Gray Mudstone Research Articles

Sedimentary dynamics and depositional model for mud accumulation in deep lake basins: A case study in the Upper Triassic Chang-7 Member, Ordos Basin, northern China

Shales in deep lake basins have become the main focus of continental shale oil and gas exploration. In order to highlight the sedimentary dynamics of mud deposition in deep lake basins, a combination of core observation, thin section examination, X-ray diffraction, and QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy) was used to analyze the depositional characteristics of mudrocks in the Chang-7 Member from the Yanchang Formation (Upper Triassic) in Ordos Basin, and to establish a depositional model for mud accumulation in deep lake basins. This study recognizes four mudrock lithofacies in the Chang-7 Member: (1) the laminated silt-bearing mudstone, which generally develops a binary composition of “silt–clay” or a ternary composition of “silt–clay–organic matter”; (2) the graded mudstone, mainly composed of dark gray and gray-black mudstone sandwiched by silt-bearing mudstone; (3) the massive mudstone, internally showing a uniform distribution of quartz, clay, and carbonate minerals, with also a small amount of organic detritus; and (4) the laminated shale, which is generally composed of clay laminae, and organic laminae of the former two. Sediment supply, topographic slope, and flood intensity combine to control the evolution of gravity flows and the transport and deposition of the mudrock in the Chang-7 Member. The influence of orogeny provides terrain gradient, water depth, abundant sediments at source areas, and triggering mechanism for the formation of gravity flows. Floods triggered by wetting events provide the impetus for sediment transport. Mud deposition in the Chang-7 Member was mainly related to the transport and sedimentation of mud by hyperpycnal flows and rapid sedimentation by buoyant plume flocculation. A comprehensive evolutionary model for shale accumulation in the deep lake basin is established by integrating various triggering mechanisms and mud transport sedimentary processes.

Mineralogical and geochemical characteristics of mudstones from the Lower Cretaceous Prosopis Formation in the Bongor basin, Chad: Implications for provenance, paleoenvironment and organic matter enrichment

The Cretaceous greenhouse represents the largest global warming event since the Phanerozoic, significantly affecting the provenance, weathering, regional climate, and paleoenvironment of lake basins globally. During the period of global warming, the control of regional paleoenvironment over organic matter enrichment is still unclear. The Lower Cretaceous Prosopis Formation in the Bongor Basin features thick lacustrine organic-rich mudstones as major effective hydrocarbon source rocks with excellent hydrocarbon potential in the basin. However, the provenance, paleoclimate, paleoenvironment and mechanism of organic matter accumulation in this location have not yet been systematically studied. In this study, 22 dark gray mudstone samples were collected from three cored wells. Clay mineralogy, along with organic and inorganic elemental geochemical analyses were jointly carried out to determine the characteristics of the provenance, paleoclimate and paleoenvironment as well as the major controlling factors for organic matter enrichment. The results imply that the Prosopis Formation mudstones are mainly composed of clay minerals such as illite, kaolinite, chlorite, and illite/smectite mixed layers (I/S). Organic geochemistry indicated that all the studied mudstones reached the standard of good to excellent source rocks (the total organic carbon content ranged from 1.27% to 7.41%). Inorganic elemental geochemistry is characterized by high enrichment of Fe, Mg, P, Mn, Ti, Na, Ca, V, Co, Cu, Sr, Ba and B relative to UCC and PAAS, whereas rare earth elements (REEs) show enrichment of light rare earth elements (LREEs) and a deficiency of heavy rare earth elements (HREEs) relative to chondrite. The source composition of the mudstones is predominantly intermediate-felsic igneous rocks that have not undergone sediment recycling. The parent rocks experienced weak to moderate weathering, corresponding to semiarid paleoclimatic conditions of transitional nature. The organic-rich mudstones were deposited in fresh to brackish waters with paleoredox conditions ranging from weakly oxic to anoxic semi-deep to deep lake environments having relatively low initial productivity. Comprehensive study indicates that paleoredox and paleoclimate are important parameters affecting the enrichment of organic matter in lake basins during global warming. The aridification of climate is one of the most important reasons for the high input and efficient preservation of organic matter. The relatively semiarid climate both increased the supply of terrigenous nutrients to the lake basins and provided a driving force for the increase in salinity, thereby providing better sources and preservation conditions for organic matter.

Lacustrine environment evolution in the Mesozoic North Yellow Sea Basin, eastern China: Insight from the transition of Jurassic grey mudstones to Cretaceous red successions

During the Cretaceous, terrestrial red beds appear on a global scale. Previous reports have suggested that these red beds were intricately linked to hot paleoclimates and unique paleogeographic configurations. This study focuses on petrological, mineralogical, and geochemical analyses conducted on samples that capture the transition from grey to red mudstone successions dating from the Middle Jurassic to Early Cretaceous within the North Yellow Sea Basin, with a focus to investigate the evolution of lacustrine environments and paleoclimates during their deposition. The findings revealed four episodes of hydrothermal activity occurring during the deposition of the Jurassic grey to Cretaceous red mudstone successions. Some mudstone samples have identified positive Eu anomalies, but the hydrothermal signal may not be as strong due to the open lake environment. Hf vs La/Th and ∑REE vs La/Yb graphs indicate that most grey mudstones originated primarily from upper crustal felsic sources, while certain red mudstones contain mantle material, and the provenance of both has changed significantly. Noteworthy indicators like Sr/Ba, B/Ga, U/Th, Ni/Co, V/Cr and V/(V + Ni) values denote that the sedimentary environments of both grey and red mudstones were oxidized, deviating from conventional viewpoints. However, this deviation may stem from hydrothermal contamination impacting trace elements such as Ni, Co, and Sr. Assessment using indicators like Rb/Sr and Sr/Cu reflects a transition from a warm and wet Jurassic climate to a hot Early Cretaceous climate. Variations in lake levels, prompted by climate fluctuations, led to increased oxygenation on the lake bottoms, resulting in the deposition of red mudstones. These findings suggest a close correlation between the red mudstones and the hot climate, driven by the tectonic evolution of Eastern China. The transformation from grey to red mudstones during the Middle Jurassic to Early Cretaceous periods signifies a response to paleoclimatic and paleoenvironmental changes in East Asia.

Characterizing mudstones and fault structure in Bumita Quarry, Perlis through integrated geophysical techniques

The parameters of seismic refraction and electrical resistivity play a crucial role in geological studies, as they provide insights into the composition of rocks or soil beneath the Earth’s surface. Previous researchers have established reference ranges for seismic velocity and resistivity values based on different rock types, which are presented in tabulated forms. However, the wide variability in these values sometimes leads to challenges in interpretation due to overlapping ranges. In the region of Perlis, Malaysia, a comprehensive geophysical investigation involving seismic refraction and electrical resistivity methods was conducted within the Chepor Member of the Kubang Pasu Formation at the Bumita Quarry and Utan Aji. The results were then correlated with porosity and permeability data. The Chepor Member primarily comprises both red mudstone and grey mudstone. Interestingly, the seismic velocities of these two mudstone types are quite similar, differing only by a small margin of approximately 200 m/s. The resistivity method employed utilized a pole-dipole array configuration. In terms of resistivity values, the red mudstone exhibited lower readings (ranging from 15 to 100 Ωm) compared to the grey mudstone (ranging from 120 to 500 Ωm). Assessing porosity, the red mudstone displayed a value of 0.95%, alongside a permeability of 5.58×10-5 μd, while the grey mudstone indicated a slightly higher porosity value of 1.9% and a permeability of 2.06×10-5 μd. Consequently, the study successfully established seismic velocity and resistivity benchmarks for the mudstones within the Chepor Member geological unit.

STRUCTURAL POSITION AND DISTRIBUTION OF THE LOWER CRETACEOUS BLACK SHALE DEPOSITS WITHIN THE SILESIAN NAPPE OF THE UKRAINIAN CARPATHIANS

The article presents the results of studying the structural position and distribution of potentially oil-generating, organic-enriched Lower Cretaceous black shale deposits of the Shypot Formation, developed within the Silesian Nappe, using the methods of geological mapping and structural analysis. We concluded, that these deposits, which are exposed in the Vicha River basin near the town of Volovets among a continuous field of Oligocene flysch, are the part of the stratigraphic succession of the Silesian Nappe, and they are not a large olistolith or an allochthonous tectonic remnant of another, more internal nappe. Shypot Formation fills a large (1 × 3 km) subvertical tectonic lens. The rocks of the Shypot Formation are represented here by medium-rhythmic flysch: sandstone beds alternating with dark gray and black foliated mudstone (black shale) layers. They are lithified products of both medium-density turbidite flows and hemipelagic deposition of clayey material with a significant amount of organic matter. In the contact zone of the tectonic lens with the surrounding rocks, the deposits are intensively deformed, almost isoclinal folds with subvetrical hinges are observed, which suggest the strike-slip movements. A tectonic mélange is recorded here, which is represented by small blocks (clastolites) of brittle sandstones, sometimes similar to Shypot sandstones, placed in a silt-pelitic ductile matrix. Foliation of the mélange matrix is also subvertical and close to the meridional and/or subcarpathian direction. The structural studies carried out show that in the zone of this contact there were compressive stresses perpendicular to the Carpathian orogen, and stress fields that caused strike-slip movements, most likely right-lateral along the Carpathian thrusts. The tectonic lens is located within the broad Latorytsa-Stryi shear zone, and probably extruded out from deep horizons by transpressive movements (compression and strike-slip) and forms a “positive flower structure”. The vertical foliation in the mélange suggests that small blocks of Shypot sandstones were extruded up together with the ductile mélange matrix during diapirism. Similar processes probably led to the rise of a large lens of Shypot sandstones as the positive flower structure. The studies suggests that the black shale deposits of the Shypot Formation, enriched with organic matter, are widespread at some depth within the Silesian Nappe, and in some places are pushed up to the level of the Oligocene flysch. As a result, the Shypot Formation and Oligocene flysch are now in direct contact along faults/mélange zones. The presence of Lower Cretaceous potentially oil- and gas-generating black shale deposits within the Silesian Nappe significantly increases the prospects for oil and gas exploration in this area.

Mechanical properties of rock under uniaxial compression tests of different control modes and loading rates

To study the influence of control mode and loading rate on mechanical property of rock, uniaxial compression tests of four types of rocks (gray sandstone, red sandstone, mudstone and granite) are carried out under axial strain control mode and lateral strain control mode respectively. The characteristics of complete stress and strain curves, strength, brittleness and failure modes are analyzed. The results show that control mode has little influence on the pre-peak deformation, stress thresholds, while it has a greater impact on post-peak stress and strain curve, which makes the post-peak deformation stable and controllable, and shows the feature of Class II behavior. With lateral loading rates decrease, post-peak stress and strain curves appear more and more obvious fluctuations in the post-peak stage, and the time required for rock failure increases sharply, but the lateral control rate has little effect on the brittleness of rock. The failure mode of rock samples under axial strain control mode is mainly splitting failure, while that under lateral strain control is gradually changed to shear failure. The smaller the lateral loading control rate is, the more obvious the characteristics of shear failure is. Compared with uniaxial compression tests, under high confining pressure, the lateral dilation deformation is restricted, so peak strength is larger and stress redistribution can be better adjusted and stress fluctuation reduced accordingly in post-peak stage. The research results are an effective supplement to rate-dependent property of rocks and can provide some reference for deformation and strength characteristics research of brittle rock under lateral control mode.

Response to palaeoclimate by Early Cretaceous terrestrial organic-rich shales in the Yin'e Basin: Evidence from sporopollen, n-alkanes and their compound carbon isotopes

The Cretaceous period is a typical representative of a ‘greenhouse climate’ in geological periods, and is also an important period for the formation of hydrocarbon source rocks, which has become a hot spot in the study of palaeoclimate and organic matter enrichment. In order to further explore the mechanism of action between paleoclimate and organic matter enrichment, the response mechanism of organic matter enrichment to palaeotemperature and palaeorainfall is finely characterised by synthesising the analysis of sporopollen, biomarkers and n-alkane carbon isotopes in this study. The sporopollen indicates that the palaeoclimate is generally temperate, semiarid to semihumid, and the n-alkane and n-alkane carbon isotopes indicate that the palaeoclimate and palaeorainfall have obvious fluctuations and synchronous characteristics of rainfall and heat. Based on the differences in palaeotemperature and palaeorainfall, the palaeoclimate can be further divided into three stages: cool-temperate and semiarid, warm and semihumid, and cool-temperate and semiarid. The trend of palaeoatmospheric pCO2 recovered from the carbon isotopes of normal alkanes C27, C29, and C31 is roughly the same as that of palaeotemperature, indicating that pCO2 is important in regulating palaeotemperature. According to the distribution pattern of organic carbon, different layers are divided into three sedimentary Units: I, II and III. Different sedimentary Units have good corresponding relationships with different palaeoclimatic stages. Under the cool-temperate and semiarid climate, lower palaeotemperature and higher palaeosalinity are not conducive to the prosperity of lake organisms. The formation of light grey mudstone, silty mudstone and carbonate has lower TOC, and the organic matter types are mostly II1 and II. Under a warm and semihumid climate, higher palaeotemperature and moderate palaeorainfall are conducive to the prosperity of lake organisms. The formation of oil shale and shale has higher TOC, and the organic matter types are mostly I and II1. However, excessive palaeorainfall brings much terrigenous debris, leading to the organic matter dilution. Therefore, palaeoclimate and palaeorainfall not only affect the abundance of organic matter, but also affect the types of organic matter. In the paper, the mechanism of organic matter enrichment is improved by delicately characterising the palaeotemperature and palaeorainfall.

Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift

Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.

Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco

The Ocean Anoxic Event II (OAE2) was a significant global event associated with a positive carbon isotope excursion, occurring from the Late Cenomanian to the Early Turonian (C/T). It has been commonly associated with enhanced organic carbon preservation. Carbon isotopic analysis carried out over the C/T interval in the Agadir Basin, Morocco, integrated with previous biostratigraphic data, allows a refined correlation of the OAE2 interval. Detailed sedimentary facies analysis identifies ten lithofacies that record the transition from shallow marine environments in the Late Cenomanian to relatively deeper conditions in the Early Turonian. The variation in lithofacies can be correlated to relative sea level changes that show a correlation to the global eustatic curve.The OAE2 interval comprises dark grey mudstones beds that display low total organic carbon (TOC) values. Trace element and facies analysis suggest dilution from high detrital influx, along with oxic water conditions and low productivity. OM-rich black mudstones are identified in post-OAE2 Early Turonian strata. Trace element analysis suggests this increase in organic matter accumulation was related to increased sea surface productivity and oxygen-depleted bottom water conditions, which facilitated organic matter preservation. Deposition of OM-rich black mudstones is widely reported during the global Early Turonian marine transgression, suggesting the very high sea level was a major control on organic matter generation and preservation.

Alternations of open and closed lakes in the Lower Aptian Codó Formation (Parnaíba Basin, Brazil)

Lakes can be classified as open, semi-closed, and closed based on the balance between accommodation space and sediment–water fill. Different lake types can coexist in adjacent basins and rapidly change throughout time from one type to another. Understanding the processes that result from shifts in a lake's hydrological properties can be challenging, as well as characterizing its sedimentary products. The Aptian Codó Formation, in the Parnaíba Basin, north–northeast Brazil, contains in its record a complex facies succession deposited in a continental lacustrine system. In order to better understand the depositional aspects of different lake types, we characterized the lower portion of the Codó Formation using a multi-proxy approach. We combined litho- and microfacies analysis, ichnology, bulk chemical data, mineralogy, isotopic geochemistry, and organic matter analysis in the recently drilled, fully recovered borehole, 2-CO-1-MA. We defined eight lithofacies: bioturbated siltstones, dark gray mudstones, laminites, stratiform and domal stromatolites, heterolites rich in ostracods and organic muddy, and anhydrite and gypsum rocks. The isotopic data and the palynofacies analysis pointed out the existence of restricted conditions (TOC up to 19.6 wt%) with intermittent connections with seawater. Integrating all techniques, we interpreted five depositional stages: expanded lake, brackish ephemeral lake, perennial shallow lake, evaporitic sabkha, and stratified lake. These stages were influenced by the relation between inflow and evaporation rate, which were the main controls on sedimentation in the lacustrine-sabkha system studied. Our findings provide a detailed description and interpretation of the paleoenvironmental conditions related to the alternation from open to closed lakes, combining sedimentological, geochemical, and palynological characteristics of an inland lake system.

Genesis of Pyrite in Clastic Rocks of Deep Salt-Related Strata in the Simao Basin and Its Implication for Potash Mineralization: A Case Study of the Well MK-3

In depth of the Simao Basin (2390 to 2650 m depth interval), many gray mudstone or carbonate rocks are developed in the red salt-related strata, and pyrite crystals are found in the fissure and matrix. In this study, petrology, mineralogy, element geochemistry, and LA-MC-ICP-MS in situ sulfur isotope analysis were used to constrain the genesis of pyrite, and the influence of relevant geological activities on potash mineralization was discussed. The results show that: (1) In the upper part of the salt layer, particle size of the pyrite ranges from 50 to 300 μm. The crystal morphology is mainly pentagonal-dodecahedral and irregularly granular, with a small amount of cuboidal pyrite. In the interlayer between rock salt, particle size of the pyrite is from 50 to 100 μm, and the crystals are mainly octahedral. (2) The S/Fe value of pentagonal-dodecahedral pyrite is significantly greater than 2; the S/Fe value of octahedral and cubic pyrite is less than 2; and the S/Fe value of irregular granular pyrite is close to 2. (3) The δ34SV-CDT values of pyrite in the upper salt-related strata range from −15.65‰ to 11.81‰, and the average δ34SV-CDT values of all samples range from 0.79‰ to 8.20‰. The δ34SV-CDT values of pyrite interlayer between rock salt range from −15.02‰ to −6.36‰, with an average value of −10.66‰. The above results indicate that the pyrite in the upper part of the rock salt layer was formed in a medium-low temperature hydrothermal environment, and the ore-forming sulfur elements have hydrothermal sources, bacterial sulfate reduction (BSR) sources, and thermochemical sulfate reduction (TSR) contributions. The pyrite between the rock salt layers is of sedimentary origin, and the ore-forming sulfur element comes from BSR. At present, there is no evidence of the influence of hydrothermal activities on deep potash-rich salt bodies, and the influence on the Mengyejing potash deposit has continued since the metallogenic period.

Milankovitch cycle of continental deep-water fine-grained sedimentary rocks in the lower submember of Es3 of Well FY1 in Dongying Sag and its significance for shale oil exploration

China has recently faced significant difficulties in the exploitation of its shale oil and gas resources. An essential geological obstacle preventing the breakthrough of Chinese shale oil exploration is the precise identification of productive oil and gas pools and ideal formation. Therefore, it is crucial to examine the properties of shale reservoirs. Deep-water fine-grained sedimentary rocks in the lower member of the Well FY1 in Dongying Sag are analyzed using the Milankovitch cycle based on core, geochemical analysis, and gamma logging data. The findings demonstrate that: (1) The entire Milankovitch cycle is preserved in the Es3x of Well FY1 in Dongying Sag, and that the long Eccentricity of 405 ka and the Precession cycle of 23.2 ka are the key controlling factors in the deposition. (2) The “three-end-member” method is used to divide eight different types of lithofacies. The main vertical changes in these lithofacies are from organic massive gray mudstone to organic lamellar callitic mudstone to organic massive gray mudstone to organic lamellar gray mudstone to organic lamellar gray mudstone and back again. From shallow to deep to deeper, the entire water depth fluctuated. (3) Each of the four lengthy Eccentricity cycles has a half-cycle of warm, humid weather and cold, dry weather. Analysis was done on how the lithofacies and organic matter concentration changed with high and small eccentricities. The enrichment of biological materials in warm, wet, dry, and cold climates was hypothesized by examining the response of fine-grain sedimentary rocks to eccentricities and Precession periods. Larger Eccentricity is thought to be more suitable for storing shale oil.

Palynology of Permian red-bed successions of Colorado and Wyoming and its influence on Laramide strata

ABSTRACTThe first reported Permian (Kungurian to Roadian) palynomorphs are described from Colorado, recovered from bedded gypsum and rare organic-rich shale intercalated in the red siltstone-dominated Lykins and State Bridge Formations. Surprisingly, these units generally lack the taeniate, saccate pollen that typifies most Permian continental rocks elsewhere, yet they contain abundant terrestrially derived palynomacerals, a low-diversity suite of sphaeromorph and acanthomorph acritarchs, and extremely rare non-taeniate, bissacate pollen grains. Acritarchs, known from one stratigraphic interval, are well-preserved and interpreted to represent autochthonous deposition during a marine incursion into the depocenter. This interpretation is consistent with their occurrence in a gray mudstone that is mantled by a mollusk-dominated coquina that bears conodonts, palaeoniscoid scales, and actinopterygian teeth. In contrast, most studied samples are dominated by wood fragments, charcoal, cuticles, and unidentified phytoclasts—all interpreted to represent dispersed plant cuticle and wood of continental origin. Fossils occur in black paper shale, gray fetid calcareous siltstone, and rhythmically bedded gypsum that is closely associated with thin limestone. Palynofacies analyses suggest that non-acritarch palynomacerals were deposited in dysoxic to anoxic waters that received minimal suspended terrigenous input. When combined with sedimentologic information, these non-acritarch fossils are hypothesized to have been deposited in shallow epicontinental lake-like settings that were periodically alkaline, hypersaline, and/or emergent.

Sequence stratigraphic divisions and correlation of the middle sub-member of Eocene Shahejie Formation in the Bonan Sag of Bohai Bay Basin (China): Implication for facies and reservoir heterogeneities

The middle sub-member of the Eocene Shahejie Formation is mainly associated with deltaic environment, and considered as hydrocarbon bearing zone in the southeast of the Bonan Sag, Bohai Bay Basin, China. Previous research and the exploration work in an area were carried out separately, and inconsistency in understanding of stratigraphic division, correlation, and facies was encountered. Furthermore, the facies evolution in the targeted formation is still questionable. To fill these short-comings, sequence stratigraphic correlation and interpretation were conducted on the middle sub-member (Es3z) of the third member (Es3) of Eocene Shahejie Formation in the study area. Integrative facies and paleo-environment analyses are carried out through the correlation study of wireline logs, seismic data and core sedimentology. The sedimentological interpretation reveals that sandstone groups in the Es3z show five different facies associations in both lateral and vertical successions developed in the inner and outer delta front. These depositional facies include deposits in distributary channels, interdistributary channels, mouth bars, natural levees, and sheet sandstones. The Es3z sandstone intervals are characterized by five depositional sequences in terms of the third-order sequence, including Es3z2+3, Es3z4, Es3z5, Es3z6+7, and Es3z8. They are found in all wells consisting of the lowstand, transgressive, and highstand system tracts. These system tracts are separated by the maximum flooding surfaces and correlation boundaries of sandstone groups with unique characteristics of well log motifs, calibrated with the cores by particular lithology. Facies and reservoir heterogeneities are defined within a sequence stratigraphic framework approach for hydrocarbon reservoir prediction. Our results show that sediments were sourced from the southeast toward the northwest. Due to tectonic activity and lake level's fluctuations, reservoir facies are prograding towards the basin side. The steepness of the region impacts the facies and reservoirs evolution together with tectonics and fluctuations in lake level, which turns from the sandy conglomeratic reservoirs into a thick succession of sandstone reservoirs towards the basin side in LST HST and TST system tracts. These reservoir facies are considered as good, and capped by grayish mudstone (inner delta front) to dark gray mudstone (outer delta front) in TST system tracts. The facies architecture of all sandstone groups possesses the hydrocarbons potential in the inner and outer delta front region in which interpreted oil, oil-water layer and water-oil layers show variation in progradation and retrogradation stacking pattern. Among them lower sandstone units of Es3z8 and Es3z6+7 show dominancy in oil rich reservoirs, followed by Es3z2+3, Es3z4 and Es3z5 in LST, HST and TST system tracts. These hydrocarbon units can be correlated laterally with exploration wells for getting new well sites in third order sequence stratigraphic framework. Our findings shed new light on the facies and reservoir heterogeneities, which provide a useful aid for hydrocarbon exploration in the study area.

An integrated study of core and borehole image data: palaeokarst development in late Permian siliceous dolomites and the structural evolution of the Loppa High from the Gohta discovery well, SW Barents Shelf

Abstract The Gohta discovery well 7120/1-3, drilled on the Loppa High in the SW Barents Shelf in northern Norway encountered a meteoric karst system hosted by the late Permian lower Røye Formation that formed during a period of sub-Triassic exposure. The karst system is preserved as a series of interbedded collapse breccias representing formerly open palaeocaves, matrix-rich breccias that represent palaeocave fills with in situ and reworked stalactites and stalagmites. Layers of ‘Swiss cheese’ texture are interpreted as zones of dissolution in marine-meteoric mixing zones that may have controlled the levels of palaeocave development within the Gohta structure. These other cavities and the matrix of karst breccias were infilled by layered fine doloarenite matrix interpreted as an internal fill generated by collapse. This was followed by a later matrix of dark grey argillaceous mudstone that is interpreted as an external fill that pre-dates the overlying Klappmyss Formation. The karst system has undergone complex early diagenesis that includes extensive dissolution, early leaching of silica, the precipitation of pyrite on internal surfaces and sediment and the precipitation of carbonate and silica as speleothems and vadose cement. A borehole imaging log (BIL) was run across the lower Røye and Triassic Klappmyss formations, including the cored interval that provides a record of inclination and azimuth of the karst fills and in situ Røye Formation. The core provides information about the origin and timing of the different karst fills, and the BIL provides information about their azimuth and inclination. This integrated use of core and BIL data allows the incremental tilting history of the Loppa High structure during exposure to be reconstructed. At present, the Gohta structure has a finite tilt of 5–10° to the SE. The plunge of in situ stalagmites and stalactites suggests that the karst system was initiated while the Røye Formation was slightly tilted to SE. The dip of karst components and the host stratigraphy indicate several phases of incremental tilting during deposition of the doloarenite and argillaceous mudstone karst matrix. There appears to have been a reversal of tilt of the Gohta structure prior to its onlap by the Klappmyss Formation.

Assessment of Reservoir Quality of the Cretaceous Yolde Sandstone: A Case Study of Doma Stream, Gongola Basin, Northern Benue Trough, Northeast Nigeria.

Sedimentology, petrography and petrophysical studies were applied to the Cretaceous Yolde Formation sediments along the Doma stream, to evaluate its reservoir quality. The Yolde Formation along the Doma Stream is composed of lithologies that are laterally extensive and have reached a thickness of about 80m. They are lithologically made of sandstones with dark grey mudstones, laminated shales and grey to pale yellow limestones. The Sandstones are structurally characterized by planar, tabular and trough cross-beds, convolute beds, and hummocky cross-stratifications with reactivation surfaces. Some of the sediments are bioturbated. The petrophysical evaluation indicated porosity values ranging from 6% to 26%, and permeability values between 12.7mD and 108mD. These values are comparable to the reservoir rocks of proven petroleum basins of the North Sea and Niger Delta. Therefore, the Yolde Sandstones of the study area can have great potential to serve as good reservoir rock in the Gongola Basin.

The role of Triassic source rocks in the formation of oil accumulation in the Fukang Depression (China)

This article presents the results of geological and geochemical studies of the organic matter of source rocks and oil from reservoirs in the Karamay Formation (Anizian–Ladinian stages) in the eastern part of the Fukang Depression in Junggar petroleum basin. The quantity, quality, and degree of maturity of the organic matter in clayey mudstones in upper part of the Karamay Formation (T2k) were considered and the composition of the original organic matter, the conditions of its accumulation and the degree of maturity were reconstructed using pyrolytic parameters and biomarkers. The dark gray and gray mudstone in upper part of the Karamay Formation accumulated in fresh or low salinity lakes with relatively reducing conditions. At present, the organic matter of the Formation within the depression is in the zone of mesocatagenesis MK1–MK3, and therefore has not fully realized its oil-geological potential. It has been previously established that mudstones in upper part of the Karamay Formation provided the hydrocarbon fl uids for the Karamay reservoirs located in the submerged part of the Fukang Depression.

Theropod teeth from the upper Barremian (Lower Cretaceous) of Vadillos-1, Spain

The upper Barremian (Lower Cretaceous) palaeontological site of Vadillos-1 is located in the North of the Cuenca Province, Spain. It includes a sedimentary succession in “Weald” facies of brown and grey mudstones and red clays, corresponding to an alluvial-palustrine muddy floodplain. Among the collected fossils, there are several vertebrate remains belonging to theropod dinosaurs, including some teeth (VD1-197, VD1-178, VD1-179, and VD1-180) that are described and identified for the first time. The taxonomic assignment was approached following two methods beside the classical consideration of their overall morphology: a cladistic analysis performed on a tooth-based data matrix, and a discriminant analysis performed on a large dataset including measurements of non-avian theropod teeth. The DSDI > 1.2 and the braided enamel present seen in VD1-197 suggest that the specimen belongs to a Dromaeosauridae or a non-tyrannosaurid Tyrannosauroidea, but the cladistics analysis classifies this tooth in the latest. The analysis showed that VD1-178, VD1-179 and VD1-180 can be ascribed to Dromaeosauridae. The dental features characteristic of this clade and present in our specimens include a DSDI > 1.2, labial and lingual depressions, and a braided enamel surface texture, which allows to place these teeth more specifically within the Eudromaeosauria. The results show, for the first time, the presence of dromaeosaurids and possibly non-Tyrannosaurid Tyrannosauroidea at Vadillos-1, thus providing new data to the European Barremian record, and contributing to the updating the geographic distribution of these dinosaurs.

Palynostratigraphy, biochronology and palaeobathymetry of a section of Awaizombe-1 well, eastern Niger Delta, Nigeria

ABSTRACTDitch cuttings (69 samples) from a section of Awaizombe-1well located in the Northern Depobelt of the eastern Niger Delta Basin were used for this study. The lithology of the studied interval 1373–1812 m (439 m thick) consists of fissile grey shale and mudstone units. The scid method of sample preparation for palynormorphs’ recovery was adopted. Analysis recorded a well-preserved and diverse assemblage of palynomorphs, rich in pollen, spores and dinoflagellate cysts (dinocysts). First and last occurrences of marker and age diagnostic species were used for palynostratigraphic interpretation. Four palynostratigraphic interval range zones were established: Psilatriporites sp.–Racemonocolpites hians Zone; early Oligocene (Rupelian age), Praedapollis africanus–Doualaidites laevigatus Zone; late Eocene (Priabonian age), middle Eocene (Lutetian and Bartonian ages), Doualaidites laevigatus–Praedapollis flexibilies Zone; and early Eocene (Ypresian age), Verrucatosporites usmensis–Retitricolpites ituensis Zone. The first downhole occurrence of D. laevigatus at the 1482 m marks the late Eocene/early Oligocene boundary. Established zones are useful for inter and intra basins correlation. Lithology and age of the studied section are suggestive of the lower Agbada Formation. Palaeoenvironmental interpretations using diagnostic species revealed two environments: brackish and inner neritic to upper bathyal (0–600 m) under relatively warm-water marine condition indicated by thermophilic dinocyst taxa, such as Lingulodinium machaerophorum, Polysphaeridium zoharyi and Homotryblium spp. The lithology and these types of environments are good sites for hydrocarbon generation.

Characterization of Pennsylvanian Paleosols in Indiana with a Special Reference to Rare Earths (REE) and Lithium

This study characterizes Pennsylvanian paleosols from coal-bearing strata in Indiana, with the main emphasis on the abundance and distribution on rare earths (REE) and lithium (Li). Eleven locations in southwest Indiana were selected for this study, targeting paleosols of both the Carbondale Group and the Raccoon Creek Group. Most paleosols were directly underlying coal beds. Lithologically, paleosols range from grey mudstones to greenish-gray claystones and occasional siltstones. In the Carbondale Group, the thickest paleosol horizons occur under the Danville and the Houchin Creek Coal Members, with their thickness reaching close to 10 feet. In the Raccoon Creek Group, the paleosol under the Wise Ridge Coal appears to be thickest (4.6 to 11.4 feet) and best developed. Total REE content in paleosol samples ranges from 138.8 ppm to 728.55 ppm, with an average range of 208.3 ppm (Colchester paleosol) to 342.9 ppm in the paleosol under the Upper Block Coal. Light REE dominate over heavy REE, and the variation in the light REE is responsible for the largest variations of the total REE. The largest amount of the heavy REE (61 ppm) occurs in the paleosol under the Lower Block Coal, whereas the paleosol under the Upper Block Coal has the largest variation in the heavy REE content (49 to 148 ppm). REE patterns for paleosol samples show very weak to no fractionation among light, medium, and heavy REE, with no major anomalies present. The majority of paleosols have REE values that are roughly representative of the upper continental crust. Only some paleosols are slightly enriched and these are paleosol samples under the Danville Coal and the Upper Block Coal. Lithium content in the paleosol samples shows a variation from 21 ppm to 530 ppm. On average, the Wise Ridge Coal is richest in Li (249.3 ppm), followed by Viking B Coal (190.8 ppm). The coals of the Brazil and Staunton Formations have higher Li content than the younger coals. Although this study has not uncovered large concentrations of REE or lithium, the wide range of concentrations and potential ease of their extraction from these clay-rich rocks imply that the paleosols deserve further examination, and the current study can be used as a screening guide for more focused investigation.