat
characteristics, however, do reflect the
degree (extent) of dolomitization in rocks. Dolomites with
polyhedral pores generally are referred to as "sucrosic." |
|
 |
Figure 9. Top - Typical lateral and
vertical compartmentalization of reservoir zones in cavernous
(karsted) carbonate rocks. Bottom - Typical porosity types and
fills of cavernous reservoirs. Cave roof rocks become
progressively more brecciated downward,
with attending fracture, dissolution-enlarged fracture, and
commonly, vuggy porosity . Cave-fill deposits variously can be: (1)
cave roof-collapse breccia, which can have
inter-clast porosity as well as
intra-clast vuggy and fracture porosity . Conversely, original
inter-clast porosity can be filled with cements and/or shale, or
can be filled with porous sand; (2) impermeable shale infiltered
into the cavern from above; or (3)
porous or tight sand infiltered into the cavern from above. Cave
roof collapse and infiltering of sand and/or shale can occur soon
after karstification or later.
|
|
 |
Figure 10. On the left is a partial
stratigraphic column of what I have sometimes encountered in the
Mississippian in Kansas a few feet of carbonate below the top of
the Mississippian, underlain by sandstones (which
are not laterally correlative for any
distance), in turn underlain by more carbonate . On the right is a
possible interpretation of such a stratigraphy that the wells in
question encountered sand-filled caverns below the top of the
Mississippian. |
|
 |
Figure 11. Processes and products of hydrocarbon generation.
Relatively minor amounts of methane (CH4)
are generated during shallow burial by
bacterial processes. As organic matter
in source rocks matures and generates
oil or gas, copious amounts of hydrogen
sulfide (H2S) and carbonate dioxide (CO2)
are generated, along with organic acids.
|
|
 |
Figure 12. Carbon dioxide, hydrogen
sulfide, and organic acids evolved during hydrocarbon generation
mix with connate waters in the burial environment to produce
acids. These acidic fluids can migrate both vertically
and laterally for great distances, even
affecting carbonate rocks that are not deeply buried (5). Acidic
fluids can migrate along faults (1), along bedding planes or
porous strata within beds (2), along formational boundaries (3),
or upward along fractures through
otherwise non-permeable beds ("cross-formational flow" (4).
|
|
 |
Figure 13. Mesogenetic porosity . A -
Fractures, which cross and therefore post-date stylolites, with
porosity from the dissolution of post-fracture cement. Adjoining
this area is selective dissolution of oolites. B - Oomoldic
porosity adjoining and post-dating the
formation of a stylolite. |
|
 |
Figure 14. Mesogenetic porosity . A -
Thin-section photomicrograph of curved saddle dolomite (with
typical undulose extinction) filling a pore. B - Core slab showing
coarse crystalline saddle dolomite that has been partially
dissolved to exhume (vuggy) porosity
|
Return to top.
Secondary Porosity Beneath Unconformities:
The Subaerial Meteoric Model
The
generation of secondary porosity in carbonate sediments or rocks in this
model is a direct
consequence of dissolution by
freshwater (ultimately rain-derived), which dissolves carbonates because
the water is undersaturated with respect to calcium carbonate . The
extent of dissolution and secondary porosity formation are controlled by
factors such as the acidity of freshwater (e.g., rain water percolating
down through a soil zone will be more acidic than in areas where soils
are not present), the amount of porosity or fractures within the
affected carbonates, the residence time of the freshwater in the
diagenetic system, the mineralogy of the sediments or rocks, and so
forth (Longman, 1980; Moore, 1989). Secondary porosity generation via
dissolution can occur relatively soon after deposition, in
unconsolidated sediments, in what Choquette and Pray (1970) refer to as
the eogenetic zone; or it can occur much later, in rocks, in the
telogenetic zone as a consequence of uplift of older, formerly
buried carbonates (Figure 2). In newly-deposited carbonate sediments
that subsequently are subaerially exposed, it is the difference in
original mineralogies of particles in the sediments that drives the
relatively rapid, selective dissolution of particles. Fragments of
corals, pelecypods and gastropods, and oolites, for example, are
originally composed mostly of the mineral aragonite (CaCO3,
orthorhombic), which is very soluble. It is for this reason that
formerly aragonitic particles in limestones usually are represented by
pores (in this case, fabric-selective pores) or cement-filled pores. In
contrast, particles such as forams, crinoid fragments, and bryozoans are
originally composed of the mineral high-magnesium calcite (CaCO3,
hexagonal-rhombohedral), which is calcite with up to 23 mole% MgCO3
in the
crystal lattice. With exposure to freshwater such particles tend merely
to lose MgCO3
and not to dissolve like
aragonitic particles. Other particles, such as brachiopod shells and
some pelecypods, build their skeletons out of low-Mg calcite (also CaCO3,
hexagonal-rhombohedral), which is calcite with less than 4 mole% MgCO3.
Particles of original high-magnesium calcite and low-magnesium calcite
mineralogy tend not to dissolve unless the freshwater is quite
undersaturated with respect to calcium carbonate , and it is for this
reason that some particles (crinoids, bryozoans, brachiopods) often are
well-preserved in ancient rocks. The eogenetic exposure to freshwater of
newly-deposited carbonate sediments, which are generally highly porous
and polyminerallic (i.e., as discussed above, composed of
mixtures of aragonite, high-magnesium and low-magnesium calcite),
results in the formation of cemented limestones, of varying porosity , of
stable low-magnesium calcite composition (notwithstanding dolomitization).
In contrast, late eogenetic or telogenetic freshwater exposure of older
limestones that have already been mineralogically stabilized and
cemented is not driven by such differences in the relative solubility of
aragonite, high-magnesium and low-magnesium calcite because the rocks
already are mineralogically stabilized to low-magnesium calcite, and
further dissolution can occur only if the fluids are quite
undersaturated with respect to calcite (the least soluble of the
aforementioned carbonate minerals). Typically, such dissolution forms
vugs and caverns, which can also form in polyminerallic carbonate
sediments. Dissolution of already stabilized limestones can also result
in the formation of particle-selective pores when certain particles in
the rocks are slightly more soluble than other particles because of
differences in particle size or their micro-architectural arrangement of
component calcite crystals. For example, crinoid fragments in older
rocks are composed of single, relatively large crystals of low-magnesium
calcite, which have relatively low solubility. It is for this reason
that crinoid-rich Mississippian limestones, for example, typically have
low porosities. In contrast, foram shells are composed of myriads of
small calcite crystals, which have relatively higher solubility, and
they usually are more readily dissolved than crinoid fragments.
In either
case, it is important to note that carbonates can be affected by
meteoric dissolution not only directly beneath unconformities on land,
but also for some distance down-dip into the subsurface ("A" in
Figure 2) and some distance in a seaward direction below sea level, depending
on the extent of freshwater lenses ("B" in
Figure 2). Porosity
generation by dissolution eventually ceases, generally in a down-dip
direction within phreatic zones when that water becomes saturated with
respect to dissolved calcium carbonate . At that point, porosity can be
maintained, or if the water becomes even more saturated with respect to
dissolved calcium carbonate , it can begin to be occluded by carbonate
cement (and other cements as well, such as gypsum/anhydrite or silica).
Meteoric Porosity in
Limestones
In
limestones, common secondary pore types formed as a result of
post-depositional dissolution variously include exhumed interparticle,
intraparticle, fenestral, shelter, and growth-framework pores, all of
which are considered to be fabric-selective pores; and also not
fabric-selective vugs (Figure 3E) and dissolution-enlarged fractures.
The size of vugs (Figure 4) varies from small (but larger than component
particles in the rocks) to caverns or cavernous porosity . Vugs may
originate either by wholescale dissolution of parts of the rock or by
dissolutional enlargement of fabric-selective pores (Figure 3E). In many
cases there is coincidence between the types of fabric-selective pores
present in the rocks and the depositional environment of the rocks,
which serves as an important guide in evaluating permeability and
potential recoverable reserves from the reservoir, and in deciding on
what stimulation procedures to use but only if one knows the
depositional environment of the rocks from study of subsurface samples.
For example, carbonate sands (lime grainstones), deposited in
high-energy environments such as oolite shoals or skeletal sand shoals,
commonly have high interparticle porosity (Figure 3A) and attending
relatively high permeabilities. On the other hand, however, high
porosity but low permeability may characterize a carbonate sand
(limestone) reservoir wherein only the particles have been dissolved
(for example, in cases where the reservoir contains only molds of
oolites referred to as oomoldic porosity or by the older term oocastic
porosity : Figure 3D). In such cases there may be ample hydrocarbon
storage volume in the pores in the rocks, but in the absence of
fractures, there is little interconnected porosity . Notwithstanding
porosity associated with dolomitization (discussed later), limestones
deposited in tidal-flat environments commonly contain a specific type of
vuggy porosity referred to as fenestral pores (for example, birdseye
pores, which is one type of pinpoint porosity : Figure 3C), and unless
fractured, such rocks may have decent porosity but limited permeability .
Skeletal sands shoals wherein the particles mainly are foraminifera,
which are common in Midcontinent Pennsylvanian limestone reservoirs (Wilhite
and Mazzullo, 2000), may be highly porous and permeable because of the
presence of interparticle pores, and within the forams, of intraparticle
pores as well (Figure 3B). On the other hand, if intraparticle pores are
the only pore types present, then porosity (and hydrocarbon storage
volume) might be high, but permeability would be low (notwithstanding
fracturing). As a corollary, variations in porosity and permeability
from well-to-well within a given zone may be a consequence of different
depositional environments in that zone and/or from differing extents of
porosity generation versus occlusion between wells. Only study of
cores/cuttings and thin-sections can resolve the possible reasons for
such variations between wells.
Over-riding such generalizations about the relationships among pore
types, permeability , and depositional environments of the limestones is
the importance of pore throats in the rocks (Wardlaw, 1976). In
limestones, particularly in grainstones, for example, the nature of pore
throats and their effect on permeability is controlled by the size of
the particles in the rocks, and more importantly, by the distribution of
any remaining earlier-precipitated cement in the pores that was not
dissolved (Figure 3F). Calcite cement overgrowths on crinoid fragments
can significantly restrict pore throats as well (Figure 5), which is why
many crinoid-rich Mississippian limestones are of low- permeability
nature. The best way to determine the extent of pore-throat restriction
in the rocks under consideration is by examining the rocks
petrographically in thin-section. Clay-mineral cements are extremely
rare in carbonate reservoir rocks, and therefore, need not be considered
here.
Meteoric Porosity in
Dolomites
In
contrast to earlier postulates on the subject (e.g., Murray, 1960; Weyl,
1960), the process of dolomitization of a pre-existing limestone does
not automatically create secondary porosity . Whereas it is true that
porosity tends to increase as amount of dolomite increases (Figure 6),
it generally does so for the following reasons. In partly dolomitized
limestones exposed to telogenetic meteoric fluids, for example, any
remaining calcite (which may represent particles and/or carbonate mud
matrix) inherently is more susceptible to dissolution by freshwater
because it is more soluble than dolomite. Hence, subaerial exposure of a
partly dolomitized limestone can result in the generation of the same
types of pores as described above by dissolution of remaining calcite,
depending on the original texture of the rock (mudstone, wackestone,
packstone, or grainstone), its depositional environment, and degree of
replacement by dolomite (Figure 7). Likewise, remaining evaporites in
the rocks can also be dissolved. In more pervasively dolomitized rock
exposed to telogenetic meteoric fluids, remaining calcite (or evaporite
minerals) between dolomite crystals can be dissolved during subaerial
exposure to produce intercrystalline pores between dolomite crystals. In
completely dolomitized rocks, vugs (and sometimes dissolution-
enlarged
fractures) are common pore types present if the meteoric fluids were
highly acidic or acted on the rocks over long periods of time. Selective
dissolution of small dolomite crystals (because solubility increases as
crystal size decreases), or of more soluble dolomite phases in the
rocks, can result in the development of vugs and intercrystalline pores.
All such processes and resulting pore types can be represented in a
given reservoir. As in limestones, the nature of pore throats in
dolomites affects permeability , and as a general rule, intercrystalline
pore throat sizes decrease with decreasing crystal size and extent of
dolomitization (Figure 8).
Cavernous Porosity in
Carbonate Rocks
Cavernous
and associated vuggy porosity are major attributes of hydrocarbon
production from reservoirs such as the Arbuckle Group in Kansas
(Walters, 1946; Newell et al., 1987) and Oklahoma (Gatewood, 1970), and
from its stratigraphic correlative, the Ellenburger Group, in west Texas
and New Mexico (Holtz and Kerans, 1992). Additional examples of
hydrocarbon reservoirs in paleo-caverns are given in Mazzullo and
Chilingarian (1996). Only rarely are completely fluid-filled caverns
encountered in the subsurface. Rather, paleo-caverns usually are filled
by porous (or, unfortunately, sometimes tight) cave-roof collapse
breccia and associated sediments and/or by overlying, younger rocks
(Figure 9). Rather than being single zones, paleo-caverns typically are
labyrinthine systems characterized by extreme lateral and vertical
reservoir compartmentalization (Figure 9). Cavernous porosity
undoubtedly also locally contributes to hydrocarbon production from some
Mississippian reservoirs in Kansas. I have encountered a number of
instances in Kansas, for example, where wellsite geologists’ reports
picked the top of the Mississippian at a certain depth, and then the
limestone or dolomite directly below seemingly was underlain by a
section of sand (which I presume to be Pennsylvanian-age siliciclastic
sand) that is, in turn, underlain by more carbonate rock. Such
occurrences may indicate that the wells penetrated sand-filled paleo-caverns
(Figure 10).
Since the
late 1970s-early 1980s, geologists began to suspect that not all
secondary dissolution porosity in carbonate rocks forms or formed solely
beneath unconformities by freshwater dissolution in either the eogenetic
or telogenetic environments (e.g., Bathurst, 1980; Scholle and Halley,
1985; Choquette and James, 1987; Moore, 1989). Rather, there was growing
realization of the significance of, and processes controlling, secondary
dissolution porosity formation (and porosity occlusion) in the
deep-burial environment B which is what Choquette and Pray (1970)
referred to as the mesogenetic environment. Two important points
in this regard are the facts that: (1) not all porous carbonates are
associated with unconformities; and (2) specifically, there are a number
of examples of porous and permeable carbonate rocks deposited in
deep-water settings and which later were deeply buried and never
subaerially exposed. Hence, meteoric exposure at any time after
deposition has been ruled out for such rocks (e.g., Mazzullo and Harris,
1991; Mazzullo, 1994). Therefore, post-depositional diagenesis and the
formation of secondary dissolution porosity in such rocks must have
occurred in an environment other than the meteoric eogenetic or
telogenetic environment. Furthermore, if we are exploring for
hydrocarbon reservoirs in the subsurface, then those reservoirs must
have resided in the subsurface, variously shallowly or deeply buried,
for long periods of time. Insofar as carbonate diagenesis never ceases,
any diagenesis that occurs in the mesogenetic environment overprints
earlier diagenesis, including that which may have occurred, for example,
in subunconformity, meteoric eogenetic or telogenetic environments.
Geologists have since come to realize that deep-burial diagenesis has
significantly contributed to secondary dissolution porosity and
permeability evolution in many carbonate hydrocarbon reservoirs (e.g.,
Mazzullo and Harris, 1992).
As
discussed above, in order for dissolution of any carbonate rocks to
proceed, they must be exposed to fluids that are undersaturated with
respect to calcium carbonate . That is easy enough to do in the
subunconformity meteoric environment because rain water, the ultimate
source of near-surface freshwater, is undersaturated. In the mesogenetic
environment, however, most connate fluids are brines that typically are
saturated or even supersaturated with respect to calcium carbonate ,
which means they are not capable of dissolving carbonate rocks and
creating secondary porosity . Rather, such fluids tend to precipitate
carbonates in the form of calcite or dolomite cement, and in some cases,
they may be capable of dolomitization. How, then, can carbonate
dissolution and porosity formation proceed in the deep-burial
environment? In other words, how are fluids undersaturated with respect
to calcium carbonate generated in the mesogenetic environment?
Studies of
porosity evolution in sandstones, combined with studies of organic
matter maturation and hydrocarbon generation in source rocks (e.g.,
Foscolos, 1984; Surdam et al., 1984; Kharaka et al., 1986; Lundegard and
Land, 1986; Meshri, 1986; Sassen and Moore,
1988),
have provided answers to these questions which have been applied to
carbonate reservoir rocks, both limestone and dolomite, around the world
(e.g., Druckman and Moore, 1985; Heydari and Moore, 1989; Mazzullo and
Harris, 1992). It is known that carbon dioxide, hydrogen sulfide, and
great quantities of organic acids are generated during the maturation of
organic matter to hydrocarbons in buried source rocks (Figure 11). As
these gases and organic acids are expelled from the source rock, the
evolved CO2
combines
with subsurface water to produce carbonic acid and the H2S
similarly combines with water to produce sulfuric acid. Together, these
acids and associated organic acids can migrate great distances laterally
as well as vertically (Figure 12; e.g., Hanor, 1987) to dissolve buried
carbonates just ahead of migrating hydrocarbons. Likewise, once the
acids are spent, subsurface fluids can then precipitate carbonate
cements, which is why many examples of such cements contain hydrocarbon
inclusions (e.g., Burruss et al., 1985). In given rocks, secondary
dissolution porosity formation can alternate with cementation many times
to result in complex diagenetic histories of reservoirs (e.g., Moore and
Druckman, 1981; Mazzullo and Harris, 1989; Moore, 1989). Because such
evolved subsurface fluids can migrate great distances both laterally and
vertically, they can affect carbonates that were or are not deeply
buried. I have come across several wells in Ness County, Kansas, for
example, where there is a great concentration of pyrite and minor
sphalerite at the top of the Mississippian, which is at only about 4100
feet below the surface, clearly not in the deep-burial environment (at
least, not today). These minerals may have been emplaced along the
pre-Pennsylvanian unconformity by fluids that evolved within and
subsequently migrated out of the Anadarko Basin.
Porosity
formed in the mesogenetic environment is represented by the same types
of pores that can form in the eogenetic and telogenetic freshwater
environment (Mazzullo and Harris, 1992), including even cavernous
porosity (e.g., Hall, 1990; Hill, 1992), which otherwise is generally
known as A burial karst. Therefore, an important point to
remember in this regard is that the diagenetic environment in which
porosity formation occurred cannot be determined on the basis of the
pore types present in a reservoir! How, then, does one recognize
porosity formed in the mesogenetic environment? The answer to this
question is "With careful, detailed petrographic study of
thin-sections, often combined with analysis of carbon and oxygen
isotopic values of carbonate cements associated with suspected
mesogenetic porosity .” For details, refer to the papers by Moore and
Druckman (1981), Druckman and Moore (1985), Heydari and Moore (1989),
Moore (1989) and Mazzullo and Harris (1991, 1992), and the many papers
cited therein. There are, however, some readily visible clues to the
mesogenetic origin of porosity in carbonate rocks that can be identified
in cuttings and core samples. For example: (1) porosity along and
associated with stylolites or which cuts across stylolites (Figure 13).
As pressure-dissolution features, stylolites form most commonly in the
deep-burial environment. So, if the timing of porosity formation can be
determined by the law of cross-cutting relationships to be younger
than the stylolites, and the rocks are still buried, then porosity
formation must be the result of mesogenetic dissolution; (2) pores that
cut across cements that contain hydrocarbon inclusions; (3) pores that
cut across or which are intimately associated with fluorite, metal
sulfides such as galena and sphalerite, and sometimes pyrite/marcasite,
which are common Mississippian Valley-type precipitates from migrating
deep-basinal fluids (e.g., Cathles and Smith, 1983; Clendenin and Duane,
1990); and (4) pores that cut across saddle dolomite (Figure 14). Saddle
dolomite, otherwise known as baroque dolomite or “pearl spar," is a
cement or replacive mineral that is readily identified by its
characteristic curved crystal faces, common opaque-white color, and
relatively coarse crystal size, that commonly forms in the deep-burial
environment (Radke and Mathis, 1980). Hence, if pores cut across saddle
dolomite, then porosity formation must be mesogenetic if the rocks are
still buried. Are there examples of mesogenetic porosity in carbonate
reservoirs in Kansas? Undoubtedly, but specific published instances
where mesogenetic porosity is present are not available.
Microporosity, Pinpoint Porosity , and Chalky Porosity
The term
microporosity refers to any very small pores that can be
recognized only with the aid of a high-powered binocular microscope or
thin-section (Choquette and Pray, 1970; Pittman, 1971). Micropores,
otherwise known as pinpoint pores, may variously represent: (1) birdseye
pores in tidal flat deposits; (2) intraparticle pores within small
particles; (3) intercrystalline pores between dolomite crystals or
between calcite cement crystals; (4) intercrystalline pores within the
nuclei or cortices of oolites; or (5) intracrystalline pores within
individual dolomite or calcite cement crystals. In some cases, whereas
matrix microporosity/pinpoint porosity may not be very permeable for
oil, it very well may be permeable enough for natural gas (e.g., Roehl,
1985; Ruzyla and Friedman, 1985). Chalky porosity is a term that
refers to microporosity that commonly forms in highly weathered or
otherwise highly diagenetically altered carbonate rocks (Pittman, 1970;
Reeckmann and Friedman, 1982) that, as a consequence of being strongly
altered, are very soft. Although true chalks (i.e., those limestones of
Cretaceous to Paleogene age that are composed dominantly of coccoliths)
commonly contain microporosity, the presence of chalky porosity does not
necessarily indicate that the rocks under consideration are chalks. In
fact, tripolitic cherts commonly have microporosity, and because the
rocks are relatively soft, such porosity can be referred to as chalky. I
point out that microporosity, and specifically chalky porosity , can form
in carbonate rocks (and cherts) beneath unconformities as well as in
deeply buried rocks, and as such, their presence does not necessarily
imply the existence of a stratigraphically-nearby unconformity.
Secondary
porosity in carbonate rocks, in both limestone and dolomite, can be
formed by: (1) freshwater dissolution either in the subunconformity
meteoric, eogenetic or telogenetic environment, or; (2) by dissolution
by chemically aggressive subsurface fluids, generated during maturation
of organic matter in source rocks, in the deep-burial (mesogenetic)
environment. Although pore types formed in these environments are
similar, their origin often can be determined by careful observation,
thin-section petrography, and stable carbon-oxygen isotope analysis.
Bathurst, R.G.C., 1980, Deep crustal
diagenesis in limestones: Revista Instituto Investaciones Geologicas,
University of Barcelona, v. 34, p. 89-100.
Burruss, R.C., Cercone, K.R., and
Harris, P.M., 1985, Timing of hydrocarbon maturation - evidence from
fluid inclusions in calcite cements, tectonics and burial history, in
N. Schneidermann and P.M. Harris, eds., Carbonate Cements: SEPM Special
Publ. 36, p. 277-289.
Cathles, L.M., and Smith, A.T., 1983,
Thermal constraints on the formation of Mississippi Valley-type
lead-zinc deposits and their implications for episodic basin dewatering
and deposit genesis: Economic Geology, v. 78, p. 983-1002.
Chilingarian, G.V., Torabzadeh, J., Rieke, H.H.,
Metghalchi, M., and Mazzullo, S.J., 1992, Interrelationships
among surface area,
permeability , porosity , pore size, and residual water saturation, in G.V.
Chilingarian, S.J. Mazzullo, and H.H. Rieke, eds., Carbonate Reservoir
Characterization: A Geologic-Engineering Analysis, Part I: Elsevier Publ.
Co., Amsterdam, Developments in Petroleum Science 30, p. 379-397.
Choquette, P.W., and James, N.P.,
1987, Diagenesis 12: diagenesis in limestones-3. The deep burial
environment: Geoscience Canada, v. 14, p. 3-35.
Choquette, P.W., and Pray, L.C., 1970,
Geologic nomenclature and classification of porosity in sedimentary
carbonates: AAPG Bulletin, v. 54, p. 207-250.
Clendenin, C.W., and Duane, M.J.,
1990, Focused fluid flow and Ozark Mississippi Valley-type deposits:
Geology, v. 18, p. 116-119.
Druckman, Y., and Moore, C.H., 1985,
Late subsurface porosity in a Jurassic grainstone reservoir, Smackover
Formation, Mt. Vernon field, southern Arkansas, in P.O. Roehl and
P.W. Choquette, eds., Carbonate Petroleum Reservoirs: Springer-Verlag,
New York, p. 371-383.
Feazel, C.T., and Schatzinger, R.A.,
1985, Prevention of carbonate cementation in petroleum reservoirs, in
N. Schneidermann and P.M. Harris, eds., Carbonate Cements: SEPM Special
Publ. 36, p. 97-106.
Foscolos, A.E., 1984, Diagenesis 7.
Catagenesis of argillaceous sedimentary rocks: Geoscience Canada, v. 11,
p. 67-75.
Friedman, G.M., 1964, Early diagenesis
and lithification in carbonate sediments: Journal of Sedimentary
Petrology, v. 34, p. 777-813.
Gatewood, L.E., 1970, Oklahoma City
field - anatomy of a giant, in M.T. Halbouty, ed., Geology of
Giant Petroleum Fields: AAPG Memoir 14, p. 223-254.
Hall, J.F., 1990, The development of
paleokarst and other solution features in the Mississippian Leadville
Dolomite, central Colorado, in D.W. Beaty, G.P. Landis, and T.B.
Thompson, eds., Carbonate -Hosted Sulfide Deposits of the Central
Colorado Mineral Belt: Economic Geology Monograph 7, New Haven, Conn.,
Economic Geology Publ., p. 108-117.
Halley, R.B., and Schmoker, J.W.,
1983, High porosity Cenozoic carbonate rocks of south Florida:
progressive loss of porosity with depth: AAPG Bulletin, v. 67, p.
191-200.
Hanor, .S., 1987, Origin and migration
of subsurface sedimentary brines: SEPM Lecture Notes, Short Course No.
21, 247 p.
Hendrickson, A.R., Thomas, R.L., and
Economides, M.J., 1992, Stimulation of carbonate reservoirs, in
G.V. Chilingarian, S.J. Mazzullo, and H.H. Rieke, eds., Carbonate
Reservoir Characterization: A Geologic-Engineering Analysis, Part I:
Elsevier Publ. Co., Amsterdam, Developments in Petroleum Science 30, p.
589-625 .
Heydari, E., and Moore, C.H., 1989,
Burial diagenesis and thermochemical sulfate reduction, Smackover
Formation, southeastern Mississippi salt basin: Geology, v. 17, p.
1080-1084.
Hill, C.A., 1992, Sulfuric acid
oil-field karst, in M.P. Candelaria and C.L. Reed, eds.,
Paleokarst, Karst Related Diagenesis and Reservoir Development: Examples
from Ordovician-Devonian age strata of West Texas and the Mid-Continent:
Permian Basin Section SEPM Publ. 92-33, p. 192-194.
Holtz, M.H., and Kerans, C., 1992,
Characterization and categorization of west Texas Ellenburger
reservoirs, in M.P. Candelaria and C.L. Reed, eds., Paleokarst,
Karst Related Diagenesis and Reservoir Development: Examples from
Ordovician-Devonian age strata of West Texas and the Mid-Continent:
Permian Basin Section SEPM Publ. 92-33, p. 45-54.
Honarpour, M.M., Chilingarian, G.V.,
and Mazzullo, S.J., 1992, Permeability and relative permeability of
carbonate reservoirs, in G.V. Chilingarian, S.J. Mazzullo, and
H.H. Rieke, eds., Carbonate Reservoir Characterization: A
Geologic-Engineering Analysis, Part I: Elsevier Publ. Co., Amsterdam,
Developments in Petroleum Science 30, p. 399-416.
Jodry, R.L., 1992, Pore geometry of
carbonate rocks and capillary pressure curves (basic geologic concepts),
in G.V. Chilingarian, S.J. Mazzullo, and H.H. Rieke, eds.,
Carbonate Reservoir Characterization: A Geologic-Engineering Analysis,
Part I: Elsevier Publ. Co., Amsterdam, Developments in Petroleum Science
30, p. 331-377.
Kharaka, Y.K., Law, L.M., Carothers,
W.C., and Goerlitz, D.F., 1986, Role of organic species dissolved in
formation waters from sedimentary basins in mineral diagenesis, in
D.L. Gautier, ed., Roles of Organic Matter in Sediment Diagenesis: SEPM
Special Publ. 38, p. 111-122.
Land, L.S., 1967, Diagenesis of
skeletal carbonates: Journal of Sedimentary Petrology, v. 37, p.
914-930.
Longman, M.W., 1980, Carbonate
diagenetic textures from near surface diagenetic environments: AAPG
Bulletin, v. 64, p. 461-487.
Lundegard, P.D., and Land, L.S., 1986,
Carbon dioxide and inorganic acids: their role in porosity enhancement
and cementation, Paloegene of the of the Texas Gulf Coast, in D.L.
Gautier ed., Roles of Organic Matter in Sediment Diagenesis: SEPM
Special Publ. 38, p. 129-146.
Mazzullo, S.J., 1994, Models of
porosity evolution in Permian periplatform carbonate reservoirs
(debris-flows and turbidites) in the Permian Basin: West Texas
Geological Society Bulletin, v. 34, no. 1, p. 5-12.
Mazzullo, S.J., and Chilingarian, G.V.,
1992, Diagenesis and origin of porosity , in G.V. Chilingarian,
S.J. Mazzullo, and H.H. Rieke, eds., Carbonate Reservoir
Characterization: A Geologic-Engineering Analysis, Part I: Elsevier Publ.
Co., Amsterdam, Developments in Petroleum Science 30, p. 199-270.
Mazzullo, S.J., and Chilingarian, G.V.,
1996, Hydrocarbon reservoirs in karsted carbonate rocks, in G.V.
Chilingarian, S.J. Mazzullo, and H.H.Rieke, eds., Carbonate Reservoir
Characterization: A Geologic-Engineering Analysis, Part II: Elsevier
Publ. Co., Amsterdam, Developments in Petroleum Science 44, p. 797-865 .
Mazzullo, S.J., and Harris, P.M.,
1991, An overview of dissolution porosity development in the deep-burial
environment, with examples from carbonate reservoirs in the Permian
Basin, in M. Candelaria, ed., Permian Basin Plays – Tomorrow’s
Technology Today: West Texas Geological Society, Publ. No. 91-89, p.
125-138.
Mazzullo, S.J., and Harris, P.M.,
1992, Mesogenetic dissolution: its role in porosity development in
carbonate reservoirs: AAPG Bulletin, v. 76, p. 607-620.
Meshri, I.D., 1986, On the reactivity
of carbonic and organic acids and generation of secondary porosity ,
in D.
L. Gautier, ed., Roles of Organic Matter in Sediment
Diagenesis: SEPM Special Publ. 38, p. 123-128.
Moore, C.H., 1989, Carbonate
Diagenesis and Porosity : Elsevier Publ. Co., Developments in
Sedimentology 46, 338 p.
Moore, C.H., and Druckman, Y., 1981,
Burial diagenesis and porosity evolution, Upper Jurassic Smackover,
Arkansas and Louisiana: AAPG Bulletin, v. 65, p. 597-628.
Murray, R.C., 1960, Origin of porosity
in carbonate rocks: Journal of Sedimentary Petrology, v. 30, p. 59-84.
Newell, K.D., Watney, W.L., Cheng,
S.W.L., and Brownrigg, R.L., 1987, Stratigraphic and spatial
distribution of oil and gas production in Kansas: Kansas Geological
Survey, Subsurface Geology Series 9, 86 p.
Pittman, E.D., 1971, Microporosity in
carbonate rocks: AAPG Bulletin, v. 55, p. 1873-1878.
Radke, B.M., and Mathis, R.C., 1980,
On the formation and occurrence of saddle dolomite: Journal of
Sedimentary Petrology, v. 50, p. 1149-1168.
Reeckmann, A., and Friedman, G.M.,
1982, Exploration for Carbonate Petroleum Reservoirs: John Wiley and
Sons, New York, 213 p.
Roehl, P.O., 1985, Depositional and
diagenetic controls on reservoir rock development and petrophysics in
Silurian tidalites, Interlake Formation, Cabin Creek field area,
Montana, in P.O. Roehl and P.W. Choquette, eds., Carbonate
Petroleum Reservoirs: Springer-Verlag, Berlin, p. 85-105.
Ruzyla, K., and Friedman, G.M., 1985,
Factors controlling porosity in dolomite reservoirs of the Ordovician
Red River Formation, Cabin Creek Field, Montana, in P.O. Roehl
and P.W. Choquette, eds., Carbonate Petroleum Reservoirs: Springer-Verlag,
Berlin, p. 41-58.
Sassen, R., and Moore, C.H., 1988,
Framework of hydrocarbon generation and destruction in eastern Smackover
trend: AAPG Bulletin, v. 72, p. 649-663.
Scholle, P.A., and Halley, R.B., 1985,
Burial diagenesis: Out of sight, out of mind!, in N.
Schneidermann and P.
M. Harris, eds., Carbonate Cements:
SEPM Special Publ. 36, p. 309-334.
Surdam, R.C., Boese, S.W., and Crossey,
L.J., 1984, The chemistry of secondary porosity , in D.A. McDonald
and R.C. Surdam, eds., Clastic Diagenesis: AAPG Memoir 37, p. 127-149.
Walters, R.F., 1946, Buried
pre-Cambrian hills in northeastern Barton County, central Kansas: AAPG
Bulletin, v. 30, p. 660-710.
Wardlaw, N.C., 1996, Factors affecting
oil recovery from carbonate reservoirs and prediction of recovery, in
G.
V. Chilingarian, S.J. Mazzullo, and H.H. Rieke, eds.,
Carbonate Reservoir Characterization: A Geologic-Engineering Analysis,
Part II: Elsevier Publ. Co., Amsterdam, Developments in Petroleum
Science 44, p. 867-903 .
Wardlaw, N.C., 1976, Pore geometry of
carbonate rocks as revealed by pore casts and capillary pressures: AAPG
Bulletin, v. 60, p. 245-257.
Weyl, P.K., 1960, Porosity through
dolomitization: conservation-of-mass requirements: Journal of
Sedimentary Petrology, v. 30, p. 85-90.
Wilhite, B.W., and Mazzullo, S.J.,
2000, Facies architecture and diagenesis of Holocene carbonate sands in
an inner-platform environment: analog of some ancient carbonate
reservoirs, in S.T. Reid, ed., Transactions of Southwest Section
AAPG, Publ. SWS 2000-107, p. 67-79.
Wilhite, B.W., and Mazzullo, S.J.,
2000, Facies architecture and diagenesis of Holocene carbonate sands in
an inner-platform environment: analog of some ancient carbonate
reservoirs, in S.T. Reid, ed., Transactions of Southwest Section
AAPG, Publ. SWS 2000-107, p. 67-79.
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