Aromatic Biomarkers Research Articles

Biomarker and isotopic characteristics of Miocene condensates and natural gases, West Delta deep marine concession, Eastern Mediterranean, Egypt

The Western Delta Deep Marine Concession (WDDM) in the Eastern Mediterranean Sea is one of northern Africa's most recent petroleum-potential regions for gas and condensate exploration. The present study aims to determine the characteristics of the 15 natural gases and 5 associated condensate samples, using molecular compositions and isotopes from the Miocene reservoir rocks in the various wells located in the WDDM. The results of this study are also used to determine the gas-condensate correlation for their probable source rocks as well as the methane-generating mechanisms (i.e., thermogenic or microbiological). Results highlighted in this research reveal that most of the natural gases in WDDM are mainly thermogenic methane gases, with small contributions of biogenic methane gases that were generated from mainly mixed sources, with a high sapropelic organic matter input for biogenic gases. The thermogenic methane gases were formed from secondary oil and oil/gas cracking at the high maturity stage of the gas window. The biogenic gases are also contributed to the Miocene reservoirs, which are formed from the primary cracking of kerogen at low maturity stage by the action of CO2 bacterial reduction. In addition, the saturated and aromatic biomarker results show that the condensate samples were generated from clay-rich source rocks. This source unit of the Miocene condensates were deposited in a fluvial deltaic environmental setting, containing mixed kerogen type II/III and accumulated during the Jurassic–Cretaceous, as evidenced by the age dating indicators. The properties of the natural gases and associated condensates in the Miocene reservoir rocks suggest that most of the thermogenic methane gases, together with the condensate, are derived primarily from mature Jurassic–Cretaceous source rocks and formed by secondary oil and oil/gas cracking at the gas generation window, as demonstrated by the 1-D basin modelling results highlighted in the prior works. Therefore, most of the natural gases in WDDM are non-indigenous and migrated from more mature Jurassic–Cretaceous source rocks in the nearby Northern Sinai provinces or the deeper sequences in the offshore Nile Delta provinces.

Oil-source correlations and petroleum accumulation in oil sands in the Carboniferous Yanghugou Formation in the Shajingzi area, Ordos Basin, China

The south-central part of the western Ordos Basin is an important structural belt. Recent drilling of Well YT2 in the Shajingzi area of this belt has led to the discovery of oil sands in the Carboniferous Yanghugou Formation. In this study, Paleozoic source rocks and oil sands were sampled from the Well YT2 and analyzed for oil-source correlations. Geochemical data including saturate and aromatic biomarker compositions were used to investigate the genetic origins of oil sands. Saturated hydrocarbon biomarkers, such as n-alkanes, isoprenoids, tricyclic terpanes and C27-C28-C29 regular steranes, indicate that the oil and source rocks in the Ordovician Wulalike Formation are similar in terms of maturity, provenance and sedimentary environment. Aromatic parameters, such as dibenzothiophene/phenanthrene and alkyldibenzothiophene/alkyldibenzofuran ratios in the oil are similar to those in source rocks in the Carboniferous Yanghugou Formation and the Permian Taiyuan Formation. Moreover, retene is direct evidence that the oil has originated in part from the Yanghugou and/or Taiyuan formations. In addition, the modelled accumulation ages indicate that that the maturity of the two types of oil inclusions is consistent with the source rocks of the Wulalike, Yanghugou and Taiyuan formations during the charging times. The thrust faults runs through the Wulalike, Yanghugou and Taiyuan formations can provide pathways for oil migration. Therefore, the oil from the Yanghugou Formation has a mixed source from the Wulalike, Yanghugou and/or Taiyuan formations. This is the first documented discovery of Ordovician marine oil in the Ordos Basin.

Paleoenvironmental characterization of the Ponta Grossa formation (Devonian) at the northwest border of the Paraná Basin, Brazil

Previously uncharacterized Devonian shale samples from the Ponta Grossa Formation (Paraná Basin) were collected from three outcrops in Mato Grosso (DA01 and DA02) and Mato Grosso do Sul (PG02) along the northwest of the basin. The depositional paleoenvironment and hydrocarbon source potential were assessed using Rock-Eval pyrolysis, saturate and aromatic biomarkers, and carbon isotope data. For the DA01 shales, poor total organic carbon (TOC) contents and low hydrogen index (HI) indicate a high predominance of type III kerogen. The biomarker analysis results suggest a transitional depositional paleoenvironment with a greater contribution of continental than marine organic matter, pointing to a regressive marine event. In DA02, the TOC contents ranged from 0.24 to 4.46%, with kerogen types II and III, S2 ranging from poor to good, HI ranging from 78 to 410 mg HC/g TOC, and biomarkers indicating a marine transitional environment and transgressive marine event. Shales from outcrop PG02 had the highest TOC contents, particularly at the top, with predominantly kerogen types II and III, S2 varying from low to excellent, and HI ranging from 100 to 350 mg HC/g TOC. Two types of paleoenvironments were identified based on the biomarkers of PG02: regressive at the base and transgressive in the middle and top of the outcrop. The DA01 outcrop was located at a more proximal position to the coast, the DA02 were at more distal position, while PG02 was located in a transitional position between DA01 and DA02. Ratios of C29αββ/(αββ+ααα) and 20S/(20S + 20R) steranes, the presence of unresolved complex mixture, and a higher proportion of polar compounds to the detriment of saturated and aromatic compounds indicated that the shales were immature for the generation of hydrocarbons. This study represents the first broad organic geochemical evaluation of the northwest Ponta Grossa Formation.

Paleoclimate-induced wildfires in a paleomire in the Ordos Basin, Northern China during the Middle Jurassic greenhouse period

Paleo-wildfires are of great significance in the study of paleomire ecosystems. The products of such fires preserved in paleomires can provide detailed information for the reconstruction of wildfire regimes. This study analyzed the distribution of charcoal and aromatic biomarkers in a coal seam from the Middle Jurassic Yan'an Formation in the Ordos Basin, China. The results showed very high content of pyrogenic inertinite (38%–62%) and common occurrence of pyrogenic polycyclic aromatic hydrocarbons (PAHs) in the studied coal seam, suggesting that wildfires occurred frequently during the period of peat accumulation. The mean reflectance of pyrogenic inertinite (1.50%) in all samples indicates a dominance of low-temperature wildfires in the paleomire, which is also supported by the high abundance of 2- to 3-ring PAHs. Furthermore, the common observations of high-reflectance pyrogenic inertinite (∼ 4.47%RIo) and 4- to 6-ring PAHs suggest the repeated occurrence of high-temperature wildfires. Various parameters of 3- to 4-ring PAHs indicate a relatively high proportion of charred material contributing to the organic matter in the paleomire. The dimethylphenanthrene ratios of the studied samples were inconsistent with those of modern experimental results, which can be probably attributed to the non-analogous vegetation composition in the Jurassic paleomire, and caution should thus be exercised when applying dimethylphenanthrene ratios to ancient sediments. The ratio of retene to 3-ring PAHs implies the contribution of conifers to the fires in the paleomire. The pyrogenic PAHs originated from both, combustion residues and smokes produced by in situ and local wildfires. The high O2 level during the Middle Jurassic promoted the frequent occurrence of wildfires in the studied paleomire. In addition, the occurrence of wildfires was collectively affected by the wet/humid climates, seasonality, and depositional environment.

Biomarker characteristics of the heterocyclic and polycyclic aromatic hydrocarbons in the crude oils from West Siberian Basin, Russia: an implication for correlation with organic matter input, depositional environment and maturity

The distribution of aromatic hydrocarbon in sediments and oils is known to be affected by thermal maturity, source input and depositional conditions. Hence, the aromatic hydrocarbon fraction of four oil samples from different wells of the West Siberian Basin (central Russia) was investigated to determine the source of organic matter (OM), depositional environment and maturity of their source rocks. Biomarker ratios and parameters of aromatic compounds indicate that the studied oils were generated from clay-rich source rocks containing a mixed organic matter of marine and terrestrial origin, which were deposited in freshwater/deltaic to marine environmental settings under suboxic to relatively oxic conditions. Furthermore, the maturity-sensitive aromatic hydrocarbon parameters indicate that the source rocks are thermally mature, and in the peak to late maturity stage of the oil generation window. Oil–oil correlation results of various established aromatic biomarker proxies for organic matter origin, depositional environment and lithology suggests that the analyzed west Siberian Basin’s oils were generated from a variety of geological sources and can be grouped into two distinct oil families. The first oil family (group I) was generated from clay-rich source rock containing mixed organic matter with relatively high amounts of terrestrial plant input and deposited in a non-marine (freshwater/deltaic) environment under relatively oxic conditions, while the second oil family (group II) was derived from mixed organic matter with high marine OM input that was deposited in marine environment under suboxic conditions. The findings of the heterocyclic and polycyclic aromatics highlighted in this study can help for better understanding of the genetic ties between oils and their sources and have an impact on traditional hydrocarbon development in the Basin.

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.

Geochemistry of oils and condensates from the lower Eagle Ford formation, south Texas. Part 4: Diamondoids

A rich variety of diamondoid indices exists that are assumed to be diagnostic of source facies, thermal maturity, the extent of oil cracking, mixed oils, and reservoir alteration processes. These parameters were developed either from empirical relationships observed in source rocks and oils produced from conventional reservoirs or laboratory experiments. Diamondoid parameters are now being routinely applied to unconventional oils; however, their accuracy and reliability have not been rigorously tested in these petroleum systems.Here, we examine diamondoid compositions in a suite of lower Eagle Ford oils and condensates with well-constrained source facies and thermal maturity established by other geochemical measurements and basin modeling. Diamondoid concentrations are low in samples from ∼0.8 to 1.35 %Ro, and then increase significantly at higher maturity due to the cracking of less stable hydrocarbons. Most published maturity-dependent diamondoid indices are not diagnostic when maturity is ≲ 1.1 %Ro and are influenced by source facies. Based on our data, their predictive accuracy is no better than ±0.2 %Ro at higher maturity. Published source indices are not diagnostic of a specific kerogen type or lithofacies but may be used to differentiate sources in complex petroleum systems. Quantitative diamondoid analysis can be used to calculate the degree of oil-to-gas conversion; however, the natural variance in the baseline concentration of 3-+4-methydiamantanes limits its accuracy at maturities ≲1.35 %Ro. Three lower Eagle Ford oils associated with Karnes Trough faults exhibit anomalous diamondoid distributions behavior that these reservoirs received secondary charges from a source other than lower Eagle Ford (likely Austin Chalk). Evidence for a secondary charge was not apparent from the study of the saturated and aromatic biomarkers. In summary, established diamondoid maturity and source indices based on compound ratios should be used with caution to characterize unconventional oils/condensates while quantitative diamondoid analysis can be used with greater confidence and can provide insights that are not apparent from other geochemical parameters provided that highly accurate determination of diamondoid concentrations can be obtained. In the case of the Eagle Ford oils and extracts, analyses must be accurate to less than 1 ppm.

Behavior of molecular maturity parameters in overmature shales affected by diabase intrusion (Irati Formation, Paraná Basin – Brazil)

Molecular parameters based on saturated and aromatic compounds are important tools to access thermal maturation, but the behavior of these compounds is affected by different rates of heating. This study used samples from the Irati Formation, one of the most important source rocks in the Paraná Basin, to assess the behavior of commonly used maturation parameters in overmature rocks, in conditions of high heating rates. The Irati Formation belongs to an atypical hydrocarbon generation system. It was matured by heat of the magmatic intrusions related to volcanism of the Serra Geral Group. The main objective of this work is to evaluate how the heat of a diabase intrusion affected the distribution of the saturated and aromatic biomarker parameters, relating to the intrusive rock distance. For this purpose, eleven shale samples from the borehole CBM-002-ST-RS of the Santa Terezinha field were analyzed. The samples were located at a maximum distance of 46.4 m from a 35.32 m-thick diabase intrusion. For the better understanding of each parameter behavior, was established four different zones according to their trends: maturity correlation, equilibrium, reversal and high thermal stress. The maturity correlation zone is where the progression of the parameter's values are correlated to the increase in maturation towards the base of the profile. The saturated parameters, such as Ts/(Ts + Tm), M30/(M30 + H30) and C29Ts/(C29Ts + H29) marked the increase in maturation of the samples more distant from the intrusion, and the aromatic parameters, such as MPI-1, MPR, DMDBTR 2.4/1.4 and TA(I)/TA(I + II), show an increase in maturation at the intermediate-distance samples. The ratio of Tric/H30 marked the increase of maturation in the largest number of samples. The parameters that reach equilibrium in the beginning of oils window entered the profile already in the equilibrium zone, such as 20S/(20S + 20R), 22S/(22S + 22R) and ββ/(αα + ββ). Furthermore, most parameters presented a reversal zone after the equilibrium or the maturity correlation zone, where they showed an inverse pattern with increase in maturation value. Finally, all parameters presented a chaotic distribution in the samples closest to the intrusion (starting at an average distance of 11 m from the intrusion), this behavior characterize the high thermal stress zone.

Geochemical assessment of upper Cretaceous crude oils from the Iranian part of the Persian Gulf Basin: Implications for thermal maturity, potential source rocks, and depositional setting

The Upper Cretaceous carbonate successions of the Sarvak Formation host giant oil reservoirs in the Persian Gulf. In this research, a total of 28 oil samples from nine oilfields located in the western, central and eastern parts of the Persian Gulf region were studied to determine the genetic relationships of oils, depositional setting of possible source rocks, thermal maturity, and source-rock ages in the Persian Gulf basin. According to the measured geochemical data, the source rocks facies vary from marine carbonates and marl/carbonates in the central and eastern oilfields to shale/carbonates in the western oilfields. The Pr/Ph ratio, steranes and terpanes suggest anoxic to dysoxic conditions of the depositional environments. The depositional environments experienced both low water stratification/low salinity and normal salinity/unstratified conditions. Evaluation of the saturated and aromatic biomarkers shows that all oil samples are mature and most of the source rocks lie within the beginning of the oil-generation window. The thermal maturity of the central oilfields is higher than that of the other samples, and has gone beyond the oil-generation stage. The C28/C29 steranes ratio suggest that the central oilfields of the Persian Gulf have Paleozoic and Jurassic source rocks, whereas the Sarvak reservoir in other parts of this region is sourced from Cretaceous carbonate rocks.

Characteristics and genesis of light oil in Tabei Area, Tarim Basin, Northwestern China

Light oil is a high-quality crude oil. The formation and chemical composition of light oil is a matter of great concern to petroleum geologists. Yakela Field and Shunbei Field, which are located in Yakela and Shunbei regions, respectively, are the two main light oil fields in the Tabei uplift of the Tarim Basin. In order to identify the causative mechanism of light oils in these two fields, 17 crude oil samples were selected and analyzed by gas chromatography (GC) for saturated hydrocarbons and gas chromatography–mass spectrometry (GCMS) for saturated and aromatic biomarkers. Then, the compounds were measured via the mass spectrometry detector after GC separation. The molecular geochemical characteristics showed that the light oils in the two fields are sourced from similar marine source rocks deposited under weak oxidative and reductive environments. The maturity of crude oil in the Shunbei Field is higher than that in the Yakela Field. Simulation results of hydrocarbon generation history of source rocks in two fields showed that the formation mechanism of the two light oils is different. The light oil in the Yakela Field is directly generated by the source rock in the late oil-generating window. The crude oil in the Shunbei Field is formed by the deep burial and maturation of the crude oil generated in the early stage of the source rock.

Biomarker geochemistry of Qaiyarah, Butmah and Ain Zala crude oils, northern Iraq: Implications for the Jurassic source rocks

Fifteen crude oils from the Qaiyarah, Butmah, and Ain Zala fields were investigated using organic geochemistry to infer organic matter maturity, redox conditions of the depositional environment, and lithology of the corresponding source rocks. Carbon isotope compositions and sulfur contents were measured for all samples and saturated and aromatic biomarkers were analyzed by GC-MS.Low pristane/phytane (0.68–0.82) and high C35 S/C34S (1.03–1.66) and homohopane indices (0.14–0.24) in the oil samples indicate anoxic or euxinic conditions for the corresponding source rocks. In addition, sulfur content, C30 norhopane/C30 hopane, and C31 R/C30 hopane, Ts/Tm and C27 diasteranes/regular steranes values suggest carbonate source rocks with variable amounts of clays. These source rocks contain mainly type II-S kerogen. Moretane/hopane, hopane and sterane isomerization ratios suggest generation from early mature organic matter. Hierarchical cluster analysis identified three families charged from carbonate source rocks with variable amounts of clays and maturity. Oil-source rock correlation (based on published data) and age-related biomarkers suggest that these oils were generated from Jurassic source rocks (Sargelu, Naokelekan, and/or Chia Gara formations). Jurassic rocks in Ain Zala and Butmah area have low potential for hydrocarbon generation, therefore, the Ain Zala and Butmah oils were charged from Jurassic formations outside the Ain Zala area, but within the neighboring area northeast of Butmah and Ain Zala.

The Birkhead and Murta formations—organic geochemistry and organic petrography of Mesozoic fluvio-lacustrine source rocks in the Eromanga Basin, central Australia

Due to a historic focus on the underlying Cooper Basin and a growing demand for energy in Australia, a reassessment of the Eromanga Basin in central and eastern Australia is necessary, including a more detailed characterization of its Jurassic-Cretaceous petroleum system elements. The Jurassic Birkhead Formation and the Cretaceous Murta Formation are considered the most important source rocks of the Eromanga Basin. To study the petroleum generation potential of these two formations, a set of 55 rock cores from eight petroleum exploration wells was analysed. The sample set was subject to multiple-organic geochemical and petrographical analyses to evaluate organic richness, hydrocarbon potential (Rock–Eval S1 and S2), maceral composition, thermal maturity (e.g. vitrinite reflectance), and saturated and aromatic biomarker composition. The investigated fluvio-lacustrine siltstones and carbonaceous shales exhibit low to excellent total organic carbon (TOC) content, very low sulphur and low inorganic carbon content. Thermal maturity of both formations is at the onset of the oil window (immature to early mature). The Cretaceous Murta Formation shows good hydrocarbon generation potential and, in part, high production indices, while the Jurassic Birkhead Formation generally shows low kerogen to bitumen conversion and a good to very good hydrocarbon generation potential. Dispersed, rather coarse organic matter of terrigenous origin in fine-grained siliciclastic matrix with well-preserved plant remnants including “cutinite flames” is typical. Liptinite (mainly lamalginite) content is generally higher in the Murta Formation (including the occurrence of Botryococcus green algae), while vitrinite is clearly dominant in the Birkhead Formation. The Birkhead solvent extracts exhibit a more heterogeneous distribution of n-alkanes with distinct maxima in the long-chain range, whereas shorter-chain n-alkanes dominate the Murta extracts. Based on their quality and quantity of incorporated organic matter as well as thermal maturity, the petroleum generation potential of the Murta and Birkhead formations is discussed in detail.

Paradigms of biomarker and PAH distributions in lower Gondwana bituminous coal lithotypes

The present study aims to illustrate the paradigms of saturated and aromatic biomarker distributions in the lithotypes of Lower Gondwana bituminous coals (R̄r = 0.50–0.68%), perhaps for the first time. The coal samples were collected from the Early Permian Barakar Formation of the Rajmahal Basin, India. These coals mostly comprise three lithotypes, i.e., vitrain, durain, and fusain. The durain shows a considerable abundance than the other two bands and represents the bulk coal characteristics. The total organic carbon (TOC), S1, and S2 parameters from the Rock-Eval pyrolysis depict insignificant variations among the lithotypes, unlike the hydrogen index (HI), which peaks in the durain samples. The micropetrography furnishes a significant amount of liptinite macerals (30.6–51.3 vol% on the mineral matter free basis) present in the samples, dominated by the sporinite and cutinite. These hydrogen-rich macerals, being one of the crucial micropetrographic constituents, would have enhanced the HI values of the durain lithotype. The waxy nature of durain is also substantiated by the long chain-length n-alkane (LCLAs) distributions, while the plausible scarcity of wax in the lignocellulosic plant materials would have led to lower abundances of the LCLAs in vitrain (detected only from the 2D-Gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS)). Further, although the saturated diterpenoids are abundant in the durain and vitrain, their extremely low abundances (identified only through the GC × GC-TOFMS) along with the degraded LCLA signature in fusain may hint toward its source from the biomass combustion. Besides, the distributions and interrelations of the 4-, 5-, and 6-ringed polycyclic aromatic hydrocarbons (PAHs; m/z 202, 206, 228, 252, 276, and 278), like fluoranthene, pyrene, chrysene, benzo[g,h,i]perylene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[e]pyrene, and the isomers of the dimethylphenanthrene, dibenzanthracene, and benzochrysene further support the pyrogenic origin of the fusain lithotype. Meanwhile, these PAHS would have derived from diagenetic/catagenetic origin to mixed sources (diagenetic/catagenetic maturation of organic matter and combustion origin) in vitrain. The durain shows a mixed to pyrogenic origin of these PAHs and represents the PAH distributions of the bulk coal samples. Besides, the thermal maturity assessment of these lithotypes may suggest that the molecular parameters respond more sensitively to the thermal maturation compared to the bulk organic geochemical proxies.

Geochemistry of oils and condensates from the lower Eagle Ford Formation, south Texas. Part 2: Molecular characterization

Fifty-five petroleum samples from unconventional wells in the self-sourced lower Eagle Ford Shale were analyzed using routine geochemical methods. The samples are broadly distributed, geographically and vertically, and range in maturity from mid-oil to wet gas. Their physical and chemical properties span most of the range observed in the entire play. Two source facies are identified: west of San Marcos Arch is a marine marl facies deposited under anoxic to euxinic conditions containing abundant type II/IIS kerogen and east of the Arch is a marine shale to marl facies deposited under mostly oxic to anoxic conditions that received an additional input of terrigenous organic matter. The unconventional oils are too mature to detect any subtle differences that could distinguish a marl from a pure carbonate source facies. Saturated and aromatic biomarkers rapidly decrease with depth and are at or near the limits of detection below ∼9,000 feet. Condensates >13,500 ft show evidence of advanced hydrocarbon cracking. All properties and molecular parameters that are dependent on thermal maturity for oils/condensates west of the San Marcos Arch exhibit two trends with true vertical depth (TVD). One is defined by samples between the Karnes Trough area and the Maverick Basin; the other trend defined by samples near the Karnes Trough is offset with equivalent maturity occurring at deeper depths. This ∼2,000 ft offset is likely due to differences in the amount of uplift and erosion experienced in the two regions after the source rocks achieved their maximum maturity.

Implications of microbial enhanced oil recovery and waterflooding for geochemical interpretation of recovered oils.

Biosurfactants and waterflooding have been widely reported thus far for enhancing oil production. Nevertheless, there is a lack of literature to explore enhanced oil recovered methods effects on its chemical composition. The aim of this work is to investigate the effects of a biosurfactant produced by Bacillus safensis and brine injection on the recovered petroleum composition, and their implications for geochemical interpretation. Original and oils recovered from displacement tests were analyzed by gas chromatography and ultra-high-resolution mass spectrometry, emphasizing saturated and aromatic biomarkers and basic and acidic polar compounds. Geochemical parameters based on some saturated compounds were subtly affected by the recovery methods, showing their reliable applicability in geochemical studies. Contrarily, parameters based on some aromatic compounds were more affected by biosurfactant flooding, mostly the low molecular weight compounds. Thus, these aromatic parameters should be applied with caution after such methods. The distribution of basic and acidic polar compounds can also be modified affecting the geochemical interpretation. In the case of the basic ones, the biosurfactant greatly influenced the N class species with favorable loss of lower aromaticity compounds. In addition to water solubilization, the compositional changes described in this study can be related to fractionation due to adsorption on reservoir rocks.

Aliphatic and Aromatic Biomarkers Applied to the Assessment of the Potential for Hydrocarbon Generation in the Pimenteiras Formation (Devonian), Parnaíba Basin (Brazil)

Aliphatic and Aromatic Biomarkers Applied to the Assessment of the Potential for Hydrocarbon Generation in the Pimenteiras Formation (Devonian), Parnaíba Basin (Brazil)

Thermal Maturation Regime Revisited in the Dongying Depression, Bohai Bay Basin, East China

To the accurate reconstruction of the hydrocarbon generation history in the Dongying Depression, Bohai Bay Basin, East China, core samples of the Eocene Shahejie Formation from 3 shale oil boreholes were analyzed using organic petrology and organic geochemistry methods. The shales are enriched in organic matter with good to excellent hydrocarbon generation potential. The maturity indicated by measured vitrinite reflectance (%Ro) falls in the range of 0.5–0.9% and increases with burial depth in each well. Changes in biomarker and aromatic hydrocarbon isomer distributions and biomarker concentrations are also unequivocally correlated with the thermal maturity of the source rocks. Maturity/depth relationships for hopanes, steranes, and aromatic hydrocarbons, constructed from core data indicate different well locations, have different thermal regimes. A systematic variability of maturity with geographical position along the depression has been illustrated, which is a dependence on the distance to the Tanlu Fault. Higher thermal gradient at the southern side of the Dongying Depression results in the same maturity level at shallower depth compared to the northern side. The significant regional thermal regime change from south to north in the Dongying Depression may exert an important impact on the timing of hydrocarbon maturation and expulsion at different locations. Different exploration strategies should be employed accordingly.

Impact of palaeo-wildfires on higher plant parameter revealed by new biomarker indicator

Palaeo-wildfires were a common phenomenon in the Earth history. Their products can influence on some biomarker indicators characterising palaeo-environment conditions. Selected plant-derived aromatic biomarkers and polycyclic aromatic hydrocarbons (PAHs) were analysed in the Jurassic strata from the Polish Basin, Jurassic strata from the Mesozoic basement of the Carpathian Foredeep and Neogene strata from the Carpathian Foredeep. Marked compounds recorded varied input from palaeoflora families and the variable intensity of palaeo-wildfires. Retene (Ret), cadalene (Cad), 6-isopropyl-1-isohexyl-2-methylnaphthalene (ip-iHMN), dehydroabietic acid methyl ester (DAME), simonellite (Sim), fluoranthene (Fl), benz[α]anthracene (BaA), benzo[α]pyrene (BaP), benzo[ghi]perylene (BgP), coronene (Cor) were identified in the analysed samples. Based on mentioned compounds, pyrolytic input ratio (PIR) defined as (Cad + Sim + TDHA+ip-iHMN+Fl + BaA + BaP + BgP + Cor)/(Cad + Sim + TDHA+ip-iHMN) was proposed. To provide broader possibility of use, the total dehydroabietic acid (TDHA) counted as sum of DAME and free dehydroabietic acid was used in PIR equation. PIR manage to define pyrolytic organic matter input. Combination of PIR with higher plant parameter (HPP) results, defined as Ret/(Cad + Ret), enables the distinction of samples deposited in a warm/hot and humid palaeo-climate from these deposited in more arid conditions, which were affected by palaeo-wildfires. Heretofore, high HPP values have been interpreted as an evidence of a warm/hot and humid palaeo-climate. This interpretation is true when PIR values are low. If the PIR values were higher, then the palaeo-wildfires influence on HPP was more significant and palaeo-climate misinterpretation was more probable. The new indicator can be used in case of immature or early mature and mostly terrestrial organic matter.

Occurrence and geochemical significance of fluorene and alkylfluorenes in crude oils and source rock extracts from Niger Delta basin, Nigeria

The crude oils and their source rocks from Niger Delta basin, Nigeria were investigated by optical microscopy for the maceral compositions and by gas chromatography - mass spectrometry (GC-MS) for the composition, distribution and abundances of saturated and aromatic biomarkers. The data obtained showed that the rock samples and crude oils were formed from the mixed input of terrestrial and marine organic matter, and deposited under oxic to sub-oxic conditions in a lacustrine-fluvial/deltaic environment. The aliphatic maturity parameters calculated for the source rocks and oil samples showed that they have immature to early mature; and early oil window maturity respectively, whereas aromatic maturity ratios indicated that they were within immature to peak of oil generation and beyond; and peak to late oil generative window, respectively. The crude oils and source rocks were characterized by the dominance of C2-fluorenes over other alkylated homologues. The predominance of C2 fluorenes over other alkylated homologues might have been influenced by source facies and depositional environments. The relative percentages of fluorenes (FLs %), dibenzofurans (DBFs %) and dibenzothiophenes (DBTs %) values for the rock samples range from 9.36 to 38.04 %, 35.82–71.60 % and 10.02–42.52 %, respectively, while the crude oils have ranges of values from 18.91 to 56.38 %, 24.90–72.34 % and 5.99–19.02 %, respectively. These values showed that the crude oils and source rocks originated from the mixture of organic matter (terrestrial and marine) with higher contribution from terrestrial organic matter and deposited in a marine/brackish-saline lacustrine/swampy environments. The abundance and distribution of fluorenes and its derivatives were found to be effective in determining the origin and depositional environments of crude oils and source rocks in the Niger Delta Basin, Nigeria.

Changes in depositional paleoenvironment of black shales in the Permian Irati Formation (Paraná Basin, Brazil): Geochemical evidence and aromatic biomarkers

Physicochemical changes that occurred during deposition of Irati black shales were investigated via outcrop samples collected in the northeastern and central-eastern Paraná Basin. Various traditional geochemical indicators were used, in addition to newly detected biomarkers: C32 lanostane, simonellite, sempervirane, and C32–C35 benzohopanes. Bulk geochemistry parameters, mineralogy, and conventional saturated biomarkers, such as tetracyclic polyprenoids (TPPs), reflect restricted and hypersaline marine deposition with freshwater input and increased terrigenous contribution toward the top of the studied outcrops. This led to stratification of the water column, establishing euxinic conditions in the photic zone and reducing conditions at the bottom of the water column, which enhanced preservation of organic matter. Principal component and hierarchical cluster analyses were performed to group the samples according to their similar geochemical characteristics: prevailing marine environment (basal outcrop samples) and higher freshwater inflow with greater terrigenous contribution (top outcrop samples). These geochemical observations were corroborated by aromatic biomarkers such as triaromatic steroids, trimethylnaphthalenes, simonellite, and sempervirane. The aryl isoprenoid ratio (AIR) indicates persistence of photic zone euxinia (PZE) during Irati black shale deposition. The presence of C32 lanostane and C32–C35 benzohopanes is directly related to the hypersaline depositional environment of the analyzed samples. Finally, the C32-33/C32-35 benzohopane ratio is proposed as a thermal maturity proxy because variations in the ratio can be linked to the presence of a diabase sill and temperature catalyst minerals near one of the outcrops.