Organic Matter Hydrocarbon Generation Research Articles

Micropore Structure of Deep Shales from the Wufeng–Longmaxi Formations, Southern Sichuan Basin, China: Insight into the Vertical Heterogeneity and Controlling Factors

The microscopic pore throat structure of shale reservoir rocks directly affects the reservoir seepage capacity. The occurrence and flow channels of shale gas are mainly micron–nanometer pore throats. Therefore, to clarify the microstructural characteristics and influencing factors of the deep organic-rich shales, a study is conducted on the marine shale from the Upper Silurian to Lower Ordovician Wufeng–Longmaxi Formation in the southern Sichuan Basin. Petrographic lithofacies division is carried out in combination with petro-mineralogical characteristics, and a high-resolution scanning electron microscope, low-temperature nitrogen and low-temperature carbon dioxide adsorption, and micron-computed tomography are used to characterize the mineral composition and pore structure qualitatively and quantitatively, upon which the influencing factors of the microstructure are further analyzed. The results show that with the increase in burial depth, the total organic carbon content and siliceous mineral content decrease in the Wufeng formation to Long-11 subsection deep shale, while clay mineral content increases, which corresponds to the change in sedimentary environment from anoxic to oxidizing environment. Unexpectedly, the total pore volume of deep shale does not decrease with the increase in burial depth but increases first and then decreases. Using total organic carbon (TOC), siliceous mineral content showed a good correlation with total pore volume and specific surface area, with correlation coefficients greater than 0.7, confirming the predominant role of these two factors in controlling the pore structure of deep shales. This is mainly because the Longmaxi shale is already in the late diagenetic stage, and organic matter pores are generated in large quantities. Clay minerals have a negative correlation with the total pore volume of shale, and the correlation coefficient is 0.7591. It could be that clay minerals are much more flexible and are easily deformed to block the pores under compaction. In addition, the longitudinal heterogeneity of the deep shale reservoir structure in southern Sichuan is also controlled by the thermal effect of the Emei mantle plume on hydrocarbon generation of organic matter and the development of natural microfractures promoted by multistage tectonic movement. Overall, the complex microstructure in the deep shales of the Longmaxi Formation in the southern Sichuan Basin is jointly controlled by multiple effects, and the results of this research provide strong support for the benefit development of deep shale gas in southern Sichuan Basin.

The Effect of Thermal Maturity on the Pore Structure Heterogeneity of Xiamaling Shale by Multifractal Analysis Theory: A Case from Pyrolysis Simulation Experiments

Shale oil and gas, as source-reservoir-type resources, result from organic matter hydrocarbon generation, diagenesis, and nanoscale pore during the evolution processes, which are essential aspects of shale gas enrichment and reservoir formation. To investigate the impact of diagenetic hydrocarbons on shale pore heterogeneity, a thermal simulation of hydrocarbon formation was conducted on immature shale from the Middle Proterozoic Xiamaling Formation in the Zhangjiakou area, covering stages from mature to overmature. Nuclear magnetic resonance (NMR) instruments analyzed the microstructure of the thermally simulated samples, and the multifractal model quantitatively assessed pore development and heterogeneity in the experimental samples. The results reveal that the quartz and clay mineral contents show alternating trends with increasing temperature. Organic matter dissolution intensifies while unstable mineral content decreases, promoting clay mineral content development. Pyrolysis intensity influences Total Organic Carbon (TOC), which reduces as hydrocarbons are generated and released during simulation. Porosity exhibits a decreasing–increasing–decreasing trend during thermal evolution, peaking at high maturity. At maturity, hydrocarbon generation obstructs pore space, resulting in higher levels of bound fluid porosity than those of movable fluid porosity. Conversely, high maturity leads to many organic matter micropores, elevating movable fluid porosity and facilitating seepage. Shale pore heterogeneity significantly increases before 450 °C due to the dissolution of pores and the generation of liquid and gas hydrocarbons. In the highly overmature stage, pore heterogeneity tends to increase slowly, correlated with the generation of numerous micro- and nano-organic matter pores.

Thermal and nonthermal effect of microwave irradiation on the pore microstructure and hydrocarbon generation of organic matter in shale

In situ heating conversion is the most effective means to develop medium-low maturity shale oil. Herein, the effects of microwave irradiation on the shale pore structure and hydrocarbon generation of organic matter were studied using an independent development microwave device in combination with Rock-Eval, Leco analysis, X-ray diffraction, field emission scanning electron microscopy, low-temperature N2 adsorption/desorption, nuclear magnetic resonance, Fourier transform infrared spectroscopy and kinetic simulation of hydrocarbon generation. The results show that microwave irradiation can effectively heat shale and the temperature can rise to hundreds of degrees Celsius in a short time (10 min), with 400 °C as the best temperature for in situ conversion of medium-low maturity shale, at which the pore volume and surface area reach the maximum. Microwave irradiation is conducive to pyrolysis and hydrocarbon generation of aliphatic structure in shale organic matter. There are differences in the response of different organic components to the nonthermal effect of microwave irradiation, which reduces the activation energy of hydrocarbon generation during organic matter pyrolysis. Heating shale by microwave irradiation can effectively promote organic matter pyrolysis and hydrocarbon generation, improve rock reservoir and seepage capacity, and is of great significance for the efficient development of shale oil.

Hydrocarbon accumulation model influenced by “three elements (source-storage-preservation)” in lacustrine shale reservoir-A case study of Chang 7 shale in Yan’an area, Ordos Basin

The organic-rich shales of the Chang 7 Member in the Yan’an Formation of the Yan’an area, Ordos Basin is a hot spot for lacustrine shale gas exploration. In this paper, taking the Chang 7 Member shale in the Yan’an area as an example, the main controlling factors of lacustrine shale gas accumulation and the prediction of “sweet spots” are systematically carried out. The results show that the Yanchang Formation shale has the complete gas generating conditions. Shale gas accumulation requires three necessary accumulation elements, namely gas source, reservoir and good preservation conditions. The dynamic hydrocarbon generation process of the Chang 7 shale reservoirs is established according to the thermal simulation experiments of hydrocarbon generation, and the mechanism of catalytic degradation and gas generation in the Chang 7 Member under the background of low thermal evolution degree is revealed. The enriched authigenic pyrite can catalyze the hydrocarbon generation of organic matter with low activation energy, thereby increasing the hydrocarbon generation rates in the low-mature-mature stage. Different types of pores at different scales (2–100 nm) form a multi-scale complex pore network. Free gas and dissolved gas are enriched in laminar micro-scale pores, and adsorbed gas is enriched in nano-scale pores of thick shales, and silty laminates can improve the physical properties of the reservoir. This is because the laminar structure has better hydrocarbon generation conditions and is favorable for the migration of oil and gas molecules. The thickness of the lacustrine shale in the Chang 7 Member is between 40 and 120 m, which has exceeded the effective hydrocarbon expulsion thickness limit (8–12 m). At the end of the Early Cretaceous, the excess pressure of the Chang 7 shale was above 3 MPa. At present, horizontal wells with a daily gas production of more than 50,000 cubic meters are distributed in areas with high excess pressures during the maximum burial depth.

Genetic Mechanism of Pyrite in the Shale of the Longmaxi Formation and Its Influence on the Pore Structure: A Case Study of the Changning Area, South Sichuan Basin of SW China

Pyrite is a mineral that is commonly found in shale gas reservoirs. Its genetic mechanism and impact on pore and organic matter in shale gas reservoirs are critical for shale gas exploration. This study selects the Longmaxi shale (Lower Silurian) in the Changning area of the southern Sichuan Basin by comprehensively using a scanning electron microscope (SEM), X-ray diffraction (XRD), total organic carbon (TOC), and image processing technology. The type and characteristics of pyrite in shale reservoirs are studied, the sedimentary environment and genetic mechanism of pyrite are analyzed, and the influence of pyrite formation on organic matter enrichment and reservoir formation is evaluated. The results showed that pyrite in shale primarily forms framboidal pyrite, euhedral pyrite, and subhedral pyrite, with particle sizes ranging from 1 to 15 μm. The maximum framboid diameter (MFD) is less than 20 μm, with the average particle size distribution of 3–5 μm. These parameters indicate the vulcanization and blocking environment. The reducing environment promotes organic matter enrichment and preservation. Framboidal pyrite has two genetic sequences: rich organic matter and poor organic matter. The development of organic matter will limit the continuous radial growth of pyrite and is conducive to the protection of pores, and the formation of pyrite can reduce the activation energy of kerogen reaction and catalyze the hydrocarbon generation of organic matter, resulting in higher gas content. The framboidal pyrite content can be used to predict high-quality shale gas reservoirs.

Effect of Volcanic Events on Hydrocarbon Generation of Lacustrine Organic-Rich Shale: An Example of the Upper Triassic Galedesi Formation in the Hala Lake Depression, South Qilian Basin, China

The thermal evolution process of organic matter is associated with the complete hydrocarbon generation and expulsion process in shale, however, the thermal evolution of organic matter is a long process and cannot be realized without experimental simulations. Although several scholars have substantially studied the thermal evolution of organic matter, it remains a challenging and much debated issue in the studies of organic geochemistry. Volcanic events are crucial in the enrichment of organic matter, and appropriate heating accelerates the thermal evolution of organic matter. However, how strong-rock baking restricts the evolution of organic matter in shale has not been specifically studied. The South Qilian Basin in China is a typical superimposed basin where complex tectonic movements have induced multiple volcanic events, which makes it a favorable location to perform the aforementioned research. This study used the Galedesi Formation shale in the Hala Lake Depression of the South Qilian Basin as an example for investigating the constraints of the volcanic events related to the thermal evolution of organic matter by integrating the results obtained using the geochemical and petrological methods. Our results demonstrate that the lacustrine Galedesi Formation shale of the Hala Lake Depression in the Late Triassic is a typical deep-lake facies deposit with good hydrocarbon generation potential. However, because of the influence of regional tectonic evolution, the burial depth of shale is not deep and the thermal evolution of organic matter is insufficient. Due to the influence of multiple volcanic thermal events in the later stages, the thermal maturity of organic matter in the Galedesi Formation shale generally exceeds 3.0%, which is abnormally high. The apparent carbonization of organic matter can be observed via scanning electron microscopy. Rapid magma baking typically cannot effectively promote the hydrocarbon generation of shale organic matter. Finally, the burial depth of lacustrine shale of the Galedesi Formation in the Hala Lake Depression of South Qilian Basin is too shallow. Organic matter hydrocarbon generation and later shale preservation conditions are not conducive to the enrichment, accumulation, exploration, and development of shale gas.

Diagenesis and Its Impact on the Reservoir Quality of Continental Shales: A Case Study of the Lower Jurassic Da’anzhai Member of the Ziliujing Formation in the Sichuan Basin, China

The Jurassic continental shale oil (gas) is a favorable unconventional resource in the Sichuan Basin of China. In this paper, analysis methods such as core and outcrops observation, thin section identification, scanning electron microscopy (SEM), and X-ray diffraction (XRD) are used to describe the characteristics of lacustrine fine-grained rock reservoir in Da’anzhai Member of the Ziliujing Formation in Sichuan Basin, and it discussed the influence and control of lacustrine fine-grained rock diagenesis on the quality of the reservoir, in order to determine the shale reservoir control factors. The results show that there are three types of rocks in the Da’anzhai Member, which are mudstone, siltstone, and limestone, and the rock combination of shell shale intercalated with shell limestone is developed in the Da 2 submember. Fine-grained sedimentary rocks in this section have undergone compaction, cementation, dissolution, metasomatism, transformation of clay minerals, and hydrocarbon generation of organic matter, which are currently in the middle diagenetic stage A or B substage. Compaction and cementation are the main factors that control the physical use of shale and limestone, and strong cementation is the main reason for the tightness of limestone reservoirs in the Da’anzhai Member in the northern and eastern parts of Sichuan Basin. The difference between dissolution and cementation is the main control factor for the formation of limestone or argillaceous limestone reservoirs in the same section in central Sichuan Basin. The organic-rich shale and shell shale of the Yuanba and Fuling area are the most favorable reservoirs of the Ziliujing Formation in this region. The shell limestone that experienced favorable dissolution in the Da’anzhai Member in the central of the Sichuan Basin has become a limestone reservoir. Discussing the impact of continental fine-grained rock diagenesis on the reservoir can better explore and develop similar intervals, enrich unconventional shale oil and gas accumulation and storage theories, and provide basic theoretical support for finding favorable unconventional shale oil and gas reservoirs.

Evaluation of the Accumulation Conditions and Favorable Areas of Shale Gas in the Upper Palaeozoic Marine-Continental Transitional Facies in the Daning-Jixian Area, Ordos Basin

This study focuses on the organic-rich mud shale in the Upper Palaeozoic transitional facies in the southeastern margin of the Ordos Basin. It systematically analyzes the shale gas accumulation conditions of the organic-rich mud shale in the Lower Permian Shanxi-Taiyuan Formation, including the thickness, distribution, organic matter type and content, thermal maturity, reservoir space, gas-bearing property, and rock brittleness. The results show that the thick dark mud shale contains a high organic matter content, is a suitable kerogen type for gas generation, and exhibits moderate thermal evolution, providing excellent conditions for hydrocarbon accumulation. Residual primary pores formed by shale compaction, secondary pores formed by organic matter hydrocarbon generation, clay mineral transformation and dissolution, and fractures provide suitable reservoir spaces for shale gas. The shale in the study area has a higher gas content than the shale strata in the marine basins of the United States. In addition, the content of brittle minerals such as quartz is higher, and Poisson’s ratio is lower, facilitating the subsequent transformation. The accumulation conditions indicate the high potential of the study area for shale gas exploration and development. The geological analogy method is used to compare the study area with five major shale gas basins in the United States. The results indicate that the shale gas resources of the Shanxi-Taiyuan Formation in the study area are in the range of 2800– 3200 × 10 8 m3. The primary controlling factors affecting shale gas reservoirs in this area are the abundance of organic matter, thermal maturity, shale thickness, and quartz content. Favorable areas are predicted based on these factors.

Dominant products and reactions during organic matter radiolysis: Implications for hydrocarbon generation of uranium-rich shales

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–CH3 rupture contributed to H2 and CH4 generation, respectively. O-containing group cleavage caused CO2 generation, and O2 presence could enhance the oxidation of OM by joining in the structure of OM or by facilitating the formation of new oxides. H2O addition could not only provide O, enhancing the oxidation of OM, but could also provide H, thus promoting H2 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.

Influence of different thermal evolution stages on shale pore development

ABSTRACT Different thermal evolution degrees on pore development were studied by cryogenic nitrogen adsorption experiment and microscopic photometric analysis. The organic matter Ro, the pore volume, and the proportion of different pore types are different in the Qingshankou Formation, Longmaxi Formation, and Niutitang Formation. However, the relationship between pore development and thermal evolution is consistent. With increasing the degree of the thermal evolution of organic matter holes appear three different change rules. When the organic matter Ro is less than 1.2, shale porosity is mainly influenced by physical compaction and cementation, shale organic hole filling. With the increase of Ro, the porosity tends to decrease, and when Ro is between 1.2 and 3.0, the organic matter of pyrolysis hydrocarbon generation and organic matter hydrocarbon generation hole and corrosion hole a lot of development, the stage total shale porosity is lower before the rising trend, organic matter porosity increased with the increase of Ro. When Ro is greater than 3.0 (overmaturity stage), graphitization occurs. Under the influence of deep burial and slow compaction, the porosity is decreasing continuously.

The time-temperature-maturity relationship: A chemical kinetic model of kerogen evolution based on a developed molecule-maturity index

Kerogen maturity is an important indicator for evaluating source rocks. We propose a kerogen maturity index (molecule-maturity index, MMI) based on the changes in the molecular structure of organic matter during thermal evolution. The MMI has a positive correlation with vitrinite reflectance. Molecular dynamics simulations indicate that the weight loss is with a positive linear correlation with the MMI, demonstrating the ability of the index to reflect the hydrocarbon generation of organic matter. In addition, a new conversion is introduced by the MMI to analyze the kinetics of kerogen thermal evolution. The activation energy is expressed as a continuous function of the MMI linking by the new conversion. The activation energy was calculated at the isoconversional points of previous conversion, which was not defined as a continuous function before. A linear relationship between the activation energy of kerogen is observed, and a new chemical kinetic model of kerogen thermal maturity is established based on the MMI (kinetic model of the maturity evolution, MEKM). The MEKM equation has a simple form and is convenient for engineering applications. Our findings provide insights into the kinetics of kerogen thermal maturation and guide the choice of temperature and time for artificial maturity.

Kinetic simulation of hydrocarbon generation and its application to in-situ conversion of shale oil

Abstract The kinetic parameters of hydrocarbon generation are determined through experimental simulation and mathematical calculation using four typical samples selected from the Cretaceous Nenjiang Formation in the northwest of Songliao Basin, Chang 7 Member of Triassic Yanchang Formation in the southwest of Ordos Basin, Paleogene in the southwest of Qaidam Basin, and Lucaogou Formation of Jimusar Sag in the east of Junggar Basin. The results show that activation energy of hydrocarbon generation of organic matter is closely related to maturity and mainly ranges between 197 kJ/mol and 227 kJ/mol. On this basis, the temperature required for organic matter in shale to convert into oil was calculated. The ideal heating temperature is between 270 °C and 300 °C, and the conversation rate can reach 90% after 50—300 days of heating at constant temperature. When the temperature rises at a constant rate, the temperature corresponding to the major hydrocarbon generation period ranges from 225 to 350 °C at the temperature rise rate of 1—150 °C/month. In order to obtain higher economic benefits, it is suggested to adopt higher temperature rise rate (60—150 °C/month). The more reliable kinetic parameters obtained can provide a basis for designing more reasonable scheme of in-situ heating conversion.

Pore Structure and Fractal Characteristics of Distinct Thermally Mature Shales

To investigate the shale pore structure and fractal characteristics during thermal evolution, 30 shale samples from two wells with contrasting maturities (the Ro values of Yanchang samples are 0.71–1.06% and the Ro values of Longmaxi samples are 2.79–3.41%) were subjected to field emission-scanning electron microscopy observation, low-pressure N2 adsorption experiment, and other analytical measurements. The Frenkel–Halsey–Hill model was used to obtain the double-fractal dimensions (D1 and D2) of shale pores for different suits of samples. Analysis shows that the shale pores undergo a complicated evolution process with the increase of thermal maturity. Hence, the evolution trends of pore structure parameters with distinct thermal maturity are not in the same trend. The pore volume and porosity have a trend of increasing first and then decreasing with increase of Ro, which is mainly related to hydrocarbon generation of organic matter (OM). With the increase of Ro, the average pore size and specific surface ...

Influence on the oil‐gas accumulation potential of the laminated algal micritic dolomite in Jixian system from Mesozoic magmatic activities at the south‐western margin of the Ordos Basin, China

The Mesoproterozoic Jixian System is widely distributed at the south‐western margin of the Ordos Basin. It is mainly dominated by siliceous laminated algal micritic dolomite that is a set of potential carbonate source rocks. The magmatic activities at the south‐western margin of the Ordos Basin were frequent and intensive, and the Tongcheng intrusion exposed in this area has an intrusive contact relationship with the Jixian System. This paper uses geochemical methods to study the influence of magmatism to lithology, physical properties, hydrocarbon generating property, and geochemistry of the Jixian System. Analysis data reveal that, with the increasing distance to the intrusion, the porosity, permeability, and content of dolomite, trace elements, and rare earth elements in surrounding rock decrease while the density and silica content increase gradually. In addition, the parameters have a weakening trend about adsorbed hydrocarbon (S1), pyrolysis hydrocarbon (S2), hydrocarbon generating potential (S1 + S2), and pyrolysis peak temperature (Tmax). The results suggest that the magmatic hydrothermalism and the thermal effect of magmatic intrusion changed the physical and geochemical characteristics of the surrounding rock and promoted the hydrocarbon generation of organic matter. However, these characteristics reflect that the magmatic activities only have a relatively small effect on the laminated algal micritic dolomite in Jixian System without a destructive effect at the south‐western margin of the Ordos Basin. The petroleum potential of the Jixian System is worthy to be further explored.

Diagenetic Process and Pore Evolution of Jurassic Continental Shale in Tarim Basin, Northwest China

The Jurassic shale in the Tarim Basin, as one of the important source rocks within the basin, is identified as a set of typical continental sedimentation with big hydrocarbon-generating potential. The diagenetic process and pore evolution characteristics of the Jurassic shale were studied based on geochemical and reservoir bed properties analyses and scanning electron microscope (SEM) observation. The results show that the Jurassic shale shows low-porosity and low-permeability, the mineral composition is mainly of clay mineral and quartz; the shale underwent three diagenetic evolutionary stages, namely syngenetic to Period A of early diagenetic stage, Period B of early diagenetic stage and Period A of middle diagenetic stage; during this process, the shale underwent a series of changes such as hydrocarbon generation of organic matter, conversion from original minerals to illite, formation of authigenic minerals, recrystallization and corrosion; meanwhile, porosity and permeability also underwent the evolutionary processes including rapid reduce of primary pore and development of secondary pore. The pore development within shale is mainly controlled from three aspects, namely material composition of rock, diagenesis and later tectonic reworking.

A method of calculating hydrocarbon generation history - hydrogen index method (TTPCI-IH method)

The traditional TTI-Ro method uses virtinite reflectance as the indicator of the organic matter maturity, and then makes it a dominant parameter during the simulation of organic matter hydrocarbon generation history. However, this method only takes virtinite reflectance as a function of time and temperature, but ignores not only the influence of pressure and catalyst, but also the follow-up generation contradiction. Actually, this method is difficult to correctly describe the organic matter hydrocarbon generation history. By a variety of analysis and comparison, hydrogen index (IH) is found to be the parameter with the above characteristics. IH is also the synthetical effect result of time, temperature, pressure and catalyst and their dynamic change. So it is appropriate to use IH, instead of Ro, as the dominant parameter during the hydrocarbon generation history of organic matter. Then it is possible to establish a method on the basis of the hydrogen index to calculate the hydrocarbon generation history – the comprehensive simulation method of time, temperature, pressure, catalyst and index, called TTPCI-IH method. And it is successfully applied in hydrocarbon generation history simulation. The actual simulation results in Shengli oilfield show that TTPCI-IH method is obviously better than TTI-Ro method.

Classification and origin of natural gases from Lishui Sag, the East China Sea Basin

Natural gases discovered up to now in Lishui Sag, the East China Sea Basin, differ greatly in gaseous compositions, of which hydrocarbon gases amount to 2%–94% while non-hydrocarbon gases are dominated by CO2. Their hydrocarbon gases, without exception, contain less than 90% of methane and over 10% of C2+ heavier hydrocarbons, indicating a wet gas. Carbon isotopic analyses on these hydrocarbon gases showed that δ13C1, δ13C2 and δ13C3 are basically lighter than −44‰, −29‰ and −26‰, respectively. The difference in carbon isotopic values between methane and ethane is great, suggesting a biogenic oil-type gas produced by the mixed organic matter at peak generation. δ13\( C_{CO_2 } \) values of nonhydrocarbon gases are all heavier than −10‰, indicating a typical abiogenic gas. The simulation experiment on hydrocarbon generation of organic matter in a closed gold-tube system showed that the proportion of methane in natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit is obviously higher than that in natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit, consequently the proportion of heavier hydrocarbons of the former is remarkably lower than that of the latter. Moreover, δ13C1 values of natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit are about 5‰ heavier than those of natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit while δ13C2 and δ13C3 values of the former are over 9‰ heavier than those of the latter. Currently the LS36-1 oil-gas pool is the only commercial oil-gas reservoir in Lishui Sag, where carbon isotopic compositions of various hydrocarbon components differ greatly from those of natural gases produced by the Lingfeng Formation organic matter but are very similar to those of natural gases derived from the Yueguifeng Formation organic matter, therefore, natural gases in the LS36-1 oil-gas pool are mainly derived from the Yueguifeng Formation lacustrine source rock rather than the Lingfeng Formation marine or Mingyuefeng Formation coal-measures source rocks.