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A new approach is presented in this paper for the dynamic modeling of the chemical and isotopic evolution of C1–3 during the hydrocarbon generation process. Based on systematic data obtained from published papers for the pyrolysis of various hydrocarbon sources (type I kerogen/source rock, type II kerogen/source rock, type III kerogen/source rock, crude oil, and asphalt, etc.), the empirical evolution framework of the chemical and isotopic composition of C1–3 during the hydrocarbon generation process was built. Although the empirical framework was built only by fitting a large amount of pyrolysis data, the chemical and isotopic composition of C1–3 derived from the pyrolysis experiments all follow evolution laws, convincing us that it is applicable to the thermal evolution process of various hydrocarbon sources. Based on the simplified formula of the isotopic composition of mixed natural gas at different maturities (δ13Cmixed), δ13Cmixed = X × n i A × δ 13 C i A + Y × n i B × δ 13 C i B X × n i A + Y × n i B , it can be derived that the cumulative isotopic composition of alkane generated in a certain maturity interval can be expressed by the integral of the product of the instantaneous isotopic composition and instantaneous yield at a certain maturity point, and then divided by the cumulative yield of alkane generated in the corresponding maturity interval. Thus, the cumulative isotopic composition (A(X)), cumulative yield (B(X)), instantaneous isotope (C(X)), and instantaneous yield (D(x)) in the dynamic model, comply with the following formula during the maturity interval of (X0~X). A(X) = ∫ X 0 X C X × D X d x B (X) , where A(X) and B(X) can be obtained by the fitting of pyrolysis data, and D(x) can also be obtained from the derivation of B(X). The dynamic model was applied on the pyrolysis data of Pingliang Shale to illustrate the quantitative evolution of the cumulative yield, instantaneous yield, cumulative isotope, and instantaneous isotope of C1–3 with increasing maturity. The dynamic model can quantify the yield of methane, ethane, and propane, as well as δ13C1, δ13C2, and δ13C3, respectively, during the hydrocarbon generation process. This model is of great significance for evaluating the natural gas resources of hydrocarbon source rock of different maturities and for identifying the origin and evolutionary process of hydrocarbons by chemical and isotopic data. Moreover, this model provides an approach to study the dynamic evolution of the isotope series of C1–3 (including reversed isotopic series), which is promising for revealing the mechanism responsible for isotopic reversal when combined with post-generation studies. [ABSTRACT FROM AUTHOR]

Based on the recent oil and gas discoveries and geological understandings on the ultra-deep strata of sedimentary basins, the formation and occurrence of hydrocarbons in the ultra-deep strata were investigated with respect to the processes of basin formation, hydrocarbon generation, reservoir formation and hydrocarbon accumulation, and key issues in ultra-deep oil and gas exploration were discussed. The ultra-deep strata in China underwent two extensional-convergent cycles in the Meso-Neoproterozoic Era and the Early Paleozoic Era respectively, with the tectonic-sedimentary differentiation producing the spatially adjacent source-reservoir assemblages. There are diverse large-scale carbonate reservoirs such as mound-beach, dolomite, karst fracture-vug, fractured karst and faulted zone, as well as over-pressured clastic rock and fractured bedrock reservoirs. Hydrocarbons were accumulated in multiple stages, accompanied by adjusting and finalizing in the late stage. The distribution of hydrocarbons is controlled by high-energy beach zone, regional unconformity, paleo-high and large-scale fault zone. The ultra-deep strata endow oil and gas resources as 33% of the remaining total resources, suggesting an important successive domain for hydrocarbon development in China. The large-scale pool-forming geologic units and giant hydrocarbon enrichment zones in ultra-deep strata are key and promising prospects for delivering successive discoveries. The geological conditions and enrichment zone prediction of ultra-deep oil and gas are key issues of petroleum geology. [ABSTRACT FROM AUTHOR]

It has been acknowledged that carbonate was identified as the source rocks of a series of oil-gas fields worldwide. For evaluating the carbonate source rocks, total organic carbon (TOC) contents act as an important indicator. However, the acid solution, which has been generated during conventional TOC measurements, contain organic matters. Hence, the released organic matters in acid solution during carbonate decomposition may lead to underestimate the hydorcarbon generation potential. In this study, rock-eval pyrolysis technique was applied to bulk rock and residue after acid treatment. Meanwhile, the organic matters in acid solution were measured by Gas Chromatography-Mass Spectrometer (GC-MS) to investigate the geochemical characteristics. In addition, the hydrocarbon generation and alteration of TOC contents of released organic matters by acid treatment were studied by hydrous pyrolysis experiments. The results show that the acid solution contains organic compounds, including n-alkanes, saturated fatty acids and fatty acid methyl esters. Meanwhile, total organic carbon (TOC) contents and hydrocarbon generation potential (S1+S2) significantly decrease for bulk rocks after acid treatment in low maturity samples. Moreover, organic CO2 (S3) decreased after treating of acid, revealing that acidolysis process can affect and release organic matters containing oxygen-bearing functional groups. The S1, S2, S3, and TOC loss are positive correlation with the proportion of rock loss during acidolysis, indicating that the organic matters in acid solution are associated with carbonate minerals. The organic fractions may exist as adsorption state on the surface of carbonate minerals and (or) exist as organic acid salts. Moreover, the thermal simulation experiments reveal that the organic matter in acid solution, which is not recovered by the conventional measurement approach, could contribute to hydrocarbon generation. [ABSTRACT FROM AUTHOR]

Abstract A series of thermochemical sulfate reduction (TSR) simulation experiments were carried out involving different organic matter (crude oil, solid bitumen, type II kerogen, type III kerogen) and different sulfate species (anhydrite and MgSO 4) to address the chemical and carbon isotopic variations of the hydrocarbon, H 2 S and CO 2. The increase of main peak carbons of residual saturated hydrocarbon and the decrease of gaseous hydrocarbons from control treatment to corresponding sulfate treatments suggest that TSR promote the consumption of both liquid and gaseous hydrocarbons. The δ13C 1 -3 values generally shift positively from control treatment to sulfate treatments and the (δ13C ethane -δ13C methane) values in sulfate treatments are higher than that in control treatments. The variation of δ13C 1 -3 in TSR is controlled by the isotope fractionation during the generation and consumption of C 1 -3 in TSR. MgSO 4 is more reactive than anhydrite in TSR. A certain amount of H 2 S is incorporated into solid bitumen as the result of secondary alteration. We believe that the reactivity order of different organic matter in TSR is crude oil > solid bitumen > type II kerogen > type III kerogen. The reactivity of organic matter in TSR depends on the hydrocarbon generation kinetics of each organic matter. The dissolution/decomposition and precipitation of carbonate control the yield of CO 2 in sour reservoirs. The negative shift of δ13CO 2 with increasing TSR extent is mainly due to the inheritance effect of carbon isotope from hydrocarbons. Inorganic CO 2 sourced from the thermal decomposition or acid dissolution of carbonate mineral impose significant influence on δ13CO 2. The H 2 S yields decrease with CH 4 yields and increase with δ13CH 4 value, the δ13CH 4 values increase with residual amount of gaseous alkane (1- H 2 S/(residual alkane + H 2 S)), suggesting that methane acted as reactant in TSR. The (δ13CO 2 -δ13CH 4) values decrease significantly with increasing temperature, and the δ13CO 2 is even more negative than δ13CH 4 in 450 °C MgSO 4 treatments involving type II and type Ⅲ kerogen. Accordingly, methane acted as a predominant reactant in 450 °C sulfate treatments in our experiment. It is possible for natural gas with high gas dryness to experience methane-dominated TSR in geological reservoirs. Highlights • Organic matter type imposes significant influence on TSR. • Dissolution and precipitation of carbonate control CO 2 yield in TSR. • δ 13CO 2 shift negatively with increasing TSR extent due to the inheritance of 13C/12C. • Methane can act as predominant reactant in TSR. [ABSTRACT FROM AUTHOR]

The mechanism of hydrogen sulfide (HS) generation plays a key role in the exploration and development of marine high-sulfur natural gas, of which the major targets are the composition and isotope characteristics of sulfur-containing compounds. Hydrocarbon source rocks, reservoir rocks, natural gases and water-soluble gases from Sichuan Basin have been analyzed with an online method for the content of HS and isotopic composition of different sulfur-containing compounds. The results of comparative analysis show that the sulfur-containing compounds in the source rocks are mainly formed by bacterial sulfate reduction (BSR), and the sulfur compounds in natural gas, water and reservoir are mainly formed by thermal sulfate reduction (TSR). Moreover, it has been shown that the isotopically reversion for methane and ethane in high sulfur content gas is caused by TSR. The sulfur isotopic composition of HS in natural gas is inherited from the gypsum or brine of the same or adjacent layer, indicating that the generation and accumulation of HS have the characteristics of either a self-generated source or a near-source. [ABSTRACT FROM AUTHOR]

Using high pressure and geological condition simulation vessels, we conducted hydrous pyrolysis experiments of kerogen, solid bitumen and liquid hydrocarbons in southern China in order to study the processes of gas generation and derive geochemical indicators of gas genesis under approximate pressure and temperature. The results indicate that gas generation productivity of different marine material decreased in the order of crude oil (light oil and condensate), dispersed soluble organic matter (solid bitumen and heavy oil), and kerogen. Under identical temperature-pressure regimes, pyrolysates derived from kerogen and dispersed soluble organic matter display drastically different geochemical characteristics. For example, the $\delta ^{13} C_{CO_2 } - \delta ^{13} C_1 $ values of gaseous products from dispersed soluble organic matter are greater than 20‰, whereas those from kerogen are less than 20‰. The δC values of pyrolysates from different marine hydrocarbon sources generally increase with pyrolysis temperature, but are always lower than those of the source precursors. The δC values of ethane and propane in the pyrolysates also increase with increasing pyrolysis temperature, eventually approaching that of their sources, at peak hydrocarbon generation. At high-over mature stages, the δC values of ethane and propane are often greater than those of their sources but close to those of coal gases, and thus become ineffective as gas genetic indicators. Ln(C/C) can clearly distinguish kerogen degradation gas from oil cracking gas and Ln(C/C)-( δC− δC) can be an effective indicator for distinguishing oil cracking gas from dispersed soluble organic matter cracking gas. [ABSTRACT FROM AUTHOR]

The age-accumulation effect of Ar in hydrocarbon source rocks was discussed in accordance with the decay law of radioactive elements. In terms of the mean values of Ar/Ar, the old Sinian gas reservoirs (mean values of Ar/Ar: 7009) were definitely distinguished from the Permian gas reservoirs (mean values of Ar/Ar: 1017) in Weiyuan, Sichuan Province, and the gas source of the Permian gas reservoir (mean values of Ar/Ar: 5222) in well Wei-7 with the Weiyuan structure is defined as the Sinian system. Based on the values of Ar/Ar, the coal-type gases (The source rocks are of the C-P system; mean values of Ar/Ar: 1125) are definitely distinguished from the oil-type gases (The source rocks are of the Tertiary system; mean values of Ar/Ar: 590) in the Tertiary reservoirs of the Zhongyuan Oilfield. Besides, Ar/Ar values also have a positive effect on the oil-source correlation of oil reservoirs in ancient hidden mountains. According to the crust-mantle interchange information reflected by He/He values, petroliferous provinces in China can be divided into three major tectonic regions. (1) The eastern active region: The crust-mantle volatile matter exchanges actively, and the He/He values are mainly around 10, partly around 10. (2) The central stable region: The He/He values are all around 10. (3) The western sub-stable region: The He/He values are mainly around 10, and around 10 on the edges of the basins. Helium contents of some gas wells in China's eastern petroliferous region reach the industrial abundance (He≈0.05%-0.1%), the He/He values reach 10, and the equivalent values for the mantle-source components in helium gas can reach 30%-50%. As viewed from this, a new type of crust-mantle composite helium resources has been proposed. Geneses of some CO gas reservoirs in the east of China and some issues concerning mantle-source methane were discussed in the light of the helium and carbon isotopes of CO and CH in natural gases. In the discussion on helium isotopic characteristics of inclusions in the reservoirs, it was discovered that the He/He values are close to those in natural gases. That is to say, this phenomenon is related to regional tectonism. The He/He, CO/He and CH/He data were used to discuss the tectonic activities of fault zones in a certain number of regions in China. [ABSTRACT FROM AUTHOR]

This study analyzed the developing environments of hydrocarbon source rocks in the Ordos Basin and evaluated carbonate rocks as hydrocarbon source rocks and their distributions on account of the fact that China’s marine carbonate rocks as hydrocarbon source rocks are characterized by the intensive thermal evolution and relatively low abundance of organic matter, by taking the Lower Paleozoic of the Ordos Basin for example and in light of the calculated enrichment coefficients of trace elements, the profile analysis of trace element contents, ratios of relevant elements, and stable isotopic compositions and their three-dimensional diagrammatization in combination with the necessary organic parameters. As for the Ordos Basin, TOC=0.2% is an important boundary value. Studies have shown that in the strata TOC>0.2%, V/(V+Ni)>0.50, Zr/Rb<2, Rb/K(× 104)>30, Z<122, Th/U>0.80, Zn and Mo are enriched with a positive δ13Ccarb excursion. All these indicated a stagnant and stratified sedimentary environment that has low energy, anoxia and high salinity in bottom water. In these strata the geological conditions are good for the preservation of organic matter, hence favoring the development of hydrocarbon source rocks. These strata have δ13Corg <−28‰ (I–II type) and high hydrocarbon-generated potential. The Klimory and Wulalik formations show certain regularities in those aspects, therefore, they can be regarded as the potential effective hydrocarbon source rocks. In the strata TOC⩽0.2%, Zr/Rb>1, V/(V+Ni)<0.50, Rb/K<30, Th/U<0.80, Cu, Zn, etc are depleted, and δ13Corg values range from −24‰ to −28‰. All these facts showed that most of the carbonate rocks or mudstones were formed in high-energy oxidizing environments, thus unfavorable to the development of hydrocarbon source rocks. It is feasible to make use of the geochemical method to comprehensively assess the highly evolved marine carbonates rocks as potential hydrocarbon source rocks and their distributions. [ABSTRACT FROM AUTHOR]

The Jiyang Sag and the Liaohe Basin are the two important areas where immature nil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, Ro, for source rocks varies from 0.25% (immature) to 0.65% (mature). Studies have shown that within this range of Ro values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e. , light → heavy → light → heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bund-breaking ratio of 12C-13C also increases, making the relatively 12C-rich materials at the low mature stage evolve again towards 13C enrichment. [ABSTRACT FROM AUTHOR]

A low-mature coal (R0 = 0.4%, from the Manjia'er depression, Tarim Basin, China) was subjected to closed system pyrolysis, in sealed gold tubes, under isothermal temperature conditions. The carbon isotopic compositions of the pyrolyst fractions (hydrocarbon, CO2, CO, etc.) at two temperature points (350°C and 550°C) were measured. The results showed that δ 13CCH4 value is generally heavier at 350°C than that at 550°C, because the high abundance of CO generated at low temperature would greatly influence δ 13CCH4 value, and the retention time of CO in gas chromatograph is close to that of CH4. But CO is formed through chemical reaction of the oxygen-containing functional group -C = O, e.g. lactones, ketones, ether, etc. at low temperature, while CO2 comes mainly from decarboxylization. The carbon isotopic composition of coal gas from Lanzhou Coal Gas Works as definitely different from that of thermally pyrolysed products from coal. The δ 13CCH4 value of coal gas was abnormally heavier than δ 13CCO. At the same time, the reversed sequence (δ 13C1 > δ 13C2) of δ 13C1 and δ 13C2 happened. The bond energy of free ions generally decides the sequence of generation of hydrocarbon fractions according to the chemical structure, whereas the stability of pyrolysate fractions and their carbon isotope fractionation are affected by the C-C bond energy. [ABSTRACT FROM AUTHOR]

The total organic carbon of carbonate sediments includes a 5%–45% contribution from acid-soluble organic matter, particularly carboxylic acid salts. However, there is little information available on hydrocarbon generation from carboxylic acid salts. The objectives of this study were to investigate the extent and timing of hydrocarbon generation from calcium stearate. Thus, gold-tube nonhydrous pyrolysis experiments of calcium stearate were conducted from 250 to 600 °C at 50 MPa. Infrared spectroscopy and δ13C testing were also performed on solid pyrolysate and gaseous products, respectively, to trace changes of functional groups and stable carbon isotopes. Pyrolysis of calcium stearate produced hydrocarbons, CO 2 , CaO, CaCO 3 , coke, and other inorganic compounds of low molecular weight. A Van Krevelen diagram indicated that calcium stearate falls in the same region as hydrogen-rich type I kerogen. The maximum yield of methane generated from calcium stearate was as high as that from North Sea oil. Hydrocarbon generation from calcium stearate was related to cracking of the alkyl moiety and similar to that occurring in pyrolysis of n- C 18. As a consequence of kinetic carbon isotope fractionation, with increasing temperature CO 2 becomes enriched in 13C and CaCO 3 becomes enriched in 12C. Notably, CaCO 3 with very low δ13C values is probably an effective proxy indicating hydrocarbon generation from carboxylic acid salts, if no other CaCO 3 is produced via thermochemical sulphate reduction processes. The optimized kinetic parameters demonstrate that calcium stearate had higher methane generation activation energies than those of n -C 18 and type I kerogen. Hence, methane generation from calcium stearate is delayed when extrapolated to geological conditions. This retarded methane generation illustrates the vital role of carboxylic acid salts in the generation of deep oil and gas. Future research should concern the abundance of carboxylic acid salts in carbonate formations and the depositional environments that facilitate the formation of carboxylic acid salts. • Closed gold-tube pyrolysis of calcium stearate was conducted. • Calcium stearate had gas potential as high as crude oil. • Negative δ13C CaCO3 may indicate hydrocarbon generation from carboxylic acid salts. • Methane generation from calcium stearate was behind that of n- C 18. [ABSTRACT FROM AUTHOR]

Organic-rich shales are generally characterized by high uranium (U) contents. The effects of U irradiation on evolution and hydrocarbon generation of organic matter (OM) have attracted substantial attention. However, details on the generation mechanisms of products and the identification of dominant reactions occurring during OM radiolysis remain understudied. In this study, γ ray was used to irradiate immature kerogen and bitumen, and electron beam was for decane. The contents and carbon (C) isotopic compositions of gaseous products, the liquid products of decane, and the functional groups alterations of kerogens and bitumen were detected. The results showed that, the product generation corresponded well to the structural alteration of OM during radiolysis. C–H and R–CH 3 rupture contributed to H 2 and CH 4 generation, respectively. O-containing group cleavage caused CO 2 generation, and O 2 presence could enhance the oxidation of OM by joining in the structure of OM or by facilitating the formation of new oxides. H 2 O addition could not only provide O, enhancing the oxidation of OM, but could also provide H, thus promoting H 2 generation. We identified cracking, cross-linking, and oxidation reactions during the OM radiolysis. C isotopic composition changes in solid OM could facilitate identification of dominant reactions. Non-closed geological environment of shales promoted the occurrence of cracking reaction. The substantial radiation energy generated from U decay in U-rich shales was significant for OM evolution via decrease in H/C, increasing O/C, and via enhancement of dealkylation, aromatization and unsaturation, which were similar to the thermal effects. Moreover, U irradiation could also promote conventional gas and immature gas generation and could provide exogenous H for hydrocarbon generation of OM, especially for OM in an over mature stage. It is essential to consider the effects of U irradiation on the evolution, hydrocarbon generation, and resources assessment of U-rich shales. • C–H, R–CH 3 and O-containing bonds rupture contribute to H 2 , CH 4 and CO 2 generation. • H 2 O can provide exogenous H for H 2 and hydrocarbons generation. • C isotopic composition changes of OM can be an indicator of dominant reaction. • Radiation like thermal effect, promotes evolution and hydrocarbon generation of OM. [ABSTRACT FROM AUTHOR]