Primary paleoproductivity provides basic organic matter for the formation of high-quality hydrocarbon source rock. A progressive method to quantitatively calculate paleoproductivity on the basis of ...total organic carbon (TOC) has been proposed to evaluate the contribution of primary paleoproductivity to a hydrocarbon source rock with high organic matter abundance. In this study, pyrite contents, organic maceral compositions and high-resolution sedimentation rates (SRs) were used to recover the amount of organic matter oxidation and to exclude the effect of terrestrial clastic input during the deposition of the Chang 7 sediment. Multiple calculation methods were used to quantitatively reconstruct paleoproductivity, and the results suggest that the Ordos Basin was a eutrophic lacustrine basin (approximately 2 × 104 mg C cm−2 kyr−1 to 10 × 104 mg C cm−2 kyr−1) during the Chang 7 stage compared with modern lake productivity. The paleoclimatic analysis indicates that a warm humid paleoclimate was beneficial for increasing the weathering intensity, which could have provided more nutrient inputs to the depositional environment with incoming terrestrial debris. In addition, the plate collision between the North China Block and Yangtze Block generated moderate volcanic ash that spread to the study area and provided large amounts of essential nutrients for algae blooms. Moreover, a brackish water condition promoted upward mixing of deep water, which inhibited water mass stratification and then facilitated primary paleoproductivity. In general, the Chang 7 high-quality hydrocarbon source rock is a result of increased paleoproductivity resulting from suitable paleoclimate conditions, brackish water masses and abundant essential nutrient inputs.
•The productivity is recovered after assessing oxidation and dilution of organic matter.•The productivity of Ordos Basin amount to 105 mg C cm−2 kry−1 indicating a eutrophic basin.•Paleoclimate and volcanism were the two main factors affecting high productivity.
The Upper Triassic Chang 7 unit consists of more than 110 m of lacustrine sediments, including mudstone and shale, and it is considered to be the most important hydrocarbon source rock in the Ordos ...Basin. Power spectral, wavelet, evolutionary fast Fourier transform (FFT), and correlation coefficient analyses of the gamma ray (GR) and density (DEN) logging series from well Y56 in the Chang 7 unit reveal strong astronomical signals of long eccentricity (405 kyr), short eccentricity (100 kyr), and obliquity (34 kyr) cycles. Those provide an excellent opportunity to establish floating astronomical time scales (ATS) and to discuss how the variations in the sediment accumulation rate (SAR) impact organic matter abundance. Based on the 405 kyr tuning ATS results, the Chang 7 unit in well Y56 covers a 7.26 Ma period from 239.50 Ma to 232.24 Ma. The ages of each subunit in the Chang 7 unit are estimated to be 232.24–234.25 Ma, 234.25–236.90 Ma and 236.90–239.50 Ma, respectively. The high-resolution SARs of the Chang 7 unit vary from 1.18 cm/kyr to 2.08 cm/kyr, with an average of 1.57 cm/kyr. According to the chemical index of alteration (CIA) and the correlation between the SAR and the Al2O3/SiO2 or Al2O3/TiO2 ratio, the main factor affecting the SAR of the Chang 7 unit is the amount of terrestrial debris input, which is the result of the warm, humid paleoclimate and the intense volcanic activity during phase I of the Indosinian. In addition, the total organic carbon per unit of paleoproductivity (TOCpup) was applied to investigate the impact of the SAR on the organic matter enrichment in order to eliminate the effect of paleoproductivity on the organic matter enrichment. We conclude that an SAR is sufficient to protect the organic matter and prevent oxidation but not too high to dilute the organic matter, which reduces the organic matter abundance of the source rock.
•Astronomical time scale of the Chang 7 unit provides high-resolution SARs.•Correlation coefficient method is used to test variable SARs.•TOCpup was applied to study the impact of the SAR on the organic matter abundance.•SARs are related to the intensity of chemical weathering and tectonic activity.
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•The mean size of pyrite framboids is related to the paleoredox conditions.•Redox-sensitive trace elements show strongly correlation with mean size of framboids.•The size distribution ...of pyrite framboids is related to relative sedimentation rate.•A favourable sedimentation rate is beneficial to organic matter enrichment.
Pyrite framboids are spherical or ellipsoidal compact aggregates of submicron pyrite microcrystals. The various lithologies in the Chang 7 Member of the Upper Triassic Yanchang Formation contain abundant pyrite framboids, which provide an opportunity to understand the fluctuation in paleoredox conditions and relative sedimentation rates during deposition. The measured diameters of the pyrite framboids in the Chang 7 Member are characterized by large mean diameters (9.7–16.6 μm) with a wide distribution range that plots in the area indicative of dysoxic to oxic paleoredox conditions during deposition of the Chang 7 sediments. Based on correlation analysis, the strong correlation between paleoredox-sensitive trace elements ratios (V/Cr, U/Th, and Cu/Zn) and the mean size of framboids indicate that the latter can serve as a reliable indicator of paleoredox conditions. The results of the framboid size distribution can also be interpreted through Crystal Size Distribution Theory (CSDT). According to CSDT, the relative sedimentation rate of the Chang 7 Member fluctuated during deposition in parallel with fluctuation in lacustrine levels. Furthermore, the strongly positive correlation between total organic carbon (TOC) and paleoredox conditions indicates that although relatively high sedimentation rate may have contributed to the dysoxic-oxic conditions that were harmful to the preservation of organic matter, but it may also have brought nutrients to increase productivity. Similarly, the correlation between TOC and relative sedimentation rates suggests that the relative sedimentation rates can affect organic matter enrichment in several ways. A high sedimentation rate is favourable for the preservation of organic matter, which can be prevented from oxidation, but large amounts of sediments can dilute the organic matter, which reduces the organic matter abundance in a potential source rock.
Shales in the Carboniferous–Permian Fengcheng (FC) and Lucaogou (LCG) formations in Junggar Basin are important organic rich rocks containing significant oil resources. To evaluate the difference in ...sedimentary environment conditions and hydrocarbon-generating potential between the FC and LCG formations. Total organic carbon (TOC), Rock-Eval pyrolysis, solvent extraction, column fractionation, stable carbon isotope, gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons and organic petrology from the source rocks of FC and LCG formations. were analyzed. The biomarker composition indicates that during the deposition of FC, LCG-1 to LCG-2, the sedimentary environment for the source rock formations changed with gradual decrease of salinity, from anoxic to dyoxic/suboxic in redox conditions, and from strong stratification to weakened stratification of water. The FC Formation source rock, with main telalginite (planktonic green algae), archaebacteria and minor terrestrial organic matter, deposited in the environment characterized by high salinity and strongly reducing condition. Its TOC content is relatively low with a high original hydrocarbongenerating potential of unit organic material. The LCG Formation source rock deposited in the environment with low salinity and large variations, the organic matter is mainly sourced from telalginite (planktonic green algae), lamalginite, bacteria and higher plants, resulting in strong heterogeneity of the source rock. The abundance of TOC is high, but the original hydrocarbon generation potential of unit organic matter is lower than that of FC Formation. The results provide a geochemical basis for further study of saline-brackish water sedimentary environment shales in the Junggar Basin.
The pyrite sulfur isotope in the black shale of the Chang 7 unit are anomalous positive compared with those of the current paradigm, in which the value of pyrite sulfur isotopes should be negative ...due to sulfur isotopic fractionation through bacterial sulfate reduction (BSR). A series of geological and geochemical analysis (FeT/Al and δ34Spy, average of 0.51 and 6.5‰, respectively) are carried out to confirm that sulfur speciation is derived from sulfate in watermass rather than hydrothermal fluids, and the δ34Spy values are effective records of the original sedimentary environment. Integrated geochemical data including redox sensitive elements (RSE) ratios, ratio of organic carbon to phosphorus (Corg:P, molar, average of 85.4), organic sulfur to carbon (organic S:C, average of 10.47%) and pyrite framboid size distribution (range in 9.7–16.6 μm), are represented to reconstruct the paleoredox chemical conditions during the deposition of the Chang 7 unit black shale. The results suggest that the paleoredox chemical conditions of the middle and lower subunits (the Ch 72 and Ch 73 subunits) were dominated by suboxic environments with intermittent anoxic environments and that changed to oxic with intermittent suboxic environments in the upper subunit (the Ch 71 subunit). Based on the analysis of sulfur isotope fractionation during the sulfur cycle, the paleoredox chemical conditions are confirmed to be the trigger for the anomalous sulfur isotopic compositions. The limited sulfate in watermass and overlying fine-grained sediments resulted in a relatively closed system for BSR, where limited sulfur isotope fractionation lead to the formation of heavy pyrite. Subsequently, due to the dissolved oxygen penetrated into the sediment, the reactivation of anaerobic oxidants resulted in reoxidation of hydrogen sulfide derived from BSR, then to “heavier” pyrite via Rayleigh-type distillation in the Ch 71 subunit.
•The paleoredox conditions of the Ch7 unit changed from suboxic to oxic upwardly.•BSR occurring in a closed system led to positive δ34Spy values in the Ch7 unit.•Reactivation of anerobic oxidant formed “heavier” pyrite in the upper Ch7 unit.
A geochemical investigation was carried out on 32 crude oil samples to investigate the origin of the oil and to trace the migration direction in the Linyi fault area, Huimin Depression, Bohai Bay ...Basin, China. The oils, which were characterized by low gammacerane content and a dominance of C27 over C29 regular steranes, originated from the source rock of the third member of the Shahejie Formation (Es3) in the Linnan Sag. A sequential charge of low- and high-maturity oils from the Es3 source rocks resulted in a decrease in oil maturity in the migration direction. The petroleum migration direction in the footwall of the Linyi fault was northward, away from the fault, and the migration direction at the southwest end (in the hanging wall) of the Linyi fault was westward, along the fault, as evidenced by lateral gradients of biomarker parameters of various maturity, density, and viscosity.
The Lower Cretaceous Xiagou Formation in Qingxi Sag consists of more than 1000 m of lacustrine sediments, including dolomitic mudstone/argillaceous dolomite and sandstone/mudstone, which is ...considered to be the most important source rock in the Jiuquan Basin. Power spectra, evolutionary fast Fourier transform (FFT), and correlation coefficient analyses of the gamma-ray (GR) series from four wells, including Liu 8 well, Long 14 well, Q2-36 well and Liu 9 well, through the Xiagou Formation reveal strong astronomical signals of long eccentricity (405 kyr), short eccentricity (100 kyr), obliquity (37 kyr) and precession (22 kyr). These provide an excellent opportunity to establish a floating astronomical time scales (ATS) then to determine variations in the sediment accumulation rate (SAR). Based on the floating ATS of the Xiagou Formation, the Xiagou Formation covers a 10.3 myr period depositional duration, and depositional duration of three members in the Xiagou Formation are 3.5 ± 0.1 myr, 3.1 ± 0.1 myr and 3.65 ± 0.15 myr, respectively. The high-resolution SARs of the Xiagou Formation vary from 8.5 cm/kyr to 19.8 cm/kyr. The different lithologies in the Xiagou Formation exhibit the same variation tendency in SARs, but the mechanisms of astronomical forcing that affect the SARs are different. In addition, the correlation between total organic carbon (TOC) and SARs can be used to investigate the impact of the SAR on organic matter abundance and conclude that relatively high SAR can protect organic matter and prevent oxidation, but rapid SAR can also dilute organic matter abundance of source rock.
Chrysophyte cyst fossils were widely pyritized and preserved in black shales from the seventh member (Ch7 Mbr) of the Yanchang Formation in the Ordos Basin. The age, pyritization, and preservation ...model of these fossils have not been studied previously. In this study, the astronomical orbital cycles of the Ch7 Mbr were determined based on the gamma ray series of the Yan56 and Zhen 421 wells. An astronomical time scale (ATS) analysis revealed that the depositional duration of Ch7 Mbr was approximately 5 Ma. According to the 206Pb/238U radiometric dating of zircons using laser ablation inductively coupled plasma mass spectrometry (La-ICP-MS), the tuffs at the bottom of Ch7 Mbr were crystallized at 234 Ma, which served as a geological anchor. The ages of three submembers in Ch7 Mbr were estimated at 234.0–232.4 Ma, 232.4–230.8 Ma, and 230.8–229.1 Ma based on ATS analysis. In addition, chrysophyte cyst fossils were well preserved by pyritization in the Ch7 Mbr black shales. There were six types of microscopic morphologies with different pores, collars, and surface ornamentation under scanning electron microscopy (SEM). The age of the chrysophyte cyst fossils was at least 233.6 Ma in the Triassic Carnian Pluvial Episode (CPE) based on the 405 kyr tuned ATS. Moreover, the paleoredox conditions in Ch7 Mbr were reconstructed, and a preservation model of chrysophyte cyst fossils was established based on geochemical analyses. Fossil pyritization was caused by bacterial sulfate reduction near the water-sediment interface under suboxic to anoxic environmental conditions. Pyritization was initiated on the walls of the chrysophyte cysts by the formation of microcrystalline pyrite. Because of the gradual pyritization of the chrysophyte cyst wall, the organic matter in the interior of the fossil was well preserved.
•Sedimentary conditions in the Lucaogou Formation source rocks are described.•The telalginite generated hydrocarbons earlier than the lamalginite.•New parameter index to identify effective source ...rocks is proposed.
The Middle Permian Lucaogou Formation in the Jimusaer Sag is a hotspot for exploring and developing lacustrine tight oil in the Junggar Basin, NW China. In this study, we evaluated the tight oil potential from the perspective of source rock hydrocarbon generation and expulsion. Based on a detailed organic, petrological, and geochemical characterization of the target interval within the key well JHBE, kinetic experiments were performed on representative shale samples from the well JHBE and the threshold of hydrocarbon expulsion of the source rock was established. The findings of this study revealed that the Lucaogou Formation shales were deposited in a dysoxic to anoxic and clay-poor lacustrine environment with variable salinity and their organic matter (OM) was contributed from both lamalginite and telalginite with minor vitrinite and inertinite, resulting in good to excellent source rock potential. The lamalginite was deposited in water with low salinity, whereas the telalginite developed in water with relatively higher salinity. Although the greater contribution of lamalginite resulted in a higher OM content than that of telalginite, the former generated a lower amount of hydrocarbons than the latter, because telalginite is capable of generating hydrocarbons earlier than lamalginite, which is indicated by the higher HCI (Hydrocarbon index, S1 × 100/TOC), EOM/TOC (extraction of organic matter/total organic carbon), C29 ββ/(αα + ββ), and C29 ααα20S/(20S + 20R) sterane values for telalginite than for lamalginite and is also evident from the difference in activation energy distributions between lamalginite and telalginite source rocks. As determined from the relationships of sterane maturity parameters C29 ββ/(αα + ββ) and C29 ααα20S/(20S + 20R) vs EOM/TOC ratio and HCI values, the lower C29 ββ/(αα + ββ) and C29 ααα20S/(20S + 20R) limits for active source rocks were approximately 0.28 and 0.44, respectively.
