Freshwater Lacustrine Environment Research Articles

Detection of Early-Middle Devonian biotic crises in East Gondwana, Paraná Basin, Brazil: An organic geochemical approach

Changes detected in the physicochemical conditions of the depositional palaeoenvironment of the Ponta Grossa Formation (Early-Middle Devonian), East Gondwana, were correlated with significant biotic crises that preceded the great Devonian extinction event (Frasnian-Famennian), culminating in the extinction of the Malvinokaffric fauna. The present study identified Zilchov, Daleje, Choteč, and Kačák biotic crisis events through geochemical analyses. Fifteen outcrop shale samples were analysed for the distributions of well-established geochemical parameters, such as those based on saturated and aromatic biomarkers and polycyclic aromatic hydrocarbons. Parameter results were correlated to polar compound distributions using atmospheric pressure photoionisation (APPI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in positive ionisation mode. A decrease in the summation of n-alkanes, αβC30 hopane, and gammacerane concentrations was observed at the Pragian-Emsian interval, related to the Zilchov event and the first decline of the Malvinokaffric fauna. At the Late Emsian, a severe change in paleoenvironment depositional conditions was detected by the increase in αβ C30 hopane, tetrahydrophenanthrene and gammacerane concentrations, suggesting water column stratification. Moreover, Tricyclic Index values indicated a change to a more freshwater lacustrine environment, which was confirmed by the occurrence ofn-alkyl cyclohexanes series. In addition, high percentages of O1 and N1 class compounds indicated that this interval was probably composed of a mixture of bacterial and terrigenous organic matter. These results indicated a large inflow of fresh and continental waters due to a regression related to the Daleje event and the second decline of the Malvinokaffric fauna. A significant decrease of the pristane/phytane ratio, summation of n-alkanes, αβ C30 hopane and gammacerane concentrations was observed, along with the absence of N1 compounds at the Emsian-Eifelian interval, representing a rapid transgression, corresponding to the Choteč event. At the Eifelian-Givetian interval, the detection of n-alkanes and biomarker low concentration values, low pristane/phytane values (anoxia), as well as the non-detection of N1 class compounds, indicated a great transgression related to the Kačák event and the disappearance of the Malvinokaffric fauna.

The Significances of Cretaceous Petrified Wood Fossils from Padangcermin, Lampung in Paleoenvironmental Perspectives

Reports on the presences of petrified wood fossils in Padangcermin, Lampung, are rare, no strong examination that expose the scientific concern on its paleoenvironment perspective. By the published geological map, these fossils was found in the area that labelled as Cretaceous sedimentary rock (145-66 mya). Recently, the known oldest petrified wood fossil in Indonesia, is from Miocene epoch (23.03-5.33 mya). It is an interesting circumstance to examine. By this study, we are trying to confirm this circumstances by conducting the geological mapping, collecting, and identifying the petrified wood fossils that could be found as in situ fossils in the area. This study reveals a new update in paleoenvironmental perspective. The petrified woods in this area are commonly preserved in cherty layer. There are also several fossilized aquatic freshwater plant remains that presences in this area. We suggest the fossilization of the petrified wood fossil in this area is related to high alkaline-freshwater lacustrine. Supposedly, it associated with the Cretaceous terrestrial landmass and freshwater lacustrine environment in southern paleo-Sumatra by paleontological interpretation of data in this study.

Geochemical Characteristics and Organic Matter Provenance of Shale in the Jurassic Da’anzhai Member, Northeastern Sichuan Basin

Rock pyrolysis, organic petrology, vitrinite reflectance, gas chromatography-mass spectrometry (GC-MS) analysis, and biomarker compound analysis were performed to comprehensively analyze the organic geochemical characteristics of the Jurassic Da’anzhai Member (J1da) shale strata in Yuanba and Puguang areas in the northeastern Sichuan Basin. Then the organic matter provenance and sedimentary environment were further analyzed. Finally, the significance of oil and gas exploration in J1da shale strata was discussed. Results show that the second section of the Da’anzhai Member (J1da2) has relatively high organic matter abundance (1.24%TOC), type Ⅱ-dominated organic matter type, which is the most favorable section of wells Y1 and T1 in the study area. The organic matter maturity and the hydrocarbon phases are quite different, which is 1.01%Ro dominated by oil generation in Puguang area, while it is 1.67%Ro dominated by gas generation in Yuanba area. Content and chromatograms of biomarkers including n-alkanes, tricyclic terpanes, C24 tetracyclic terpanes, and C27-C28-C29 regular steranes show that the organic matters of J1da shale strata derive from both terrestrial higher plants and lower aquatic organisms, with slightly differentiated mixed ratio of each sublayer. Characteristics of Pr/Ph, γ-cerane and hopanes compounds indicate that the overall depositional environment of J1da is a freshwater lacustrine environment, with saline lake deposits in local areas and intervals in the study area. The rapid changes of sedimentary environment have resulted in obvious stratification of water body, frequent interbeds, and strong heterogeneity of J1da shale strata. Comprehensive analysis shows the shale/mud microfacies in the semi-deep lake subfacies and shale/mud interbedded with siltstone and shell bank microfacies in the shallow lake subfacies are the most favorable sedimentary facies for J1da hydrocarbon enrichment. Deeper burial depth and higher maturity make for oil and gas enrichment with higher gas/oil ratio (GOR); moreover, the thicker intervals with organic-rich shale are favorable targets for geochemical evaluation.

BIOMARKER GEOCHEMISTRY OF PALEOGENE CRUDE OILS AND SOURCE ROCKS FROM THE RAOYANG SAG, BOHAI BAY BASIN, NE CHINA: AN OIL – SOURCE ROCK CORRELATION STUDY

This study presents a systematic geochemical analysis of Paleogene crude oils and source rocks from the Raoyang Sag in the Jizhong sub‐basin of the Bohai Bay Basin (NE China). The geochemical characteristics of fifty‐three oil samples from wells in four sub‐sags were analysed using gas chromatography (GC) and gas chromatography – mass spectrometry (GC‐MS). Twenty core samples of mudstones from Members 1 and 3 of the Eocene‐Oligocene Shahejie Formation were investigated for total organic carbon (TOC) content and by Rock‐Eval pyrolysis and GC‐MS to study their geochemistry and hydrocarbon generation potential. The oils were tentatively correlated to the source rocks.The results show that three groups of crude oils can be identified. Group I oils are characterized by high values of the gammacerane index and low values of the ratios of Pr/Ph, Ts/Tm, 20S/(20S+20R) C29 steranes, ββ/(ββ+αα) C29 steranes, C27 diasteranes/ C27 regular steranes and C27/C29 steranes. These oils have the lowest maturity and are interpreted to have originated from a source rock containing mixed organic matter deposited in an anoxic saline lacustrine environment. The biomarker parameter values of Group III oils are the opposite to those in Group I, and are interpreted to indicate a highly mature, terrigenous organic matter input into source rocks which were deposited in suboxic to anoxic freshwater lacustrine conditions. The parameter values of Group II oils are between those of the oils in Groups I and III, and are interpreted to indicate that the oils were generated from mixed organic matter in source rocks deposited in an anoxic brackish–saline or saline lacustrine environment.The results of the source rock analyses show that samples from Member 1 of the Shahejie Formation were deposited in an anoxic, brackish – saline or saline lacustrine environment with mixed organic matter input and are of low maturity. Source rocks in Member 3 of the Shahejie Formation were deposited in a suboxic to anoxic, brackish – saline or freshwater lacustrine environment with a terrigenous organic matter input and are of higher maturity. Correlation between rock samples and crude oils indicates that Group I oils were probably derived from Member 1 source rocks, while Group III oils were more likely generated by Member 3 source rocks. The Group II oils with transitional characteristics are likely to have a mixed source from both sets of source rocks.

Application of chemometrics to quantitative source assessment of crude oils from the Zhanhua Depression, Bohai Bay Basin, northeast China

The chemometric methods alternating least squares and multidimensional scaling were employed in this study to quantify the relative contribution of the sources and identify the affinities of sixty–five crude oil samples collected in the Zhanhua Depression. The circos diagram was used to reflect the relative contributions of different source rocks. Based on these techniques, two end–members (EMs) were identified which represent the geochemical characteristics of two regional source–rocks (Es3L and Es4U). The EM1 composition assigned to Es3L is characterized by relatively low C35/C34 homohopane ratio and relatively low abundance of gammacerane. This EM is consistent with a source facies that was deposited in freshwater lacustrine environment that contained a non–stratified water column and suboxic to dysoxic bottom waters. Distributions of the regular steranes and tricyclic terpanes, the relatively low steranes/hopanes ratio suggest the organic matter input into this source facies were combined microalgal material and terrestrial organic matter. Oils from the Bonan Sag and Gubei Sag were derived mainly from this source facies. In contrast, EM2 has been assigned to the Es4U source rock and is characterized by relatively high abundance of gammacerane, C35/C34 homohopane ratio greater than one, suggesting deposited in a saline to hypersaline lacustrine environment that contained a stratified water column with strongly reducing bottom waters. Distributions of the regular steranes and tricyclic terpanes as well as relatively high steranes/hopanes ratio suggest the organic matter deposited in this setting was mainly derived from microalgal/bacterial sources. Oils from the Luojia Nose and Chanjiazhang High were derived mainly from this source facies. Mixed contributions of these two source facies (end–members) account for the compositions of crudes produced in the Yihazhuang Arch and Gudao High. In the case of the Yihezhuang Arch the crudes appear to have near equal contributions from the two source faces, whereas crudes from the Gudao High appear to have slightly higher contributions from the Es3L source facies than from the Es4U source facies.

Biostratigraphic and petrological characteristics of Cretaceous\u2013Paleogene sediments in the eastern Cuu Long delta

The basic structure of reservoir models is strongly affected by the stratigraphic interpretation and the properties of reservoir rock. The estimation of the volume of hydrocarbon accumulation will depend on the accuracy of predicting and distributing reservoir quality. Biostratigraphic and sedimentary petrologic results provide geological information to clarify the stratigraphy and properties of sedimentary rocks on the eastern margin of the Cuu Long delta plain trough. On the basis of stratigraphic correlation of the wells and studying the structures of the area, the deposition, thickness of sediments and the ability to correlate with prospect rocks on the southeastern Vietnam continental shelf were determined. The results have identified the basement rock of wells TC-1 and TC-2, which are Cretaceous metasandstones and were deposited in freshwater fluvial environments with high-energy conditions. The upper Paleogene sediments were overlaid directly on the Cretaceous basement rock in the TC-2 well but were completely absent in the TC-1 well. The lithologic composition is mainly feldspathic litharenite sandstone, which is formed in freshwater fluvial and freshwater lacustrine environments. This study provides new data on stratigraphic column of the eastern Cuu Long delta plain. The data show that the age of the sedimentary basement rock is Cretaceous which has not been determined in previous studies. The findings of this study can help for better understanding of the geological development history and the completely stratigraphic column of the Cuu Long delta to correlate with objects that are hydrocarbon accumulation in Cuu Long sedimentary basin in order to improve efficiency in petroleum exploration activities.

Lake basin evolution from early to Middle Permian and origin of Triassic Baikouquan oil in the western margin of Mahu Sag, Junggar Basin, China: Evidence from geochemistry

Large-scale oil and gas fields have been discovered after years of petroleum exploration in Mahu Sag, Junggar Basin, confirming the significant resource potential in its western margin. However, the origin of oil in the Triassic Baikouquan (T1b) reservoir and lacustrine basin evolution during the Permian are still poorly understood. In this study, Permian source rocks and T1b crude oil were analyzed using Rock-Eval pyrolysis, vitrinite reflectance, stable carbon isotopes, and biomarker compositions. Results show that the Lower Wuerhe source rock (P2w) was developed in a freshwater lacustrine environment with high oxygen content, higher plant input, and now contains typical humic (type-III) kerogen at the low thermal maturity stage. The Fengcheng source rock (P1f) was developed in an evaporative lake after mixed seawater (sea and lake). Deposition included anoxic conditions, terrigenous input, high salinity, and intense evaporation, and was dominated by sapropelic (type-II) kerogen, which is currently at the thermal maturity stage. The Jiamuhe source rock (P1j) was developed in a transitional marine and terrigenous environment with terrigenous input, moderate salinity, and reducing water, and now contains mainly type-III kerogen at the high thermal maturity stage. The Junggar region's climate was humid–arid–humid for the Early to Middle Permian. Mahu Sag was not completely closed until the end of P1j deposition and did not completely evolve into a lacustrine basin with large-scale freshwater input until the P2w deposition. Crude oil of the T1b formation is very mature and is derived from the source rock of the transitional marine and terrigenous environments deposited in the center of Mahu Sag, with high salt content, weak reducibility, and low evaporite content. Using biomarkers, carbon isotopes, and fine lithologic analysis, the oil-source correlation shows that T1b crude oil came from P1f calcareous and tuffaceous source rocks.

Quaternary Stratigraphic Division and Paleoenvironmental Evolution Observed from Core LZK1 on Hengsha Island, Shanghai

AbstractThe Quaternary sediments in the Yangtze delta are loose and lack precise stratification marks in the lithology. Moreover, due to the limitations of dating methods, it is difficult for Quaternary cores to deliver accurate age constraints. Thus, it is a challenge to establish the Quaternary stratigraphic framework. Gravity core LZK1 was drilled on Hengsha Island, Shanghai, in the Yangtze delta, in 2012. The core was terminated at 403.83 m below the local land surface, the uppermost 291.2 m comprising a thick sequence of Quaternary sediments. This study investigated the stratigraphic subdivision and paleoenvironmental change of the Quaternary sediments. From bottom to top, the Quaternary stratigraphic sequence can be subdivided into the lower Pleistocene Anting Formation, Middle Pleistocene Jiading Formation, Upper Pleistocene Chuansha Formation and Nanhui Formation, Holocene Loutang Formation, Shanghai Formation, and Rudong Formation. According to this study, the Hengsha Island area was dominated by a freshwater lacustrine environment during the early Pleistocene, an alternation of shallow lake and shore lake environment during the Middle Pleistocene, a delta plain to lagoonal environment during the early Upper Pleistocene, a fluvial channel to floodplain environment from the LGM (Last Glacial Maximum) to the end of the Upper Pleistocene, and a delta environment during the Holocene.

Organic geochemical characteristics and depositional setting of Paleogene oil shale, mudstone and sandstone from onshore Penyu Basin, Chenor, Pahang, Malaysia

The eastern Chenor Pahang area of the Penyu Basin contains an untapped potential, in part due to poor knowledge of its organic facies characteristics The sedimentary succession of eastern Chenor Pahang area is dominated by fine to coarse grained sandstone occurring in association with oil shale and mudstone in a predominantly fining upward sequence. Detailed geochemical and petrographic characterization was carried out on oil shale and mudstone samples to evaluate the origin and type of organic matter (OM), as well as the paleoenvironmental conditions and organic richness of the Paleogene sedimentary succession. The facies are only enriched in total organic carbon (TOC), potential hydrocarbon yield (S2) and extractible organic matter (EOM) the types of organic matter are important index reflecting the potential of the source rock(s). Based on elemental ratios, the analyzed samples are shown to contain Types I and II kerogens. This finding is supported by the high hydrogen index (HI) of >300 mg HC/g TOC and the presence of abundant liptinitic materials. Biomarker signatures [n-alkanes >n-C25, high C29/C30 17α (H) hopane ratios (0.8–3.3) and high C29 steranes (49–78%)]. Low total sulfur and V/(V + Ni) ratio] suggest a predominantly terrestrial source input with some contribution from aquatic microorganisms as indicated by the common occurrence of amorphous organic matter. Combining this results with n-alkane patterns, kerogen types and organic petrography, predominance of C29 sterane probably indicates a contribution of aquatic organisms such as green and brown algae (consistent with Botrycoccus braunii race A and Pediastrum contribution) and some influence of higher plants OM. Integration of biomarker studies with bulk geochemical and petrographic analysis suggests that the analyzed samples were deposited under suboxic conditions in a fresh water lacustrine environment. This finding is corroborated by the ratios of pristane/phytane, V/Ni and Sr/Br. High bioproductivity and a stratified fresh water column with suboxic bottom water condition enhanced OM preservation in the analyzed mudstone and shale facies. The high OM content in these samples maybe related to the fine-grained matrix of the mud rocks and the paleoclimate in the area.

MAJOR AND TRACE ELEMENTS GEOCHEMISTRY OF ĐỒNG HO SEDIMENTS (QUẢNG NINH, VIET NAM): IMLICATION FOR PALEOENVIRONMENTAL CONDITION

The Dong Ho sedimentary formation consists of gravel, sand and sandstone, mudstone interbeded with asphalt layer or oil shale cropping out at Quang Ninh is considered as outcrop of petroleum potential source rock and correlated to source rock of the Cenozoic basins on the continental shelf of Southeast Asia. Geochemical investigation of major and trace elements content variation from 14 typical samples selected from diferent layers leads to divide the Dong Ho formation into two parts: the lower part characterized by unclear variation while the upper part exposing a more clear trend. The paleoenvironmental proxy and the CIA, CIW, PIA and CPA indices of the Dong Ho formation revealed high weathering intensity. V/Ni and C/Cr s vary from 0.14 to 1.52; and from 0.02 to 0.52 respectively indicate to oxic depositional environment. The provenance of the Dong Ho sedimentary layers come from the recycling of sedimentary source rocks and deposited within freshwater lacustrine environment dominated with humid climate with estimated mean annual rainfall of 1533 mm / yr ± 181 mm before changing to wet and reductioin environment during diagenesis.

Sedimentary Environment and Geochemical Characteristics of Source Rocks from Later Carboniferous and Middle Permian in Santanghu Basin, NW China

There is a long-term controversy on sedimentary environment of Late Carboniferous-Permian in Santanghu Basin. In this paper, we study on the geochemical characteristics of source rocks from Harjiawu Formation of Upper Carboniferous and Lucaogou Formation of Middle Permian in Santanghu Basin. The results show that the abundance and maturity of organic matter of source rocks in Harjiawu Formation are both high, which shows excellent source rocks. In contrast, the abundance of organic matter from source rocks in Lucaogou Formation is relatively low and the maturity of organic matter is between immature and mature, which suggests good-excellent source rocks. The organic geochemical characteristics indicate the freshwater lacustrine environment of Late Carboniferous, while Lucaogou Formation is the high salinity lacustrine environment. In addition, magmatic-hydrothermal activity may result in the water salinization of Permian.

Geochemical characterisation of Early Cretaceous lacustrine sediments of Bima Formation, Yola Sub-basin, Northern Benue Trough, NE Nigeria: Organic matter input, preservation, paleoenvironment and palaeoclimatic conditions

The Bima Formation is the oldest lithogenetic unit occupying the base of the Cretaceous successions in the Yola Sub-basin of the Northern Benue Trough. It is differentiated into Lower (B1), Middle (B2) and Upper (B3) Bima Members. Integrated organic and inorganic geochemical studies were applied on the lacustrine sediments of the Bima Formation with the aim of reconstructing the palaeodepositional environment and organic matter input in response to climate and tectonism. The analysed sediments were deposited in a freshwater lacustrine environment with a low-salinity stratified water column and suboxic to relatively anoxic conditions, as indicated by the bulk geochemical parameters. The biomarkers provide evidence for a contribution of aquatic algae and microorganisms, with a significant amount of terrigenous organic matter input. The preservation of the organic matter is possibly attributed to the stratified water column with low salinity and suboxic to relatively anoxic conditions. Based on the geochemistry of major and trace elements, the sediments were deposited during semi-arid climatic conditions within passive continental margin setting. This agrees with the tectonic events in the West and Central African Rift System (WCARS) during the Early Cretaceous period.

Early Eocene fossil plants from the Mwadui kimberlite pipe, Tanzania

An early Eocene (52±2Ma) flora from the Mwadui kimberlite pipe in Tanzania includes ten leaf morphotypes, small seeds, fossil wood putatively related to Cynometra (Detarieae, Fabaceae), and a sparse palynoflora. The leaf flora is characterised by microphyllous and notophyllous entire margined leaves that are suggested to be related to the Euphorbiaceae and Fabaceae, although many cannot be firmly placed in modern families. The wood is the oldest member of Cynometroxylon and confirms that the Detarieae were diverse by the early Paleogene. The palynoflora is dominated by Ailanthipites daedaleus and Triporopollenites mwaduiensis with lesser amounts of small tricolpate (Tricolpites) and monosulcate (Lilliacidites) grains together with spores (Cyathidites spp., Triporoletes laevigatus) and freshwater algal cysts. Other components of the palynoresidue include abundant charcoalified tracheids and fungal hyphae. The low diversity and sparse palynoflora suggests a restricted catchment with little input from the regional vegetation. The sediments accumulated in a freshwater lacustrine environment with palynological input from the crater walls that were covered by low diversity vegetation. This is supported by the leaf floras. Although the leaf floras are more diverse with nine morphotypes identified, the small leaves (microphyll to notophyll) suggest a water-limited palaeoenvironment. This is supported by the low diversity and abundance of fern and bryophyte spores. The presence of a single wood taxon not only indicates that there were large trees around the crater-lake but that the aboreal component was of low diversity. These observations suggest an open woodland-like setting, while the taxonomic affinities of the wood (Cynometra) suggest a humid microclimate possibly with marked wet–dry seasonal contrasts.

Geochemical characteristics of crude oil in fluvial deposits from Maling oilfield of Ordos Basin, China

Maling oilfield is one of the earliest discovered oilfields in the Ordos Basin. It is also a typical representative of oil reservoirs in fluvial deposits. However, due to the deficiency of geochemical study on the crude oils and source rocks, the interpretation of the oil origin is largely based on the geological situation. Maling crude oils (21) and source rocks (14) from the basin have been systematically analyzed. The distribution of the biomarkers indicates that the crude oils mainly originated from source rocks deposited under a weakly reducing to sub-oxic, freshwater lacustrine environment. Besides algae, higher land plants have also made an important contribution to the source organic matter. Comparison of the distributions of the biomarkers suggests that there is a correlation between the Maling oilfield oils and the source rocks of the Yanchang Formation.

Organic facies of the Oligocene lacustrine system in the Cenozoic Taubaté basin, Southern Brazil

Abstract A quantitative study of palynofacies and organic facies was carried out in the Cenozoic sedimentary succession of the Taubate basin. The palaeoenvironmental distributions of particulate organic components were examined in relation to various potential controlling factors (lithology, depositional environment, and stratigraphic sequence). About 30 samples were analyzed from a single borehole drilled in Sao Paulo State. This involved rigorous kerogen counts supported by 30 TOC and sulphur, 20 Rock-Eval pyrolysis and 20 biochemistry analyses. In the studied area the Oligocene deposits are approximately 600 m thick. Data was obtained by counting 300 to 500 kerogen particles per sample, and then recalculating the percentages of the different constituent groups (phytoclast, palynomorph, and amorphous organic matter — AOM). The kerogen assemblage is palynomorph-dominated (> 70%), consisting mainly of freshwater lacustrine algae ( Pediastrum and Botryococcus ). Non-opaque phytoclasts predominate in most of the kerogen samples. However, significant variations in the proportion of biostructured and non-biostructured brown phytoclasts occur in some intervals. The fluorescent AOM content occurs in some levels and is composed of two main components: an intensively fluorescing structureless organic matter, which also has high lipid and protein content indicating a probable bacterial origin for the organic matter (microbial mats); and terrestrially-derived amorphous organic matter, which has been reworked to a different extent exhibiting a high content of carbohydrates. Comparison of the mean kerogen assemblages from different lithologies revealed that the finer-grained lithologies generally have higher palynomorph percentages, whereas coarser-grained lithologies present higher percentages of phytoclasts, emphasizing the important role of grain size in organic-matter distribution. The TOC content is generally less than 10%, but reaches as high as 44% in some intervals, due mainly to the presence of coaly seams. The T max , Spore Coloration Index, and spectral fluorescence measurement values obtained from these samples indicate a low maturity stage. The integration of these results, together with the organic facies parameters, indicate that the analyzed sedimentary section represents a variation from a freshwater lacustrine environment with an oxidant regime to a restricted saline, lacustrine environment with a dysoxic–anoxic regime. There was an excellent correlation between the optical and geochemical parameters.

A Danian subtropical lacustrine palynobiota from South America (Bororó Formation, San Jorge Basin, Patagonia - Argentina)

New results on a lacustrine Danian palynological association in extra-andean northern Patagonia (lower member of the Cerro Bororo Formation, San Jorge Basin, Chubut province, Argentina) are presented. This is a contribution to the largely unexplored field of Paleogene freshwater environments. The palynobiota includes representatives of the kingdoms Protoctista and Plantae. Twelve of the aquatic species are Protoctista. From the nine Chlorophyta form-species, four correspond to Botryococcaceae, one to Coelastraceae and four to Zygnemataceae (Gelasinicysta, Schizosporis, Ovoidites, and Pseudoschizaea). The remaining forms are dinoflagellate cysts (Peridinium and dinocyst indet.) and prasinophyceans (Leiosphaeridia). Plants are represented by spores and pollen of Bryophytes (Sphagnaceae and Ricciaceae), Pteridophytes (Salviniaceae), and Angiosperms (Arecaceae, Araceae and Sparganiaceae/Typhaceae, the latter representing macrophytes living in or near the swampy areas). Other palynomorph groups closely related with biotopes of permanent moisture are dominated by spores of the families Selaginellaceae, Lycopodiaceae, Cyatheaceae, Schizaeaceae, Polypodiaceae and Psilotaceae. The palynological assemblage reflects a freshwater lacustrine environment that developed in a marine coastal zone nearby brackish coastal swamps. Based on the composition of the palynoflora including palm pollen and other thermophilous taxa, humid, warm climatic conditions are inferred for the area of deposition.

COMPOSITION AND PETROLEUM POTENTIAL OF LAKE FACIES IN THE FA‐MS‐48‐73 WELL, MAE SOON STRUCTURE, FANG OILFIELD, NORTHERN THAILAND

The Fang Basin is one of a series of Cenozoic rift‐related structures in northern Thailand. The Fang oilfield includes a number of structures including the Mae Soon anticline on which well FA‐MS‐48‐73 was drilled, encountering oil‐filled sandstone reservoirs at several levels. Cuttings samples were collected from the well between depths of 532 and 1146 m and were analysed for their content of total organic carbon (TOC, wt%), total carbon (TC, wt%) and total sulphur (TS, wt%); the petroleum generation potential was determined by Rock‐Eval pyrolysis. Organic petrography was performed in order to determine qualitatively the organic composition of selected samples, and the thermal maturity of the rocks was established by vitrinite reflectance (VR) measurements in oil immersion. The TOC content ranges from 0.75 to 2.22 wt% with an average of 1.43 wt%. The TS content is variable with values ranging from 0.12 to 0.63 wt%. Rock‐Eval derived S1 and S2 yields range from 0.01–0.20 mg HC/g rock and 1.41–9.51 mg HC/g rock, respectively. The HI values range from 140 to 428 mg HC/g TOC, but the majority of the samples have HI values >200 mg HC/g TOC and about one‐third of the samples have HI values above 300 mg HC/g TOC. The drilled section thus possesses a fair to good potential for mixed oil/gas and oil generation. On an HI/Tmax diagram, the organic matter is classified as Type II and III kerogen. The organic matter consists mainly of telalginite (Botryococcus‐type), lamalginite, fluorescing amorphous organic matter (AOM) and liptodetrinite which combined with various TS‐plots suggest deposition in a freshwater lacustrine environment with mild oxidising conditions. Tmax values range from 419 to 436°C, averaging 429°C, and VR values range from ∼0.38 to 0.66% R0, indicating that the drilled source rocks are thermally immature with respect to petroleum generation. The encountered oils were thus generated by more deeply buried source rocks.

Insights into the morphology, geometry, and post-impact erosion of the Araguainha peak-ring structure, central Brazil

The 40-km-wide Araguainha structure is the largest and best-exposed complex impact crater in South America. It was excavated in flat-lying sediments of the intracratonic Parana Basin, where target rocks are well exposed and have remained undeformed since the impact event ca. 245 Ma. Despite the excellent state of preservation and exposure, information available on the target rock stratigraphy, post-impact erosion, and morphology of the structure is limited. Our combined field observations and remote sensing analysis demonstrate that Araguainha preserves all features of a shallowly eroded peak-ring crater. The interior of the structure exposes a central peak surrounded by a 5-km-wide annular basin and two main ring features 10–12 and 14–18 km from the center. Analysis of the pre-impact stratigraphy, present morphology, and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin, but minimal to nonexistent beyond the inner ring feature. The formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls. Regional stratigraphic data combined with field observations indicate two periods of post-impact erosion associated with exhumation of the Parana Basin. We estimate that 250–350 m of fallback deposits and target rocks have been removed by erosion. Erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure. In contrast to previous suggestions that the impact took place in a shallow-marine environment, our observations are more consistent with an extremely shallow, brackish to freshwater lacustrine environment, with little effect on the developing impact crater.

Geochemical study of crude oils from the Xifeng oilfield of the Ordos basin, China

The Xifeng oilfield is the largest newly-discovered oilfield in the Ordos basin. In order to determine the possible source, crude oils collected systematically from the oilfield and an adjacent oilfield have been examined isotopically and molecularly. The predominance of long-chain n-alkanes, high abundance of C 29 sterane, lower ratios of C 25/C 26 tricyclic terpane and C 25 tricyclic terpane/C 24 tetracyclic terpane and high C 24 tetracyclic terpane/(C 24 tetracyclic terpane + C 26 tricyclic terpanes ratio in the studied oils suggest generation from a source with mixed terrigenous and algal–bacterial organic matter. The presence of diterpenoid hydrocarbon with abietane skeletons is characteristic of the main contribution of higher land plants to the oils. The biomarker distributions in the oils show that they were formed under a weakly reducing freshwater environment. Molecular maturity parameters indicate that the crude oils are mature. The oil–source rock correlation and oil migration investigation suggest that the oils in the Xifeng oilfield originated from the source rocks of the Yanchang formation deposited in a shallow to deep freshwater lacustrine environment, especially Chang-7 source rocks. The data from the distribution of pyrrolic nitrogen compounds indicate that the charging direction of the Chang-8 crude oils is mainly from the Zhuang 12 well northeast of the oilfield toward the southwest. This direction of oil migration is consistent with that indicated by regional geological data.

BIOMARKER GEOCHEMISTRY OF CRUDE OILS FROM THE QAIDAM BASIN, NW CHINA

A suite of 16 crude oil samples from 13 oilfields in the Qaidam Basin were analyzed using techniques including gas chromatography and gas chromatography ‐ mass spectrometry. Biomarker compositions and parameters were used to investigate the palaeoenvironmental and depositional conditions and to correlate the oils with eachother. Oils from the western Qaidam Basin have pristane/phytane (Pr/Ph) ratios of less than 0.7, and contain abundant gammacerane, C27 steranes, 4‐methyl steranes and long‐chain tricyclic terpanes. C29 sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios show that the western Qaidam oils have variable maturities ranging from immature to mature. Oils from the northern Qaidam Basin, by contrast, have Pr/Ph ratios greater than 3, low gammacerane contents, and relatively abundant C29 steranes, bicyclic terpanes and alkylcyclohexanes. C29 sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios indicate that the northern Qaidam oils are mature.δ13C values, which range from ‐25.4‰ to ‐28.3‰ with the exception of one oil from the north (‐3l.6‰), are similar for oils from both the northern and western parts of the Qaidam Basin. The oils'carbon isotope compositions are similar to those of the organic matter in potential source rocks.The western Qaidam oils are inferred to have originated from Tertiary source rocks deposited under anoxic and saline‐hypersaline lacustrine conditions with dominant algal organic matter. The northern Qaidam oils are interpreted to be derived from Jurassic source rocks which were deposited in a freshwater lacustrine environment and which are dominated by terrigenous organic matter.