High Gammacerane Research Articles

An enhanced presence of tricyclic terpanes in the Triassic oils from the tight conglomerate reservoir in the Mahu Sag, Junggar Basin, NW China

A major exploration breakthrough has been made in the Mahu Sag, Junggar Basin. However, the origin of oils from Triassic Baikouquan Formation (T1 b) remains debated. In this study, 20 oils and 44 cores were selected to investigate their distributions of tricyclic terpanes (TTs). The results show that the relative abundance of TTs in oils is higher than those of hopanes due to a high maturity. Permian Wuerhe Formation (P2 w) cores are dominated by C20TT. Permian Fengcheng Formation (P1 f) samples, predominated by C23TT, are mature to high-mature, while Permian Jiamuhe Formation (P1 j) samples, having the same abundance of C21TT and C23TT, are in the over-mature stage. Maturity may be a likely possible reason for the different occurrences of C19–C23TTs between P1 f and P1 j as their organic matter source and depositional environment were similar. High gammacerane and C29 sterane occurred in T1 b oils, P1 f and P1 j cores, while those in P2 w are little, indicating P2 w had a minor contribution for T1 b oils. The δ13C values of oils and P1 f are similar and lower than those of P2 w and P1 j source rocks. Therefore, it is preliminarily speculated that the oils may be sourced from the deep buried P1 f in the over-mature stage.

Main controlling factors evolution on high-quality source rocks development in the Shanan Sag, Bohaibay Basin, NE China: Implication from structure, depositional environment, and organic matter

Lacustrine source rocks have a significant oil potential and contribute to 95 % of oil reserves in China's offshore areas, Nature of these rocks is related to climate and structure and they are commonly heterogeneous. Mechanism of organic matter accumulation in these rocks has been a matter of debate. Based on Rock-Eval pyrolysis, organic petrographic examinations, gas chromatography-mass spectrometry (GC–MS) analysis, and trace elements analysis, this paper illustrates the paleoclimate, tectonic activity, paleo-productivity, and preservation conditions recorded in high-quality source rocks of the Shanan Sag. The aim is to understand the controlling factors on the deposition of these source rocks. The results indicated that the organic matter of source rocks in middle segment of the third member of the Shahejie Formation (Es3m) was mainly from phytoplankton (average content = 52.2 %), the development of source rocks was controlled by high paleo-productivity. The primary productivity was low during the period of the first and second members of the Shahejie Formation (Es1-2), and the high gammacerane index, and low Pr/Ph ratios indicated that the source rocks were mostly controlled by a good reduction environment with high salinity. The source rocks of the third member of the Dongying Formation (Ed3) were characterized by high terrestrial materials (average exinite content = 62 %) and low to moderate Pr/Ph ratios, which indicated the source rocks were affected by both paleo-productivity and preservation environment. A productivity-controlling model in Es3m was established along with a co-controlling model of productivity and preservation in Ed3, which was largely developed under the background of a warm-humid climate and strong depression-controlled fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity.

Hydrocarbon generation and migration in the Fuxin Basin during the Cretaceous evolution of the North China Craton, NE China

The extensive thinning and destruction of North China Craton (NCC) during the Early Cretaceous led to the development of numerous rift basins and petroleum systems. However, the specific relationship between NCC evolution and hydrocarbon generation and accumulation in these sedimentary basins is still unclear. In this study organic geochemistry and oil-source correlation analyses were conducted to investigate the hydrocarbons generation potential and migration pathway in the representative Fuxin Basin. Results demonstrate that the semi-deep lacustrine source rocks were developed in the Jiufotang Formation (K1jf) in the west and the upper Shahai Formation (K1sh) in the east, both of which are enriched with organic matter content and hydrocarbon generation potential of kerogen Types II to III. Based on the hierarchical cluster analysis of crude oils biomarker fingerprints, two crude oil types (A and B) were deduced. The Type A crude oil is characterized by high gammacerane, low pristane/phytane, relatively high C29 regular sterane and methylphenanthrene index, which is consistent with the K1jf biomarker characteristics in the west. Therefore, the Type A crude oil is likely generated from the K1jf source rock that accumulated in turbidite sand bodies of the K1jf and/or migrated to the K1sh reservoir through strike-slip faults. Type B crude oil is dominated by low gammacerane, moderate pristane/phytane, high C27 regular sterane and 1,2,5-trimethylnaphthalenes, which is in good agreement with the K1sh4 features. The basin simulation revealed that hydrocarbon generation of source rocks in the Jiufotang and Shahai formations was linked to a rapid subsidence of the basin, which was induced by intense extension with the NCC destruction during the Early Cretaceous. Subsequently, the rapid subduction of the Western Pacific plate during the earliest Late Cretaceous led to the development of extensive epigenetic fractures, enabling extensive hydrocarbon migration in the Fuxin Basin. This study sheds light on oil sources in sedimentary basins in an extensional setting and provides insights into the dynamic process of hydrocarbon generation and migration associated with NCC evolution.

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 classification and secondary alteration of crude oil in the southern thrust belt of Junggar Basin

This study analyzed, 18 oil samples from four representative oil-bearing secondary tectonic units in the southern thrust belt of the Junggar Basin. The genetic types and the secondary alteration of oil were discussed based on the characteristics of light hydrocarbon compounds, adamantanes, sterane/terpene biomarkers, and stable carbon isotopic composition of the bulk oil. . The results show that the oil in the study area can be divided into five categories. Type I oil is characterized by 13C-depleted carbon isotopes, a low pristane to phytane ratio, and high gammacerane and low diasterane content, whereas Type II oil exhibits the opposite characteristics. Type III is very similar to Type II oil but has a higher 13C-depleted bulk carbon isotopic composition and a wider carbon number of tricyclic terpenes. Type IV and V oils are similar to Type I oils, except for relatively lower gammacerane content. Further, Type IV oils also have high ααα 20R regular sterane content, 13C-enriched bulk oil carbon isotopes, and a higher pristane to phytane ratio than Type I oil. In contrast, the relative content of C27 regular steranes of Type V oils is considerably lower than that of Type I oils. The oil in the study area has not suffered from biodegradation. Maturity information indicated by regular biomarkers (i.e., steroid, terpene, and phenanthrene) and adamantane differs significantly, with the latter exhibiting a strong cracking characteristic. This reflects the charging and mixing of oils formed at different evolution stages. The other types of oils are in the normal oil-generating window and do not suffer from intense cracking. The oil in the study area is generally subjected to evaporative fractionation, but there are obvious differences in the degree of alteration between different oil-bearing structures and different reservoirs with the same structure. This study is of great significance for understanding the origin and accumulation process of oil and gas in a complex structural zone.

Orbital-scale cyclicity and organic matter production in a Permian shallow mixed carbonate-siliciclastic system: Paraná Basin, Brazil

Mixed deposition in shallow seas is highly sensitive to climate variations and sea-level changes, which are the primary controls of high-frequency cycles in the stratigraphic record. The widespread Lower Permian mixed carbonate-siliciclastic succession of the Paraná Basin is composed of rhythmic organic-rich shale interbedded with dolostone. The strata are organized into several orders of magnitude of superimposed asymmetrical and symmetrical facies stacking patterns that developed in a broad epeiric sea, which extended over 106 km2 of Permian Gondwanan land. Detailed sedimentological and stratal stacking analysis allow the hierarchical organization to be identified in large-scale, medium-scale, small-scale, and elementary stacking patterns. The high-frequency depositional cycles compose four 3rd-order sequences. They comprise gray to black shale, marls, and peloidal to micritic dolostone. Due to the shallow water setting, the minimal available accommodation is responsible for the reduced cycle thickness and the overall mixed succession. The rhythmic deposits host highly productive hydrocarbon source rocks. Two widespread, thicker intervals of bituminous shale that develop during 3rd-order sequence transgression events are highly organic-rich oil shale (27 wt%). In addition, thin black shale laminae, which are interbedded with dolostone in high-frequency stacking patterns, are also rich in organic carbon (2–8 wt%). The alternation of high carbonate content and organic matter production was controlled by wet‒dry seasons and was related to total solar radiation maxima and minima. The total solar radiation maxima are translated into elevated carbonate precipitation due to shallower and hypersaline conditions. Total solar radiation minima induce freshwater discharge in shallow and hypersaline environments during wet stages, resulting in water column stratification. The occurrence of kerogen types III and IV, a high gammacerane index, variable Pr/Ph ratios, and C35 homohopane indices indicate this condition. In addition, the input of nutrient-rich terrigenous sediments favor algal blooms, enhancing anoxia in the bottom water, which is responsible for the organic-rich shale facies, as indicated by kerogen types I and II, high hydrogen index values, high C27/C29 sterane ratios, and higher β-carotane values. Based on the hierarchical organization and previous U–Pb SHRIMP zircon ages of volcanic ash layers, it is possible to correlate the observed succession with the global sea-level curve. A time span of 8.0 Ma is assessed for the Irati Sea, with an average of 2.7 Ma for the large-scale cycles. Furthermore, the high-frequency patterns that are compatible with the precession and eccentricity of the short (±100 ka) and major (±400 ka) periodicities of the Earth's orbit are estimated to be 26.6 ka, 135.5 ka, and 400 ka. The high-frequency cycles of organic-rich shales and carbonate that are deposited over thousands of years under long-term sea-level changes and are correlated along the basin suggest climatic control driven by the Earth's orbital perturbation, which is possible in the Milankovitch frequency band.

Effects of Magmatic-Hydrothermal Activities on Characteristic of Source Rocks from Beipiao Formation in the Jinyang Basin, NE China

The Jinyang Basin is a typical volcanic-sedimentary basin, located in the southern peripheral area of the Songliao Basin. Hydrothermal activity is often closely related to the intrusion or eruption of magma. It was recently suggested that dolomite in the Jurassic Beipiao Formation was formed under the influence of magmatic-hydrothermal activity and magmatic-hydrothermal activity might have impacts on the organic matter of the source rocks. No investigation has been aimed at the effect of magmatic-hydrothermal activities on the accumulation of organic matter in the Beipiao Formation source rocks and a comprehensive study is urgent, which would be indicative in unravelling the accumulation mechanism of organic matter and useful in further petroleum exploration. To provide important insights into these issues, we carried out a detailed investigation of geological and geochemical analysis for Wolong (WL) and Dongkuntouyingzi (DK) outcrop shales from the Lower Jurassic Beipiao Formation in the Jinyang Basin. The hydrothermal indicator discrimination diagram (Zn-Ni-Co triangular plot) and rare earth element anomalies (δEu and δCe) indicate that the formation of WL samples is associated with hydrothermal activity, but DK is not. The TOC values suggest that most of the WL and DK samples are good to very good and fair to good source rocks, respectively. The Ro values suggest that both WL (Ro = 1.17%) and DK (Ro = 1.01%) samples have entered the oil-generating stage, and WL samples were influenced by the magmatic activity with higher maturity. The biomarkers such as high steranes/hopanes, high 4-methyl steranes/C29 steranes, low Pr/Ph values and high gammacerane index suggest that WL samples were deposited in an anoxic-prone saline environment with significant contributions of algal sources. Contrarily, the DK samples were deposited in oxic-prone and freshwater paleolake with significant contributions of terrigenous organic matter. The magmatic-hydrothermal activities in the Wolong area brought numerous nutrients to the lake basin, which may facilitated the reproduction of aquatic organisms. At the same time, the magmatic-hydrothermal activities increased the salinity of water and promoted the formation of a water reducing environment, which provided an excellent environment for the preservation and enrichment of organic matter. Therefore, the magmatic-hydrothermal activities in the Wolong area promoted the formation of organic-rich source rocks and the hydrocarbon generation process.

The geochemical characteristics of the Paleogene lacustrine source rock and Cenozoic oil in the eastern Huanghekou Sag, Bohai Bay Basin, China: An oil–source rock correlation

Oil source correlations were conducted through bulk carbon isotope composition, source-related biomarkers, and maturity-related parameters to characterize the differences between two Paleogene lacustrine source rocks the Shahejie third (Es3) and fourth (Es4) members, utilizing seven Cenozoic crude oils. Es3 is dominated by type II1 sapropel kerogen, and Es4 is dominated by type II2 humic kerogen. The Es4 source rock has high gammacerane content, low Pr/Ph, high C30 steranes, high dinosteranes, and low β-/γ- and α-/γ-methyltrimethyltridecylchroman (MTTC), indicating a transgression in a saline lake during sedimentation. The low abundance of short-chain n-alkanes, high carbon peak number, abundant perylene, cadalene, and retene indicate that the Es4 input included significant amounts of ferns and gymnosperms. In contrast, the Es3 source rock was deposited in a moderately deep freshwater lake and had relatively more angiosperm and algal input with high oleanane to C30 hopane and high 1,2,7-/1,2,6- trimethylnaphthalene (TMN) ratios. The oils have relatively high sulfur content except oil O1. The larger population of oils were derived from the Es4 system, and were mixed with the low sulfur oil produced by Es3. The oil maturity parameters are generally higher than those seen in the two source where sampled, indicating that the oils were generated from source kitchens of higher thermal maturity in the east and south. The results of this study will contribute to the understanding of the evolution of Paleogene sedimentary environments and regional oil and gas exploration.

The Shahejie Formation in the Dongpu Depression, Bohai Bay Basin, China: Geochemical investigation of the origin, deposition and preservation of organic matter in a saline lacustrine environment during the Middle Eocene

The Eocene Shahejie Formation in the Bohai Bay, Eastern China is known to contain important petroleum source rocks. Here, a new, large set of geochemical, petrographical and mineralogical data obtained on 41 samples from 9 wells within the North Dongpu Depression is presented in order to investigate organic matter precursor material, thermal maturity as well as paleoenvironment during deposition of the Es3U, Es3L, Es3M and Es4U members within the Eocene Shahejie Formation. These shales and marlstones from the four sub-members show low to moderate TOC contents (up to 5%) with highest values in the Es3U member; quartz content is at about 10% and carbonate content is variable, partly reaching more than 40%. A range of different maturity parameters was determined indicating early mature to peak oil mature stages, and maturity increase with burial depth. Throughout, alginite is the predominant maceral, whereas both vitrinite and inertinite are rare. Hydrogen index values are highly variable, ranging from about 100 to 700. Distinctly different biomarker assemblages indicate variable depositional environments and organic matter inputs between the four intervals. The Eocene Shahejie shales and marlstones in the study area were deposited under episodic photic zone anoxia as revealed by the presence of aryl isoprenoids. Bottom water conditions were anoxic during most of the time, except for Es3M, as indicated by several parameters, e.g. low pristane/phytane ratios. In north Dongpu Depression Es4U displays a fairly uniform biomarker composition, which indicates relatively stable hypersaline lake conditions with a shallow chemocline based on the high Gammacerane Index > 0.6 and abundant aryl isoprenoids. Water salinity changed during deposition of Es3L according to variable C24 tetracyclic terpanes related parameters and evaporite minerals content. The aquatic algal organisms, i.e. haptophyte algae, diatoms and bryophytes which are reflected by abundant nC37–39 alkanes, a variable sterane pattern and other biomarkers, were flourishing during Es3L. Additionally, specific PAHs, i.e. retene, cadalene, fluoranthene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene also have been identified in small concentration in Es3L, which may be attributed to limited terrigenous organic matter input. Es3M was deposited in a normal lacustrine environment under rather low salinity conditions with substantial input of organic matter derived from algae. During Es3U, saline lake conditions were re-established accompanied by input of both algal and microbial organic matter, especially cyanobacteria as deduced from MMAs and 2α-MeHs, methanogens as deduced from abnormally abundant phytane, PMI, squalane, and methanotrophs from 3β-MeHs.

Molecular Characterization of NSO Compounds and Paleoenvironment Implication for Saline Lacustrine Oil Sands by Positive-Ion Mass Spectrometry Coupled with Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry.

NSO compounds mainly exist in geological bodies in the form of nonhydrocarbons and asphaltenes with abundant geological and geochemical information. Combined with the gas chromatography mass spectrometry (GC–MS) technology, positive-ion electrospray ionization Fourier-transform ion cyclotron resonance MS (FT-ICR MS) was used to understand the composition and distribution characteristics of NSO compounds in the oil sands of the Dongpu Depression and to explore their paleoenvironmental significance. The results show that n-alkanes are characterized by an even carbon number and phytane dominance, suggesting a saline lacustrine environment. Certain abundance of nC37 and β-carotane, high gammacerane content, and low diasterane content are detected in the analyzed samples, reflecting the saline-reducing paleoenvironment for the organic matter. Nine types of heteroatom compounds are detected: N1, N1O1, N1S1, O1, O1S1, O2, O2S1, S1, and S2. The main compounds are S1 and N1 compounds, followed by O1S1 compounds. The double-bond equivalent (DBE) value of S1 compounds is mainly distributed between 3 and 12, and the carbon number is mainly distributed between 18 and 35. The DBE value of N1 compounds is mainly distributed between 4 and 14, and the carbon number is mainly distributed in the range 15–35. Among the S1 compounds, DBE3 compounds (thiophenes) have relatively more sulfur-containing carotenoids (C40). The abundance of C40 S1 and the ratio of pyridine and its homologue DBE4–8/DBE9–12 N1 compounds show a good contrast with the paleoenvironment indicators of gammacerane/C30 hopane and diasterane/regular sterane. They can be used as a reference for the paleoenvironment index. Maturity is another factor affecting the distribution of NSO heteroatoms in the oil sands. NSO compounds are enriched in the DBE area with higher condensation, and the main peak carbon shifts forward. As the maturity increases, the relative abundance of N1 compounds increases, the aromatization intensifies, and carbon is broken into short chains. The research results shed light on the potential application of NSO compounds in petroleum exploration based on FT-ICR MS.

Characteristics and deposition models of the paleocene source rocks in the Lishui Sag, east China sea shelf basin: Evidences from organic and inorganic geochemistry

The Lishui Sag, situated at the East China Sea Shelf Basin, is regarded as one of the most prospecting oil exploration zones. Low exploration in this region requires detailed identification of source rocks to enhance further oil and gas exploration. Three potential source rocks, including the Paleocene Yueguifeng (E1y), Lingfeng (E1l) and Mingyuefeng (E1m) Formations, were investigated by integrating organic and inorganic geochemistry to estimate hydrocarbon generation potential, and to document original organic matter input and deposition environments. Two deposition models for source rocks were established considering tectonic subsidence and climate variation. The result shows that the E1y mudstones are dominated by high TOC content, oil/gas-prone kerogen at moderate maturity stage with relatively high hydrocarbon generation potential, whereas E1l and E1m mudstones are dominated by low TOC content, gas-prone kerogen at low to moderate maturity stage with low hydrocarbon generation potential, and coals with high hydrocarbon generation potential. The original organic matter input and deposition environment are obviously different among these three potential source rocks and are controlled by the coupling of tectonic subsidence and climate variation. Insufficient material supply lead mudstone deposition in a hemi-deep/deep lake during the E1y period, resulting in inefficient water mixing and stable water column stratification (high Gammacerane Indexs). The weak water inflow supplied minor oxygen and terrigenous organic matter to the lake due to arid-hot climate, as indicated by low Pr/Ph, C19 T/C23T, C20 T/C23T and C24Tet/C26T ratios, low Oleanane Indexs, and high C35 Homohopane Indexs and V/Ni ratios. Water algae, including pediastrums and chlorellas, which contributed to the development of high-quality source rocks, could be efficiently preserved in dysoxic condition. High subsidence rate together with sufficient sediment supply during the transgression in the E1l and E1m periods developed a coastal marine or paralic environment. The alternated climate brought unstable water inflow against stable water column stratification (low Gammacerane Indexs). Strong water inflow due to warm-humid climate provides abundant terrigenous organic matter and oxygen to the lake as indicated by high Pr/Ph, C19 T/C23T, C20 T/C23T and C24Tet/C26T ratios, high Oleanane Indexs, low C35 Homohopane Indexs and V/Ni ratios, and rich C29 steranes. Algal might be preferentially oxidized and degraded in aerobic conditions. Thus, mudstone is regarded as general-quality source rocks but coal is regarded as high-quality source rocks. Weak water inflow under arid-hot climate provided subtle aquatic and terrigenous organic matter to the lake, resulting in poor-quality source rocks.

Understanding the geobiology of the terminal Ediacaran Khatyspyt Lagerstätte (Arctic Siberia, Russia).

The Khatyspyt Lagerstätte (~544Ma, Russia) provides a valuable window into late Ediacaran Avalon-type ecosystems with rangeomorphs, arboreomorphs, and mega-algae. Here, we tackle the geobiology of this Lagerstätte by the combined analysis of paleontological features, sedimentary facies, and lipid biomarkers. The Khatyspyt Formation was deposited in carbonate ramp environments. Organic matter (0.12-2.22 wt.% TOC) displays characteristic Ediacaran biomarker features (e.g., eukaryotic steranes dominated by the C29 stigmastane). Some samples contain a putative 2-methylgammacerane that was likely sourced by ciliates and/or bacteria. 24-isopropylcholestane and 26-methylstigmastane are consistently scarce (≤0.4% and ≤0.2% of ∑C27-30 regular steranes, respectively). Thus, Avalon-type organisms occupied different niches than organisms capable of directly synthesizing C30 sterane precursors among their major lipids. Relative abundances of eukaryotic steranes and bacterial hopanes (sterane/hopane ratios=0.07-0.30) demonstrate oligotrophic and bacterially dominated marine environments, similar to findings from other successions with Ediacara-type fossils. Ediacara-type fossils occur in facies characterized by microbial mats and biomarkers indicative for a stratified marine environment with normal-moderate salinities (moderate-high gammacerane index of 2.3-5.7; low C35 homohopane index of 0.1-0.2). Mega-algae, in contrast, are abundant in facies that almost entirely consist of allochthonous event layers. Biomarkers in these samples indicate a non-stratified marine environment and normal salinities (low gammacerane index of 0.6-0.8; low C35 homohopane index of 0.1). Vertical burrowers occur in similar facies but with biomarker evidence for stratification in the water column or around the seafloor (high gammacerane index of 5.6). Thus, the distribution of macro-organisms and burrowers was controlled by various, dynamically changing environmental factors. It appears likely that dynamic settings like the Khatyspyt Lagerstätte provided metabolic challenges for sustenance and growth which primed eukaryotic organisms to cope with changing environmental habitats, allowing for a later diversification and expansion of complex macroscopic life in the marine realm.

Lacustrine conditions control on the distribution of organic-rich source rocks: An instance analysis of the lower 1st member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin

The lower 1st Member (Es1L) of the Oligocene Shahejie Formation in the Raoyang Sag, the particularly noteworthy source rock interval in the Jizhong Subbasin, were analyzed with sedimentological and geochemical techniques to describe the distribution of organic-rich source rocks and reveal the lacustrine environment and lake types during source rocks deposition. Results of the Raoyang Sag demonstrate that the organic-rich source rocks are mainly distributed in the north-central region with a maximum thickness of up to 120 m in the Maxi Subsag and gradually decreases to the southern tectonic regions such as Liuxi Subsag. These horizontal and vertical heterogeneities are attributed to depositional environments and lake types during the sedimentary period of the Es1L. Lakes in the Raoyang Sag are generally characterized by saline water during the Es1L deposition. The samples representing the south part of the Raoyang Sag display high pristane/phytane (Pr/Ph), C19/C23 TT, and C24 Tet/C26 TT ratios as well as low gammacerane index, 4-methyl sterane index, and sterane/hopane ratio. These findings suggest that hydrological open lake type has unstable water-column stratification, less reducing bottom water, and significant terrigenous supply. However, the samples representing the north-central part of the Raoyang Sag display high gammacerane index, 4-methyl sterane index, and sterane/hopane ratio as well as low Pr/Ph, C19/C23TT, and C24 Tet/C26 TT ratios. A strong correlation exists between δ13C and δ18O, indicating hydrological closed lake-type with excellent water stratification, anoxic bottom water, and enhanced algae productivity. This lake-type is of great significance for continuing research on organic matter enrichment in the Bohai Bay Basin.

Cretaceous and Paleogene saline lacustrine source rocks discovered in the southern Junggar Basin, NW China

Saline lacustrine source rocks in China provide the resource basis for many giant oil fields. Here, we report the first discovery of such source rocks in the piedmont area of the northern Tianshan Mountains in the southern Junggar Basin of NW China. These source rocks are Cretaceous and Paleogene in age. Organic geochemistry and trace-element data suggest that these mudstones were formed in a brackish–saline lacustrine environment, characterized by deep-water and reducing conditions, and in a warm and dry paleoclimate. Evidence for this environment includes mudstone Pr/Ph values of <1.0, high gammacerane index ratios of >0.4, and negative Ce and Eu anomalies. The total organic carbon (TOC) contents of these Cretaceous and Paleogene mudstones are typically >0.5%, with maximum values of 1.45% and 2.22%, respectively, signifying moderate to high organic matter abundance. Rock-Eval and kerogen analyses suggest that the organic matter was dominated by type II kerogen. The mudstones are generally thermally mature, suggesting favorable conditions for hydrocarbon (particularly oil) generation. Oil–source correlation confirms the oil generation of these two sets of source rocks, whose development are controlled mainly by reducing environments. These Cretaceous and Paleogene mudstones should therefore be a target for regional exploration in the southern Junggar Basin in addition to the known gas-producing Jurassic rocks. This oil-bearing potential is unique amongst the Early Cretaceous and Eocene–Oligocene central Asian petroliferous basins, as the eastern portion of this realm is considered to host predominantly Jurassic coal-based source rocks, giving rise to numerous large gas fields rather than oil resources.

Source rocks geochemistry and oil-source correlation in the Aershan and first member of Tengge'er formations of the Wulan-Hua sag in south of Erlian Basin, Northeastern China

The Wulan-Hua Sag in the south of Erlian Basin has large oil and gas resource, whereas the source rocks genetic potential and oil-rock correlation in its Aershan Formation (K1ba) and First member of Tengge'er Formation (K1bt1) are still unclear. We performed organic geochemistry analyses of oil and mudstone samples to divide crude oil types, evaluate source rock potential, and establish relatively accurate oil-source correlation. The results indicate that the source rocks in K1ba and the Lower interval in First member of Tengge'er Formation (LK1bt1) belong to good source rocks, characterized by high organic matter abundance, oil-prone kerogen type, and relatively high thermal maturity. K1ba and K1bt1 crude oil samples were divided into two types sourced from different Wulan-Hua source rocks. Type A oil is distributed in the K1ba and LK1bt1 of Saiwusu uplift and Hongjing uplift, and featured by a low gammacerane amounts (the majority of gammacerane/C30H < 0.30), and high Pr/Ph (ranging from 0.18 to 1.18, with mean value of 0.78). It has high mature organic matter mainly originated from terrestrial plant and dominant terrestrial plant. This type of oil was sourced from the K1ba source rocks in the Saiwusu uplift. Type B oil occurs in the LK1bt1 of the Tumuer Uplift and north sub-sag, and have high gammacerane abundance (the majority of gammacerane/(C30 > 0.3), and low Pr/Ph (ranging from 0.38 to 0.76, with mean value of 0.63). Its organic matter includes dominant terrestrial plant source, and this oil type should be sourced from the LK1bt1 source rocks in the Saiwusu uplift.

BIOMARKER GEOCHEMISTRY OF CRUDE OILS AND NEOGENE BITUMINOUS SHALES IN THE YENIKÖY AREA, EREĞLI‐ULUKIȘLA BASIN, CENTRAL ANATOLIA, TURKEY

In the Ereğli‐Ulukıșla Basin, southern Turkey, crude oil shows have been observed in the subsurface in the shale‐dominated non‐marine Upper Miocene – Pliocene succession. Based on analyses of samples from four boreholes, the shales’ organic matter content, thermal maturity and depositional characteristics are discussed in this study. Geochemical correlations are established between shale extracts and a crude oil sampled from the shale succession. The shales have moderate to high hydrogen index (HI) and very low oxygen index (OI) values. Pyrolysis data show that the shales contain both Types I and II kerogen, and n‐alkane and biomarker distributions indicate that organic matter is dominated by algal material. Very high C26/C25 and C24/C23, and low C22/C21 tricyclic terpane ratios and C31 R/C30 hopane, C29/(C28+C29) MA and DBT/P ratios in shale extracts indicate that deposition occurred in a lacustrine setting. High gammacerane and C35 homohopane concentrations and low diasterane/sterane ratios with a very low Pr/Ph ratio suggest that both the shales and the source rocks for the oil were deposited in a highly anoxic environment in which the water column may have been thermally stratified.Although the shales analysed have very low Tmax values, the production index is quite high which suggests that the shales are early‐mature to mature. Biomarker ratios including C32 22S/(22R+22S) homohopanes, C29 20S/(20R+20S) and ββ(ββ+αα) steranes, moretane/hopane, TA(I)/TA(I+II) and MPI‐3 all suggest that the shales are within the oil window. Heavy components of free hydrocarbons (S1) within the shales may have been recorded as part of the Rock‐Eval S2 peak resulting in the low Tmax values.The oil and shale extracts analysed are similar according to their sterane and triterpane distributions, suggesting that the oil was generated by the shales. However burial depths of the Upper Miocene – Pliocene shale succession are not sufficient for thermal maturation to have occurred. It is inferred that intense volcanism during the Pliocene – Pleistocene may have played an important role in local maturation of the shale succession.

Chemometric differentiation of crude oil families in the southern Dongying Depression, Bohai Bay Basin, China

Fifty-seven crude oil samples from the southern Dongying Depression were selected for detailed geochemical analysis. High C26/C25 tricyclic terpanes (0.81–1.26) and low C31 homohopane (R)/C30 hopane (0.15–0.26) ratios indicate that the oils are from lacustrine source rocks. The presence of C30 4α-methyl-24-ethylcholestanes and C30 tetracyclic polyprenoids imply the contribution of microalgal organic matter. Large variations of depositional environment-related biomarker parameters suggest differences in organofacies and water environment. Four genetically distinct oil families were differentiated by chemometrics of hierarchical cluster and principal component analyses using the parameter dataset of three isotope ratios and 15 molecular parameters, all related to source and depositional environment. Family I is mature and does not match any available source rocks. The inferred source rock contains microalgal material that could produce large quantities of wax, C29 steranes and tricyclic terpenoids in a reducing, salinity stratified water column. Family II is characterized by a strong phytane dominance, relatively high gammacerane and C35 homohopanes, and low diasterane and 4-methyl sterane contents. It has high values of sterane/hopane (S/H = 1.67–4.14), C35/C34 homohopane (C35H/C34H = 0.81–1.66) and gammacerane/C30 hopane (G/C30H = 0.57–1.13) and low pristane/phytane (Pr/Ph = 0.30–0.59), suggesting that the inferred source rock is the upper Es4 interval. Family III has characteristics distinct from families I and II: low S/H (0.15–0.27), C35H/C34H (0.39–0.49) and G/C30H (0.03–0.12) ratios and high Pr/Ph (0.48–1.90), C30-4M/C29 ααα20R (0.54–1.18) and C30 tetracyclic polyprenoid (0.54–0.66) ratios, suggesting that the source rock is the lower Es3 interval. The family IV oils are compositionally in-between the ranges of families II and III oils, and in line with them in plots of geochemical parameters. We suggest that family IV consists of mixed oils derived from the upper Es4 and lower Es3 intervals. Family II has a relatively low thermal maturity and the inferred source rock may be located in the Niuzhuang Sag and/or the southern gentle slope zone. Family III is mature and its inferred source rock may be situated in the Lijin and Boxing sags. The upper Es4 and lower Es3 source rocks in the three sags contribute the crude oils of family IV that occurs in the middle and eastern parts of the southern Dongying Depression.

Biomarker paleo-reconstruction of the German Wealden (Berriasian, Early Cretaceous) in the Lower Saxony Basin (LSB)

During the Early Cretaceous (Berriasian; Wealden 3–4), Northwestern Germany was covered by an east–west elongated tentatively brackish lake in which locally more than 700 m-thick black shales were deposited. While the distribution of organofacies’ in the basin is relatively well documented, the paleoenvironmental conditions in the basin center (e.g., occurrence and spread of water column stratification) and the spatial record of biomarkers in Wealden 3–4 shales (and coals) are rarely known. We here present respective data from the entire basin. In large areas, total organic carbon (TOC) contents are above 5 wt% and HI values above 700 mg hydrocarbons (HC)/g TOC, supporting the high potential of shales in the central basin as petroleum source rocks. Furthermore, bulk geochemical data as well as biomarkers clearly mirror the Wealden 3–4 facies distribution with the differentiation of a predominantly terrestrial setting east of the Weser River and an aquatic and brackish lake setting in the west. Certain biomarkers such as isorenieratane, specific for green sulfur bacteria, indicate that the basin consisted of a permanently stratified water column with a brackish/marine deep water body and an oxic–anoxic transition zone in the photic zone. In the southwestern gate of the lake (including the Isterberg area) and towards the east, no water column stratification developed. Characteristic of Wealden 3–4 black shale organic matter are: high relative abundances of saturated versus aromatic hydrocarbons (most likely due to high Botryococcus algal input), highly negative δ13C values in the extract fractions, low isotopic “canonical variables” (sensu Sofer in AAPG Bull 68:31–49, 1984), and high gammacerane, dinosterane, and C35-homohopane relative abundances. Interpreting those data, the different sub-facies of the environmental setting can be excellently documented. Particularly, in the western part of the basin, Wealden 3–4 shales are important petroleum source rocks. However, an overlap of biomarker signatures with those from Jurassic Posidonienschiefer Formation (“Posidonia”) shales from the same area shows that oil–source rock correlations in this area remain challenging.

Revelation of organic matter sources and sedimentary environment characteristics for shale gas formation by petrographic analysis of middle Jurassic Dameigou formation, northern Qaidam Basin, China

The 7th member of Middle Jurassic Dameigou Formation (J2d7) in northern Qaidam Basin, China was proposed to have good hydrocarbon generating potential in previous studies. Here we apply an integrated petrographic and geochemical analysis to investigate the depositional environment, real thermal maturity and shale gas potential of organic-matter (OM)-rich assemblage. The relatively high content of C29 regular sterane (56.8%–62.8%) and vitrinite predominance in maceral composition suggesting plenty of higher-plant input, combined with the high pristane/phytane ratio (Pr/Ph)(2.8–8.2) and low gammacerane index (0.2–0.4) reveals oxic water column with in-situ depositional condition of carbonaceous-mudstone when the lower J2d7 was deposited. The maceral composition of oil shale at the upper part of J2d7 is mainly amorphous OM with small proportion of reworked vitrinite and inertinite, consistent with a mainly algea and small amount of exogenous OM input. When associated with low Pr/Ph (0.8) and high gammacerane index (4.3), the oil shale was deposited in an anoxic saline water column with mainly zooplankton OM source. The geochemical and petrographic result implies a transitional environment ranging from suboxic semi-saline to oxic fresh water environment with varying proportion of higher-plant and algae input by in-situ deposition and distant transportation for the mudstone at the middle part of J2d7. By use of random reflectance (Ro) distribution, two sedimentary environment microcycles which are confirmed by maceral composition, from the depth of 1983 m to 1962 m and 1954 m to 1931.9 m, are differentiated within the homogenous mudstone section. The variation of OM input condition and alternation of depositional environment led to accumulation and deposition of various kinds of abundant OM when J2d7 was deposited. Although the OM has actually low maturity at oil window with vitrinite reflectance around 0.6% according to the petrographic results, the liptinite inherited from higher-plants especially suberinite is the main maceral to generate gaseous hydrocarbons at the low maturity, which implies that the Dameigou formation (J2d7) of Qaidam Basin is of gas potential.

Families of reservoired crude oils from the Cangdong Sag, Bohai Bay Basin, China

The organic geochemistry of 35 crude oils from six oil fields across the Cangdong Sag, Bohai Bay Basin, was characterized and an oil family classification was carried out for the first time. Four oil families were classified based on 18 parameters related to source and depositional environment using hierarchical cluster analysis. The families have broadly similar geochemical characteristics which suggest oils derived from lacustrine source rocks deposited in a reducing environment. Family A has relatively abundant C27 steranes and C30 4α-methyl-24-ethylcholestanes, high Pr/Ph, C22/C21 tricyclic terpane and C31R/C30 hopane ratios, and low ETR, C26/C25 tricyclic terpane and C35S/C34S homohopane ratios. Family A shows higher thermal maturity than families C and D, but lower than family B. The most distinct feature for Family B is the high gammacerane index; they also show slightly higher C22/C21 tricyclic terpane, C31R/C30 hopane and C29/C30 hopane, lower dibenzothiophene/phenanthrene, and higher maturity than the other families. Families C and D oils are low maturity and they are more similar to each other than families A and B based on most biomarker ratios. However, family C can be distinguished from family D by slightly higher C27/C29 steranes and C30 4Me/C29 steranes C30 4α-methyl-24-ethylcholestanes/C29 steranes, and lower C29 steranes/C30 hopane ratios. A preliminary oil–source rock correlation suggests that the oil families originated from various intervals having different organofacies within the second member of the Kongdian Formation. Family A is more widespread than the other families, supporting extensive distribution of the source rock. Family B was only found in the southern part of Kongdong slope, which suggests limited geographical distribution of the related source rock. Families C and D show similar geographical distributions and dominate Wangguantun, Shenjiapu and Shenvsi fields, suggesting that the source kitchens are near these regions.