Oil Cracking Gas Research Articles

Geochemical Evidence for Differential Accumulation of Natural Gas in Shiwu Fault Depression, Songliao Basin, China

The carbon, helium and argon isotopes and composition data of natural gas sampled from the Shiwu fault depression indicated complex origin of natural gas. Carbon isotope reversal was possibly caused by mixing of sapropelic and humic sourced gases and leakage from gas reservoir. The shallow natural gas in both Bawu and Houwujiahu gas fields came from deep source rocks. In the Bawu gas reservoir, it was kerogen pyrolysis gas with a low degree of maturity and originated from type II kerogen in deep formation, whereas the high maturity shallow gas in the Houwujiahu reservoir was oil cracking gas and originated from type II kerogen and type III kerogen. The deep primary gas reservoir was destroyed, and natural gas migrated through the deep faults at the early stage and was accumulated in the shallow reservoir. The fault activities, source rock types, degree of thermal evolution and leakage from gas reservoir resulted in the differential accumulation of natural gas vertically and areally.

Genetic origins of marine gases in the Tazhong area of the Tarim basin, NW China: Implications from the pyrolysis of marine kerogens and crude oil

Abstract Although abundant natural gases have been discovered in Carboniferous and Ordovician reservoirs in the Tazhong area, central Tarim basin, their genetic origins is still an open question because of significant variations in chemical and carbon isotopic compositions. In the present study, three samples representing the possible gas sources were pyrolyzed using sealed gold tubes to investigate their gas potentials, and chemical and carbon isotopic signatures. The samples were a crude oil, a low maturity kerogen and a high maturity kerogen. The results show that the chemical and isotopic compositions of gaseous pyrolysates from the three samples are quite different. Generally, the gases from oil cracking are compositionally wetter and isotopically lighter than those from kerogen cracking at similar thermal levels. The large quantities of methane experimentally observed in oil cracking gases are mainly associated with the re-cracking of wet gases at very high maturity levels. Methane in gases derived from kerogen, especially high maturity kerogen, in contrary, is mainly generated by the demethylation reaction. The diagrams of δ13C1 vs. δ13C2–δ13C3 and C2/C3 vs. δ13C2–δ13C3, along with natural gas plots, have proven to be a very effective tool in the genetic interpretation of natural gases from kerogen and oil cracking in the Tarim basin. Their combined application shows that two types of gas source can be identified in the Tazhong area. The first type is oil cracking gas, mainly discovered in Carboniferous reservoirs of the central fault horst area, but also partly in Ordovician reservoirs, such as in the wells TZ45, TZ44b and TZ 16. The other is kerogen cracking gas occurring largely in Ordovician reservoirs, especially in the North Slope area. The two types of gases, however, are mixed in many reservoirs as their generation is overlapped in time and/or space. This mixing process is especially obvious for methane.

Geochemical study on oil-cracked gases and kerogen-cracked gases (II)—Discrimination methods between oil-cracked gases and kerogen-cracked gases

In the processes of discrimination between oil-cracked gases and kerogen-cracked gases, Behar and Pinzgofer et al.’s results were adopted in the former researches, in which the ratio of C2/C3 is basically a constant while the ratio of C1/C2 gradually increases in the course of primary cracking of kerogen. Otherwise in the course of secondary cracking of oil, the ratio of C2/C3 increases rapidly while C1/C2 keeps relatively stable. Our study on analogue experiment shows that, whether it is oil or kerogen, in its process of gas generating by cracking, the ratios of C2/C3, C1/C2 or C1/C3 will all be increased with the growth of thermal conditions. In comparison, the ratio of C2/C3, which is affected by genetic type to some comparatively less extent, mainly responds to the maturity of gases, while the value of C2/C3 is about 2, and that of C2/iC4 is about 10, and the corresponding value of Ro is about 1.5%–1.6%. The influence of gas source on C2/C3 is less than that of gas maturity, otherwise C1/C2 (or C1/C3) is obviously affected by cracking matrices. The ratios of C1/C2, C1/C3 of oil-cracked gases are less than that of kerogen-cracked gases, under the condition that the ratios of C2/C3 are similar in value, so are the value of dryness indexes. There exists wide diffidence between this view and the former discrimination method in theory. The analysis of the spot sample indicates that we can apply the above basic view to dealing efficiently with the problem of the discrimination between oil-cracked gas and kerogen-cracked gas.

Genetic features of gas-condensate reservoirs in the Kekeya Field, southwest depression in the Tarim Basin, China

Oils, condensates and natural gases in the Kekeya Field, southeast depression of the Tarim Basin were studied for their geochemical characteristics. According to the distribution analysis of the C2/C3 values with C1/C2 values, C2/C3 values with C1/C3 values, as well as C2/C3 values with dryness index, there are two different types of natural gases in the studied field, which are spatially regularly distributed. One is the oil cracking gas, located on shallow reservoirs over X52 reservoir, namely Upper oil legs; the other is kerogen cracking gas, located on X72 reservoirs, X8 reservoirs and E2k reservoirs, namely Lower oil legs. In addition, the distribution patterns of molar concentration of oils and condensates with different carbon numbers of the n-alkanes in the Kekeya Field indicate that the crude oils have experienced several kinds of secondary alterations, which were closely related to the charging of gaseous hydrocarbons after petroleum accumulation. These results indicate that, based on the research of δ13C values of individual hydrocarbons, heptane values and isoheptane values of light hydrocarbons and aromatic maturity parameters for oils, condensates and natural gases, oils and gases were charged at different geological time in the Kekeya Field.

The generation and its sealing condition of natural gas in the Tadong area

Focusing on the two natural gas exploration geological problems with abundant source of oil cracking gas in the late stage and the sealing condition of the oil cracking gas reservoir, the kinetics of oil cracking gas and the evaluation parameters of gas cap rock are adopted to the study on the natural gas accumulation conditions in the Tadong area. Both the study on the kinetics of oil cracking gas and the statistical results of reservoir bitumen reveal that the geological formation of oil cracking gas in the Tadong area is located in the top of Cambrian. Two kinds of oil cracking gas geological models at least, namely well Mandong-1’s early rapid generation model (Middle Ordovician-end Silurian) and peak cracking model (with the natural gas conversion rate >90%), namely well Yingnan-2’s two-stage generation model of oil cracking gas, have been set up. The oil cracking gas of Yingnan-2 in the late stage is very significant in the evaluation of natural gas exploration in the Tadong area. The evaluation results of the cap rock show that the microscopic parameters of cap rock from the lower assemblage of Cambrian-Ordovician are better than those from the upper assemblage. The former has strong capillary sealing ability and higher cap rock breakthrough pressure than the upper assemblage, with strong sealing ability, so that natural gas dissipates mainly by diffusion. According to the above investigations, the lower assemblage Cambrian-Ordovician natural gas of Kongquehe slope, Tadong low uplift and Yingjisu depression in the Tadong area prospects well.

Pyrolysis of oil at high temperatures: Gas potentials, chemical and carbon isotopic signatures

Although the gas cracked from oil has been believed to be one of the important sources in highly matured marine basins, there are still some debates on its resource potentials and chemical and isotopic compositions. In this study a Cambrian-sourced marine oil sample from the Silurian reservoir of well TZ62 in the central Tarim basin was pyrolyzed using sealed gold tubes with two different pyrolysis schemes: continuous pyrolysis in a closed system and stepwise semi-open pyrolysis. The results show that the maximum weight yield of C1–5 gases occurs at EasyRo=2.3% and the residual gas potential after this maturity is only 43.4 mL/g, about 12% of the yield of 361 mL/g at EasyRo=2.3%. Combined with the results of kinetic modeling, the main stage of gas generation from oil cracking is believed within the EasyRo=1.6%–2.3%. The increase in the volume yield of C1–5 gases at EasyRo>2.3% in a closed system is mainly related to the re-cracking of previously formed C2–5 wet gases, not the direct cracking of oil. The stepwise pyrolysis experiments show that the gas from the cracking of residual oil at EasyRo>2.3% is characterized by very high dryness index (higher than 92%) and heavy methane carbon isotopes ranging from −28.7% to −26.7%, which is quite different from the gases from the continuous pyrolysis in a closed system. The kinetic modeling of methane carbon isotope fractionation shows that the carbon isotopes of methane within the main stage of gas generation (EasyRo<2.3%) are far lighter than the carbon isotopes of the precursor oils under a geological heating rate of 2 °C/Ma. The above observations and results provide some new clues to the accurate recognition and objective resource evaluation of oil cracking gas in highly mature marine basins.

Identification of marine natural gases with different origin sources

Kinetic experiments of gas generation for typical samples of marine gas precursors including low-maturity kerogen, residual kerogen and oil as well as dispersed liquid hydrocarbon (DLH) in source rocks were performed by closed system, and the evolution trends of molecular and isotopic compositions of natural gases from different precursors against the maturity (R 0%) at laboratory conditions were analyzed. Several diagrams of gas origin were calibrated by using the experimental data. A diagram based on the ratio of normal and isomerous butane and pentane (i/nC4 − i/nC5) was proposed and used to identify the origins of the typical marine natural gases in the Sichuan Basin and the Tarim Basin, China. And the maturities of natural gases were estimated by using the statistical relationships between the gaseous molecular carbon isotopic data and maturities (δ13C-R 0%) with different origins. The results indicate that the molecular and isotopic compositions of simulated gases from different precursors are different from each other. For example, the dryness index of the oil-cracking gas is the lowest; the dryness indices of gases from DLH and kerogen in closed system are almost the same; and the dryness index of gases from residual kerogen is extremely high, indicating that the kerogen gases are very dry; the contents of non-hydrocarbon gases in kerogen-cracking gases are far higher than those in oil-cracking and DLH-cracking gases. The molecular carbon isotopes of oil-cracking gases are the lightest, those of kerogen in closed system and GLH-cracking gases are the second lightest, and those of cracking gases from residual kerogen are the heaviest. The calibration results indicate that the diagrams of In(C1/C2)-In(C2/C3) and δ4 3C2-δ4 3C3-In(C2/C3) can discriminate primary and secondary cracking gases, but cannot be used to identify gas origin sources, while the diagram of i/nC4 − i/nC5 can differentiate the gases from different precursors. The application results of these diagrams show that gas mixtures extensively exist in China, which involved the gases from multiple precursors and those from different maturity stages. For example, marine gases in the Sichuan Basin involve the mixture of oil-cracking gases and high-over-maturated kerogen gases, while those in the Tarim Basin involve not only the mixture of gases from multiple precursors, but also those from different maturity gases and post-reservoir alternations such as oxidized degradation and gas intrusion processes.

Natural gas origins of large and medium-scale gas fields in China sedimentary basins

China sedimentary basins present abundant natural gas resource thanks to its unique geological settings. Marine highly-matured hydrocarbon source rocks, widespread coal-measure strata and low temperature Quaternary saline strata, etc., indicate the wide foreground of China natural gas resources. Up to now, most of the petroliferous basins have been discovered to have wholesale natural gas accumulation from Precambrian, Paleozoic, Mesozoic to Cenozoic in the east, the central, the west and the coast of China. These large and medium-scale gas reservoirs are mainly composed of hydrocarbon gas with big dry coefficient, tiny non-hydrocarbon, wide carbon isotope distribution and varying origin types, the hydrocarbon gas includes coal-formed gas, oil-formed gas, biogenic gas and inorganic gas, etc. Coal-formed gas is the main type of China natural gas resources, in particular several explored large-scale gas fields (>100 billion cubic meter) of Kela 2, Sulige and Daniudi, etc., they all belong to coal-formed gas fields or the gas fields consisting mostly of coal-formed gas. Oil-formed gas is also abundant in China marine basins, for example marine natural gas of Sichuan Basin generated from crude oil cracking gas. Primary and secondary biogenic gas fields were discovered respectively in the Qaidam Basin and Western Slope of Songliao Basin. In addition, inorganic gases are mainly distributed in the eastern China, in particular the Songliao Basin with abundant carbon dioxide accumulation, indicating that the eastern China present large exploration potential of inorganic gas.

Kinetics of hydrocarbon generation for Well Yingnan 2 gas reservoir, Tarim Basin, China

Well Yingnan 2, an important exploratory well in the east of Tarim Basin, yields high commercial oil and gas flow in Jurassic. Natural gas components and carbon isotopic composition indicate that it belongs to sapropel type gas. Because this region presents many suits of hydrocarbon source rocks, there are some controversies that natural gases were generated from kerogen gas or crude oil cracking gas at present. By using the kinetics of hydrocarbon generation and carbon isotope, natural gas of Well Yingnan 2 is composed mainly of crude oil cracking gas, about 72%, it is generated from secondary kerogen gas of Cambrian-Lower Ordovician source rock and crude oil cracking gas of Mid-Upper Ordovician oil reservoir. The main oil and gas filling time is 65 Ma later in the Jurassic gas reservoir of Well Yingnan 2, so the gas reservoir belongs to late accumulation and continuous filling type.

Cracking conditions of crude oil under different geological environments

There are mainly 3 kinds of existing states of oil generating from source rocks, that is, dispersive liquid hydrocarbon inside of source rock, dispersive liquid hydrocarbon outside of source rock and concentrated liquid hydrocarbon outside of source rock. Because of the differences in thermal history and medium conditions around, and the interaction of organic and inorganic matter, the liquid hydrocarbon with 3 kinds of existing state has different cracking conditions. The gas generation dynamics experiments of crude oil matching different mediums indicate that the distribution of activation energy of methane changes a lot according to medium difference. The carbonate has a main influence on oil cracking conditions and can largely reduce its activation energy, which reflects the lower cracking temperature of crude oil. The mudstone takes a second place and the sandstone is the smallest. The catalytic cracking function to the oil of the carbonate, of the mudstone and of the sandstone changes weaken in turn. The corresponding Ro values of main gas generation period in different mediums are as follows: 1.5%–3.8% with pure crude oil, 1.2%–3.2% with dispersive crude oil in carbonate, 1.3%∼3.4% with dispersive crude oil in mudstone and 1.4%–3.6% with dispersive crude oil in sandstone. The influence of pressure to crude oil cracking is relatively complicated. In the low heating speed condition, pressure restrains the oil cracking and gas generation, but in the high heating speed condition, pressure has an indistinctive influence to the oil cracking and gas generation. Pressure also makes a different effort in different evolvement stage. Taking the middle and lower Cambrian source rocks in the Tarim Basin as an example, primary oil generating quantity is 2232.24×108t, residual oil and oil cracking gas quantity is 806.21×108t and 106.95×1012m3 respectively.

TSR promotes the formation of oil-cracking gases: Evidence from simulation experiments

TSR is an interaction between sulfate and hydrocarbons, occurring widely in carbonate reservoirs. Because this process can produce a large amount of noxious acidic gases like H2S, it has drawn serious concern recently. This paper reports an experiment that simulated an interaction between different minerals and hydrocarbon fluids under different temperature and time using a confined gold-tube system. The results showed that the main mineral that initiates TSR is MgSO4, and adding a certain amount of NaCl into the reactive system can also promote TSR and yield more H2S. The H2S produced in TSR is an important incentive for the continuous oxidative degradation of crude oils. For instance, the yield of oil-cracking gases affected by TSR was twice of that not affected by TSR while the yield of TSR-affected methane was even higher, up to three times of that unaffected by TSR. The carbon isotopes of wet gases also became heavier. All of the above illustrated that TSR obviously motivates the oxidative degradation of crude oils, which makes the gaseous hydrocarbon generation sooner and increases the gas dryness as well. The study on this process is important for understanding the TSR mechanism and the mechanism of natural gas generation in marine strata.

Discussion of gas enrichment mechanism and natural gas origin in marine sedimentary basin, China

There are abundant natural gas resources in Chinese marine sedimentary basin. The exploration hot shots of natural gas are the Palaeozoic marine strata here in recent years, and several large scale gas fields have been discovered. Chinese Palaeozoic high-post matured and coal measure hydrocarbon source rocks are mainly prone to gas generation in the present. This research considered that gas source rocks and TSR are the key cause of gas enrichment of marine strata. High-quality argillaceous and coal measure hydrocarbon rocks are distributed widely in the Palaeozoic marine strata, which have been in highly matured phase in the present. The argillaceous source rock generally contains various sulfates that could accelerate crude oil cracking to gas for TSR occurrence, and coal measure source rock mainly generates gas, so Chinese marine basin gives priority to accumulating gas. Marine strata have not founded oil reservoirs in the Sichuan Basin and Ordos Basin, and they consist mainly of dry gas. Marine natural gases are the mixed gases of oil cracking gas and coal-formed gas in a general way, oil cracking gases contain usually some H2S and CO2. Hydrocarbon carbon isotopes are very complicated, and methane and ethane isotopic values bear apparent reversal caused by thermal evolution and mixing among different genetic types of natural gas. Coal-formed gases are the main component of Chinese marine natural gas. The Upper Permian of the Sichuan Basin and the Carboniferous-Permian of the Ordos Basin coal measure hydrocarbon source rock present large hydrocarbon generation potential, which are the prospecting highlight of marine natural gas hereafter. Oil cracking gas exploration will be paid much attention to in the Tarim Basin because of the lack of coal measure hydrocarbon source rock.

A discussion on gas sources of the Feixianguan Formation H2S-rich giant gas fields in the northeastern Sichuan Basin

In recent years several H2S-rich oolite giant gas pools have been discovered in the Lower Triassic Feixianguan Formation of the northeastern Sichuan basin, and their explored gas reserves have been over 5000×108 m3. However, gas sources remain unsolved due to multiple source horizons with high maturity in this area and TSR alterations. By integrating analytical data of natural gas samples with conprehensive investigations on many factors, such as oil-gas geology, distribution and evolution of source rocks, charging and adjustment of gas pools, mixture of natural gases and secondery alterations, the present study concluded that the dominant source for the Feixianguan Fr. gas pools is the Permian Longtan Fr. source rock and secondly the Silurian Longmaxi Fr. source rock. Natural gases from the various gas pools differ genetically due to the matching diversity of seal configurations with phases of hydrocarbon generation and expulsion by different source rocks, among which natural gases in Puguang Gasfield are dominated by the trapped gas generated from the Longtan Fr. source rock and commingled with the gas cracked from the Silurian crude oil, while those in Dukouhe, Tieshangpo and Luojiazhai Gasfields are composed mainly of the Silurian oil-cracking gas and commingled with the natural gas derived from the Longtan Fr. source rock.

Mass balance calculation of the pyrolysates generated from marine crude oil: A prediction model of oil cracking gas resources based on solid bitumen in reservoir

Oil cracking gas plays an important role in the resources of natural gas in the basins with high and over mature marine source rocks in China. The prediction of the oil cracking gas resources becomes necessary and urgent in the gas exploration in these basins. A marine crude oil sample was pyrolyzed using sealed gold tubes system. The pyrolysates including gas, liquid and solid were quantitatively analyzed. Based on the pyrolysis data and kinetic calculation, the yield correlativity among gas, liquid and solid products was regressed with high correlative coefficients to establish a prediction model suitable for the resource estimation of oil cracking gas. The verification formula for this model was also established on the principle of mass conservation. The affecting factors and the application preconditions of this prediction model were discussed. This model would enlighten and provide some new ideas for the resource assessment of natural gas in the high and over mature marine carbonate source rock areas in China. It is expected to be valuable in gas exploration.

Geochemical features and genesis of the natural gas and bitumen in paleo-oil reservoirs of Nanpanjiang Basin, China

Bitumen from the Nanpanjiang Basin occurs mainly in the Middle Devonian and Upper Permian reef limestone paleo-oil reservoirs and reserves primarily in holes and fractures and secondarily in minor matrix pores and bio-cavities. N2 is the main component of the natural gas and is often associated with pyrobitumen in paleo-oil reservoirs. The present study shows that the bitumen in paleo-oil reservoirs was sourced from the Middle Devonian argillaceous source rock and belongs to pyrobitumen by crude oil cracking under high temperature and pressure. But the natural gas with high content of N2 is neither an oil-cracked gas nor a coal-formed gas generated from the Upper Permian Longtan Formation source rock, instead it is a kerogen-cracked gas generated at the late stage from the Middle Devonian argillaceous source rock. The crude oil in paleo-oil reservoirs completely cracked into pyrobitumen and methane gas by the agency of hugely thick Triassic deposits. After that, the abnormal high pressure of methane gas reservoirs was completely destroyed due to the erosion of 2000–4500-m-thick Triassic strata. But the kerogen-cracked gas with normal pressure was preserved under the relatively sealed condition and became the main body of the gas shows.

Gas sources of the YN2 gas pool in the Tarim Basin—Evidence from gas generation and methane carbon isotope fractionation kinetics of source rocks and crude oils

The YN2 gas pool in the eastern Tarim basin has been believed to be an accumulation of gases resulting from oil-cracking. However, this gas source model cannot explain such geochemical characteristics of the natural gases as heavier carbon isotope values ( δ 13C 1=−37.5‰ to −36.2‰; δ 13C 2=−34.7‰ to −30.9‰) and high content of nitrogen gas (15.5% on average). In this study, we performed pyrolysis experiments on typical source rock and crude oil samples selected from the basin, evaluated their kinetic parameters of gas generation and methane carbon isotopic fractionation, and extrapolated them to geological conditions. The results show that neither kerogen-cracking gas nor oil-cracking gas can act as the single gas source of the YN2 gas pool. By contrast, a mixing model of 55% kerogen-cracking gas and 45% oil-cracking gas results in an excellent match with the geochemical characteristics of the gas pool. The gas condensate from the YN2 gas pool originated from the cracking of oils in mid-upper Ordovician strata, and the nitrogen gas was mainly derived from the cracking of kerogen in Cambrian–lower Ordovician mudstones in the Yingjisu depression center during the very high level of maturation. The YN2 gas pool went through three stages of formation: (1) oils from Cambrian–lower Ordovician source rocks accumulated in Cambrian and Ordovician reservoirs during the middle Ordovician, and gases were generated from both kerogen and paleo-oil pools in the Cambrian–lower Ordovician strata during late Ordovician to early Devonian; (2) gases leaked during either late Devonian to Triassic due to uplifting, or late Jurassic due to intense faulting activity in this area; (3) gases were regenerated from the late cracking of kerogen in the Cambrian–lower Ordovician strata as well as from oil-cracking in mid-upper Ordovician reservoirs with rapid subsidence after the Cretaceous. The two types of gas mixed and accumulated in the Jurassic traps to form the present wet gas pool with high nitrogen content and a heavier methane carbon isotopic value.

Origin of natural sulphur-bearing immiscible inclusions and H2S in oolite gas reservoir, Eastern Sichuan

Based on results of microscopic observation and laser Raman analysis about fluid inclusions, multiple special forms of immiscible inclusions that contain sulphur, liquid hydrocarbon, bitumen, etc. were discovered in samples collected from the H2S gas reservoir-containing carbonates in the Lower Triassic Feixianguan Formation in the Jinzhu-Luojia area, Kai County, Sichuan Province. Based on the lithology and burial history of the strata involved as well as measurement results of homogenization temperature of fluid inclusions, bitumen reflectivity, etc., it is concluded that the H2S in the gas reservoir resulted from the thermal reaction between hydrocarbons in reservoir and CaSO4 in the gypsum-bearing dolostone section at the high temperature (140°C–17°C) oil-cracked gas formation stage in Late Cretaceous. Thereafter, research on a great number of immiscible inclusions in the reservoir reveals that elemental sulphur resulted from oxidation of part of the earlier-formed H2S and further reaction between sulphates, hydrocarbons and H2S in geological fluids in H2S-bearing gas reservoir at a temperature of 86°C–89°C and a pressure of 340×105Pa and during the regional uplift stage as characterized by temperature decrease and pressure decrease in Tertiary. Meanwhile, gypsum, anhydrite and calcite formed at this stage would trap particles like elemental sulphur and result in a variety of special forms of immiscible inclusions, and these inclusions would contain information concerning the complexity of the fluids in the reservoir and the origin of H2S and natural sulphur in the gas reservoir.

Oil cracking: An important way for highly efficient generation of gas from marine source rock kitchen

The potentials of gas generation by kerogen in the late period and by crude oil cracking are closely related to the origin of natural gas in the high-to over mature marine area and their exploration perspectives. The carbon structure of kerogens, with different types and at different evolution stages, have been experimentally studied using the high magnetic field solid13C nuclear magnetic resonance technique in order to determine the oil and gas potential of kerogens. Results show that the contents of gas potential carbon(GPC) of types I, II, III kerogens at the high-to over mature stage are very low, indicating their weak gas-generating capacity and limited gas production; however, the content of oil potential carbon(OPC) of the low mature type I kerogen is much higher, implying that a large amount of crude oil generated during the oil-generating period will be the material for later gas generation by oil cracking. The kinetic experiment of gas generation by crude oil cracking shows that, when the temperature is about 160°C (Ro=l.6%), the crude oil will start to produce large amounts of gas; the temperature range for major gas generation of crude oil is higher than that of the kerogens, and the gas production is 2 to 4 times higher than that of kerogens. The natural gas derived from oil cracking (called oil-cracked gas) is much abundant in methyl hexamethylene, which is quite different from the natural gas produced by thermal degradation of kerogens (named kerogen degradation gas) at high-to over mature stage.

Genetic type and maturity of Lower Paleozoic marine hydrocarbon gases in the eastern Tarim Basin

Hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity, genetic types and sources of Lower Paleozoic marine hydrocarbon gases from the eastern Tarim Basin. All of the Lower Paleozoic gases are relatively dry (dryness coefficient >0.9) and belong to the marine sapropelic type of cracked gases. These gases are characterized by low δ 13 C 1 and low Δ(δ 13 C 2−δ 13 C 1) values due to a lack of terrestrial source input. Thermal simulation experiments on Upper Proterozoic to Lower Paleozoic source rocks resulted in a δ 13 C 1–R o regression equation that was used to discriminate the maturities and sources of the natural gases. These natural gases are divided into four genetic types: (1) humic, coal-derived pyrolytic gas; (2) lacustrine sapropelic oil-cracking gas; (3) the marine sapropelic, highly mature oil-cracking gas; and (4) overmature oil-cracking gas. Types (1) and (2) were mainly generated from the Upper Triassic-Middle and Lower Jurassic; types (3) and (4) are mainly generated from the Lower Paleozoic. The data can be used to describe multiple sources and multiple generating stages of natural gas.