Cambrian Reservoir Research Articles

Reservoir solid bitumen-source rock correlation using the trace and rare earth elements–implications for identifying the natural gas source of the Ediacaran-Lower Cambrian reservoirs, central Sichuan Basin

The hydrocarbon-source correlation in highly-mature and over-mature stages is a challenging task in natural gas exploration. The Ediacaran-Lower Cambrian natural gas in the central Sichuan Basin is typical oil-cracking gas. With a burial depth of more than 4650 m, the gas is now in the highly-mature to the over-mature stage (equivalent Ro>1.8%) with apparent alteration by thermochemical sulfate reduction (TSR), leading to difficulties for gas-source correlation. Conventional hydrocarbon-source correlation mainly relies on organic geochemical methods, taking media such as biomarkers and carbon isotope for correlation. However, in the highly-mature and over-mature stages, the characteristics of conventional biomarkers tend to be homogenised. Furthermore, the gas components and carbon isotope of natural gas are frequently altered by TSR in the deep burial stage. Thus, conventional organic geochemical methods are significantly restricted. This paper explored the source of highly-mature natural gas from the DY and LWM Formations in the central Sichuan Basin by studying the inorganic geochemical information (trace elements and rare earth elements) of the reservoir bitumen and potential source rocks. The results show that the depositional environment of the potential source rocks (the QZS and the D-3 source rocks) in the central Sichuan Basin were different. The redox-sensitive parameters (V/(V + Ni), Mo/Ni, δCe and Ce/Y) and the provenance-sensitive parameters (La/Co and La/Sc) of the source rocks have apparent differences, which can be used as the basis for gas-source correlation. The correlation results show that the LWM gas was derived from the QZS source rock, while the D-2 and D-4 gases were mainly derived from the QZS source rock, with a partial contribution of the D-3 source rock. During geologic evolution, the crude oil generated from the two sets of source rocks were thoroughly mixed in the paleo-oil reservoir of the D-4 Member. As a result, the characteristics of the D-3 source rock were obscured by the QZS. In contrast, the solid bitumen in the D-2 paleo-oil-reservoir recorded the contribution of the D-3 source rock. However, this contribution was limited within the range of 50 m adjacent to the D-3 Member by a more significant oil generation from the QZS source rock located in Deyang-Anyue Rift Trough. This study clarified the gas source differences between the D-2 Member, D-4 Member and LWM Formation, providing a new insight for in-depth study on the gas accumulation of the Ediacaran-Lower Cambrian in the central Sichuan Basin.

Characteristics of natural gas accumulation in the Cambrian weathered crust in southwestern Ordos Basin

The Cambrian weathered crust formed in the Early Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration, and important breakthroughs in oil and gas exploration have been made there. In order to further accelerate the process of natural gas exploration in this area, this paper based on systematic analysis of source-reservoir assemblage and trap type in the Cambrian weathered crust in southwestern Ordos Basin, and then evaluates its natural gas exploration potentials and favorable exploration zones. The following research results are obtained. First, the Upper Paleozoic coal-measure source rocks in the basin is the main gas source rocks, forming a source-reservoir assemblage of “upper source and lower reservoir” with Cambrian reservoir. Second, the types of the reservoir in the Cambrian weathered crust are mainly dolomite intercrystalline (dissolution) pores and karst fractures and vugs, whose development is controlled by grain shoal microfacies and karstification in the weathered crust stage, respectively. Third, the traps in the Cambrian weathered crust are mainly lithologic-stratigraphic traps, which were formed in the tectonic uplifting setting of the eastern Ordos Basin during the Yanshanian. There are geological conditions of lithologic and stratigraphic barriers in the updip direction, which is beneficial to the short-distance natural gas migration and accumulation. In conclusion, Huanxian-Zhengning area on the east margin of the central paleouplift in southwestern Ordos Basin is a favorable exploration zone of Cambrian weathered crust gas reservoirs, and the Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.

P-047. Elevated placental histone H3K4 methylation via upregulated histone methyltransferases in preeclampsia and its possible involvement in hypoxia-induced pathophysiology

The Cambrian weathered crust formed in the Early Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration, and important breakthroughs in oil and gas exploration have been made there. In order to further accelerate the process of natural gas exploration in this area, this paper based on systematic analysis of source-reservoir assemblage and trap type in the Cambrian weathered crust in southwestern Ordos Basin, and then evaluates its natural gas exploration potentials and favorable exploration zones. The following research results are obtained. First, the Upper Paleozoic coal-measure source rocks in the basin is the main gas source rocks, forming a source-reservoir assemblage of “upper source and lower reservoir” with Cambrian reservoir. Second, the types of the reservoir in the Cambrian weathered crust are mainly dolomite intercrystalline (dissolution) pores and karst fractures and vugs, whose development is controlled by grain shoal microfacies and karstification in the weathered crust stage, respectively. Third, the traps in the Cambrian weathered crust are mainly lithologic-stratigraphic traps, which were formed in the tectonic uplifting setting of the eastern Ordos Basin during the Yanshanian. There are geological conditions of lithologic and stratigraphic barriers in the updip direction, which is beneficial to the short-distance natural gas migration and accumulation. In conclusion, Huanxian-Zhengning area on the east margin of the central paleouplift in southwestern Ordos Basin is a favorable exploration zone of Cambrian weathered crust gas reservoirs, and the Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.

Study and Application on ROP-Enhancing Technology of Ordovician-Cambrian Dolomite Strata in Tarim Basin

The Ordovician-Cambrian dolomite formation in Tarim basin has certain characteristics such as poor drillability, long drilling cycle, difficulty in selecting drill bit and accelerating tool. In order to meet the requirements of efficient exploration and development of Cambrian oil and gas reservoirs, there is an urgent need to design high ROP bit and acceleration tool suitable for dolomite formation, optimize drilling parameters and improve ROP to form the technical scheme of drilling acceleration in dolomite formation. Therefore, the drillability of the Cambrian dolomite strata was studied. The experimental analysis was carried out according to the geological characteristics and rock mechanical parameters. Dual index drillability analysis method was used to evaluate the formation compactness, abrasiveness and other formation drillability characteristics. A new type of PDC bit was designed to improve the impact resistance and eddy resistance of single tooth, a three-dimensional vibration acceleration tool was selected to reduce the vibration of the bit, and combined with the optimization of drilling parameters, the technical scheme for acceleration of the Ordovician -Cambrian dolomite formation in Tarim Basin was finally formed, which was successfully field tested in Well A of Tarim Basin. The research results show that compared with the adjacent well B section, the average footage and the average ROP are increased by 47.8% and 32.0% respectively. Compared with the adjacent well C section, the average footage and average ROP increased by 37.0% and 29.5% respectively. The results provide technical reference and support for the efficient exploration and development of Ordovician - Cambrian reservoirs in Tarim Basin.

Potential of the Middle Cambrian Aquifer for Carbon Dioxide Storage in the Baltic States

The importance of CO2 removal from the atmosphere has long been an essential topic due to climate change. In this paper, the authors aim to demonstrate the suitability of the underground reservoirs for CO2 storage based on their geological characteristics. The research addressed the potential of geological formations for fossil CO2 storage in the Baltic States to support the goal of achieving carbon neutrality in the region. The geological, technical, and economic feasibility for CO2 storage has been assessed in terms of carbon sequestration in geological structures and the legal framework for safe geological storage of fossil CO2. Results indicate that prospective structural traps in the Baltic States, with reasonable capacity for CO2 storage, occur only in Southwestern Latvia (onshore) and in the Baltic Sea (offshore), whilst other regions in the Baltics either do not meet basic geological requirements, or have no economically feasible capacity for CO2 storage. Based on the examination of geological characteristics, the most fitting is the middle Cambrian reservoir in the Baltic sedimentary basin, and one of the most prospective structural traps is the geological structure of Dobele, with an estimated storage capacity of 150 Mt CO2. This study revealed that the storage capacity of the middle Cambrian reservoir (up to 1000 Mt CO2) within the borders of Southwestern Latvia is sufficient for carbon capture and safe storage for the whole Baltic region, and that geological structures in Latvia have the capacity to store all fossil CO2 emissions produced by stationary sources in the Baltic States for several decades.

Organic Matter Accumulation Mechanism in the Lower Cambrian Strata from Well Luntan 1 in the Tarim Basin, NW China

A large amount of light crude oils have been found within 4000 to 7000 m deep strata of Cambrian, Ordovician, and Silurian reservoirs in the Tarim Basin, NW China. To enhance the understanding of parental materials of deep oils, a set of high-quality source rocks from the Yuertusi Formation in well Luntan 1 (maximum drilling depth of 8882 m) was studied in terms of their sedimentary condition and mechanism of organic matter enrichment. Total organic carbon (TOC) content, carbon isotope of kerogen (δ13Cker), and major and trace elements of the rocks from the Sinian Qigebulake, Cambrian Yuertusi, and Xiaoerbulake Formations in well Luntan 1 were analysed. The results showed that the δ13Cker value of the Yuertusi Formation barely changed with an average of -31.19‰. High TOC contents accompanied by enrichments of the bioessential trace elements, such as cadmium, chromium, copper, nickel, and zinc, occurred in the lower part of the Yuertusi Formation. Excess barium (Baxs) and phosphorus concentrations revealed high primary productivity during the deposition of the Yuertusi Formation. Moreover, variations in the enrichment factors of molybdenum, uranium, and vanadium and molybdenum-uranium covariation pattern indicated suboxic-anoxic conditions in the Qigebulake Formation, anoxic-euxinic conditions in the Yuertusi Formation, and suboxic-oxic conditions in the Xiaoerbulake Formation. The TOC contents were significantly correlated with the paleoproductivity and paleoredox parameters, indicating that high productivity and reducing conditions jointly controlled the organic matter accumulation and preservation in well Luntan 1.

Origin and Formation of Pyrobitumen in Sinian–Cambrian Reservoirs of the Anyue Gas Field in the Sichuan Basin: Implications from Pyrolysis Experiments of Different Oil Fractions

Plenty of pyrobitumens were found throughout the Sinian Dengying and Cambrian Longwangmiao reservoirs of the Anyue gas field in the central Sichuan Basin. However, their genetic sources and formati...

Deepest oil in Asia: Characteristics of petroleum system in the Tarim basin, China

The petroleum exploration well Luntan-1 (LT1) located in the Tabei Uplift, Tarim Basin, China reached vertical depth 8882 m and is now the deepest well in Asia. A significant petroleum flow (134 m3 of oil and 45,900 m3 of gas) was obtained from the Cambrian Wusongger Formation perforated in the interval 8203–8260 m, with a pressure of 90.8 Mpa and interval logging temperature is 162 °C. The well LT-1 is the deepest producer of oil from Paleozoic reservoirs in the world. According to the analysis of the effect of acidizing fracturing reservoir reconstruction, the Lower Cambrian Wusongger Formation has low pump stop pressure, large pressure drop, fast deceleration, and good reservoirs and the lithology are limestone dolomite, lime-bearing dolomite, and dolomite. The source rock is the Lower Cambrian Yuertusi shale containing significant amount of total organic carbon (2–30 wt%) and located in the late oil/early gas maturity window (vitrinite reflectance 1.4–1.7%). Through GC × GC-TOFMS analysis of this oil showed low concentration of diamondoids, which indicates lack of thermal cracking of oil to gas. Thiadiamondoids, typically indicative of thermochemical sulfate reduction, were not detected. Using kinetic calculations, we estimated the temperature threshold for thermal cracking of well LT1 oil to gas at ~210–220 °C. Based on the thermal evolution and burial history of the Tarim Basin, the depth threshold for the disappearance of liquid oil is ~9000 m. Low geothermal gradient (19.6 °C/km) and rapid subsidence after petroleum accumulation jointly contribute to the preservation of liquid petroleum in ultra-deep strata in the Tarim Basin. Our findings suggest that the favorable exploration area in ultra-deep, high-temperature and high-pressure Cambrian reservoirs in the Tarim Basin exceeds 20,000 km2 and may contain over 1 billion tons of oil resources. Furthermore, the significant discovery of oil in the well LT1 provides important insights for global exploration of Paleozoic ultra-deep oil.

Effect of thermochemical sulfate reduction on carbonate reservoir quality: Cambrian and Ordovician oilfield, Tazhong area, Tarim basin, China

Deep (4000–6000 m) and ultra-deep burial (>6000 m) carbonate reservoirs are becoming important petroleum exploration targets of China and worldwide basin. Thermochemical sulfate reduction (TSR) may be pervasive in the deep and ultra-deep burial petroliferous carbonate strata associated with evaporite. However, there is still considerable confusion on the effect of TSR on reservoir quality. Petrography, geochemistry and fluid inclusion data from carbonate reservoirs of Tarim Basin were integrated to resolve the disagreement. Based on distribution of sulfur-bearing mineral, SO42− concentrations and SO42−/Cl− ratios in both formation water and fluid trapped in fluid inclusion, we propose that SO42− in formation water is likely major reactant involved in TSR reaction of North Slope, whereas coarsely diagenetic anhydride is likely major sulfate reactant involved in TSR reaction of East Burial Hill Belt. Silurian fracture-filling pyrite has δ34S values similar to those of Ordovician fracture-filling pyrite, H2S and sulfur, suggesting that TSR dissolved sulfate involved in (North Slope) may occur in a partly open system for H2S and CO2. However, TSR diagenetic anhydride involved in (East Burial Hill Belt) may occur in a closed diagenetic system. TSR in a partly open-system, associated with gas charge and flow of cross formational fluids, probably increased fluid pressure, and further transported H2S, CO2 and solute from dissolution through fracture. A closed system of anhydrite-bearing strata seems to hinder transports of TSR-derived CO2 or bicarbonate, and result in precipitation of TSR calcite, pyrite and bitumen close to the TSR sites. In combination with petrographic observations, intense dissolution in North Slope have released more inorganic CO2 leading to TSR calcite and CO2 in the natural gas showing δ13C values much heavier relative to δ13C values of TSR calcites in East Burial Hill Belt as light as −13.4‰. TSR-induced dissolution of carbonate minerals during oil-dominated TSR may be related to the reaction of dolomite with anhydride and release of H+ due to pyrite precipitation, rather than new formed water during TSR. Similar to geological settings of Ordovician TSR, anhydrite-poor Lower Cambrian dolostones underlying evaporites seals may thus be the most promising target for future exploration success in ultra-deep Cambrian reservoirs of Tarim Basin. The controversy about the effects of TSR on reservoir quality can partly be ascribed to differently geological settings where TSR occurred.

Multifractal Analysis of Pore Structure and Evaluation of Deep-Buried Cambrian Dolomite Reservoir with Image Processing: A Case from Tarim Basin, NW China

Reservoir pore space assessment is of great significance for petroleum exploration and production. However, it is difficult to describe the pore characteristics of deep-buried dolomite reservoirs with the traditional linear method because these rocks have undergone strong modification by tectonic activity and diagenesis and show significant pore space heterogeneity. In this study, 38 dolostone samples from 4 Cambrian formations of Tarim Basin in NW China were collected and 135 thin section images were analyzed. Multifractal theory was used for evaluation of pore space heterogeneity in deep-buried dolostone based on thin section image analysis. The physical parameters, pore structure parameters, and multifractal characteristic parameters were obtained from the digital images. Then, the relationships between lithology and these parameters were discussed. In addition, the pore structure was classified into four categories using K-means clustering analysis based on multifractal parameters. The results show that the multifractal phenomenon generally exists in the pore space of deep-buried dolomite and that multifractal analysis can be used to characterize the heterogeneity of pore space in deep-buried dolomite. For these samples, multifractal parameters, such as αmin, αmax, ΔαL, ΔαR, Δf, and AI, correlate strongly with porosity but only slightly with permeability. However, the parameter Δα, which is usually used to reveal heterogeneity, does not show an obvious link with petrophysical properties. Of dolomites with different fabrics, fine crystalline dolomite and medium crystalline dolomite show the best petrophysical properties and show significant differences in multifractal parameters compared to other dolomites. More accurate porosity estimations were obtained with the multifractal generalized fractal dimension, which provides a new method for porosity prediction. The various categories derived from the K-means clustering analysis of multifractal parameters show distinct differences in petrophysical properties. This proves that reservoir evaluation and pore structure classification can be accurately performed with the K-means clustering analysis method based on multifractal parameters of pore space in deep-buried dolomite reservoirs.

Мониторинг фронта заводнения карбонатных коллекторов нижнего кембрия методом 4D электроразведки

Мониторинг фронта заводнения карбонатных коллекторов нижнего кембрия методом 4D электроразведки

Affecting factors and application of the stable hydrogen isotopes of alkane gases

Abstract To study the composition, affecting factors of the stable hydrogen isotopes of alkane gases and their application to identification of the natural gas origin and maturities, the chemical and isotopic compositions of 118 gas samples of Carboniferous- Permian in the Ordos Basin, and of Triassic in the Sichuan Basin, combined with 68 gas samples from the Sinian and Cambrian reservoirs in the Sichuan Basin, and Ordovician and Siliurian reservoirs of Tarim Basin, are analyzed comprehensively. The following conclusions are obtained: (1) Natural gases in the study area and strata of the Ordos and Sichuan basins are dominated by alkane gases, and the dryness coefficients and maturities of the Carboniferous-Permian gases in the Ordos Basin are higher than the gases in the Triassic Xujiahe Formation of the Sichuan Basin, while the hydrogen isotopes of the latter ones are much enriched in 2 H than the former. (2) The δ 2 H CH4 -C 1 /C 2+3 genetic identification diagram of natural gas was drawn, and the diagrams of hydrogen isotopic differences between the heavy alkane gases and methane vs. hydrogen isotopes of alkane gases can also be used in natural gas genetic identification. (3) The δ 2 H CH4 -R o formulas of coal-formed gas in different areas of the two basins are given, and the δ 2 H C2H6 - δ 2 H CH4 is a new index for maturity, and the ( δ 2 H C2H6 — δ 2 H CH4 )-R o formula of the coal-formed gas can be used to calculate the maturity of the natural gas. (4) The stable hydrogen isotopes of alkane gases are affected by parent materials in source rocks, maturity, mixing and the aqueous medium conditions, among which the aqueous paleo-salinity is the key factor. To sum up, the hydrogen isotopes of alkane gases are affected by multiple factors, and they are significant to the identification of the origin, and maturity of natural gas, and the water environment during the deposition of source rocks.

Evaluation of the subsurface compressed air energy storage (CAES) potential on Gotland, Sweden

Wind energy is an important field of development for the island of Gotland, Sweden, especially since the island has set targets to generate 100% of its energy from renewable sources by 2025. Due to the variability of wind conditions, energy storage will be an important technology to facilitate the continued development of wind energy on Gotland and ensure a stable and secure supply of electricity. In this study, the feasibility of utilizing the Middle Cambrian Faludden sandstone reservoir on Gotland for Compressed Air Energy Storage (CAES) is assessed. Firstly, a characterization of the sandstone beneath Gotland is presented, which includes detailed maps of reservoir thickness and top reservoir structure. Analysis of this information shows that the properties of the Faludden sandstone and associated cap rock appear favorable for the application of CAES. Seven structural closures are identified below the eastern and southern parts of Gotland, which could potentially be utilized for CAES. Scoping estimates of the energy storage capacity and flow rate for these closures within the Faludden sandstone show that industrial scale CAES could be possible on Gotland.

ХАРАКТЕРИСТИКА РЕЗЕРВУАРОВ НЕФТИ И ГАЗА В КЕМБРИЙСКИХ ОТЛОЖЕНИЯХ ЦЕНТРАЛЬНОЙ ЧАСТИ СИБИРСКОЙ ПЛАТФОРМЫ

The article describes Cambrian reservoirs in the territory of the Turukhano-Irkutsk-Olekminsky facies region, which covers the territory of the southern and central parts of Eastern Siberia and Western Yakutia. In recent years, a significant amount of geological and geophysical work has been carried out on this territory, which made it possible to significantly clarify the oil and gas content of the subsalt and inter-salt Cambrian sediments. It has been shown that Cambrian carbonate horizons are characterized by incompetence of reservoir properties, which is associated with different facial conditions of sediment accumulation, as well as with the development of secondary processes.

Trace element characterization of bitumen constraints on the hydrocarbon source of the giant gas field in Sichuan Basin, South China

The exploration of high production, natural gas wells (>106 m3/d) has been extremely successful in the Gaoshiti and Moxi areas of the Leshan–Longnvsi paleo‐uplift, in the west‐central Sichuan Basin. This paper focuses on the Sinian and Cambrian reservoirs from the Gaoshiti and Moxi areas, where a dataset of new trace elements in bitumen and potential source rock is reported and where hydrocarbon‐source correlation is attempted based on trace elements characteristics of the 12 potential source rocks and 22 bitumen samples. Results show that the bitumen from the three reservoirs can be distinguished from each other using trace element distribution patterns and redox‐ and provenance‐sensitive elemental ratios. Bitumen in the second and fourth members of the Sinian Dengying Formation were derived primarily from the third member of the source rocks of the Dengying Formation, which represents a self‐sourced paleo‐petroleum system. In contrast, bitumen in the reservoirs of the Cambrian Longwangmiao Formation is derived from the Cambrian Qiongzhusi Formation and third member of Dengying Formation. The trace element method used in this study improves our understanding of the complex hydrocarbon–source relation and confirm the potential target for petroleum exploration in the study area.

Molecular markers, carbon isotopes, and rare earth elements of highly mature reservoir pyrobitumens from Sichuan Basin, southwestern China: Implications for PreCambrian-Lower Cambrian petroleum systems

The Moxi–Gaoshiti Bulge of the Sichuan Basin in southwestern China is unique because it provides insights into the geochemistry of Lower Cambrian reservoir bitumens and their geological significance for paleo-oil reservoirs. Analyses of biomarkers, carbon isotopes, trace elements (TE) and rare earth elements (REE) were conducted on reservoir bitumens and the associated source rocks to constrain sedimentary environments and bitumen/paleo-oil sources. Based on biomarkers, carbon isotopes, TE ratios and REE distribution patterns, both of the Cambrian Qiongzhisi (Є1q) and Sinian Dengying (Z2dn) source rocks can be hydrocarbon sources for the Cambrian Longwangmiao (Є1l) bitumens in the MG Bulge (Lower Cambrian Doushantuo Formation not discussed here). Abundant n-alkanes relative to isoprenoids, significant C21 and C22 steranes, predominance of dibenzothiophene (DBT), and low ratios of biomarkers, such as Pr/Ph, DBT/P, C22TT/C21TT and C35H(22S)/C34 H(22S) indicated that the source rock was characterized as shale from an overall highly reducing marine depositional environment. The average δ13C values from the compositions of the solid bitumens show an abnormal order: saturate hydrocarbons < asphaltenes < N-S-O compounds < aromatic hydrocarbons, suggesting that high temperature might affect the normal fractionation among different organic matter fractions. The reservoir bitumens are enriched in Mo, Tl, Pb and Ba, strongly depleted in Bi Compared to PAAS, and mostly well correlated in the REE distribution patterns. In the primary migration pathways suggested by the DBT concentration, the parameters ∑REE and Zr/Cu show an overall increase, and the ratios V/Ni, Ni/Co, V/Zr and V/Cr have decreasing trends. The comprehensive analyses of molecular biomarkers, carbon isotopes, TE and REE in pyrobitumens of the highly mature reservoirs in this study suggest a potential means to study paleo-oil reservoirs, and can thus provide valuable information for deep oil and gas exploration in Precambrian – Lower Cambrian petroleum systems.

Detection and significance of higher thiadiamondoids and diamondoidthiols in oil from the Zhongshen 1C well of the Tarim Basin, NW China

Oil and gas breakthroughs have been achieved in the Zhongshen 1 (ZS1) and 1C (ZS1C) wells in Cambrian pre-salt from the Tarim Basin in northwest China. However, Middle and Lower Cambrian reservoirs reveal substantial differences in the geochemistry and secondary alteration characteristics between the oils collected from the two wells. High concentrations of thiadiamondoids and diamondoidthiols, including thiatetramantanes, tetramantanethiols, thiapentamantanes, and pentamantanethiols, are detected in the organic sulfur compound fraction of concentrated oil collected from the ZS1C well, which samples the Lower Cambrian Xiaoerbulake Formation. Higher diamondoids, such as tetramantanes, pentamantanes, hexamantanes, and cyclohexamantane, also occur in the saturate fractions of the concentrated ZS1C oil. The presence of these compounds is verified by mass spectra analysis and comparison with previous studies. During thermochemical sulfate reduction (TSR), the cage of higher diamondoids is interpreted to open because of sulfur radicals forming open-cage higher diamondoid-like thiols, followed by cyclization that leads to the formation of high thiadiamondoids. Using D16-adamantane as an internal standard, the concentrations of lower diamondoids and thiadiamondoids of non-concentrated Cambrian oil from well ZS1C are 83874 and 8578 μg/g, respectively, which are far higher than Cambrian oil from well ZS1 and most Ordovician oils in the Tarim Basin. The high concentrations of lower thiadiamondoids and occurrence of higher thiadiamondoids and diamondoidthiols support that the oil from well ZS1C is a product of severe TSR alteration.

High abundance of alkylated diamondoids, thiadiamondoids and thioaromatics in recently discovered sulfur-rich LS2 condensate in the Tarim Basin

Molecular and stable sulfur isotopic (δ34S) analysis of petroleum recently discovered in the Bachu uplift of the Tarim Basin (China) was conducted to characterize the oil and assist future regional petroleum correlation studies. Sulfur-rich condensate from the Lower Ordovician – Upper Cambrian reservoir rock in the LS2 well showed abundant aromatic hydrocarbons and organic sulfur compounds. Extensive distributions of alkylated caged hydrocarbons (i.e., diamondoids) and caged sulfur compounds (thiadiamondoids), including many additional isomers not previously detected in petroleum, were resolved by GC × GC–MS analysis. The thiadiamondoid products and thioaromatics (i.e., benzothiophenes, dibenzothiophenes), also detected in LS2, products were attributed to a major thermochemical sulfate reduction (TSR) event, which resulted in elevated H2S (3.66%) and dry gas in the condensate. TSR might also have contributed to the generally heavier δ34S values of the benzothiophenes (+26 to +28‰) compared to dibenzothiophenes (+20 to +27‰). The LS2 reservoir has no sulfate evaporites and the temperature (144 °C) is relatively low for TSR, implying that it is a secondary reservoir for migrated TSR-derived gas, possibly derived from deeper and hotter (>200 °C) Cambrian strata.

Origin of deep sour natural gas in the Ordovician carbonate reservoir of the Tazhong Uplift, Tarim Basin, northwest China: Insights from gas geochemistry and formation water

Deep sour gases are found in the Ordovician carbonate reservoir of the Tazhong Uplift in the Tarim Basin, northwest China. All sour gases in this field are dominated by gas hydrocarbons (C1–C4) with varying amounts of H2S. Most of the gases in the eastern Tazhong are characterized by high gas dryness coefficients (>0.95), 13C-enriched methane (δ13C1 ranging from −44.6‰ to −35.8‰), and small difference between values of the δ13C2 and δ13C1 (δ13C2-1). In contrast, the gases in the western Tazhong are characterized by relatively low gas dryness coefficients (mostly < 0.90), 13C-depleted methane (δ13C1 ranging from −54.9‰ to −35.7‰), and high values of δ13C2-1, indicating relatively lower maturity. Gas compositions, carbon isotope ratios, and diamantane data together with thermal pyrolysis experiments using sealed gold tubes suggest that the Ordovician gases in the Tazhong Uplift belong to oil cracking gases mixed with high-maturity 13C-enriched dry gases, which originated from the deeper paleo-reservoirs cracking. The H2S (0.0008%–23.1000%) is attributed to thermochemical sulfate reduction (TSR); in contrast to typical TSR-altered gases, the natural gases in the western Tazhong are characterized by low dryness coefficients, relatively 13C-depleted methane, and high δ13C2-1, suggesting a low degree of TSR alteration. Observations suggest that the H2S contents are generally high in the western Tazhong, with positive correlation to Mg2+ concentrations and total dissolved solids (TDS) in the Ordovician formation water, indicating that TSR has entered early period of SO42− contact ion pairs (CIPs) oxidation. Compared with the eastern Tazhong, the Mg2+ concentrations and TDS in the western Tazhong are higher and form more CIPs that accelerate TSR and produce H2S. In addition, based on the analysis of the Cambrian formation water and the Cambrian gypsiferous salt, TSR might have also exist in the deeper Cambrian reservoirs, and the TSR-altered gases with high content of H2S migrated along faults to Ordovician reservoirs.

Geochemical characteristics of water-dissolved gases and implications on gas origin of Sinian to Cambrian reservoirs of Anyue gas field in Sichuan Basin, China

The Anyue gas field is a supergiant gas field which was discovered recently in the Sichuan Basin, China where the Sinian Dengying Formation and the Cambrian Longwangmiao Formation are principal gas reservoirs. Gas in the Anyue field contains mainly methane (>98%) and a low content of heavy hydrocarbons, with minor H2S component. Reservoirs commonly contain bitumen and produce formation water. It is currently suggested that natural gas in the Anyue field is derived from the secondary cracking of crude oil. However, carbon isotopic ratio of methane is significantly less negative than that of bitumen, which contradicts the isotopic signature oil cracking process would predict. Besides, this phenomenon also cannot be explained by the process of thermochemical sulphate reduction (TSR), because there is no remarkable correlation between the δ13C1 values and H2S contents as suggested in previous studies. Both free gas and water-dissolved gas from the Anyue gas field were analyzed in this study for their stable isotopes. Results show that the δ13C1 of the gas derived from the formation water is much less negative than that of free gas. Therefore, it is inferred that the less negative δ13C1 values in the free gas reservoir are attributable to addition of natural gas that is previously dissolved in formation water. Geological setting of the gas reservoirs in Dengying and Longwangmiao formations of the Anyue gas field is favorable for the formation and preservation of water-dissolved gas. In addition, both formations had experienced significant structural uplifting before the gas reservoirs were formed. Reduced temperature and pressure in the formations by geological uplifting could cause super-saturation of methane in the formation water, and as a consequence, water-dissolved methane gas could exsolve from water phase into gas phase. Therefore, it is concluded that natural gas in the Anyue field is a mixture of free gas and gas exsolved from the formation water. As a result, the carbon isotope of methane gas in the Anyue field became less negative than that of a single free gas component would predict.