Lower Cretaceous Section Research Articles

Mapping Cretaceous faults using a convolutional neural network – A field example from the Danish North Sea

The mapping of faults provides essential information on many aspects of seismic exploration, characterisation of reservoirs for compartmentalisation and cap-rock integrity. However, manual interpretation of faults from seismic data is time-consuming and challenging due to limited resolution and seismic noise. In this study, we apply a convolutional neural network trained on synthetic seismic data with planar fault shapes to improve fault mapping in the Lower and Upper Cretaceous sections of the Valdemar Field in the Danish North Sea. Our objective is to evaluate the performance of the neural network model on post-stack seismic data from the Valdemar Field. Comparison with variance and ant-tracking attributes and a manual fault interpretation shows that the neural network predicts faults with more details that may improve the overall geological and tectonic understanding of the study area and add information on potential compartmentalisation that was previously overlooked. However, the neural network is sensitive to seismic noise, which can distort the fault predictions. Therefore, the proposed model should be treated as an additional fault interpretation tool. Nonetheless, the method represents a state-of-the-art fault mapping tool that can be useful for hydrocarbon exploration and CO2 storage site evaluations.

Quantitative Evaluation of CO2 Storage Potential in the Offshore Atlantic Lower Cretaceous Strata, Southeastern United States

The geological storage of CO2 in the Earth’s subsurface has the potential to significantly offset greenhouse gas emissions for safe, economical, and acceptable public use. Due to legal advantages and vast resource capacity, offshore CO2 storage provides an attractive alternative to onshore options. Although offshore Lower Cretaceous reservoirs have a vast expected storage capacity, there is a limited quantitative assessment of the offshore storage resource in the southeastern United States. This work is part of the Southeast Offshore Storage Resource Assessment (SOSRA) project, which presents a high-quality potential geological repository for CO2 in the Mid- and South Atlantic Planning Areas. This is the first comprehensive investigation and quantitative assessment of CO2 storage potential for the Lower Cretaceous section of the outer continental shelf that includes the Southeast Georgia Embayment and most of the Blake Plateau. An interpretation of 200,000 km of legacy industrial 2D seismic reflection profiles and geophysical well logs (i.e., TRANSCO 1005-1-1, COST GE-1, and EXXON 564-1) were utilized to create structure and thickness maps for the potential reservoirs and seals. We identified and assessed three target reservoirs isolated by seals based on their effective porosity values. The CO2 storage capacity of these reservoirs was theoretically calculated using the DOE-NETL equation for saline formations. The prospective storage resources are estimated between 450 and 4700 Mt of CO2, with an offshore geological efficiency factor of dolomite between 2% and 3.6% at the formation scale.

Распределение ароматических углеводородов в нижнемеловых и юрских отложениях месторождения Боровое

This research continues a series of studies on the migration of various classes of petroleum hydrocarbons in productive strata (these are Jurassic and Lower Cretaceous deposits) on the Borovaya Area in Western Siberia. In the bitumen obtained from well core samples, the content and molecular mass distribution of hydrocarbons were determined by chromatography-mass spectrometry: alkylbenzenes of the composition C8–C34; biarenes, including naphthalene, methylnaphthalenes, dimethylnaphthalenes; triarenes, including phenanthrene, methylphenanthrenes. The research considered the interlayers movement of arenes from the Upper Jurassic oil accumulation zone to the over- and underlying strata. According to the prospects of Cretaceous deposits of the Borovoye oil field, the study confirms the conclusions of the authors’ early publications. Hydrocarbons come from the Jurassic reservoirs to the Cretaceous very little. The migration of light (up to C20) components is recorded, which are dispersed over the rock layers, which may indirectly indicate the absence of geological conditions for the accumulation of hydrocarbons and the formation of non-structural deposits in the Lower Cretaceous section of the Borovaya Area.

ВОЗРАСТ (U-Pb, LA-ICP-MS) ДЕТРИТОВЫХ ЦИРКОНОВ ИЗ ОСАДОЧНЫХ ПОРОД КОМСОМОЛЬСКОЙ СЕРИИ (CЕВЕРНЫЙ CИХОТЭ-АЛИНЬ)

A geochronological study of detrital zircons from sedimentary rocks of the Gorinskaya (91 zircons with concordant ages), Pionerskaya (117 zircons with concordant ages), and Pivanskaya (115 zircons with concordant ages) formations of the Komsomolskaya Group was carried out. Comparison of the 206Pb/238U isotopic age of the youngest zircon populations from these formations with their stratigraphic age revealed that they are consistent only for one of them (Pivanskaya Formation). For the other two formations, the youngest zircons turned out to be significantly younger than the upper limit of their stratigraphic age. New evidence shows that: 1) the age of the «Berriassian-Valanginian» deposits of the Pionerskaya Formation is not older than the Barremian; and 2) the age of the «Titonian-Berriasian» deposits of the Gorinskaya Formation is not older than the Hauterivian. The division of deposits in the Komsomolskaya Group into formations is based primarily on lithological criteria, and since the composition of the formations is very similar, similar deposits in other Lower Cretaceous sections of the region may have been assigned to other stratigraphic units, so the above conclusion is valid only for the sections we studied.

Вертикальная зональность насыщенных углеводородов в разрезе нижнемеловых и юрских отложений Борового месторождения

The distribution of alkanes in the Lower Cretaceous and Jurassic deposits of the Borovoye oil field, which is part of the East Sabun structural-formation zone of local compression in the eastern part of the West Siberian Plate, is considered. The core material of exploration wells was studied. The bitumoid of rocks contains C8–C40 hydrocarbons, including n-alkanes, isoprenoids, n-alkylbenzenes, naphthalenes, and phenanthrenes, this is determined by chromatography-mass spectrometry, and the molecular mass distribution of n-alkanes and isoprenoids is considered. The lithological and petrographic characteristics of the rocks were studied by optical microscopy. Based on the results of the analysis of hydrocarbons in the Jurassic sediments, a closed oil and gas complex is identified, it’s including: the parent rocks of the Tyumenskaya and possibly Marianovskaya formations, a reservoir in the upper Tyumenskaya and Naunakskaya formations, fluid barriers — the Marianovskaya formation at the top and the Radomskaya pack at the bottom. In the marginal parts of the complex, there is a drop in the concentration and depletion of the composition of hydrocarbons, which is characteristic of filtration. In the Cretaceous sediments, the transit movement of alkanes up to C21 from the Jurassic zone of oil occurrence is recorded. The composition and content of heavier hydrocarbons vary from layer to layer, probably a function of the syngenetic bitumoid of the rocks. In the considered sandstones of Cretaceous deposits, low concentrations of extracted organic substances are consistently recorded, without a significant trend within the section, which may indirectly indicate the absence of geological conditions for the accumulation of hydrocarbons and the formation of non-structural deposits in the Lower Cretaceous section of the Borovoye field.

Upper Jurassic–Lower Cretaceous Source Rocks in the North of Western Siberia: Comprehensive Geochemical Characterization and Reconstruction of Paleo-Sedimentation Conditions

The present work is devoted to geochemical studies of the Bazhenov Formation in the north of the West Siberian Petroleum Basin. The object is the Upper Jurassic–Lower Cretaceous section, characterized by significant variations in total organic carbon content and petroleum generation potential of organic matter at the beginning of the oil window. The manuscript presents the integration of isotopic and geochemical analyses aimed at the evaluation of the genesis of the rocks in the peripheral part of the Bazhenov Sea and reconstruction of paleoenvironments that controlled the accumulation of organic matter in sediments, its composition and diagenetic alterations. According to the obtained data, the sediments were accumulated under marine conditions with a generally moderate and periodically increasing terrigenous influx. The variations in organic matter composition are determined by redox conditions and terrigenous input which correlate with the eustatic sea level changes during transgressive/regressive cycles and activation of currents. Transgression is associated with an intensive accumulation of organic matter under anoxic to euxinic conditions and insignificant influence of terrigenous sources, resulting in the formation of rocks with oil-generating properties. During the regression periods, the terrigenous sedimentation increased along with the dissolved oxygen concentration, and deposits with low organic matter content and gas-generating properties were formed.

Sequence-stratigraphic model of the West Siberia Lower Cretaceous

Apparently, conceptual base of the sequence stratigraphy is one of the most acknowledged methodologies in the geological world at the present time for the sedimentary strata structure prediction. It is based on the complex analysis of the seismic, stratigraphic and sedimentary data on the depositional bodies where the structure and facies filling is regulated by the relative sea level changes. The Lower Cretaceous section of the Western Siberia in this regard is the unique object, as it is represented by the full range of the clastic depositional environments – from relatively deep-water to the continental, which are very sensitive to the conditions changes. Sequence-stratigraphic analysis results can be used to complete the pragmatic tasks in the petroleum geology as the analysis is based on the reconstruction of the sedimentogenesis processes in the past, and the understanding of that processes is the key to the generalized depositional model development. This model can be used to create the models, which can be applied to the unique local objects that occur in the Lower Cretaceous section.

Results of studying the azimuthal anisotropy of the Paleozoic basement with VSP technique at the Bystrovka vibroseismic test site

At present due to depletion of reserves in the majority of Western Siberia oil and gas fields, with reservoirs mainly related to lower Cretaceous and Jurassic section interval, the basement intervals present primary scientific and practical interest for prospecting and exploration. At the Bystrovka vibroseismic test site (Novosibirsk region), the Paleozoic basement occurs at depth of about 10 m. This allowed investigating its elastic properties with shallow seismic technique using VSP. Results of these investigations are presented in the present paper.

Palynomorph biostratigraphy, palynofacies, thermal maturity and paleoenvironmental interpretation of the Bajocian–Aptian succession in the OBA D-8 Well, Matruh Basin, Egypt

A palynological study of 35 cutting samples from the Middle Jurassic–Lower Cretaceous section encountered in the OBA D-8 Well in the north Western Desert of Egypt yielded palynomorphs and particulate organic matter used to interpret the biostratigraphy, palynofacies, paleoenvironment, thermal maturation, and hydrocarbon potential of the Matruh Basin. The stratigraphically significant dinoflagellate cyst and sporomorph taxa recovered, which include Murospora florida, Aequitriradites spinulosus, Pilosisporites trichopapillosus, Callialasporites trilobatus, Tenua anaphrissa, Muderongia simplex, Systematophora sp., Gonyaulacysta jurassica, Korystocysta gochtii, Pareodinia ceratophora, and Dissiliodinium sp., permit an age assignment of the studied section as Bajocian to Aptian. Seven formal dinoflagellate cyst zones and four formal sporomorph zones are proposed for this time interval, in addition to differentiating four palynofacies assemblages and using the Rock-Eval Pyrolysis method to visually and geochemically evaluate the source rock potential. The 0.45–2.82 wt% TOC values of the studied samples indicate fair to good source rocks. An inner neritic depositional environment due to relatively high sea level is deduced for the Khatatba and Masajid formations using palynological proxies, as reflected in the lower percentages of sporomorphs relative to dinoflagellate cysts. During the Lower Cretaceous, a predominance of terrestrial sporomorphs in the Alam-El Bueib Formation suggests the advance of a fluvio-deltaic environment, possibly related to a significant sea level fall and regressive phase.

Applications of miospore quantitative stratigraphy and cyclostratigraphy in the Neocomian - Cenomanian sediments in the Western Desert, Egypt: A step in the way to biochronostratigraphy

The Lower Cretaceous section in the Western Desert witnesses regular petroleum exploration activities and contains active petroleum systems. The main tool for biostratigraphic classification of these important strata is the palynology based biozonation. The classical biozonation methods that rely on the ranges between only selected zonal palynomorphs markers or bioevents cause many problems in the correlation of the stratigraphic units. These problems slowed the development of a unified scheme and constructions of a time scale. In this study, another approach is introduced for the first time to overcome these problems. This approach depends on analytical stratigraphy that is based on a number of quantitative methods in the framework of the geo-mathematics. These quantitative methods include statistical constrained single linkage cluster analysis, graphic correlation, constrained optimization, ranking and scaling and unitary associations. These methods take into account all of the identified taxa bioevents or their co-existences. Each of these methods produces a biozonation that predict the real distribution of the palynomorphs in time and space in a different approach. In addition, these methods rank and scale the taxa bioevents in a way that can be calibrated to a time scale. The resulted biozonations are comparable through the methods. The study is based on the miospore records of 15 wells that are closely spaced to become the center for future correlations. This is an important step toward the biochronology that add the time dimension to the biostratigraphic analysis. In addition, the present study introduces for the first time the astronomical time calibration in Bassel-1 well based on the principles of cyclostratigraphy including time series analysis and age model tuning. The G-ray log of this well is tuned to 405 kyr eccentricity cycle to produce an age model that anchored on the OAEs. The age model can be developed by additional calibration and tuned to a higher resolution and used in other applications.

Palaeoenvironment, sequence stratigraphy and palaeogeography of the Lower Cretaceous deposits of Mehdi Abad, Yazd Block, Central Iran

Four Lower Cretaceous sections in the southern Yazd Block were measured and studied to interpret the palaeoenvironments, synsedimentary tectonics, and sequence stratigraphy. The Early Cretaceous sedimentary record of this block, consisting of the Sangestan, Taft, Abkuh, and Darreh Zanjir formations, was mainly influenced by synsedimentary tectonic activities in a tectonically unstable basin. Field observations and laboratory studies were used to identify lithofacies and microfacies, based on which six depositional environments were identified: upper coastal plain (alluvial fans), shore, tidal flat, lagoon, shoal, and open marine. A carbonate-siliciclastic shallow platform including an alluvial-coastal plain and an inner platform is suggested for the depositional environment of the Sangestan Formation. The depth of the overall shallow sedimentary basin of Sangestan Formation increases from west to east and deposition was controlled by long- term sea-level changes. A carbonate platform consisting of inner and outer parts, including tidal flat, lagoon, open marine belts, is suggested for the depositional environment of the Taft and Abkuh formations, while the Darreh Zanjir Formation accumulated in a deep basin. The predominant facies demonstrate an overall transgression-regression cycle (the 2rd order cycle) during the depositional time of these formations in the southern Yazd Block.

The Gol’chikha Formation (Upper Bathonian–Lower Boreal Berriasian) of the Yenisei–Khatanga Depression (West of the North Siberian Lowland)

Abstract —The uppermost Bathonian–lowermost Boreal Berriasian clay horizons (Gol’chikha Formation) of the Yenisei–Khatanga regional depression are regarded as probable oil source strata. Considerable core recovery in the Middle Jurassic to Lower Cretaceous sections from the boreholes drilled in the Paiyakhskaya well site and the presence of oils in the overlying strata of the Shuratovo Formation permit us to carry out integrated stratigraphic (bio-, litho-, chemo-, and seismostratigraphic) and geochemical (organic matter and oils) studies of the entire section of the Gol’chikha Formation and boundary beds, to reveal oil-producing horizons, and to compare the genotype and maturation level of their oils with those of the potentially oil source organic matter (OM) of the rocks. A detailed biostratigraphic zonation of the sections of the Gol’chikha Formation based on microfossils has been carried out. Comparison of δ13Corg variations in the Volgian and in the lower beds of the Boreal Berriasian with those in the Barents Sea shelf and in the northeast of East Siberia provided the basis for more accurate definition of the boundaries of stages and substages in the intervals free of fossils in the Paiyakhskaya area. The studied section of the Gol’chikha Formation has been divided into eight lithologic members calibrated with bio- and seismostratigraphic units. The distinctive features allowing the definition of the upper boundary of the Gol’chikha Formation are proposed using GIS data. Analysis of bio- and chemostratigraphic data allowed the correlation of the seismic reflecting horizons defined in the Gol’chikha Formation and its boundaries with the geologic section and relevant litho- and biostratigraphic units. It has been established that the strata with the highest content of organic matter consist of the upper part of the Gol’chikha Formation (the Upper Volgian and basal Boreal Berriasian). According to the vitrinite reflectance data, the OM catagenesis in the Upper Volgian interval corresponds to the oil window, which is confirmed by pyrolysis data. Thus, these strata can be considered oil-producing. The low δ13Corg values confirm the predominantly marine OM composition. Analysis of oils from the Cretaceous productive strata of the Paiyakhskaya area shows that they formed from the marine OM of the upper part of the Gol’chikha Formation at the same accumulation stage. The comprehensive studies of the Gol’chikha Formation in the Paiyakhskaya well site and complete stratigraphic coverage of the sections confirm that they can be considered a hypostratotype.

Paleoenvironmental characterization of a Lower Cretaceous section of the Recôncavo Basin, Bahia, Brazil

Paleoenvironmental characterization of a Lower Cretaceous section of the Recôncavo Basin, Bahia, Brazil

Organic Geochemistry and Microfossils of the Upper Jurassic and Lower Cretaceous Strata in the Lower Reaches of the Olenek River (Northeastern Framing of the Siberian Platform, Arctic Siberia)

Abstract —The organic-geochemistry data reveal two levels in the reference Upper Jurassic–Lower Cretaceous section of the lower reaches of the Olenek River: lower (Volgian–lower Boreal Berriasian (diasterene)) and upper (Boreal Berriasian–Valanginian (hopane)). The Volgian beds are composed of clays with abundant prasinophytes Leiosphaeridia and Tasmanites and various dinocyst assemblages and have the highest content of organic carbon (Corg), up to 9%. Isoprenoids, in particular, pristane and phytane, are highly predominant among aliphatic hydrocarbons; their content is more than three times higher than that of coeluting n-alkanes, which is typical of buried chlorophyll-containing plankton (dinocysts and prasinophytes). Sedimentological, biofacies, and paleoecological analyses show that the highly carbonaceous beds of the Buolkalakh Formation formed under oxygen deficit conditions. An integrated analysis demonstrated that the pristane/phytane ratio does not always reliably reflect the reducing or oxidizing conditions of organic-matter accumulation and diagenesis. The discrepancy between the geochemical identification of organic matter according to the pristane/phytane ratio and the biofacies and sedimentological data is due to the low catagenetic maturity of OM. The Volgian was marked by a significant transgression of the Anabar–Lena sea, which was gradually changed by a successive regression of its basin at the end of this stage and in the Boreal Berriasian. The Corg contents in the coastal and subcontinental sediments decreased. Diasterenes and 4-methyldiasterenes disappeared from the balance of biomarker molecules, and the portion of hopanoids increased. Aerobic environments prevailed in the subbottom waters. Earlier, three biomarker horizons were identified according to geochemical criteria in the synchronous sections of Anabar Bay (Laptev Sea coast): terpane, diasterene, and hopane ones. In the section of the Olenek basin, the upper two horizons are well identified by specific biomarkers, and the lower one is absent because of the sedimentation break. Stratigraphic analysis of the location of these geochemical levels in different parts (and bathymetric zones) of the Anabar–Lena basin shows their diachronous formation. According to all geological and geochemical criteria, the Volgian Stage and the lower beds of the Boreal Berriasian Stage of this basin have a high petroleum potential. In the axial zone of the basin and, especially, on the Laptev Sea shelf, there were probably favorable conditions for the generation and accumulation of hydrocarbons genetically related to the Upper Jurassic highly carbonaceous rocks.

Sedimentation Conditions and Provenance Composition of the Upper Jurassic–Lower Cretaceous Deposits of the Upper Pegtymel Depression, Chukotka Terrane

The deposits of the Imlekin Formation of the Upper Pegtymel depression were formed in the southern margin of the Chukotka Terrane synchronously with accretion of the Kulpolney island arc to the Chukotka–Arctic Alaska microplate. The sedimentological structural features of the Upper Jurassic–Lower Cretaceous sections indicate their accumulation within various parts of submarine fans of a shallow marine basin. The submarine slump origin of the boulder–pebble (tillite-like) conglomerates is substantiated. The petrographic, geochemical, and geochronological methods identified two dominant provenances: Triassic clastic rocks and Volgian suprasubduction volcanic rocks. The Middle Volgian–Early Berriasian age of the Imlekin Formation is specified. The geodynamic model proposed explains the change of the sources and direction of the sediment transport in the Volgian.

Sequence Stratigraphy and Rudist Facies Development of the Upper Barremian‐Lower Cenomanian Platform, Northern Sinai, Egypt

AbstractThe Lower Cretaceous sections in northern Sinai are composed of the Risan Aneiza (upper Barremian‐middle Albian) and the Halal (middle Albian‐lower Cenomanian) formations. The facies reflect subtle paleobathymetry from inner to outer ramp facies. The inner ramp facies are peritidal, protected to open marine lagoons, shoals and rudist biostrome facies. The inner ramp facies grade northward into outer ramp deposits. The upper Barremian‐lower Cenomanian succession is subdivided into nine depositional sequences correlated with those recognized in the neighbouring Tethyan areas. These sequences are subdivided into 19 medium‐scale sequences based on the facies evolution, the recorded hardgrounds and flooding surfaces, interpreted as the result of eustatic sea level changes and local tectonic activities of the early Syrian Arc rifting stage. Each sequence contains a lower retrogradational parasequence set that constituted the transgressive systems tracts and an upper progradational parasequence set that formed the highstand systems tracts. Nine rudist levels are recorded in the upper Barremian through lower Cenomanian succession at Gabal Raghawi. At Gabal Yelleg two rudist levels are found in the Albian. The rudist levels are associated with the highstand systems tract deposits because of the suitability of the trophic conditions in the rudist‐dominated ramp.

Berriasian to lower Hauterivian palynostratigraphy, U.K. onshore and Outer Moray Firth

Detailed palynofloral analysis of Lower Cretaceous sections at Speeton (northeast England) and in the UKCS Outer Moray Firth has involved 31 outcrop samples and over 1100 well samples fromGolden Eagle Field (UKCSblocks 14/26a and 20/01), including extensive conventional core.Very close sampling, particularly within theValanginian, has allowed some assessment of inter- and intra-species trends, some very short-lived and others of longer duration and the possible origins of several species are discussed. Paleoenvironmental change is seen as a possible stimulus to these evolutionary trends. The zonation scheme proposed by Duxbury (2001) has been re-visited and significantly refined. Recognition of additionalmarker events has allowed the further subdivision of several zones to sub-zonal level, allowing a better understanding of the stratigraphy, particularly of the Valanginian. Over 100 key palynofloral events have now been recognised between the top Berriasian and lower Hauterivian, including many first occurrences. One genus and twenty-one species of stratigraphically significant dinoflagellate cysts are described as new. Eight new combinations have been effected and the diagnoses of one genus and three species have been emended. The very high resolution achievable by applying the refined scheme is clearly demonstrated in two transects across the Golden Eagle Field.

Integrated methodology for determining provenance of detrital quartz using optical petrographic microscopy and cathodoluminescence (CL) properties

The provenance of detrital quartz is a useful predictor in frontier basin sandstone reservoirs of the availability of coarse-grained quartz grains of plutonic or high grade metamorphic origin. The standard approach is to use scanning electron microscope (SEM) panchromatic cathodoluminescence (CL) imagery and observations from an optical petrographic microscope. We describe a work flow, modified from previous literature proposals, to determine quartz provenance in detrital grains using optical petrographic microscopy and CL properties from both a hot-cathode cathodoluminescence microscope attachment (HCMA) and panchromatic SEM-CL. HCMA analysis permits better discrimination of different types of metamorphic quartz. This method is applied to a Middle Jurassic to Lower Cretaceous section in the Bandol-1 wildcat well, located in the Laurentian sub-basin south of Newfoundland. Two types of high-grade metamorphic quartz are distinguished, one with low luminescence as described in previous literature and one with medium blue CL colour and a moderate colour shift that is also known from granulite in the hinterland. Three phases of detrital supply are identified, suggesting progressive unroofing of higher grade metamorphic sources through the middle Jurassic and major input of plutonic quartz in the early Cretaceous.

Documentation and characterization of the Lower Cretaceous (Valanginian) Calvin and Winn carbonate shelves and shelf margins, onshore northcentral Gulf of Mexico

The Lower Cretaceous stratigraphic section in the northern Gulf of Mexico (GOM) comprises several well-documented carbonate systems that have prominent shelf-margin buildups, including the Berriasian Knowles Limestone ramp and shelf system, Barremian Sligo Formation shelf system, Aptian Pearsall Formation ramp system, and Albian Glen Rose, Edwards, and Stuart City Formations shelf systems. Two lesser known but large-scale Lower Cretaceous Valanginian carbonate shelf to shelf-margin systems are documented in this investigation using core, wireline-log, and regional 2D seismic data. The platform-margin succession of the older Calvin section comprises a shelf-margin buildup (back reef, reef, and fore reef facies) that may be as thick as 2000 ft (610 m). The seaward margin of the shelf, as displayed by seismic data, suggests shelf-to-basin relief of as much as 1000 ft (305 m) and margin-to-lagoon relief of 200 to 500 ft (60 to 140 m) (relief estimates are not decompacted). Major reef-building organisms are Lithocodium, stromatoporoids, and corals. The Winn limestone is younger than the Calvin limestone, and at the shelf margin, it may have been 600 to 800 ft (180 to 245 m) thick. It is composed of organisms similar to those of the Calvin reef complex. Considering the vertical position of the Calvin platform, succeeding the major bypass of Calvin lowstand siliciclastic sands, the platform probably records a transgression, which is consistent with its highly aggradational depositional style. The Winn shelf margin was initiated landward of the Calvin shelf margin and did not prograde as far seaward as the Calvin shelf margin. The recognition of these two Lower Cretaceous carbonate systems is important because they further detail the stratigraphic history and architecture of this deeply buried, lowermost Lower Cretaceous section in the GOM. These formations are also potential deep-gas, tight-carbonate (low-porosity) plays.

Source rock potential of the Middle Jurassic to Middle Pliocene, onshore Nile Delta Basin, Egypt

Organic geochemical characterization of cutting samples from the Abu Hammad-1 and Matariya-1 wells elucidates the depositional environment and source rock potential of the Jurassic and Lower Cretaceous successions and the Middle Miocene to Pleistocene section in the southern and eastern Nile Delta Basin. The burial and thermal histories of the Mesozoic and Miocene sections were modeled using 1D basin modeling based on input data from the two wells. This study reveals fair to good gas-prone source rocks within the Upper Jurassic and Lower Cretaceous sections with total organic carbon (TOC) averaging 2.7% and hydrogen index (HI) up to 130 mg HC/g TOC. The pristane/n-C17 versus phytane/n-C18 correlation suggests mixed marine and terrestrial organic matter with predominant marine input. Burial and thermal history modeling reveals low thermal maturity due to low heat flow and thin overburden. These source rocks can generate gas in the western and northern parts of the basin where they are situated at deeper settings. In contrast, the thick Middle Miocene shows fair source rock quality (TOC averaging at 1.4%; HI maximizing at 183 mg HC/g TOC). The quality decreases towards the younger section where terrestrial organic matter is abundant. This section is similar to previously studied intervals in the eastern Nile Delta Basin but differs from equivalents in the central parts where the quality is better. Based on 1D modeling, the thick Middle Miocene source rocks just reached the oil generation stage, but microbial gas, however, is possible.