BASIC CARBONATE GEOLOGY: ONE DAY SHORT COURSE FOR GEOLOGISTS AND ENGINEERS

Based on a workshop sponsored by PTTC's Appalachian Region and the Pittsburgh Association of Petroleum Geologists held in Washington, PA on March 8, 2005.

BOTTOM LINE

For integrated carbonate reservoir characterization the answers to reservoir questions lie in understanding the rocks. With carbonates, understanding stratigraphy, diagenesis and pore type distribution is especially important. Analysis of outcrops, cores and/or cuttings and linking findings to logs and seismic can improve the bottom line.

PROBLEM ADDRESSED

The need for increased unconventional natural gas production in the next 10 to 20 years has created a market, which independent operators in Kansas want to share in. However, information on the resources and techniques to evaluate them has been minimal. The Kansas Geological Survey has a responsibility to the people of Kansas to provide information on the development and economic production of resources vital to Kansas' economy.

KEY WORDS:

Carbonate Platforms, Carbonate Sedimentology, Diagenesis, Rimmed Shelves, Carbonate Ramps, Sequence Stratigraphy, Shoal Complexes

SPEAKERS:

Taury Smith, New York State Museum,
Albany, NY

Fred Read, Virginia Polytechnic Institute and State University, Blacksburg, VA
 

TECHNOLOGY OVERVIEW

Carbonate sediments may be defined by their mineralogy, grain types, pore types and diagenesis. Unique fabric and textural combinations may be interpreted in terms of their original depositional setting. Carbonates may also be interpreted in terms of their high-frequency cycles, which can be grouped into a meaningful sequence stratigraphic hierarchy. The sequence stratigraphy of carbonate systems is somewhat different from that in terrigenous systems and provides the basis for bridging the gap between simple geological description and interdisciplinary reservoir characterization.

Mineralogy and Carbonate Grain Types

The basic minerals in carbonate sediments include aragonite, low-Mg calcite (0-4 mol % MgCO₃), high-Mg calcite (10-25 mol % MgCO₃), and dolomite. In many carbonates the dominant grains are created by skeletal material. Fossils are identified according to their shape, microstructure, internal structure, and mineralogy (e.g. rudists, stromatoporoids, and foraminifera). Skeletal grains are often reduced by abrasion to microscopic size. In addition, biological alteration is common. Because of the strong biological control of carbonates, a working knowledge of major fossil types in thin section and in outrop or core is necessary for lithofacies identification. Also, because dominant forms of life have changed through time a nearly infinite number of facies may be defined that can be interpreted as part of a spectrum of relatively few depositional settings. Other grain types include ooids (concentrically laminated grains of aragonite or calcite), pisoids (which are gravel size concentrically coated grains including marine and vadose forms), oncoids (microbially coated granule to gravel size grains), rhodolites (concentric and radially laminated red algal nodules often as large as softballs), intraclasts (syndepositional or weakly lithified rip-up clasts), lithoclasts (fragments of lithified carbonate sediment that is reworked across an unconformity), peloids (sand size micrite grains formed from a variety of sources including fecal origin, erosion of poorly lithified sediment, and biological degradation of skeletal clasts). Technically, lime mud (micrite) is not a type of grain; however it can form from physical breakdown and micritization of grains as well as from direct precipitation from seawater.

Carbonate Rock Types

The most commonly used system for classifying carbonate rock types was devised by Dunham (1962). This system recognizes the well known textural classes: boundstone (the "reefal" facies including components bound together during deposition) and all other textural types that were not bound together during deposition. This second large group includes grain-supported rocks (grainstone with essentially no micrite and packstone that contains micrite matrix) and mud supported rocks (micrite with <10% grains and wackestone with >10% matrix). A wide variety of reef rocks exist with sheet to mounded external morphology and an internal rigid skeletal framework. Embry and Klovan extended the Dunham classification system by substituting specialized terms bafflestone, bindstone, and framestone for Dunham's "boundstone." In addition, floatstone (gravel size grains floating in matrix) and rudstone (gravel size grains free of matrix) are often (but not always) associated with reefal carbonates.

It is useful to distinguish carbonate banks from reefs. Banks are sheet to mound like in-place accumulations of skeletal carbonate lacking a rigid internal skeletal framework. They have wackestone to mudstone core facies and generally have lime grainstone/packstone flanking and capping facies or channel fill facies.

Reefs and banks can have the geometry of mounds (less than 30 times wider than high) or biostromal sheets (more than 30 times wider than high).

Carbonate Depositional Settings

A variety of important carbonate depositional settings can be recognized. Because of the limited time for this duration only selected settings were discussed.

Platform interiors are often highly cyclical with burrowed shallowing-upward lagoonal carbonate units capped by tidal flat laminites or fenestral sands. Eolian facies on platform interiors often have extremely sharp cross lamination of quartz-peloid sand that sometimes is inversely graded. Erosive surfaces (microkarst) are common in updip settings at the top of cycles deposited during humid conditions. In arid conditions tops of parasequences comprise tidal flats associated with evaporates. Intraclasts eroded from the top of the cycle are common in the base of the next higher parasequence. Microbially laminated dolomites commonly form beneath tidal flat laminites in arid settings. Microbial mounds form in shallow subtidal to intertidal conditions often on thin-bedded subtidal muds and pelletal to oolitic grainstones. Tidal channels cutting through tidal flats are a common feature. There are several vertical sequences in tidal channels that depend on the depth and geometry of the channels, as well as the strength of tidal flux. Grainstones may occur at the bottoms of tidal channel fills, at the tops, or they may be completely absent.

Shoal complexes include oolitic sand shoals, banks, and reefs.

Carbonate ramps are characterized by less than 1 degree of dip, shallow water wave-agitated facies passing downdip uninterrupted to deeper water facies. Downslope breccias are commonly absent and reef rims are absent on carbonate ramps. Updip on ramps pelletal-oolitic barrier complexes are present, such as along the Trucial Coast. Ooid sand bodies can occur as oolitic tidal ridges (generally oriented at high angles to the shelf margin) or marine belts, such as Joulter's Cay. Marine oolitic belts tend to be a few to several km across while tidal bar belts are comprised of narrow, elongate ridges up to 20 km long.

Carbonate banks consist of in place accumulations that lack a rigid skeletal frame (this is reserved for reefs). Barrier banks on ramps often consist of updip channeled skeletal sands and muds that are separated from the mainland by a narrow lagoon. Carbonate banks can also form against the mainland without an intervening lagoon. These banks are termed fringing banks and are generally cut by tidal channels that terminate downdip in vegetated packstone to wackestone. Reservoir facies on ramps include fringing and barrier banks, downslope buildups, ooid-pellet shoal complexes and downslope reefs. Seals are provided by regressive peritidal carbonates or evaporates, or transgressive deep slope/basinal facies.

Reefs are in place skeletal accumulations with a rigid framework. They and skeletal sand belts commonly form the barrier found on rimmed shelves. On rimmed shelves the wave agitated reefal (and sand) facies mark a strong break in slope into deeper water. Slopes range from a few degrees to tens of degrees and deep-water breccias and turbidites are common. Reservoir facies on rimmed shelves include reef rims and associated sands, pinnacle reefs in the deep lagoon and on the basin slope, and deep-water detrital carbonates. Seals are regressive carbonates, evaporates and shale as well as transgressive deep-water muds.

Deep ramp settings tend to form near or below storm wave base. Deep ramp lithologies are often thin bedded, nodular argillaceous wackestone to mudstone. They are often nodular due to interlayering of shaly lime and lime layers, burrowing, nodular marine cementation and compaction/stylolitization. Slope facies tend to consist of very thin, evenly bedded lime mudstone with thin shale partings. Fore-reef breccias downlap onto basinal muds and contain clasts that vary from mm to house-size. Basinal rhythmites consist of graded lime sands (grainstone-packstone) and intervening black limey shale with soft sediment deformation.

Diagenesis of Carbonate Sediments

The primary product of most diagenesis is cement. Common morphologies of cement may be used to interpret the environment within which it formed. For example, bladed to fibrous aragonite fans (often neomorphosed to coarse calcite) form in the marine environment. Bladed crusts of high-mg calcite also typically reflect original marine cement. In contrast fine equant calcite overlain by coarse equant sparry calcite indicates meteoric to burial cementation.

More importantly, diagenesis can completely change the pore system in carbonates—either positively or negatively. Despite the attitudes of many explorationists, diagenesis is a primary control of reservoir properties. This is because diagenesis includes all the post-depositional biological, chemical, and physical changes that occur to the sediments. Saturation or solubility of limestone (calcite) is affected by temperature, pressure, pH, salinity, and the partial pressure of CO₂ (P_CO2). Subtle changes in these parameters can cause a fluid to dissolve or to precipitate calcite and other minerals.

A lot of effort is spent determining where diagenesis occurred (its "environment") so that the vertical and lateral extent of the reservoir can be predicted. Near surface or "early" diagenesis includes marine cement, meteoric leaching and cement, and near-surface dolomitization. Subsurface or burial diagenesis includes compaction and pressure solution, hydrothermal, and burial or geothermal cements. The most common environments in which diagenesis occurs include marine, meteoric, deep burial and hydrothermal settings.

Incomplete early cementation (e.g., marine cements) can help preserve porosity and permeability because it helps resist compaction. Marine cements (aragonite and Mg-calcite) are generally identified due to their bladed to fibrous nature. In contrast, meteoric diagenesis occurs when carbonates are subaerially exposed and altered by fresh water. Leaching tends to occur when mildly acid rainwater flows through limestone and when it mixes with seawater. Carbonate that was dissolved is reprecipitated as meteoric cement elsewhere. Epikarst refers to karst processes that occur in the vadose zone, at the water table and at the mixing zone between meteoric and marine fluids. Karst breccias commonly have a clay-rich or micritic matrix and generally make poor reservoirs. Modern cave fills are a mix of layered cave fill and some roof collapse (clasts). They also commonly include speleothem deposits (stalagmites and stalactites) of calcite. These tend to be less commonly recognized in ancient cave systems.

Many authors believe that when meteoric water migrates through carbonates it causes extensive leaching, microporosity and recrystallization. Others believe that meteoric waters are channeled along high permeability zones (enhanced fractures, caves) and does not significantly leach or recrystallize the surrounding matrix.

Meteoric cements include vadose (meniscus, vadose silt, and gravity oriented cements) or phreatic products (isopachous and blocky cements). Epitaxial overgrowths have been shown to occur in both meteoric subzones.

Mixing of seawater and freshwater can produce an undersaturated solution capable of significant leaching due to the non-linear relationship between calcite saturation and percent marine water—this model may be underapplied in reservoir modeling.

In most carbonate reservoirs, burial diagenesis results in progressive porosity destruction with depth due to compaction, pressure solution and cementation.

Many reservoirs are dolomitized when Mg-rich fluids migrate through pre-existing limestone. Matrix dolomitization occurs in many settings: there is no single dominant environment for dolomitization. All models for dolomitization require a magnesium source and a mechanism to deliver it. Because there is a lot of Mg+2 in seawater, it is probably the original source for most dolomitizing fluids. Seepage reflux and sabkha models for dolomitization are well accepted as is reflux or tidal flat dolomitization. Tidal flat dolomitization is probably the most applicable model for early regional dolomitization. The concept of mixing-zone dolomitization (between marine and meteoric waters) has been over-sold. Very little dolomite is currently forming in the world's mixing zones.

Burial mineralization is sometimes easily confused with early stage hydrothermal dolomitization. The determination of one mechanism or the other really boils down to whether the mineralizing fluids were migrating laterally through the formation or had a component of vertical (hydrothermal) flow. It is becoming more and more obvious that some reservoirs would simply not be reservoirs without hydrothermal alteration or enhancement of porosity by fault-related fluids.

Tools for understanding diagenesis include petrography or observations from thin sections used to determine the paragenetic sequence. Stable isotopes and fluid inclusions can be used to determine the composition and temperature of fluids that precipitated cements. Crossplots of stable isotopes of C and O can be used to interpret diagenetic environments. Trace elements are used to determine the oxidation state of the fluids. Strontium isotope ratios (87Sr/86Sr) help to differentiate burial fluids from seawater and can help to date some younger carbonates.

Pore Types

Carbonate reservoirs are often complex because they have multi-component pore systems. Common pore types include interparticle, intercrystalline, interclast, moldic, intraparticle, intraskeletal, intracrystalline, fracture, vuggy, cavernous and microporosity. Primary porosity is porosity that remains from when the rocks were deposited. Secondary porosity is porosity that was created during diagenesis through leaching or recrystallization. Crossplots of carbonate porosity and permeability have different trends for different pore types. This concept led Lucia to propose that pore classification should be based on grain-dominated vs. mud-dominated fabric and dolomite crystal size. Crossplots show that grainstones, mud-lean packstone, mud-rich packstone, and mud-dominated fabrics (wackestones and mudstones) have unique and decreasing relationships.

Sequence Stratigraphy

Sequence stratigraphy is a powerful tool to determine the framework of reservoirs. This tool provides unconformity bounded time slices rather than simple lithological correlations. Therefore, important controls on fluid flow within the reservoir are recognized. Also, high-frequency cycles in 3rd order sequences are at or below the level of resolution by biostratigraphy. This makes it difficult to correlate such cycles world wide, but makes them ideal for determining reservoir- and seismic-scale stratigraphic frameworks in carbonate sections.

All of the fundamental concepts of sequence analysis (sequences, systems tracts, etc.) designed for terrigenous systems may be applied to the sequence stratigraphic analysis of carbonate systems. However, the strong effect of biological controls in carbonates does provide some complications in interpretation. For one thing the concept of updip terrigenous depocenters must be contrasted with shallow marine "carbonate factories." One of the most marked features of sequence stratigraphy on carbonate platforms is that the lowstand systems tract is often below the shelf margin. This results in alternating transgressive and highstand systems on the platform. When sea level does not fall beyond the shelf edge, the shelf margin systems tract forms a prograding carbonate wedge above the previous highstand resulting in a Type II sequence boundary.

The fundamental unit of terrigenous sequences is the parasequence. The carbonate equivalent is the high frequency cycle, which is generally a meter scale shallowing-upward succession bounded by flooding surfaces downdip and subtle unconformities overlain by flooding surfaces updip.

 

CONNECTIONS:

Taury Smith
New York State Museum
3140 Cultural Education Center
Albany, NY 12230
Ph: 518-473-6262
Email: lsmith@mail.nysed.gov

Fred Read
Virginia Polytechnic Institute and State University
4044 Derring Hall
Blacksburg, VA 24061
Ph: 540-231-5124
Email: jread@vt.edu

For information on PTTC’s Appalachian Region and its activities contact:

Douglas G. Patchen, Program Director
West Virginia University, Appalachian Basin Regional Lead Organization
P.O. Box 6064, Evansdale Drive, Morgantown, WV 26506-6064
Voice: (304) 293-2867 ext. 5443; Fax: (304) 293-7822
Email: Doug.Patchen@mail.wvu.edu

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