•The δD values of n-alkanes were analyzed for Cretaceous lacustrine sediments.•The profile of the δD values of n-alkanes is consistent with that of δ13C.•ΔδDLW-MW and ΔδDC23 ‐ long values are used to determine paleoclimatic conditions.•Correlation analysis shows that δD value of n-alkanes is a better source indicator.Hydrogen isotopic composition of n-alkanes was measured in sediments from an excavated profile of the Early Cretaceous Yixian Formation in Liaoning Province, NE China, aiming to assess the significance of the δD value of n-alkanes in ancient lacustrine sediments as the indicator for determining the source inputs of organic matters and paleoclimatic conditions. The δD values of n-alkanes are in the range of − 250‰ to − 85‰ and display an obvious three-stage variation pattern through the profile, which is consistent with the distribution of the dominated n-alkanes and the profile of their δ13C values. The δD and δ13C values of n-alkanes suggest that short-chain n-alkanes are primarily derived from photosynthetic bacteria and algae; n-C29 and n-C31 are mainly originated from terrestrial higher plants; n-C28 and n-C30 may be derived from the same precursor but via the different biological mechanism of hydrogen isotopic fractionation; while the source inputs of medium-chain n-alkanes are more complicated, with n-C23 being derived from some specific algae or biosynthesized by various aquatic organisms. The paleoclimatic conditions are reconstructed via two approaches. The reconstructed hydrogen isotopic values of lake water and meteoric water (expressed as δDLW and δDMW, respectively) were at the intervals of − 51.8‰ to 17.0‰ and − 118.1‰ to − 43.5‰, respectively, indicating a general climate transition from semi-arid to arid. The calculated ΔδDLW-MW values vary from 37.0‰ to 89.1‰ and display a similar but a significant large-scale variation trend with the ΔδDC23 ‐ long (− 28.8‰ to 85.0‰; long represents long-chain n-alkanes) and ΔδDmid-long (− 15.4‰ to 43.4‰; mid represents medium-chain n-alkanes) values. The discrepancy may be attributed to the source input overlap for n-alkanes and the uncertainties of εwater/lipid values. The coupling of ΔδDC23 ‐ long, ΔδDmid-long and ΔδDLW-MW values with the paleoclimatic evidence indicates that the δD values of n-alkanes could be more sensitive to the change of paleoclimatic conditions.Download high-res image (342KB)Download full-size image

•Well preserved low maturity lacustrine sediments containing volcanic ash were studied.•Si-he-tun sediments were deposited in an anoxic water column, at times stratified.•C24 17,21-secohopane has a distinct source associated with the Jehol Biota fossils.•The dominant n-alkane varied as the climate changed from warm and humid to arid.•δ13C values for long-chain n-alkanes reflect an influence of volcanic activity.The abundance and composition of organic matter and the δ13C compositions of some specific individual compounds were analysed in sediments collected from an excavated profile of the Early Cretaceous Yi-xian Formation in Liaoning Province, NE China, in order to provide organic geochemical information on palaeoclimate and environmental changes. The distribution of n-alkanes varies steadily through the profile, with long-chain n-alkanes dominating in the lower and upper sections, and medium chain-length n-alkanes dominating in the middle section of the profile. The δ13C values of n-alkanes, hopanes and steranes show similar upward positive trends through the profile, suggesting that these compounds originated from the same source material and also reflect consistent changes in the environment of the sedimentary basin. The C24 17,21-secohopane was only detected in the lower and middle sections of the profile and has a more positive carbon isotopic composition by about 3.7–6.7‰ than that of associated n-alkanes, hopanes, steranes and C30 8,14-secohopane. This implies that the C24 17,21-secohopane had a different origin, possibly connected with the volcanic ash deposits and massive vertebrate remains. Reconstruction of the δ13C composition of atmospheric CO2 based on plant-derived C29 and C31n-alkanes shows a change from −10‰ to −3‰ from the bottom to the middle of the profile. This upward positive excursion through the profile is consistent with the worldwide variation of atmospheric CO2 during the Early Cretaceous and indicates an upward warming trend, likely related to globally intensified volcanic activity.

The species and distributions of secondary compounds generated from eight organic sulfur compounds by way of hydrous pyrolysis were investigated. The results indicate that the formation of the secondary compounds and their structures and distribution depend on their thermal stability and the types of initial model compounds, as well as hydrous pyrolysis temperatures, while a large number and higher abundance of the secondary compounds appear to be formed mainly between 200 and 270 °C. Assignment of these secondary compounds indicates that alkyl thiols and sulfides are the most reactive compounds, producing a large number and relatively high amount of secondary organic thiols, sulfides, disulfides, sulfoxides and sulfones; while the secondary compounds generated from the thiophenic compounds are mainly low abundant methylated isomers of their own. Disulfidization, sulfidization and oxidation are the most significant mechanism(s) associated with the transformation of the initial thiols and sulfides model compounds. Alkyl thiophenes are only found to be formed from the alkyl thiol and sulfides, while it is noticed that thiophene, benzothiophene and dibenzothiophene are not genetically connected as they are not precursors of each other. Methylated thiophenic compounds are quantitatively insignificant but commonly present in the pyrolysates of thiophenes, benzothiophenes and dibenzothiophenes.Highlights► Species and distributions of secondary derivatives generated from hydrous pyrolysis of eight organic sulfur compounds investigated. ► The secondary thiols, sulfides, disulfides and sulfonyls were largely formed from the alkyl thiols and sulfides. ► Disulfurization, sulfidization and oxidation are the most significant mechanism(s) for the transformation of the initial model compounds. ► Thiophenes, benzothiophenes and dibenzothiophenes were not genetically correlated and are not precursors for each other.

The geochemical transformations of a number of individual model sulfur compounds were investigated using laboratory hydrous pyrolysis under conditions of constant temperature between 200 and 330 °C and in the presence of low-sulfur brown coal. The results show that most of the compounds were reactive and could eventually be transformed largely into secondary sulfur species, whereas only aryl sulphides and condensed sulfur compounds were virtually inactive. Analysis of sulfur species in the pyrolysis products of the reactive compounds suggests that the geochemical transformation of organic sulfur could be principally characterised in terms of the formation and distribution of hydrogen sulphide, secondary sulfur compounds and macromolecular sulfur. The great variation in the proportions of secondary sulfur species indicates that there are significant differences in the geochemical behaviour of the various sulfur compounds or structures. However, as hydrogen sulphide is the predominant pyrolysis product, its formation from the decomposition of organic sulfur compounds proves to be the primary and the most significant mechanism for organic sulfur transformation during the maturation process.

•The methane sorption of Paleozoic shales from Sichuan Basin were investigated.•A general positive correlation exists between TOC and methane sorption capacity.•The clay minerals show no or even negative correlation with sorption capacity of these shales.The methane sorption capacities of Paleozoic shales from the Sichuan Basin were investigated. The primary results show they have sorption capacities ranging between 0.94 and 4.29 cm3/gRock at STP, comparable to the sorption capacity (1.27–2.41 cm3/gRock at STP) of Barnett shale. However, the sorption isotherms indicate that most of the Sichuan Basin shales would reach their approximate sorption equilibrium at 3–4 MPa, in contrast to the continuously upward trend of North American shales. A general positive relationship between TOC content and methane sorption capacity indicates that TOC content is the major controlling factor for the methane sorption of the Sichuan shales. A negative relationship between the clay mineral content and the sorption capacity of these shales may suggest that clay minerals play an insignificant role in the methane sorption of these Paleozoic shales. Sorption capacities and variation ranges are generally ordered as to geological age, viz Cambrian < Silurian < Permian.

•The pore structure and sorption capacity of the Lower Cambrian shales was studied.•The shales are generally very low in their surface area, porosity and permeability.•TOC shows a positive correlation against porosity, surface area, micropore volume.•MSC of shales display a fan-shaped positive correlation with the BET surface area.•The Pa-lang Formation shales appear to be more prospective shale gas reservoirs.The organic carbon content (TOC), mineral composition, specific surface area, porosity, pore volume and methane sorption capacity (MSC) of the Lower Cambrian Niu-ti-tang and Pa-lang shales from north Gui-zhou Province were investigated, aiming to characterize the pore structure and its controlling factors and the sorption capacity of these shales. The results indicate that these shales are characterized by low porosity and permeability, with a BET specific surface area ranging from 5.64 to 28.29 m2/g and micropore and mesopore volumes ranging from 0.02 to 0.54 cm3/100 g and 0.53–3.38 cm3/100 g, respectively. The positive correlations between porosity, specific surface area, micropore volume and TOC content suggest that organic matter is a primary factor in the pore structure development. However, the shales with TOC content of above 5% display a much low micropore volume and specific surface area. The Lower Cambrian shales display an excess MSC at 12 MPa (nex12MPa) and Langmuir MSC (nL) ranging from 0.30 cm3/g to 3.71 cm3/g and 0.41 cm3/g to 4.22 cm3/g, respectively. Both of the MSCs exhibit a positive correlation with TOC but no correlation among these shales with TOC content above 5% due to the lesser degree of organic pore development. In addition, the sorption capacity of these shales displays a fan-shaped positive correlation with the BET specific surface area and suggests that the higher BET specific surface area does not always result in high MSC in shales. Finally, the Pa-lang shales display a better pore property and higher MSC than that of the Niu-ti-tang shales and appear to be superior prospective shale gas reservoirs.

The abundance and composition of total organic carbon, aliphatic and aromatic hydrocarbons including biomarkers, and the δ13C composition of total organic matter and individual compounds in core samples of the Nenjiang Formation from the SK-1 borehole of the Songliao Basin provide information on the paleoenvironmental conditions of lacustrine sediments formed during the Upper Cretaceous. The distribution of n-alkanes, and their δ13C composition, and 4-methylsteranes and dinosteranes in the first member of the Nenjiang Formation (K2n1) indicate that the organic matter largely originated from algal organisms and is dominated by types I and II kerogen. The occurrence of gammacerane, aryl isoprenoids isorenieratane and low pristane/phytane and diasterane/sterane ratios further suggest a salinity stratified water column associated with anoxic bottom waters, as well as possible photic zone oxygen depletion conditions during major source rock deposition of the K2n1. The organic geochemical data also indicate significantly different depositional environments for the second member of the Nenjiang Formation (K2n2) sediments, which are interpreted to have been deposited under a uniform salinity (non-stratified) oxic water column. Consequently, although the organic matter in the K2n2 sediments was largely derived from algal and bacterial sources, the kerogen types are dominantly IIb and III due to the oxidative degradation of organic matter, combined with a gradual increase in land–plant input. Furthermore, the occurrence and shift of two saw-toothed distribution patterns of the δ13C composition of C29 to C33 n-alkanes, with “W” and “M” patterns being isotopically heavy odd- and even-numbered alkanes, respectively, is suggestive of organic input shift or an unknown mechanism of carbon uptake or isotopic fractionation. In particular, the heavy even carbon numbered high molecular weight n-alkanes is rare and needs further investigation.Highlights► The occurrences of biomarker in the Cretaceous lacustrine sediments are investigated. ► The source rocks were formed under salinity and anoxic stratified water column. ► The type and quality of OM are largely controlled by palaeodepositional conditions. ► An unusual heavier δ13C composition pattern of n-C30 and n-C32 alkanes is identified.