Porosity In Carbonate Rocks Research Articles

Biogeochemical ecology ofThiothrixspp. In underwater limestone caves

Thiothrix spp., sulfide‐oxidizing mixotrophic bacteria, were sampled from visible colonies in the Floridan aquifer in several underwater caves, sinkholes, and springs below the water table in North Florida. Bacteria samples were collected by cave divers certified by the National Speleological Society/Cave Diving Section. Sites sampled were ecological niches in the aquifer where visible colonies had a white slimy or filamentous appearance indicative of Thiothrix spp. Sterile sampling methods were adapted to the underwater cave setting. Bulk water samples for media preparation were collected by divers from bacteria sampling sites. Bacteria were isolated and cultured in growth media prepared with cave or spring water. Thiothrix spp. were identified by microbiological and immunological methods. Monoclonal antibodies specific for Thiothrix spp. were utilized in fluorescent antibody assays and enzyme‐linked im‐munosorbent assays (ELISA). Thiothrix was found in six of eight underwater caves sampled. Three of these caves had no discernible water flow at the time of sampling, indicating that Thiothrix in the Floridan aquifer does not necessarily require constantly flowing water. Most of the visible bacterial colonies that tested negative for Thiothrix were biofilms growing on limestone and iron oxyhydroxide substrates on the walls of clear‐water and high‐flow caves. The sulfur cycle in phreatic limestone conduits is described. The reactions and bacteria involved in the HS‐ cycle and pyrite cycle are discussed. Thiothrix generates sulfuric acid, which has the potential to dissolve limestone below the water table. Results of this study should contribute to a better understanding of the role of colorless sulfur bacteria in the development of porosity in carbonate rocks and microbial ecology in these karst aquifer settings.

Geochemical constraints on CaCO 3 transport in subsurface sedimentary environments

Simple models for carbonate transport in subsurface sedimentary environments that do not carefully consider solution chemistry are unable to produce even order of magnitude predictions of the volume of water or time needed to transport calcium carbonate into or out of a carbonate rock unit. We feel that the time intervals necessary for creation or destruction of porosity cannot usually be well established, because the solution chemistry (e.g., P CO 2 ) and flow velocities of ancient waters are generally not determinable. Our calculations also indicate that major changes in porosity of medium- to large-sized carbonate rock units cannot take place by transporting a significant fraction of the calcium carbonate into or out of the rock unit. This conclusion is based on both the time needed for sufficient volumes of water to flow through the unit, and the probable reaction kinetics between the solution and rock that will usually result in relatively sharp reaction fronts. Based on these considerations, we suggest that most changes in porosity in carbonate rocks are the result of the redistribution of material within the rocks, rather than massive importation or exportation of calcium carbonate. However, in some sequences, allochthonous cementation of limestones may occur, because of mobilization of CaCO 3 brought about by high effective tectonic stresses and high pore-water fluid pressures.

Petrographic image analysis and petrophysics: Analysis of crystalline carbonates from the Permian basin, west Texas

Classification of porosity in carbonate rocks from thin section can be performed quickly and objectively using computer-based image acquisition and classification procedures. Pore type information (size, shape and volumetric abundance) is determined with high precision, equivalent to millions of point counts. Such pore type information is always relevant because each pore type has a different distribution of associated pore throat sizes. Five pore types occur in a sequence of highly recrystallized dolomites from the Reinecke Field in the Late Carboniferous Horseshoe Atoll Complex in the northern Midland basin. These pore types represent an expanded version of conventional classification, representing two kinds of intercrystalline porosity and three kinds of channel porosity. The channel pore types represent secondary porosity which survived cementation by dolomite, and can be classified into channel pores of three distinctly different sizes. When combined with capillary pressure data the image-based pore type data reveals that the intercrystalline pores have very small throats while the channel pores have throats which are closer in size to the pore body. The product of the numbers of pores per unit volume of each type and the fourth power of the associated mean throat diameter is an index of the relative contribution of each pore type to discharge. In the Reinecke Field rocks, the smallest type of channel pore is the major control on flow. Its relatively small throat size is compensated by its great numerical abundance. The preference of throat size for pore type in these highly recrystallized dolomites, is just as strong as previously observed in detrital sandstones. Relatively tight limestones found with the dolomites also have a strong relationship between pore type and throat size, though their maximum throat radius is 7 microns and some pore types have mean throat sizes less than 1 micron. The Reinecke Field data, coupled with data from studies on sandstones, suggests that throat size in all sedimentary rocks can be expected to be non-randomly associated with pore type. The non-random pore/throat association means that the pore types defined by image analysis represent fundamental elements of the porous microstructure and that variability in a wide range of physical properties is tied to variation in pore type abundance.

A simple method for producing high-quality porecasts of carbonate rocks

The study of porecasts is an important technique for evaluating the distribution, and quantity, of porosity in carbonate rocks. This paper sets out a method by which high-quality porecasts may be produced, quickly and easily. The procedure consists of impregnation firstly under vacuum, then under pressure with low viscosity (6 × 10 −3 Pa s) epoxy resin. Examination under SEM reveals penetration of intragranular porosity at a submicron scale without disruption of the rock fabric.

Cathodoluminescence Microscopy: A Valuable Technique for Studying Ceramic Materials

Over the past 10 to 20 years, cathodo-luminescence microscopy (CLM) has become an important, standard technique for studying many geological materials, but its application to ceramic material has not been fully realized. CLM has been found to be especially valuable in determining original porosity in carbonate rocks, a feature that was extremely important in determining the ability of petroleum and related fluids to migrate through the rocks and concentrate in the favorable structures where petroleum is found today. CLM constitutes one of the most significant techniques employed in the exploration for petroleum, but the results of those investigations remain largely unpublished in company files. A number of investigations have discussed the application of cathodoluminescence (CL) to studies of porosity in carbonates.Other applications of CLM to geology involve metals exploration. Halos of red CL feldspar in the rocks around carbon-atite deposits, which may contain rare earths, copper, and other metals, contrast with feldspar crystals with blue or blue-green CL that are distal from the ore deposits. Dolomite associated with lead-zinc ores may exhibit growth zoning by CL that can be used to trace the path of the ore fluids for distances of 100 miles or more. The detection of host rock limestone alteration in the Tri-State zinc-lead district, potassic and argillic alteration at the Henderson molybdenite mine, and ore guides in other metallic deposits by CL was discussed by Hagni.

Porosity Advisor--Expert System Used as Aid in Interpreting Origin of Porosity in Carbonate Rocks: ABSTRACT

Porosity Advisor--Expert System Used as Aid in Interpreting Origin of Porosity in Carbonate Rocks: ABSTRACT

Zur Bestimmung des Gesamtporenvolumens von Karbonatgesteinen

Zur Bestimmung des Gesamtporenvolumens von Karbonatgesteinen

Modelling Carbonate Reservoir Porosity For Enhanced Recovery Schemes

Abstract Most carbonate rock porosity classifications are generic and do not account for features of the pore systems that may affect fluid movement and withdrawal-important factors in the production of hydrocarbons from these rocks. Mercury injection capillary pressure (MICP) analyses of carbonate reservoirs supply valuable information on the interaction between non-wetting fluids and reservoir porosity. Some 200 analyses of a variety of Mississippian carbonate reservoirs in southern Saskatchewan indicate that porosity can be simplified into three types; inter-crystal, cavity and mixed (inter-crystal and cavity). The categories can be recognized by the degree of uniformity of pore throat sizes in the reservoir rock; a high degree of uniformity indicates inter-crystal porosity and a wide variation in pore throat size is representative of cavity porosity. Entry pressure is a second MICP factor that tells something about the reservoir rock. High entry pressures are commonly related to calcitic reservoir rocks with inter-crystal porosity: intermediate entry pressure indicates dolomite with inter-crystal porosity; and low pressures are representative of calcitic or dolomitic rocks with abundant large cavities (mesopores). Pore system/fluid interaction can be shown to influence efficient withdrawal of hydrocarbons from a reservoir, thus productivity may vary markedly due to differences in porosity types over the lateral extent of a producing zone. Introduction During the past ten years, several graduate students in the Department of Geology, University of Regina, and the author have investigated several geological aspects of the Mississippian reservoir rocks in southeastern Saskatchewan. These include studies into the microfacies, diagenesis and nature of the pore systems in the Benson (Midale Beds)(1), Glen Ewen (Midale Beds)(2), Innes (Frobisher Beds)(3,4) and Hummingbird (Ratcliff Beds)(5) oilfields. In addition to these completed investigations, a study is at present being carried out on three Ratcliffe Beds producing areas: Flat Lake, Lake Alma, and Oungre. Most of these studies have been generously supported by the Saskatchewan Department of Energy and Mines, in an effort to establish a readily available source of information on carbonate rock reservoirs for enhanced recovery schemes. The Hummingbird study was financed through a grant to Dr. L. W. Vigrass from Hudson's Bay Oil and Gas Co. Ltd. The pore systems of the reservoirs in these fields have been studied using mercury injection capillary pressure analyses, to establish pore throat size distribution and a variety of fluid response characteristics, including entry pressure, ejection efficiency and unsaturated pore volume. The results of these analyses were enhanced by detailed examination of the reservoir rocks using scanning electron and light microscopy. The variety of carbonate rock types constituting the reservoirs in the Mississippian fields studied have made it possible for the writer to establish a simplified classification of porosity for carbonate reservoir rocks. Because Mississippian rocks are used as the basis for this classification a brief review of their important characteristics is presented below. Geologic Setting Mississippian rocks in southern Saskatchewan were deposited on the northern flank of the Williston Basin in a marine seaway in which paleobathymetry exerted an influence on the distribution of facies.

Seismic Prediction of Porosity and Hydrocarbon Traps in Carbonate Rocks: ABSTRACT

Seismic stratigraphy has been used in many areas to identify stratigraphic carbonate traps such as shelf margins, pinnacle reefs, mounds, and updip porosity pinch-outs. Many large oil fields are the result of oil entrapment in these types of stratigraphic traps. Examples include fields from the Gulf Coast, Permian basin, Mid-Continent, and Rocky Mountain provinces in the United States, as well as fields in the Middle East, Canada, North Sea, and the Far East. Where the trap shows geomorphological relief, evidence such as draping, pull-ups, dim spots, data dispersion, and other criteria can be used. Stratigraphic carbonate traps can be localized with some precision using these criteria. However, the specific definition and measurement of porosity in carbonate rocks are much more difficult. With very careful integration of geologic (rock data from cores and petrophysical data from logs) and geophysical (high resolution seismic) data, it is possible to estimate both the thickness and, in a qualitative way, the amount of porosity in a potential carbonate reservoir. During the Carboniferous in the Mid-Continent, a sequence of depositional and diagenetic events created irregular pods of porosity in otherwise tight limestones. The areal extent, thickness, and quality of this porosity are the primary factors that determine the location, geometry, and productivity of major oil fields in the area. A twenty-million-barrel oil field has been studied in detail, and the initial production rates and overall production richness correspond closely to measurable seismic phenomena. A seismic line shot through the producing interval at a depth of 4,000 ft, using 30-fold, broad band (20-120 hertz) data has allowed the recovery of frequencies over 100 hertz. These data confirm (1) the presence of porosity and (2) field limits that correspond to the field limits k own from subsurface information. The Geoquest System work station was used to model (1) the key porous interval as known from core and petrophysical data in the analog field and (2) evaluate similar phenomena in the surrounding play area and measure both porosity thickness and quality in prospective stratigraphic traps. Two specific trap types occur regionally. The first type evidences porosity that developed locally and has an acoustically recognizable event over it (and between it and an overlying shale). The second type shows evidence of local porosity extending vertically to the shale seal. In both types, the lateral limits can be mapped seismically. The trap types have very different characteristics; both trap types and variations of them can be modeled successfully. The trap types discussed are very subtle and have historically been End_Page 1442------------------------------ discovered only by operators drilling for structure or by random drilling. Many areas within the onshore United States are underexplored for these subtle type traps. With the availability of very high frequency/high-resolution data, these type plays can now be made. End_of_Article - Last_Page 1443------------

New Wave Carbonate Research: What Paleontologists Can Learn from It: A Status Report from Cancun, Mexico

Carbonate research in recent years has moved a long way from the emphasis on facies and paleoecology that dominated work in the '60s and early '70sso far, indeed, that traditional paleontologists may view it as an unrelated discipline. Yet much that has been happening of late in carbonate circles has significant application to problems in the mainstream of paleontologic thought, including determination of paleosalinities, paleotemperatures, and the calibration of secular changes through Phanerozoic time. The benefits of interaction between carbonate researchers in industry and academia were crystallized at Cancun last August, as 65 geologists gathered to parade, review, and joust over current research in an SEPM Research Symposium convened by Paul Harris (Chevron Research), Lynton Land (University of Texas), and Peter Scholle (Chevron Research). The symposium title 'Spar Cements and Secondary Porosity in Carbonate Rocks' ensured strong financial support from oil companies and drew almost equal participation from geologists in industry and academia. Participants came from many countries on both sides of the Atlantic, including U.S.A., Canada, Venezuela, Belize, Great Britain, West Germany, and Poland. Four consecutive days of focussed discussion challenged brains as well as derrieres, for sessions began promptly at 8:00 a.m. each morning and continued well past 10:00 p.m. in the evenings, and opportunities were provided for informal discussion still later (should the mind, body, and soul prove capable!). Presentations were organized in four sequential sessions: (1)Surficial Processes, (2) Burial Processes, (3) Analytical Methods, and (4) Review: Comparison of Diagenetic Environments. Understandably, most of the papers were primarily oriented to carbonate petrologists, but several should be of considerable interest to PALAIOS readers for specific new data, for techniques, and for some significant theoretical implications. In the 'specific new data' category, algae were implicated as catalysts of some rather esoteric cements in two different Caribbean habitats. Brian Jones (University of Alberta) documented evidence that algae, fungi and lichen promoted the formation of 'lichen stromatolites' and microdendritic calcite growths in sinkholes on exposures of the Bluff Dolomite (Oligocene to Miocene) on Grand Cayman Island. John Kaldi (Shell Canada Limited) illustrated how algal filaments in beachrock pores of northern Jamaica have promoted the formation of low-magnesian meniscus cements by trapping fine detritus and retaining droplets of marine water in intergranular spaces.

Crystallite Size of Diagenetic Chert in Carbonate Rocks--Probable Indicator of Prelithification Porosity: ABSTRACT

Silicification and dolomitization in seven constituent lithofacies of the Meramecian carbonate sequence in central eastern Missouri are genetically related and are facies dependent. In the arenite facies with variable lime-mud matrix (0-30%), both were probably caused by phreatic freshwater and seawater mixing. Silicification is characterized by microcrystalline quartz-replaced (MCQ-replaced) bioclasts and pore-occluding fibrous chalcedony (CHY); the latter is common as micronodules. In these facies-specific, silica-replaced bioclasts, MCQ has abnormally large crystallite size in parts of the profile characterized by blocky low-matrix cement. In contrast, the matrix-rich facies, characterized by extensively dolomitized matrix, have smaller crystalline size in the MCQ-replaced bioclasts. The conventional model of thermal annealing does not adequately explain the observed, intermittently anomalous, large crystallite size of the diagenetic chert in the Meramecian arenite facies, considering the regional thermal and burial history of the rocks. Also, the chert crystallite size inversely correlates with the matrix mud content in the facies. Therefore, the authors propose that the observed variation in the size of the chert crystallites reflects the degree of silica saturation in the pore water. The degree of silica saturation is a function of permeability controlled by the effective prelithification porosity in these carbonates. Preliminary data suggest that chertmore » crystallite size can be used to map the distribution of prelithification porosity in carbonate rocks that were substantially modified in texture during diagenesis. Oxygen isotopic composition of these cherts may help delineate the fluid-migration paths in these rocks. This information is useful in exploring for energy resources.« less

Porosity Prediction in Shallow vs. Deepwater Limestones

Summary Limestones can be divided arbitrarily into shallow- and deepwater types. Shallow-water limestones characteristically have complex depositional facies patterns and originally are composed primarily of unstable minerals (aragonite and high-Mg calcite), which make them highly susceptible to post-depositional alteration. Small differences in burial-uplift-water chemistry histories can produce radically different trends of porosity preservation, destruction, or creation as a consequence of the great susceptibility of unstable minerals to alteration by fresh (nonmarine) pore fluids. These factors make porosity prediction of shallow-water limestones very difficult. Only through careful and detailed mapping of both primary facies and mineralogical trends, coupled with equally painstaking determinations of diagenetic histories and patterns, can porosity predictions be made for shallow-water limestones. Deepwater, pelagic limestones, on the other hand, commonly are composed of a much more stable primary mineral (low-Mg calcite) and have more uniform and predictable facies patterns. Studies performed on Mesozoic and Cenozoic chalks throughout the U.S. and Europe have shown that porosity is lost as a direct function of burial depth. The rates of porosity loss vary with the pore-water chemistry and, more importantly, as a function of pore-fluid pressure. If these factors are known with reasonable certainty, then reasonable predictions can be made of average subsurface porosity of deepwater limestones. Only where such limestones have never been buried deeply, where they have had an early input of oil, or where they have elevated pore-fluid pressure can high porosities and high petroleum-reservoir potential be expected. Scanning electron microscopy (SEM), petrography, and isotopic analysis have proved useful techniques for deciphering the depositional and diagenetic histories of both shallow- and deepwater limestones. Introduction Prediction of porosity in carbonate rocks is at best a difficult undertaking. Most carbonate hydrocarbon reservoirs drilled to date have been in shallow-water limestones. In such rocks, depositional patterns are complex and variable and diagenetic alteration of original sediment textures and compositions may be extreme. Few, if any, general rules can be used to predict the porosity of such carbonate rocks in the absence of drilling data. Even where a significant number of wells have been drilled, useful porosity trends that enable accurate porosity prognostication over even relatively short distances may be difficult to discern. Porosity prediction in deepwater limestones, on the other hand, generally is easier. Deepwater strata have more uniform depositional facies, more predictable primary chemical compositions, and simpler diagenetic alteration patterns than their shallow-water equivalents. Alteration of deepwater limestones is controlled mainly by their maximum burial depth. Average rates for such diagenetic porosity loss can be determined as a function of burial.

Now You See It, Now You Don't--Seismic Expressions of the Subtle Trap: ABSTRACT

Exploration geologists and geophysicists have much in common in terms of tools and vocabulary when it comes to searching for hydrocarbons, but there are clear differences in approach. Certain occurrences of hydrocarbons are subtle in terms of their geologic setting. Others may be obvious in terms of geology, but subtle in their seismic expression. Little has been written about such traps other than what has appeared in works on stratigraphic applications for seismic data. It is also important to distinguish between subtle seismic expressions of traps and complicated seismic expressions. The complicated seismic expression, while difficult to interpret, is not often overlooked. Noting subtle seismic signature in the first instance End_Page 912------------------------------ is another matter. A challenge is presented when both geology and seismic expression entail subtlety. These problems are encountered frequently and often are treated with great success. Detection of fracture porosity in carbonate rocks is an example. Bounds and nature of seismic visibility and detectability are developed for families of lithologies of exploration interest. Geometric considerations are examined in the context of subsurface definition. Tools and techniques currently available for treating seismic data for the subtle trap are described, and uses for these tools and methods are given. In reconciling geologic and geophysical views toward the subtle trap, a more complete definition of the concept and a still larger family of exploration targets are provided. End_of_Article - Last_Page 913------------

Occurrences of Tertiary Porosity in Carbonate Rocks: ABSTRACT

Occurrences of Tertiary Porosity in Carbonate Rocks: ABSTRACT

Development of Porosity in Carbonate Rocks of Evaporitic Basins, Alternative Mode of Formation: ABSTRACT

Development of Porosity in Carbonate Rocks of Evaporitic Basins, Alternative Mode of Formation: ABSTRACT

Influence of Eustatic Sea-Level Changes in Oil and Gas Accumulations in Appalachian Basin: ABSTRACT

Regional stratigraphic studies indicate a minimum of 23 eustatic sea-level fluctuations in the Appalachian region from New York to Alabama. A eustatic fluctuation is interpreted if the stratigraphic and sedimentologic records on two or more sides of the Appalachian basin show evidence of a similar simultaneous shift in relative sea level within the limits of temporal resolution by fossils, intertonguing facies, or bentonite chronology. Simultaneous sea-level shifts affecting different lobes of ancient delta complexes built into the Appalachian basin from eastern sources also are considered eustatic. For analysis of a hypothetical single basin, a eustatic sea-level change is one which affects the entire basin. The cause may be large-scale tectonics of the continental area ontaining the basin, or a true sea-level shift related to glaciation or rate of sea-floor spreading. Timing on the sea-level variation curve is related closely to ages of strata with hydrocarbon production in the Appalachian basin. Changes in sea level result in shifting of sand deposition along shorelines, solution porosity in carbonate rocks exposed along basin margins, or modifications of reef growth. The clearest relations to hydrocarbon production are in the well-explored oil and gas fields in the Devonian and Silurian, where the sea-level shifts can be used to explain permeability distribution. Superposing sea-level shifts onto sedimentary tectonics in a basin of known shape allows prediction of exploration trends. The largest fluctuations of sea level are at the base of the Sauk sequence (Cambrian transgression), the Owl Creek discontinuity (base of the Middle Ordovician), the Wallbridge discontinuity (end of the Early Devonian), and the discontinuity at the base of the Absaroka sequence (beginning of Pennsylvanian). Fluctuations associated with the Wallbridge discontinuity are related to deposition of the Oriskany Sandstone, which is the largest Appalachian gas producer. Lesser sea-level changes are related to other production, notably oil and gas from the Upper Devonian fields which were the birthplace of the American petroleum industry, and gas from the Silurian Newburg sandstones. The eustatic sea-level curve from one basin such as the Appalachian area should be compared with other basins to identify worldwide patterns and to help to focus petroleum exploration in distant basins. The major level drops at the end of the Early Ordovician, end of the Early Devonian, and the beginning of the Pennsylvanian seem established. The sea-level drop in the Appalachians at the end of the Ordovician appears related to continental glaciation centered in the African Sahara. The rise in sea level at the end of the Precambrian is possibly a result of an increase in rate of sea-floor spreading as the proto-Atlantic Ocean opened. Recognition of true global changes should permit more precise intercontinental correlations because eustatic sea-level change is not related to distribut on of faunal provinces or local tectonic processes. End_of_Article - Last_Page 2216------------

Petrophysical Approach to Origin of Porosity of Carbonate Rocks in Middle Carboniferous Windsor Group, Nova Scotia, Canada

Primary porosity in carbonate rocks consists of original pore spaces between and within grains during deposition. Secondary porosity is a modification of primary porosity as a result of subsequent physical and chemical processes after deposition. These processes are compaction, fracturing, solution, cementation, and inversions such as dolomitization and calcitization. Various physical and chemical processes affect porosity of carbonate rocks of the Carboniferous Windsor Group of Nova Scotia. The Windsor Group is a postorogenic, molasse-type sequence of redbeds with perhaps a dozen, thin carbonate-sulfate blankets. Each blanket undergoes facies changes landward from shallow-marine biodolomicrite, through a high-energy facies of biosparite or oosparite, commonly to a low-energy, lagoonal biomicrite, and finally to intertidal and supratidal lithosomes. Facies 1 of Schenk is mainly biodolomicrites with relatively high grain densities, low natural densities, and high absolute porosities. Schenk's Facies II and III are chiefly biosparites or oosparites (locally dolomitic), with relatively low grain densities, high natural densities, and low absolute porosities. In general, absolute porosities of these carbonate rocks are directly proportional to their effective porosities. Aragonite must have been the chief source material in these carbonate rocks. The depositional primary porosity of Facies I was nearly 80 percent and that of Facies II and III was almost 55 percent. The fundamental process in reducing the pore spaces is compaction, although it must be resisted by the presence of authigenic clay minerals. Other important processes that decrease the porosity are cementation and calcitization. Porosity increases as a result of dolomitization and solution in Facies I and as a result of solution in Facies II and III.

Effect of Postsedimentation Processes on Carbonate Reservoir Rocks in Volga-Urals Region, USSR

Porosity and permeability of carbonate rocks are controlled by (1) sedimentation conditions, which determine primary textural features, and (2) secondary postsedimentation processes. Bioclastic carbonates, with small amounts of cement and without any appreciable amounts of secondary minerals, form the best carbonate reservoir rocks. Postsedimentation processes appreciably alter the primary textures and thus affect the reservoir rock properties. Such processes as solution and formation of fractures and stylolites considerably improve porosity and permeability of carbonate rocks. Processes of sulfatization, calcitization, and silicification, resulting in filling and sealing of pores, cavities, and fractures, adversely affect reservoir properties. Processes of secondary mineral formation, however, indirectly improve the flow capacity of rocks by creating heterogeneous texture, which favors the formation of fractures and solution cavities. The development of carbonate rock porosity is very complex because of the alteration by solution and secondary mineral-formation processes. Porosity and permeability of carbonate reservoirs were found to be related closely, with the correlation factor ranging from 0.7 to 0.8.

Origin of Porosity in Neogene Carbonate Rocks of Akita, Japan

Primary porosity in carbonate rocks originates from primary pore-spaces between and also within particles during deposition. Secondary porosity is a modification of primary porosity influenced by subsequent physical and chemical reactions after deposition. These reactions are compaction, fracturing, solution, cementation and inversions such as dolomitization, calcitization and so on.Oil-bearing Neogene carbonate strata of Akita, Japan, of bathyal to inner-neritic origin, are composed mainly of small skeletal debris. During syn-diagenesis, calcium, magnesium and carbon dioxide from both the sea-water and also from basin volcanic detritus altered the originally siliceous sediments to dolostone and dolomitic limestone.The most fundamental and significant pore-space reducing reaction in these carbonates was compaction. However, this was resisted by the presence of authigenic clay minerals. The next most important reaction tending to reduce porosity was cementation. Factors effective in increasing pore-space were inversione, such as dolomitization and transformations of opal to calcite, dolomite and quartz.

Diagenesis and Porosity in Carbonate Rocks [Abstract

Diagenesis and Porosity in Carbonate Rocks [Abstract