Grainstone Textures Research Articles

Tectono-sedimentologic and lithostratigraphic control in Aures Region: Case study Djebel Metlili, Batna, Algeria

This research delves into the geological features of the western section of the Aures Basin, with a primary focus on Djebel Metlili. The geological characteristics span Mesozoic and Tertiary deposits, ranging from the Triassic to the Quaternary epochs. Notably, the higher Cretaceous period stands out for its substantial carbonate-rich sequence. The research relied on geological maps, field observations, core samples, and laboratory analyses, including lithostratigraphic examinations (cross-section) and thin section. Structural features show that is formed by large regular folds of ENE-WSW or E-W direction. Anticlines and synclines are often affected by transverse accidents at the axes of the folds. In its northern part is located immediately south of the Belezma-Batna mountains. Structural analysis highlights significant tectonic disturbances, oriented in a northwest-southeast direction. A detailed lithostratigraphic examination reveals marly formations interspersed with limestone-rich layers containing Inoceramus. The southern part of Dj. Metlili, particularly the Santonian-Campanian series, unveils three distinct meso-transgressive sequences, linked to sea-level fluctuations associated with sedimentary basin subsidence. The studied area exhibits three distinct facies: one characterized by gray phosphate limestone with crisscrossed stratifications and agitated bioclastic sand, another featuring a mollusk-rich bioclastic limestone indicating a turbulent intertidal environment, and a third presenting a clay limestone bank with fine to medium grains and lumachels rich in oysters and gastropods. The associated grainstone texture in the microfacies suggests an internal platform environment marked by dissolution, bioturbation, and ferruginization. This comprehensive exploration provides valuable insights into the geological history of the region, significantly contributing to our understanding of its evolution over time.

Analyzing the impact of geological features on reservoir heterogeneity using heterogeneity logs: A case study of Permian reservoirs in the Persian Gulf

Carbonate reservoir heterogeneity has a major impact on fluid flow and hydrocarbon recovery. It is challenging to characterize due to the complex and heterogeneous nature of carbonate reservoirs. It is important to express heterogeneity as a quantitative parameter for comparing various reservoirs. It can also be used for investigating the influence of different factors on the degree of heterogeneity. Heterogeneity log is one of the effective methods for quantifying heterogeneity. The aim of the present study is to investigate the application of heterogeneity log to determine the geological features affecting heterogeneity in Upper Dalan Member (Late Permian) in the central Persian Gulf. The gamma ray, bulk density, neutron porosity, acoustic, and deep resistivity well logs data were used to calculate the heterogeneity logs. A total of 1093 thin sections were studied and primary depositional as well as secondary diagenetic changes were recorded. Porosity and permeability of the same number of samples were measured on the plugs. Heterogeneity logs were calculated by Lorenz coefficient method. The changes in heterogeneity logs were classified into different ranges and compared with frequency of geological features in corresponding intervals. Findings show that heterogeneity logs can help identify areas of the reservoir with distinct reservoir behavior (permeability and/or porosity). These areas can be targeted for increased oil and gas production. This can help reduce exploration and development costs. Dolomite samples are observed in both low and high heterogeneity values. Anhydrite lithology reduces porosity and increases heterogeneity. High heterogeneity values correspond with mudstone texture from supratidal, peritidal, and lagoon subenvironments with low porosity. Grainstone textures from shoal subenvironment and crystalline carbonates are associated with low values of heterogeneity. These microfacies have high porosity and permeability as well as a high rate of marine cementation. Also, interparticle and moldic pores along with stylolites and fractures are observed in low values of heterogeneity. The increase in the ratio of pore space to the solid component of samples has resulted in a reduction in the value of heterogeneity. By creating different subenvironments with different microfacies and average porosities, environmental energy controls the heterogeneity of the studied member. Therefore, the amount of heterogeneity is mainly controlled by two factors namely, porosity and pore type.

Characterization of proven Late Cretaceous Reservoir rocks in the Gulf of Gabes: Integrated case study

Microstructural features control the petrophysical properties of rocks. Of these, pore size is particularly sensitive when non-wetting fluids saturate the reservoir. The pore networks and physical properties (helium, water and mercury saturation porosity, bulk density, nitrogen permeability, P-wave velocity and thermal conductivity) of different rock types from a productive Upper Cretaceous (Coniacian) reservoir in the tunisian offshore are measured on hydrocarbon-washed samples. The facies sampled of Douleb Member are wackestone, packstone and grainstone textures as well as dolostone, dolomitized packstone and anhydrite textures. Based mainly on the results obtained by mercury injection, the porous facies of the Coniacian Douleb Member are characterized by a complex pore system with a large morphological and pore size variability in the rock. Porosity values range from 0.3 to 23.6%, bulk densities vary from 2.05 to 2.92 g.cm−3. The permeability is variable from 3760 mD to values below 0.01 mD (measurement limit of the device). P-wave propagation velocity values range from 2060 to 6084 m.s−1 and thermal conductivity varies from 1.43 to 3.77 W.m−1.K−1. The oil-impregnated facies with the best petrophysical characteristics are mainly the rudist-rich limestones and dolomites of the first unit (U1) of the Douleb Member. The well-sorted grainstones with small rudist debris and peloids have the best reservoir qualities. Porosity is the first order characteristic that controls petrophysical properties. The variability of permeability values around this first-order relationship is attributed to variations in the size of the pore access thresholds and connectivity. The variability in velocities is due to the shape of the voids, while the variability in thermal conductivity measurements is due to the nature of the contacts between the grains.

Comparison Between the Classical Classification and Digital Classification for Selected Samples of Igneous and Carbonate Rocks

As igneous rocks have widely chemical and mineralogical compositions, there are many ways to classify these rocks. These ways are classical approved methods to give a reliable classification and nomenclature of rocks. Some igneous rocks may be classified by digital image processing to assist in classical methods. Five igneous samples were cut, prepared of thin sections, and polished to classify them by classical methods and digital image processing by ENVI software. Moreover, part of these samples crushed an analysis of major oxides. The current igneous samples have referred to the basic and mesocratic rocks based on the classical methods and this has corresponded to ENVI software. The igneous samples have reflected the leucogabbros when classify them by classical and ENVI classifications, except the G5 sample, which has been referred to as gabbro by ENVI. There is a clear similarity between the classical and ENVI classifications. ENVI classification is a reliable classification to assist the classical methods in the nomenclature of igneous rocks, especially, plutonic rocks, it can be also applied to thin sections of volcanic rocks to classify and nomenclature classification by ENVI has been applied on fifty thin sections of limestones to identify microfacies which are classified beforehand by classical (optical) classification. According to optical classification, microfacies have classified as mudstone, wackestone, packstone, and grainstone. When the digital classification is applied to them, there is no grainstone texture found in these them. Digital thin sections, where the true name of these microfacies is packstone. Therefore, the positive sides of the digital image processing by ENVI software appeared and contrasted to the optical classification which contained some mistakes when applied to the nomenclature of these microfacies.

Cased-Hole Standalone Evaluation Succeeds in the Absence of Openhole Data

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 202894, “Cased Hole Standalone Evaluation: Breaking the Barrier To Successfully Evaluate Challenging Deep Carbonate Reservoirs,” by Pradeep Menon and Carey Mills, ADNOC, and Suvodip Dasgupta, SPE, Schlumberger, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually from 9-12 November. The paper has not been peer reviewed. Accurate petrophysical evaluations (formation lithology, porosity, and water saturation) are essential in characterizing potential reservoir zones and estimating resources in place. Typically, these evaluations rely on acquisition of openhole logging measurements; however, this is not always possible. The complete paper outlines two examples from tight gas reservoirs in two separate fields offshore Abu Dhabi in which openhole data could not be acquired and petrophysical analysis was undertaken using cased-hole log data. These evaluations successfully identified gas-saturated porous intervals in each well, one of which was successfully flow-tested. Introduction A growing need exists to increase gas production in the UAE. As a result, specific gas-production targets have been mandated from development of currently undeveloped deep gas carbonate reservoirs such as the Permo-Triassic Khuff formation, the middle Jurassic Araej formation, and the Permian Pre-Khuff Unayzah and Berwarth formations. Recent appraisal wells have aimed at evaluating these reservoirs systematically by acquiring a good suite of openhole logs, cutting conventional cores, and conducting well-testing operations. These well data are combined with an evolving regional understanding to better assess and ultimately develop these complex formations. An accurate petrophysical evaluation requires the petrophysicist to develop a realistic evaluation of formation lithology, porosity, and water saturation. These parameters provide the foundations for further work such as static modeling stands, and they must be robust. The Upper Khuff is composed of dolomite occasionally grading to calcareous dolomite with minor interbeds of claystone and anhydrite. In core and cuttings, the dolomite in the uppermost section exhibits a grainstone texture with poor intercrystalline/intergranular porosity. The Lower Khuff is composed of very hard dolomite in part grading to calcareous dolomite, medium-to-dark grey-brown in places, with occasional very-fine-to-medium grainstone texture and very poor intercrystalline porosity. In this paper, the Upper and Lower Araej members are interpreted to have been deposited in an open, marine- circulation shelfal environment, while the Uweinat member is considered to have been deposited in a more- restricted circulation setting with-in a similar shelfal environment. The Barrier Openhole logging generally is the preference for formation evaluation because it represents the simplest environments and benefits from a comprehensive list of available measurements. The variety of tools and diversity of output data available make openhole log acquisition the gold standard for formation evaluation. However, in certain situations in which openhole logging is not possible because of borehole conditions (re-entry of old cased wells, wellbore instability, over-pressure), no option exists other than acquiring petrophysical data in a cased-hole environment.

3D forward stratigraphic modelling of the Lower Jurassic carbonate systems of Kuwait

This paper describes the architectural and texture/facies evolution of a complex carbonate system through the construction of a regional scale 3D stratigraphic forward model (252 × 152 km2, 4 km × 4 km grid size) encompassing onshore and offshore Kuwait. The model mimics the development of a Jurassic carbonate ramp system at a 2nd to 3rd sequence stratigraphic order (1–80 Myr duration; Schlager, 2010) by simulating sedimentary processes and ecological parameters such as carbonate production versus depth, time, wave energy, as well as erosional and gravitational transport. Multi-scale 1D, 2D, and 3D geological and geophysical constraints have been used to calibrate the Lower Jurassic Marrat stratigraphic model, which resulted in the identification of (1) a dominantly transgressive lower unit composed of onlapping packages of mudstone to wackestone textures, with anhydritic intercalations; (2) a dominantly regressive middle unit composed of prograding inner ramp shoal and lagoonal facies with wackestone to grainstone textures, and finally (3) a transgressive onlapping upper unit composed of mudstones to wackestones textures in a mid-ramp system. The identification of potential reservoir facies like oncolitic shoals located onshore Kuwait, as well as possible porous reworked material transported away from the shoal loci in a relatively deeper mid ramp settings provides a better insight on further model improvement needed to de-risk stratigraphic trapping mechanisms. The identification of porous features in 3D provides crucial insights on the influence of ecological constraints on the complex Jurassic carbonate ramp systems of Arabia.

Microfacies, diagenesis and hydrocarbon potential of Eocene carbonate strata in Pakistan

The role of microfacies and diagenesis was focused to assess the hydrocarbon potential of Eocene carbonate strata exposed in Sulaiman–Hazara mountain ranges. Thin- to thick-bedded Eocene limestone exhibits wackestone, packstone, mudstone, and grainstone texture and comprises larger benthic forams such as Nummulite, Assilina, Lockhartia, Discocyclina, Orbitolites and Operculina. The accumulation of the strata occurred in lagoon, inner to deeper shelf. The microfacies and textural variations revealed stable shallow shelf conditions in early Eocene. The marine sedimentation ceased in Hazara and Salt ranges, but Kohat and Sulaiman regions remained under water during middle Eocene. The deposition of Habib Rahi Formation in deeper shelf attests to local scale normal faulting due to the northward movement of the Indian Plate. The Eocene reservoir zones were sourced by Patala and Ghazij Formations and capped by thick cover of fluvial shale, establishing favourable conditions for stratigraphic traps. Thick limestone packages of foraminiferal wackestone/packstone and dolomitized/dolomitic wackestone microfacies are regarded as estimable targets for hydrocarbons exploration in the region. Several diagenetic processes were observed in the studied limestone which includes: micritization, dolomitization, neomorphism, cementation, mechanical and chemical compaction, fracturing and dissolution which were caused in marine, meteoric and burial diagenetic environments. Compaction was the main factor for the destruction of primary porosity along with cementation effect. The porosity in the limestone was significantly enhanced by late stage dissolution, fractures enclosed by calcite cement and dolomitization.

RESERVOIR CHARACTERIZATION OF AN ISOLATED MIDDLE MIOCENE CARBONATE PLATFORM: CA VOI XANH FIELD, OFFSHORE VIETNAM

The Ca Voi Xanh (CVX) gas field is located offshore Vietnam along the eastern margin of the southern Song Hong Basin. Reservoir rocks are carbonates of Middle Miocene (Langhian and Serravallian) age which developed on an isolated platform (length approximately 100 km, and width approximately 15 km) on top of the Triton Horst structural high. Shallow‐water corals and large and small benthic foraminifera are the main faunal constituents of the Langhian carbonates, whereas overlying Serravallian carbonates, the principal reservoir at CVX, are dominated by deeper‐water coralline red algae (rhodoliths) and large benthic foraminifera (LBF). The Serravallian carbonates consist of rhodolith‐LBF grainstones and packstones to mud‐lean packstones. Langhian carbonates consist of coral‐LBF grainstones‐packstone/rudstones.This paper documents the workflow used to develop an integrated sequence‐stratigraphic and reservoir rock‐type framework for the Ca Voi Xanh reservoir, and the impact of this work on reservoir quality prediction and modelling. Sequence stratigraphic interpretations of the Serravallian carbonates are based on data from three wells including detailed sedimentological core descriptions tied to well‐log character. The carbonate succession consists of three third‐order depositional sequences (Ser1, Ser2 and Ser3). Two well‐developed exposure surfaces can be identified: the base‐Serravallian sequence boundary (Ser1 SB), and the base‐Tortonian sequence boundary (Tor1 SB). The Serravallian shows an overall shallowing‐upward trend from carbonates with more horizontally‐oriented coralline red algae (encrusted and bored pavements/hardgrounds) in the lower part of the section, to carbonates with large, roundish irregular rhodoliths towards the upper part. Petrographic thin section, stable isotope (oxygen and carbon), and fluid inclusion analyses confirm a freshwater (vadose and phreatic) diagenetic overprint of the carbonates below the exposure surfaces (sequence boundaries).Partial dolomitization of the Serravallian carbonates was observed at Wells‐2 and ‐3 in the CaVoi Xanh field. Dolomite formation could be related to hydrodynamic fluid interactions associated with the development of a freshwater lense during the top‐Serravallian sea‐level fall (Tor1 SB), favoring dolomitization by sea water during the subsequent sea‐level rise. Alternatively, dolomitization could be related to CO2 and hydrocarbon charging, causing secondary leaching of the Mg‐rich coralline red algae and large benthic foraminifera, leading to an enrichment of the water with Mg‐ions and thus favoring downward dolomitization.A reservoir rock type (RRT) scheme was developed for the Langhian and Serravallian carbonates at the Ca Voi Xanh field based on a combination of depositional environment interpretations together with diagenetic characteristics and reservoir parameters (porosity and permeability).The Langhian is characterized by two RRTs depending on the degree of cementation (RRT‐3L) and dissolution (RRT‐5L). Serravallian RRTs are separated into dominantly packstone (RRT‐1) and dominantly grainstone textures (RRTs‐2, ‐3, ‐5 and ‐6). RRTs with grainstone texture show varying degrees of cementation (RRTs‐2 and ‐3) and dissolution (RRTs‐5 and ‐6). Early diagenetic dolomitization (RRT‐4) preferentially affects packstones (RRT‐1) but also grainstones (RRT‐2, ‐3 and‐5).The geologic (static) model consists of both matrix and non‐matrix components. Sequence stratigraphic concepts in combination with seismically‐derived palaeo‐reconstructions were used to guide the vertical and lateral distribution of reservoir rock types, which comprise the matrix component of the geological model. Different karst geometries and features consistent with known hydrogeologic processes were identified in the seismic discontinuity (variance) cube, and were used to interpret karst regions with different degrees (and/or types) of karst/fractures. These karst regions were used to populate the non‐matrix component of the geologic model along the Ser1 and Tor1 sequence boundaries.

Geology of the ‘Sénia stone’ from Ulldecona, Catalonia (Aptian, Maestrat Basin, Iberian Chain) and its implications for regional stratigraphy

The municipality of the town of Ulldecona (Catalonia) is notable for extensive quarrying activities, which exploit limestone, popularly named Stone from Ulldecona, for ornamental and building purposes. The Stone from Ulldecona, commercially known as Sénia stone, is one of the most important ornamental and building stones quarried in Catalonia, and is used worldwide in all kinds of public and private buildings. Little is known about the geological nature of this stratigraphic interval of commercial value. Therefore, this study explores the geology of the Stone from Ulldecona in open pit quarries and natural outcrops. The Stone from Ulldecona consists of limestones of upper lower Aptian age, including wackestone, packstone and grainstone textures containing peloids, miliolids, Palorbitolina lenticularis, Orbitolinopsis simplex, Paracoskinolina maynci, Lithocodium aggregatum, Choffatella decipiens, Salpingoporella muehlbergi, Chondrodonta, Toucasia carinata, Polyconites sp. and Mathesia darderi. These platform carbonates rich in orbitolinids and rudists belong to the Villarroya de los Pinares Formation of the Maestrat Basin. Locally, the limestones are highly bioturbated and/or dolomitized. Dolomitic limestones, calcitic dolostones and dolostones are stratabound tabular geobodies with thicknesses of up to 60 m. Dolomitic limestones and calcitic dolostones corresponding to initial and intermediate stages of dolomitization mainly exhibit isolated euhedral dolomite crystals or idiotopic mosaics. Dolostones (advanced dolomitization stages) are sucrose, exhibit vacuolar and cave porosities, and are characterized by idiotopic and hypidiotopic mosaics, which indicate temperatures lower than 50-60 °C during dolomitization. Dolomite textures are mainly fabric-destructive and pervasive, but locally retentive and/or selective fabrics also occur. The limestones of the Villarroya de los Pinares Formation, the underlying marls of the Forcall Formation and the overlying marls and platform carbonates of the Benassal Formation examined can be arranged into two high-rank, low-frequency transgressive-regressive sequences. Similar coeval long-term transgressive-regressive trends have been reported from other basins, indicating that eustatism largely controlled accommodation of the Aptian succession studied in Ulldecona.

Characterization and Evaluation of the Potential of Limestone Jebels Bent Saidane, Bou Garnine Raous and Bridge of Fahs (North East Tunisia)

At Zaghouan locality, north eastern Tunisia, significant sedimentary ores took place (e.g. sand, clay, limestone…). This sedimentary ores were largely extended and covered partially by quaternary deposits. In this paper, multidiscipline approaches with petrographic, mineralogical, chemical and geotechnical analyses were involved to valorize these materials in various industrial uses. The carbonates of the Lower Jurassic (Oust Formation) present good criteria and higher quality to be use in engineering application moreover huge reserves. The gray carbonates of the Lower Jurassic (Oust Formation) are constituted by limestones with packstone or grainstone texture, with relatively abundant lithoclasts which are well coated by a micritic matrix. Mineralogical studies reveal the presence of calcite with a small content of quartz and dolomite. The major element compositions of this limestone show higher content in CaO, ranging from 53% to 55%, and from 94.65% and 98.21% expressed as CaCO3. The limestone of Oust Formation display low amount of Al2O3, SiO2 and Fe2O3 which ranging from 2% to 5%. Other minor elements (Na2O and K2O) are also detected. Geotechnical tests (e.g. Los Angles, Humid Micro Deval, Density Index) conducted on the Lower Jurassic limestone at studied area show dense materials (2,55 to 2,81 g/cm3) and well resistant to the fragmentation. Depending on the provenance of the limestone rock of the Oust Formation, different employments can be proposed. The Bent SaN—dane and Bou Garnine Mountains can be used to produce the aggregates while Rouas Mountains for marble manufactures. Either this applications, Lower Jurassic limestone from studied area can be used in the manufacture of Portland cement when mixed with Lower Cretaceous clay from the Zaghouan locality.

Lithologic and diagenetic attributes of the Sharwayn (Maastrichtian) and Umm Er Radhuma (late Paleocene–Eocene) formations and their significance to the K-T unconformity, Jabal Samhan area, Dhofar, Sultanate of Oman

The boundary separating Maastrichtian Sharwayn Formation from late Paleocene Eocene Umm Er Radhuma (UER) Formation in Dhofar, southern Oman, is characterized by a regionally extensive unconformity. The Jabal Samhan escarpment, north of Marbat-Sadh transect, preserves this unconformable boundary. This paper addresses the lithologic and diagenetic differences of the strata across the boundary and discusses their significance and link to the development of the upper Maastrichtian to lower Paleocene unconformity. The upper part of the Sharwayn Formation is characterized by lower thickly bedded, bioclastic, and peloidal mudstone to wackestone lithofacies overlain by a thick ledge (~5.5 m) of medium to coarse crystalline, (sucrosic) dolostone. Poorly preserved outlines of the framework grains suggest an original peloidal and bioclastic grainstone texture for the dolostone unit. The contact with the overlying UER Formation is sharp. The UER Formation is characterized by thickly bedded, bioclastic mudstone to wackestone overlain by coarse-grained, foraminiferal grainstone. The dolomitization process of the dolomite unit at the top of the Sharwayn Formation is envisaged to a shallow subsurface mixed meteoric and sea water diagenetic realm. This interpretation is supported by an inferred timing of dolomitization of latest Maastrichtian (post-Sharwayn Formation) to early Paleocene (pre-EUR Formation), preservation (although poorly recognizable) of the original depositional texture and diagenetic features that postdate the sucrosic dolomite. Pervasive dolomitization of the dolomite unit was controlled by its original grainstone texture, which permitted efficient percolation of the dolomitizing fluids. Correlation between the reference section of the formation and the section studied in this work raises the thickness of the formation to 28.5 m (relative to a thickness of 22 m at the reference section). Analysis of the new composite section suggests that deposition of the formation took place in a shallowing-upward setting where low-energy subtidal sediments (the lower limestone unit) were succeeded by a high energy sand shoal (upper dolomitized unit).This shallowing-upward succession is attributed to a third-order sea-level drop. The later is also recognized in many parts of the Arabian Peninsula, as well as globally, and interpreted as eustatic sea-level fluctuation.

Upper Aptian calcareous algae from Padurea Craiului (Northern Apuseni mountains, Romania)

A study of calcareous strata previously assigned to the Barremian-Early Aptian interval in the northwestern part of P?durea Craiului, (Apuseni Mountains), led to the identification of a micropaleontological association indicative of a Late Aptian age. Unequivocal evidence for the Late Aptian assignment of these limestones is the presence throughout the sequence of two orbitolinid species, Mesorbitolina texana (ROEMER) and Mesorbitolina subconcava (LEYMERIE). The most interesting sections are located in the neighbourhood of Subpiatr?, where both outcrops and a quarry facilitated detailed analyses. In this area, the Upper Aptian succession consists basically of three types of macrofacies: 1, limestone with rudists; 2, limestone with Bacinella and 3, limestone with corals, each of them showing several types of microfacies. Bacinella structures are the most common feature in the whole succession, irrespective of the macrofacies. This paper focuses on an algal association that was identified in several levels within the succession. Dasycladalean algae are more frequent, and are commonly found in grain-dominated fabrics (mostly grainstone textures), in association with orbitolinid foraminifera and bioclasts of corals, rudists and gastropods. However, a few species are present only in mud-dominated fabrics (i.e. lower-energy intervals). The dasycladalean association from the Upper Aptian deposits of P?durea Craiului is of special interest, for this group registered a dramatic decline at the Lower Aptian/Upper Aptian boundary, as confirmed by the relative scarcity of the Dasycladales in the Upper Aptian carbonate deposits.

Modern non-tropical mixed carbonate-siliciclastic sediments and environments of the southwestern Gulf of California, Mexico

Modern mixed carbonate-siliciclastic warm-temperate environments were studied in the southern Gulf of California (GOC), Mexico. The combined heterozoan and photozoan character is defined by the absence of true coral reef structures, calcareous green algae and non-skeletal grains, and the dominance of rhodoliths (unattached coralline red algae), mollusks and small coral patch reefs. The major carbonate producing environments are pocket bays, a shallow carbonate bioherm, high-energy rocky and sandy shorelines and mid-shelf settings. Sediments characterizing the pocket bays display heavily bioturbated bioclastic wackestone and grainstone textures. The rhodolith dominated bioherm is composed of coarse sediments exhibiting grainstone and rudstone textures with fine material being winnowed away by tidal currents. Rocky and sandy shorelines are characterized by erosion and mixing of up to boulder size granitic and volcaniclastic material with coralline red algal and molluskan carbonates. The mid-shelf settings are defined by a silt and fine sand sized non-carbonate matrix mixed with abundant molluskan fragments and foraminifera. Peak carbonate production takes place between 10 and 15 m water depth; below 40 m production significantly decreases, combined with a steep increase in the amount of siliciclastics. This is in contrast to many temperate and cool-water mixed-carbonate siliciclastic systems, where most carbonate production occurs in mid- to outer shelf settings. A full spectrum of sediments from pure carbonates (90–100% carbonate), allochem carbonates (50–90% carbonate) and allochemic mud or sand (<50% carbonate) was encountered in the study area. The absence of early diagenetic binding and cementation of the non-tropical carbonates facilitates mixing of carbonates and siliciclastics. The latter are mainly derived from adjacent Tertiary volcaniclastics and Mesozoic granites. Mixing takes place (1) as punctuated mixing during periodic short-term storm or hurricane events, when coarse granitic and volcaniclastic derived material eroded from rocky shorelines is transported into the shallow carbonate factories, and (2) as facies mixing on the outer carbonate shelf below 40 m water depth where carbonates interfinger with olive-gray sandy and silty siliciclastics.

Stylolitization as Source of Cement in Mississippian Salem Limestone, West-Central Indiana (1)

The Mississippian Salem Limestone of west-central Indiana is a homogeneous cross-bedded grainstone containing numerous stylolites with amplitudes ranging up to 25 cm. Petrographic and geochemical analyses of closely spaced samples from four 1-m thick stylolite-bounded units document spatial trends in grainstone texture and composition, which correlate with proximity to bounding solution seams. Textural data indicate that stylolitization was locally preceded by grain compaction and that seam solution preferentially occurred within layers where grain packing was tightest. Amount of cement largely corresponds to volume of available pore space, and remaining porosity varies inversely to stylolite proximity. Trace-element compositions demonstrate that intergranular spar is enriched in Mn and depleted in Mg relative to grains, and suggest a significant contribution of carbonate cement to grainstone pores from bounding solution seams. Data on grainstone and stylolite insoluble contents indicate that stylolite amplitude records 43% of actual section shortening. On average, seam solution within the Salem Limestone could have provided no less than 47% and no more than 90% of the CaCO[3], Fe, and Mn mass now in grainstone pores as intergranular spar cement. As such, stylolitization has played an important role during burial diagenesis, porosity occlusion, and permeability reduction within this Mississippian grainstone sequence.

Influence of Early Cementation on Dolomitization and Compaction in Cambro-Ordovician Carbonates of Central Appalachians: ABSTRACT

The Cambro-Ordovician platform carbonates of the Great Valley, central Appalachians, are notable for the interlayering of limestones and dolostones with well-preserved primary sedimentary structures. Despite such primary structures there are several observations that demonstrate a secondary origin for much of the dolomite, the most significant being that dolostone-limestone boundaries commonly crosscut bedding planes. The nature of such dolomitization is clearly revealed by certain mottled dolostones in which mudcracks and burrows are haloed by limestone. Stratification can be traced through the 2 to 10-mm wide limestone halos into the dolostone without break. The limestone halos have a peloidal grainstone texture with high minus-cement porosity and unbroken shells wherea the dolostone is a sucrosic mudstone with fragmented shells. We interpret these observations to mean that early cementation around permeable mudcracks and burrows has protected these zones from later compaction and dolomitization. We extend this idea of selective early cementation of permeable sediments to explain why the limestones are invariably grainstones while the dolostones are mudrocks. The idea is supported by the observation that laminae of dolostones are bent around limestone mudcrack halos, ripple lenses, etc, showing that the limestones have acted competently and the dolostones incompetently during compaction. The timing of this post-early cementation dolomitization is not unequivocal, but it is certainly pre-stylolitization, as demonstrated by dissolution of dolomite rhombs along bedding-plane stylolites. Our observations do not rule out the possibility of a syndepositional origin for at least some dolomite in these rocks (there are dolomites and dolomites!) but they do negate the pressure-solution origin of dolomite advanced by Wanless (1979). End_of_Article - Last_Page 577------------

Smackover Formation of Gulf Coast Region: ABSTRACT

The Smackover Formation in the United States Gulf Coast region has been the target of continuing, and indeed renewed, exploratory interest of the oil industry. It is important that geologists understand the depositional and diagenetic realms that affected the Smackover, because of the influence they have on reservoir development. The Smackover was deposited during a major transgressive-regressive cycle. High-energy fossiliferous, pelletal, and oolitic grainstones deposited during the regressive End_Page 534------------------------------ phase form carbonate reservoirs that are widespread in Texas, Arkansas, Louisiana, Mississippi, Alabama, and Florida. Although much of the upper Smackover displays packstone and grainstone textures, there are variations in the type of allochems, percentage of micrite, and clastic sand content. For example, there are oolitic grainstones in the upper Smackover of east Texas, Arkansas, Louisiana, and southeastern Mississippi; micritic, pellet packstones are most common in parts of Alabama and the Florida panhandle; and deltaic, beach, and offshore-bar clastic sands are intermixed with carbonate beds in much of Mississippi. Of greater significance to the exploration geologist are variations in the amount and type of porosity within similar Smackover lithologies. These variations result from differences in diagenetic regimes. In Arkansas, Louisiana, and eastern Mississippi, the best reservoirs display incompletely cemented primary porosity. The preservation of porosity may be the result of early freshwater cementation, which occurred in areas that were uplifted and exposed during salt movement. Most of the updip Smackover carbonate grainstones contain leached moldic porosity, developed as extensive freshwater flushing occurred near the regional shoreline. Combinations of dolomitized intercrystalline and leached moldic porosity are prevalent in east Texas and Alabama-Florida. Dolomite porosity is less depe dent on early salt tectonics, although salt structures may still affect the shape of the productive reservoir. End_of_Article - Last_Page 535------------