C29 Steranes Research Articles

Petroleum in the Jurassic Reservoirs within the Eastern Fukang Sub‐depression, Junggar Basin, NW China: Correlation and Source Rock

AbstractThe origin and source of the petroleum in the Jurassic reservoirs within the eastern Fukang sub‐depression were geochemically investigated. They show thermal maturities matching the peak generation stage, while the condensates are at the early stage of intense cracking. Oils and condensates may have experienced mild evaporative fractionation, while mixing of severely biodegraded with non‐biodegraded oils has occurred. Using biomarkers and isotopes, petroleums were classified into Group I, II and III genetic groups, with Group III further divided into IIIa and IIIb subgroups. Group I petroleum displays heavy carbon isotopes, a strong predominance of pristine over phytane, high C19 and C20 tricyclic and C24 tetracyclic terpanes, low gammacerane, and dominant C29 steranes, while Group II shows light carbon isotopes, a predominance of phytane over pristine, high C21 and C23 tricyclic with low C24 tetracyclic terpanes, high gammacerane and dominant C27 steranes. Group IIIa petroleum shows mixing compositions of Group I and II, while Group IIIb displays similar compositions to Group I, but with significantly higher Ts, C29Ts and C30 diahopane proportions. Oil‐source rock correlation suggests Group I and II petroleums originate from Jurassic and Permian source rocks, respectively, while Group IIIa are mixtures sourced from these rocks and IIIb are mixtures from Jurassic and Triassic source rocks.

Geochemical evidence for Alberta Oil Sands contamination in sediments remote to known oil sands deposits in Alberta, Canada

Oil spills and natural oil seeps are sources of petrogenic hydrocarbons in soils and sediments. To determine the source of hydrocarbon contamination in the environment the geochemical signature of the contaminant needs to be characterised. Here, we present biomarker and other molecular marker diagnostic ratios of Alberta Oil Sands using gas chromatography-mass spectrometry to characterise the deposits and detect their incorporation in surficial sediments. Diagnostic ratios of steranes, terpanes, and aromatic steroids (e.g. C27, C28, and C29 regular sterane abundance, Gammacerane Index, Ts/Tm, TAS/(TAS + MAS), and MPI-2) were measured in samples of Alberta Oil Sands providing a set of criteria for their identification. Seven surficial sediment samples from central and southeast Alberta were then analysed using these criteria to detect Alberta Oil Sands contamination and other petrogenic and pyrogenic source inputs. Geochemical signatures consistent with Alberta Oil Sands hydrocarbons were identified in surficial sediments in Lamont County and glacial sediments from a moraine in Beaver County. Both sites are in Central Alberta, ∼300 km south of any oil sands extraction sites and natural exposures in northern Alberta, indicating long-distance sediment transport processes mobilised the deposits. These results show that the oil sands have been eroded and transported beyond the boundaries of their current known limits. This is important for understanding sediment transport processes as well as for remediation and reclamation purposes.

The Depositional Environment of the Lacustrine Source Rocks in the Eocene Middle Number of the Liushagang Formation of the Weixinan Sag, Beibuwan Basin, China: Implications from Organic Geochemical Analyses

The Eocene middle number of the Liushagang Formation (LS2) of the Weixinan Sag, Beibuwan Basin, characterized by a thick succession of excellent quality source rocks, is composed of lacustrine organic-rich shales, mudstones, and shales (mudstones/shales). However, the complex and specific depositional environment in the source rocks of LS2 raise questions about the mainly controlling factors of lacustrine organic matter (OM) accumulation. In this study, total organic carbon (TOC) contents, Rock-Eval pyrolysis, as well as biomarker data are used to investigate the nature of the depositional environment and the enrichment mechanism of OM in the source rocks of LS2. The values of Tmax, CPI, C29 steranes αββ/(ααα+αββ), and the 22S/(22S+22R) ratios of the 17α, 21β(H)-C31 hopane together confirm that the OM in the source rocks of LS2 is immature to of low maturity, which suggests that the nature of biomarkers may not be affected by thermal maturity. The hydrocarbon potential was higher in the organic-rich shales (with a mean of 20.99 mg/g) than in the mudstones/shales (with a mean of 7.10 mg/g). The OM in organic-rich shales is type I and II kerogen and that in mudstones/shales is type II kerogen. The C27/C29 regular steranes ratios and 4-methylsterane indices (4MSI) further confirmed the difference in the source of OM between organic-rich shales and mudstones/shales; that is, that the OM of organic-rich shales is mainly derived from the lake algae and aquatic macrophytes and the OM of mudstones/shales is mainly from the higher plants. The values of the gammacerane index and ratios of C21/C23 TT and C24 Tet/C26 TT all indicate that the source rocks from LS2 are deposited in freshwater to a low salinity water column. Moreover, a cross-plot of C21–22/C27–29 sterane versus dia/reg C27 sterane ratios and Pr/Ph ratios suggests that the source rocks from LS2 are recorded to have sub-oxic to oxic conditions. Based on those analyses, two dynamical formation models were proposed: a high-productivity and oxic-suboxic dynamical formation model (Model A) and a low-productivity and oxic-suboxic dynamical formation model (Model B).

Elemental Geochemistry and Biomarker Measurements of the Silurian Shale of Qusaiba Formation, Tayma Area, Northwestern Saudi Arabia: Implication for Organic Matter Input and Paleoenvironmental Conditions

This study systematically analyzes the dark shale samples of the Silurian Qusaiba Formation from the Tayma Quadrangle outcrop section in the northwest of Saudi Arabia, and assesses the source and nature of its organic matter and the main sedimentary environmental conditions during accumulation of organic matter. The Qusaiba shale samples are characterized by total organic carbon (TOC) and sulfur contents with total values in the range of 0.87–1.76 wt. % and 0.59–4.64 wt. %, respectively, indicating a marine setting ranging from dysoxic to anoxic environmental conditions. The biomarkers are characterized by a relatively low Pr/Ph ratio between 0.50 and 1.24. The abundance of tricyclic terpanes and high C27 and C29 regular steranes equated to C28 regular sterane, providing evidence that the organic matter derived primarily from marine organisms, including algal and other aquatic organic matter and some terrigenous land plants, and was deposited under dysoxic to anoxic environmental conditions. The significant low oxygen environmental conditions may contribute to preservation of organic matter during deposition. The inorganic geochemical indications suggest that the Qusaiba shales were accumulated in a warm–humid climate and with low salinity stratification conditions of the water columns.

Global sea level changes or local tectonics? Pliocene, Miocene and Oligocene biomarkers in cored sedimentary rocks from IODP Expedition 317, Canterbury Basin, New Zealand

The various influences of global sea level (eustasy) and local tectonic changes on sedimentation processes in continental margin deposits are important topics in sedimentary research. International Ocean Discovery Program Expedition 317 to the Canterbury Basin, on the eastern margin of the South Island of New Zealand, provided an opportunity to study sediment geochemistry in contrasting depositional settings, from mid-shelf to upper continental slope sedimentary rocks from the Eocene to the Holocene. This paper provides the first examination of the organic geochemical record from Pliocene, Miocene, and Oligocene sediment samples recovered during the expedition from the U1351, U1352, and U1353 sites, using bulk geochemistry and hydrocarbon distributions, including biomarkers. The main aim of this research was to correlate changes in organic accumulation with local tectonic activity and global climate changes that occurred in the Miocene. There is good preservation of C11 to C35n-alkanes, with varying predominance of odd-over-even chain length n-alkanes, and much variation in the terrigenous/aquatic ratio over the cores. Based on biomarker and Rock Eval parameters, thermal maturity varies from low in the Pliocene, to early oil window in the deeper Miocene and Oligocene sedimentary rocks. The pristane/phytane ratios for the three cores indicate anoxic to oxic depositional environments. Marine organic matter input is indicated by the high C30 sterane index, whilst high relative amounts of oleanane, C24 tetracyclic terpane, and C29 steranes indicates major terrigenous organic matter input. Local tectonic activity is suggested to have had a significant influence on the accumulation of the organic matter in the Canterbury Basin. The influence of eustasy and global sea temperature variations are suggested for the periods < 14 Ma, 12–7 Ma, and ∼ 6 Ma. An increase in terrigenous organic matter input during the late Miocene correlates with the uplift of the Southern Alps and an increase in the continental slope angle for the South Island of New Zealand.

Redox conditions and ecological resilience during Oceanic Anoxic Event 2 in the Western Interior Seaway

Oceanic Anoxic Events (OAEs) are important geological events that may be analogues to future climate-driven deoxygenation of our oceans. Much of the global ocean experienced anoxic conditions during the Cenomanian–Turonian OAE (OAE2; ∼94 Ma), whereas the Western Interior Seaway (WIS) experienced oxygenation at this time. Here, organic geochemical and palynological data generated from Cenomanian–Turonian age sediments from five sites in the WIS are used to investigate changing redox and ecological conditions across differing palaeoenvironments and palaeolatitudes. Heterogeneity across the sites is apparent, but important relationships and trends among oceanographic variables are recognised: 1) Increasing total organic carbon (TOC) and CaCO3 percentages indicate the onset of a sea-level maximum towards the end of OAE2; 2) C28 sterane is shown to be a useful marker for prasinophyte abundance, and concurrent increases in this marker and overall sterane abundance indicate prasinophyte-driven increase in algal productivity in a stratified water column; and 3) sterane ratios can be a more reliable geochemical proxy than redox proxies for assessing the Benthic Oxic Zone. Our redox data do not always follow established trends for the WIS overall, particularly for proximal settings. We therefore surmise that local effects, such as nutrient-driven expansion of the oxygen minimum zone and/or sedimentation-driven anoxia just below the sediment-water interface, have overprinted regional trends.

Reclassification and distribution patterns of discovered oils in the Dongying Depression, Bohai Bay Basin, China

The Dongying Depression is an important petrolifeous province, with diverse source rocks and complex petroleum distribution patterns. A total of 32 crude oils were analyzed by the gas chromatography–mass spectrometry and isotopic compositions to better understanding the petroleum systems in the study area. Three oil types were classified by hierarchical cluster analyses. Type I and II oils have closely correlation with the discovered source rocks, which have been confirmed to be mainly derived from the lower third and upper forth member of the Eocene Shahejie Formation source rocks (Es3L and Es4U), respectively. Obviously, type III oils contain abundant gammacerane, tricyclic terpanes and C29 steranes and have lower values of δ13C than type I and II oils, indicating a completely different source rock and biological origins. Until recently, type III oils fail to match any of the discovered source rock, which contains main contribution of aquatic organism or/and bacteria inputs. In addition, the spacial distribution of these three oil types were discussed. Type I oils mainly distributed in the Es3 and Es4 reservoirs that closed to the generative kitchens. Type II oils occurred in the Es4 reservoirs in the sourthern slope of the depression, which probably caused by lateral migration along the horizontal fractures and sandstone layers within the Es4 interval. Differently, type III oils in the sourthern slope of the depression were mainly discovered in the Eocene Kongdian or Ordocician reservoirs, which suggests great exploration potential of deep underlying strata.

Geochemistry of hydrocarbons-biomarkers and carbon isotopes of organic matter in the Domanik Deposits of the Timan-Pechora Basin

It has been established that the average values of some hydrocarbons biomarker coefficients of bitumens from the Domanik deposits and oils from the Domanik–Tournaisian complex coincide. These are the proportions of C27–C29 steranes, tri-/pentacyclanes ratio, and coefficients 2 × С17/С16 + С18 and 2 × С27/С26 + С28 calculated from n-alkane composition. Such indicators as the Pr/Ph, (Pr + Ph)/(n‑C17 + n‑C18), dia-/reg-steranes, and the sterane/hopane ratios in oils are slightly lower than in the bitumen. The average values of δ13C in asphaltene, resin, aromatic hydrocarbons, and aliphatic hydrocarbons fractions from oils of the Domanik genotype and in the studied bitumens are similar. The carbon isotope composition of asphaltenes in oils is depleted in 13С compared to those of resins and aromatic hydrocarbons, and vice versa in bitumens. A heterogeneous composition of the bacterial flora in the sediments of the Domanik Sea is suggested. In some places, species with high concentrations of diplopterol or hexafunctional bacteriohopanes likely dominated over bacteriohopantetrol.

Preservation of molecular fossils in carbonate concretions in cretaceous shales in the songliao basin, northeast China

Paleoenvironmental information is better preserved in carbonate concretions. In this study, carbonate concretions in the Cretaceous Nenjiang shale, Songliao Basin, were examined to determine whether molecular fossils reflective of the paleoenvironment were better preserved at these sites. Organic and inorganic geochemical characteristics of the concretions and surrounding rocks were analyzed using a series of techniques, including SEM, LA-ICP-MS, GC-MS-MS, and GC-IRMS. The concretions are composed of high content microcrystalline dolomite. The δ13Ccarb and δ18Ocarb values of the concretionary dolomite were significantly higher than those of the surrounding rocks. The dolomite show enrichment in the LREEs and have a negative Eu anomaly. The concretion biomarkers showed distribution characteristics similar to those of surrounding rocks. This suggested that the molecular fossils preserved in concretions were mainly inherited from surrounding rocks. However, the concretions contained more C27 sterane and hopanes, with the hopane/sterane ratio being significantly higher than that of surrounding rocks (1.49 v. 0.86). Moreover, the relative content of 2-methylhopane was 2.4–6.6 times that of the surrounding rocks. This indicated changes in the biological equilibrium of source organisms within and outside the concretions. It was possible that the unstable organic matter at the core increased the bacterial concentration and activity inside the concretions. Both the hydrogen index and biomarker-derived indicators implied that the transformation of organic matter in concretions was minimized when compared with their host rock. The isotope δ13C16-30 was 1‰–3‰ more prevalent in individual N-alkane hydrocarbons in the concretions than in surrounding rocks, likely owing to differences in lithology, bacterial action, and degree of weathering. The study concluded that carbonate concretions could preserve molecular fossils better than the surrounding rocks, and the in-depth organic geochemical analysis of concretions could provide a valuable reference for research into early life forms.

Long eccentricity forcing of the Late Pliensbachian to Early Toarcian (Jurassic) terrestrial wildfire activities in the Tarim Basin, northwestern China

Wildfires are an important disturbance driver for terrestrial ecosystems and even Earth systems, which reflect crucial information about the interactions between the atmosphere and the biosphere. However, the relatively long-term secular evolution controlled by climate change in the Early Jurassic wildfires are poorly understood. Here, we conduct cyclostratigraphic, chemostratigraphic, and molecular compound analyses on the Late Pliensbachian to Early Toarcian terrestrial stratigraphy of the Kuqa River section in the Tarim Basin, northwestern China, in order to obtain a holistic understanding the evolutionary processes of wildfire on long time scales during a past warming event, the Pliensbachian-Toarcian event (Pl-To). Based on the long eccentricity cycle identified in the time series analysis, we establish a ∼ 1.8 Ma high-resolution floating astronomical timescale (FATS) and reconstruct the variable lake level by using the sedimentary noise model. The Pl-To warming event associated with the Karoo-Ferrar Large Igneous Province (K-F LIP) may have led to a shift in climate from warm and wet to dry and hot, with a significant decrease in lake levels in the Tarim Basin. Besides, polycyclic aromatic hydrocarbons (PAHs), C29 sterane and dibenzofuran evidence indicates that the global warming event (Pl-To event) also may regulate the wildfires by affecting the fuel abundance (vegetation). Notably, the astrochronology and PAHs suggest that the periodically enhanced wildfires correspond to the long eccentricity cycles, indicating that astronomical forcing plays a key role in triggering the wildfires. Our results report the first documentation of multiple full long eccentricity cycles forced wildfires during the Late Pliensbachian to Early Toarcian and reveal orbital forcing as a driving mechanism for periodically enhanced wildfires superimposed on a long-time gradual weakening trend associated with Pl-To warming event.

Biological Marker Fingerprints of Crude Oils from Three Oilfields in the Central Niger Delta: Implication to Source Input, Conditions of Deposition, and Thermal Maturation

Crude oil samples recovered from three oilfields distributed within the central Niger Delta were used to investigate biological marker fingerprints, sources of organic matter, conditions of deposition, and thermal maturation. This was achieved by the application of gas chromatography-mass spectrometric (GC-MS) analyses on the saturated components of the crude oils. The biological marker ratios show low sterane/hopane (steranes/hopane ˂ 1) in the scope of 0.29 to 0.61 indicating that the crude oils initiated from rocks containing high terrigenous biota (mainly land plants). The n-alkanes distribution and the pristane/phytane ratios (0.63 to 2.08) suggested source deposition under oxic to suboxic environmental conditions. The percentage composition of C27, C28, and C29 steranes and oleanane index (18α (H)-oleanane/ C30 17α (H)-hopane) also indicated major contributions from terrigenous organic biota with minor marine source input. The studied crude oils have C32:22S/ (22S+22R) ratios in the scope of 0.55 to 0.64, and these values suggested high maturation level. This agrees with the oleanane index (0.64 to 0.95), CPI (0.92 to 1.06), C29: 20S/ (20S + 20R) sterane ratios (0.29 to 0. 61), Ts/(Ts+Tm) ratio (0.45 to 0.83) and moretane/C30 hopane proportion (0.16 to 0.24). Keywords: Biological marker; Depositional conditions; Niger Delta; Organic matter; Thermal maturation.

Assessing marine communities and carbon cycling that sustained shallow-marine ecosystems on Silurian, reef-rimmed carbonate platforms

A detailed middle Silurian to Early Devonian biomarker stratigraphic record was obtained from thermally well-preserved strata, sampled from a > 400 m long drill-core 25 from Podillya, Ukraine, deposited in the Podillyan peri-continental basin. These lipid biomarker records provide useful insights into the temporal changes in microbial ecology and ocean chemistry for this ancient, tropical shallow-marine reefal carbonate platform. The paleoenvironmental conditions favored sustenance of bacteria over algae (as indicated by elevated hopane/sterane ratios (average: 10.7, maximum value of 36.9); and appreciable 2α-methylhopanes and 3β-methylhopanes) in this nutrient-depleted, marine reefal habitat. The setting behind the reef tract in a lagoon and muted nutrient supply from the open ocean likely contributed to the persistently low primary productivity and low net biomass over millions of years of deposition. The most prolific algal primary producers were green algae consistent with a C29 sterane dominance. No detectable contributions of 24-n-propylcholestane (24-npc) and/or other C30 regular sterane compounds were found with GC–MRM-MS. Elevated 3β-methylhopane index values (3-MeHI; range: 3–13%, mean: 6.4%) were observed throughout the core interval, which is a characteristic associated with source contributions from methanotrophic bacteria and indicative of an enhanced marine methane cycle. This extends the previous findings of consistently high abundances of 3-methylhopanes for Ordovician marine environments to include a tropical, marine shelf setting persisting through the Silurian Period.

Organic matter accumulation in lacustrine shale of the Permian Jimsar Sag, Junggar Basin, NW China

The lacustrine organic-rich shale in the Permian Lucaogou (LCG) Formation of the Jimsar Sag, Junggar Basin, is one of the main shale oil plays in China. In this paper, geological and geochemical research techniques were employed to evaluate the geochemical variability of the lacustrine shale and the production of organic matter and its preservation conditions. The LCG Formation is characterized by its complex mineral compositions and a wide range of organic matter richness and quality. The presence of high proportions of β-carotane and C29 steranes, indicates that the organic matter mainly originated from phytoplankton and aquatic algal-bacterial organisms, especially cyanobacteria. This study found that the productivity of the Lower LCG Member (P2l1) was highest, and the Middle LCG Member (P2l2) was the lowest. During the deposition of the Lower LCG Member, the lake's bottom water was predominantly a reducing environment, and the degradation of organic matter was largely a result of bacterial sulfate reduction. During the deposition of the Middle and Upper LCG members, the lake's bottom water was mainly oxidizing, and the degradation of organic matter was likely to be caused by aerobic processes. Based on a comprehensive analysis of the origin and production of organic matter, as well as its depositional environment and preservation conditions, two organic matter accumulation models were proposed to explain the distribution of the organic-rich shale. In model A, the high influx of volcanic ash released nutrients and brought abundant sulfate into the water, the accumulation of organic matter was mainly controlled by the preservation of organic matter, which was mainly controlled by BSR. In the model B, the influx of volcanic ash was small, organic matter was mainly degraded by oxygen and the accumulation of organic matter is mainly determined by the production of organic matter.

Origin and Characteristics of the Crude Oils and Condensates in the Callovian-Oxfordian Carbonate Reservoirs of the Amu Darya Right Bank Block, Turkmenistan

Abstract The Amu Darya Right Bank Block is located northeast of the Amu Darya basin, a large petroliferous sedimentary basin, with abundant natural gas resources in carbonate rocks under the ultra-thick gypsum-salt layer. Oil fields producing crude oils have recently been found around large gas fields. Unraveling the origins of the crude oils is crucial for effective petroleum exploration and exploitation. The origin of gas condensates and crude oils was unraveled through the use of comprehensively analytical and interpretative geochemical approaches. Based on oil-source correlation, the reservoir forming process has been restored. The bulk geochemical parameters of the local source rocks of the ADRBB indicated that the local sources have hydrocarbon generation and accumulation potential. The middle-lower Jurassic coal-bearing mudstone is gas prone, while the mudstone of the Callovian-Oxfordian gap layer is oil prone, and the organic matter type of Callovian-Oxfordian carbonate rocks is the mixed type between the two previous source rocks. The interpretation schemes for compositions of n-alkanes, pristane and phytane, C27–C28–C29 sterane distributions, C19+C20–C21–C23 tricyclic terpane distributions, extended tricyclic terpane ratio, and δ13C indicated that crude oil is likely from marine organic matter, while condensates mainly originate from terrestrial organic matter. However, from the perspective of the 18α-trisnorneohopane/17α-trisnorhopane and isomerization ratio of C29 sterane, condensates are too mature to have originated in the local source rocks of the ADRBB, whose maturity is well comparable with that of crude oils. The geochemical, geologic, and tectonic evolutions collectively indicate that the crude oils were most likely generated and migrated from the relatively shallow, lowly mature gap layer and Callovian-Oxfordian carbonate rocks of the ADRBB, while the condensates mostly originated from the relatively deep and highly mature middle-lower coal-bearing mudstone and Callovian-Oxfordian carbonate rocks in the Murgab depression in the southeast of the basin. Basement faults are the key factors affecting the types of oil and gas reservoirs. During the periods of oil and gas migration, traps with basement faults mainly captured natural gas and condensates and traps without basement faults were enriched with crude oils generated from local source rocks.

Organic geochemistry and oil-source rock correlation of the Cretaceous succession in West Wadi El-Rayan (WWER) concession: Implications for a new Cretaceous petroleum system in the north Western Desert, Egypt

Mesozoic rift basins of the north Western Desert host the majority of proven oil reserves in Egypt. These basins are bounded by poorly-explored structural highs (Kattaniya high, Qattara ridge) with unknown petroleum system. The recently-discovered oil in West Wadi El-Rayan (WWER) concession in the southern part of the Kattaniya high highlighted a need for understanding the petroleum potential of these transit regions. The present study integrates geochemical and basin modelling results to define potential source rocks and decipher the mechanism of oil generation, expulsion and migration. In WWER concession, oil is produced from the Upper Cretaceous (Upper Bahariya) sandstones. Rock-Eval pyrolysis results demonstrate that the Lower Cretaceous Alam El Bueib (AEB) and Upper Cretaceous Lower Bahariya shales below the reservoir contain mature source rocks (S2 up to 28.76, Tmax > 430 °C). WWER oil displays a uniform composition with high pristane/phytane (Pr/Ph > 3), saturates/aromatics (>5.5) and canonical variable values (CV > 6) pointing to a generation from a terrestrial source rock. This is consistent with high contents of C30 hopane, C29 steranes and diasteranes as well as the very low values of dibenzothiophane (DBT)/Phenantherane (<0.1) confirming oil generation from a clay-rich source rock deposited in a fluvio-deltaic environment.Oil-source correlation highlights a robust similarity in compositional biomarkers between the WWER oil and AEB rock extracts. Additionally, the maturity-relevant biomarkers such as Ts/Tm trisnorhopanes and H32 S/(R + S) homohopanes favor an oil expulsion phase during the early stage of oil window in accordance with the basin modelling results and the maturity level of AEB organic-rich facies. These findings shed the light on a new petroleum system in the north Western Desert where the AEB fluvio-deltaic shales are capable of generating early mature oil which migrates through the Cretaceous deep-seated extensional faults into the Bahariya sandstone reservoir.

Characterization and possible geochemical implications of thiolane steroids in sulfur-rich lacustrine crude oils from Huanghekou Depression, Bohai Bay Basin

Thiolane steroids can provide abundant geochemical information as long as their molecular structures, biological origins, formation pathways, sulfurization environments and sulfurization timing are understood. Steroids in saturates, aromatics and sulfidic fraction are investigated in five low-mature sulfur-rich oils in Huanghekou Depression, Bohai Bay Basin. The high abundances of C27 sterane, C30 4-methylsterane, dinosterane and triaromatic steroids indicate the abundant dinoflagellate and other algae inputs. In addition, the distribution of C29 steranes is also influenced by the unique algae inputs. Algae blooms offer abundant precursors of steroids for sulfurization. Two types of thiolane steroids were identified in the oils: (i) thiophane ring merging at D-ring and (ii) thiophane ring at side-chain of steroids. The double bonds at C-22 position of precursors are crucial for intramolecular sulfurization of steroids. Therefore, the C28 and C29 steroids are easier to form thiolane steroids in type I than C27 species, further, C30 species are mostly transferred to counterparts in type II via isomerization and H2S addition/elimination. Thiolane steroids are diagenetic products, and the saline lacustrine, with stratified water column, might be the suitable depositional environments for formation of thiolane steroids. Even though the evolution pathways occur under the sulfur-rich conditions, the thiolane steroids are still sensitive to the reducing environments, resulting in good correlations between parameters of the percentage of C27 thiolane steroids, (C28a + C29a)/C27a and biomarkers that indicate reducing environment.

Origin of organic matter and depositional characteristics of early Cambrian Niutitang Formation from South China: New insights using molecular fossils

The early Cambrian Niutitang Formation is broadly developed in South China, especially in the Yangtze region. This research explains the nature of hydrocarbons through bitumen fractions of the early Cambrian sediments of the Niutitang Formation. For this purpose, source rock samples from two diverse outcrop sections are utilized for total organic carbon content (TOC), solvent extraction, and gas chromatography–mass spectrometry (GC–MS). Results show a lower quantity of extractable organic matter in these older sediments because of higher maturation history. The presence of monomethyl alkanes suggests the contribution of prokaryotic organisms, whereas the presence of hopanoids and C27–C29 steranes shows the occurrence of eukaryotic algal sources in organic matter. Likewise, C30‐4‐methyl sterane, dinosteranes, and ß‐carotene also suggested incorporating prokaryotic and eukaryotic sources during the early Cambrian period. Various hopanoid parameters, alkylated aromatic isomers, triaromatic steroids, and vitrinite reflectance via methyl‐phenanthrene index reveal mature to higher thermal evolution levels of these lower Palaeozoic sediments. Similarly, a higher abundance of polyaromatic pyrene and fluoranthene indicates mature organic matter, and their presence could be due to hydrothermal action. The higher gammacerane index, lower pristine/phytane, and the presence of C30 dimethyl steranes indicate the anoxic saline sedimentary condition throughout the deposition of these lower Palaeozoic Niutitang Formation. The early Cambrian Niutitang Formation is deposited in an oxygen‐deficient environment and contains an adequate amount of mature‐high mature organic matter that could have the potential to produce gaseous hydrocarbons.

Geochemical characteristics of crude oil in the eastern of Qinnan sag, Bohai Bay Basin, Eastern China

The oil and gas exploration degree of Qinnan sag is low. In this paper, all the 17 crude oil samples from the four reservoirs in the eastern Qinnan sag were taken as the research object. Research methods include GC-MS, stable carbon isotopic of crude oil and group components and other analytical methods. The geochemical characteristics of crude oil were studied by the main parameters, Pr/ph, γ/C31 hopane, Ts/Tm, 4MSI and C29 sterane isomerization. The results show that most of the hydrocarbon generating parent materials of all the 17 crude oils are reductive lacustrine sedimentary organic matter with higher salinity. The crude oil of Es12 is mainly derived from low aquatic algae, and the crude oil of Ed3, Ek and Mz shows the characteristics of a mixed source. The maturation characteristics of crude oils are low mature-mature. The stable carbon isotope of oil and the components is in the range of −30‰ to −26‰, which shows the distribution characteristics of δ13CST &amp;lt;δ13CASP &amp;lt;δ13CNOS &amp;lt;δ13CAR. The characteristics in B-6, C-1 and C-3 of crude oils show the heavy δ13CASP and high abundance of hopane, indicating that there is more contribution from bacterial sources. The crude oil from Es12 is mainly the contribution of Es1 source rocks. The crude oil of Mz, Ek and Ed3 is mainly the mixed source of Es1 and Es3 source rocks, and Es3 is the main source, and the oil source may mainly come from the southeast subsag. The carbon isotope of the component in Block D can be divided into two kinds of crude oil, which may contribute to the oil source of the Bozhong sag. The eastern of Qinnan sag has great hydrocarbon potential.

Geochemical Characterization of Coal, Carbonaceous Shale, and Marine Shale As Source Rock in West Sulawesi, Indonesia

DOI:10.17014/ijog.9.3.303-314The eastern part of the Makassar Strait has not achieved significant success in the petroleum exploration stage. In general, exploration wells in this area have not found hydrocarbons to date. Most of the problems experienced by these wells are geochemical-related, including the absence of a source rock where the hydrocarbon is formed, immature source rock, and low volume of hydrocarbon. Therefore, this study aims to find potential Eocene source rock in West Sulawesi. It commenced by determining the paleofacies followed by the geochemical analysis of well cutting and surface samples. The geochemical analyses conducted are total organic carbon (TOC), rock eval pyrolysis (REP), kerogen type, vitrinite reflectance (VR), and gas chromatography/mass spectrometry (GC/MS). The samples that have a potential as an Eocene source rock were coal, as well as carbonaceous and marine shale. Based on the plotting of relative abundance for sterane (C27 - C29), the coal and carbonaceous shale had the input from terrestrial organic material with a contribution from marine, while marine shale had the input from marine organic material and contribution from terrestrial. Furthermore, coal and carbonaceous shale had a relatively high proportion of C29 sterane compared to C27 and C28, while marine shale had a relatively high proportion of C27 compared to C28 and C29. The higher plants also contributed to the organic matter in carbonaceous shale and coal samples, as indicated by the abundance of oleanane/C 30 hopane and the other significant content of resin biomarker. In accordance with the objectives of the study, it was found that during the Eocene, the type of source rock in the eastern side of the province varied, starting with coal and carbonaceous shale in the upper intertidal-fluviodeltaic environment, and marine shale sediments from the source facies of marine algal.

Corroborative insights into genetically related kerogen, asphaltenes and hydrocarbons using free and bound biomarkers

Unlike the free biomarkers, which can be routinely recovered via solvent extraction, the bound biomarkers are covalently linked to the kerogen or asphaltene macrostructures and are more resistant to secondary alteration processes. Although similarities have been identified between these two types of biomarkers using different techniques, it remains unclear how the kerogen- and asphaltene-bound biomarkers are complicatedly corelated with their free counterparts, thus limiting the applications of bound biomarkers in geoscience research. In this study, the characteristics of free biomarkers recovered from shale samples, from 10 sedimentary basin around the world, and petroleum samples, from the North Sea, have been systematically compared with their kerogen- and asphaltene-bound biomarkers, which were released by the newly developed Microscale Sealed Vessel Catalytic Hydrogenation (MSSV-Hy) method. Although the kerogen-bound biomarkers are characterized by the absence of rearranged biomarkers (e.g., Ts and diasteranes) and elevated C29/C30 hopane ratios, many key source-related parameters show strong positive correlations between free and bound fractions, including the steranes/hopane, C27/C29 sterane, C30/C29 sterane, C29/C30 hopane (αβ + βα), C35/C34 homohopanes and C24 tetracyclic terpane/C26 tricyclic terpane ratios. These results suggest that bound biomarkers can reveal critical information about deposits and organic matter precursors of source rocks. Thermal maturity indicators, such as C29 sterane ββ/(αα + ββ), C29 sterane S/(S + R), C31 hopane S/R, and Tβ/Tm ratios, also display excellent correlations between the free and bound counterparts in the source rocks. Although the correlations are not 1:1, empirical formulae, which are based on the correlations between free and bound biomarkers, can be used to calibrate bound biomarkers into values that can be directly compared to routine free biomarkers. Importantly, asphaltene-bound biomarkers of North Sea samples exhibit similar characteristics to kerogen-bound biomarkers, and the conversion between free and bound biomarkers of the source rock and reservoir samples is the same. In summary, the bound biomarkers are found highly effective in determining the organic facies, lithology, and thermal maturity of the parent kerogen and/or asphaltene, and, thus, have a great potential to be widely used in geoscience research when information from free biomarkers is hampered by oil mixing, contamination or biodegradation.