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Overview of Previous HitPorosityNext Hit Evolution in Carbonate Reservoirs*

By

S. J. Mazzullo1

 

Search and Discovery Article #40134 (2004)

 

*This online version is basically a reprint of an article by the author and with the same title, published in Kansas Geological Society Bulletin, v. 79, nos. 1 and 2 (January-February and March-April, 2004) and available on the KGS website (http://www.kgslibrary.com/bulletins/bulletins.htm). Appreciation is expressed to the author and to KGS for permission to present this article on Search and Discovery.  

1Department of Geology Wichita State University, Wichita, Kansas ([email protected])   

 

Introduction 

Carbonate rocks (limestone and dolomite) account for approximately 50% of oil and gas production around the world. Of the carbonates, a slightly greater percentage of world hydrocarbon reserves has been produced from dolomites because such rocks commonly, but not always, have more Previous HitporosityNext Hit and permeability than limestones (Halley and Schmoker, 1983). Unlike most Previous HitsandstoneNext Hit reservoirs, which typically are single-Previous HitporosityNext Hit systems (i.e., interparticle pores) of relative uniform (homogeneous) nature, reservoirs in carbonate rocks commonly are multiple-Previous HitporosityNext Hit systems that characteristically impart petrophysical heterogeneity to the reservoirs (Mazzullo and Chilingarian, 1992). Hence, the specific types and relative percentages of pores present, and their distribution within the rocks, exert strong control on the production and stimulation characteristics of carbonate reservoirs (e.g., Jodry, 1992; Chilingarian et al., 1992; Honarpour et al., 1992; Hendrickson et al., 1992; Wardlaw, 1996). Pore types in carbonate rocks can generally be classified on the basis of the timing of Previous HitporosityNext Hit evolution (Choquette and Pray, 1970) into: (1) primary pores (or depositional Previous HitporosityNext Hit), which are pores inherent in newly-deposited sediments and the particles that comprise them. Such pore types include interparticle pores in, for example, carbonate sands (but also in muddy carbonates), intraparticle pores (within particles such as foraminifera or gastropod shells), fenestral pores (formed by gas bubbles and sediment shrinkage in tidal-flat carbonates), and shelter and growth-framework pores (common in reef buildups); and (2) secondary pores, which are those that form as a result of later, generally post-depositional dissolution. Such pore types include all of those mentioned above, but only when it can be demonstrated that primary pores which subsequently were occluded by cement later had all or some of that cement dissolved (resulting in the generation of exhumed pores - Figure 2), as well as vugs (large pores that transect rock fabric, that is, dissolution was not fabric-selective) and dissolution-enlarged fractures. Most of these primary and secondary pore types can readily be identified in cores, and with the possible exception of shelter and growth-framework pores, also in well cuttings samples.  

Because the natural tendency in most carbonate sediments is that primary Previous HitporosityNext Hit is substantially reduced by cementation and compaction during post-depositional burial (Figure 1; Halley and Schmoker, 1983), many workers would argue that most Previous HitporosityNext Hit in limestone and dolomite reservoirs is of secondary origin (e.g., Mazzullo and Chilingarian, 1992). Exceptions to this statement are cases where primary Previous HitporosityNext Hit is preserved because of the early influx of hydrocarbons into pores (e.g., Feazel and Schatzinger, 1985). Early on in the study of carbonate sediments and their diagenesis, the subaerial meteoric diagenetic (freshwater) model was promoted as a means of explaining Previous HitporosityNext Hit evolution in carbonates, specifically in shallow-water carbonates that lie beneath unconformities in paleo-vadose and paleo-phreatic freshwater zones (e.g., Friedman, 1964; Land, 1967). This model still is heavily applied today, especially in sequence stratigraphic-related diagenetic studies of reservoirs. This model presupposes that if porous carbonate rocks are present beneath unconformities, then that Previous HitporosityNext Hit must have been created by freshwater dissolution during subaerial exposure. Of course, as explorationists, we all can probably list a large number of wells that were drilled into non-porous carbonate rocks beneath unconformities. Hence, the corollary pertaining to subaerial exposure is not true - meteoric exposure does not always create Previous HitporosityNext Hit, and even if it did, that Previous HitporosityNext Hit may be occluded during later burial (Figure 1). A most critical constraint on evaluating, or more importantly, on Previous HitpredictingNext Hit Previous HitporosityNext Hit in carbonate rocks utilizing only the subaerial meteoric diagenetic model is that one must call upon fluids capable of dissolving carbonate to come from above; that is, from rain water percolating down into sediments or rocks beneath unconformity surfaces. Ostensibly, then, many might consider that if there is not an unconformity in the section, then the carbonates will not be porous. Again, as explorationists we can all probably compile a list of wells in which porous carbonates that were not associated with unconformities were encountered in the subsurface.  

The foregoing analysis therefore begs the following questions: (1) can secondary Previous HitporosityNext Hit in carbonate rocks be generated by processes other than subaerial meteoric exposure, and if so, what are those processes?; (2) how might Previous HitreservoirNext Hit Previous HitporosityNext Hit formed by such alternative processes differ from reservoirs created by meteoric dissolution along unconformities?; (3) how can we recognize and determine the origin of Previous HitreservoirNext Hit Previous HitporosityNext Hit?; and (4) can the subsurface occurrence of Previous HitporosityNext Hit formed by any such alternative models of Previous HitreservoirNext Hit origin be predicted? The purpose of this contribution is to address these questions by demonstrating the multi-faceted evolution of secondary Previous HitporosityNext Hit in carbonate hydrocarbon reservoirs. In the following discussions attention will be focused on the recognition and origin of pore types in shallow-water limestones and dolomites, as observed mainly in cores and well cuttings samples.

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uSubaerial meteoric model

  uPrevious HitPorosityNext Hit in limestones

  uPrevious HitPorosityNext Hit in dolomites

  uCavernous Previous HitporosityNext Hit

uMesogenetic model

uMicroporosity

uConclusions

uReferences

 

 

 

Figures Captions

Figure 1. Processes by which Previous HitporosityNext Hit 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 Previous HitporosityNext Hit in a carbonate grainstone. B - thin-section photomicrograph of intraparticle Previous HitporosityNext Hit (arrows) within a fusulinid and B' core showing intraparticle Previous HitporosityNext Hit within a coral. C- Fenestral Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit.  

Figure 6. Relationship of Previous HitporosityNext Hit to percent dolomite in carbonate rocks (after Murray, 1960).

Figure 7. Origin of secondary dissolution Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitreservoirNext Hit zones in cavernous (karsted) carbonate rocks. Bottom - Typical Previous HitporosityNext Hit types and fills of cavernous reservoirs. Cave roof rocks become progressively more brecciated downward, with attending fracture, dissolution-enlarged fracture, and commonly, vuggy Previous HitporosityNext Hit. Cave-fill deposits variously can be: (1) cave roof-collapse breccia, which can have inter-clast Previous HitporosityNext Hit as well as intra-clast vuggy and fracture Previous HitporosityNext Hit. Conversely, original inter-clast Previous HitporosityNext Hit 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 Previous HitporosityNext Hit. A - Fractures, which cross and therefore post-date stylolites, with Previous HitporosityNext Hit from the dissolution of post-fracture cement. Adjoining this area is selective dissolution of oolites. B - Oomoldic Previous HitporosityNext Hit adjoining and post-dating the formation of a stylolite.

Figure 14. Mesogenetic Previous HitporosityNext Hit. 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) Previous HitporosityNext Hit 

Return to top.

Secondary Previous HitPorosityNext Hit Beneath Unconformities: The Subaerial Meteoric Model 

The generation of secondary Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit, 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). Previous HitPorosityNext Hit 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, Previous HitporosityNext Hit 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 Previous HitPorosityNext Hit 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 Previous HitporosityNext Hit. 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 Previous HitreservoirNext Hit, 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 Previous HitporosityNext Hit (Figure 3A) and attending relatively high permeabilities. On the other hand, however, high Previous HitporosityNext Hit but low permeability may characterize a carbonate sand (limestone) Previous HitreservoirNext Hit wherein only the particles have been dissolved (for example, in cases where the Previous HitreservoirNext Hit contains only molds of oolites referred to as oomoldic Previous HitporosityNext Hit or by the older term oocastic Previous HitporosityNext Hit: 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 Previous HitporosityNext Hit. Notwithstanding Previous HitporosityNext Hit associated with dolomitization (discussed later), limestones deposited in tidal-flat environments commonly contain a specific type of vuggy Previous HitporosityNext Hit referred to as fenestral pores (for example, birdseye pores, which is one type of pinpoint Previous HitporosityNext Hit: Figure 3C), and unless fractured, such rocks may have decent Previous HitporosityNext Hit 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 Previous HitporosityNext Hit (and hydrocarbon storage volume) might be high, but permeability would be low (notwithstanding fracturing). As a corollary, variations in Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitreservoirNext Hit rocks, and therefore, need not be considered here.

 

Meteoric Previous HitPorosityNext Hit 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 Previous HitporosityNext Hit. Whereas it is true that Previous HitporosityNext Hit 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 Previous HitreservoirNext Hit. 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 Previous HitPorosityNext Hit in Carbonate Rocks  

Cavernous and associated vuggy Previous HitporosityNext Hit 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 Previous HitreservoirNext Hit compartmentalization (Figure 9). Cavernous Previous HitporosityNext Hit 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). 

 

Alternative Origin Of Secondary Previous HitPorosityNext Hit: The Mesogenetic Model 

Since the late 1970s-early 1980s, geologists began to suspect that not all secondary dissolution Previous HitporosityNext Hit 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 Previous HitporosityNext Hit formation (and Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit. 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitreservoirNext Hit 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 Previous HitporosityNext Hit 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.  

Previous HitPorosityNext Hit 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 Previous HitporosityNext Hit (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 Previous HitporosityNext Hit formation occurred cannot be determined on the basis of the pore types present in a Previous HitreservoirNext Hit! How, then, does one recognize Previous HitporosityNext Hit 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 Previous HitporosityNext Hit.” 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 Previous HitporosityNext Hit in carbonate rocks that can be identified in cuttings and core samples. For example: (1) Previous HitporosityNext Hit 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 Previous HitporosityNext Hit formation can be determined by the law of cross-cutting relationships to be younger than the stylolites, and the rocks are still buried, then Previous HitporosityNext Hit 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 Previous HitporosityNext Hit formation must be mesogenetic if the rocks are still buried. Are there examples of mesogenetic Previous HitporosityNext Hit in carbonate reservoirs in Kansas? Undoubtedly, but specific published instances where mesogenetic Previous HitporosityNext Hit is present are not available.

 

Microporosity, Pinpoint Previous HitPorosityNext Hit, and Chalky Previous HitPorosityNext Hit 

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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit can be referred to as chalky. I point out that microporosity, and specifically chalky Previous HitporosityNext Hit, 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.

 

Conclusions 

Secondary Previous HitporosityNext Hit 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.

 

Selected References 

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, Previous HitporosityNext Hit, pore size, and residual water saturation, in G.V. Chilingarian, S.J. Mazzullo, and H.H. Rieke, eds., Carbonate Previous HitReservoirNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit in a Jurassic grainstone Previous HitreservoirNext Hit, 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 Previous HitporosityNext Hit Cenozoic carbonate rocks of south Florida: progressive loss of Previous HitporosityNext Hit 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 Previous HitReservoirNext Hit 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 Previous HitReservoirNext Hit 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 Previous HitReservoirNext Hit 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 Previous HitReservoirNext Hit 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 Previous HitReservoirNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit, in G.V. Chilingarian, S.J. Mazzullo, and H.H. Rieke, eds., Carbonate Previous HitReservoirNext Hit 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 Previous HitReservoirNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit, 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 Previous HitPorosityNext Hit: Elsevier Publ. Co., Developments in Sedimentology 46, 338 p.

Moore, C.H., and Druckman, Y., 1981, Burial diagenesis and Previous HitporosityNext Hit evolution, Upper Jurassic Smackover, Arkansas and Louisiana: AAPG Bulletin, v. 65, p. 597-628.

Murray, R.C., 1960, Origin of Previous HitporosityNext Hit 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 Previous HitreservoirNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit, 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 Previous HitReservoirNext Hit 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, Previous HitPorosityTop 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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