Characteristics and Origins of the Natural Gas and Implications for Gas-Source Correlation in Deep Formations of the Songliao Basin, NE China

: The Songliao Basin is the most productive petroliferous lacustrine basin in NE China, and numerous large gas ﬁelds with large proven reserves occur in its deep formations. However, considerable challenges remain: (1) the origins and genetic types of the natural gases are controversial; (2) the gas-source correlations are poorly studied; and (3) the migration distance is vague. In this study, these problems are addressed by the study of the gas compositions, light hydrocarbons, and stable hydrogen and carbon isotopes. The gases are predominantly of organic and thermogenic origins. The Huoshiling (J 3 h) and Shahezi (K 1 sh) gases are mainly mixtures of coal-derived and oil-associated gases and the mixed-sources of primary kerogen degradation and secondary oil cracking, while the Yingcheng (K 1 yc) gases are mainly coal-derived gases and predominantly derived from primary kerogen degradation. The gases in di ﬀ erent sags are derived from the source rocks developed in the same sags where the gases accumulated, characterized by the proximal-source accumulation. Vertically, the gases in the J 3 h and K 1 sh are predominantly sourced by the proximal J 3 h and K 1 sh mudstones, while the gases in the K 1 yc are mainly derived from either the J 3 h or the K 1 sh source rocks, suggesting the gas migration with short distances.


Introduction
Natural gases in sedimentary basins contain hydrocarbons (e.g., methane and its higher homologues), non-hydrocarbons (e.g., CO 2 , N 2 , H 2 , H 2 S, and Hg), and noble gases (e.g., He and Ar). Generally, hydrocarbon gases are proved to be the result of organic matter conversion by thermal As a result of synrift extension, the SLB in the synrift stage was characterized by "basin-andrange" fault blocks [56]. The Dehui Depression, located in the uplift zone of southeastern SLB, is NE trending and fault-controlled, with an area of 4053 km 2 . The tectonic evolution stages of the Dehui Depression are approximately consistent with those of the SLB. Tectonic movements of multiepisodes have resulted in complex internal structures, and the Dehui Depression contains seven sags: the Nong'an sag, the Huajia sag, the Baojia sag, the Nong'annan sag, the Helong sag, the Longwang sag, and the Lanjia sag ( Figure 1).

Stratigraphy and Lithology
As a consequence of mantel upwelling and doming, the Dehui Depression evolved into the Mesozoic rifting and subsidence, including the synrift subsidence and post-rift thermal subsidence. Highlands located in the west and south were the main sediment source areas [86]. The succession deposited during the synrift subsidence includes the Huoshiling (156-145Ma), Shahezi (145-133.9Ma), Yingcheng (133.9-125Ma), and Denglouku (125-116Ma) Formations [56,86] (Figure 2). The Huoshiling-Yingcheng Formations (H-YFs) include clastic floodplain, fluvial, delta, and lacustrine rocks intercalated with pyroclastic and volcanic rocks [56,86], consisting of volcanic, conglomerates, fine sandstones, mudstones, coals and tuff interlayers ( Figure 2). The thickness of the H-YFs is As a result of synrift extension, the SLB in the synrift stage was characterized by "basin-and-range" fault blocks [56]. The Dehui Depression, located in the uplift zone of southeastern SLB, is NE trending and fault-controlled, with an area of 4053 km 2 . The tectonic evolution stages of the Dehui Depression are approximately consistent with those of the SLB. Tectonic movements of multi-episodes have resulted in complex internal structures, and the Dehui Depression contains seven sags: the Nong'an sag, the Huajia sag, the Baojia sag, the Nong'annan sag, the Helong sag, the Longwang sag, and the Lanjia sag ( Figure 1).

Samples and Methods
Firstly, the gases were flushed for 15-20 minutes to remove air contamination, then gas samples were collected directly from the wellheads. Stainless steel cylinders (Φ = 10.0 cm) equipped with two shut-off valves (P max = 22.5 MPa) were utilized to contain the gas samples. The inside pressure of Energies 2019, 12,4641 5 of 27 the container should be maintained over 5.0 MPa. After the collection, the cylinders were subjected to leakage checks in water. Thirty-six samples from 11 wells in H-YFs were collected for molecular composition analysis. Twenty-seven samples from 11 wells in H-YFs were collected for stable carbon isotope analysis. Two samples from Well DS12 in the J 3 h and K 1 sh Formations were analyzed for stable hydrogen isotopes. Eleven mudstone samples from four wells and 23 gas samples from eight wells are collected out for light hydrocarbon analysis.

Analysis of Molecular Composition
A Trace GC Ultra gas chromatograph (GC) (Thermo Fisher, USA) equipped with flame ionization and thermal conductivity detectors was utilized to ascertain the gas chemical components. The C 1 -C 4 components were isolated utilizing a porous-layer open-tabular capillary column (PLOT, Al 2 O 3 50 m×0.53 mm). The carrier gas was helium (He, flowrate=1 mL/min). At the beginning, the GC oven temperature was 30 • C and kept there for 10 minutes, then raised to 180 • C (rate of 10 • C/min) and held for 20-30 minutes [87][88][89][90][91][92][93].

Analysis of Stable Carbon and Hydrogen Isotope
A Finnigan Mat Delta S mass spectrometer (Thermo Fisher, USA) interfaced with a HP 5890II chromatograph (HP, USA) can be used to ascertain the stable carbon isotopes. A gas chromatograph (a fused silica capillary column (PLOT Q 30 m×0.32 mm)), with helium as the carrier gas (1 mL/min) was utilized to separate the gas components (C 1 -C 4 and CO 2 ). The gases were converted into CO 2 in a combustion interface, then transferred into the mass spectrometer. The GC oven temperature was initially heated from 35 • C to 80 • C (8 • C/min), then heated up to 260 • C (5 • C/min) and held 10 minutes. Gas samples were analyzed in duplicate. The stable carbon isotope values are recorded in the customary "δ" notation in per mil (% ) relative to the Vienna Pee Dee Belemnite (VPDB) standard. The value of ± 0.5% should be the reproducibility and analytical precision with respect to the VPDB standard [87][88][89][90][91][92][93]. A Finnigan MAT 253 mass spectrometer with the GC-TC-IRMS method was used to ascertain the stable hydrogen isotope. A chromatographic column (HP-PLOTQ column, 30 m × 0.32 mm × 20 mm) was used to separate the gas components. The accuracy should be ±3% , and stable hydrogen isotope results are recorded relative to Vienna Standard Mean Ocean Water (VSMOW) [87][88][89][90][91][92][93].

Analysis of Light Hydrocarbons
A HP5890A gas chromatograph and a PONA capillary column (50 m × 0.25 mm × 0.5 µm) were utilized to analyze the light hydrocarbons. Helium was used as the carrier gas. A container with liquid nitrogen was used to trap the low content of light hydrocarbons. The liquid nitrogen container can be removed after a large volume of gas (15-20 mL) was injected after 20 minutes. Utilizing a FID at 320 • C, the eluted light hydrocarbons were analyzed. At the beginning, the temperature was held at 30 • C for 10 minutes, then heated up to 70 • C (1 • C/min), and then raised to 160 • C (3 • C/min), and finally heated up to 270 • C (5 • C/min) and maintained 20 minutes. An Agilent PONA gas chromatograph was utilized to analyze the light hydrocarbons qualitatively. 53 individual compounds (from isobutane to n-octane) were tested in the experiment. The peak areas of individual compounds on the gas chromatograph were utilized to quantify the light hydrocarbon compounds [87][88][89][90][91][92][93].

Stable Hydrogen and Carbon Isotopes
The stable carbon isotope values for gases in the H-YFs of the Dehui Depression are shown in Figure 4 and Table 2  The stable hydrogen isotope values of CH 4 in the J 3 h and K 1 sh are −211% and −216% , respectively, and those of ethane in the two formations are −213% and −215% , respectively (Table 3).

Gas Origins
The gas origins and the deposition environments of its parent material can be favorably identified utilizing gas chemical compositions and stable hydrogen and carbon isotopes [1,42,[78][79][80][81][82]94]. Generally, a gas can be identified as organic or inorganic type. Dai et al. [41] proposed that: a) for the organic origin, the CO 2 content is less than 15% and δ 13 C CO2 value is <−10% ; b) for the inorganic origin, the CO 2 content is over 60% and δ 13 C CO2 value is >−8% . Moreover, the thermogenic gas is predominantly characterized by δ 13 C 1 <δ 13 C 2 <δ 13 C 3 <δ 13 C 4 (called normal carbon isotopic distribution pattern), while the negative carbon isotopic distribution pattern (δ 13 C 1 >δ 13 C 2 >δ 13 C 3 >δ 13 C 4 ) generally indicates the inorganic type of natural gas [3,46,95,96]. In the Dehui Depression, the CO 2 contents and δ 13 C CO2 values of the deep natural gas are mainly less than 5% and lower than −10% , respectively, and the carbon isotopic distribution patterns of the gas hydrocarbons predominantly show the normal patterns, indicating that the origins of the deep natural gas are predominantly organic sources. However, as shown in Table 1 and Figure 5, parts with reversed δ 13 C series, including δ 13 C 1 <δ 13 C 2 <δ 13 C 4 <δ 13 C 3 , δ 13 C 1 <δ 13 C 3 <δ 13 C 2 and δ 13 C 2 <δ 13 C 1 <δ 13 C 3 , exist in the study area, suggesting that secondary processes may have happened to the gas.

Gas Origins
The gas origins and the deposition environments of its parent material can be favorably identified utilizing gas chemical compositions and stable hydrogen and carbon isotopes [1,42,[78][79][80][81][82]94]. Generally, a gas can be identified as organic or inorganic type. Dai et al. [41] proposed that: a) for the organic origin, the CO2 content is less than 15% and δ 13 CCO2 value is <−10‰; b) for the inorganic origin, the CO2 content is over 60% and δ 13 CCO2 value is >−8‰. Moreover, the thermogenic gas is predominantly characterized by δ 13 C1<δ 13 C2<δ 13 C3<δ 13 C4 (called normal carbon isotopic distribution pattern), while the negative carbon isotopic distribution pattern (δ 13 C1>δ 13 C2>δ 13 C3>δ 13 C4) generally indicates the inorganic type of natural gas [3,46,95,96]. In the Dehui Depression, the CO2 contents and δ 13 CCO2 values of the deep natural gas are mainly less than 5% and lower than −10‰, respectively, and the carbon isotopic distribution patterns of the gas hydrocarbons predominantly show the normal patterns, indicating that the origins of the deep natural gas are predominantly organic sources. However, as shown in Table 1 and Figure 5, parts with reversed δ 13 C series, including δ 13 C1<δ 13 C2<δ 13 C4<δ 13 C3, δ 13 C1<δ 13 C3<δ 13 C2 and δ 13 C2<δ 13 C1<δ 13 C3, exist in the study area, suggesting that secondary processes may have happened to the gas.  According to the gas parent materials, the oil-associated gas and coal-derived gas should be the two types. The δ 13 C 2 and δ 13 C 3 mainly reflect the inheritance of parent material, which can be utilized to differentiate between coal-derived gas and oil-associated gas [1,[97][98][99]. It is favorably effective to use this method for gas derived from a single source, however, it is more complicated to identify the mixed sources of gas [42,94,100,101]. According to a number of analyses in China, Dai et al. [42] pointed out that gases derived from humic and sapropelic kerogens are characterized by the δ 13 C 2 > −27.5% and δ 13 C 2 < −29.0% , respectively, while the gases of mixed origins are featured by δ 13 C 2 values between −27.5% and −29.0% . The cross-plots of δ 13 C 1 -δ 13 C 2 show that the J 3 h gases are mainly oil-associated gas derived from sapropelic kerogens, with a bit of coal-derived gas, and the K 1 sh gases are mixed-sources of coal-derived and oil-associated gases, while the K 1 yc gases are predominantly coal-derived gas derived from the humic kerogens ( Figure 6). Moreover, according to the δ 13 C 1 -δ 13 C 2 -δ 13 C 3 cross-plots for the discrimination of coal-derived and oil-associated gases proposed by Dai [3], the results are similar that the gases in the J 3 h and K 1 sh are mainly the mixed-sources of coal-derived and oil-associated gases, while the K 1 yc gases are predominant coal-derived gases (Figure 7). According to the gas parent materials, the oil-associated gas and coal-derived gas should be the two types. The δ 13 C2 and δ 13 C3 mainly reflect the inheritance of parent material, which can be utilized to differentiate between coal-derived gas and oil-associated gas [1,[97][98][99]. It is favorably effective to use this method for gas derived from a single source, however, it is more complicated to identify the mixed sources of gas [42,94,100,101]. According to a number of analyses in China, Dai et al. [42] pointed out that gases derived from humic and sapropelic kerogens are characterized by the δ 13 C2 > −27.5‰ and δ 13 C2 < −29.0‰, respectively, while the gases of mixed origins are featured by δ 13 C2 values between −27.5‰ and −29.0‰. The cross-plots of δ 13 C1-δ 13 C2 show that the J3h gases are mainly oilassociated gas derived from sapropelic kerogens, with a bit of coal-derived gas, and the K1sh gases are mixed-sources of coal-derived and oil-associated gases, while the K1yc gases are predominantly coal-derived gas derived from the humic kerogens ( Figure 6). Moreover, according to the δ 13 C1-δ 13 C2δ 13 C3 cross-plots for the discrimination of coal-derived and oil-associated gases proposed by Dai [3], the results are similar that the gases in the J3h and K1sh are mainly the mixed-sources of coal-derived and oil-associated gases, while the K1yc gases are predominant coal-derived gases (Figure 7).   According to the gas parent materials, the oil-associated gas and coal-derived gas should be the two types. The δ 13 C2 and δ 13 C3 mainly reflect the inheritance of parent material, which can be utilized to differentiate between coal-derived gas and oil-associated gas [1,[97][98][99]. It is favorably effective to use this method for gas derived from a single source, however, it is more complicated to identify the mixed sources of gas [42,94,100,101]. According to a number of analyses in China, Dai et al. [42] pointed out that gases derived from humic and sapropelic kerogens are characterized by the δ 13 C2 > −27.5‰ and δ 13 C2 < −29.0‰, respectively, while the gases of mixed origins are featured by δ 13 C2 values between −27.5‰ and −29.0‰. The cross-plots of δ 13 C1-δ 13 C2 show that the J3h gases are mainly oilassociated gas derived from sapropelic kerogens, with a bit of coal-derived gas, and the K1sh gases are mixed-sources of coal-derived and oil-associated gases, while the K1yc gases are predominantly coal-derived gas derived from the humic kerogens ( Figure 6). Moreover, according to the δ 13 C1-δ 13 C2δ 13 C3 cross-plots for the discrimination of coal-derived and oil-associated gases proposed by Dai [3], the results are similar that the gases in the J3h and K1sh are mainly the mixed-sources of coal-derived and oil-associated gases, while the K1yc gases are predominant coal-derived gases (Figure 7).   Besides, the δ 2 D CH4 value can be an indicator to determine the deposition environment (δ 2 D CH4 < −190% for marine and lacustrine brackish environments and δ 2 D CH4 > −170% for terrigenous fresh water environments [82,102]), while the δ 2 D C2H6 value can be used to ascertain the parent material types [81,103,104]. The hydrogen isotope distribution of the J 3 h gas shows a normal pattern, while that the K 1 sh gas shows a negative pattern, indicating that the gases in the K 1 sh may be affected by bacterial action or be mixed-sources of coal-derived and oil-associated gases. Moreover, the gases in the deep formations of Dehui Depression are produced by both terrigenous and lacustrine organic matters.
Generally, the relative contents of heptane (n-C 7 ), MCH, and DMCP are useful parameters to analyze the gas origins [88,[105][106][107]. Heptane (n-C 7 ) from algae and bacteria is sensitive to thermal maturation. As a result of a high thermodynamic stability, methylcyclohexane (MCH), mainly derived from components of terrestrial higher plants, is a good gas origin indicator, suggesting that a gas with a high MCH content should be the typical humic-type [106]. On the contrary, a gas with a high abundance of dimethylcyclopentane (DMCP), mainly sourced by aquatic microorganisms, is mainly considered to be of sapropelic-type [108]. In China, the ternary chart of n-C 7 , MCH and DMCP has been broadly utilized to differentiate the coal-derived and oil-associated gases [106,108]. The J 3 h and K 1 sh gases are mainly mixed sources of coal-derived and oil-associated gases, while the K 1 yc gases are predominantly coal-derived gases, as shown in Figure 8. Consequently, natural gases in the deep formations of Dehui Depression are predominantly of organic origin. The J 3 h and K 1 sh gases are predominantly mixed-sources of coal-derived and oil-associated gases and the proportion of oil-associated gas in the J 3 h is larger, while the K 1 yc gases are predominantly coal-derived.

Genetic Types of Gas Hydrocarbons
Hydrocarbon gases are predominantly viewed to be from organic matter conversion by kerogen thermal-degradation, oil and gas cracking, and even bacterial action [1][2][3][4][5][6][7][8][9][10]. Generally, the δ 13 C 1 values and dryness coefficients, mainly affected by thermal maturity, can be as the effective indexes of gas thermal maturity [42,94,109]. Bernard et al. [110] proposed using the cross-plots of C 1 /(C 2 + C 3 )-δ 13 C 1 to distinguish the gas origins of both kerogen type and thermal maturity. As shown in Figure 9a,b, the majority of the gases in the H-YFs belong to the thermogenic gas, and the parent materials of the J 3 h and K 1 sh gases are the mixed-sources of kerogen type II (sapropelic-humic) and type III (humic), while those of the K 1 yc gas are mainly kerogen III (humic).  [45,110]) and (b) is the cross-plots of δ 13 C 2 −δ 13 C 3 vs. C 2 /C 3 (modified after [111]).
As the extensively studied before, the stable carbon isotopes of hydrocarbon gases have favorable correlations with the thermal maturity of their source rocks [1,40,78,112]. According to the geological observations and numerous statistics in the lacustrine basins of China, Dai [81] has proposed a semi-logarithmic equation between δ 13 C 1 value and vitrinite reflectance (%Ro): (1) Equation (1) is for oil-associated gas and Equation (2) is for coal-derived gas. According to these two equations, the source rock maturity can be estimated ( Table 6). The thermal maturities of the J 3 h gas range from 0.96 %Ro to 2.73 %Ro with a mean of 1.57 %Ro, and those of the K 1 sh gas are 0.78-2.49 %Ro with a mean of 1.49 %Ro, while those of the K 1 yc gas are 0.73-1.77 %Ro with an average of 1.37 %Ro. These indicate that these gases were mainly the products of the mature to over-mature source rocks, predominantly generated by thermal degradation and cracking.
James [113] and Jenden et al. [114] proposed that the values of δ 13 C 3 -δ 13 C 2 and δ 13 C 2 -δ 13 C 1 reduce with the increasing thermal maturity of gas. As shown in Figure 10a, the gases in the H-YFs predominantly belong to the mature gas class (Figure 10a). As shown in Figure 10b, the J 3 h gas is mainly located on the trend line of kerogen type II with 0.6-1.2 %Ro, and the K 1 sh gas is mainly located on the trend lines of kerogen type II and type III with 0.7 %Ro and 0.8 %Ro, respectively, while the K 1 yc gas is mainly located in the trend line of kerogen type III with 0.7-0.9 %Ro, indicating that all the deep gas hydrocarbons are thermogenic and mature gases.    [112,113]) and (b) is the cross-plots of δ 13 C2 vs. δ 13 C3 (modified after [98]).
James [112] and Jenden et al. [113] proposed that the values of δ 13 C3-δ 13 C2 and δ 13 C2-δ 13 C1 reduce with the increasing thermal maturity of gas. As shown in Figure 10a, the gases in the H-YFs predominantly belong to the mature gas class (Figure 10a). As shown in Figure 10b, the J3h gas is mainly located on the trend line of kerogen type II with 0.6-1.2 %Ro, and the K1sh gas is mainly located on the trend lines of kerogen type II and type III with 0.7 %Ro and 0.8 %Ro, respectively, while the K1yc gas is mainly located in the trend line of kerogen type III with 0.7-0.9 %Ro, indicating that all the deep gas hydrocarbons are thermogenic and mature gases.
Prinzhofer and Huc [43] established the cross-plots of ln(C2/C3) vs. ln(C1/C2) to distinguish gases generated from both primary degradation (also called primary cracking) and secondary cracking. The C1/C2 ratios rise gradually in the primary degradation and mainly keep constant in the secondary cracking, while the C2/C3 ratios mainly keep constant in the primary degradation and rise dramatically in the secondary cracking. The gases in the J3h are mainly sourced from kerogen type II and type III and generated by the primary kerogen degradation and secondary oil cracking, and the gases in the K1sh are mainly sourced by kerogen type III and generated by primary kerogen degradation, while the K1yc gases are mainly sourced by kerogen type II and type III and generated by primary kerogen degradation (Figure 11).  ) is the cross-plots of δ 13 C 2 −δ 13 C 1 vs. δ 13 C 3 −δ 13 C 2 (modified after [113,114]) and (b) is the cross-plots of δ 13 C 2 vs. δ 13 C 3 (modified after [98]).
Prinzhofer and Huc [43] established the cross-plots of ln(C 2 /C 3 ) vs. ln(C 1 /C 2 ) to distinguish gases generated from both primary degradation (also called primary cracking) and secondary cracking. The C 1 /C 2 ratios rise gradually in the primary degradation and mainly keep constant in the secondary cracking, while the C 2 /C 3 ratios mainly keep constant in the primary degradation and rise dramatically in the secondary cracking. The gases in the J 3 h are mainly sourced from kerogen type II and type III and generated by the primary kerogen degradation and secondary oil cracking, and the gases in the K 1 sh are mainly sourced by kerogen type III and generated by primary kerogen degradation, while the K 1 yc gases are mainly sourced by kerogen type II and type III and generated by primary kerogen degradation ( Figure 11). Lorant et al. [114] established the cross-plots of C2/C3 vs. δ 13 C2−δ 13 C3 to distinguish gases generated by primary degradation and secondary cracking. The gases in the J3h and K1sh mainly plot in the secondary oil cracking area, with a bit in the primary degradation area, while the gases in the K1yc are located in the primary degradation area. Consequently, the gases in the deep formations of Dehui Depression are predominantly thermogenic types. The gases in the J3h and K1sh are mainly generated by primary kerogen degradation and secondary oil cracking, while the gases in the K1yc are mainly produced by primary kerogen degradation.

Gas-source Correlation
Although many features of the Cretaceous petroleum system of the SLB are known, the gassource correlation in the deep formations are not fully understood. Huang et al. [9], Zhang et al. [72], Chen [77] and Shen and Liang [83] have proposed that the mudstones in J3h, K1sh, and K1yc, with high organic abundance (TOC = ~2.5 wt.%), high thermal maturity (Ro = ~1.5-2.0%) and type II-III kerogens, are the main source rocks for gas in the deep formations of the SLB. The main deficiency in the understanding of the multi-source, multi-reservoir gas system in the deep formations is gassource correlation. Considering the huge lateral extension of the basin and sets of verticallydeveloped source rocks and reservoir rocks, the gas-source correlation is challenging. A recent attempt, which tried to determine possible source rocks for the J3h gases using their chemical components and stable carbon isotopes [77], showed that the J3h gas was mainly sourced by the J3h mudstones. However, the sources of the gases in the K1sh and K1yc are still unknown.
Generally, the gas chemical components and stable carbon isotopes are influenced by the parent materials, thermogenic actions, migration, and bacterial actions. As shown in Figure 3 and Figure 4, the C1 contents and dryness coefficients rise with the increasing burial depth, while the C2+ contents reduce with the increasing burial depth, indicating that the gases in the deep formations were predominantly generated by thermogenic actions and accumulated in/approaching the source rocks. Besides, corresponding to the results, the density of gases in the deep formations decrease with the increasing burial depth. The results suggest that the gases in the deep formations should be mainly sourced by the J3h and K1sh mudstones, or just the J3h mudstones.
The relative contents of light hydrocarbon components with similar boiling points can be the fingerprints to ascertain the gas genetic type and access gas-source correlation. The fingerprint trends of the light hydrocarbons in the mudstones of the J3h and K1sh do not display any obvious differences, while those in the K1yc show some differences ( Figure 12). In Figures 12a-b, the fingerprint trends of the gases in the middle J3h of Well DS11 and the lower J3h of Well DS12 show high coherence with those of the mudstone samples in the J3h of the Huajia Sag, indicating that the gases accumulated in the low-middle J3h are predominantly sourced by the J3h source rocks, corresponding to the results Lorant et al. [111] established the cross-plots of C 2 /C 3 vs. δ 13 C 2− δ 13 C 3 to distinguish gases generated by primary degradation and secondary cracking. The gases in the J 3 h and K 1 sh mainly plot in the secondary oil cracking area, with a bit in the primary degradation area, while the gases in the K 1 yc are located in the primary degradation area. Consequently, the gases in the deep formations of Dehui Depression are predominantly thermogenic types. The gases in the J 3 h and K 1 sh are mainly generated by primary kerogen degradation and secondary oil cracking, while the gases in the K 1 yc are mainly produced by primary kerogen degradation.

Gas-Source Correlation
Although many features of the Cretaceous petroleum system of the SLB are known, the gas-source correlation in the deep formations are not fully understood. Huang et al. [9], Zhang et al. [72], Chen [77] and Shen and Liang [83] have proposed that the mudstones in J 3 h, K 1 sh, and K 1 yc, with high organic abundance (TOC =~2.5 wt.%), high thermal maturity (Ro =~1.5-2.0%) and type II-III kerogens, are the main source rocks for gas in the deep formations of the SLB. The main deficiency in the understanding of the multi-source, multi-reservoir gas system in the deep formations is gas-source correlation. Considering the huge lateral extension of the basin and sets of vertically-developed source rocks and reservoir rocks, the gas-source correlation is challenging. A recent attempt, which tried to determine possible source rocks for the J 3 h gases using their chemical components and stable carbon isotopes [77], showed that the J 3 h gas was mainly sourced by the J 3 h mudstones. However, the sources of the gases in the K 1 sh and K 1 yc are still unknown.
Generally, the gas chemical components and stable carbon isotopes are influenced by the parent materials, thermogenic actions, migration, and bacterial actions. As shown in Figures 3 and 4, the C 1 contents and dryness coefficients rise with the increasing burial depth, while the C 2+ contents reduce with the increasing burial depth, indicating that the gases in the deep formations were predominantly generated by thermogenic actions and accumulated in/approaching the source rocks. Besides, corresponding to the results, the density of gases in the deep formations decrease with the increasing burial depth. The results suggest that the gases in the deep formations should be mainly sourced by the J 3 h and K 1 sh mudstones, or just the J 3 h mudstones.
The relative contents of light hydrocarbon components with similar boiling points can be the fingerprints to ascertain the gas genetic type and access gas-source correlation. The fingerprint trends of the light hydrocarbons in the mudstones of the J 3 h and K 1 sh do not display any obvious differences, while those in the K 1 yc show some differences ( Figure 12). In Figure 12a,b, the fingerprint trends of the gases in the middle J 3 h of Well DS11 and the lower J 3 h of Well DS12 show high coherence with those of the mudstone samples in the J 3 h of the Huajia Sag, indicating that the gases accumulated in the low-middle J 3 h are predominantly sourced by the J 3 h source rocks, corresponding to the results proposed by Shen and Liang (2015) [77]. In Figure 12c,d, about 70% of the total fingerprint parameters of the gases in the upper J 3 h of Well DS12 and the K 1 sh of Well DS15 show similar trends with those of the mudstones in the J 3 h and K 1 sh, suggesting that the gases in the K 1 sh and the upper J 3 h are mainly produced by the mudstones both in the J 3 h and K 1 sh. In Figure 12e,f, less than 50% of the total fingerprint parameters of the gases in the J 3 h of Well DS19 and the K 1 yc of Well DS17 show similar trends with those of the mudstones in the deep formations of the Huajia Sag, indicating that the gases in the Baojia and Nong'annan sags are not sourced by the mudstones of the deep formations in the Huajia Sag. Combined with the geological conditions in Dehui Depression and the gas chemical components and density, the gases in the K 1 yc may be derived from the three sets of source rocks (J 3 h, K 1 sh, and K 1 yc), or one or two of them through the vertical faults shown in Figure 1.   Rooney et al. [44] proposed that if the alkane gas comes from a single source, a linear correlation of carbon number (1/n) vs. δ 13 Cn exists. The slope changes of regression line can ascertain the gasmixing between different types at different thermal-maturity stages, and also indicate the bio-genetic gas and methane seepage [44]. In Figure 5d, the trends of 1/n vs. δ 13 Cn of the J3h alkane gases in the Huajia sag mainly show linear relationships with bits of nonlinear relationships and can be divided into three groups (Groups A, B, and C), indicating the J3h gases may have two types (coal-derived and oil-associated gases) with two sources (maybe both the J3h and the K1sh mudstones). According to the Fig. 5e, the trends of 1/n vs. δ 13 Cn of the alkane gases in the K1sh of the Huajia (Well DS15 and Well N101) and Lanjia (Well DS39-3) sags mainly indicate predominant linear relationships and can be divided into three groups (Groups D, E, and F), and the slopes of the samples in each sag are similar. The Group D is similar with the Group B, and the Group E is similar with the Group A, and  Rooney et al. [44] proposed that if the alkane gas comes from a single source, a linear correlation of carbon number (1/n) vs. δ 13 C n exists. The slope changes of regression line can ascertain the gas-mixing between different types at different thermal-maturity stages, and also indicate the bio-genetic gas and methane seepage [44]. In Figure 5d, the trends of 1/n vs. δ 13 C n of the J 3 h alkane gases in the Huajia sag mainly show linear relationships with bits of nonlinear relationships and can be divided into three groups (Groups A, B, and C), indicating the J 3 h gases may have two types (coal-derived and oil-associated gases) with two sources (maybe both the J 3 h and the K 1 sh mudstones). According to the Figure 5e, the trends of 1/n vs. δ 13 C n of the alkane gases in the K 1 sh of the Huajia (Well DS15 and Well N101) and Lanjia (Well DS39-3) sags mainly indicate predominant linear relationships and can be divided into three groups (Groups D, E, and F), and the slopes of the samples in each sag are similar. The Group D is similar with the Group B, and the Group E is similar with the Group A, and the Group F is similar with the Group C. Combined with the gas origins and genic types studied above (Sections 5.1 and 5.2), these three groups (Group D, E, and F) suggest that the two types gases (coal-derived and oil-associated gases) derived from two sources (two sags) may be mixed. Additionally, in Figure 5f, the trends of 1/n vs. δ 13 C n of the alkane gases in the K 1 yc of the Huajia (Well DS21) and Lanjia (Well DS35, Well H3, and Well H4) sags mainly indicate predominant linear relationships and can be divided into two groups (Group G and H). The Group G is similar with the Group A, and the Group H is similar with the Group C, suggesting that: 1) the two types (oil-associated and coal-derived) of gases may be mixed in the K 1 yc; 2) the gases were derived from two sources (two sags) with one-type gas; and 3) the gases of two stages (mature or charging) were mixed. According to the study above (Section 5.1), the gases in the K 1 yc are mainly the coal-derived gases and came from the Baojia sag, so the two-group samples (Group F and G) suggest that the gas samples are mainly controlled by the mixed-sources of two mature or charging stages.
According to the geological and geochemical analyses, as shown in Figures 13 and 14, the gas reservoirs in Well DS17 are mainly distributed in the upper K 1 yc, and the lithology of the K 1 yc is predominantly igneous rocks (dacite) in the location of Well DS17. The gas reservoirs in Well DS11 are mainly distributed in the middle of J 3 h and K 1 sh, and the reservoir beds are mainly igneous rocks in the K 1 sh and sandstones in the J 3 h. The source rocks of both Wells DS17 and DS11 are mainly located in the J 3 h-K 1 sh, and the geochemical characters mainly suggest the good to excellent source rock potentials ( Figure 13). The gas reservoirs in the J 3 h of Well DS11 are mainly sourced by the underlying J 3 h source rocks through the faults, and the gas reservoirs in the K 1 sh of Well DS11 may be sourced by both underlying J 3 h and overlying K 1 sh source rocks through faults and carrier beds, and the gas reservoirs are mainly sourced by the source rocks in the Huajia sag (Figure 14a). The gas reservoirs in the K 1 yc of Well DS17 are mainly sourced by the underlying J 3 h and K 1 sh source rocks through faults and carrier beds, and the gas reservoirs are mainly sourced by the source rocks in the Baojia sags ( Figure 14b).
Consequently, the gases in the lower-middle J 3 h are predominantly derived from the J 3 h mudstones, and the gases in the K 1 sh and the upper J 3 h are mainly the mixed-sources of both the J 3 h and the K 1 sh mudstones, while the gases in the K 1 yc are mainly derived from either the J 3 h mudstones or the K 1 sh mudstones, or both the two sets of mudstones. Moreover, the gas accumulations in different sags are mainly controlled by the J 3 h and K 1 sh mudstones developed in the sags where the gases accumulated, indicating that the gas migration with a short distance was the main migration type in the deep formations of SLB.
underlying J3h source rocks through the faults, and the gas reservoirs in the K1sh of Well DS11 may be sourced by both underlying J3h and overlying K1sh source rocks through faults and carrier beds, and the gas reservoirs are mainly sourced by the source rocks in the Huajia sag (Figure 14a). The gas reservoirs in the K1yc of Well DS17 are mainly sourced by the underlying J3h and K1sh source rocks through faults and carrier beds, and the gas reservoirs are mainly sourced by the source rocks in the Baojia sags (Figure 14b).  Consequently, the gases in the lower-middle J3h are predominantly derived from the J3h mudstones, and the gases in the K1sh and the upper J3h are mainly the mixed-sources of both the J3h and the K1sh mudstones, while the gases in the K1yc are mainly derived from either the J3h mudstones or the K1sh mudstones, or both the two sets of mudstones. Moreover, the gas accumulations in different sags are mainly controlled by the J3h and K1sh mudstones developed in the sags where the gases accumulated, indicating that the gas migration with a short distance was the main migration type in the deep formations of SLB.

Conclusions
Integrated geochemical study suggests the methane contents and stable carbon isotopes and gas dryness coefficients increase with the increasing burial depth in the Dehui Depression, lacustrine SLB. The δ 13 C series mainly show normal carbon isotopic distribution pattern, with parts of reversal. The gases in the deep formations (generally the H-YFs (156-125Ma)) of the Dehui Depression are predominantly organic and thermogenic. According to the determination charts utilizing gas components and stable carbon isotopes and the gas thermal maturity calculated by equation of δ 13 C 1 -Ro%, the gases in the J 3 h (156-145 Ma) and K 1 sh (145-133.9 Ma) are mixed-sources of kerogen thermal degradation and oil secondary cracking, while the gases in the K 1 yc (133.9-125 Ma) are predominantly derived from kerogen thermal degradation.