Lacustrine Rocks Research Articles

An operational classification system for the South Atlantic pre-salt rocks

Abstract The giant Aptian reservoirs and associated lacustrine rocks of the South Atlantic pre-salt section present a series of unique characteristics, which make the systems created for marine carbonate rocks quite inadequate for their classification. Based on our experience with the characterization of thousands of samples of the pre-salt reservoirs and associated deposits from the Brazilian basins, and on a wide literature evaluation, we propose an objective and operational system for the classification of the unusual yet extremely important pre-salt lacustrine rocks. The system allows the coherent record of structure, fabric, primary texture and composition, and main diagenetic modifications of the in situ and resedimented rock types. The in situ rocks are directly classified according to the original proportion among calcite spherulites, fascicular shrubs, and mud matrix. The resedimented rocks are classified according to the original volume of > 2 mm particles and the proportion between sand and mud, using calcirudite, calcarenite, and calcilutite as unbiased names. Such an approach can also be applied to the classification of other particulate carbonate rocks, avoiding the conceptual problems and dubious interpretation of depositional environment and reservoir quality of currently used classification systems. The direct, objective, and intuitive proposed classification system shall contribute to the understanding, exploration, and production of the extraordinary South Atlantic pre-salt petroleum province.

The enhanced presence of tricyclic terpanes in the lacustrine rock extracts of the Upper Triassic Yanchang Formation, Qingcheng area, Ordos Basin, China

The predominance of tricyclic terpanes over hopanes was observed for the first time in the lacustrine rock extracts of the Upper Yanchang Formation collected from Well CYX in the Qingcheng area, Ordos Basin, China. Specifically, hopanes are dominant terpanes in the Chang 7-2 member, while tricyclic terpanes ranging from C19 to C39 predominate over hopanes in the Chang 7–3 and Upper Chang 8-1 member. The distributions of terpanes, steranes, as well as n-alkanes vary significantly in these thermally mature rock extracts, however, are closely correlated in each sample. Generally, rock extracts comprising more abundant tricyclic terpanes and their lower molecular weight homologues are characterized by having lower Pr/n-C17 ratios and by containing more abundant low molecular weight n-alkanes (n-C21-) and short-chain C21-22 steranes. Furthermore, among the Chang 7–2 and Chang 7-3 samples, black shales contain more abundant low molecular weight tricyclic terpanes and n-alkanes (n-C21-), and short-chain C21-22 steranes when compared with siltstones and mudstones, which comprise more abundant hopanes, diasteranes and regular C27-29 steranes. The similar geochemical characteristics could also be found in other rock extracts that have variable abundances of tricyclic terpanes and hopanes, especially in the East Berlin Formation (Hartford Basin, Connecticut, U.S.A), which suggests that the causes for the variable distributions of terpanes, steranes and n-alkanes in these strata are similar.On the one hand, it is determined by the proportion of aquatic versus terrigenous source input and depositional environment. The predominance of tricyclic terpanes over hopanes and short-chain C21-22 steranes over regular C27-29 steranes in the Chang 7-3 member, especially in black shales, could be resulted from algal bloom and possibly associated microbial metabolic process in stable or stratified water column, which may be triggered by plentiful nutrients provided by runoff, volcanic or hydrothermal events. On the other hand, high hydrocarbon-generation intensity and petroleum expulsion efficiency for source rocks, especially for black shales, may be another significant driving force for the enrichment of tricyclic terpanes. In such cases, significant oils have been generated and expelled, an early or faster rate of generation/expulsion of hopanes relative to tricyclic terpanes may occurred, which is evidenced by the extended hopanes are depleted in all the studied rock extracts. Furthermore, to our knowledge, the predominance of tricyclic terpanes over hopanes has not been observed in source rocks of the Yanchang Formation in other regions of the Ordos Basin. The observation of the enhanced presence of tricyclic terpanes in the current study may suggest that the hydrocarbon-generating intensity and petroleum expulsion efficiency of source rocks in the Qingcheng area are higher than that of other regions in the basin. Thus, the Qingcheng area has tremendous potential for petroleum resources and is an important area for future exploration.

Characterization, Classification, and Evaluation of the Reservoir Pore Structure Features of Lacustrine Fine-Grained Sedimentary Rocks. A Case Study of the Fourth Member of the Shahejie Formation in the Chenguanzhuang Area of the Southern Gently Sloping Zone of the Dongying Depression, Bohai Bay Basin

With the development of unconventional oil and gas exploration “from sea to land,” lacustrine fine-grained sedimentary rocks (FSR) have gradually attracted the attention of scholars and become an important topic in the field of unconventional oil and gas, but the research is still in its initial stage. In this study, lacustrine FSR in the Dongying Depression of the Bohai Bay Basin are used as the research object, and nuclear magnetic resonance (NMR) and quantitative image characterization are used to characterize the pore structure of the reservoir in the study area on multiple scales, analyze the reservoir characteristics control factors, and classify and evaluate the reservoir. The results show that: 1) the favorable petrographic phases of the FSR reservoir can be classified into six types of organic-rich lime mudstone, organic-rich laminoid lime clay rock, organic-rich laminoid clay micritic limestone, organic-bearing banding clay micritic limestone, organic-rich banding lime clay rock, and organic-bearing lumpy clay micritic limestone. With an average porosity of 12.3% and an average permeability of 10.58 mD, the overall reservoir is a typical low-porosity-low-permeability type; 2) the reservoir space types are diverse, with strong microscopic inhomogeneity; pores with a pore size of less than 2 nm almost have no contribution to the reservoir space; the pore volume and pore area are mainly provided by organic matter pores at the 100 nm level, mineral intergranular pores, and clay mineral shrinkage pores/slits. The FSR reservoirs in the study area are classified into three categories, and the pore structure of the reservoirs from categories I to III deteriorates in turn. This study provides a basis for the microscopic characterization, classification, and evaluation of lacustrine FSR reservoirs and their exploration.

Diagenesis and Its Impact on the Reservoir Quality of Continental Shales: A Case Study of the Lower Jurassic Da’anzhai Member of the Ziliujing Formation in the Sichuan Basin, China

The Jurassic continental shale oil (gas) is a favorable unconventional resource in the Sichuan Basin of China. In this paper, analysis methods such as core and outcrops observation, thin section identification, scanning electron microscopy (SEM), and X-ray diffraction (XRD) are used to describe the characteristics of lacustrine fine-grained rock reservoir in Da’anzhai Member of the Ziliujing Formation in Sichuan Basin, and it discussed the influence and control of lacustrine fine-grained rock diagenesis on the quality of the reservoir, in order to determine the shale reservoir control factors. The results show that there are three types of rocks in the Da’anzhai Member, which are mudstone, siltstone, and limestone, and the rock combination of shell shale intercalated with shell limestone is developed in the Da 2 submember. Fine-grained sedimentary rocks in this section have undergone compaction, cementation, dissolution, metasomatism, transformation of clay minerals, and hydrocarbon generation of organic matter, which are currently in the middle diagenetic stage A or B substage. Compaction and cementation are the main factors that control the physical use of shale and limestone, and strong cementation is the main reason for the tightness of limestone reservoirs in the Da’anzhai Member in the northern and eastern parts of Sichuan Basin. The difference between dissolution and cementation is the main control factor for the formation of limestone or argillaceous limestone reservoirs in the same section in central Sichuan Basin. The organic-rich shale and shell shale of the Yuanba and Fuling area are the most favorable reservoirs of the Ziliujing Formation in this region. The shell limestone that experienced favorable dissolution in the Da’anzhai Member in the central of the Sichuan Basin has become a limestone reservoir. Discussing the impact of continental fine-grained rock diagenesis on the reservoir can better explore and develop similar intervals, enrich unconventional shale oil and gas accumulation and storage theories, and provide basic theoretical support for finding favorable unconventional shale oil and gas reservoirs.

Analysis of Sedimentary Environment Conditions for Lacustrine Fine-Grained Sedimentary Rocks and Its Control of Lithofacies Development: A Case Study of the Lower Submember of Member 3 of Shahejie Formation in FY-1 Well, Dongying Sag, Bohai Bay Basin, China

In recent years, the studies on fine-grained sedimentation mainly focus on the rock type, sedimentary environment, sedimentation, and sequence stratigraphy, while those on the relationship between sedimentary environment and lithofacies development are rare. However, a clear understanding on the relationship is of great significance to the muddy shale oil and gas exploration. This paper studied the muddy shale of semideep and deep lacustrine facies in the lower submember of Member 3 of Shahejie Formation in FY-1 Well, Dongying Sag, Bohai Bay Basin, East China. Based on geochemical data, the sedimentary environment media conditions and vertical changes of this submember were analyzed by means of core description, thin section authentication, X-ray diffraction (XRD), and other technologies, and the relationship between sedimentary environment and lithofacies development was discussed. The results show that this environment underwent three stages and is featured by cyclicity. From the bottom up, it experienced the semideep, deep, and deep/semideep waters under the relatively dry-cold/relatively warm-wet, warm-wet, and warm-wet/relatively warm-wet paleoclimates, respectively. Correspondingly, the paleoredox transited three stages from reducibility to high reducibility to high reducibility/reducibility; the paleosalinity changed from saline water to brackish water to brackish/saline water. Paleoproductivity was low/relatively high at the beginning, then became high, and finally relatively high. In this submember, the lithofacies primarily includes organic-rich mudstone, organic-rich lime mudstone, organic-rich lime-bearing mudstone, and organic-contained argillaceous limestone/lime mudstone. The sedimentary environment controls the mineral composition and content in the lithofacies and thus determines the lithofacies types. Meanwhile, the cyclicity of environmental change not only leads to that of the lithofacies development but also affects the positions of the lithofacies and its assemblages in sequence stratigraphic framework.

Logging evaluation of lamina structure and reservoir quality in shale oil reservoir of Fengcheng Formation in Mahu Sag, China

The Fengcheng Formation in the Mahu Sag of the Junggar Basin is composed of a set of fine-grained sedimentary rocks located under a layer of alkaline lacustrine rocks. There are alternating dolomite lamina, siltstone and organic matter laminae that show strong heterogeneity. The lamina structure determines reservoir quality and oil-bearing potential in shales. Therefore, the identification of lamina structure is a crucial basis for identifying reserves of shale oil. Identified as massive, layered and laminated, three different types of lamina structures are divided according to core observations in terms of the density of the laminae and the overlapping relationships. High-resolution processing is also performed on dynamic image log data to form slab images for recognizing the various types of lamina structures. Compared to core slab photos, it is believed that slab images offer an excellent thin-layer resolution of up to a millimeter in scale and can effectively identify the lamina structure of the shale reservoir. There are no evident beddings on the core of the massive pattern, and the slab image shows a bright or dark block pattern. There are centimeter-level felsic bands of fine silt sand on the thin-layered sedimentary fabric core, and the slice image shows a band-like pattern. A large number of millimeter-level dolomitic laminae are enriched on the core of the laminated type, and the slab image shows millimeter-level light and dark lamina. Slab images were used for the division and identification of lamina structures in a single well. Then, the relationships between the three types of lamina structures and the two-dimensional nuclear magnetic resonance logs and oil test data were recorded. The results show that massive and laminated pores are dominated by intergranular pores with low porosity and movable oil content. Therefore, only low oil flow can be obtained during an oil test. However, the layered type due to centimeter-level silty bands, T1 and T2, is bimodal with high porosity, movable oil content, and high levels of hydrocarbon production. Therefore, the optimal areas of shale reservoirs are closely related to the layered type. The more significant the proportion of layered types in shale reservoirs, the higher the quality of the reservoir. The present study is intended to identify lamina structure-controlled reservoir quality and oil accumulation and to provide insights into the exploration and development of shale oil.

Lacustrine carbonate lithofacies characterization, paleontological content and depositional processes in the Mayrán Basin System

Lithofacies analyses were undertaken on a suite of well-exposed carbonate deposits from the (Pliocene) Mayrán Formation, in the Mayrán Basin system, northeast Mexico to investigate the controls on carbonate lithofacies variability in a series of linked lacustrine subbasins, with a common hydrology yet very different inputs. The lacustrine carbonates contain variable proportions of production-derived components, with subordinate proportions of detrital and early diagenetic components. The lithofacies contain, inorganic and organic calcite (average 91% CaCO3), and organic matter (10.5%–13.8% average 12.6% TOC), together with minor detrital minerals. Nine sub-facies were identified: ostracod mudstone-wackestone, clotted boundstones, gastropod wackestone-packstones, oncoid wackestone-packstones, charophyte wackestone-packstones, crystalline carbonate mudstone, stromatolites, crystalline carbonate, marlstone, and associated evaporite minerals. Isotopic composition of specific components such as micritic laminations in stromatolites (δ13Ccalcite −6.3‰ and δ18Ocalcite −8.0‰), and micrititic matrix from samples in all the subbasins (δ13Ccalcite −5.7‰ and δ18Ocalcite −7.8‰), indicate that overall calcite deposition occurred in freshwater lakes. Non luminescent (e.g. charophytes) to orange dull luminescencent micrite strongly suggest in situ carbonate production. Together these processes contributed to the local production of lime mud. Additionally, microtextural evidence of scour surfaces however, indicates that occasionally the sediment was reworked and transported laterally and basinward. Fossil content and diversity composed by Limnaeidae, Physidae, and Planorbidae types, together with phosphatised bone fragments, micro-oncoids, stromatolites, microbial morphotypes, charophytes, macrophyte phytoherms, and microcodium structres, are good references for lithofacies characterization and indicators of specific paleoenvironmental conditions. Intense to moderate bioturbation (BI 1 to 4), indicates that deposition took place under predominantly oxic conditions. Significant organic matter was preserved in these lacustrine rocks, likely controlled by moderate rates of carbonate mud production. The lacustrine lithofacies were deposited in proximal and distal lacustrine settings, making possible to recognize basin scale variability. Strata staking patterns are organized into shallowing upwards cycles (0.3–2.5 m thick), which are capped either by evaporites or beds with a pedogenic overprint. Their stacking patterns reflect lake level variations which can be recognized in all the subbasins. The lithofacies architecture within each subbasin forms parasequences that overall exhibit aggradational geometries. The lithofacies and their large scale geometries also indicate deposition occurred in a series of relatively shallow (<10 m), lake systems with ramp-like margins. The aggradational stacking patterns of the carbonates and the presence of subordinate evaporites suggest that basin hydrology (water supply and precipitation/evaporation ratios) had an important influence on facies architecture and cyclicity. The Mayrán Formation lacustrine lithofacies deposited under overfilled to balance-filled lake conditions; although balance-filled conditions with an open hydrology were more likely. Overall, facies variability in the lacustrine carbonate lithofacies was controlled by basin inherited geomorphology, accommodation, and basin hydrology, which ultimately controlled clastic trapping, solute inputs, water chemistry, primary production, and lake level changes. We interpret that all these controls were largely driven by climate.

A Study on Astronomical Cycle Identification and Environmental Response Characteristics of Lacustrine Deep-Water Fine-Grained Sedimentary Rocks: A Case Study of the Lower Submember of Member 3 of Shahejie Formation in Well Fanye-1 of Dongying Sag, Bohai Bay Basin, China

With the further exploration and development of shale oil and gas, cycle division of fine-grained sedimentary rock strata has been increasingly highly focused on by scholars. Owing to the application of the theory of classical sequence stratigraphy based on water depth changes and its technical methods being unsatisfactory in the isochronous division and correlation of deep-water fine-grained sedimentary strata, the cycle division of fine-grained sedimentary rock strata has always been a difficult point in the study of sequence stratigraphy. In this paper, the Milankovitch cycle recorded from the study interval and the environment response characteristics were studied, with the lacustrine shale in the lower third submember of the Paleogene Shahejie Formation (lower Es3 submember) in Well Fanye-1 of the Dongying sag, Bohai Bay Basin, as the object of study, by such technical means as thin section identification and X-ray whole rock diffraction, based on such data as logging data and geochemistry, combining the methods of spectral analysis, wavelet transform, and modulus extremum. The results showed that the stratigraphic cycle thicknesses caused by long eccentricity, short eccentricity, and obliquity periods were 38.95 m, 12.98 m, and 4.10 m, respectively, and a total of 16 short eccentricity periods and 4.5 long eccentricity periods were identified in the study interval. Thus, it was further calculated that the sedimentation time was approximately 1.905 Ma, and the average sedimentation rate was estimated to be 0.105 m/ka. Studies have shown that the sedimentary environment of lacustrine fine-grained sedimentary rocks is controlled by the astronomical period, based on which the climate as a whole changes from relatively dry and cold to warm and wet when the eccentricity increases. The identification of the Milankovitch cycle of the lacustrine fine-grained sedimentary strata will provide references for the study of high frequency sequence and the division of high-resolution sequence strata, which can effectively solve the scientifically difficult isochronous division and correlation of lacustrine shale strata.

Investigating biological nitrogen cycling in lacustrine systems by FT-ICR-MS analysis of nitrogen-containing compounds in petroleum

Biological nitrogen cycling plays an important role in bio, environmental and petroleum geochemistry. Until now most research has focused on marine environments using nitrogen isotopes, and in contrast, nitrogen cycling in lacustrine environments remains poorly understood. To fill this knowledge gap, we present a case study of crude oils from Permian brackish and saline alkaline lacustrine rocks in the Junggar Basin, northwestern China. To complement the large variation in nitrogen isotope values in lacustrine sediments, we analyzed the pyrrolic N-containing compounds of crude oil using negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). Our results show that pyrrolic compounds derived from biological sources are more abundant in oils from brackish lacustrine settings (i.e., the Jimusar Sag oils of the Junggar Basin) than in oils from alkaline lacustrine settings (i.e., the Mahu Sag oils of the Junggar Basin), which is mainly due to greater higher plant contributions to the latter. Higher plants provide abundant alkaloids and chlorophyll that are the biological precursors of pyrrolic N-containing compounds. In addition, anoxic, sulfate-type, brackish lacustrine settings provide abundant ferrous iron and sulfide that promote N fixation. This study provides the first insights into biological N cycling in brackish and alkaline lacustrine systems, which has significance for fossil fuel resources and their environmental impacts. Our data suggest that the N cycling in lacustrine systems might have an important role in entire Earth system N budget.

Geochemical characteristics of the Chang7 organic-rich fine-grained sedimentary rocks and its relationship with the tight oil in Longdong area, Northwest China

In order to study the geochemical characteristics of fine-grained sedimentary rocks and clarify its relationship with the tight oil reservoir, the Chang7 Member of Triassic Yanchang Formation in Longdong area was taken as an example to be studied by rock pyrolysis, isotopic determination, maceral analysis, gas chromatography and gas chromatography–mass spectrometry. Lacustrine organic-rich fine-grained sedimentary rocks can be divided into organic-rich shale and organic-rich mudstone according to their lithology and texture. The results show that the organic-rich shale was deposited in a quiet and anoxic deep lacustrine environment, the organic matter was of high abundance and mainly sourced from plankton and benthic algae (type I–II1). Organic-rich mudstone mainly deposited in the semi-deep lacustrine environment, where the water was relatively shallow and the salinity was low, the organic matter was of low abundance and came from the mixed source of plankton and terrestrial clastic (type II1–II2). The Chang7 tight oil is widely distributed in the turbidite sand bodies, and it was mainly sourced from the Chang7 organic-rich shale.

Effects of lacustrine organic-rich fine-grained sedimentary rocks on the accumulation of tight oil

In order to establish the distribution pattern of fine-grained sedimentary rocks and clarify the coupling relationship between lacustrine organic-rich fine-grained sedimentary rocks and tight oil reservoir, the Chang7 Member of Triassic Yanchang Formation in Longdong area was taken as an example to be studied by core description and log interpretation, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Lacustrine organic-rich fine-grained sedimentary rocks were divided into shale and mudstone according to their lithology and texture. The results show that the shale is mainly developed in Chang73 Sub-member, while the mudstone is developed in each sub-member of Chang7 Member. The organic-rich fine-grained sedimentary rocks in Bazhu-Maling-Qingcheng-Huachi area are the thickest in Longdong area. During the Chang73 sedimentary period, the shale was widely distributed in the center of the lake. In the Chang72 sedimentary period, the mudstone and turbidite were interstratified and superimposed. During the Chang71 sedimentary period, the mudstone was only distributed as thin layers between the thick turbidite. The Chang7 tight oil is widely distributed in the turbidite sand bodies, and it is mainly sourced from the Chang73 shale. The distribution of tight oil is controlled by the development of organic-rich fine-grained sedimentary rocks.

Basic geochemical characteristics of lacustrine rocks in the Neogene Ka\u011f\u0131zman\u2013Tuzluca Basin, Northeastern Turkey

The Neogene Kağızman–Tuzluca Basin is located in the NE Anatolia and extends eastward into Armenia Oktemberian Basin. The Halıkışlak (Late Oligocene), Turabi and Tuzluca formations (Lower–Middle Miocene) are time equivalents of the organic-rich Oligo-Miocene Maikop Series in the Caspian region. However, depositional conditions within the KBT are appreciably different and source rocks are not as richer as it. The Halıkışlak Formation has little source rock potentials, with very low values of TOC (0.04–0.25%), HI (14–90 mg HC/TOC) and genetic potential-GP (i.e. Rock–Eval S1 + S2 = 0.05 to 0.22 mg HC/g rock). Although a peat facies sample has a relatively high TOC (6.78%), Tmax of 428 °C, HI values are very low (45 mg HC/TOC). TOC and GP values for the Tuzluca Formation are between 0.06 and 0.39 wt%, and 0.16–1.51 mg HC/g rock, respectively. Some levels of the Turabi Formation have TOC contents up to 6.14 wt%; however, mainly Tmax values are low 435 °C, and HI range from 25 to 54 mg HC/TOC. The extract of one low-maturity Turabi sample possesses biomarker distributions of an immature rock. Low relative abundance of tricyclic terpane and dominant C29 ααα (20R) are in accordance with the immature source rock. Some samples collected from the outcrops nearby Pliocene/Pleistocene lava series show localized maturation stage. We suggest that the Oligocene to Middle Miocene units could be more studied beneath the volcanic plateau.

Mineralogy and Geochemistry of Sedimentary Huntite Deposits in the Egirdir-Hoyran Lake Basin of Southern Turkey

ABSTRACT &amp;gt;The sediment-hosted huntite-magnesite deposits are located in the Egirdir-Hoyran lake basin in the Isparta Angle (southern Turkey). The deposits occur at two different localities in the region: (1)Kemersirti huntite deposit, (2) Köytepe huntite-magnesite deposit. The huntite-magnesite occurrences are found in shallow lacustrine rocks of the Miocene-Pliocene Kizilcik Formation and formed as a result of Neogene tectonic activity. Based on X-ray diffraction and scanning electron microscopic studies, the mineral assemblage of huntite deposits contains mostly huntite, less magnesite, dolomite, very little calcite, illite, simectite, brucite, and quartz in the Kemersirti area but contain huntite, magnesite, dolomite, and calcite in the Köytepe area. In the huntite and magnesite-bearing huntite samples, MgO varies from 32.70 to 37.95 wt. %, CaO from 7.83 to 15.10 w.t. %, and SiO2 from 0.99 to 10.60 w.t. %. Ba and Sr are dominant minor elements in the deposits. Ba and Sr for huntite and magnesite bearing huntite in the study area vary from 11 to 233 ppm and from 325 to 765 ppm, respectively. As, U, Zr, V and Ce contents ranged from 11.5-146 ppm, 0.5-3.7 ppm, 1.4-13.2 ppm, 7-34 ppm, and 0.9-2.7 ppm respectively. The huntite-magnesite is characterized by relatively lower Ni (0.5-2.4 ppm) and Co (0.5-1.1 ppm) contents. The huntite and magnesite-bearing huntite occurrences have higher Ba, Sr, As, Zr, V, and U contents than those of the other elements. The δ13C isotope values vary between 7.8‰ to 8.8‰ PDB for huntite+magnesite, 8.2‰ PDB for huntite, 1.4‰ PDB for magnesite+dolomite, and 4.0‰ PDB for limestone from deposits in the study area. The δ18O isotope values of the huntite deposits ranged from 30.4 to 35.5‰ SMOW for huntite+magnesite, 32.4‰ SMOW for huntite, 29.8‰ SMOW for magnesite+ dolomite, and 26.9‰ SMOW for limestone. The presence of nodular huntite and the abundance of gastropod, ostracoda and Chura shells in the carbonate units indicate that the huntite occurrences are precipitated at shallow, alkaline (8.5-9.5 pH) and lower temperature (approximately 25°C) lake conditions. The Mg++, Ca++ and Si++ ions for the huntite formation were derived from the surrounding rocks such as ultrabasic rocks, dolomite, dolomitic limestone, and limestone in the Egirdir-Hoyran lake basin. Also, the C isotope ratios indicate that the CO2 source for the huntite formations results to sedimentary basin from metamorphic CO2, carbonate rocks, fresh water carbonates, and ground water. The source of oxygen for the huntite formation may come from marine limestone, fresh water carbonates and meteoric water.

Trace element contents and C-O isotope geochemistry of the different originated magnesite deposits in Lake District (Southwestern Anatolia), Turkey

Vein-stockwork magnesite in the Madenli area, sedimentary huntite-magnesite in the Asagitirtar area, and lacustrine hydromagnesite in the Salda Lake area are located in the Beysehir-Hoyran and Lycian nappe rocks around Isparta and Burdur, Southwest Anatolia. The aim of this study is to understand trace element contents and carbon-oxygen isotope ratios in different originated magnesite, magnesite bearing huntite, and hydromagnesite deposits. Also, the element contents and isotope ratios of the magnesite occurrences are to compare with each other and similar magnesite occurrences in Turkey and world. It is found that the Madenli magnesite occurrences in the Şarkikaraagac ophiolites, Asagitirtar magnesite bearing huntite deposits in the lacustrine rocks of the Miocene-Pliocene, and the Salda hydromagnesite deposits in lacustrine basin on the Yesilova ophiolites. The paragenesis contains a common carbonate mineral magnesite, less calcite, serpentine, smectite, dolomite, and talc in the Madenli magnesite occurrences, mostly huntite and locally magnesite, dolomite, calcite, illite, quartz, and smectite in the Asagitirtar huntite-magnesite occurrences, and only hydromagnesite mineral in the Salda Lake hydromagnesite occurrences. Vein and stockwork Madenli magnesite deposits were recognized by higher total iron oxide concentrations (mean 1.10 wt%) than sedimentary Asagitirtar magnesite bearing huntite (mean 0.13 wt%) and lacustrine Salda hydromagnesite (mean 0.22 wt%) deposits. It is suggested that high Fe content (up to 5%) in the magnesite associated with ultramafic rocks than those from sedimentary environments (≤1% Fe). Based on average Ni, Co, Ba, Sr, As and Zr contents in the magnesite deposits, average Ni (134.63 ppm) and Co (15.19 ppm) contents in the Madenli magnesite and Salda hydromagnesite (36.85 ppm for Ni, 3.15 ppm for Co) have higher values than Asagitirtar huntite + magnesite (7.67 ppm for Ni and 0.89 ppm for Co). Average Ni-Co contents of these deposits can have close values depending on ophiolite host rock. Average Ba values of the Madenli (108.09 ppm) and Asagitirtar (115.88 ppm) areas are higher than those of Salda hydromagnesite (13.15 ppm). Sediment-hosted Asagitirtar magnesite-huntite deposits have the highest Sr contents (mean 505.81 ppm) as reasonably different from ultrabasic rock-related Madenli magnesite (mean 38.76 ppm) and Salda hydromagnesite (mean 36.70 ppm). The highest Sr content of sedimentary Asagitirtar deposits reveals that Sr is related to carbonate rocks. As and Zr contents have the highest average values (As 52.76 ppm and Zr 9.67 ppm) in the Asagitirtar deposits different from Madenli magnesite (As 0.54 ppm and Zr 1.67 ppm) and Salda hydromagnesite (As 0.5 ppm and Zr 2.58 ppm) deposits. High As and Zr concentrations in the Asagitirtar magnesite-huntite deposits may come from volcanic rocks in near country rocks. The δ 13C (PDB) isotope values vary between −10.1 and −11.4‰ in the Madenli magnesite, 7.8 to 8.8‰ for huntite, 1.7 to 8.3‰ for huntite + magnesite and 4.0‰ for limestone + magnesite in the Asagitirtar huntite-magnesite deposits, and 4.4 to 4.9‰ for Salda Lake hydromagnesite. The sources of the CO2 are hydrothermal solutions, meteoric waters, groundwater dissolved carbon released from fresh water carbonates and marine limestone, soil CO2, and plant C3 in the Madenli magnesite, and may be deep seated metamorphic reactions in limestone and shales of rich in terms of organic matter. The sources of CO2 in Asagitirtar huntite and Salda hydromagnesite were meteoric water, groundwater dissolved inorganic carbon, fresh water carbonates, and marine limestone. The δ 18O (SMOW) isotope composition ranges from 26.8 to 28.1‰ in the Madenli magnesite, 30.4 to 32.4‰ for huntite and 29.8 to 35.5‰ for huntite + magnesite and 26.9‰ for limestone + magnesite in the Asagitirtar area, and 36.4 to 38.2‰ in the Salda Lake hydromagnesite. The Salda Lake hydromagnesite has heavier oxygen isotopic values than others. The sources of oxygen in the Madenli magnesite deposits are hydrothermal solutions, meteoric water, freshwater carbonates, and marine limestone, but the sources of oxygen of the Asagitirtar magnesite-huntite are meteoric water, fresh water carbonates, and marine limestone. The Salda Lake hydromagnesite has very high δ18O isotope values indicating a strong evaporitic environment. Magnesium (Mg+2) and silica are released by disintegration of very weathered-serpentinized ultrabasic rocks of all magnesite deposits and from partly dolomite and dolomitic limestone in the Asagitirtar magnesite bearing huntite deposits. In the Asagitirtar area, calcium (Ca+2) for huntite mineralization is provided by surrounding carbonate rocks. Based on isotopic data, host rocks, petrographic properties of the Madenli magnesite can be described as an ultramafic-associated hydrothermal vein mineralization corresponding to “Kraubath type” deposits, but Asagitirtar ve Salda Lake deposits are sedimentary mineralization (lacustrine/evaporitic) corresponding to “Bela Stena type” deposits. The estimated temperature using average δ18O isotope values is about 33.51 °C for Madenli magnesite, 48.33 °C for Asagitirtar huntite-magnesite, and 25 °C for Salda hydromagnesite. Based on isotope data, we can be say that the Madenli magnesite, Asagitirtar magnesite-huntite, and Salda hydromagnesite occur at low to moderate-low temperature water and alkaline (pH 8.5–10.5) under surface or near-surface conditions.

Facies, rock attributes, stratigraphy, and depositional environments: Yanchang Formation, Central Ordos Basin, China

The Late Triassic Yanchang Formation (Fm) is a major target of drilling for hydrocarbons in the Ordos Basin. Although most of the early focus on this thick succession of lacustrine rocks has been the dominant deltaic sandstones and siltstones, which act as local reservoirs of oil and gas, more recent consideration has been given to the organic-rich mudstone source rocks. We used modern chemostratigraphic analysis to define vertical facies successions in two closely spaced cores through the Chang 7 Member, the primary source rock for the Yanchang hydrocarbon system. We used integrated high-resolution X-ray fluorescence and X-ray diffraction measurements to define four dominant facies. Variations in stable carbon isotopes mimic facies stacking patterns, suggesting that terrigenous organic matter (although minor in volume) is associated with the arkoses and sandstones, whereas aquatic organic matter is dominant in the mudstones. Facies stacking patterns define three major depositional cycles and parts of two others, each defined by basal mudstone facies that document basin flooding and deepening (i.e., flooding surfaces). Unconfined compressive strength measurements correlate with clay mineral abundance and organic matter. Comparisons of core attributes with wireline logs indicate that although general variations in clay mineral volumes (i.e., mudstone abundance) can be discerned from gamma-ray logs, organic-matter distribution is best defined with density or resistivity logs. These findings, especially those established between the core and log data, provide a powerful linkage between larger scale facies patterns and smaller scale studies of key reservoir attributes, such as pore systems, mineralogy, diagenesis, rock mechanics, hydrocarbon saturation, porosity and permeability, and flow parameters. This first application of modern chemostratigraphic techniques to the Yanchang Fm reveals the great promise of applying these methods to better understand the complex facies patterns that define this lacustrine basin and the variations in key reservoir properties that each facies displays.

Perennial Lakes as an Environmental Control on Theropod Movement in the Jurassic of the Hartford Basin

Eubrontes giganteus is a common ichnospecies of large dinosaur track in the Early Jurassic rocks of the Hartford and Deerfield basins in Connecticut and Massachusetts, USA. It has been proposed that the trackmaker was gregarious based on parallel trackways at a site in Massachusetts known as Dinosaur Footprint Reservation (DFR). The gregariousness hypothesis is not without its problems, however, since parallelism can be caused by barriers that direct animal travel. We tested the gregariousness hypothesis by examining the orientations of trackways at five sites representing permanent and ephemeral lacustrine environments. Parallelism is only prominent in permanent lacustrine rocks at DFR, where trackways show a bimodal orientation distribution that approximates the paleoshoreline. By contrast, parallel trackways are uncommon in ephemeral lacustrine facies, even at sites with large numbers of trackways, and those that do occur exhibit differences in morphology, suggesting that they were made at different times. Overall, the evidence presented herein suggests that parallelism seen in Hartford Basin Eubrontes giganteus is better explained as a response to the lake acting as a physical barrier rather than to gregariousness. Consequently, these parallel trackways should not be used as evidence to support the hypothesis that the trackmaker was a basal sauropodomorph unless other evidence can substantiate the gregariousness hypothesis.

A 25 million year macrofloral record (Carboniferous–Permian) in the Czech part of the Intra-Sudetic Basin; biostratigraphy, plant diversity and vegetation patterns

The Intra-Sudetic Basin is essentially a post-orogenic (Variscan) continental basin with a depositional history that ranges from the late Viséan to the Triassic. The fossil record in the Czech part of the basin, although discontinuous, spans the interval from the late Viséan to the Asselian. Middle Baskhirian (late Namurian) to Asselian adpression floras collected from boreholes, coal mines and outcrops permit reconstruction of the stratigraphic ranges of individual species in considerable detail, and reveal changes in plant diversity and vegetation patterns throughout this ~25Myr interval at high resolution, even to the scale of individual coal-bearing cycles. Species diversity recorded in the rock record throughout the studied interval mainly was controlled by climatically-driven potential for preservation of plant material, which was highest in poorly drained/waterlogged habitats (wetlands, lakes) concentrated in basinal lowlands and representing major windows of preservation. Low fossilization potential is typical for well-drained fluvial habitats, now represented by red beds deposited under drier (seasonal) climate. Diversity in major windows of preservation was highest during the late Langsettian and Duckmantian (>50 species), lower in the Late Pennsylvanian (25–40 species) and the lowest in the Asselian (~20 species). The diversity in red bed intervals with poor preservation potential varies between 11 and 21 species. The two distinct habitat types, with high and low preservation potential, were dominated by different plant groups. Wetland habitats flourishing during the (per)humid to slightly seasonal intervals were colonized mainly by cryptogamic plants and some early gymnosperms, mainly pteridosperms and cordaitaleans. Habitats represented by red beds were dominated by cordaitaleans; from late Asturian time walchian conifers also occurred in these plant assemblages. However, conifers only became common in red beds in the Cantabrian. In the Saberian, conifers are rarely found in discrete beds associated with coal-bearing strata, however, their common occurrences start in lacustrine rocks of Asselian age. Temporal changes in vegetation patterns that show an increasing proportion of gymnosperms, in both wetland and dryland habitats are in agreement with the well known aridification trend in late Paleozoic tropical Pangea. Delays between the first appearance of walchian conifers in red beds (late Asturian) versus coal-bearing deposits (Saberian) supports the hypothesis that evolutionary innovations took place outside the windows of preservation, i.e. in well-drained, moisture-deficient areas.

The origin and occurrence of subaqueous sedimentary cracks

Abstract The rock record attests that sediments have cracked at or below the sediment–water interface in strictly subaqueous settings throughout Earth history. In recent decades, a number of hypotheses have been advanced to explain this phenomenon, but these are widely regarded as being mutually exclusive and there is little consensus about which model is correct. In this paper, we first review the geometries, lithologies and range of facies in which subaqueous sedimentary cracks occur in the geological record, with particular attention to cracks in carbonates. We then evaluate current models for subaqueous cracking, emphasizing that different models may be correct with respect to different sets of cracks, but that cracking is generally a two-step process involving sediment stabilization prior to disruption. We also present the results of some simple new experiments designed to test the dominant models of crack formation. These results demonstrate for the first time that microbial mats can produce thin, shallow cracks at the sediment–water interface. We conclude that the presence of cracks in marine, brackish and lacustrine rocks should not be used uncritically to infer fluctuations in salinity in the depositional environment. Supplementary material: A video showing a micro-CT scan of a hand-sample from the Monteville Formation, South Africa is available at https://doi.org/10.6084/m9.figshare.c.3580673

New petrified forest in Maranhão, Permian (Cisuralian) of the Parnaíba Basin, Brazil

This work presents a new fossil plant-bearing area located in the municipalities of Duque Bacelar and Coelho Neto, Maranhão State, Brazil, recovered from lower Permian (Cisuralian) strata of the Pedra de Fogo Formation, northeastern portion of the Parnaíba Basin. The area comprises more than five exposures with assemblages formed mostly of large gymnosperm woods, a number of them in life-position (reaching up to ∼2.30 m in height and 1.15 m in diameter) and, in lesser degree, of horizontal tree-fern stems (up to 5 m in length), some of them being referable to Psaronius sp. The fossils are recorded in sedimentary beds of continental origin that accumulated in shallow, nearshore areas of large lakes, which eventually were affected by rapid burial episodes generated by non-channelized, high energy fluvial systems. The new fossil assemblages are included within lacustrine rocks placed at the base of the Pedra de Fogo Formation, i.e., Sílex Basal Member, near the contact with the underlying Piauí Formation (Pennsylvanian). This observation contrasts with previous studies at the southwestern portion of the basin, where the stratigraphic position of plant fossils is referred to the upper Pedra de Fogo Formation (Trisidela Member) or even to the overlying Motuca Formation. The new sites currently suffer damage from human activities and require urgent actions in order to protect them. Based on the current laws, some measures of protection for these sites are discussed and proposed herein.

Composition of conglomerates analyzed by the Curiosity rover: Implications for Gale Crater crust and sediment sources

AbstractThe Curiosity rover has analyzed various detrital sedimentary rocks at Gale Crater, among which fluvial and lacustrine rocks are predominant. Conglomerates correspond both to the coarsest sediments analyzed and the least modified by chemical alteration, enabling us to link their chemistry to that of source rocks on the Gale Crater rims. In this study, we report the results of six conglomerate targets analyzed by Alpha‐Particle X‐ray Spectrometer and 40 analyzed by ChemCam. The bulk chemistry derived by both instruments suggests two distinct end‐members for the conglomerate compositions. The first group (Darwin type) is typical of conglomerates analyzed before sol 540; it has a felsic alkali‐rich composition, with a Na2O/K2O &gt; 5. The second group (Kimberley type) is typical of conglomerates analyzed between sols 540 and 670 in the vicinity of the Kimberley waypoint; it has an alkali‐rich potassic composition with Na2O/K2O &lt; 2. The variety of chemistry and igneous textures (when identifiable) of individual clasts suggest that each conglomerate type is a mixture of multiple source rocks. Conglomerate compositions are in agreement with most of the felsic alkali‐rich float rock compositions analyzed in the hummocky plains. The average composition of conglomerates can be taken as a proxy of the average igneous crust composition at Gale Crater. Differences between the composition of conglomerates and that of finer‐grained detrital sediments analyzed by the rover suggest modifications by diagenetic processes (especially for Mg enrichments in fine‐grained rocks), physical sorting, and mixing with finer‐grained material of different composition.