Jurassic Formations Research Articles

The origin and evolution of waters in the Jurassic formations of the Marginal Trough area (Poland)

The Permian–Mesozoic cover of the Western European Platform includes numerous aquifers containing mineralized water. An attempt has been made to determine both the genesis of water from the Jurassic formations of the Marginal Trough (tectonic unit in the Zechstein–Mesozoic structural floor, located in central Poland) and the processes to which they were subjected. The results of archival chemical analysis of waters from deep boreholes located in the Marginal Trough have been studied. Hydrogeochemical ratios of Na/Cl, Cl/Br, (SO 4 × 100)/Cl, HCO 3 /Cl and Na/(Ca + Mg), together with their variability depending on water depth, were presented and discussed. The majority of waters from the Jurassic formations are Na–Cl waters, typical for areas isolated from hypergene zones. Their mineralization is closely related to the depths of the Jurassic formations. The analyses of hydrochemical parameters of the Jurassic waters have shown that the vast majority of these waters are brines or chloride waters, genetically related to seawater. The diversity in mineralization and hydrochemical parameters, together with their dependence on the depth of occurrence of the analysed waters, indicate that waters from the Lower Jurassic formations originate from isolated geological structures, while the Middle and Upper Jurassic waters are/or were affected by infiltration waters.

Study of the low-frequency dispersion of permittivity and resistivity in tight rocks

The road to understanding the frequency dispersion (relaxation) of permittivity and resistivity in tight rocks remains relatively uncharted. Our team from Da'anzhai Group, Jurassic formation, Sichuan Basin carried out practical research to explore this phenomenon. The research was conducted under laboratory conditions for a selection of low frequencies, with ranges between 0.1Hz to 1kHz.Our research has shown that, although both the permittivity and resistivity decrease as the frequency increases, the two individual metrics display different behaviours when compared with each other. While the degree of resistivity variation is minimal, to the point that it is redundant, the permittivity, on the other hand, demonstrates something that is scientifically noteworthy. Permittivity has a distinctive dispersion degree across the entire sample of frequencies and the difference between the minimum and maximum frequencies is several orders of magnitude. An additional, and unexpected, learning from our research is that the level of frequency dispersion increases as the water saturation and concentration increases.In this paper, a collection of equations has been formulated to describe this relationship. These equations particularly shed light on the areas of rock porosity and saturation. They also show that the degree of frequency dispersion of permittivity or resistivity can be used as a function of water saturation and concentration.Two new variables are introduced here, DR and DC, to demonstrate the relaxation law quantitatively. In our practical research, we have characterised the relationship between the saturation and concentration with dielectric relaxation, using three different concentrations of DR and DC and five different saturations of NaCl solution. In difference to conventional Archie's multiple experimental parameters, we have established a new formula to derive the saturation from Rp and Cp, or from DR and DC directly. Two important frequencies were also further investigated for Cp dispersion: first is the critical frequency, which marks the dispersion speed change from steep phase to steady phase, and second is the zero-frequency, which marks the dispersion when it approaches zero. All tight rocks were measured under the same conditions, with the results displaying the same pattern of variations. The results have led us to believe that Cp's frequency dispersion at low-frequencies provides a new methodology to characterise tight rocks.

AJRM as basis for design and construction of more than 70 km of tunnels of the Railway Project Stuttgart‐Ulm

AbstractThe tunnels of the railway project Stuttgart‐Ulm are located in sedimentary rock of the Keuper Formation, the Black Jurassic, the Brown Jurassic and the White Jurassic formation. The two longest tunnels are excavated by tunnel boring machine over most of their length. The remaining tunnels, which the authors are involved in, are excavated by conventional tunnelling. All tunnels are designed on the basis of the AJRM‐method. This method is based on an anisotropic, elasto‐viscoplastic model for jointed rock and corresponding FE‐software, which has been extended to seepage and swelling. The method has been applied to practical projects for the last 40 years. By means of rock mechanical test programs and back‐analyses of projects in a variety of rocks, the reliability of the predictions is quite high and the application leads to safe and economic solutions.Special emphasis is given to tunnels located in swelling gypsum Keuper. The phenomena of swelling and leaching are described and a swelling model is outlined. The evaluation of the rock mechanical parameters and a method of analyses are explained. On this basis, the design principles and package of measures applied for the tunnels located in anhydrite‐bearing rock are described. Some important aspects with regards to tunnelling in leached Gypsum Keuper and in the other Keuper layers as well as the Black Jurassic are addressed. For the 8.8 km long Boßler Tunnel, located in the Brown and White Jurassic, for an approx. 900 m long section in the Aalenium 2 with a maximum overburden of 280 m squeezing rock conditions had to be anticipated. By means of a vertical exploration shaft and a connecting exploration gallery, an improvement of the predicted characteristic rock mechanical parameters could be justified, thus enabling TBM‐tunnelling also in this section, with an increase of the segment thickness from 45 to 65 cm. For the Steinbühltunnel located in White Jurassic, special emphasis was placed on karstic rocks. The corresponding measures are described.

Reservoir features and exploration potential of the 1st member of Shaximiao Formation of Middle Jurassic in central Sichuan Basin

Reservoir features and exploration potential of the 1st member of Shaximiao Formation of Middle Jurassic in central Sichuan Basin

Influence of natural fractures on gas accumulation in the Upper Triassic tight gas sandstones in the northwestern Sichuan Basin, China

The Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin, China, is a typical tight gas sandstone reservoir that contains natural fractures and has an average porosity of 1.10% and air permeability less than 0.1 md because of compaction and cementation. According to outcrops, cores and image logs, three types of natural fractures, namely, tectonic, diagenetic and overpressure-related fractures, have developed in the tight gas sandstones. The tectonic fractures include small faults, intraformational shear fractures and horizontal shear fractures, whereas the diagenetic fractures mainly include bed-parallel fractures. According to thin sections, the microfractures also include tectonic, diagenetic and overpressure-related microfractures. The diagenetic microfractures consist of transgranular, intragranular and grain-boundary fractures. Among these fractures, intraformational shear fractures, horizontal shear fractures and small faults are predominant and significant for fluid movement. Based on the Monte Carlo method, these intraformational shear fractures and horizontal shear fractures improve the reservoir porosity and permeability, thus serving as an important storage space and primary fluid-flow channels in the tight sandstones. The small faults may provide seepage channels in adjacent layers by cutting through layers. In addition, these intragranular and grain-boundary fractures increase the connectivity of the tight gas sandstones by linking tiny pores. The tectonic microfractures improve the seepage capability of the tight gas sandstones to some extent. Low-dip angle fractures are more abundant in the T3X3 member than in the T3X2 and T3X4 members. The fracture intensities of the sandstones in the T3X3 member are greater than those in the T3X2 and T3X4 members. The fracture intensities do not always decrease with increasing bed thickness for the tight sandstones. When the bed thickness of the tight sandstones is less than 1.0 m, the fracture intensities increase with increasing bed thickness in the T3X3 member. Fluid inclusion evidence and burial history analysis indicate that the tectonic fractures developed over three periods. The first period was at the end of the Triassic to the Early Jurassic. The tectonic fractures developed during oil generation but before the matrix's porosity and permeability reduced, which suggests that these tectonic fractures could provide seepage channels for oil migration and accumulation. The second period was at the end of the Cretaceous after the matrix's porosity and permeability reduced but during peak gas generation, which indicates that gas mainly migrated and accumulated in the tectonic fractures. The third period was at the end of the Eogene to the Early Neogene. The tectonic fractures could provide seepage channels for secondary gas migration and accumulation from the Upper Triassic Xujiahe Formation into the overlying Jurassic Formation.

Organic geochemistry and hydrocarbon potential of source rocks from the Mohe Formation of the Upper Jurassic in the Mohe Basin, northeast China

The Mohe Formation is one of the important source rocks for oil-gas and gas hydrates in the Mohe Basin. In this paper, mudstone from cores of the M-1 and M-3 wells and argillite from cores of the MK-2 well of the Mohe Basin were studied. The depositional environment, organic matter source, thermal maturity, kerogen type, and hydrocarbon generation potential of the core samples were determined using organic and inorganic geochemistry. The dual sources of organic matter were terrestrial plants and aquatic organisms. The organic matter was deposited in a reducing lacustrine environment. The lake water salinity was higher when argillite was deposited. The late stage of the sedimentary filling of the lacustrine basin was characterized by high sulfate contents. The source rocks primarily contained Type II2–III kerogen. The present-day thermal maturity parameters reveal that the organic matter of the Mohe Formation is mature to overmature, indicating that this unit is capable of generating gas condensate and dry gas. In addition, we compared the organic geochemistry characteristics of mudstone and argillite. The results indicate that TOC, S1 + S2, and HI are lower in argillite than in mudstone. Although the biomarker maturity parameters cannot reflect the thermal evolution of the source rocks, they reflect the source of the organic matter and the depositional environment. The results provide important constraints for the evaluation of the hydrocarbon potential of lacustrine source rocks of the Mohe Formation from the Upper Jurassic and for the location of petroleum resources in the Mohe Basin.

Geological map of the partially dolomitized Jurassic succession exposed in the core of the Montagna dei Fiori Anticline, Central Apennines, Italy

In this contribution, we present a new geological map of the Jurassic formations exposed in the central sector of the Montagna dei Fiori anticline, in the Central Apennines. Mapping activity was devoted to better detail the extensional fault patterns still preserved in the anticlinal core and to separate dolostones from the parent limestones. The results of our work indicate that the lower half of the Corniola Fm. is systematically dolomitized in the study area. On the other hand, the underlying Calcare Massiccio Fm. is strongly dolomitized in the Castel Manfrino and Osso Caprino hill area, whereas limestone dominates to the east and to the south. Our new map supports block faulting induced by Jurassic rifting as the cause of the Calcare Massiccio Fm. map pattern, rather than gravity-driven block collapse, as alternatively proposed.

3D stratigraphic forward numerical modelling approach for prediction of organic-rich deposits and their heterogeneities

The source rock characteristics (e.g. thickness, lateral extension, richness) are known to be highly variable in both time and space. The Lower Jurassic formations of north-western Europe contain source rocks with organic-rich intervals showing different characteristics from one region to another: the Paris Basin differs from the South-East Basin of France, but organic content and hydrogen index also vary within a single basin. During the Early Jurassic, the overall depositional environment of north-western Europe corresponded to an epicontinental domain at the western extremity of the Tethys Ocean. The early transgressive phase of the Jurassic induced flooding of this European realm. Because of the evolution of the connections and threshold of the European basins and their associated sedimentary settings, this domain occupied a key position for the deposition of organic-rich layers. Using a forward modelling approach, we aim to predict the heterogeneous characteristics of such sediments. It is widely accepted that primary productivity and preservation are key factors favouring the accumulation of organic-rich layers. However, the roles of these factors remain to be assessed and the processes leading to accumulation and preservation need to be quantified.We focus on the Lower Jurassic (Hettangian to Toarcian) in France, since this provides a vast amount of data to calibrate the 100-kyr time step model in terms of an accurate framework including paleogeography, stratigraphy, lithology, palaeontology, mineralogy and geochemistry.Results show that favourable conditions for the deposition of organic-rich sediment can occur even with moderate primary productivity and without requiring anoxic conditions. These windows are firstly dependent on internal factors directly linked to the basin morphology, such as sedimentation rate, bathymetry and distance from the coastline. We do not need to evoke external factors such as oceanic anoxic events or enhancement of primary production to account for the deposition of organic-rich sediments.

Hydrogeochemical Modelling of Forming of Geothermal Waters in the Synclinorium of Łódź

The Synclinorium of Łódź, is one of the most prospective areas of geothermal water in central part of Poland. Currently there are two geothermal centers in this region in Uniejów and Poddębice. Moreover, the use of geothermal energy projects have been developed for Łódź, Poddębice, Kleszczów, Piotrków Trybunalski, Radom, Zduńska Wola and Rogóźno. Geothermal water in the Synclinorium of Łódź may be used for the purpose of balneology, recreaction and heating. This unit has an asymmetric structure, which comprises permian-mesosoic sediments placed on the older rocks. First of the all thermal waters are accumulated in the Lower Cretaceous, where the temperature is more than 700C and Lower Jurassic formations with a temperature of about 1000C. Hydrogeochemical zonation of the study area is characterized by the increase of water mineralization and the water chemical type change: HCO3-Ca→ HCO3-Na→Na-Cl→ Cl-Na. All the waters studied have total dissolved solids TDS from 0,2 to 157 g/dm3. Minerals regulating the hydrogeochemical equilibrium in thermal waters in the basin of Łódź are principally quartz and calcite. The hydrogeochemical modelling study of these thermal waters can be explained by the polygenetic origin. In general, the chemical composition of thermal waters in this region is determined by the interaction between the groundwaters (meteoric waters of modern hydrological cycle), the rock environment and the brines from leaching chloride salt ions. The process may depend on pH, mineralization and the occcurrence of other ions and temperature.

Application of Microbial Geochemical Exploration Technology in Identifying Hydrocarbon Potential of Stratigraphic Traps in Junggar Basin, China

Even though three-dimension seismic has been widely applied in the eastern slope of Fukang sag with several fluvial channel sand bodies identified as in Jurassic Toutunhe group, the hydrocarbon charge of these stratigraphic traps is hard to predict and still of great uncertainty. Microbial geochemical exploration technology (MGCE), which detects the microbial and geochemical anomalies in surface soil, was applied to delineate three hydrocarbon play fairways, identify six favorable prospects and to predict the geochemical properties of the reservoirs at depth. The results of MGCE survey agreed well with the drilling results of 10 wells and demonstrated the applicability and effectiveness of MGCE in the exploration of stratigraphic traps in this area. Furthermore, the integration of geology, geophysics, geochemistry and geo-microbiology, so-called 4G exploration, led to the discovery of the Qigu group as a new hydrocarbon-bearing play in late Jurassic formation in this area. MGCE technology and 4G integrated exploration has not only made the breakthrough of Qigu discovery, but also opened a new model for stratigraphic reservoir exploration in the area.

Perspectives of using the geological subsurface for hydrogen storage in Poland

The increasing share of renewable energy in electricity production in Poland is part of the growing interest in the hydrogen underground storage. In the search for suitable locations for this purpose it is useful to have the knowledge on the geological structures for underground storage of gases, including carbon dioxide.This article presents the possibilities of geological storage of hydrogen in underground geological structures in Poland (salt caverns, depleted oil and natural gas fields, and deeply seated aquifers). The study results confirm that there are favorable conditions to achieve this goal – in many areas of Poland it is possible to realize some type of hydrogen underground storage.The paper presents prospective areas for different options of hydrogen underground storage at specified conditions. For locations in salt caverns, rock salt deposits from the following areas are proposed: Łeba Elevation, Fore-Sudetic Monocline, Carpathian Foredeep, and salt stocks in the Polish Lowlands. For depleted oil and natural gas fields in the Polish Lowlands, Carpathians and Carpathian Foredeep, a few tens of fields are indicated, most of them are natural gas deposits. In the Polish Lowlands, several anticlinal structures in the Lower Cretaceous and Lower Jurassic formations have been selected. These pre-designated locations may be the basis for further work and detailed considerations in order to find the best and most appropriate locations intended for hydrogen underground storage.

The use of seismic interpretations to discover and evaluate the new hydrocarbon prospects of Jurassic formations in JG field, NEAG concession, North Western Desert

Khatatba Formation is Middle Jurassic and act as a completely hydrocarbon system in the Northeast Abu Gharadig basin (NEAG) especially in JG field. This study is an attempt to discover and evaluate the new hydrocarbon prospects in Khatatba Formation and define the nature of the source rock in this investigation area. This study was done by interpreting forty (40) 3D seismic sections in addition eight wells drilled in this area. The geochemical reports of many samples from JG-1 well have been carried out to identify the maturity of the source rock. Four horizons are interpreted from this study and represented as two way time (TWT) and depth structure contour maps. These horizons are arranged from the top to the bottom as; top Masajid, top Khatatba, Top Kabrit and top lower Safa. The lower Safa sand member in JG1-ST well act as the main reservoir in Khatatba Formation that is characterized by high porosity 22%, low GR and SW is 12% that is led to the presence of oil zone. The lower Safa depth is ranges between 3120m in the southern part of the JG field and gradually increases toward the northwest to 3580m and to 3680m in the northeast part of the study area. Two main faults are interpreted in the NE-SW and in NW-SE directions and form a pattern of horst and graben blocks. Four and three-way dip structural closures (A, B, C and D) are representing the fruitful locations for high production and new prospect wells in the JG oil field. The Khatatba Formation consists of organic-rich shales, revealing mainly type II-III and type III kerogen and TOC content ranges from 2.7 to 4.98wt%. Vitrinite reflectance data indicate the mature to over mature stages of oil window for significant hydrocarbon generation. The overburden rocks of Masajid carbonates perform sufficient levels of thermal maturity for hydrocarbon generation.

Geological framework and mineral occurrences in the Georgia segment Eastern Greater Caucasus

The eastern part of the Caucasus orogen, termed the Georgian segment, is mainly underlain by highly deformed Lower-Middle Jurassic shales, sandstones, and volcaniclastic rocks, and cut by numerous intrusive bodies of various compositions. All units contain a variety of mineral deposit types and their surrounding zones of hydrothermal alteration. Detailed geological and metallogenic research, after 30 years stagnation, has led to important new information regarding the evolution of this region. Three main stages of magmatic activity are now clearly distinguished. The oldest magmatism comprises early Jurassic events, related to extensional tectonism, with volcanism and subvolcanic plutonism that ranged from rhyolitic, through dacitic and andesitic, and to basaltic. Additional extensional processes (in Bajocian times) were characterized by intrusion of a gabbro and diorite dike system into slightly older late Early Jurassicearly Middle Jurassic formations. The final magmatic event was associated with Middle Jurassic folding and uplift during which multiphase diorite plutons were emplaced. Intense hydrothermal activity related to this latest magmatism was responisble for important polymetallic mineralization including more than 100 historically recognized outcropping occurrences. A detailed study of 11of the more significant of these occurrences has indicated anomalous concentrations of base metals as well as gold, thorium, yttrium, cobalt, cadmium, and bismuth. In addition, several new and potentially significant mineral occurrences were discovered, including two designated here as Gelia and Lechuri.

Geochemistry characteristics and significance of two petroleum systems near top overpressured surface in central Junggar Basin, NW China

Most hydrocarbons discovered in the central Junggar Basin are endowed in limited areas below and above top overpressured surface. By studying geochemical and distribution characteristics of hydrocarbons in reservoirs near top overpressured surface, it was found that the lacustrine dark mudstone in Middle-Lower Permian and coal measures in Jurassic are the major source rocks. Crude oils from these two sets of source rocks have apparently different geochemical indexes. Their maturity values are 0.7–1.0% (higher in Permian). The natural gas is mainly coal-derived wet gas from Jurassic formation, with general maturity of 0.9–1.0%. Hydrocarbons in tectonic units have different distribution features depending on sources. Crude oil above top overpressured surface in western Well Pen 1 Sag is from both Middle and Lower Permian, while crude oil in overpressure compartment is mainly from Middle Permian, and natural gas is from hybrid sources. Crude oil near top overpressured surface in western Changji Sag is mainly from Middle Permian, including some oil from hybrid sources; the natural gas is mainly coal-derived wet gas. Hydrocarbons in northern Dongdaohaizi Sag and eastern Changji Sag are mainly coal-derived oil and gas in overpressure compartment. Hydrocarbons in the central Junggar Basin underwent two stages of accumulation, and experienced two large-scale chargings at the later stage. To summarize geochemical and distribution characteristics of hydrocarbons in reservoirs near top overpressured surface in the central Junggar Basin can provide reference for understanding hydrocarbon accumulation regularity and predicting favorable exploration targets near top overpressured surface in this region and similar high overpressure and hydrocarbon generation basins.

A preliminary spatial-temporal study of some soil characteristics in the calcareous massif of Sicó, Portugal.

The mountainous massif of Sicó, in the centre of Portugal, is an extensive area composed of calcareous Jurassic formations. Hillside calcareous soils, with high pH, present chemical restrictions to support plant growth and are subjected to important erosion processes leading to their degradation if not protected by vegetation. In a first year of study some soil physicochemical characteristics have been measured in some geo-referenced locations of a larger design experiment and an exploratory spatial analysis has been performed. The objective of this study was to present some suggestions in order to give sustainable phosphorus fertiliser recommendations aiming to establish pastures in these soils and thus support traditional livestock activity. Ten years apart, those soil characteristics have been measured again in the same locations and comparisions have been made. The objective was to understand the variability of the soil properties under study in order to better adequate the fertiliser soil management regarding the area restoration.

Research on Dinoasur Footprints in Zizhou, Shaanxi Province, China

AbstractIn recent years, the discoveries of dinosaur footprints have been successively reported from Dianshi Town, Zizhou City, Shaanxi Province. The footprints include the tracks of theropod, ornithopod and sauropod as well. Actually, the dinosaur footprints were found by local inhabitants much earlier in history, but not for science. The slabs bearing the dinosaur footprints were collected for domestic use, such as building stones, millstones, cellar covers, sheepfold fences, windlass holders, etc. This paper is to describe the dinosaur footprints on both sides of three slabs used for cellar covers, sheepfold fences and windlass holders by the local people. 24 dinosaur footprints and 4 trackways have been recognized and all of them belong to theropod. Four kinds of dinosaur footprints are identified, including 1 new ichnogenus and 2 new ichnospecies: (1) Shanbeipus caudatus ichnogen. et ichnosp. nov.; (2) Pengxianpus yulinensis ichnosp. nov.; (3) Eubrontes ichnosp; (4) Kayentapus ichnosp. Dinosaur‐footprint‐bearing beds were initially identified as the Fuxian Formation of the Lower Jurassic. Multipal dinosaur footprints associated with fragment plant fossils suggest a humid fluvio‐lacustrine environment setting in the Ordos Basin during the early period of the Jurassic.

Sedimentary Petrology Characteristics and Their Implications for Provenance of Flowerpot Basin Jurassic System in Yanqing County, Beijing

The characteristics of sedimentary petrology are significant in determining geological provenance. By using field outcrop observation and indoor identification of thin sections, the characteristics of Jurassic system Tuchengzi Formation clastic rocks exposed in the Beijing Yanqing “flowerpot” basin, which include rock types, clastic composition and cement type, etc. have been analyzed systematically to identify the provenance direction and tectonic setting and nature of the provenance. Research shows that the sedimentary rocks in this area belong to analluvial fan and braided river sedimentary rock system and include mainly conglomerates, lithic sandstone, and feldspathic lithic sandstone. The main compositions of the conglomerate are dolomite and limestone. Nearly 50% to 70% of the sandstones are mainly polycrystalline quartz; 10% to 15% are feldspar; and 40% to 55% are composed of debris from mainly magmatic and metamorphic rocks. A Dickinson triangular diagram indicates that the provenance type in this area is recycled orogny evolving from clastic recycled orogeny such as a collisional orogenic belt to a transitional recycled orogeny. On the basis of the geographic position and tectonic setting, three types of provenance can be identified in the Jurassic system Tuchengzi Formation. The far source provenances are Archean erathem basement metamorphic rocks in the northern basin and granite or adamellite that intruded rocks of the Late Jurassic epoch of the Yanshanian period in the northeast. The nearby provenance is the Mesoproterozoic erathem Changchengian system and Jixianianian system dolomite in both the east and west.

The presence of geogenically saline waters in the area of salt dome Rogóźno (central Poland)

Rogoźno salt dome is one of the most recognizable diapirs in the Polish Lowland. Despite its shallow occurrence depth, increased salinity has not been detected in surface water. The presented hydrogeochemical research aimed at determining if there is the presence of increased geogenical salinity in the groundwater and shallow water. Increased geogenical salinity has not been detected in the Moszczenica River and the Czerniawka River, but were displayed in the Moszczenica River valley in the specific electrical conductivity (SEC) spatial distribution. Hydrogeochemical research indicated the presence of a higher mineralized water ascent in the north-eastern part of the dome. The presence of mixing of saline waters around the dome with fresh waters was determined. Ionic composition and hydrochemical indexes of waters sampled from different levels indicate that the saline structure is not completely isolated from surrounding waters. The dome structure is in contact with water from Upper Jurassic formation which influences the final water composition. Active anthropogenic processes partly mask the influences of geogenic salinity in the study area.

Integrating Core and Wireline Log Data to Evaluate Porosity of Jurassic Formations of Injra-1 and Nuryal-2 Wells, Western Potwar, Pakistan

Abstract: Petrophysical properties of rocks can be better understood by core and well log analysis. Exact porosity estimation in the Potwar sub-basin of Pakistan is one of the main issues because of complex tectonic settings. Core and wireline log data of two wells: Injra–1 and Nuryal–2 drilled in the study area are used for the porosity evaluation in the Jurassic reservoirs. The log interpretation and laboratory analysis show that the Datta Formation of the early Jurassic is mainly sandstone with minor shaly beds. A workflow is developed to correlate the calculated porosity from logs with the measured porosity from core plug samples. The results show that log porosity and core porosity of Datta sandstone are comparable. Porosity in the younger formations is higher than in the older formations, which is an indication of presence of secondary porosity and structurally deformed fractures in sandstone. The Datta sandstone encountered in Nuryal–2 is about 20–30 % less porous than in Injra–1 well.

Middle–Late Mesozoic sedimentary provenances of the Luxi and Jiaolai areas: Implications for tectonic evolution of the North China Block

Provenances of sedimentary rocks may provide important constraints on the tectonic evolution of the North China Block (NCB). Previous studies have demonstrated that the northern NCB (NNCB) and the Xing-Meng orogenic belt (XMOB) supplied massive detritus southward into the hinterland of the NCB during the Jurassic. In order to study the evolution of sedimentary provenance during the Middle–Late Mesozoic, U–Pb geochronology and Hf isotopic geochemistry of detrital zircon grains and chemical compositions of detrital garnets from sandstones in the Luxi and Jiaolai areas, eastern NCB, were analyzed in combination with published data on the Jurassic sandstones. The Late Paleozoic–Mesozoic (367–139Ma) zircons in the lowermost Cretaceous Mengyin Formation samples from the Luxi area show εHf(t) values of −15.3 to −3.2 and +1.3 to +10.0, which are very similar to the results of analyses of the Jurassic formations. Further, the increased amount of Mesozoic zircons and granulite-derived garnets in the Mengyin Formation samples, compared to those in the Jurassic samples, indicates there was more detritus supply from the NNCB than from the XMOB. In the overlying Qingshan Formation samples, zircon grains do not exhibit Paleozoic ages, but most of them have Early Cretaceous ages and negative εHf(t) values, which are similar to the zircon grains extracted from the widespread Early Cretaceous igneous rocks in the NCB. This suggests that the provenance might have changed to a locally derived source. In contrast, the zircon population of the Early Cretaceous sandstones from the Jiaolai basin is dominated by grains of mid-Neoproterozoic age (700–900Ma) which signifies contribution from the Sulu orogen. Moreover, the detrital garnet assemblages of sandstones in the Luxi area are not consistent with those from representative metamorphic rocks in the Sulu orogen. The above results seem to confirm that the Mesozoic sedimentary provenance of the Luxi area had no evident connection with the Sulu orogen. Instead, the differential uplifting and exhumation of deep crustal rocks in the NNCB and the XMOB, associated with closure of the Mongol–Okhotsk Ocean, might exert a dominant control on sedimentary provenances in the wide hinterland of the NCB from the Jurassic period to ∼136Ma (the upper boundary age of the Mengyin Formation).