Mixed Siliciclastic-carbonate Rocks Research Articles

Chloritization of mixed siliciclastic-carbonate rocks: case study from Gydan Peninsula, Russia

Chloritization of mixed siliciclastic-carbonate rocks: case study from Gydan Peninsula, Russia

Pore Structure and Fractal Characteristics of Mixed Siliciclastic-Carbonate Rocks from the Yingxi Area, Southwest Qaidam Basin, China

Summary Evaluating reservoir properties at the pore scale is vital to better estimate hydrocarbon reserves and plan field development. The lacustrine mixed siliciclastic-carbonate deposits of the Upper Paleogene Xiaganchaigou Formation in the west Yingxiongling area form one of the most important hydrocarbon reservoirs in the southwestern Qaidam Basin (China). In this study, we analyzed well samples with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and mercury injection capillary pressure (MICP) data in integration with scanning electron microscopy (SEM) images to decipher the mineral composition and pore structure characteristics of the Xiaganchaigou Formation. We also calculate the fractal dimensions using MICP, NMR T2 spectrum, and SEM images based on fractal theory models. The results indicate that the mixed siliciclastic-carbonate samples of the upper section of the Xiaganchaigou Formation are mainly formed by dolomite and clay minerals with low siliceous and calcite content. Porosity is relatively low (2.01−9.83%) and positively correlated with dolomite content, thus indicating that the dolomite intercrystalline pores formed by infiltration and reflux dolomitization control the reservoir characteristics. The size of dolomite intercrystalline pores varies between several and hundreds of nanometers. The porosity has a poor correlation with permeability, which indicates that the pores are mostly primary, which lack the transformation of late dissolution. Three types of mixed siliciclastic-carbonate reservoirs are identified according to pore size distribution (<20 nm, 20−300 nm and multiple distribution), calculated using the NMR T2 spectrum. Fractal curves calculated by combining the MICP and NMR data are characterized by multisegments. The number of segments depends on the degree of heterogeneity of pore structure: two segment for high heterogeneity and three segment for low heterogeneity, also indicating a multifractal feature in mixed rock reservoirs. There is a negative correlation trend between porosity and fractal dimensions, and larger pores often have larger fractal dimensions. These results show that MICP-based fractal values are higher than those of NMR-based, which result from unconnected pores that the MICP is unable to reach. Fractal dimensions obtained from SEM have a small and narrow distribution range and are negatively correlated with the number of pores with smaller sizes. In essence, this study shows that the fractal dimension can be a concise index to evaluate the heterogeneity of lacustrine mixed siliciclastic-carbonate reservoirs, which can serve as an important reference for hydrocarbon development plans.

Pore Structure and Connectivity of Mixed Siliciclastic-Carbonate Tight Reservoirs in the Palaeogene from Qaidam Basin, NW China

The pore structure and connectivity in petroleum reservoirs are controlled in part by their petrological properties. Mixed siliciclastic-carbonate rocks have complex compositions and heterogeneous spatial distributions of the various minerals. As a result, the study of the pore structure and connectivity of mixed siliciclastic-carbonate tight reservoirs has been limited. In this study, methods such as thin section microscopy, X-ray diffraction, X-ray computed tomography, low pressure N2 adsorption, and spontaneous imbibition were adopted to comprehensively analyze the petrological properties, pore structure, and connectivity of the mixed siliciclastic-carbonate tight reservoirs in the upper member of the Xiaganchaigou Formation in the Yingxi Area, Qaidam Basin. The results showed that micrometer-sized pores in mixed siliciclastic-carbonate tight reservoirs are mainly dissolution pores, and that the spatial distribution of the pores is highly heterogeneous. The average pore radius range, average throat radius range, and average coordination number range of micronmeter-sized pores are 2.09~3.42 μm, 1.32~2.19 μm, and 0.48~1.49, respectively. Restricted by the concentrated distribution of local anhydrite, the connectivity of micronmeter-sized pores develops well only in the anhydrite, showing negligible contribution to the overall reservoir connectivity. In contrast, nanometer-sized pores in the mixed siliciclastic-carbonate tight reservoirs are mainly intercrystalline pores in dolomite. The range of nanometer-sized pores diameters is mainly distributed in 1.73-31.47 nm. The pores have a smooth surface, simple structure, and relatively homogeneous spatial distribution. The dissolution of dolomite intercrystalline pores by acidic fluids increases the connectivity of the nanometer-sized pores. This paper presents genetic models for microscopic pore structures and connectivity of mixed siliciclastic-carbonate rocks, making possible the evaluation on the quality of the mixed siliciclastic-carbonate tight reservoirs.

Upper Cretaceous (Coniacian -Campanian) siliciclastic/carbonate ramp depositional sequences and relative sea-level changes in Sinai, Egypt: Implications from facies analysis

Through the analysis of biostratigraphy, facies types and distribution, lithofacies associations, and bounding surfaces, we derived high-resolution sequence stratigraphy of the mixed siliciclastic–carbonate rocks for four sections in the Coniacian–Campanian succession in Sinai (Egypt). These stratigraphic sections provide a framework for studying the sedimentary-facies evolution, the paleogeography, and the role of eustatic and tectonic processes in Egypt and surrounding countries. Twenty-six foraminiferal species and four foraminiferal zones (the Dicarinella concavata Interval Zone in the upper Coniacian, Dicarinella asymetrica Total Zone in the Santonian, Globotruncanita elevata Partial Range Zone in the lower–middle Campanian, and Globotruncanita ventricosa Partial Range Zone in the middle Campanian) are noted. Twenty-three lithofacies and microfacies types into six facies associations ranging from progradational shoreface to outer ramp are detected and categorized. These facies associations exhibit lateral variation and deepening toward the north from siliciclastic–carbonate (Matulla Formation) to dominant carbonate (Themed Formation). Three sequences separated by four sequence boundaries have been determined in the field and/or from strong vertical facies changes associated with short time gaps representing the Turonian–Coniacian; the lower–middle and upper Coniacian; the upper Coniacian–Santonian; and the Santonian–Campanian stage boundaries. Comparisons of the examined sequence boundaries in Egypt and in some parts of Europe, within the Arabian Plate, and in global charts suggest that eustatic and tectonic effects were simultaneous.

Mixed siliciclastic-carbonate deposition and cyclical evolution of the Upper Shahejie Formation and its impact on reservoir development in the Eocene Huanghekou Sag, Bohai Bay Basin, East China

We have systematically investigated a typical lacustrine mixed siliciclastic-carbonate sedimentary system in this study, which primarily deposited in the Eocene Upper Shahejie Formation (Es1 formation) of the Huanghekou Sag, Bohai Bay Basin. Using synthetic lithologic data sets, we summarized eight lithofacies from mixed siliciclastic-carbonate rocks and classified them into siliciclastic- and carbonate-dominated groups, indicating the variation of relative intensity between fluvial/deltaic process and carbonate productivity. Combining petrological and mineral component analysis, most lithofacies are compositionally replaced by micrite dolomite and show vertically different scales of cyclicity. The high-level cyclicity suggests the alternation between siliciclastic- and carbonate-dominated mixed rocks, whereas the lower level indicates interbedding of different dolomite-dominated lithofacies. The controlling factors on both levels are different. The higher level is probably controlled by climate changes, which finally resulted in the relative variations of siliciclastic supply, although the lower level of cyclicity changes more frequently and is considered to have been controlled by fluctuating hydrodynamic conditions. Typical mixed rock depositing patterns are summarized as the “sandwich type,” which are mainly distributed at siliciclastic-dominated sections and the “interbedded type” at carbonate-dominated sections. Besides, reservoir physical properties among lithofacies display significant differences as well, and the grain-supported framework of carbonate-dominated lithofacies suggests most constructive in keeping intergranular pores. The investigated lacustrine cyclical sedimentation in mixed siliciclastic-carbonate rocks has been less concerned in previous studies and may serve as a useful reference for lacustrine mixed rocks analysis in other rifted basins.

Pore structure characteristics and their influencing factors: A case study from the middle jurassic mixed siliciclastic carbonate rocks, Turpan-Hami basin, Northwest China

The mixed siliciclastic-carbonate rocks of the second member of the Qiketai Formation (J2q2) in the Turpan-Hami Basin are lacustrine tight oil reservoir with vitrinite reflectance values (%, Ro) ranging from 0.64% to 0.81%. X-ray diffraction (XRD), thin sections, field emission scanning electron microscopy (FE-SEM) of argon-milled thin sections, low-temperature N2 adsorption (LNTA), and nuclear magnetic resonance (NMR) analyses were performed to investigate the pore structure characteristics and their influencing factors of the mixed siliciclastic-carbonate reservoirs. The results show that the tight reservoir has a medium-high total organic carbon (TOC) content, and the sedimentary textures include laminated, massive, and intraclasts-rich types. Nanometer scale interparticle pores and microfractures are dominant in the tight reservoirs. The small pores (2–100 nm) make a dominant contribution to the nanoscale storage space according to the comprehensive analysis of the T2 spectra of NMR. Reversed S-shaped isotherms obtained from N2 adsorption are type IV, and hysteresis loops indicate that the shape of pores include slit-or plate-like, ink-bottle-shaped and mixtures of the two. BET surface areas and BJH total pore volume vary from 1.707 m2/g to 8.556 m2/g and 1.13 cm3/100 g to 3.0 cm3/100 g, with an average of 4.31 m2/g and 1.848 cm3/100 g, respectively. The massive and intraclasts-rich rocks have higher NMR porosities, pore sizes and pore volumes than the laminated rocks. Based on the Frenkel-Halsey-Hill model of low-temperature N2 adsorption, the fractal dimensions D1 and D2 are 2.4305–2.7081 and 2.2667–2.6070, respectively. The correlations between the pore structure parameters and the mineralogical composition, TOC content, and fractal dimension reveal that dolomite makes greater contribution to the pore volume and movable fluid saturation than the siliciclastic gains and organic matter, resulting in higher fractal dimensions and more complicated pore structure. In addition, the clay minerals increase pore specific surface area and its complexity in the tight reservoirs. The samples with high silicate mineral and TOC contents tend to contain lower specific surface areas and pore volumes, resulting in lower fractal dimensions and more simplistic pore structure than clay rich samples. Therefore, the complex mineralogical composition, various sedimentary textures and organic matter content all play important roles in the development of pore network.

Late Jurassic (Oxfordian‒Kimmeridgian) brachiopods of the El Bayadh Area (Central Saharan Atlas, Algeria): biostratigraphy, stratigraphic revision, Paleobiogeography and paleoecology

ABSTRACT The Upper Jurassic rocks of the El Bayadh Area (Central Saharan Atlas, Algeria) are composed of mixed siliciclastic-carbonate rocks, which were roughly assigned to a Kimmeridgian-Hauterivian age in previous studies. Based on our studies, the succession can be subdivided into five new formations; from base to top: the Theniet Et-Temar, Ksel, Laguremi, Boualem, and the Sfissifa formations. Brachiopod faunas have been recorded from two levels. The lower one is located in the Theniet Et-Temar Formation, and the upper one in the Sfissifa Formation. Two brachiopod species Lacunosella (Lacunosella) arolica and Moeschia alata from the first level and Xestosina arguta from the second level have been recorded for the first time not only in Algeria but also in Africa. Based on the new findings, the age of the studied section can be restricted to the middle Oxfordian at the base and to the upper Kimmeridgian at the top. The stratigraphic successions between the two levels are nearly barren except for dinosaur footprints, which were tentatively assigned to a Lower Cretaceous age by previous workers. However, this succession can be attributed to an upper Oxfordian-lower Kimmeridgian age now based on its brachiopod faunas.

Lithology identification and reservoir characteristics of the mixed siliciclastic-carbonate rocks of the lower third member of the Shahejie formation in the south of the Laizhouwan Sag, Bohai Bay Basin, China

In the Shahejie Formation in the D oilfield area of the Laizhouwan Sag, Bohai Bay Basin, a mixed sedimentary structure of terrigenous clastic and carbonate rocks has developed. Scholars at home and abroad have performed much research on clastic and carbonate lithology identification. However, studies on lithology identification and reservoir characteristics of mixed siliciclastic-carbonate rocks in the study area have not been conducted thus far. Due to the complex logging response characteristics of mixed sedimentary rocks, it is difficult for conventional lithology identification methods to clearly reflect the lithological characteristics of mixed siliciclastic-carbonate rocks. In this study, the sensitivity of acoustic time difference, natural gamma ray, density, deep lateral resistivity and compensated neutron logging curves to each lithology was analysed using the conventional cross-plot method in the D oilfield. On this basis, a decision tree method is used to further screen and identify the lithology layer by layer, and the process and results of identifying a mixed lithology with logging curves are clearly reported. In addition, imaging logging and core data are used to verify the identified lithology, and a lithology identification template and technical method consisting of conventional logging cross plots, decision trees, imaging logging and core calibration are established to improve the accuracy of lithology identification of mixed rocks. Moreover, the influence of different lithologies on the reservoir characteristics in the study area is analysed.

Determination of NMR T2 Cutoff and CT Scanning for Pore Structure Evaluation in Mixed Siliciclastic–Carbonate Rocks before and after Acidification

Nuclear magnetic resonance (NMR) is used widely to characterize petrophysical properties of siliciclastic and carbonate rocks but rarely to study those of mixed siliciclastic–carbonate rocks. In this study, 13 different core samples and eight acidified core samples selected amongst those 13 from the Paleogene Shahejie Formation in Southern Laizhouwan Sag, Bohai Bay Basin, were tested by scanning electron microscopy (SEM), micro-nano-computed tomography (CT), and NMR. SEM and CT results revealed a complex pore structure diversity, pore distribution, and pore-throat connectivity in mixed reservoirs. Sixteen groups of NMR experiments addressed changes in these properties and permeabilities of mixed siliciclastic–carbonate rocks before and after acidification to determine its effects on such reservoirs. NMR experimental results showed no “diffusion coupling” effect in mixed siliciclastic–carbonate rocks. Distributions of NMR T2 cutoff values (T2C) are closely related to the pore structure and lithologic characteristics before and after acidification. The T2C index separates irreducible and movable fluids in porous rocks and is a key factor in permeability prediction. Centrifugation experiments showed that, before acidification, the T2C of mixed siliciclastic–carbonate rocks with 60–90% siliciclastic content (MSR) ranged widely from 1.5 to 9.8 ms; the T2C of mixed siliciclastic–carbonate rocks with 60–90% carbonate content (MCR) ranged from 1.8 to 5.6 ms. After acidification, the T2C of MSR ranged widely from 2.6 to 11.6 ms, the T2C of MCR ranged from 1.5 to 5.6 ms, and no significant difference was observed between MCR reservoirs. Based on an analysis of the morphology of NMR T2 spectra, we propose a new T2 cutoff value prediction method for mixed siliciclastic–carbonate rocks based on a normal distribution function to predict various T2C values from morphological differences in NMR T2 spectra and to calculate the irreducible water saturation (Swir), i.e., the ratio of irreducible total fluid volume to effective porosity. The reliability of the proposed method is verified by comparing predicted T2C and Swir values with those from NMR experimental results. New experiments and modeling demonstrate the applicability of NMR for the petrophysical characterization of mixed siliciclastic–carbonate rock reservoirs. Our results have potential applications for identification and evaluation of mixed siliciclastic–carbonate rock reservoirs using NMR logging.

Genetic sequence stratigraphy on the basis of ichnology for the Middle Jurassic basin margin succession of Chorar Island (eastern Kachchh Basin, western India)

Abstract Synrift basin margin successions are greatly influenced by eustatic sea level changes, tectonics and accommodation space filled in by sediments. The Middle Jurassic (Bajocian–Callovian) of Chorar Island (western India) comprises a ~109-m-thick synrift basin margin succession of clastic, non-clastic and mixed siliciclastic-carbonate rocks which are here analysed and categorised into nine lithofacies. The succession is bioturbated to varying intensities; 16 identified ichnogenera can be assigned to environmentally related groups of five trace fossil assemblages, which include Gyrochorte, Hillichnus, Rhizocorallium, Skolithos and Thalassinoides. These ichnoassemblages document the Skolithos and Cruziana Ichnofacies which marks a change in energy conditions, sedimentation dispersal patterns and bathymetry in a shallow-marine environment. The Bajocian–Callovian succession is further analysed on the basis of sedimentological and ichnological data that show two genetic sequences consisting of Transgressive Systems Tract and Highstand Systems Tract bounded by Maximum Flooding Surface. The synrift basin margin succession of the Middle Jurassic of Chorar Island shows cyclicity in deposition; the Bajocian–Bathonian succession represents progradational to retrogradational coastlines, while the Callovian succession documents an aggrading progradational coastline.

The Antimonio ramp in Sonora, Mexico

The Upper Permian to Lower Jurassic El Antimonio Group has been considered as part of the NW–SE-trending tectonostratigraphic Antimonio terrane in Sonora, Mexico. The Upper Triassic Rio Asuncion Formation of the El Antimonio Group comprises a shallow-marine detrital-carbonate succession at three major localities near Caborca and Hermosillo. Previous reconstructions have proposed differing depositional environments for the Rio Asuncion Formation (e.g., carbonate platform, carbonate ramp) and remained therefore nonconsensual. The present study has its focus on the Rio Asuncion Formation and includes the analyses of 129 thin-sections of carbonate and mixed siliciclastic–carbonate rocks from three localities (Barra los Tanques, Sierra del Alamo, and Sierra Santa Teresa). In Sierra del Alamo, ammonite findings allowed us to confirm the position of the Triassic–Jurassic boundary. Considering the lack of biostratigraphic markers and the clastic nature of the samples, a statistical approach was used to propose a depositional model that can provide more detailed insights into this sedimentary succession. Hierarchical cluster analyses were performed on the gathered abundance data to identify microfacies and to compare the results for each outcrop. Through these analyses, 22 microfacies were defined, which describe the depositional environments of the two main localities. Furthermore, it could be demonstrated through this approach that not only the localities of Barra los Tanques and Sierra del Alamo but also the outcrops near Sierra Santa Teresa belong to the Antimonio depositional system. To gain insights into the relative depositional conditions among the microfacies, non-metric multidimensional scaling was performed. The resulting trends of water energy and proximity to the shoreline of Laurentia were then used to propose a depositional model for the mixed siliciclastic–carbonate Antimonio ramp system.

Factors controlling the evolution of a wedge-top temperate-type carbonate platform in the Miocene of the northern Apennines (Italy)

An integrated study including stratigraphy, composition and biostratigraphy was performed on temperate-type shallow-water carbonates of early middle Miocene age deposited in a wedge-top basin within the Apenninic chain (Romagna–Tuscan area). The succession rests unconformably on the deep-water sediments of allochthonous Ligurian units and is represented by up to 350m thick carbonates and mixed siliciclastic-carbonate rocks. Two main units are recognized, each characterized by a distinct facies association. The basal unit (upper Burdigalian) consists of rhodalgal rudstones and packstones of shelf environment, dominated by echinoids, bryozoans, coralline algae and benthic foraminifera, deposited in temperate conditions and showing a transgressive pattern. It gradually passes into mixed carbonate-siliciclastic shallow water-facies, (Langhian in age) characterized by an increase in terrigenous components and planktonic and benthic taxa. Sediment starvation during the Langhian is suggested by the occurrence of glaucony-rich packstones with abundant planktonic foraminifera. The succession grades upwards into upper Langhian-lower Serravallian planktonic foraminifera-rich marls. Increasing detrital discharge and water depth up section results in drowning of the carbonate shelf. Debris-flow deposits including material of extraformational origin are intercalated at different levels, in particular from the uplifting south-western margins, close to the Ligurian thrusts. The inception and demise of these temperate rhodalgal-foramol carbonate sediments located in wedge-top Epiligurian basins are primarily controlled by synsedimentary tectonics, which in turn affects relative sea-level rise, and nutrient and detrital input. In particular, tectonics controlled the initial stage of the transgression, determining the configuration of the basin, subdivided in narrow submerged palaeotopographic lows and relatively elevated areas. Synsedimentary tectonics related to the thrust migration was also the main controlling factor in the demise of carbonate sedimentation, through the combined effect of two processes: increasing the subsidence of the basin, and triggering the terrigenous discharge from the erosion of the uplifted Apenninic relieves.

Peritidal to intrashelf basin, facies transition of the Adgham Formation (Late Triassic) Al Qasim Province, Saudi Arabia

The Upper Triassic Adgham Formation outcropping on the western margin of the Arabian Basin comprises shallow mixed siliciclastic-carbonate rocks deposited on the Arabian Plate passive margin during the Neo-Tethys. It presents an example of three coeval environments: peritidal environment in the southeast, shoal environment in the middle and intrashelf basin towards the northeast and northwest. In the peritidal environment, the facies consists of shallowing-upward cycles, each of which commences with claystone, overlain by ferroan dolomitic quartz arenite, skeletal dedolomitic quartz arenite, microbial laminite, and sometimes caliche at the top. These cycles were deposited in intertidal, supratidal and terrestrial with intermittent subaerial exposure by climatic and allocyclic mechanisms. The shoal environment represents the barrier between the peritidal environment in the southeast and the intrashelf basin in the northeast. In the basal part, it consists of shallowing-upward cycles, each starts with thin claystone, capped with burrowed dolomitic quartz arenite, dedolomitic quartz arenite, while the upper part comprises shallowing-upward cycles, each of which begins with claystone, followed by intraclastic oolitic wackestone and dolomitic oolitic packstone. It was deposited on the uplifted block that was synchronous with the deposition of the Adgham Formation. The intrashelf basin interfingers with the shoal facies and deepening northeastwards and is restricted from the open marine in the northeast by reefal facies at the shelf edge from the east. It comprises shallowing-upward subtidal cycles; each consists of a lower claystone that gradationally passes upward into pelletal–peloidal dolomicrites, sandy oncolitic-peloidal dolomicrite, evaporitic dolomicrite and/or spheroidal dolomite. These facies were deposited in restricted lagoonal and intrashelf basin, in which the subtidal cycles were formed by climatic changes and autocyclic mechanism.

Facies and fauna of the Pennsylvanian Buckhorn Asphalt Quarry deposit: a review and new data on an important Palaeozoic fossil Lagerstätte with aragonite preservation

The Pennsylvanian Buckhorn Asphalt Quarry contains the best-preserved Palaeozoic mollusc fauna in the world. Early impregnation of mixed siliciclastic–carbonate rocks (mudstones, pack to grainstones, shell beds, and conglomerates) with hydrocarbons prevented aragonite destruction (“Impregnation Fossil Lagerstatte”). The exceptional preservation comprises shell microstructures, microornaments and early ontogenetic shells. Most gastropods had planktotrophic larval development indicating a high primary production although the remains of phytoplankton are very rare in this and other Late Palaeozoic deposits. Deposition occurred close to a shallow-water coastal area. Mass flow processes (density currents) triggered by storms were involved in the transport mechanisms of some units. Shells of benthic molluscs yield the most diverse known Palaeozoic microboring assemblage, indicating at least partly euphotic conditions. The invertebrate fauna comprises about 160 species and is dominated by molluscs, which is unusual for a Palaeozoic deposit, suggesting that aragonite dissolution produces a major bias in the fossil record. However, most mollusc genera in the Buckhorn deposit are also known from other Pennsylvanian occurrences as recrystallised shells. This shows that preservation bias via preferential aragonite dissolution may be overestimated.

Cenomanian rocks in the Sinai Peninsula, Northeast Egypt: Facies analysis and sequence stratigraphy

Five stratigraphic sections of the Cenomanian rocks exposed in the Sinai Peninsula are described and interpreted on the basis of field observations and facies analysis in order to reconstruct their depositional environments and sequence stratigraphic framework. Based on their lithologic characteristics, the Cenomanian successions consist of mixed siliciclastic–carbonate rocks. The Cenomanian successions are dominated by carbonate production in the northern and eastern parts of the study area. Siliciclastic supply increases southward. Detailed petrographic investigations made it possible to recognize several clastic and carbonate facies types. The facies recognized and their related palaeoenvironments document a lateral transition between inner- and mid ramp settings. The inner-ramp setting occurs in the south and west central Sinai where the peritidal flat, lagoonal, high-energy shoals facies dominate. The mid-ramp setting is assumed to have developed in the north and east central Sinai where intertidal and low-energy subtidal facies interfingers with a few storm-influenced deposits occur. The main factors controlling ramp deposition were eustatic sea-level fluctuations combined with environmental influences such as autochthonous carbonate productivity and siliciclastic supply. In terms of sequence stratigraphy, the Cenomanian successions in Sinai exhibit two superimposed depositional sequences, each of which shows retrogradational (transgressive systems tract) and aggradational to prograditional (highstand systems tract) packages of facies. The retrograditional facies display a predominance of subtidal carbonate facies, whereas the prograditional and aggradational facies show an increase of peritidal carbonates and siliciclastic deposits.

Diagenesis, Depositional Environments, and Hydrocarbon Potential of the Basal Limestone, Green River Formation, Uinta Basin, Utah: ABSTRACT

The basal limestone of the Eocene Green River Formation is well exposed along the southern margin of the Uinta basin. Surface and subsurface stratigraphy evidence a wide spectrum of mixed siliciclastic-carbonate rocks within a well-defined sequence. Although numerous stratigraphic and petrographic investigations of Green River rocks have been completed, none have dealt specifically with the basal limestone facies in detailed stratigraphic context. Rocks exposed along the southern basin margin reflect deposition in relatively shallow-water, mud-flat conditions. Shallowest conditions are marked by dolomitized ostracod and pellet grainstones. Shoreward of the grainstones in typically fresher water, sandy pelecypod-gastropod grainstones are presented and are commonly interbedded with silty mudstones. Deepest offshore conditions resulted in the deposition of dark-gray, kerogen-rich carbonates. Cores, electric logs, and subsurface mapping indicate rapid facies changes to the north (toward the basin center). Kerogen-rich carbonate potential source beds predominate toward the basin center and interfinger southward with lakeshore and lagoonal deposits, which are composed of ostracod packstone and sandy grainstone reservoirs. Interpretation of the detailed petrography of various environments has evolved a paragenetic sequence. Several authigenic clay and carbonate mineral phases were precipitated in the pore systems.