Sandstone Lithofacies Research Articles

Palynofacies as a palaeoenvironment and hydrocarbon source potential assessment tool: An example from the Cretaceous of north Western Desert, Egypt

Optical examination employing transmitted light and UV-fluorescence microscopy of palynological preparations of eighteen cutting samples representing the Alam El Bueib Member (Hautervian-Barremian), Kharita/lower Bahariya (Cenomanian), and Abu Roash (Turonian-Santonian) formations collected from the Faghur Hj5-1 well, north Western Desert, Egypt, allows the identification of three different palynological assemblages from the studied rock units. These assemblages are mainly non-marine but apparently marine at the base of the Alam El Bueib Member, as evidenced by dinocyst occurrence. In addition, the presence of the Pediastrum and chlorophycean algae ecozone, recognised in previous works, is a good datum for the Abu Roash Formation in the north Western Desert of Egypt. Three associations of palynofacies linked to lithofacies changes are recognised and employed in identification of depositional environments. The Alam El Bueib samples yielded mixed kerogen assemblages of non-marine and marine organic facies. The Kharita/lower Bahariya interval is mostly barren, possibly due to prevailing sandstone lithofacies, except for one sample at its upper part which contains a diverse palynological assemblage. The overlying Abu Roash Formation has a homogeneous kerogen composition comprising mainly granular fluorescent AOM and algae as well as rare palynomorphs. Qualitative as well as quantitative variations of palynofacies allow the reconstruction of the depositional environment. The obtained data have the potential for discriminating spatial and redox status differences and providing also information about terrestrial/freshwater influxes. Results support the model that the Alam El Bueib Member was deposited in a marginal dysoxic-anoxic to distal suboxic-anoxic basin. The Kharita/lower Bahariya unit in the studied well was deposited under marginal dysoxic-anoxic conditions whereas the overlying Abu Roash Formation in a distal suboxic-anoxic basin. Palynofacies results also show that the studied material comprises two distinct facies of kerogen. First, Type II > I kerogen (AOM-rich) is overwhelmingly dominant in the Abu Roash Formation and a few samples from the Alam El Bueib Member which are presumed highly oil-prone, whereas Type III kerogen (phytoclast-rich) is particularly common in the Alam El Bueib Member and Kharita/lower Bahariya unit which are considered gas-prone. Thermal maturity determinations obtained from colour changes of smooth-walled palynomorphs reveal that Alam El Bueib samples belong to immature to mature stages; however, Kharita/lower Bahariya and Abu Roash samples are within the immature phase.

Reservoir characterization and production history matching of Lower Cretaceous, Muddy Formation in Ranch Creek area, Bell Creek oil field, Southeastern Montana, USA

A detailed reservoir description of the Lower Cretaceous, Muddy Formation is an essential step for developing an enhanced oil recovery project in the Bell Creek field. The applied workflow highlighted the role of key uncertainties that were not well addressed during early field development. Several challenges including: stratigraphic subdivision of the reservoir units, rock type complexity due to diagenesis, petrophysical data inconsistencies and fluid contacts have historically led to hurdles in reservoir development. The study outlines a workflow used to refine some of these uncertainties and generate better inputs for the geologic model. The Ranch Creek area at Bell Creek field in southeastern Montana is characterized by a clastic sequence of mixed rock types from argillaceous mudstone, bioturbated tightly cemented to clean sandstone lithofacies. Delta front to fluvial-dominated depositional settings as well as diagenesis played an important role in controlling reservoir quality and geometry. The facies complexity is reflected in rock properties controlling fluid migration through the reservoir intervals. The lithofacies types are clustered into three groups, each with characteristic porosity and permeability cutoffs based on core plugs measurements. The rock types are linked to depositional facies obtained from the petrographic description and calibrated to the clusters derived from the capillary pressure measurements. Porosity distribution within the reservoir is biased to the diagenetic complexity and rock type distribution. Three different transforms were used to calculate the corresponding permeability to the modeled porosity parameter. Modeling water saturation for the three different capillary pressure groups using the J-function approach improves the spatial distribution of water saturation and leads to a better match of historical oil and water production rates.Reservoir simulation is crucial to improve our understanding of rock and fluid properties. Integrating dynamic data such as pressure, production and injection rates helps to reduce the uncertainty of the model parameters. The history match process highlights the need to tune the output of the static model. Iterations on the static model, focused on changing the ranges of key parameters such as facies volume fractions and water saturation that contain the most uncertainties. Applying these changes results in an improved geological model that provides a better data to the simulation model. The tuned static model was able to reproduce the observed results. Matching the pressure data supports a valid oil-in-place which is consistence with historical volumetric calculation.

Diagenetic history and reservoir evolution of tight sandstones in the second member of the Upper Triassic Xujiahe Formation, western Sichuan Basin, China

The second member of the Upper Triassic Xujiahe Formation is an important target for tight gas exploration in the western Sichuan Basin. The diagenetic history and reservoir evolution were investigated using integrated analyses of petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), cathodoluminescence (CL), fluid inclusion microthermometry and stable carbon and oxygen isotopes. The sandstones consist mainly of litharenites, sublitharenites and feldsparthic litharenites with fine-to medium-sand grain sizes, moderate to good sorting and subangular to subrounded roundness. The reservoir properties of the sandstones are generally poor with low porosity and matrix permeability, small pore-throat radii, and high displacement pressure. Eodiagenesis is composed mainly of mechanical compaction, precipitation of siderite, stage-I quartz overgrowths, chlorite, calcite and ferroan calcite. Mesodiagenesis consists mainly of mechanical and chemical compaction, calcite and ferroan calcite precipitation, stage-II and stage-III quartz cementation, feldspar dissolution and precipitation of illite, dolomite and ankerite. Three types of sandstone lithofacies are defined according to sandstone textures and composition, diagenetic minerals, grain sizes, sorting and pore types: lithofacies I, II and III. Compaction appears to reduce the reservoir quality more significantly compared to cementation. Authigenic chlorite coats were constructive in preserving primary porosity, while carbonate cementation decreased reservoir quality. Diagenetic evolution pathways and reservoir quality prediction models for the Upper Triassic Xujiahe Formation sandstones have been proposed to shed light on the different reservoir quality evolution patterns of various sandstone lithofacies. Lithofacies I ─ medium and coarse-grained sandstones characterized by extensive chlorite coating, feldspar dissolution and a low content of clay matrix and cement, appears to have the best reservoir properties and may serve as a high-quality reservoir facies in these tight sandstone reservoirs.

Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have good to very good reservoir potential. Despite the excellent reservoir quality of the continental sandstone lithofacies, the effective stratigraphic seal is leaking. The interbedded calcareous and carbonaceous claystone lithofacies may provide an excellent stratigraphic seal for these siliciclastic reservoirs. Based on these analyses, a model is proposed that can be used as a template for subsurface Jurassic reservoir characterization and reservoir discrimination.

Integrated sedimentological and petrophysical rock-typing of the Messinian Abu Madi formation in South Batra gas field, onshore Nile Delta, Egypt

The Messinian incised-valley fill (IVF) sediments of the Abu Madi Formation host the most prolific gas-bearing sandstones in the onshore Nile Delta, Egypt. This study provides the first integrated geological, geophysical and petrophysical approach in order to link the Abu Madi sandstones’ reservoir properties with their geological framework, and thereby optimize the predictability of the most prospective reservoir rock types (RRT). Sandstone-bearing reservoir levels are mostly associated with the fluvial-estuarine valley-fill sediments (Abu Madi levels II and III). The intergranular volume of some sandstones (e.g. arkoses) is occupied by compacted ductile argillaceous rock fragments (pseudomatrix), and therefore have dominantly microporosity with anisotropic, poorly-connected pore system.Three RRT were recognized in Abu Madi reservoir levels. RRTI has the best poro-perm characteristics with excellent capillary properties (e.g. megapores, lowest initial displacement pressure ~10 psi air/mercury and irreducible water saturation Swir below 20%). Pore system heterogeneity increases in RRTII and RRTIII. The latter contains dominant micropores and exhibits the highest initial displacement pressure ~500 psi air/mercury and Swir exceeding 50%. RRTI often comprises subarkoses associated with the cross-bedded, massive and cross-laminated lithofacies of the fluvial channel-fill and fluvial channel sand bars facies. Whilst RRTII and III mainly comprise the sandstone lithofacies deposited in the abandoned fluvial, estuarine and restricted marine successions. Excellent reservoir quality facies follows the depositional trend of the valley-fill fluvial channels and pinches-out where repeated avulsion and channel abandonment processes dominate. The spatial distribution of Abu Madi RRT highlights the paramount impact of syn-depositional attributes and canyon-floor relief on the sediments’ reservoir heterogeneity and productivity. Moreover, the present integrated RRT approach could serve as a model for exploration in analogues settings where autogenic processes induce widespread sedimentary heterogeneity, which largely impacts the reservoir quality.

Characterization of favorable lithofacies in tight sandstone reservoirs and its significance for gas exploration and exploitation: A case study of the 2nd Member of Triassic Xujiahe Formation in the Xinchang area, Sichuan Basin

Abstract By using core, logging curves, and experiment data, favorable lithofacies types in the 2nd Member of Triassic Xujiahe Formation in the Xinchang area, Sichuan Basin were classified, standard of the favorable lithofacies was established, planar distribution regularities of the favorable lithofacies were identified, and forming mechanisms of the favorable lithofacies and their control effect on production were examined. (1) The 2nd Member of Xujiahe Formation has twelve types of lithofacies, among which multiple layer medium-coarse grain sandstone lithofacies, parallel bedding medium-coarse grain sandstone lithofacies, massive bedding medium-coarse grain sandstone lithofacies, inclined bedding medium-coarse grain sandstone lithofacies, and charcoal-bearing medium-coarse grain sandstone lithofacies with better physical properties and higher gas content are favorable lithofacies; they feature low gamma, low neutron porosity, low resistivity, and high acoustic travel time on logging curves. (2) The sedimentary process controls spatial distribution of sand bodies which are the material basis of the favorable lithofacies; post diagenetic fluids would differentially reconstruct the favorable lithofacies; tectonic activities and abnormal formation pressure made strata slide along the weakness plane, giving rise to fractures in different types of rocks, which can enhance the reservoir permeability significantly. (3) The development degree of favorable lithofacies is a major factor affecting stable production of gas well.

Sedimentology of mixed siliciclastic-carbonate system in a structurally constrained Paleogene basin: Eocene Musawa Formation in Eastern Oman Mountains

The Middle Eocene sedimentary succession in Abat Basin contains a thick mixed siliciclastic and carbonate sequence belonging to the upper member of the Musawa Formation. Over 300 m thick deltaic sediments of the Bartonian to Priabonian upper member of the Musawa Formation show two main facies successions over ten km distance, grading from a proximal delta-plain to a distal delta-front succession. Four major lithofacies associations based on their depositional significance are identified in the basin, namely the conglomerate, sandstone, mudstone, and carbonate. Each of these lithofacies associations is further divided into several individual lithofacies based on their depositional characteristics. The lithofacies association assemblage represents fluvially-dominated deltaic sediments that accumulated as a local siliciclastic feature in a carbonate dominated tropical environment. The conglomerate and sandstone lithofacies associations were deposited by channels and shoreface processes, whereas coastal processes in delta-plain setting deposited mudstone lithofacies association (including coal). The carbonate lithofacies association was deposited in an open marine shelf conditions during transgressive events associated with highstands of sea level and delta-lobe switching. The occurrence of thick siliciclastic sediments is interpreted as the result of activation of the North Ja'alan Fault resulting in a transtensional basin during the Middle to Late Eocene.

Stratigraphic Analysis of Gercus Formation in Dohuk area, Northern Iraq

A surface section of the Gercus Formation (Middle-Late Eocene) was studied in Berafat area, Dohuk Governorate, Northern Iraq. The Gercus Formation consists of a mixed siliciclastic sediments, evaporates and carbonate sequences in the studied region, predominantly in the upper and middle parts. Nevertheless, it usually consists of upward-fining carbonate-rich sandstone cyclothems, marl, conglomerate and siltstone along with a gypsum lens and thin micrite carbonate beds. The Gercus Formation was deposited in delta and delta front of occasionally depositional environment which is represented by red-brown claystone and reddish-brown mudstone lithofacies. Cross bedded pebbly sandstone, trough cross-bedded sandstone and laminated cross-bedded sandstone lithofacies are deposited in braided delta environment. Marl lithofacies and gypsum lithofacies are deposited in intertidal and supratidal environments. The sea level fluctuation caused the river base level to occasionally rise and fall. In addition, the process changed from erosion to deposition, while the grain size also changed at different environments from gravel to sand and clays. The Gercus succession of northern Iraq was developed during the Middle-Late Eocene in an active margin basin, where the last stage closure of the New-Tethys and its collision with the Eurasian plate took place between the northeastern Arabian plate. It caused major episodes of uplifts and subsidence along with base level variations due to eustatic ups and downs. Within Gercus Formation, several fourth order cycles can be recognized, reflecting generally asymmetrical cycles, as well as the difference between sediment supply and accommodation space.

Application of machine learning tool to predict the porosity of clastic depositional system, Indus Basin, Pakistan

Porosity is one of the key factors of a reservoir system that is typically distributed in a spatially non-uniform and non-linear manner. Nevertheless, spatial distribution of porosity in stratigraphic traps (channel sand bodies) is challenging due to the variations in depositional environments, shale intercalation, and repetition of textural changes in the pore sizes. This paper presents a strategy of joint inversion that combines support vector machine (SVM) and particle swarm optimization (PSO) algorithms to predict the spatial distribution of porosity using well logs and seismic data. In addition, Gaussian simulation algorithms (sequential Gaussian and Gaussian indicator simulations) were used to predict lithology and porosity distribution using wireline logs and core data. Rock physics modeling was performed to bridge the gap between the elastic and petrophysical properties for seismic-inversion based reservoir characterization. The results show that the joint inversion strategy leads to the most stable prediction of AI and porosity distribution in the lower Goru heterogeneous reservoir of Sawan Gas Field, Pakistan. The calibration of the individual spatial distribution of lithology and porosity from wireline logs data using Gaussian simulation algorithms and post-stack seismic inversion using SVM and PSO; revealed favorable matching. In fact, the spatial pattern of low AI corresponds with the high porosity and sandstone lithofacies. The study results provide valuable insights for identifying the sweet spots in the heterogeneous lower Goru clastic packages of Sawan Gas Field through quantitative analysis of rock physics, petrophysics, seismic inversion, and fuzzy optimization. The proposed joint inversion strategy may be considered useful for the prediction of porosity in similar context of extremely heterogeneous stratigraphic traps in other basins of the world.

Heterolithic Lower Rewa Sandstone of the Neoproterozoic Rewa Group, Vindhyan Basin, U. P., India: An example of tidal point bar

The Lower Rewa Sandstone of the Rewa Group exposed in Drummondganj ghat area is about 15 m thick; it shows well developed features of tidal and wave origin. The sandstone succession characterized by Inclined Heterolithic Strata (IHS) is composed of three lithofacies associations namely, i) Sandstone lithofacies association (A), ii) Siltstone lithofacies association (B) and iii) Shale lithofacies association (C). These three lithofacies associations are repetitive and signify vertically stacked parasequences, which are further divisible into smaller hemicycles showing fining upward in terms of grains size and lithology. The studied section is characterized by heterolithic facies, lateral accretion elements (LA surfaces) with tide generated features such as cross-beddings with mudstone drapes, lenticular and wavy bedding, often of wave origin, suggesting a tidal point bar of a sinuous estuarine channel formed along coast line having momentous tidal action, superimposed by significant wave action. The upper part of the section, characterized by erosionally based cross-bedded sand-bodies with occasional mudstone drapes and current modified wave ripples, indicates translation of river-dominated inner part of an estuarine channel over the mixed-energy zone of the estuary in the sense of Dalrymple et al. (1992). The complete Lower Rewa Sandstone represents a 3rd order Highstand Systems Tract (HST). Palaeocurrent pattern is polymodal suggesting subtidal to intertidal sedimentation whereby sediment was moved mainly by tidal action with intermittent wave activity. Petrographic study reveals that the fine to very-fine grained sandstone is quartzarenite, which has been derived from a stable cratonic uplifted basement.

Facies, petrography, reservoir heterogeneity and quality of the late Carboniferous-Permian Juwayl Member, Wajid Sandstone, SW Saudi Arabia

The Paleozoic Wajid Sandstone in southwestern Saudi Arabia comprises four members known as Dibsiyah, Sanamah, Khusayyayn and Juwayl. The study characterizes the sedimentary facies of the Juwayl Member from five outcrop localities in Wadi Ad-Dawasir area. The objective is to reveal the influence of facies heterogeneity on the reservoir distribution and quality. The field investigations involve facies description, sampling and architectural elements identification. The second step involves the interpretation of facies associations to predict the depositional environments and controls. The petrographic analysis, of thin section, SEM and XRD, helped to define the diagenetic controls that affected the quality and heterogeneity of rock properties such as porosity and permeability. The facies analysis revealed ten lithofacies corresponding to glacio-fluvial and braided-fluvial depositional environments. The glacio-fluvial deposits consist of pebbly to conglomeratic sandstone lithofacies that suggest high to medium energy conditions within a proximal fluvial setting. The braided-fluvial deposits are of medium to coarse-grained, trough to planner cross-bedded sandstone lithofacies. These lithofacies indicate a lower energy distal condition of laterally switching channels. The remaining lithofacies include overbank deposits of fine-grained argillaceous-ferruginous sandstone lithofacies, observed at the top of fining upward sequences. The study classified Juwayl sandstone as mineralogically mature quartz arenite with good to very good porosity and permeability. It dominates by (90%) quartz, and other components of calcite, clay and iron oxides with subordinate K-feldspars. The diagenetic controls including cementation, grain-size reduction, quartz overgrowth and clay minerals are affecting the Juwayl sandstone porosity and permeability. The depositional environments of Juwayl sandstone indicate a system switching from high energy glacio-fluvial to lower energy braided-fluvial setting. The thickness and width of the sandstone bodies tend to vary in this model implying vertical and lateral sandstone heterogeneity. Clay, hematite and calcite cements contributed to porosity reduction, while grain dissolution, clay coats and rims contributed to porosity enhancement. Macro-scale outcrop heterogeneity in terms of facies, paleonvironments and stratigraphic architecture and microscopic scale in terms of texture, composition and diagenesis, all might provide guides for subsurface reservoir assessment, comparison and correlations.

Lithofacies, Palynostratigraphy and Paleoecology of the Outcropping Rock Succession at Ogbunike Old Toll Gate, Niger Delta Basin, Nigeria

Aim: Sedimentary succession exposed at Ogbunike old toll gate is part of the outcropping sediments of the Niger Delta Basin and its age is controversial. The outcrop was studied for the purpose of age determination, lithostratigraphic placement and interpretation of paleoecology, paleoclimatology and depositional environment.
 Methodology: Lithofacies and biofacies analyses were integrated in the study.
 Results: Thirteen lithofacies identified include; bioturbated sandstone, ripple laminated sandstone, ripple laminated claystone, dark shale, ferruginized sandstone, carbonaceous sandstone, greyish shale, very fine sandstone, mudstone, massive claystone, coarse sandstone, cross bedded sandstone and flaser bedded sandstone lithofacies. Four lithofacies associations consisting of lower shoreface to inner neritic, fluvial channel, lagoonal/mixed flat and subtidal sandwave associations were delineated. Middle Eocene age is assigned to the succession based on the high abundance of marker pollen such as Margocolporites foveolatus, Ctenelophonidites costatus, Monocolpites marginatus, Retibrevitricolporites triangulatus, Proxapertites cursus, Bombacacidites sp. and common occurrences of Scrabratisporites simpliformis, Anacolosidites luteoides, Psilatricolporites crassus, Gabonisporis viaourouxii, Striatricolporites catatumbus and Retistephanocolporites williamsi. These co-occur with Cordosphaeridium cantharellus. Palynofloral group recovered are dominated by mangrove and palm pollen. Pteridophyte spores are also abundant whereas the hinterland pollen group is very low. Benthic foraminiferal assemblages of Textularia, Miliammina,Ammobaculites Haplophragmoides, Fursenkoina, Heterolepa, Reophax, Nodosaria, Florilus, Uvigerina, Cibicides and Bolivina recovered from the dark shale suggest deposition in an inner neritic setting. Trace fossil suite of Skolithos- Cruziana ichnofacies is an attribute of the sedimentary units.
 Conclusion: The sedimentary succession is part of the Ameki Group (Nanka Formation) deposited under varied environmental setting. Paleoclimate is tropical.

Sedimentology of Shahejie Formation, Bohai Bay Basin: a case study of Es1 member in Nanpu Sag

The Es1 sandstone is a fruitful hydrocarbon zone of Eocene Shahejie Formation. Therefore, the present study comprises the subsurface facies analysis established on the description of seven wells. Methods include core observation, thin section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and X-ray diffraction to evaluate the microfacies, lithofacies, and rock characteristics. Two microfacies of lithofacies have been established as sandstone microfacies and mudstone microfacies on the basis of grain size. Among them, sandstone is further divided into four sub-microfacies on the basis of microscopic studies, rock fragments and mineral composition, which are arkose sandstone, lithic arkose, feldspathic litharenite, and litharenite. These microfacies consisting quartz, feldspar as abundant detrital grains followed by rock fragments and micas, whereas calcite, ferro-calcite, quartz overgrowth, and clay are the main cement. The Es1 sandstone consists of good primary porosity and permeability as well as good secondary porosity caused by fracturing and dissolution of unstable minerals that enhance the reservoir quality. Whereas cementation and compaction reduce the reservoir characteristics. Studied formation divided into five lithofacies including conglomerate sandstone lithofacies, medium to coarse-grain sandstone lithofacies, fine sandstone lithofacies, siltstone lithofacies, and mudstone lithofacies. Core observation, sedimentary structures, facies sequences of the well log, and sedimentary composition reveal that Es1 deposited into the lacustrine delta fan depositional environment. Rock grain size indicates that Es1 contains mixtures of grains including pebbles, sand-size as well as minor silt and clay size. Grain size varies from conglomerate to clay size. The Es1 sandstone was deposited in braided river (conglomerates sandstone), channel bar, lacustrine, fluvial channel (massive bedding, cross-bedding, and graded bedding sandstone), deltaic, and flood-plain (siltstone and mudstone) sedimentary environment. It is concluded that higher transport energies in channels deposited coarse-grain sediments in shallowest part of the lake setting. However, the energy decreases towards greater depth which deposited mudstone in central part of the lake.

Microfacies and Palaeoenvironment of Avanah Formation (Middle Eocene) Geli Bessri in Dohuk City / North of Iraq

This research is based on (23) Samples selected from the Avanah Formation (Middle Eocene) in the Geli Bessri section in Dohuk to recognize the microfacies and depositional environments. The Avanah Formation consists of rocks of marly limestone and limestone containing Alveolina and thin layers of sandstone lithofacies found in the lower part of the Formation. The depositional environment was determined depending on skeletal and non- skeletal grains. The facies of the Avanah Formation were divided into four main microfacies and four submicrofacies based on skeletal and non- skeletal grains: The mudstone and wackestone, which is divided into benthic foraminifera wackestone and Rotaliids wackestone, the Formation also contains the packstone which is divided into the Rotaliid - Miliolid Lime packstone submicrofacies and benthic foraminifera lim packstone submicrofacies, in addition to the lime grainstone and also contains the Sandstone lithofacies located at the bottom of the Formation. The Avanah Formation in the middle and upper part is subjected to diagenesis process early dolomization and it is observed this through floating dolomite rohmb. According to the results of the facies analysis and the presence of foraminifera fossils, the Formation was deposited in open marine platform to restricted platform interior.

Interpreting the Subsurface Lithofacies at High Lithological Resolution by Integrating Information From Well‐Log Data and Rock‐Core Digital Images

AbstractSpectral facies interpretation and classification methods have been proposed to improve the sophistication of interpretation of the subsurface heterogeneity. In the spectral facies interpretations, the intensity values of the RGB spectrum and the local entropy from rock‐core digital images are used, and the results are compared to conventional electrofacies and expert petrophysical interpretations. During the classification, a practically applicable model that identifies the more detailed types of lithofacies is constructed by using a multilayer neural network model, with the interpreted spectral facies and well‐log data from the corresponding depths used as response and explanatory variables, respectively. Core digital images and five types of well‐log data from the Satyr 5 well in Western Australia are applied for the actual implementation. Through comparative interpretations, three spectral facies are identified as separable lithofacies (i.e., shale, shaly‐sandstone, and sandstone lithofacies), which is supported by detailed HyLogger mineralogy along the tested cores. On the other hand, two electrofacies (i.e., shale‐dominant and sand‐dominant facies) are identified by a conventional method. In the classification based on the spectral facies, the trained multilayer neural network model showed high prediction accuracy for all the lithofacies. Based on these observations, it is confirmed that more precise lithofacies interpretation and classification can be conducted with the developed methods. The developed methods have the potential to improve subsurface characterization when high lithological resolution is essential.

Depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China

A succession of tight-oil mixed siliciclastic-volcaniclastic sandstones (MSVS) deposited in the Lower Cretaceous in the Erlian Basin has increasingly attracted attention for its significant discovery of oil and gas. The mixed compositions have a significant influence on diagenesis and reservoir properties. Understanding the relationship between lithofacies and diagenesis is of great value to providing an understanding of hydrocarbon enrichment potential in MSVS. The current investigation utilizes a multi-method approach including core analysis, petrography, SEM, microthermometry, and stable carbon- and oxygen-isotope geochemistry to better understand the mixed sandstones' lithofacies and controls on diagenesis. Three main sandstone lithofacies are composed of non-tuffaceous sandstone deposited in fan delta front (FFD-NTss), non-tuffaceous sandstone deposited in pro-fan delta (PFD-NTss), and tuffaceous sandstone deposited in pro-fan delta and lake (PFD-Tss). The results suggest that the early diagenetic stage of the MSVS includes the formation of chlorite and subhedral micritic-crystalline dolomite-1 cement (DI). The late stage includes the precipitation of quartz and calcite and subhedral to euhedral microcrystalline dolomite-2 cements (DII). FFD-NTss suffered from strong compaction due to abundant silt-sized and clay matrix whereas PFD-NTss was influenced by abundant calcite cementation due to weak compaction and more rigid debris. PFD-Tss contains abundant volcanic materials, alteration of which not only provided abundant ions but enhanced methanogenesis during the shallow burial, therefore, leading to formation of early diagenetic DI, analcime, and chlorite. Clay minerals are predominant in FFD-NTss whereas the late diagenetic calcite and dolomite cements are common in PFD-NTss that suffered from weak compaction. With the alteration of volcanic material and clay mineral transformations, the late diagenetic carbonate and clay minerals are widely distributed in the PFD-Tss.

The sedimentology and paleoclimatology of Early Triassic regional marine oxic event (Beduh Formation), Kurdistan region – Northern Iraq

Sedimentological study of Beduh Formation (Lower Triassic) is carried out through three outcrop sections at the Northern Thrust Zone – northern Iraq. Lithologically, the formation consists of alternation of dominantly purple and reddish brown shale with subordinate greenish grey shale and wisps of sandstone and/or siltstone.Beduh Formation comprises two lithofacies, dominantly shale lithofacies and wisps of sandstone lithofacies. These facies are divided into five subfacies on the basis of colour, composition, texture and sedimentary structures. Deposition took place in oxic offshore and deep shelf environment and lapping on the shoreface zone (shale-dominated ramp.X-ray diffractometry reveals that the shales are composed mainly of clay minerals such as illite, in a major proportion, kaolinite, chlorite and interstratified clays in minor proportions. In addition to clay minerals, the non-clay minerals are quartz and calcite with minor proportions of feldspar and hematite. Glass shards, foraminifera and conodonts are extracted by washing 26 samples. Petrographically the sandstone and siltstone beds are composed essentially of quartz with little proportions of rock fragments, feldspars and mica, in addition to some fossil remains (posidonia and microconchids).The clays originated from terrigenous influx from land and explosive volcanoes. The predominance of reddish colour and marine fossil content infer deposition in oxic condition during halmyrolysis with minor alternations with anoxic conditions.

The Upper Permian volcanic-sedimentary succession in northern Qamdo Block, central Qinghai-Tibet Plateau and its sedimentary, paleogeographic and tectonic significance

A combined sedimentological and geochemical study has been carried out on a volcanic-sedimentary succession from a borehole newly drilled in the northern Qamdo Block (QMB, in central Qinghai-Tibet Plateau) in order to improve the knowledge of the sedimentary and tectonic environment. The lithologic data allow to separate four main lithofacies, including massive volcanic lithofacies (LF1), tuffaceous lithofacies (LF2), sandstone lithofacies (LF3) and mudstone lithofacies (LF4). LF1 likely reflects the products of felsic lava flows that were formed during the process of magma ascent onto the seafloor at an earlier stage. LF2 contains two sublifhofacies that respectively represents the proximal and distal accumulation of tuffaceous turbidity flows. LF3 could be the arc-sourced submarine fan deposits, and the sublithofacies LF3a and LF3b represent mid-fan distributaries and interchannel deposits in fan lobes. LF4 is mainly composed of suspension deposits, which represents the sedimentation of post-volcanic activities. Furthermore, the lithofacy associations reveal the evolution of a volcano-sourced submarine fan in the back-arc basin. Geochemically, LF1 and LF2 are characterized by LREE-enriched patterns with negative Eu anomalies, enrichment in large ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSEs), which correspond with the previously published island arc environment. However, LF2 shows more distinctive patterns, which represents a later evolution stage than that of LF1. This volcanic-sedimentary succession reveals that the sedimentary-tectonic setting could be consistent with the subduction of Jinshajiang oceanic slab in the late Permian period.

Carbon Storage and Enhanced Oil Recovery in Pennsylvanian Morrow Formation Clastic Reservoirs: Controls on Oil–Brine and Oil–CO2 Relative Permeability from Diagenetic Heterogeneity and Evolving Wettability

The efficiency of carbon utilization and storage within the Pennsylvanian Morrow B sandstone, Farnsworth Unit, Texas, is dependent on three-phase oil, brine, and CO2 flow behavior, as well as spatial distributions of reservoir properties and wettability. We show that end member two-phase flow properties, with binary pairs of oil–brine and oil–CO2, are directly dependent on heterogeneity derived from diagenetic processes, and evolve progressively with exposure to CO2 and changing wettability. Morrow B sandstone lithofacies exhibit a range of diagenetic processes, which produce variations in pore types and structures, quantified at the core plug scale using X-ray micro computed tomography imaging and optical petrography. Permeability and porosity relationships in the reservoir permit the classification of sedimentologic and diagenetic heterogeneity into five distinct hydraulic flow units, with characteristic pore types including: macroporosity with little to no clay filling intergranular pores; microporous authigenic clay-dominated regions in which intergranular porosity is filled with clay; and carbonate–cement dominated regions with little intergranular porosity. Steady-state oil–brine and oil–CO2 co-injection experiments using reservoir-extracted oil and brine show that differences in relative permeability persist between flow unit core plugs with near-constant porosity, attributable to contrasts in and the spatial arrangement of diagenetic pore types. Core plugs “aged” by exposure to reservoir oil over time exhibit wettability closer to suspected in situ reservoir conditions, compared to “cleaned” core plugs. Together with contact angle measurements, these results suggest that reservoir wettability is transient and modified quickly by oil recovery and carbon storage operations. Reservoir simulation results for enhanced oil recovery, using a five-spot pattern and water-alternating-with-gas injection history at Farnsworth, compare models for cumulative oil and water production using both a single relative permeability determined from history matching, and flow unit-dependent relative permeability determined from experiments herein. Both match cumulative oil production of the field to a satisfactory degree but underestimate historical cumulative water production. Differences in modeled versus observed water production are interpreted in terms of evolving wettability, which we argue is due to the increasing presence of fast paths (flow pathways with connected higher permeability) as the reservoir becomes increasingly water-wet. The control of such fast-paths is thus critical for efficient carbon storage and sweep efficiency for CO2-enhanced oil recovery in heterogeneous reservoirs.

Sedimentology, geochemistry and reservoir potential of the organic-rich Qusaiba Shale, Tabuk Basin, NW Saudi Arabia

Abstract The Qusaiba Shale Formation of the Qalibah Group is the most prolific source rock for the Paleozoic petroleum system in Saudi Arabia and is recently considered as a potential unconventional shale gas reservoir. Detailed sedimentological, petrographical, and geochemical analyses were undertaken on an ~31 m thick outcrop of this organic-rich shale exposed in the northwest of Tayma City, NW of Saudi Arabia to provide a better understanding of the vertical and lateral lithofacies variations and the effects of such variations on reservoir properties. X-ray diffraction analysis reveals that clay minerals (average: 50%) and quartz (average: 28%) dominate the bulk mineralogical composition while K-feldspar, plagioclase, and pyrite occur as common to minor components in the analyzed samples. An integration of the sedimentological description, petrography and mineralogical composition allows the subdivision of the investigated outcrop section into six lithofacies. These are: Laminated organic-rich shale, Graptolitic concretion, Massive organic-rich shale, Organic-lean shale, Siltstone-shale interbedded, and Cross-stratified siltstone/sandstone lithofacies. TOC content in the laminated and massive organic-rich shale lithofacies ranges between 0.5 and 6.1 wt% (average 2.52 wt%) then it decreases upward (0.04–0.35 wt%, average 0.18 wt%) in the overlying organic-lean shale and siltstone lithofacies. Trace elements (Cu, Mo, As, Ni, Co, U and V) show positive excursions in the lowermost 20 m of the organic-rich, dark-grey shale interval and display abrupt depletions at the contact with overlying organic-lean shale. The enrichment factors of these trace elements are characterized by similar distribution patterns as their measured concentrations. Like the elements, these factors also exhibit a significant depletion at the contact between the organic-rich shale and the overlying organic-lean shale. Out of all the redox indices, V/(V + Cr), V/(V + Ni), Th/U and [Mo]s/TOC were found to best distinguish between the organic-rich and the overlying organic-lean intervals. The laminated organic-rich shale lithofacies indicate deposition of hemipelagic sediments by a relatively low energy quiet anoxic bottom water, most likely in a sulfidic conditions that were associated with a strongly restricted bottom water mass circulation at the time of its deposition. This restricted hydrographic condition was likely enhanced in the study area due to the paleotopography that was created by the deglaciation of the Early Silurian time. The TOC richness, common occurrence of pyrite and high enrichment of redox elements Mo, U and V in the organic-rich shale intervals support the existence of anoxic bottom water at the time of deposition. The abrupt decrease of the TOC and the redox sensitive elements and their associated indices from the underlying organic-rich lithofacies to the overlying organic-lean shale lithofacies is likely due to a gradual increase in oxygen level. This improved bottom water oxygenation condition might have led to a reduction in organic productivity and limited organic matter preservation in the organic-lean shale lithofacies. The graptolitic concretion lithofacies found in the sequence, refers to an in-situ, syngenetic to early diagenetic origin formed close to the sediment-water layer when there was little or no deposition or at periods characterized by low sedimentation rates. The siltstone - fine sandstone lithofacies in the topmost part of the studied section, is interpreted to indicate the dominance of high energy and highly oxygenated shallow bottom water conditions during deposition, probably in a middle to inner shelf depositional settings. The kerogen in QS is a mixture of type III (gas prone) and type IV (inert kerogen), Tmax values with an average of 430 °C, and VRo values (average 0.58) calculated from the Tmax, assigned the studied section as immature. Qualitative evaluation of porosity allows the identification of three different types of porosity including inorganic porosity, organic porosity and natural fracture porosity. Although the ductile clay minerals are the most abundant minerals in the lower part of the studied QS (average 56%), the brittle minerals (Quartz, Feldspar, and Pyrite) form an average of 42% and exhibit a uniform distribution of brittleness in the organic-rich interval.