Triaromatic Steroids Research Articles

The Distribution and Geochemical Significance of Triaromatic Steroid Series in Crude Oils from the Chepaizi Area, Junggar Basin, NW China

The Distribution and Geochemical Significance of Triaromatic Steroid Series in Crude Oils from the Chepaizi Area, Junggar Basin, NW China

Geochemical characteristic of the Lutetian black shale (Bayburt, NE-Turkey): amount, nature, origin of organic matter and palaeo-environment conditions

Abstract To define the amount, thermal maturity and type of organic matter (OM), a comprehensive analysis of major and trace elements, organic carbon isotope and organic geochemistry was conducted on Lutetian black shales found in the Everek (Bayburt) region of northeastern Turkey. Total organic carbon (TOC) levels in the shale samples range from 0.62% to 6.75%, and type II–III to type III kerogen is generated, displaying a combination of high terrigenous and low marine OM. The δ13C values (ranging from –28.22‰ to –28.23‰), aromatic hydrocarbon compounds (methyl phenanthrene, dibenzothiophene, tri-aromatic and monoaromatic steroids), saturated hydrocarbon compounds (sterane and terpane), acyclic isoprenoids, n-alkane distribution (n-C13–n-C36) and inorganic geochemical characterization support that the black shales were deposited in a terrestrial-marine transition environment and had a high proportion of terrestrial OM with small amounts of marine OM preserved in relatively arid to hot climate and oxic to suboxic conditions. The analysis of biomarker thermal maturity markers, Tmax (ranging from 449–458 oC) and estimated vitrinite reflectance (varying from 0.92 to 1.08%) values suggest that the black shales have reached the oil window. As a result, black shales are thought to contain low to high amounts of TOC, have a mixed kerogen type, reach a high thermal maturity level and produce little hydrocarbons.

ORIGIN OF OIL IN UPPER PERMIAN (ZECHSTEIN) CARBONATE RESERVOIR ROCKS AT THE JARVIS STRUCTURE UNDERLYING THE ETTRICK FIELD, OUTER MORAY FIRTH, UK NORTH SEA

Oil in the Jarvis structure underlying the main Upper Jurassic reservoir at the Ettrick oilfield (Outer Moray Firth, UK northern North Sea) is present in Upper Permian (Zechstein) carbonates. The origin of this “Jarvis oil” is investigated in this paper using a multidisciplinary approach based on data from well‐logs and cores from wells 20/02‐2 and 20/02‐3. Reservoirs at the Jarvis structure consist of carbonates in the upper part of the Halibut Carbonate Formation (Ca2) and in the Carbonate Member of the Turbot Anhydrite Formation (Ca3). These carbonates are typical Zechstein dolomites composed of a range of facies from mudpackstones with storm beds deposited at moderate water depths to shoreface bioclastic‐oolitic packstones to shallow‐subtidal and intertidal microbial laminites. Interbedded anhydrites replace sabkha and lagoonal selenitic gypsum. Several shallowing‐upward units are recognised. Molecular analysis of the Jarvis oil, and comparisons with biomarker and other geochemical data from extracts of Zechstein cores and published data from different source rocks from the North Sea area, suggest that the oil was generated by marine, OM‐rich shales in the Upper Jurassic Kimmeridge Clay Formation. The oil was generated at peak oil window maturity and is characterised by high Pr/Ph, BNH/H and DBT/P ratios, and abundant C28 steranes and C28+29 monoaromatic and C26R + C27S triaromatic steroids. The molecular composition of organic material in extracts of core samples of Zechstein carbonates from wells in the Jarvis structure differs significantly from that of the Jarvis oil. Biomarkers such as BNH are absent in the core extracts, and there are different distributions and abundances of saturated and aromatic hydrocarbons, likely controlled by thermal maturity.

N-AlKanes Base Exploration as Potential Petroleum Source Rocks in ILullemmeden Basin (Sokoto Basin) North west Nigeria

The aim of this work is to investigate the potential petroleum shales of Dukamaje formation in Sokoto basin for their petroleum potentials. A total of sixteen samples were obtained from different parts of the basin. The samples were subjected to sohxlet extraction to extract the bitumen. The bitumen was subjected to column chromatography and aliphatic and aromatic fractions were collected from each sample by using DCM and n-hexane as solvents. The fractions were subjected to GC/MS analyses with the application of SIM method (selected ion monitoring) which revealed the wide variety of hydrocarbons compounds including n-alkanes, hopanes, sterenes, phenanthrenes, methyl-phenanthrenes and triaromatic steroids. Interpretation biomarker parameters revealed that the rocks under studies are endowed with organic matter. Also biomarker correlation parameters were employed to compute the quantity of organic matter, type of organic matter, thermal maturity of the organic matter and to assess the depositional environment of the rocks samples. The biomarker parameters indicated that the source organic matter is mostly from marine environment, the depositional condition found to be anoxic reducing condition. The shales are reach in organic matter and the shales are found to be partially matured.

Weathering impacts on petroleum biomarker, aromatic, and polar compounds in the spilled oil at the northeast coast of Brazil over time

After the wide oil spill reached the northeast of Brazil, the resurgence of oil was recorded and to evaluate this oil in detail, two samples collected in the state of Pernambuco in 2019 and 2021 were submitted to multiple analytical techniques. For both, we have found similar saturated biomarkers and triaromatic steroid ratios, implying that they are from the same spilled source. The n-alkanes, isoprenoids, and cycloalkanes were almost completely degraded due to evaporation, photooxidation, and/or biodegradation processes. The preferential loss of less alkylated PAHs than the more alkylated ones suggests that biodegradation was the most active process. This hypothesis is reinforced by the formation of mono and dicarboxylic acids assessed by GC × GC-TOFMS and ESI(−) FT-ICR MS high-resolution techniques. Furthermore, based on the ESI(−) FT-ICR MS results, three new ratios were proposed to evaluate the progress of the biodegradation process over time: Ox>2/O, SOx/SO, and SOx/N.

Geochemical characteristics and significance of aromatic hydrocarbons in crude oil from the East Fukang Sag, Junggar Basin, NW China

The aromatic compounds of twenty crude oil samples from different formations in the East Fukang Sag were studied by the gas chromatography–mass spectrometry technique. Through analysis of the composition of the main aromatic hydrocarbons, the studied crude oil samples were generally divided into two types: Oil type I had the relatively high content of the naphthalene series, biphenyl series and dibenzofuran series, while the samples from Oil type II contained relatively high abundance of phenanthrene series, fluorene series, triaromatic steroid series, and obvious methyltrimethyltridecylchromans. The geochemical parameters reflecting the source and sedimentary environment in naphthalene series, phenanthrene series, biphenyl series, three-fluorene series, and triaromatic steroid series were systematically analyzed. Analysis of the results showed that studied oil samples from the East Fukang Sag were of continental origin and deposited in a weakly oxidizing to weakly reducing environment with fresh-brackish water. Oil type I was characterized by considerably abundant terrestrial higher plant organic matter inputs, while Oil type II was typical with obviously lower aquatic organisms. Maturity-related parameters calculated from alkyl naphthalenes, alkyl phenanthrenes, alkyl dibenzothiophenes, and methyltrimethyltridecylchromans showed that all the studied oil samples have reached the mature stage.

The diagenetic continuum of steroid hydrocarbon transformation from early diagenesis into the oil window

Steroids and their derivatives are ubiquitous in the sedimentary rock record, where they are widely applied as biological or geochemical markers to determine organic facies as well as thermal maturation of bitumen and oils covering the diagenetic to catagenetic stages. The diagenetic continuum from early diagenesis into catagenesis of steroids has not been extensively studied despite the importance of these reactions in influencing the successive continuum of late diagenetic transformations.This study utilizes a stratigraphically constrained transect of 9 wells from the Upper Cretaceous Second White Specks and Belle Fourche formations of south-central Alberta, Canada ranging in thermal maturity from 0.2 %VRE to 0.8 %VRE. Biomarkers were examined from 69 samples from 8 wells and thermal maturity was determined from 87 samples from 9 wells using HAWK programmed pyrolysis analysis. Relative and absolute abundances of regular sterane, diasterene, diasterane, monoaromatic steroid, and triaromatic steroid biomarkers and changes in the proportions of steroid compound classes were determined and evaluated for thermal maturity. Additionally, isomerization ratios within each of the compound classes were examined with increasing thermal maturation.Rearranged diasterenes initially dominate the steroidal hydrocarbons, followed by diasteranes in the late diagenetic and catagenetic stages and at peak oil maturity (0.8 %VRE) diasteranes were most abundant. The relative abundance of a steroid is initially controlled by its rate of production or isomerization from a biological precursor, but at higher thermal maturities selective catagenetic destruction alters the relative abundance of steroid classes, with aromatic steroids and regular steranes being degraded or expelled earliest. Thermal maturity parameters based on the isomerization of regular steranes, and the aromatization of monoaromatic steroids are effective thermal maturity indicators in specific but overlapping ranges beginning at 0.2 %VRE. This work represents a novel examination of a natural thermal maturity transect and helps to consolidate our understanding and application of the diagenetic transformations of steroid compounds.

Comprehensive Fluid Compositional Analysis Program to Support the Interpretation of Chichimene Field In-Situ Combustion Pilot

Summary The evaluation of enhanced oil recovery (EOR) methods in Colombia has been very active during the past decade. One of the most recent and promising pilots is the in-situ combustion (ISC) in Chichimene field, starting in September 2019. Based on international ISC field experiences, this pilot represents a unique case study given the depth (≈8,000 ft) of this heavy crude oil (9 °API) reservoir. The pilot project consists of one injector, seven producers, and two temperature observation wells between the injector and first-line wells. Production response shows encouraging results. Its interpretation is supported by a comprehensive fluid compositional analysis, which is the main objective of this paper. This paper describes the compositional analysis of produced fluids (gas, oil, and water) and the influence of the current flow assurance program. Geochemical simulations support the evaluation of scaling tendencies, and possible corrosion trends are based on iron and manganese concentrations following the National Association of Corrosion Engineers (NACE) standards. Crude oil analysis is based on conventional techniques (i.e., acid number, distillation curves, etc.) and biomarkers to infer possible thermal maturation changes in the produced oil. Results confirm predicted cycles of CO2 and H2S during the planning of the monitoring program. The solubility of both gases in water leads to their acidification and the formation of carbonate and sulfate scales characterized in production wells. The precipitation of solids was also influenced by the N2-based H2S scavenger decomposition downhole due to the water pH increment observed with the dosage increases. The scaling tendencies did not impact the productivity due to the high reservoir permeability. The precipitation of iron species made it difficult to interpret NACE standards to infer corrosion except for wells shut in for more than 2 months showing a higher concentration of Fe and Mn. However, a recent casing inspection job at one of the first-line producers shows no corrosion signs. Heavy metals such as nickel and vanadium were analyzed in water to infer the possible corrosion or thermal cracking of porphyrins in crude oil. Changes in the paraffinic fractions and biomarkers (i.e., methyl phenanthrene index, mono- and tri-aromatic steroids) suggest an increased thermal maturity of the produced oil. The robust monitoring program has provided important insights from the ISC process and flow assurance strategy supporting possible expansion plans. This study provides valuable guidelines for monitoring programs based on compositional analysis of produced fluids, including the influence of production chemistry. Lessons learned through the Chichimene ISC monitoring program can be valuable in interpreting thermal and potentially nonthermal EOR projects.

TSR Action and Genesis Mechanism of Antimony Deposit: Evidence from Aromatic Hydrocarbon Geochemistry of Bitumen from Paleo-Oil Reservoir in Qinglong Ore Field, Southwestern Guizhou Depression, China

In Qinglong ore field, the paleo-oil reservoir is found to be associated with antimony deposits, and they have a close genetic relationship. In this study, the aromatics geochemistry of paleo-oil reservoir bitumen was studied to further discuss the thermochemical sulfate reduction (TSR) reaction and the mechanism of antimony mineralization. A total of 124 aromatic compounds were identified by gas chromatography–mass spectrometry (GC–MS) analysis in bitumen samples, including abundant phenanthrene series, dibenzothiophene series, fluoranthene series, chrysene series, and a small number of fluorene series, naphthalene series, dibenzofuran series, biphenyl series, and triaromatic steroid series. Aromatic parameters such as trimethylnaphthalene index (TMNr), methylphenanthrene index (MPI), methylphenanthrene distribution fraction (MPDF, F1, and F2), methyldibenzothiophene parameter (MDR), C28TAS-20S/(20R + 20S), and benzofluoranthene/benzo[e]pyrene indicate that the Qinglong paleo-oil reservoir is in over maturity level. The abundance of phenanthrene and chrysene aromatic compounds and a small amount of naphthalene series, benzofluoranthene, fluoranthene, pyrene, anthracene, retene, perylene, and biphenyl suggest that the organic matter source of the paleo-oil reservoir was mainly low aquatic organisms, mixed with a small amount of higher plant. They detected a certain number of compounds, such as retene, triaromatic steroid series, and perylene, the ternary diagram of DBF–DBT–F and binary plot of Pr/Ph–DBT/P, DBT/(F + DBT)–DBF/(F + DBF), and Pr/Ph–DBT/DBF reveal that the source rock of the paleo-oil reservoir was formed in the marine environment of weak oxidation and weak reduction. The comprehensive analysis shows that the Qinglong paleo-oil reservoir originated from Devonian source rocks, just like other paleo-oil reservoirs and natural gas reservoirs in the Nanpanjiang basin. Abundant dibenzothiophene series were detected, indicating that the paleo-oil reservoir underwent a certain degree of TSR reaction. We believe that the gas reservoir formed by the evolution of the oil reservoir in the ore field participated in antimony mineralization; that is, hydrocarbon organic matter acted as a reducing agent and transformed SO42− in oilfield brine into H2S through TSR, providing reduced sulfur and creating environmental conditions for mineralization.

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.

Relative thermodynamic stabilities of biomarkers – Evidence from drill-bit metamorphism in the Iving Discovery wells 25/8-19 S & A, Norwegian North Sea

In this study, the effects of drill-bit metamorphism (DBM) on oil in a kerogen-free system is examined to provide an insight into the relative thermodynamic stabilities of biomarkers and aromatic hydrocarbons, free from the contributions of newly generated hydrocarbons that complicate studies based on source rock intervals. Mud gas from the upper 20 m of fractured basement in the 25/8-19 A well of the Iving Discovery exhibits compositional variations attributable to DBM, such as abundant alkenes and neopentane, and C isotopic fractionation. Molecular, and particularly biomarker, distributions from corresponding core extracts are compared to those in unaltered test oil from immediately overlying Triassic sandstone, after normalization to diahopane, which is suggested to be relatively resistant towards thermal degradation. The variations in these relative abundances, determined by GC–MS analysis, correspond to some of the previously observed or theoretically postulated thermodynamic stability trends among biomarkers, such as depletion increasing with C-number among extended hopanes, and enrichment of rearranged vs regular hopanes. Some other trends are not as expected, such as the similar sensitivities of diasteranes towards removal as regular steranes, and greater depletion of short-chain triaromatic steroids compared to their C26–C28 homologues. Triaromatic dinosteroids and their 28-nor counterparts are enriched relative to diahopane, whereas during normal maturation their concentrations have been found to decline towards the end of the oil window in a similar way to biomarkers in general. Among cheilanthanes, C29 and C30 members are particularly depleted when compared to C28. Methylated naphthalenes are more readily removed than corresponding phenanthrenes and dibenzothiophenes, but all such polycyclic aromatics are at least as depleted as regular hopanes and steranes, suggesting a level of thermal destruction similar to that observed above ∼1.2% vitrinite reflectance under natural maturation conditions. The influence of oil-based mud (OBM) on core extracts is limited, although contributions are recognized for C29 and C30 17α-hopanes and 17β-moretanes, 5α,14α,17α-20R isomer of the C27–C30 steranes, C20 and C21 cheilanthanes, gammacerane and 3β-methyl-17α-hopanes.

Depositional environment, age determination and source diagnose of oils from the western Chepaizi Uplift (Junggar Basin) constrained by molecular biomarkers

The western Chepaizi Uplift of petroleum-enriched Junggar Basin hosted multiple oil-bearing intervals, i.e., Neogene Shawan Formation (N1s), Lower Cretaceous Tugulu Group (K1tg), and Upper Carboniferous Xibeikulasi Formation (C2x). However, their origins had always been debated, due to the occurrence of Jurassic, Cretaceous and Paleogene source rocks in the neighboring Sikeshu Sag. In this paper, two oil groups, i.e., group A (N1s and K1tg oils from the west margin of western Chepazi Uplift), and group B (oils from the east margin of the western Chepaizi Uplift). The latter can further subdivided into family B1(Upper Jurassic Qigu Formation (J3q) oil and N1s oils from east margin of western Chepaizi Uplift close to the Sikeshu Sag, and oil sands from the Sikeshu Sag) and family B2 (N1s and C2x oils from eastern margin of the western Chepaizi Uplift away from the Sikeshu Sag) can be defined. Age-specified biomarker ratios (24-norcholestane, Oleanane index and extended tricyclic terpane ratio) implied Jurassic and Paleogene source ages with minor Cretaceous contributions. Depositional environment-diagnosed biomarkers (Pr/Ph, Pr/nC17, Ph/nC18, methyldibenzothiophene/methylphenanthrene, C22/C21 tricyclic terpanes, C24/C23 tricyclic terpanes, C29αβ/C30αβ hopane, C31 hopane (22R)/C30αβ hopane, and gammacerane/C30 hopane) showed lacustrine source input with high hypersalinity for family B1 but freshwater lacustrine facies for group A and family B2. C27–C28–C29 regular steranes and triaromatic steroids distributions indicated ascending order of high plant contribution from family B1 to family B2 to group A. Combined with stable carbon isotope, the uplift-scale petroleum contribution to multiple oil-bearing intervals by multiple source intervals were lined out. Group A oils implied origins from source rocks deposited in freshwater lacustrine facies under weak oxidation conditions with predominant organic input of high plants, and genetically matched with the Lower Jurassic Badaowang Formation (J1b) mudstone source rock. Family B1 oils showed provenance from source rocks occurred in reducing source facies with high hypersalinity and great contribution of lower organisms, and well corelated with both J1b and Paleogene Anjihahe Formation (E2-3a) source rocks. However, where away from the generating Sikeshu Sag, especially for family B2 oils, the hydrocarbon contribution of E2-3a source rocks was decreased, mainly controlled by their variable distribution and thermal evolution.

Bitumens from Tell Yarmuth (Israel) from 2800 BCE to 1100 BCE: A unique case history for the study of degradation effects on the Dead Sea bitumen

Twenty-one archaeological samples (2800 BCE-1100 BCE) from Tell Yarmuth (Israel) and two samples (700–800 BCE) from Tall Dhiban (Jordan) were analyzed by geochemical techniques. Dead Sea bitumen was identified in all samples.The sample set affords a unique opportunity to study the effects of weathering on Dead Sea bitumen. Lumps of pure bitumen were compared to bitumens which coat the interior face of potsherds. Bitumen on potsherds was recognized as more altered than lumps of pure bitumen.Saturates of potsherd bitumens were enriched in both 2H and 13C. Resins and asphaltenes were depleted in 2H. Resin was the only fraction depleted in 13C. The δ13C of asphaltenes was almost unchanged: −29.3 ± 0.07 (‰/VPDB) compared to an average −29.3 ± 0.16 (‰/VPDB) for present day Dead Sea asphalt. This stability confirms the δ13C of asphaltenes as a reliable parameter for tracing the sources of archaeological bitumens. Both resins and asphaltenes were enriched in 15N. Biodegradation and oxidation proceed through the cleavage of the N-C bond with preferential removal of 14N.Steranes and terpanes were also affected by degradation. C27steranes were preferentially removed. C30αβ hopane was degraded. 18α-22,29,30-trisnorneohopane (Ts) /17α-22,29,30-trisnorhopane (Tm) and Gammacerane/C31αβHopane ratios increased due to the selective biodegradation of the αβ hopane family. Significant changes were recorded among phenanthrenes, dibenzothiophenes and also in triaromatic steroids.

Comparison of biomarkers in retained oil and expelled oil of lacustrine marlstone determined by hydrous pyrolysis: Application to hydrocarbon migration determination in the tight oil reservoir

Oil in an unconventional tight reservoir is considered to migrate from a very adjacent reservoir or be impregnated in situ within source rocks, but their large hydrocarbon molecular fractions during short‐distance migration in the tight system are poorly understood. In order to explore the evolution characteristics and differences of the geochemical properties, especially the biomarkers, between expelled oil (just detached from source rock and not undergoing long‐distance migration) and retained oil under different thermal evolution stages of lacustrine marlstone. Semi‐open hydrous pyrolysis was conducted on an immature lacustrine marlstone collected from the third member of the Eocene Shahejie Formation in Shulu Sag, Bohai Bay Basin and the hydrocarbon product expelled from and remained in the rock during pyrolysis were collected and analysed with gas chromatography–mass spectrum instrument. The results indicate that the saturates compositions show expelled oil having commonly lower thermal maturity than retained oil because expelled oil is a mixture of multiple‐term hydrocarbon expulsions, while retained oil is the remained final hydrocarbon product of pyrolysis in semi‐open experimental system. The hydrocarbon migration has a significant effect on the maturity‐related parameters distribution such as 20S/(20S + 20R) and αββ/(αββ + ααα) for C29 steranes. Hydrocarbon expulsion enables Ts/(Ts + Tm) ratio in expelled oil is much greater than that of retained oil at lower matural stage. The saturated hydrocarbon maturity biomarkers in retained oil and expelled oil vary in a subtly different pathways with increasing maturity, comparatively the aromatic hydrocarbon biomarkers, such as triaromatic steroids and methyl‐phenanthrene (MPI1 and MPI2), regularly changed during maturation, which is not obviously affected by migration effects and can be used not only for thermal maturity predications but also for oil–oil or oil–source correlation. C29 20S/(20S + 20R) and C29 αββ/(αββ + ααα) commonly are preferable maturity parameters for shale or mudstone, reflecting that mineral matrix and oil in source rock are not affected by migration and supposed to have an oil thermal maturity corresponding to depth. Combined with some tight reservoir cases in North America and China, the biomarker ratios variation along depth and its abnormal value can be used to predict sweet spots in tight reservoirs.

SEVERE BIODEGRADATION OF CRUDE OILS FROM THE QUIRIQUIRE FIELD, EASTERN VENEZUELAN BASIN

Five crude oil samples from the Quiriquire field (Maturin sub‐basin, Eastern Venezuelan Basin) were analysed to evaluate their levels of biodegradation. The oils were obtained from coarse sandstones and conglomerates of the Pliocene Quiriquire Formation at depths <1000 m. Analyses of the samples’ bulk physicochemical parameters indicate variations in API gravity and in the content of saturated hydrocarbons and NSO+asphaltenes, and also in the saturate/aromatic ratio which increases in more biodegraded oils. n‐Alkane distributions are characterized by a dominant unresolved complex mixture (UCM) or hump under an envelope of peaks which lack the acyclic isoprenoids pristane and phytane. The alteration of steranes and terpanes together with the presence of 25‐norhopanes and 17‐nor‐tricyclic terpanes, and the alteration of low molecular‐weight (C20‐C21) triaromatic steroids, phenanthrene, methyl‐phenanthrene, dibenzothiophene and methyl‐dibenzothiophene, indicate that the oils have undergone severe biodegradation.The oils contain compounds with different susceptibilities to biodegradation which is probably a consequence of the mixing of different oil charges in the Quiriquire Formation reservoir. The oils were derived from underlying source rocks in the Upper Cretaceous Guayuta Group (Querecual and San Antonio Formations), and migration into the shallow reservoir at Quiriquire field likely occurred continuously through time. Although the oils have undergone severe biodegradation, it was possible to make some inferences about their origin. Thus, the analyzed oils are interpreted to have originated from marine shale or marl source rocks containing mixed organic matter deposited under anoxic‐suboxic conditions and were generated at near peak oil window maturities.

GEOCHEMICAL COMPOSITION OF ORDOVICIAN OILS FROM THE TUOPUTAI REGION, NORTHERN TARIM BASIN, NW CHINA: SOURCE ROCK CORRELATION

Molecular and stable carbon isotope compositions of 46 Ordovician crude oil samples from wells in the Tuoputai region of the northern Tarim Basin were investigated using GC–MS, MRM GC–MS and IRMS to determine their genetic relationships and to identify possible source rocks. Thirty‐three source rock samples from outcrops and cores were also investigated. The oil samples varied from light to heavy crudes and showed very narrow δ13C value ranges for the whole oil, saturated and aromatic fractions. The majority of the oils displayed very similar molecular compositions with relatively high concentrations of n‐alkanes and isoprenoids and low concentrations of terpenoids and steroids. Comparison of the compositions of these crude oils strongly suggested their genetic affinity, while maturity parameters indicated maturity variations from the peak to the late oil generation stages. The samples also showed the characteristics of mixtures of biodegraded and fresh oil charges.Bitumen extracts from Cambrian and Ordovician source rocks were studied in detail. The oil compositions suggested a marine marl source deposited in anoxic, hypersaline conditions with significant bacterial and algal organic matter inputs. The distributions of C26–C28 triaromatic steroids, tricyclic terpanes and regular steranes appear to have been greatly influenced by thermal maturation, making them unreliable for correlating the oils and the source rocks. In contrast, dinosteranes and triaromatic dinosteroids seem not to have been affected by maturation and were more useful for correlation studies. They indicated that there was no or little genetic relationship between the Cambrian – Lower Ordovician source rocks and the oils, but in general suggested a possible Middle – Upper Ordovician source for the oil accumulations in the Tuoputai field. However, the occurrence of triaromatic dinosteroids in oil from well TP28XCX may also suggest a minor contribution from Cambrian – Lower Ordovician source rocks.

A distinct oil group in the Dongying Depression, Bohai Bay Basin, China: New insights from norcholestane and triaromatic steroid analyses

The lower third and upper fourth members of the Eocene Shahejie Formation (lower Es3 and upper Es4) are known as two sets of effective source rocks in the Dongying Depression, Bohai Bay Basin. In this study, the saturated and aromatic steroids of 20 crude oil samples from the Dongying Depression were systematically investigated using gas chromatography–mass spectrometry (GC–MS) and GC–MS metastable reaction monitoring. Based on the distribution patterns of the steranes, norcholestanes, and triaromatic steroids, the Group I and II oils were clearly identified and shown to be derived from the lower Es3 and upper Es4 source rocks, respectively. Significantly, it was discovered that three oil samples collected from the Eocene Kongdian Formation and the Ordovician reservoirs exhibit completely different steroid and isotopic compositions from those of the Group I and II oils. These oils, as a distinct oil group (Group III), have the characteristics of abnormally high concentrations of C29 steranes, 24-nordiacholestane, 27-nordiacholestane, C28 triaromatic steroids, C27 4α-methyltriaromatic steroids, and C29 4α-methyl-24-ethyltriaromatic steroids, and low concentrations of 24-norcholestane, 27-norcholestane and triaromatic dinosteroids, and 13C depletion. Although the related source rock of Group III oils needs to be further clarified in the study area, the 13C depletion, abundant C29 steranes, and low 24-norcholestane content may suggest greater bacteria/algae inputs (e.g., green algae rather than dinoflagellates) than both the lower Es3 and upper Es4 source rocks. Identification of the Group III oils indicates that the norcholestanes and triaromatic steroids may be more effective molecular fossils for the classification of crude oils in complicated petroleum exploration fields.

Molecular Markers of Biogenic and Oil-Derived Hydrocarbons in Deep-Sea Sediments Following the Deepwater Horizon Spill

Following the Deepwater Horizon oil spill (DWHOS), the formation of an unexpected and extended sedimentation event of oil-associated marine snow (MOSSFA: Marine Oil Snow Sedimentation and Flocculent Accumulation) demonstrated the importance of biology on the fate of contaminants in the oceans. We used a wide range of compound-specific data (aliphatics, hopanes, steranes, triaromatic steroids, polycyclic aromatics) to chemically characterize the MOSSFA event containing abundant and multiple hydrocarbon sources (e.g., oil residues and phytoplankton). Sediment samples were collected in 2010–2011 (ERMA-NRDA programs: Environmental Response Management Application – Natural Resource Damage Assessment) and 2018 (REDIRECT project: Resuspension, Redistribution and Deposition of Deepwater Horizon recalcitrant hydrocarbons to offshore depocenter) in the northern Gulf of Mexico to assess the role of biogenic and chemical processes on the fate of oil residues in sediments. The chemical data revealed the deposition of the different hydrocarbon mixtures observed in the water column during the DWHOS (e.g., oil slicks, submerged-plumes), defining the chemical signature of MOSSFA relative to where it originated in the water column and its fate in deep-sea sediments. MOSSFA from surface waters covered 90% of the deep-sea area studied and deposited 32% of the total oil residues observed in deep-sea areas after the DWHOS while MOSSFA originated at depth from the submerged plumes covered only 9% of the deep-sea area studied and was responsible for 15% of the total deposition of oil residues. In contrast, MOSSFA originated at depth from the water column covered only 1% of the deep-sea area studied (mostly in close proximity of the DWH wellhead) but was responsible for 53% of the total deposition of oil residues observed after the spill in this area. This study describes, for the first time, a multi-chemical method for the identification of biogenic and oil-derived inputs to deep-sea sediments, critical for improving our understanding of carbon inputs and storage at depth in open ocean systems.

Aromatic hydrocarbons as parameters of oil formation conditions

Aromatic hydrocarbons in the oils of Absheron and Baku archipelago, as well as in coastal zones have been identified via the method of chromato-mass-spectrometry. Geochemical estimation of distribution character of main biomarkers and their correlations as well is presented. The similarity in the distribution of monoaromatic and triaromatic steroid hydrocarbons, as well as benzogopane and sekabenzogopans has been specified, which suggests that studied oils are associated with one source rock or organic facies. The varieties observed in the distribution of saturated steranes are most likely the result of differences in the values of thermal maturity or the processes of secondary alterations.