Lower Cretaceous Age Research Articles

Impact of pore fluid heterogeneities on angle-dependent reflectivity in poroelastic layers: A study driven by seismic petrophysics

The present study demonstrates the application of seismic petrophysics and amplitude versus angle (AVA) forward modeling to identify the reservoir fluids, discriminate their saturation levels and natural gas composition. Two case studies of the Lumshiwal Formation (mainly sandstone) of the Lower Cretaceous age have been studied from the Kohat Sub-basin and the Middle Indus Basin of Pakistan. The conventional angle-dependent reflection amplitudes such as P converted P (RPP) and S (RPS), S converted S (RSS) and P (RSP) and newly developed AVA attributes (deltaRPP, deltaRPs, deltaRSS and deltaRSP) ) are analyzed at different gas saturation levels in the reservoir rock. These attributes are generated by taking the differences between the water wet reflection coefficient and the reflection coefficient at unknown gas saturation. Intercept (A) and gradient (B) attributes are also computed and cross-plotted at different gas compositions and gas/water scenarios to define the AVO class of reservoir sands. The numerical simulation reveals that deltaRPP, deltaRPS, deltaRSS and deltaRSP are good indicators and able to distinguish low and high gas saturation with a high level of confidence as compared to conventional reflection amplitudes such as P-P, P-S, S-S and S-P. In A-B cross-plots, the gas lines move towards the fluid (wet) lines as the proportion of heavier gases increase in the Lumshiwal Sands. Because of the upper contacts with different sedimentary rocks (Shale/Limestone) in both wells, the same reservoir sand exhibits different response similar to AVO classes like class I and class IV. This study will help to analyze gas sands by using amplitude based attributes as direct gas indicators in further gas drilling wells in clastic successions.

Serpentines as the indicators of mesozoic peridotites metamorphic and geodynamic transformations in the Internal Ukrainian Carpathians

Formulation of the problem. Peridotites of ophiolite complexes,being the fragments of the oceanic upper mantle that have undergone several stages of partial melting, brought to the surface by tectonic movements, also have undergone metamorphic transformations almost immediately after its formation. Because of serpentinization, the mineral composition of the rocks became more complicated. The analysis of the final structure and composition of apoperidotites allows obtaining data for geodynamic reconstructions regarding the stage of their formation. Review of previous publications. It has been determined that serpentines are the most common secondary minerals of peridotites of the Uholskyi complex in the Ukrainian Carpathians, and the processes of serpentinization took place at a depth of 40–50 to 100 km (?) at T = 450–600 °C and P = 13–16 kbar (Stupka O., 2013). The study of serpentinites of the Main Ural Fault (Panas'yan L. et al., 2014) revealed that high alumina and high chromium serpentinites have ultrabasic protoliths formed in the mantle, and medium alumina and low chromium varieties – the protoliths of the basic composition which were born in the conditions of the crust. Based on the study of serpentinites in the orogenic Qinling belt (China), researchers (Wu K. et al., 2018) determined their mantle origin: magnetite-enriched antigorite serpentinites were formed as a result of the interaction of serpentinized apoperidotites of mantle protoliths with molten rock in the subduction channel. Purpose. We have investigated secondary serpentines in order to reconstruct the geodynamic conditions of the formation and transformations of the peridotites (Uholskyi complex), localized in the Marmarosh rocky zone in the Internal Ukrainian Carpathians, and are most widely spread in the interfluve of Velyka and Mala Uholka-rivers. Methods. The work is based on the results of geological observations of the Uholskyi complex rocks in natural outcrops, as well as petrographic, mineralogical (including X-ray diffraction, thermal and microprobe analysis), and geochemical studies. Results. The paper presents the study results of serpentinized apoperidotites of the ophiolite Uholskyi complex in the Internal Ukrainian Carpathians. Serpentinized apoperidotites (T2–K1?) form olistoliths in the Soimulska olistostrome-conglomerate strata of the Lower Cretaceous age. The investigated serpentines are the rock-forming minerals of lizardite and antigorite serpentinites. Lizardite serpentinites are characterized by lenticular-looped textures formed by α-lizardite and non-altered chrome-spinellids. Antigorite serpentinites, recognized by striped-shale textures, contain antigorite, β-lizardite and magnetite. Lizardite serpentinites are characteristic of the regressive metamorphism of the greenschist facies upper part, and antigorite serpentinites are a typical formation of the progressive metamorphism of the lower greenschist – upperlower epidote-amphibolite facies. Regressive metamorphism occurred under geodynamic conditions of spreading and the progressive ones – under subduction conditions between the terrains of Dacia and Tisza, which led to the closure of the Transylvanian-Mureş Paleocean. It has been concluded, that the protoliths of lizardite serpentinites were the primary mantle rocks of the ultrabasic (restitic) composition, and the protoliths of the antigorite serpentinites were the lithospheric rocks of the basic composition. Conclusions. The complex study of serpentinized apoperidotites of the Uholskyi complex makes it possible to reconstruct the metamorphic transformations of the primary mantle protoliths and to determine the stages of lithosphere formation within the fold-nappe structures. The obtained results can be used for prediction of serpentinite mineralization.

Biostratigraphic partition of the Upper Jurassic and Lower Cretaceous (Oxfordian - Lower Berriasian) deposits of the Crimean Mountains for foraminiphers

Introduction. Upper Jurassic and Lower Cretaceous deposits in the Crimean Mountains are widespread and are represented by all stages ‑ Oxfordian, Kimmeridgian, Tithonian, and Berriasian. These deposits have been unevenly studied. The most studied deposits are the Oxfordian, Tithonianian and Berriasian stages. The biostratigraphic division of the Upper Jurassic and Lower Cretaceous deposits of the Mountain Crimea is based on "Stratigraphic scheme of the Upper Jurassic and Lower Cretaceous deposits of the Mountain Crimea" 2013. Purpose. Detailed biostratigraphic devision of the Upper Jurassic and Lower Cretaceous (Oxfordian ‑ Lower Berriasian) of the Mountain Crimea as results of foraminiferal analysis. Results of the study. There are five foraminiferal complexes in sections that characterize Upper Jurassic (Lower Upper Oxfordian, Lower Kimmeridgian, Upper Tithonian) and Lower Cretaceous (Lower Berriasian) age. Analysis of foraminiferal complexes allowed us to establish biostratigraphical zones in each section: oxfordiana in the Lower Oxfordian Lenticulina quenstedti-Globuligerina; in the Upper Oxfordian Lenticulina russiensis-Epistomina uhligi; in the Upper Tithonian Anchispirocyclina lusitanica-Melathrokerion spirialis; in the Lower Berriasian (lower zone) Protopeneroplis ultragranulatus-Siphoninella antiqua and layers in the Lower Kimmeridgian with Epistomina praetatariensis-Globuligerina parva. Specified foraminiferal zones were traced by us in different structural-facial zones (zone Lenticulina quenstedti-Globuligerina oxfordiana ‑ in Sudak-Feodosia; zone Lenticulina russiensis-Epistomina uhligi ‑ in Ai-Petri-Babugansk and Sudak-Feodosia; strata layers with Epistomina praetatariensis-Globuligerina parva ‑ in Sudak-Feodosia; zone Anchispirocyclina lusitanica-Melathrokerion spirialis ‑ in Sukhorychensk-Baidar, Demerdzhi-Karabijskaya and Sudak-Feodosiya; the zone of Protopeneroplis ultragranulatus-Siphoninella antiqua in Demerdzhi-Karabijskaya and Sudak-Feodosiya). Characteristic features of each foraminiferа zone include a complex of species, disappearing species, and species that continue to exist, as well as particular features of the complexes. According to lithological peculiarities and paleontological characteristics, suite and sub suite are defined. Conclusions. As a result of the study, the biostratigraphic characteristics of the sections have been specified and the paleontological characteristics of the Upper Jurassic and Lower Cretaceous deposits of the Mountain Crimea have been supplemented.

Zircon U-Pb geochronology of the mesozoic to lower Cenozoic rocks of the Coastal Cordillera in the Antofagasta region (22°30′-23°00′ S): Insights to the Andean tectono-magmatic evolution

This work reports on new zircon U-Pb ages (U-Pb LA-ICPMS Geochronology) obtained from volcanic, intrusive and sedimentary rocks which have been exposed in the Coastal Cordillera of the Antofagasta region (22°30′-23°00′S). A frequency diagram was performed with all the obtained ages. The diagram showed peaks representing emplacement or crystallization, depositional and inherited zircon ages. The zircons showed to be 300 Ma and younger form four major peaks. These peaks are consistent with previously dated emplacement and depositional ages, as well as, with the ages of the rock units recognized in this work. The oldest cluster ranges between 250 and 230 Ma and represents the first volcanic episode in the study area that corresponds to the Middle Triassic Sierra Miranda-Cerro Camaleón Rhyolites which yielded a zircon age of 240 ± 1.2 Ma. A second age cluster, between 220 and 130 Ma, can be associated with the Lower Jurassic volcanic and plutonic rocks, to Middle-Upper Jurassic plutonic rocks and Lower Cretaceous subvolcanic rocks. The Lower volcanic rocks correspond to La Negra Formation, which yielded ages that range between ca. 196 and 177 Ma. These ages are older than the Middle-Upper Jurassic ages commonly reported for this formation. The La Negra Formation and the interbedded marine Rencoret Beds are in stratigraphic continuity with the Middle Triassic Sierra Miranda-Cerro Camaleón Rhyolites. The ages that range between 170 and 150 Ma can be associated with the larger intrusive rock bodies from the Middle-Upper Jurassic Coastal Batholite of Northern Chile and Southern Perú. Lower Cretaceous ages (ca. 140 Ma) corresponds to subvolcanic rocks related to the metallogenic belt of the easternmost Coastal Cordillera of the Antofagasta region. A zircon ages gap is observed in the Lower Cretaceous (between 130 and 100 Ma). This gap indicates that either the study area did not undergo magmatic and volcanic activity at that time, or that the Peruvian tectonic phase allowed for the erosion of the geological record of such activity. An Upper Cretaceous cluster occurs at ca. 100-80 Ma and is correlated with the Cerro Empexa, Quebrada Mala, and Hornitos volcanic formations. The last volcanic activity occurred at about ca. 70-60 Ma. Lower Paleocene (ca. 63 Ma) volcanic unit indicates that the products of the Paleocene volcanism reached as far west as the Coastal Cordillera at the latitude of Antofagasta. Even the presence of lava indicates the proximity of effusive volcanoes, which, in turn, suggests that the Paleocene volcanic arc extended west away from the Domeyko Cordillera. Both Upper Cretaceous and Lower Paleocene volcanic units are restricted to the west by the Atacama Fault System.

Structural and Stratigraphic Study of the Nahr Umr Formation (Lower Cretaceous) Using 3D seismic Survey in Abu Amood Oil Filed, Southern Iraq

3D seismic reflection study was applied to Abu Amood oil field which is located at the southern part of Iraq within DhiQar province that carried out by oil Exploration Company to an area of 1534.88 Km2 for studying Nahr Umr Formation. Synthetic seismogram was prepared by using available data of well (AAM-1) in order to define and picking the reflectors on the seismic section. These reflectors are (Top of Nahr Umr Formation and middle unit of Nahr Umr Formation which represents the layer of sand). The seismic section time slice maps confirmed that the Nahr Umr Formation was not affected by faults and the faults may probably present in the Ratawai and Yamama Formations, where the variance attribute applied on seismic sections showed that the area was affected by normal fault, this fault was very deep. The time, velocity, and depth maps are drawn for the top of Nahr Umr Formation to get the structural picture: these maps showed that Abu Amood oil field was a semi symmetrical structure that has an axis trending NW-SE, the slope of NE limb is greater than SW limb. Isochrone and Isopach maps are drawn for Nahr Umr Formation to display thickness of Nahr Umr Formation which gradually increases to the west as well as NW and SW parts of the study area and more gradually increases to the East and NE part of the study area. Seismic attributes were applied to the study area (instantaneous phase, instantaneous frequency and variance attribute) these attributes showed the presence of Direct Hydrocarbone Indicator at Nahr Umr Formation in Lower Cretaceous age as a Dim spot at In-lines 51500 and 49500 by applying instantaneous phase and bright spot at AAM-1 well by applying instantaneous frequency.

Halocinese and Halotectonics in the Northern Atlas: Case Study of the Triassic Outcrop of J. El Matria (Oued Zarga Area)

Apart from the Triassic bodies, there are several structural elements that characterize the Oued Zarga area. The new field data allowed us to deduce that the structural evolution of this region is essentially based on two generations of dextral strike-slip faults: an E-W direction to WNW-ESE and a later NW-SE second trend. In the study area, the Triassic material of J. El Matria is framed by two sedimentary contacts. Lower contact in clansayesian clays characterized by a glauconic to Triassic insoluble conglomerates and an upper contact where the Triassic material is covered by a thick pelagic series attributed to the lower Albian. This configuration results from a rapid lateral outpouring during the Lower Cretaceous in an extensive tectonic context and re-folded during the tertiary compressive phases, interpreted like most Triassic structures in northern Tunisia, in terms of the salt glacier with Lower Cretaceous age.

Structural Interpretation and Petrophysical Analysis for Reservoir Sand of Lower Goru, Miano Area, Central Indus Basin, Pakistan

The main purpose of this research is to estimate the structural analysis and hydrocarbon potential of Miano Block by using seismic and well log techniques. Miano area hosts a number of gas fields with structural and stratigraphic traps. The area is located in Central Indus Basin which is a part of an extensional regime exhibiting normal faulting due to the split of the Indian Plate firstly from Africa and then from Madagascar and Seychelles. Miano area recognized as a proven petroleum province which has complex tectonic history of Cretaceous extensional and overprints of Tertiary strike-slip tectonics. The area has prospect with accumulation of hydrocarbons in structural and stratigraphic traps including pinchouts. NW-SE oriented Khairpur and Mari Highs are main structural features with impact on the fault system. The sands of Lower Goru of Lower Cretaceous age are acting as a reservoir in the area. The area has great potential of hydrocarbons for which more exploratory wells are required to be drilled with better insight of structural and stratigraphic traps.

Geochemical Study of Oil Shales Sargelo Formation in Kuh-e Kaino Section (Northern Masjed-Soleiman), Iran

Oil (bituminous) shales source rocks with variable amounts of solid kerogen type II are the conditions for changing the oil that is not provided for them. Each of these shales affected in temperature about 150 to 170 gallons of oil produced. Therefore by reducing hydrocarbon resources, it can be a good energy source for the future. To study the geochemistry of this shale, one section of outcrops in the Kuh-e Kaino (Northern Khozestan) was selected which was located in the central Zagros. Chemical analysis and thin sections were prepared from samples of carbonate shale. These deposits with a thickness of 115 m of the Lower Jurassic formations Neyriz and Sargelo relative age and their relationship are gradual and interfinger. The upper border after an erosional unconformity, Fahliyan Formation of Lower Cretaceous age on them covered. Analysis results show that the percentage of organic matter this shales in Kuh-e Kainois is equivalent to 8.44 percent. The index has gained economic value of metal elements Sr, V, Ba, Ni, and also has economic value. Bituminous materials formed stabilize in shaley source rock of Sargelo formation, but not with continuous deposition on them, as a result of temperature and pressure not increasing and immature kerogen. The causes of this event are as follows: the basin tectonic condition, Stratigraphy position and faults activity in the basin.

Reservoir zonation based on statistical analyses: A case study of the Nubian sandstone, Gulf of Suez, Egypt

Both reservoir engineers and petrophysicists have been concerned about dividing a reservoir into zones for engineering and petrophysics purposes. Through decades, several techniques and approaches were introduced. Out of them, statistical reservoir zonation, stratigraphic modified Lorenz (SML) plot and the principal component and clustering analyses techniques were chosen to apply on the Nubian sandstone reservoir of Palaeozoic – Lower Cretaceous age, Gulf of Suez, Egypt, by using five adjacent wells. The studied reservoir consists mainly of sandstone with some intercalation of shale layers with varying thickness from one well to another. The permeability ranged from less than 1 md to more than 1000 md. The statistical reservoir zonation technique, depending on core permeability, indicated that the cored interval of the studied reservoir can be divided into two zones. Using reservoir properties such as porosity, bulk density, acoustic impedance and interval transit time indicated also two zones with an obvious variation in separation depth and zones continuity. The stratigraphic modified Lorenz (SML) plot indicated the presence of more than 9 flow units in the cored interval as well as a high degree of microscopic heterogeneity. On the other hand, principal component and cluster analyses, depending on well logging data (gamma ray, sonic, density and neutron), indicated that the whole reservoir can be divided at least into four electrofacies having a noticeable variation in reservoir quality, as correlated with the measured permeability. Furthermore, continuity or discontinuity of the reservoir zones can be determined using this analysis.

Organic geochemistry of crude oils from the Intisar oil field (East Sirte Basin, Libya)

Four crude oil samples from the oil fields Intisar A, Intisar D and Intisar E (Sirte Basin, Libya) were investigated in order to define depositional environment, lithology, thermal maturity and geologic age of the corresponding source rocks. Saturated biomarkers (n-alkanes, isoprenoids, steranes and triterpanes) were determined using gas chromatography – mass spectrometry (GC–MS) and gas chromatography – mass spectrometry – mass spectrometry (GC–MS–MS). Aromatic hydrocarbons (phenanthrene, methylphenanthrenes, methyl-dibenzothiophenes and trimethyl-naphtalenes) were analyzed by GC–MS.The Intisar crude oils were generated from siliciclastic source rocks containing a mixture of marine and terrestrial organic matter. Maturation parameters indicated high level of thermal maturity corresponding to the main phase of oil generation. The analysis of the age-specific biomarker ratios suggested Cretaceous, most probably Lower Cretaceous age for the Intisar oils. Two source rocks for the Intisar oils were identified, similar in the precursor organic matter type, lithology, maturity and geologic age but different in the redox conditions in the environment during their deposition. The oils generated from these different sources probably migrated over different migration pathways.

Assessments of oil characterization, source affinities, and hydrocarbon dynamic of East Baghdad oil fields, Central Iraq

Bulk properties, stable carbon isotopes, whole crude Gas-Chromatography and oil mode ratios of sterane and terpanes data of analyzed samples of oil from the Cretaceous Formation reservoirs of East Baghdad oil field are from five producing wells (EB21, EB68, EB92, EB5 and EB31) from reservoirs of Zubair, Khasib, and Tanuma Formations. They are discussed with aids of characters, hopane, tricyclic terpane, isotopic δC13 saturate, aromatics, and pristine-phytane diagrams. The analized oil have indicated light and medium oil in the Zubair reservoirs and heavy oil for the Khasib and Tannuma oil, with about 15% asphalt and porpherine range of 16.7–35.1 Ni and 68–117 ppm, giant reservoirs. The oil source environment and lithology are of marine algal type II and deltaic type II/III that are non biodegraded deposited in anoxic environments of carbonate and shale. Source maturation at the time of the oil generation, aided by pristine-phytane and Tmax equivalent from MDR (Methyldibenzothiophene ratio) diagrams, are showing mature with Tmax 430–435 °C while source age assessment are taken from δC13 (%) and the calculated C28/C29 sterane ratio to be of Middle and Upper Jurassic as well as Lower Cretaceous age that could be correlated mainly with the Chia Gara and Zubair Formations as well as the Middle Jurassic Sargelu Formation source rocks for oil that are accumulated in the reservoirs of Zubair, Khasib and Tannumah Formations.Source rocks are mainly Upper Jurassic-Lower Cretaceous Chia Gara Formation with contribution from the Middle Jurassic Sargelu and Lower Cretaceous Zubair Formations. The richness of the source rocks, mostly due to shale rich in TOC, and the type of kerogen is mixed type II/III and type III. The formations have good petroleum potentiality (PP) and the OM reaches the Early of hydrocarbon generation. Oil, and gas are the most probable product, and the generated hydrocarbons can be expelled at this level of thermal maturity which is confirmed by petromod software basin modeling to suggest migration and accumulation during the Upper Cretaceous time.Hydrocarbon dynamic in East Baghdad Oil Field are performed using seismic section, logs and porosities. Model assessments of the hydrocarbon generation, migration paths and accumulation sites as well as places of enriched oil are used to assess reservoir location and suggestion for drilling sites.

Trace-element geochemistry and U–Pb dating of perovskite in kimberlites of the Lunda Norte province (NE Angola): Petrogenetic and tectonic implications

Perovskite (CaTiO3) has become a very useful mineral for dating kimberlite eruptions, as well as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the first work of in situ U–Pb geochronology and Sr–Nd isotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskite were identified, differing in texture, major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6wt% Nb2O5) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts.U–Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2±6.5Ma), Mulepe 2 (123.0±3.6Ma), Calonda (119.5±4.3Ma) and Cat115 (133±10Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (>300km) during the break-up of Gondwana. Sr–Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations.

Petrography and geochemistry of fault-controlled hydrothermal dolomites in the Riópar area (Prebetic Zone, SE Spain)

The present paper reports the first detailed petrographical and geochemical studies of hydrothermal dolomites related to MVT Zn-(Fe–Pb) deposits in the Riópar area (Mesozoic Prebetic Basin, SE Spain), constraining the nature, origin and evolution of dolomitizing and ore-forming fluids. Mapping and stratigraphic studies revealed two stratabound dolostone geobodies connected by other patchy bodies, which replace carbonate units of Upper Jurassic to Lower Cretaceous ages. These dolostones are associated to the W–E trending San Jorge fault, indicating a main tectonic control for fluid flow. Seven different dolomite types were identified: i) matrix-replacive planar-s (ReD-I); ii) matrix-replacive planar-e (ReD-II); iii) planar-e sucrosic cement (SuD); iv) non-planar grey saddle dolomite cement (SaD-I) pre-dating Zn-(Fe–Pb) sulfides; v) non-planar milky to pinkish saddle dolomite cement (SaD-II) post-dating Zn-(Fe–Pb) ores; vi) ore-replacive planar-e porphyrotopic (PoD); and vii) planar-s cloudy cement (CeD). Meteoric calcite types were also recognized. The different dolomite types are isotopically characterized by: i) depleted δ18O (from +25.1 to +27.6‰ V-SMOW) and δ13C (from −2.3 to +0.9‰ V-PDB) values compared to Upper Jurassic to Lower Cretaceous limestone signature (δ18O: +27.6 to +30.9‰ V-SMOW; δ13C: +0.5 to +3.2‰ V-PDB); and ii) 87Sr/86Sr ratios for the main dolomitization phases (ReD and SuD: 0.70736–0.70773) close to the Jurassic and Cretaceous carbonate values (0.70723–0.70731) whereas more radiogenic values (0.70741–0.70830) for saddle dolomites (SaD) related to the Zn-(Fe–Pb) sulfide mineralization prevailed after fluid interaction with Rb-bearing minerals. Microthermometrical studies on two-phase liquid and vapor fluid inclusion populations in planar and non-planar dolomites and sphalerite show homogenization temperatures between 150 and 250 °C. These data indicate that both planar and non-planar dolomite textures formed at high-temperatures under hydrothermal conditions in deep-burial diagenetic environments. The main dolomitizing phase (ReD-I/ReD-II and SaD-I) shows low to moderate fluid inclusions salinity (5–14 wt.% eq. NaCl), whereas the dolomitization related to ore precipitation (sphalerite and SaD-II) spreads to higher salinity values (5–25 wt.% eq. NaCl). These data may respond to a mixing between a low salinity fluid (fluid A, less than 5 wt.% eq. NaCl) and a more saline brine (fluid B, more than 25 wt.% eq. NaCl) at different fluid proportions.

Structural Styles and Petroleum Potential of Miano Block, Central Indus Basin, Pakistan

The main purpose of this research article is to evaluate the structural styles and hydrocarbon potential of Miano Block using seismic and well log data. The Miano Block discovered in 1993 is located in the Central Indus Basin which is a part of an extensional regime exhibiting normal faulting, formed as a result of split of the Indian Plate firstly from Africa and then from Madagascar and Seychelles. Tectonically, the Miano Block lies on the Panno-Aqil graben between two extensive regional highs i.e. Jacobabad-Khairpur High and Mari Kandhkot High. Four migrated seismic lines were used for structural enhancement; P2092-111, P2092-113 and P2092-115 (dip lines) and P2092-110 (strike line). Time and depth contours were generated for four horizons, HabibRahi Formation, Sui Main Limestone, Ranikot Formation and Lower Goru Formation which showed the presence of horst and graben structures in the subsurface. The interpretation of horst and graben structures is based on a parallel set of NS-oriented high-angle planar normal faults with dips either towards SE or SW with majority of the faults dipping towards the SW. The faults are observed to exhibit slight disruption of strata with limited displacement to the order of about 50 m in Lower Goru Formation. Thus, horst and graben structures with NS trend in the study area are interpreted. The NS trend of these structures along with similar structural dip is inconsistent to the NW-SE orientation of regional structures i.e. Panno- Aqil graben and the Jacobabad-Kharipur and Mari-Kandhkot Highs indicating about the presence of a second minor set of faults and complexity of deformation. In this setting, Maino-02 well was drilled with the primary target as B-Sands of Lower Goru Formation belonging to Lower Cretaceous age while secondary target was A-Sands of Lower Goru belonging to the same age. The result of the petrophysical analysis supports two potential zones (zone 1 and zone 2) within the Lower Goru Formation. Potential reservoir zones were marked with average hydrocarbon saturation of zone 1 calculated as 63.5% and that of zone 2 as 68.9%.

Recognition features of felsic pyroclastics of Serra do Tombo formation, Minas Gerais, Brazil

In this contribution we report a study of poorly exposed, rhyodacitic welded-ignimbrite deposit from Minas Gerais. A petrographic study of textures indicate high temperature of emplacement. Key features include eutaxitic texture, flattened and agglutinated lapilli and glass menisci. Most of the feldspar minerals and glass are extensively altered to clay minerals, which pseudomorph the original volcanic textures. Glass menisci and spherules suggest a possible process of liquid immiscibility. Immobile trace element distribution indicates a possible link with other post-Palaeozoic felsic volcanic rocks in Brazil, a magmatism interpreted as due to basaltic underplating and partial melting of a hydrous continental crust. A peculiar feature is a high Light REE/Heavy REE ratio. Depletion in heavy rare earth elements is possibly due to a residual HREE-bearing phase in the source. The geologic context of these rocks suggests a Lower Cretaceous age and a tectonic relationship with a continental rifting event.

Lower Cretaceous (Albian) Asteroidea (Echinodermata) from Tepexi de Rodriguez, Puebla, Mexico

Seven asteroids are described from the Tlayua quarries (18° 35´ N - 97° 56´ W), recovered from the Middle Member of the Tlayua Formation. Tlayua quarries are 94 km southeast of Puebla and 2 km northeast of the town of Tepexi de Rodriguez in the Eastern part of central Mexico. The Middle Member is fossiliferous and composed mainly of a micritic lithographic limestone with layers having packs that consist of as much as 30 % hematite. The fossils are confined to these iron rich layers. These Middle Member are about 28 m thick. Due to the abundance, variety and conservation of its paleobiota, the Tlayua quarry is one of the most important fossiliferous localities from North America’s Cretaceous. About 5 000 samples of invertebrate and vertebrate fossils have been collected from this locality and have been studied. The seven specimens belong to the Cretaceous strata and are believed to be of Lower Cretaceous (Albian) age. The asteroids described are referred to the genus Astropecten, Plutonaster, Tamaria and Echinaster. This is the first report of Cretaceous asteroids for Mexico.

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

Cenozoic strata from forearc basins in southern Alaska record deposition related to two different types of shallow subduction: Paleocene–Eocene spreading-ridge subduction and Oligocene–Recent oceanic plateau subduction. We use detrital zircon geochronology (n = 1368) and clast composition of conglomerate (n = 1068) to reconstruct the upper plate response to these two subduction events as recorded in forearc basin strata and modern river sediment. Following spreading-ridge subduction, the presence of Precambrian and Paleozoic detrital zircon ages in middle Eocene–lower Miocene arc-margin strata and Early Cretaceous ages in lower Miocene accretionary prism–margin strata indicates that sediment was transported to the basin from older terranes in interior Alaska and from the exhumed eastern part of the Cretaceous forearc system, respectively. By middle-late Miocene time, diminished abundances of these populations reflect shallow subduction of an oceanic plateau and associated exhumation that resulted in an overall contraction of the catchment area for the forearc depositional system. In the southern Alaska forearc basin system, upper plate processes associated with subduction of a spreading ridge resulted in an abrupt increase in the diversity of detrital zircon ages that reflect new sediment sources from far inboard regions. The detrital zircon signatures from strata deposited during oceanic plateau subduction record exhumation of the region above the flat slab, with the youngest detrital zircon population reflecting the last period of major arc activity prior to insertion of the flat slab. This study provides a foundation for new tectonic and provenance models of forearc basins that have been modified by shallow subduction processes, and may help to facilitate the use of U-Pb dating of detrital zircons to better understand basins that formed under changing geodynamic plate boundary conditions.

Evaluation of the petroleum composition and quality with increasing thermal maturity as simulated by hydrous pyrolysis: A case study using a Brazilian source rock with Type I kerogen

Hydrous pyrolysis (HP) experiments were used to investigate the petroleum composition and quality of petroleum generated from a Brazilian lacustrine source rock containing Type I kerogen with increasing thermal maturity. The tested sample was of Aptian age from the Araripe Basin (NE-Brazil). The temperatures (280–360°C) and times (12–132h) employed in the experiments simulated petroleum generation and expulsion (i.e., oil window) prior to secondary gas generation from the cracking of oil. Results show that similar to other oil prone source rocks, kerogen initially decomposes in part to a polar rich bitumen, which decomposes in part to hydrocarbon rich oil. These two overall reactions overlap with one another and have been recognized in oil shale retorting and natural petroleum generation. During bitumen decomposition to oil, some of the bitumen is converted to pyrobitumen, which results in an increase in the apparent kerogen (i.e., insoluble carbon) content with increasing maturation.The petroleum composition and its quality (i.e., API gravity, gas/oil ratio, C15+ fractions, alkane distribution, and sulfur content) are affected by thermal maturation within the oil window. API gravity, C15+ fractions and gas/oil ratios generated by HP are similar to those of natural petroleum considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. API gravity of the HP expelled oils shows a complex relationship with increasing thermal maturation that is most influenced by the expulsion of asphaltenes. C15+ fractions (i.e., saturates, aromatics, resins and asphaltenes) show that expelled oils and bitumen are compositionally separate organic phases with no overlap in composition. Gas/oil ratios (GOR) initially decrease from 508–131m3/m3 during bitumen generation and remain essentially constant (81–84m3/m3) to the end of oil generation. This constancy in GOR is different from the continuous increase through the oil window observed in anhydrous pyrolysis experiments. Alkane distributions of the HP expelled oils are similar to those of natural crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. Isoprenoid and n-alkane ratios (i.e., pristane/n-C17 and phytane/n-C18) decrease with increasing thermal maturity as observed in natural crude oils. Pristane/phytane ratios remain constant with increasing thermal maturity through the oil window, with ratios being slightly higher in the expelled oils relative to those in the bitumen. Generated hydrocarbon gases are similar to natural gases associated with crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous, with the exception of elevated ethane contents. The general overall agreement in composition of natural and hydrous pyrolysis petroleum of lacustrine source rocks observed in this study supports the utility of HP to better characterize petroleum systems and the effects of maturation and expulsion on petroleum composition and quality.

The application of multilayer perceptron neural network in volume of clay estimation: Case study of Shurijeh gas reservoir, Northeastern Iran

Volume of clay is an important component in the assessment of shaly sand reservoirs, due to its significant impact on the production characteristics. The Shurijeh sandstone Formation of Lower Cretaceous age, with subordinate shales is one of the most challenging gas reservoirs to be properly characterized in the eastern Kopet-Dagh sedimentary Basin, Northeastern Iran. This paper describes the improvement achieved in estimating the volume of clay in the Shurijeh reservoir Formation, with an application to a gas producing well and another non-producing well in a joint field between Iran and Turkmenistan. A clear comparison between estimates from several conventional petrophysical methods and actual laboratory measured data of 76 core samples showed very large estimation errors; therefore an attempt has been made to improve the estimation with developing a multilayer feedforward backpropagation neural network. Six types of well logs were selected, through a sensitivity analysis, as the most relevant input data to the volume of clay (network output). Data were then standardized and randomly divided into three sets of 70% for training, 15% for validation and 15% for testing. Three different training algorithms of genetics, particle swarm optimization, and Levenberg–Marquardt were tested on a network with certain topology, and the latter was chosen, due to the better performance. The number of hidden layer neurons, was efficiently determined 8 through a trial and error process. The developed network with the architecture of 6-8-1 was then successfully validated with 16 unseen core data. Mean squared error of 2.8069 E−3 and a correlation coefficient of 0.9013, as the network criteria; clearly showed a multilayer neural network can significantly improve the estimation of volume of clay by 53% within data set from the Shurijeh Formation.

Deformations in the Upper Cretaceous — Neogene sediments of the South-Western Crimea on the base of new tectonophysical data

Deformation structures (tectonic planes with slickensides, folds, parageneses fractures аre described) in the Upper Cretaceous—Neogene sediments of the south-western Crimea. The purpose of the study is to clarify the scope and sense of deformation in the Alpine orogeny stage. Strike-slip planes are dominated in the studied structures. Slickensides of reverse and thrust account for 25 % of all fractures. Deformations of various types are concentrated in the most plastic rocks and weak zones. The rest of the Upper Cretaceous and Cenozoic sediments have a gentle monoclinal bedding. It is shown that the part of the deformations in the rocks of the Tavrick series and of the rocks of Middle Jurassic—Lower Cretaceous age could occur during alpine period. Given geological data confirms compression structures for all the mountainous Crimea during Alpine stage. The orientation of the restored axes stress field shows that compression deformation is caused by pressure of the Black Sea microplate toward Crimea. The Strike-slip-type deformations, within the studied area, dominate over the reverse-type one. Folded-thrusted deformations are located more to the south — in the continental slope and shelf respectively, in the interaction area of the Black Sea microplate and the Crimea.