- Email: [email protected]
The geochemistry and origin of lower Permian gas in the northwestern Sichuan basin, SW China
Accepted Manuscript The geochemistry and origin of lower Permian gas in the northwestern Sichuan basin, SW China Jungang Lu, Jie Ma, Shijia Chen, Bingyan Wu, Yong Li, Benjian Zhang, Hui Han, Ruopeng Lin PII:
S0920-4105(17)30617-4
DOI:
10.1016/j.petrol.2017.07.068
Reference:
PETROL 4154
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 24 February 2017 Revised Date:
20 June 2017
Accepted Date: 27 July 2017
Please cite this article as: Lu, J., Ma, J., Chen, S., Wu, B., Li, Y., Zhang, B., Han, H., Lin, R., The geochemistry and origin of lower Permian gas in the northwestern Sichuan basin, SW China, Journal of Petroleum Science and Engineering (2017), doi: 10.1016/j.petrol.2017.07.068. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Abstract： ：The lower Permian series in northwestern Sichuan basin of China is a favorable region for natural gas accumulation with several potential source rocks and high-quality reservoirs. However, the tectonic evolution in this area has gone through long-term and complicated activities and the origin of gases in the lower Permian series is complex. For the wise exploration strategies planning, the origin and
RI PT
accumulation of natural gas in lower Permian reservoir have been addressed. And the results reveal that: (1) Three sets of potential source rocks, including the mudstone of Lower Cambrian Qiongzhusi formation, Lower Silurian Longmaxi formation and the
SC
marine carbonate source rocks of Lower Permian, have been identified as the gas sources in Lower Permian reservoirs. (2) The gases mainly contain by hydrocarbon gases, with the ratio of C1/C2+ > 0.99, and the characteristics of carbon isotopes are
M AN U
various and several wells’ are reversed or close to reversed. The δ13C2 values of Hewanchang and Wujiaba gas fields are ranging from -33.35‰ to -36.34‰（PDB）, similar to the production of sapropelic organic matter. The Shuangyushi and Kuangshanliang gas fields, with the δ13C2 values ranging from -28‰ to -30‰（PDB）, are source mixed gas. And the δ13C2 values of Jiulongshan gas field are the heaviest
TE D
with the value range from -25.23‰ to -28.4‰（PDB）. (3) The biomarkers of reservoir extracts are consistent with those of the lower Permian source rock, but the bitumen scales in lower Permian are not as much as those in the lower Cambrian or Lower Silurian series, which reveals that the crude oil creaked gas formed in lower Permian
EP
series falls a great deal than that in Cambrian and Silurian series. Based on the comprehensive researches of natural gas composition, carbon isotope, biomarkers of
AC C
extract and other parameters, the natural gases in lower Permian series are mixed by the crude oil creaked gas from deep formation and the lower Permian source rock, and the scale of fracture determines the amount of creaked gas mixed in the lower Permian reservoir.
KEYWARDS: Gas geochemistry； Natural gas origins； Gas accumulation model； Lower Permian series； Southwest Sichuan basin.
ACCEPTED MANUSCRIPT
The Geochemistry and Origin of Lower Permian Gas in the Northwestern Sichuan Basin, SW China Jungang LUa,b,c*, Jie MAa, Shijia CHENa,b,c, Bingyan WUa, Yong LIa,Benjian ZHANGd, Hui
RI PT
HANa,b,c, Ruopeng LINa a. School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China; b.Sichuan Natural Gas Geology Key Laboratories, Chengdu,
SC
Sichuan 610500, China; c. State Key Laboratory of Oil and Gas Reservoir Geology and
M AN U
Exploitation, Chengdu, Sichuan 610500, China; d.Northwest Sichuan Gas Field, PetroChina Southwest Oil and Gasfield Company, Mianyang, Sichuan 621700, China;
1 INTRODUCTION
The Sichuan basin, China’s largest gas-bearing basin, is characterized by containing different sediment environments, which changed from deep marine to
TE D
shallow terrestrial condition (Dai et al., 2003； Wang et al., 2002；Zhai G. M., 1992). For the past few decades, more than 300 gas fields and 20 gas producing layers have been found, and part of them were under the deep marine condition(Liu et al., 2000；
EP
Liu et al., 2004；Zhu et al., 2006). The study area is located in northwestern Sichuan basin, where several gas fields have been found in the Permian lower formation like Hewanchang, Shuangyushi, Jiulongshan and so on. With the testing production of
AC C
Well L16 (251.74×104m3/d, P1m)，Well L004-X1 (111.65×104m3/d, P1m) and Well ST1 (126.77×104m3/d, P1m) and so on，the lower Permian reservoirs are believed to have an excellent exploration potential. Some potential source rocks were formed in certain sedimentary cycles. And the
hydrocarbon-generating materials and periods are different for the several source rocks.（Dai et al., 2000；Li et al., 2012；Tenger et al., 2008；Xu et al., 2011）. Furthermore, multi-stage adjustment by tectonic movement also have a significant influence on gas characters，which make the gas-source relationships more complex. Several geologists believe that the gas was source from the lower Permian source 1
ACCEPTED MANUSCRIPT rock itself, (Cai et al., 2003；Li et al., 2013). Also, there were those who argued that Permian gas was sourced from Silurian crude oil cracked gas (Huang et al., 2011). Associated with other areas, it is significant to do researches on deep gas for its meaningful influence in gas reservoirs. And it’s necessary to dig into more details on the sources of Low-Permian gas. Based on comprehensive analysis of Permian gas
natural gas and establish the reservoir forming model.
SC
2 GEOLOGICAL SETTING
RI PT
geochemistry and source rocks as well as structural features, we discuss the cause of
Sichuan basin is located in the southwest of China, and the study area is near the northwest margin of the basin. The study area is bound by the Micang mountain to the
M AN U
north, Jiangyou to the south, Central Sichuan uplift to the east, and Longmenshan nappe fault fold belt to the west(Fig.1) (Li et al, 2002). The tectonism of study area is intense for its location on the edge of three secondary tectonic belt as Longmenshan piedmont fault fold, Micangshan fault zone and northern Sichuan tectonic belt. Before the Indosinian, northwestern Sichuan area was deposited of marine
TE D
carbonate on the passive continental margin in the western Sichuan craton. Cambrian sedimentary thickness was getting thinner from the southwest to southeast for the development of NW-SE platform margin in Sinian. Uplift and erosion led to thin in northwest and thick in southeast from Ordovician-Silurian to Devonian-Carboniferous
EP
while normal faults controlling the sedimentary. Permian and Lower-Middle Triassic formed stable shallow environment including multiple transgression and regression.
AC C
Also, Middle Triassic observed the comprehensive regression in Sichuan craton, uplifted Longmenshan, reversed normal faults and part of the thrust faults as well as the certain structural shape. At the end of the Upper Triassic, strong late Indosinian movement formed large-scale nappe structures and unconformity between Jurassic and upper triassic system formation. This area restarted the compression under the distant effect of northward subduction between India and Eurasian plate during Himalayan, and therefore, the formation of the whole developed a wide and slow unchangeable structure (Fig.2, Fig.3) ( Guo et al., 1996；He et al., 2014；Luo et al., 2006；Pang et al., 2010；Yuan et al., 2010).
2
ACCEPTED MANUSCRIPT Multiple oil gas structures, like Hewanchang and Shuangyushi, result in complex source-reservoir distribution and multiple sets of favorable assemblage rocks, reservoirs and cap-rocks. The certain lack of Devonian and Carboniferous strata and distinct loss of Upper-Middle Cambrian, Lower Ordovician and Upper-Middle Silurian have been detected according to the drilling data and outcrop observation
RI PT
(Hou et al, 2003;Li et al, 1992). There are Lower-Permian Qixia and Maokou carbonate rocks reservoirs with great properties and massive fractures for favorable migration and accumulation while various cap-rocks (Davies et al, 2005;Huang et al,
SC
2004a; Lavoie and Morin, 2004; Song et al, 2005; Yang and Feng, 2000). In a word, multiple sets of assemblage rocks, reservoir and cap-rocks exactly do a great favor to
M AN U
good generation, migration and accumulation.
3 SAMPLES AND MATHODS
A total of 10 samples, comprising 2 samples from Hewanchang Gas Field, 1 sample from Wujiaba Gas Field, 3 samples from Shuangyushi Gas Field, 2 samples from Jiulongshan Gas Field, 1 sample from Kuangshanliang Gas Field, 1 sample from
TE D
Shejianhe Gas Field, were collected and analyzed for chemical composition and carbon isotopes. Besides, 60 rock samples, comprising 37 core samples and 23 outcrop samples were collected from three sets source rocks and reservoirs of lower Permian analyzed for biomarkers. Furthermore, analysis results of seven gas samples
EP
and 25 rock samples were collected from Northwest Sichuan Gas Field, PetroChina Southwest Oil and Gasfield Company.
AC C
The molecular composition of the gas samples was measured by Agilent 6890N
gas chromatograph (GC) equipped with a flame ionization detector and a thermal conductivity
detector.
A
fused
silica
capillary
column
(HP-PONA
50mX0.20mmX0.50µm) was used for individual hydrocarbon gas components (C1-C5) separated. GC oven temperature was initially set at 27℃for 10min, and then ramped to 130℃at 3℃/min. The stable carbon isotopic values of the natural gas samples were measured by MAT 252 mass spectrometer (MS) equipped with EI ion source and Dual viscous sample introduction system. The gas components were separated on a GC equipped
3
ACCEPTED MANUSCRIPT with MS and converted into CO2 in a combustion interface and then injected into MS. A fused silica capillary column (HP-PLOT Q 30mmX 0.32 mm X 20.0µm) was used for separating the individual hydrocarbon gas components (C1-C2). The GC oven was initially set at 50℃for 3min, and then ramped to 190℃at 15℃/min. The stable carbon isotopic values are reported in the δ notation in per mil (‰) relative to the VPDB
RI PT
(Vienna Pee Dee Belemnite) standards.
The core and outcrop samples were powdered (<80 mesh) and extracted by a Soxhlet apparatus with a solvent mixture of dichloromethane and methanol (93:7, v: v)
SC
for 72h. And after evaporation and concentration, the concentrated extracts were mixed with n-hexane and allowed to stand for 12 h to yield the asphaltenes. The open silica gel column chromatography were used for yield saturated hydrocarbons,
M AN U
aromatic hydrocarbons and non-hydrocarbons respectively by n-hexane, a mixture of n-hexane and dichloromethane (2:1, v:v) and methanol from the remaining extracts. The saturated hydrocarbons GC-MS was measured by Agilent 6890N GC couple to MAT 252 MS equipped with fused silica capillary column (HP-5MS 60mX 0.25mmX 0.25µm). For analyzing the saturated fractions, the GC oven temperature
TE D
was initially set at 50℃for 1min, and then ramped to 120℃at 20℃/min and then to 310℃ at 3℃/min, with a final hold time of 30 min.
4 RESULTS
EP
4.1 SOURCE ROCKS
Three sets of potential source rocks, including the mudstone of Lower Cambrian
AC C
Qiongzhusi Formation, the shale of Lower Silurian Longmaxi Formation and the marine carbonate source rocks of Lower Permian, have been seen the origins of the gas in Lower Permian reservoirs(Tab.1). The Lower Cambrian Qiongzhusi Formation source rock are mainly black
mudstone or shale and widely distributed around the study area with the thickness between 150 and 300 meters(Fig.4.a). They have relatively high organic matter abundance with average TOC of 2.7% (between0.6% and 6.2%). And the rocks are characterized by typical feature of type Ⅰ kerogen and over maturity level (Ro between 2.7% and 3.83%)(Fig.5). Based on these characteristics, the Lower Cambrian Qiongzhusi Formation source rock are favorable for gas generation. 4
ACCEPTED MANUSCRIPT The Lower Silurian Longmaxi Formation source rock consists of black shale and dark-grey mudstone. And they are distributed in the northern part of the study area with the 250m max thickness and thinning to the south until pinch-out (Fig.4 b). The source rock are characterized by the average total organic matter of 0.85% (between 0.1% and 2.33%), by the partial sapropel organic matter with the type ofⅠ-Ⅱ1, and by
RI PT
the high maturity of 2.18%-2.83%. Thus, the Lower Silurian Longmaxi Formation source rock may be limited for the distribution and relatively favorable for gas generation in the northern study area.
SC
The marine carbonate source rocks of Lower Permian are distributed around the entire study area with the thickness between 80m and 250 m (Fig.4.c). The source rock has the average TOC of 0.34 % (between 0.11% and 0.7%), the partial sapropel
M AN U
organic matter with the type of Ⅱ1 and the high maturity with the Ro between 1.79% and 2.45%. Thus, the Lower Permian source rock have the hydrocarbon producing potential but inferior to the two sets source rocks above especially the Lower Cambrian Qiongzhusi Formation source rock.
On the basis of tectonic uplift and denudation by Caledonian movement, the
TE D
hydrocarbon generation of Paleozoic source rock has obvious hysteresis. And the maturity of three sets source rocks are obviously affected by the thickness of the Jurassic stratum. The Lower Cambrian source rock reached the oil window in the Permian, intensely generated oil during the Triassic to Early Jurassic, and to the peak
EP
gas generation stage in the Middle Jurassic to the Late Jurassic. The Lower Silurian Longmaxi Formation source rock reached the oil window in the Late Triassic, and
AC C
intensely generated oil during the Early Jurassic to Middle Jurassic, and to the peak gas generation stage in the Late Jurassic to the Cretaceous. The marine carbonate source rocks of Lower Permian reached the oil window in the Late Triassic, and intensely generated oil during the Middle Jurassic to late Jurassic, and to the peak gas generation stage in the Late Jurassic to the Cretaceous(Fig.6.)
4.2 NATURAL GAS GEOCHEMISTRY For the natural gas research, the chemical compositions of natural gas are significant indexes and several factors, like gas source rock, maturity and secondary alteration, have a great effect on it (Behar et al.1992; Pallasser, 2000; Tian et al., 5
ACCEPTED MANUSCRIPT 2009). In the study area, the composition of natural gas in the Lower Permian reservoirs consists of hydrocarbon gas and non-hydrocarbon gas and characterized by hydrocarbon gas with predominance in the total gas. Among the compositions of measured hydrocarbon gases, methane gases dominate, most of them are more than 95%(between 95.22% and 98.28%) and only seldom between 90% and 95%(Tab.2).
RI PT
Ethane gases all less than 1% in the total gas with the average of 0.67% and the gas dryness (C1/∑C1-C5) all > 0.99. Thus, the gas in this area is typical dry gases and in the high-over maturation.
SC
The abundance of non-hydrocarbon gases in the study area are not high, and mainly consist by carbon dioxide and nitrogen and with little hydrogen sulfide, helium, hydrogen and so on. Among the measured gases, the abundance of CO2 is
M AN U
mainly between 0.1% and 2.0 % and several relative high in some wells like Well Wujia1(5.93%) and Well Kuang1(3.79%). The abundance of N2 are mainly between 0.3% and 1.6 %, such as Well He2 (1.38%), Well He3 (1.18%), Well Long16 (0.92%), and so on. Besides, in this area, the H2S is ubiquitous in the marine carbonate reservoirs especially in the Jiulongshan gas field (Tab.2.).
TE D
The carbon isotope compositions of natural gas are very commonly used to indicate the origin, type and maturity of the gas (Clayton, 1991; Galimov, 2006; Stahl and Koch, 1974). And the δ13C2 values is used to assess the genetic type of the gas(Dai et al,2005;Wang, 1994) and the δ13C1 is believed to be related to the maturity
EP
with given genetic types(Dai, 1992;). Based on the numerous analytical results in Sichuan basin, it is proposed that the gas are derived from sapropelic kerogen if the
AC C
δ13C2 is <-30‰, humic kerogen if >-28‰, and mixed origin if between these two values. The δ13C1 value commonly increases with the maturity and have certain ranges in different evolutionary stages (Cao et al., 2012). As shown in Tab.3, the stable carbon isotopic composition gases in this area are
complicated and various. The δ13C2 values range from -25.23‰ to -36.34‰（PDB）, indicating that the gases are derived from different genetic types. According to the values of δ13C2, the natural gases can be divided into three kinds. The natural gases in the Hewanchang and Wujiaba gas fields are derived from typical sapropelic kerogen with the δ13C2 values ranging from -33.35‰ to -35.35‰（PDB）. In contrast，The
6
ACCEPTED MANUSCRIPT natural gases in the Jiulongshan gas field seem to be derived from typical Humic kerogen with the δ13C2 values ranging from -25‰ to -28.4‰（PDB）. In addition, there are also mixed origin gases like Shuangyushi gas field and Kuangshanliang gas field with the δ13C2 values ranging from -28‰ to -30‰（PDB）.Thus, origin of the
4.3 BIOMARKER GEOCHEMISTRY
RI PT
natural gases in this area is various.
The characteristics of reservoir extract and three possible source rock are different in their m/z 191 and m/z 217 mass chromatograms. The gas and source rocks
SC
can be well correlated according to the distribution of terpanes, hopanes, steranes and so on.
M AN U
The TT（tricyclic terpane）has attracted more and more attention for their extensive distribution, better thermal stability and strong anti-biodegradation. In the study area, the relative abundance of TT in the mudstone is higher than in the carbonate rocks (Fig7.). And all of the source rocks extracts demonstrate a tricyclic terpane C20 - C21 - C23 distribution of C20 < C21 < C23.
The distribution of the pentacyclic terpane shows that the abundance of C30
TE D
hopance is highest and with the carbon number increased the abundance decrease. The ratio of Ts/Tm is affected by several parameters like maturity, the organic facies and clay minerals in the source rocks and so on. Nearly all of extracts in the three set
EP
source rocks display high Ts/Tm values with marine carbonate source rocks of Lower Permian between 0.2 and 1.19, and mudstone source of Lower Silurian Longmaxi and Lower Cambrian Qiongzhusi be 0.76 and 0.87 respectively. The abundance of
AC C
gammacerane is not high and the ratio of gammacerane/C31 hopane is generally greater than 0.01.
The m/z mass chromatogram of the three set of source rock shows that the
abundance of pregnane and higher pregnane are moderate and the regular sterane isomerization degree is relatively high. In the Lower Permian, the regular sterane C27-29 demonstrate like shape‘V’ or partial ‘L’, with the distribution of C27 ≈ C29 > C28. And in the regular sterane C27-29 demonstrate like typical shape ‘L’, with the distribution of C27 > C29 > C28. According to the fig.8, the regular sterane C27-29 from the reservoir extract and 7
ACCEPTED MANUSCRIPT bitumen demonstrate like shape‘V’ or partial ‘L’, with the distribution of C27 ≈ C29 > C28, similar with the marine carbonate source rocks of Lower Permian. Indicating that the extract has greatest possibility evolved from the marine carbonate source rocks. The reservoir extract and bitumen demonstrate a TT distribution of C20 < C21 < C23. The distribution of the pentacyclic terpane shows that the abundance of
RI PT
C30 hopance is highest and with the carbon number increased the abundance decrease.
5 DISCUSSION 5.1 The origin of natural gases
SC
5.1.1 The gas derived from not only the lower Permian source rock, but also the deep formation source rocks, including lower
M AN U
Cambrian series and Lower Silurian series.
Based on the study above, several parameters, especially the carbon isotopes and distribution of bitumen, indicated that the gas origins from not only Lower Permian source rock, but also the deep series source rocks.
（1）The gas carbon isotopes of several wells were reversed or close to reversed.
TE D
Most field and laboratory observations and all theoretical models of kinetic isotopic effects yield a normal sequence of carbon isotopic compositions with δ13C1 < δ13C2 < δ13C3 < δ13C4 observed in natural gas accumulations worldwide. However, the
EP
carbon isotopic compositions will be reserved by mixing between gases from different sources, including inorganic sources and organic sources at different levels of thermal
AC C
maturity (Burruss and Laughrey, 2010; Dai et al., 2004; Huang et al., 2004b; Jenden et al.,1993).
According to the methane and ethane carbon isotope characteristics in the study
area, the carbon isotope reversals or nearly reversals are existed in several wells gases (Fig.9). Such as the Well He1 (δ13C1: -35.4‰，δ13C2: -35.35‰)in Hewanchang gas field, Well Wujia1（δ13C1: -28.49‰ and -29.77‰，δ13C2: 34.5‰ and -34.83‰ respectively ） in Wujiaba gas field and Well Shuangtan1 （ δ13C1:-29.7‰ ， δ13C2:-29.9‰） in Shuangyushi gas field(Tab.3). Based on above, the gases were mixed by the production of deep source rock, which led the gas carbon isotopes composition to reversed or close to reversed. 8
ACCEPTED MANUSCRIPT （2）The ethane carbon isotopes in Hewanchang and Wujiaba gases field were so light that they were inconsistent with the production of Lower Permian source rock. The methane gases are affected easily by thermal maturity, and the sapropel-type cracked gas and Humic type of pyrolysis gas are always hard to distinguish for their similar methane carbon isotopes distribution. However, the heavy hydrocarbon gas
RI PT
carbon isotopes like the ethane gas had little change as the thermal maturity of source rock increasing and used to distinct different types natural gas.
The values of ethane carbon isotopes from gas fields of Hewanchang and
SC
Wujiaba are small enough （-33.35‰～-36.34‰（PDB）,Tab.3., Fig.9.）that seem to origin from typical sapropelic organic matters, and the source rock of lower Permian series belongs to humic-sapropel type. So the ethane carbon isotope of lower Permian
M AN U
source rock can’t be so small, and with the increase of thermal evolution degree, isotope will transform to be heavier. Combining the viewpoints above, gas supply must exist that from low-cambrian typical sapropelic organic matters. （3）Bitumen scale in lower Permian series shows the oil-cracked gas is rarely existed, and external gas is mixed into the reservoirs.
TE D
After liquid hydrocarbon generation by sapropelic organic matters, it will generate pyrolysis gas accompany with bitumen as the maturity increases. So the existence of bitumen demonstrated liquid hydrocarbon has pyrolysed into gas before. The scale of the bitumen amount reflected the scale of pyrolysis gas. There wasn’t
EP
evident bitumen in lower Permian series of Qixia and Maokou Formation in the studied area (Fig.10 a,b). It was only fund under the fluorescence observation (Fig.10
AC C
c,d), which showed oil cracked truly existed in lower Permian series, but the scale is small, so the gaseous hydrocarbon is small too. According to the explorative achievements, it is not enough to generate so large scale sapropelic gas by the lower Permian series source rocks, there are the mixture gas from other source rocks.
5.1.2 The product from deep formation is crude oil cracked gas instead of kerogen degradation gas or liquid hydrocarbon. The production of high-quality sapropelic source rocks could mix with Permian hydrocarbons as liquids which then crack into gas, or just as the gaseous hydrocarbons like crude oil cracked gas or kerogen degradation gas. However, 9
ACCEPTED MANUSCRIPT according to the extract characters, bitumen distributions and gas compositions, we found that the mixture from the deep formation is mainly the crude oil cracked gas. (1) The similar characters between reservoir extracts and lower permian source shows that liquids hydrocarbon generated by deep formation source rock haven’t migrated into the lower permian reservoirs.
RI PT
The Permian reservoir extracts are mainly authigenic with some mixture of cracked gas instead of liquid hydrocarbons because of the dissimilar features to that of lower source rocks. According to the analysis on the biomarker characteristics of
SC
these three sets of source rocks as well as Lower-Permian extracts, the result shows that Lower-Permian bitumen comprises similar characteristics to the source rocks
type and some partial ‘L’ type.
M AN U
with relatively high abundant C27 sterane and lowest abundant C28 sterane just like ‘V’
（2）Crude oil accumulated and cracked in deep formation and the cracked gas migrated into the lower Permian series because of less bitumen in Lower-Permian reservoir but large-scale distribution in Cambrian reservoir.
The theory of hydrocarbon generation suggests that sapropelic source rocks are
TE D
dominated by oil in mature stage but oil into gas and some mixture of heavy-component bitumen with maturity (Chilingarian, 1988；Huang and Wang, 2008； Tissot and Welte, 1984; Xu, 1997). Associated with the Permian bitumen, no large-scale bitumen has been detected in Permian which implies that there’s no
EP
massive oil cracked or migration of liquid hydrocarbon from Lower- Cambrian high-quality source rocks but the vertical migration of gas mixed with Permian.
AC C
Also, the distribution of Cambrian field bitumen confirms this point: large-scale
bitumen exposure indicates the massive accumulation of Cambrian liquid hydrocarbon in the reservoirs and then crack into oil cracked gas and bitumen with maturity increased. The Cambrian oil cracked gas could supply for Lower-Permian gas reservoir and mix with Permian gas after the adjustment and migration into Permian favorable reservoir ( Fig.11). (3)The amount of N2 indicated the gas was not the kerogen cracked gas but crude oil cracked gas. The cross plot of natural gas (δ13C2-δ13C3) and ln(C2/C3) could do a great favor to
10
ACCEPTED MANUSCRIPT effectively distinguish oil cracked gas and kerogen cracked gas (Alain, 1995; Chen, 2002). However, the application effect of this method on studies area disappoints us because the concentration of heavy hydrocarbon gas is too low to detect in high-over mature stage(Hao et. al,1995；Stahl, 1977). The natural gas in Dengying formation, Weiyuan gas reservoir, next to studied
RI PT
area, has been confirmed to be typical Cambrian kerogen cracked gas in Qiongzhusi formation. According to the comparison of natural gas in adjacent areas, the nitrogen content in Lower-Permian gas reservoir is far less than that in Weiyuan Dengying
SC
formation which indicates the obvious distinction between them (natural gas in Weiyuan Dengying formation has been confirmed to generate from Cambrian kerogen cracked gas in Qiongzhusi formation(Yin, et. al,2001))( Fig12.). Therefore, it could be
M AN U
concluded that Lower-Permian gas is accumulated from oil cracked gas instead of kerogen cracked gas in Qiongzhusi formation.
5.1.3 The amount of gas origin from deep formation is various and controlled by the scale of fractures in different gas fields.
TE D
The structural interpretation results show that the western adequate fractures, mostly deep through the bottom source rocks, provide favorable migration path for hydrocarbons such as Cambrian deep gas upward and make it mix with others, while eastern fractures are much less to channel with deep gas.
EP
The amount of deep-sourced gas varies place to place for the different tectonic fractures and it also determines the weight of ethane isotope in distinct tectonic
AC C
natural gas.
(1) The ethane isotope of natural gas in Hewanchang and Wujiaba gas fields are relatively light for the pretty mixture of deep-sourced gas. The ethane isotope of natural gas in Hewanchang and Wujiaba gas fields show
typical sapropelic organic matter because the large-scale fractures which are mostly faults deep cross Cambrian strata become favorable path for Cambrian sapropelic gas to migrate upward. Massive deep Cambrian sapropelic gas mixed with relatively few Permian autogenetic gases to make ethane isotope so light. (2) The ethane isotope of natural gas in Shuangyushi and Shejianhe gas fields are relatively heavy for the amount of mixed gas from deep-source is not as much as 11
ACCEPTED MANUSCRIPT Hewanchang gas feild. The ethane isotope of natural gas in Shuangyushi and Shejianhe gas field are not light like that in Hewanchang gas field because the adequate but small-scale fractures, which are just deep to the Upper-Middle Cambrian strata, result in few mixture of deep-sourced gas and collective mixture of deep formation and Permian.
RI PT
(3) The ethane isotope of natural gas in Jiulongshan gas field is heavy for little mixture of deep-sourced gas.
There’s no or little mixture of deep-sourced gas because few fractures in
SC
Jiulongshan block the natural gas generated in Lower-Cambrian source rocks from Permian. Natural gas in Permian is mostly autogenetic and shows heavy isotopes.
M AN U
5.2 Model of natural gas accumulation
Based on hydrocarbon generation and expulsion, tectonic evaluation and trap formation history, two major petroleum charging periods in lower Permian reservoirs are suggested: the middle to late Jurassic and Cretaceous to early Tertiary. In the first period, the lower Permian source rock was in hydrocarbon generation threshold, with
TE D
the maturity value between 0.5% and 1.1%, the homogenization temperatures between 110℃ and 120℃,and dominant by the liquid hydrocarbon and low dry coefficient accompanying gas origin from the lower Permian marine carbonate source rocks(Fig.13). In the second period, the lower Permian source rock was in high and
EP
over mature periods, with the maturity value between 1.5% and 1.7%, the homogenization temperatures between 130℃ and 140℃, the lower Permian source
AC C
rocks are dominant in peak-period of dry gas generation. In this period, the crude oil creaked gas also migrated from the deep Cambrian and Silurian formation to lower Permian reservoir and mixed. The second period is the main accumulation stage. In the Stage of Himalayan movement, the Sichuan basin underwent intense
tectonic adjustment, and the movement also had a great effect on gas accumulation in the NW Sichuan basin. Based on tectonic movement, the fissures in the reservoir were increased and interconnected, which is beneficial to gas migration and accumulation in higher position of structure. Besides, the crude oil creaked gas from deep formation were migrated vertically by the developmental fracture and mixed in the lower Permian reservoir. The scale of fractures in different tectonic regions were diverse, 12
ACCEPTED MANUSCRIPT and the Hewanchang area was most developed and the gases were mainly from the deep formation, than the Shuangyushi area, and the Jiulongshan area was the least developed and the gases were mainly from the Permian source rock(Fig.14).
RI PT
6 CONCLUSION （1）Three sets of potential source rocks, including the mudstone of Lower Cambrian Qiongzhusi formation, the shale of Lower Silurian Longmaxi formation and the marine carbonate source rocks of Lower Permian, have been seen that contributed to
SC
the accumulation of gas in Lower Permian reservoirs. The deep formation source rocks have great contribution to the gases accumulation for the high abundance of
M AN U
total organic matter, typeⅠorganic matter and high or over maturity. The lower Permian carbonate source rocks also have contribution to the gases accumulation for closing to the reservoir.
（2）The natural gases in the study area are mixed in several periods by three source rocks. The crude oil creaked gases are predominant and accompany with the high or over mature kerogen cracked gas. The natural gases composition and carbon isotope
TE D
are diverse in different gas fields. The gases in the Hewanchang and Wujiaba gases fields are characterized with light methane and ethane carbon isotope and mainly from the deep formation source rocks. The gases in the Jiulongshan gases field are heaviest
EP
and mainly the production of lower Permian source rock. And the Shuangyushi and shejianhe gas fields are mixed origin gases.
AC C
（3）On the base of gases origin and genesis, tectonic and source rocks evaluation, two major petroleum charging periods in lower Permian reservoirs are suggested. And the homogenization temperature of fluid inclusion and Microscopic fluorescence also exist two periods, which support the conclusion above. The middle to late Jurassic was the first period and dominant by the liquid hydrocarbon. The Cretaceous to early tertiary was the second period and mainly the crude oil creaked gas mixed by the lower Permian and the deep formation oil pool. The second period is the main accumulation stage. During the Himalayan movement, the gases field formed before were adjusted by the movement, and migrated and accumulated in higher position of structure. Besides, the crude oil creaked gas from deep formation were migrated 13
ACCEPTED MANUSCRIPT vertically by the developmental fissures and fracture and mixed in the lower Permian reservoir.
ACKNOWLEDGEMENTS
RI PT
The authors would like to express their gratitude to School of Geoscience and Technology, Southwest Petroleum University ，Sichuan Natural Gas Geology Key Laboratories and State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation for providing the laboratory facilities to analyze the samples described in
SC
this paper. Additionally, we would also like to thank everyone who provides valuable
M AN U
comments and suggestions during the interpretation and revision process.
FUNDINGS
Funding for this research are National Science Foundation of China (No. 41502146 & 41572137), the National key Scientific and Technological project
(No.
2016ZX05007003—007) and Southwest Petroleum University funding plan of oil and
TE D
gas accumulation geochemistry for Youth Science and technology innovation team (No.2015CXTD02) for the petroleum accumulation research.
EP
REFERENCES CITED
Alain A.P., Alain Y. H., 1995. Genetic and post-genetic molecular and isotopic
AC C
fractionations in natural gases. Chem Geol, 126:281-290.
Behar F., Kressmann S., Rudkiewicz J.L., Vandenbroucke M., 1992. Experimental simulation in a confined system and Kinetic modeling of kerogen and oil cracking. Organic Geochemistry, 19, 173-189.
Burruss R.C, Laughrey C.D, 2010. Carbon and hydrogen isotopic reversals in deep basin gas: Evidence for limits to the stability of hydrocarbons. Organic Geochemistry.41,1285-1296. Cai K.P., Wang Y.R., Yang Y.M., et al., 2003.Evaluation of hydrocarbon source rocks in Permian and Triassic at Guangyuan Wangcang region in North West Sichuan
14
ACCEPTED MANUSCRIPT basin and a primary discussion on gas source. Natural Gas Industry, 23,10-14. Cao J., Wang X.L., Sun P.A., Zhang Y.Q., Tang Y., Xiang B.L., Lan W.P., Wu M., 2012. Geochemistry and origins of natural gases in the central Junggar Basin, northwest China. Organic Geochemistry. 53, 166-176. Chen S.J., Fu X.W., Ma L.N., Chen X.L., Zhang X., 2002. Genetic identification
RI PT
method of kerogen-creaked gases and oil cracked gases. Petroleum Geology & Experiment,24,364-366+371.
Chilingarian G.V., 1988. Bitumens, asphalts and tar sands. Yu Jingfang
SC
(Trans.).Beijing: Petroleum Industry Press.
Clayton, C., 1991. Carbon isotope fractionation during natural gas generation from kerogen. Marine & Petroleum Geology. 8, 232-240.
M AN U
Dai, J.X., 1992. Identification of different hydrocarbon gas. Sci. China 2, 185-193. Dai J.X., Wang T.D., Dai H.M., et al., 2000. Gas sources of large scale Carbonate gas field. Marine Petroleum Geology. 5( 1 /2):141- 144.
Dai J.X., Chen J.F., Zhong N.N. et al., 2003.Giant gas fields and gas source in China[M].Beijing: Science Press.
TE D
Dai, J.X., Xia, X.Y., Qin, S.F., Zhao, J.Z., 2004. Origins of partially reversed alkane δ13C values for biogenic gases in China. Organic Geochemistry, 35, 405-411. Dai J.X., Qin S.F., Tao S.Z., Zhu, G.Y., Mi, J.K.2005. Developing trends of natural gas industry and the significant progress on natural gas geological theories in
EP
China. Natural Gas Geoscience. 16, 127-142. Davies，G.R., T. Boreen and. L. Smith, 2005. Hydrothermal dolomite and leached
AC C
limestone reservoirs: Representative core fabrics and facies. AAPG Annual Meeting Program, 14, A32.
Galimov E.M., 2006. Isotope organic geochemistry. Organic Geochemistry. 37, 1200-1262.
Guo Z.W., Deng K.L, Han Y.H.,1996. The formation and development of Sichuan Basin. Beijing: Geological Publishing House. Hao S.S., Chen Z.M., Gao Y.B., 1995. The series and preservation of natural gas. Beijing: Petroleum Industry Press. He L.J.，Huang F.，Liu Q.Y.，Li C.R.，Wang J.Y., 2014. Tectono-thermal Evolution
15
ACCEPTED MANUSCRIPT of Sichuan Basin in Early Paleozoic. Journal of Earth Sciences and Environment, 36, 10-17. Hou M.C., Chen H.D., Tian J.C., 2003. Sequence-Filling Dynamics--A New Study Direction on Sequence Stratigraphy. Journal of Stratigraphy, 27, 358-364. Huang X.P., Yang T.Q., Zhang H.M., 2004a. Research on The Sedimentary Facies and
RI PT
Exploration Potential Areas of Lower Permian in Sichuan Basin. Natural Gas Industry, 24, 10-12.
Huang, H.P., Yang, J.，Yang, Y.F., Du, X.J., 2004b. Geochemistry of natural gases in
SC
deep strata of the Songliao basin, NE China. International Journal of Coal Geology 58 231-244.
Huang D.F., Wang L.S., 2008. Geochemical characteristics of bituminous dike in
M AN U
Kuangshanliang area of the Northwestern Sichuan Basin and its significance. Acta Petrolei Sinica. 29, 23-28.
Huang D., Dai X., Dai Y., Zhu Y.G., Yang Y., Pei S.Q., 2011.Geochemical behaviors and gas origins in the Hewanchang Gas Field northern part of the Western Sichuan Basin. Natural Gas Industry, 23, 37-40+109.
TE D
Jenden, P.D., Drazan, D.J., Kaplan, I.R., 1993. Mixing of thermogenic natural gases in northern Appalachian basin. American Association of Petroleum Geologists Bulletin 77, 980-998.
Lavoie D, Morin C.2004. Hydrothermal dolomitization in the Lower Silurian Sayabec
EP
Series in north Gaspe-Matapedia(Quebec):Constraint on timing of porosity and regional significance for hydrocarbon reservoirs. Bulletin of Canadian Petroleum
AC C
Geology, 52, 256-269.
Li S.T., Yang S.G., Lin C.S., 1992. On the Chronostratigraphic Framwork and Basic Building Blocks of Sedimentary Basin. Acta Sedimentologica Sinica, 10, 11-22.
Li H.J.，Xie X.N.，Huang J.H.，Chen H.，Lin Z.L., 2012.Main Factors Controlling the Series of Excellent Marine Source Rocks in Permian Maokou Series of Northwest Sichuan ， China. Earth Science-Journal of China University of Geosciences,37, 171-180. Li Y.H., Chen G.S, Zhang J., Yang Y.M., 2002. Natural Gas Pool Forming Conditions and Exploration Prospect of the Foreland Basin in West Sichuan. China
16
ACCEPTED MANUSCRIPT Petroleum Exploration, 7, 34-45. Li Y.J.，Zhao S.X.，Li Y.G.，Zhang B.J.，Ma H.L.，Pei S.Q., 2013. Natural Gas Genesis and Sources in Jiulongshan Gas Field，Northwest Sichuan Basin. Natural Gas Geoscience,24, 755-767. Liu D.L., Song Y., Xue A.M.,2000. Integral study of tectonics and natural gas
RI PT
accumulation in Sichuan basin, China[M]. Beijing:Petroleum Industry Press, pp： 20-65.
Liu S.G., Xu G.S., Xu G.Q., Yong Z.Q., Li G.R., Li J.C., 2004. Primary study on the
SC
dynamics of natural gas pools in Sichuan basin, China. Gas Geoscience, 15,323-330.
Luo L., Jia D., Chen Z.X.，Hu Q.W., Jia Q.P., Li Y.Q., Zhang Y.Y., 2006. Magnetic
M AN U
fabric evolution in the northwestern Sichuan basin, China and its strain characteristics. Geological Bulletin of China, 25, 1342-1348. Pallasser R.J., 2000. Recognising biodegradation in gas/oil accumulations through the δ13C compositions of gas components. Organic Geochemistry. 31, 1363-1373. Pang Y.J.，Zhang B.J.，Feng R.W.，Wang Y.R., 2010. Evolution of Devonian
TE D
depositional environment in northern Longmenshan tectonic belt. Global Geology, 29, 561-568.
Stahl W.J., Koch J., 1974. 13C-12C relation of North-German natural-gas - maturity
27,623.
EP
characteristic of their parent substances. Erdol & Kohle Erdgas petrochem.
Stahl, W.J., 1977. Carbon and nitrogen isotopes in hydrocarbon research and
AC C
exploration, Chemical Geology., 20:121-149.
Song Z.Q., Wang X.Z., Zeng D.M., 2005. Sedimentary Facies and its relativity with petroleum gas at Qixia age of Permian in the northwest of Sichuan. Journal of Southwest Petroleum Institute, 27, 20-24.
Tenger，Qin J.Z.，Fu X.D.，Li W.，Rao D.，Zhang M.Z.,2008. Basic conditions of marine hydrocarbon accumulation in northwest Sichuan basin-high quality source rocks. Petroleum Geology & Experiment,30, 478-483 Tian H., Xiao X.M., Yang L.G., Xiao Z.Y., Guo L.G., Shen J.G., Lu Y.H., 2009. Pyrolysis of oil at high temperatures: gas potentials, chemical and carbon
17
ACCEPTED MANUSCRIPT isotopic. Chin Sci. Bull. 54. 1217-1224. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence. Springer Verlag, Berlin Heidelberg, pp. 643–644. Wang, S.Q.,1994. Geochemical characteristics of Jurassic-Sinian gas in Sichuan basin. Natural Gas Industry. 14, 1-5.
RI PT
Wang Z.C, Zhao W.Z., Zhang L., et al.,2002. The structural sequence of the Sichuan basin and the natural gas exploration[M]. Beijing: Geological Publishing House, 27-94.
SC
Xu K.D.,1997. The origin of Kangshan bituminous lode. Marine Origin Petroleum Geology, 2;54-59.
Xu S.L.，Chen H.D.，Chen A.Q.，Lin L.B.，Li J.W.， Yang J.B.,2011. Source rock
M AN U
characteristics of marine strata, Sichuan Basin. Journal of Jilin University( Earth Science Edition). 41,343-350+358.
Yang Y.Q., Feng Z.Z, 2000. Permian depositional systems in south China. Journal Of Palaeogeography, 2, 11-18.
Yin C.H., Wang T.D., Wang S.Y., Lin F.,2001. Differences between Kerogen-and
TE D
Oil-cracked Gases in Sinian Reservoirs of Weiyuan and Ziyang Area. Acta Sedimentologica Sinica. 19, 156-160.
Yuan Q.D.，Li B.L.，Liu H.T.，Liu S.J.，Wang L., 2010. The tectonics evolution and lithofacies palaeogeography in the northwest of the Sichuan Basin. Journal of
EP
Daqing Petroleum Institute, 34, 42-52. Zhai G.M., 1992. Petroleum geology of China [M].Beijing: Petroleum Industry Press.
AC C
Zhu G.Y, Zhang S.C., Liang Y.B., et al., 2006. The characteristics of natural gas in Sichuan basin and its sources. Earth Science Frontiers, 13, 234-248.
18
ACCEPTED MANUSCRIPT
muddy limestone
OM Abundance
Organic Matter Type 13
TOC/%
δ C（‰）
0.2-0.7
-28.11～-27.40
0.38(47)
-27.69(23)
P 0.11-0.51 0.25(10) 0.1-2.33 S
S1l
shale 0.85(12) 0.6-6.2
∈1q
mudstone 2.7(10)
-28.25（9）
-32.04～-28.78
AC C
∈
-28.82～-27.36
TE D
muddy limestone
EP
P1q
TI
SC
P1m
Sample Type
M AN U
Formation
RI PT
Tab.1. Parameters of three sets potential source rocks in the Northwest Sichuan basin, SW China.
Ro/% Type
43-87.25
1.79-2.45 Ⅱ1
57.12(18)
2.0(13)
41.5-80.25
1.83-2.17 Ⅱ1
58.47(10)
2.05(5)
74.5-85.5
2.18-2.83 Ⅰ-Ⅱ1
-30.23(8)
80.1(5)
-34.96～-31.59
87.75-97.3
2.37(6) 2.7-3.83 Ⅰ
-33.06（9）
92.3(7)
3.32(5)
ACCEPTED MANUSCRIPT
Kuangshanliang Shejianhe Jiulongshan Shuangyushi
N2
0.03 0.05 0.04 0.05 0.06 0.07 — — — — — 0.032 0.023 0.035 0.04 0.027 0.03
0.14 0.13 0.16 — 0.23 0.1 0.5 0.1 — 3.5 0.05 — 0.005 0.005 0.001 0.001 0
1.27 1.43 1.38 1.18 0.37 0.97 1.69 0.51 0.48 6.47 1.06 1.01 0.92 1.38 1.34 0.69 0.87
1
H2S
CO2
C1
C2+
— — 0.05 0.05 — 0.2 — — 0.3 — — 0.72 0.81 0.36 0.74 0.017 0.34
0.06 0.21 0.06 0.33 0.07 0 5.93 7.31 3.79 0.1 1.33 0.02 0.68 0.96 0.041 0.81 2
97.21 96.01 97.67 97.82 98.28 90.92 91.68 91.79 95.22 88.67 97.35 97.5 97.42 97.16 96.94 96.94 96.65
1.32 0.688 0.564 0.971 7.82 0.11 0.317 0.19 1.573 0.22 0.15 0.14 0.1 0.14 0.131 0.1
SC
H2
M AN U
P1m P1m P1m P1m P1m P1m P1q P1m P1m P1m P1m P1m P1m P1q P1m P1m P1q
He
TE D
Wujiaba
He2 He2 He2 He3 He6 WuJia1 WuJia1 Kuang1 Kuang1 Kuang3 She1 Long4 Long16 Long17 Long004-X1 ShuangTan1 ShuangTan1
Formation
EP
Hewanchang
Well
hydrocarbon gases/%
non-hydrocarbon gases/%
AC C
Area
RI PT
Tab2. Chemical compositions of lower Permian natural gases in the Northwest Sichuan basin, SW China.
ACCEPTED MANUSCRIPT Tab3. Carbon isotopes of lower Permian natural gases in the Northwest Sichuan basin, SW China.
Shuangyushi
Jiulongshan
δ13C1（‰）
δ13C2（‰）
He1 He2 He2 He2 He3 Heshen1 Shuangtan1 Shuangtan1 Long16 Long16 Long17 Wujia1 Wujia1 Kuang1
P1m P1m P1m P1m P1m P1m P1m P1q P1m P1m P1q P1m P1m P1m
-35.4 -35.52 -33.7 -34.5 -35.5 -35.65 -29.7 -30.1 -27.66 -28.8 -28.31 -28.49 -29.77 -31.55
-35.35 -33.35 — -34.7 — -36.34 -29.9 -28.2 -26.27 -28.4 -25.23 -34.5 -34.83 -29.82
M AN U
Wujiaba
Formation
RI PT
Hewanchang
Well
SC
Area
AC C
EP
TE D
Kuangshanliang
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig.1. Sketch map showing structural units and gas distribution in the Northwest Sichuan basin, SW China.( ① Hewanchang,② Kuangshanliang,③ Shejianhe,④ Shuangyushi,⑤ Wujiaba,⑥ Zhangjiabian,⑦
TE D
Jiulongshan， ，⑧Changjianggou， ，⑨Wulongzhai; ①-⑦are gas fields, ⑧、⑨ are outcrops; Hewanchang-
AC C
EP
Shuangyushi-Jiulongshan Section is showing in Fig.3)
b
c
d
SC
a
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
Fig10. Pictures show the bitumen in the lower Permian reservoirs are rarely existed.( Pic. a & b are core from well He1 and well Bian1 and no evident bitumen in them. Pic. c & d are fluorescence photographs and a little bitumen in fissure and the larger pore )
c
d
e
f
SC
b
M AN U
a
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig11. Pictures show large amount of bitumen in the Cambrian reservoirs.(Pic.a & b are the larger scale bitumen in the outcrop; c & d are the cores that filled by bitumen in the fissures and pores; e & f are the fluorescence photographs and bitumen are obvious)
ACCEPTED MANUSCRIPT
N2 in the study area is not as much as Weiyuan gas feild.
The abundance of N2 is high in Weiyuan gas feild, which is the typical Kerogen creaking gas.
6.26
RI PT
WELLS
0.87 0.69 1.34 0.92 1.01 1.06 0.48 0.51 1.69 0.97 0.37 1.18 1.38 1.43 1.27
7.08 7.55
0
2
4
6
8
M AN U
N2(%)
SC
8.33
10
AC C
EP
TE D
Fig12. The gas in study area is different from Weiyuan gas field in abundance of N2, which means the gas origins are diverse.（ （The Nitrogen content of Well Wei 106, 39, 30, 2 respectively are 6.26%,7.08%,7.55% and 8.33%,the dates are courtesy of the Yin, et al.,2000 ）.
ACCEPTED MANUSCRIPT 30
20
15
10
5
0 90-100
100-110
110-120
120-130
RI PT
Samples(n)
25
130-140
SC
Homogenization temperatures(℃)
140-150
AC C
EP
TE D
M AN U
Fig.13. The distribution of homogenization temperatures of hydrocarbon inclusion showing two temperature intervals: 110-120 ℃ and 130-140 ℃, and showing the possibility that there are two key periods of massive oil migration in this area.
SC
RI PT
ACCEPTED MANUSCRIPT
Fig.14. Model of natural gas accumulation in the Northwest Sichuan basin, SW China. The section is
AC C
EP
TE D
M AN U
Hewanchang-Shuangyushi-Jiulongshan.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
Fig.2. Generalized stratigraphic column for the Northwest Sichuan basin, SW China.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig.3. Schematic cross section of HWC-JLS at different geological times showing tectonic evolution of HWC-JLS from the Sinian to the present.( HWC is Hewanchang area, and SYS and JLS represent Shuangyushi area and Jiulongshan area respectively).
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Fig.4.Distribution of source rocks in the Northwest Sichuan basin, SW China. (a)The Lower Cambrian Qiongzhusi Formation source rock; (b)The Lower Silurian Longmaxi Formation source rock; (c)The Lower Permian marine carbonate source rocks( Guangyuan , Jiange, Changjianggou, Tianjingshan, Shuangyushi, Kuangshanliang, Hewanchang, ⑧ Shejianhe, ⑨ Wujiaba, ⑩ Zhangjiabian, ⑪Jiulongshan).
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 5. Representative geochemistry of source rocks in the Northwest Sichuan basin, SW China. ( TOC = total organic carbon; Ro = vitrinite reflectance;δ δ 13C=Kerogen carbon isotopes； ； TI is type index ， [TI=Liptinite+Exinite×0.5-Vitrinite×0.75-inertinite].
The
figure
shows
that
there
are
significant
geochemistry differences in three set of source rocks in the study area. The Qiongzhusi formation source rock, with high abundance total organic carbon and typical organic matter type Ⅰ, is superior to the
AC C
EP
TE D
source rock in the Lower Permian formation.
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Fig.6. Thermal evolution during the burial history of the Northwest Sichuan basin, SW China. The figure
AC C
EP
TE D
shows that three set of source rocks are all in highly or over maturity.
SC
RI PT
ACCEPTED MANUSCRIPT
Fig7. Representative m/z191 and 217 mass fragments showing the correlation in the potential source
AC C
EP
TE D
M AN U
rocks(CJG= Changjianggou; WLZ=Wulongzhai; Wj1= well Wujia1; Hs1=Heshen1).
SC
RI PT
ACCEPTED MANUSCRIPT
Fig8. Representative m/z191 and 217 mass fragments showing the correlation in reservoir extracts and
AC C
EP
TE D
M AN U
bitumens(Wj1= well Wujia1; CJG= Changjianggou).
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Fig9. Cross plot showing δ13C1 vs. δ13C2 values of natural gases in the Northwest Sichuan basin, SW China. The gas carbon isotopes of several wells were reversed or close to reversed.(HWC=Hewanchang, WJB= Wujiaba, SYS= Shuangyushi, KSL= Kuangshanliang, JLS = Jiulongshan)
ACCEPTED MANUSCRIPT 1. Three sets of source rock contribute to the gas accumulation and the deep formation source rock provided more. 2. The geochemistry of different gas field is diverse and the gas accumulation mechanisms are complex.
RI PT
3. Crude oil creaked gases from deep formation are proved that mixed in the lower Permian gas reservoirs. And the amounts and scales of fractures affect the amounts of
AC C
EP
TE D
M AN U
SC
gas from deep formation in different gas fields.
S0920-4105(17)30617-4
DOI:
10.1016/j.petrol.2017.07.068
Reference:
PETROL 4154
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 24 February 2017 Revised Date:
20 June 2017
Accepted Date: 27 July 2017
Please cite this article as: Lu, J., Ma, J., Chen, S., Wu, B., Li, Y., Zhang, B., Han, H., Lin, R., The geochemistry and origin of lower Permian gas in the northwestern Sichuan basin, SW China, Journal of Petroleum Science and Engineering (2017), doi: 10.1016/j.petrol.2017.07.068. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Abstract： ：The lower Permian series in northwestern Sichuan basin of China is a favorable region for natural gas accumulation with several potential source rocks and high-quality reservoirs. However, the tectonic evolution in this area has gone through long-term and complicated activities and the origin of gases in the lower Permian series is complex. For the wise exploration strategies planning, the origin and
RI PT
accumulation of natural gas in lower Permian reservoir have been addressed. And the results reveal that: (1) Three sets of potential source rocks, including the mudstone of Lower Cambrian Qiongzhusi formation, Lower Silurian Longmaxi formation and the
SC
marine carbonate source rocks of Lower Permian, have been identified as the gas sources in Lower Permian reservoirs. (2) The gases mainly contain by hydrocarbon gases, with the ratio of C1/C2+ > 0.99, and the characteristics of carbon isotopes are
M AN U
various and several wells’ are reversed or close to reversed. The δ13C2 values of Hewanchang and Wujiaba gas fields are ranging from -33.35‰ to -36.34‰（PDB）, similar to the production of sapropelic organic matter. The Shuangyushi and Kuangshanliang gas fields, with the δ13C2 values ranging from -28‰ to -30‰（PDB）, are source mixed gas. And the δ13C2 values of Jiulongshan gas field are the heaviest
TE D
with the value range from -25.23‰ to -28.4‰（PDB）. (3) The biomarkers of reservoir extracts are consistent with those of the lower Permian source rock, but the bitumen scales in lower Permian are not as much as those in the lower Cambrian or Lower Silurian series, which reveals that the crude oil creaked gas formed in lower Permian
EP
series falls a great deal than that in Cambrian and Silurian series. Based on the comprehensive researches of natural gas composition, carbon isotope, biomarkers of
AC C
extract and other parameters, the natural gases in lower Permian series are mixed by the crude oil creaked gas from deep formation and the lower Permian source rock, and the scale of fracture determines the amount of creaked gas mixed in the lower Permian reservoir.
KEYWARDS: Gas geochemistry； Natural gas origins； Gas accumulation model； Lower Permian series； Southwest Sichuan basin.
ACCEPTED MANUSCRIPT
The Geochemistry and Origin of Lower Permian Gas in the Northwestern Sichuan Basin, SW China Jungang LUa,b,c*, Jie MAa, Shijia CHENa,b,c, Bingyan WUa, Yong LIa,Benjian ZHANGd, Hui
RI PT
HANa,b,c, Ruopeng LINa a. School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China; b.Sichuan Natural Gas Geology Key Laboratories, Chengdu,
SC
Sichuan 610500, China; c. State Key Laboratory of Oil and Gas Reservoir Geology and
M AN U
Exploitation, Chengdu, Sichuan 610500, China; d.Northwest Sichuan Gas Field, PetroChina Southwest Oil and Gasfield Company, Mianyang, Sichuan 621700, China;
1 INTRODUCTION
The Sichuan basin, China’s largest gas-bearing basin, is characterized by containing different sediment environments, which changed from deep marine to
TE D
shallow terrestrial condition (Dai et al., 2003； Wang et al., 2002；Zhai G. M., 1992). For the past few decades, more than 300 gas fields and 20 gas producing layers have been found, and part of them were under the deep marine condition(Liu et al., 2000；
EP
Liu et al., 2004；Zhu et al., 2006). The study area is located in northwestern Sichuan basin, where several gas fields have been found in the Permian lower formation like Hewanchang, Shuangyushi, Jiulongshan and so on. With the testing production of
AC C
Well L16 (251.74×104m3/d, P1m)，Well L004-X1 (111.65×104m3/d, P1m) and Well ST1 (126.77×104m3/d, P1m) and so on，the lower Permian reservoirs are believed to have an excellent exploration potential. Some potential source rocks were formed in certain sedimentary cycles. And the
hydrocarbon-generating materials and periods are different for the several source rocks.（Dai et al., 2000；Li et al., 2012；Tenger et al., 2008；Xu et al., 2011）. Furthermore, multi-stage adjustment by tectonic movement also have a significant influence on gas characters，which make the gas-source relationships more complex. Several geologists believe that the gas was source from the lower Permian source 1
ACCEPTED MANUSCRIPT rock itself, (Cai et al., 2003；Li et al., 2013). Also, there were those who argued that Permian gas was sourced from Silurian crude oil cracked gas (Huang et al., 2011). Associated with other areas, it is significant to do researches on deep gas for its meaningful influence in gas reservoirs. And it’s necessary to dig into more details on the sources of Low-Permian gas. Based on comprehensive analysis of Permian gas
natural gas and establish the reservoir forming model.
SC
2 GEOLOGICAL SETTING
RI PT
geochemistry and source rocks as well as structural features, we discuss the cause of
Sichuan basin is located in the southwest of China, and the study area is near the northwest margin of the basin. The study area is bound by the Micang mountain to the
M AN U
north, Jiangyou to the south, Central Sichuan uplift to the east, and Longmenshan nappe fault fold belt to the west(Fig.1) (Li et al, 2002). The tectonism of study area is intense for its location on the edge of three secondary tectonic belt as Longmenshan piedmont fault fold, Micangshan fault zone and northern Sichuan tectonic belt. Before the Indosinian, northwestern Sichuan area was deposited of marine
TE D
carbonate on the passive continental margin in the western Sichuan craton. Cambrian sedimentary thickness was getting thinner from the southwest to southeast for the development of NW-SE platform margin in Sinian. Uplift and erosion led to thin in northwest and thick in southeast from Ordovician-Silurian to Devonian-Carboniferous
EP
while normal faults controlling the sedimentary. Permian and Lower-Middle Triassic formed stable shallow environment including multiple transgression and regression.
AC C
Also, Middle Triassic observed the comprehensive regression in Sichuan craton, uplifted Longmenshan, reversed normal faults and part of the thrust faults as well as the certain structural shape. At the end of the Upper Triassic, strong late Indosinian movement formed large-scale nappe structures and unconformity between Jurassic and upper triassic system formation. This area restarted the compression under the distant effect of northward subduction between India and Eurasian plate during Himalayan, and therefore, the formation of the whole developed a wide and slow unchangeable structure (Fig.2, Fig.3) ( Guo et al., 1996；He et al., 2014；Luo et al., 2006；Pang et al., 2010；Yuan et al., 2010).
2
ACCEPTED MANUSCRIPT Multiple oil gas structures, like Hewanchang and Shuangyushi, result in complex source-reservoir distribution and multiple sets of favorable assemblage rocks, reservoirs and cap-rocks. The certain lack of Devonian and Carboniferous strata and distinct loss of Upper-Middle Cambrian, Lower Ordovician and Upper-Middle Silurian have been detected according to the drilling data and outcrop observation
RI PT
(Hou et al, 2003;Li et al, 1992). There are Lower-Permian Qixia and Maokou carbonate rocks reservoirs with great properties and massive fractures for favorable migration and accumulation while various cap-rocks (Davies et al, 2005;Huang et al,
SC
2004a; Lavoie and Morin, 2004; Song et al, 2005; Yang and Feng, 2000). In a word, multiple sets of assemblage rocks, reservoir and cap-rocks exactly do a great favor to
M AN U
good generation, migration and accumulation.
3 SAMPLES AND MATHODS
A total of 10 samples, comprising 2 samples from Hewanchang Gas Field, 1 sample from Wujiaba Gas Field, 3 samples from Shuangyushi Gas Field, 2 samples from Jiulongshan Gas Field, 1 sample from Kuangshanliang Gas Field, 1 sample from
TE D
Shejianhe Gas Field, were collected and analyzed for chemical composition and carbon isotopes. Besides, 60 rock samples, comprising 37 core samples and 23 outcrop samples were collected from three sets source rocks and reservoirs of lower Permian analyzed for biomarkers. Furthermore, analysis results of seven gas samples
EP
and 25 rock samples were collected from Northwest Sichuan Gas Field, PetroChina Southwest Oil and Gasfield Company.
AC C
The molecular composition of the gas samples was measured by Agilent 6890N
gas chromatograph (GC) equipped with a flame ionization detector and a thermal conductivity
detector.
A
fused
silica
capillary
column
(HP-PONA
50mX0.20mmX0.50µm) was used for individual hydrocarbon gas components (C1-C5) separated. GC oven temperature was initially set at 27℃for 10min, and then ramped to 130℃at 3℃/min. The stable carbon isotopic values of the natural gas samples were measured by MAT 252 mass spectrometer (MS) equipped with EI ion source and Dual viscous sample introduction system. The gas components were separated on a GC equipped
3
ACCEPTED MANUSCRIPT with MS and converted into CO2 in a combustion interface and then injected into MS. A fused silica capillary column (HP-PLOT Q 30mmX 0.32 mm X 20.0µm) was used for separating the individual hydrocarbon gas components (C1-C2). The GC oven was initially set at 50℃for 3min, and then ramped to 190℃at 15℃/min. The stable carbon isotopic values are reported in the δ notation in per mil (‰) relative to the VPDB
RI PT
(Vienna Pee Dee Belemnite) standards.
The core and outcrop samples were powdered (<80 mesh) and extracted by a Soxhlet apparatus with a solvent mixture of dichloromethane and methanol (93:7, v: v)
SC
for 72h. And after evaporation and concentration, the concentrated extracts were mixed with n-hexane and allowed to stand for 12 h to yield the asphaltenes. The open silica gel column chromatography were used for yield saturated hydrocarbons,
M AN U
aromatic hydrocarbons and non-hydrocarbons respectively by n-hexane, a mixture of n-hexane and dichloromethane (2:1, v:v) and methanol from the remaining extracts. The saturated hydrocarbons GC-MS was measured by Agilent 6890N GC couple to MAT 252 MS equipped with fused silica capillary column (HP-5MS 60mX 0.25mmX 0.25µm). For analyzing the saturated fractions, the GC oven temperature
TE D
was initially set at 50℃for 1min, and then ramped to 120℃at 20℃/min and then to 310℃ at 3℃/min, with a final hold time of 30 min.
4 RESULTS
EP
4.1 SOURCE ROCKS
Three sets of potential source rocks, including the mudstone of Lower Cambrian
AC C
Qiongzhusi Formation, the shale of Lower Silurian Longmaxi Formation and the marine carbonate source rocks of Lower Permian, have been seen the origins of the gas in Lower Permian reservoirs(Tab.1). The Lower Cambrian Qiongzhusi Formation source rock are mainly black
mudstone or shale and widely distributed around the study area with the thickness between 150 and 300 meters(Fig.4.a). They have relatively high organic matter abundance with average TOC of 2.7% (between0.6% and 6.2%). And the rocks are characterized by typical feature of type Ⅰ kerogen and over maturity level (Ro between 2.7% and 3.83%)(Fig.5). Based on these characteristics, the Lower Cambrian Qiongzhusi Formation source rock are favorable for gas generation. 4
ACCEPTED MANUSCRIPT The Lower Silurian Longmaxi Formation source rock consists of black shale and dark-grey mudstone. And they are distributed in the northern part of the study area with the 250m max thickness and thinning to the south until pinch-out (Fig.4 b). The source rock are characterized by the average total organic matter of 0.85% (between 0.1% and 2.33%), by the partial sapropel organic matter with the type ofⅠ-Ⅱ1, and by
RI PT
the high maturity of 2.18%-2.83%. Thus, the Lower Silurian Longmaxi Formation source rock may be limited for the distribution and relatively favorable for gas generation in the northern study area.
SC
The marine carbonate source rocks of Lower Permian are distributed around the entire study area with the thickness between 80m and 250 m (Fig.4.c). The source rock has the average TOC of 0.34 % (between 0.11% and 0.7%), the partial sapropel
M AN U
organic matter with the type of Ⅱ1 and the high maturity with the Ro between 1.79% and 2.45%. Thus, the Lower Permian source rock have the hydrocarbon producing potential but inferior to the two sets source rocks above especially the Lower Cambrian Qiongzhusi Formation source rock.
On the basis of tectonic uplift and denudation by Caledonian movement, the
TE D
hydrocarbon generation of Paleozoic source rock has obvious hysteresis. And the maturity of three sets source rocks are obviously affected by the thickness of the Jurassic stratum. The Lower Cambrian source rock reached the oil window in the Permian, intensely generated oil during the Triassic to Early Jurassic, and to the peak
EP
gas generation stage in the Middle Jurassic to the Late Jurassic. The Lower Silurian Longmaxi Formation source rock reached the oil window in the Late Triassic, and
AC C
intensely generated oil during the Early Jurassic to Middle Jurassic, and to the peak gas generation stage in the Late Jurassic to the Cretaceous. The marine carbonate source rocks of Lower Permian reached the oil window in the Late Triassic, and intensely generated oil during the Middle Jurassic to late Jurassic, and to the peak gas generation stage in the Late Jurassic to the Cretaceous(Fig.6.)
4.2 NATURAL GAS GEOCHEMISTRY For the natural gas research, the chemical compositions of natural gas are significant indexes and several factors, like gas source rock, maturity and secondary alteration, have a great effect on it (Behar et al.1992; Pallasser, 2000; Tian et al., 5
ACCEPTED MANUSCRIPT 2009). In the study area, the composition of natural gas in the Lower Permian reservoirs consists of hydrocarbon gas and non-hydrocarbon gas and characterized by hydrocarbon gas with predominance in the total gas. Among the compositions of measured hydrocarbon gases, methane gases dominate, most of them are more than 95%(between 95.22% and 98.28%) and only seldom between 90% and 95%(Tab.2).
RI PT
Ethane gases all less than 1% in the total gas with the average of 0.67% and the gas dryness (C1/∑C1-C5) all > 0.99. Thus, the gas in this area is typical dry gases and in the high-over maturation.
SC
The abundance of non-hydrocarbon gases in the study area are not high, and mainly consist by carbon dioxide and nitrogen and with little hydrogen sulfide, helium, hydrogen and so on. Among the measured gases, the abundance of CO2 is
M AN U
mainly between 0.1% and 2.0 % and several relative high in some wells like Well Wujia1(5.93%) and Well Kuang1(3.79%). The abundance of N2 are mainly between 0.3% and 1.6 %, such as Well He2 (1.38%), Well He3 (1.18%), Well Long16 (0.92%), and so on. Besides, in this area, the H2S is ubiquitous in the marine carbonate reservoirs especially in the Jiulongshan gas field (Tab.2.).
TE D
The carbon isotope compositions of natural gas are very commonly used to indicate the origin, type and maturity of the gas (Clayton, 1991; Galimov, 2006; Stahl and Koch, 1974). And the δ13C2 values is used to assess the genetic type of the gas(Dai et al,2005;Wang, 1994) and the δ13C1 is believed to be related to the maturity
EP
with given genetic types(Dai, 1992;). Based on the numerous analytical results in Sichuan basin, it is proposed that the gas are derived from sapropelic kerogen if the
AC C
δ13C2 is <-30‰, humic kerogen if >-28‰, and mixed origin if between these two values. The δ13C1 value commonly increases with the maturity and have certain ranges in different evolutionary stages (Cao et al., 2012). As shown in Tab.3, the stable carbon isotopic composition gases in this area are
complicated and various. The δ13C2 values range from -25.23‰ to -36.34‰（PDB）, indicating that the gases are derived from different genetic types. According to the values of δ13C2, the natural gases can be divided into three kinds. The natural gases in the Hewanchang and Wujiaba gas fields are derived from typical sapropelic kerogen with the δ13C2 values ranging from -33.35‰ to -35.35‰（PDB）. In contrast，The
6
ACCEPTED MANUSCRIPT natural gases in the Jiulongshan gas field seem to be derived from typical Humic kerogen with the δ13C2 values ranging from -25‰ to -28.4‰（PDB）. In addition, there are also mixed origin gases like Shuangyushi gas field and Kuangshanliang gas field with the δ13C2 values ranging from -28‰ to -30‰（PDB）.Thus, origin of the
4.3 BIOMARKER GEOCHEMISTRY
RI PT
natural gases in this area is various.
The characteristics of reservoir extract and three possible source rock are different in their m/z 191 and m/z 217 mass chromatograms. The gas and source rocks
SC
can be well correlated according to the distribution of terpanes, hopanes, steranes and so on.
M AN U
The TT（tricyclic terpane）has attracted more and more attention for their extensive distribution, better thermal stability and strong anti-biodegradation. In the study area, the relative abundance of TT in the mudstone is higher than in the carbonate rocks (Fig7.). And all of the source rocks extracts demonstrate a tricyclic terpane C20 - C21 - C23 distribution of C20 < C21 < C23.
The distribution of the pentacyclic terpane shows that the abundance of C30
TE D
hopance is highest and with the carbon number increased the abundance decrease. The ratio of Ts/Tm is affected by several parameters like maturity, the organic facies and clay minerals in the source rocks and so on. Nearly all of extracts in the three set
EP
source rocks display high Ts/Tm values with marine carbonate source rocks of Lower Permian between 0.2 and 1.19, and mudstone source of Lower Silurian Longmaxi and Lower Cambrian Qiongzhusi be 0.76 and 0.87 respectively. The abundance of
AC C
gammacerane is not high and the ratio of gammacerane/C31 hopane is generally greater than 0.01.
The m/z mass chromatogram of the three set of source rock shows that the
abundance of pregnane and higher pregnane are moderate and the regular sterane isomerization degree is relatively high. In the Lower Permian, the regular sterane C27-29 demonstrate like shape‘V’ or partial ‘L’, with the distribution of C27 ≈ C29 > C28. And in the regular sterane C27-29 demonstrate like typical shape ‘L’, with the distribution of C27 > C29 > C28. According to the fig.8, the regular sterane C27-29 from the reservoir extract and 7
ACCEPTED MANUSCRIPT bitumen demonstrate like shape‘V’ or partial ‘L’, with the distribution of C27 ≈ C29 > C28, similar with the marine carbonate source rocks of Lower Permian. Indicating that the extract has greatest possibility evolved from the marine carbonate source rocks. The reservoir extract and bitumen demonstrate a TT distribution of C20 < C21 < C23. The distribution of the pentacyclic terpane shows that the abundance of
RI PT
C30 hopance is highest and with the carbon number increased the abundance decrease.
5 DISCUSSION 5.1 The origin of natural gases
SC
5.1.1 The gas derived from not only the lower Permian source rock, but also the deep formation source rocks, including lower
M AN U
Cambrian series and Lower Silurian series.
Based on the study above, several parameters, especially the carbon isotopes and distribution of bitumen, indicated that the gas origins from not only Lower Permian source rock, but also the deep series source rocks.
（1）The gas carbon isotopes of several wells were reversed or close to reversed.
TE D
Most field and laboratory observations and all theoretical models of kinetic isotopic effects yield a normal sequence of carbon isotopic compositions with δ13C1 < δ13C2 < δ13C3 < δ13C4 observed in natural gas accumulations worldwide. However, the
EP
carbon isotopic compositions will be reserved by mixing between gases from different sources, including inorganic sources and organic sources at different levels of thermal
AC C
maturity (Burruss and Laughrey, 2010; Dai et al., 2004; Huang et al., 2004b; Jenden et al.,1993).
According to the methane and ethane carbon isotope characteristics in the study
area, the carbon isotope reversals or nearly reversals are existed in several wells gases (Fig.9). Such as the Well He1 (δ13C1: -35.4‰，δ13C2: -35.35‰)in Hewanchang gas field, Well Wujia1（δ13C1: -28.49‰ and -29.77‰，δ13C2: 34.5‰ and -34.83‰ respectively ） in Wujiaba gas field and Well Shuangtan1 （ δ13C1:-29.7‰ ， δ13C2:-29.9‰） in Shuangyushi gas field(Tab.3). Based on above, the gases were mixed by the production of deep source rock, which led the gas carbon isotopes composition to reversed or close to reversed. 8
ACCEPTED MANUSCRIPT （2）The ethane carbon isotopes in Hewanchang and Wujiaba gases field were so light that they were inconsistent with the production of Lower Permian source rock. The methane gases are affected easily by thermal maturity, and the sapropel-type cracked gas and Humic type of pyrolysis gas are always hard to distinguish for their similar methane carbon isotopes distribution. However, the heavy hydrocarbon gas
RI PT
carbon isotopes like the ethane gas had little change as the thermal maturity of source rock increasing and used to distinct different types natural gas.
The values of ethane carbon isotopes from gas fields of Hewanchang and
SC
Wujiaba are small enough （-33.35‰～-36.34‰（PDB）,Tab.3., Fig.9.）that seem to origin from typical sapropelic organic matters, and the source rock of lower Permian series belongs to humic-sapropel type. So the ethane carbon isotope of lower Permian
M AN U
source rock can’t be so small, and with the increase of thermal evolution degree, isotope will transform to be heavier. Combining the viewpoints above, gas supply must exist that from low-cambrian typical sapropelic organic matters. （3）Bitumen scale in lower Permian series shows the oil-cracked gas is rarely existed, and external gas is mixed into the reservoirs.
TE D
After liquid hydrocarbon generation by sapropelic organic matters, it will generate pyrolysis gas accompany with bitumen as the maturity increases. So the existence of bitumen demonstrated liquid hydrocarbon has pyrolysed into gas before. The scale of the bitumen amount reflected the scale of pyrolysis gas. There wasn’t
EP
evident bitumen in lower Permian series of Qixia and Maokou Formation in the studied area (Fig.10 a,b). It was only fund under the fluorescence observation (Fig.10
AC C
c,d), which showed oil cracked truly existed in lower Permian series, but the scale is small, so the gaseous hydrocarbon is small too. According to the explorative achievements, it is not enough to generate so large scale sapropelic gas by the lower Permian series source rocks, there are the mixture gas from other source rocks.
5.1.2 The product from deep formation is crude oil cracked gas instead of kerogen degradation gas or liquid hydrocarbon. The production of high-quality sapropelic source rocks could mix with Permian hydrocarbons as liquids which then crack into gas, or just as the gaseous hydrocarbons like crude oil cracked gas or kerogen degradation gas. However, 9
ACCEPTED MANUSCRIPT according to the extract characters, bitumen distributions and gas compositions, we found that the mixture from the deep formation is mainly the crude oil cracked gas. (1) The similar characters between reservoir extracts and lower permian source shows that liquids hydrocarbon generated by deep formation source rock haven’t migrated into the lower permian reservoirs.
RI PT
The Permian reservoir extracts are mainly authigenic with some mixture of cracked gas instead of liquid hydrocarbons because of the dissimilar features to that of lower source rocks. According to the analysis on the biomarker characteristics of
SC
these three sets of source rocks as well as Lower-Permian extracts, the result shows that Lower-Permian bitumen comprises similar characteristics to the source rocks
type and some partial ‘L’ type.
M AN U
with relatively high abundant C27 sterane and lowest abundant C28 sterane just like ‘V’
（2）Crude oil accumulated and cracked in deep formation and the cracked gas migrated into the lower Permian series because of less bitumen in Lower-Permian reservoir but large-scale distribution in Cambrian reservoir.
The theory of hydrocarbon generation suggests that sapropelic source rocks are
TE D
dominated by oil in mature stage but oil into gas and some mixture of heavy-component bitumen with maturity (Chilingarian, 1988；Huang and Wang, 2008； Tissot and Welte, 1984; Xu, 1997). Associated with the Permian bitumen, no large-scale bitumen has been detected in Permian which implies that there’s no
EP
massive oil cracked or migration of liquid hydrocarbon from Lower- Cambrian high-quality source rocks but the vertical migration of gas mixed with Permian.
AC C
Also, the distribution of Cambrian field bitumen confirms this point: large-scale
bitumen exposure indicates the massive accumulation of Cambrian liquid hydrocarbon in the reservoirs and then crack into oil cracked gas and bitumen with maturity increased. The Cambrian oil cracked gas could supply for Lower-Permian gas reservoir and mix with Permian gas after the adjustment and migration into Permian favorable reservoir ( Fig.11). (3)The amount of N2 indicated the gas was not the kerogen cracked gas but crude oil cracked gas. The cross plot of natural gas (δ13C2-δ13C3) and ln(C2/C3) could do a great favor to
10
ACCEPTED MANUSCRIPT effectively distinguish oil cracked gas and kerogen cracked gas (Alain, 1995; Chen, 2002). However, the application effect of this method on studies area disappoints us because the concentration of heavy hydrocarbon gas is too low to detect in high-over mature stage(Hao et. al,1995；Stahl, 1977). The natural gas in Dengying formation, Weiyuan gas reservoir, next to studied
RI PT
area, has been confirmed to be typical Cambrian kerogen cracked gas in Qiongzhusi formation. According to the comparison of natural gas in adjacent areas, the nitrogen content in Lower-Permian gas reservoir is far less than that in Weiyuan Dengying
SC
formation which indicates the obvious distinction between them (natural gas in Weiyuan Dengying formation has been confirmed to generate from Cambrian kerogen cracked gas in Qiongzhusi formation(Yin, et. al,2001))( Fig12.). Therefore, it could be
M AN U
concluded that Lower-Permian gas is accumulated from oil cracked gas instead of kerogen cracked gas in Qiongzhusi formation.
5.1.3 The amount of gas origin from deep formation is various and controlled by the scale of fractures in different gas fields.
TE D
The structural interpretation results show that the western adequate fractures, mostly deep through the bottom source rocks, provide favorable migration path for hydrocarbons such as Cambrian deep gas upward and make it mix with others, while eastern fractures are much less to channel with deep gas.
EP
The amount of deep-sourced gas varies place to place for the different tectonic fractures and it also determines the weight of ethane isotope in distinct tectonic
AC C
natural gas.
(1) The ethane isotope of natural gas in Hewanchang and Wujiaba gas fields are relatively light for the pretty mixture of deep-sourced gas. The ethane isotope of natural gas in Hewanchang and Wujiaba gas fields show
typical sapropelic organic matter because the large-scale fractures which are mostly faults deep cross Cambrian strata become favorable path for Cambrian sapropelic gas to migrate upward. Massive deep Cambrian sapropelic gas mixed with relatively few Permian autogenetic gases to make ethane isotope so light. (2) The ethane isotope of natural gas in Shuangyushi and Shejianhe gas fields are relatively heavy for the amount of mixed gas from deep-source is not as much as 11
ACCEPTED MANUSCRIPT Hewanchang gas feild. The ethane isotope of natural gas in Shuangyushi and Shejianhe gas field are not light like that in Hewanchang gas field because the adequate but small-scale fractures, which are just deep to the Upper-Middle Cambrian strata, result in few mixture of deep-sourced gas and collective mixture of deep formation and Permian.
RI PT
(3) The ethane isotope of natural gas in Jiulongshan gas field is heavy for little mixture of deep-sourced gas.
There’s no or little mixture of deep-sourced gas because few fractures in
SC
Jiulongshan block the natural gas generated in Lower-Cambrian source rocks from Permian. Natural gas in Permian is mostly autogenetic and shows heavy isotopes.
M AN U
5.2 Model of natural gas accumulation
Based on hydrocarbon generation and expulsion, tectonic evaluation and trap formation history, two major petroleum charging periods in lower Permian reservoirs are suggested: the middle to late Jurassic and Cretaceous to early Tertiary. In the first period, the lower Permian source rock was in hydrocarbon generation threshold, with
TE D
the maturity value between 0.5% and 1.1%, the homogenization temperatures between 110℃ and 120℃,and dominant by the liquid hydrocarbon and low dry coefficient accompanying gas origin from the lower Permian marine carbonate source rocks(Fig.13). In the second period, the lower Permian source rock was in high and
EP
over mature periods, with the maturity value between 1.5% and 1.7%, the homogenization temperatures between 130℃ and 140℃, the lower Permian source
AC C
rocks are dominant in peak-period of dry gas generation. In this period, the crude oil creaked gas also migrated from the deep Cambrian and Silurian formation to lower Permian reservoir and mixed. The second period is the main accumulation stage. In the Stage of Himalayan movement, the Sichuan basin underwent intense
tectonic adjustment, and the movement also had a great effect on gas accumulation in the NW Sichuan basin. Based on tectonic movement, the fissures in the reservoir were increased and interconnected, which is beneficial to gas migration and accumulation in higher position of structure. Besides, the crude oil creaked gas from deep formation were migrated vertically by the developmental fracture and mixed in the lower Permian reservoir. The scale of fractures in different tectonic regions were diverse, 12
ACCEPTED MANUSCRIPT and the Hewanchang area was most developed and the gases were mainly from the deep formation, than the Shuangyushi area, and the Jiulongshan area was the least developed and the gases were mainly from the Permian source rock(Fig.14).
RI PT
6 CONCLUSION （1）Three sets of potential source rocks, including the mudstone of Lower Cambrian Qiongzhusi formation, the shale of Lower Silurian Longmaxi formation and the marine carbonate source rocks of Lower Permian, have been seen that contributed to
SC
the accumulation of gas in Lower Permian reservoirs. The deep formation source rocks have great contribution to the gases accumulation for the high abundance of
M AN U
total organic matter, typeⅠorganic matter and high or over maturity. The lower Permian carbonate source rocks also have contribution to the gases accumulation for closing to the reservoir.
（2）The natural gases in the study area are mixed in several periods by three source rocks. The crude oil creaked gases are predominant and accompany with the high or over mature kerogen cracked gas. The natural gases composition and carbon isotope
TE D
are diverse in different gas fields. The gases in the Hewanchang and Wujiaba gases fields are characterized with light methane and ethane carbon isotope and mainly from the deep formation source rocks. The gases in the Jiulongshan gases field are heaviest
EP
and mainly the production of lower Permian source rock. And the Shuangyushi and shejianhe gas fields are mixed origin gases.
AC C
（3）On the base of gases origin and genesis, tectonic and source rocks evaluation, two major petroleum charging periods in lower Permian reservoirs are suggested. And the homogenization temperature of fluid inclusion and Microscopic fluorescence also exist two periods, which support the conclusion above. The middle to late Jurassic was the first period and dominant by the liquid hydrocarbon. The Cretaceous to early tertiary was the second period and mainly the crude oil creaked gas mixed by the lower Permian and the deep formation oil pool. The second period is the main accumulation stage. During the Himalayan movement, the gases field formed before were adjusted by the movement, and migrated and accumulated in higher position of structure. Besides, the crude oil creaked gas from deep formation were migrated 13
ACCEPTED MANUSCRIPT vertically by the developmental fissures and fracture and mixed in the lower Permian reservoir.
ACKNOWLEDGEMENTS
RI PT
The authors would like to express their gratitude to School of Geoscience and Technology, Southwest Petroleum University ，Sichuan Natural Gas Geology Key Laboratories and State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation for providing the laboratory facilities to analyze the samples described in
SC
this paper. Additionally, we would also like to thank everyone who provides valuable
M AN U
comments and suggestions during the interpretation and revision process.
FUNDINGS
Funding for this research are National Science Foundation of China (No. 41502146 & 41572137), the National key Scientific and Technological project
(No.
2016ZX05007003—007) and Southwest Petroleum University funding plan of oil and
TE D
gas accumulation geochemistry for Youth Science and technology innovation team (No.2015CXTD02) for the petroleum accumulation research.
EP
REFERENCES CITED
Alain A.P., Alain Y. H., 1995. Genetic and post-genetic molecular and isotopic
AC C
fractionations in natural gases. Chem Geol, 126:281-290.
Behar F., Kressmann S., Rudkiewicz J.L., Vandenbroucke M., 1992. Experimental simulation in a confined system and Kinetic modeling of kerogen and oil cracking. Organic Geochemistry, 19, 173-189.
Burruss R.C, Laughrey C.D, 2010. Carbon and hydrogen isotopic reversals in deep basin gas: Evidence for limits to the stability of hydrocarbons. Organic Geochemistry.41,1285-1296. Cai K.P., Wang Y.R., Yang Y.M., et al., 2003.Evaluation of hydrocarbon source rocks in Permian and Triassic at Guangyuan Wangcang region in North West Sichuan
14
ACCEPTED MANUSCRIPT basin and a primary discussion on gas source. Natural Gas Industry, 23,10-14. Cao J., Wang X.L., Sun P.A., Zhang Y.Q., Tang Y., Xiang B.L., Lan W.P., Wu M., 2012. Geochemistry and origins of natural gases in the central Junggar Basin, northwest China. Organic Geochemistry. 53, 166-176. Chen S.J., Fu X.W., Ma L.N., Chen X.L., Zhang X., 2002. Genetic identification
RI PT
method of kerogen-creaked gases and oil cracked gases. Petroleum Geology & Experiment,24,364-366+371.
Chilingarian G.V., 1988. Bitumens, asphalts and tar sands. Yu Jingfang
SC
(Trans.).Beijing: Petroleum Industry Press.
Clayton, C., 1991. Carbon isotope fractionation during natural gas generation from kerogen. Marine & Petroleum Geology. 8, 232-240.
M AN U
Dai, J.X., 1992. Identification of different hydrocarbon gas. Sci. China 2, 185-193. Dai J.X., Wang T.D., Dai H.M., et al., 2000. Gas sources of large scale Carbonate gas field. Marine Petroleum Geology. 5( 1 /2):141- 144.
Dai J.X., Chen J.F., Zhong N.N. et al., 2003.Giant gas fields and gas source in China[M].Beijing: Science Press.
TE D
Dai, J.X., Xia, X.Y., Qin, S.F., Zhao, J.Z., 2004. Origins of partially reversed alkane δ13C values for biogenic gases in China. Organic Geochemistry, 35, 405-411. Dai J.X., Qin S.F., Tao S.Z., Zhu, G.Y., Mi, J.K.2005. Developing trends of natural gas industry and the significant progress on natural gas geological theories in
EP
China. Natural Gas Geoscience. 16, 127-142. Davies，G.R., T. Boreen and. L. Smith, 2005. Hydrothermal dolomite and leached
AC C
limestone reservoirs: Representative core fabrics and facies. AAPG Annual Meeting Program, 14, A32.
Galimov E.M., 2006. Isotope organic geochemistry. Organic Geochemistry. 37, 1200-1262.
Guo Z.W., Deng K.L, Han Y.H.,1996. The formation and development of Sichuan Basin. Beijing: Geological Publishing House. Hao S.S., Chen Z.M., Gao Y.B., 1995. The series and preservation of natural gas. Beijing: Petroleum Industry Press. He L.J.，Huang F.，Liu Q.Y.，Li C.R.，Wang J.Y., 2014. Tectono-thermal Evolution
15
ACCEPTED MANUSCRIPT of Sichuan Basin in Early Paleozoic. Journal of Earth Sciences and Environment, 36, 10-17. Hou M.C., Chen H.D., Tian J.C., 2003. Sequence-Filling Dynamics--A New Study Direction on Sequence Stratigraphy. Journal of Stratigraphy, 27, 358-364. Huang X.P., Yang T.Q., Zhang H.M., 2004a. Research on The Sedimentary Facies and
RI PT
Exploration Potential Areas of Lower Permian in Sichuan Basin. Natural Gas Industry, 24, 10-12.
Huang, H.P., Yang, J.，Yang, Y.F., Du, X.J., 2004b. Geochemistry of natural gases in
SC
deep strata of the Songliao basin, NE China. International Journal of Coal Geology 58 231-244.
Huang D.F., Wang L.S., 2008. Geochemical characteristics of bituminous dike in
M AN U
Kuangshanliang area of the Northwestern Sichuan Basin and its significance. Acta Petrolei Sinica. 29, 23-28.
Huang D., Dai X., Dai Y., Zhu Y.G., Yang Y., Pei S.Q., 2011.Geochemical behaviors and gas origins in the Hewanchang Gas Field northern part of the Western Sichuan Basin. Natural Gas Industry, 23, 37-40+109.
TE D
Jenden, P.D., Drazan, D.J., Kaplan, I.R., 1993. Mixing of thermogenic natural gases in northern Appalachian basin. American Association of Petroleum Geologists Bulletin 77, 980-998.
Lavoie D, Morin C.2004. Hydrothermal dolomitization in the Lower Silurian Sayabec
EP
Series in north Gaspe-Matapedia(Quebec):Constraint on timing of porosity and regional significance for hydrocarbon reservoirs. Bulletin of Canadian Petroleum
AC C
Geology, 52, 256-269.
Li S.T., Yang S.G., Lin C.S., 1992. On the Chronostratigraphic Framwork and Basic Building Blocks of Sedimentary Basin. Acta Sedimentologica Sinica, 10, 11-22.
Li H.J.，Xie X.N.，Huang J.H.，Chen H.，Lin Z.L., 2012.Main Factors Controlling the Series of Excellent Marine Source Rocks in Permian Maokou Series of Northwest Sichuan ， China. Earth Science-Journal of China University of Geosciences,37, 171-180. Li Y.H., Chen G.S, Zhang J., Yang Y.M., 2002. Natural Gas Pool Forming Conditions and Exploration Prospect of the Foreland Basin in West Sichuan. China
16
ACCEPTED MANUSCRIPT Petroleum Exploration, 7, 34-45. Li Y.J.，Zhao S.X.，Li Y.G.，Zhang B.J.，Ma H.L.，Pei S.Q., 2013. Natural Gas Genesis and Sources in Jiulongshan Gas Field，Northwest Sichuan Basin. Natural Gas Geoscience,24, 755-767. Liu D.L., Song Y., Xue A.M.,2000. Integral study of tectonics and natural gas
RI PT
accumulation in Sichuan basin, China[M]. Beijing:Petroleum Industry Press, pp： 20-65.
Liu S.G., Xu G.S., Xu G.Q., Yong Z.Q., Li G.R., Li J.C., 2004. Primary study on the
SC
dynamics of natural gas pools in Sichuan basin, China. Gas Geoscience, 15,323-330.
Luo L., Jia D., Chen Z.X.，Hu Q.W., Jia Q.P., Li Y.Q., Zhang Y.Y., 2006. Magnetic
M AN U
fabric evolution in the northwestern Sichuan basin, China and its strain characteristics. Geological Bulletin of China, 25, 1342-1348. Pallasser R.J., 2000. Recognising biodegradation in gas/oil accumulations through the δ13C compositions of gas components. Organic Geochemistry. 31, 1363-1373. Pang Y.J.，Zhang B.J.，Feng R.W.，Wang Y.R., 2010. Evolution of Devonian
TE D
depositional environment in northern Longmenshan tectonic belt. Global Geology, 29, 561-568.
Stahl W.J., Koch J., 1974. 13C-12C relation of North-German natural-gas - maturity
27,623.
EP
characteristic of their parent substances. Erdol & Kohle Erdgas petrochem.
Stahl, W.J., 1977. Carbon and nitrogen isotopes in hydrocarbon research and
AC C
exploration, Chemical Geology., 20:121-149.
Song Z.Q., Wang X.Z., Zeng D.M., 2005. Sedimentary Facies and its relativity with petroleum gas at Qixia age of Permian in the northwest of Sichuan. Journal of Southwest Petroleum Institute, 27, 20-24.
Tenger，Qin J.Z.，Fu X.D.，Li W.，Rao D.，Zhang M.Z.,2008. Basic conditions of marine hydrocarbon accumulation in northwest Sichuan basin-high quality source rocks. Petroleum Geology & Experiment,30, 478-483 Tian H., Xiao X.M., Yang L.G., Xiao Z.Y., Guo L.G., Shen J.G., Lu Y.H., 2009. Pyrolysis of oil at high temperatures: gas potentials, chemical and carbon
17
ACCEPTED MANUSCRIPT isotopic. Chin Sci. Bull. 54. 1217-1224. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence. Springer Verlag, Berlin Heidelberg, pp. 643–644. Wang, S.Q.,1994. Geochemical characteristics of Jurassic-Sinian gas in Sichuan basin. Natural Gas Industry. 14, 1-5.
RI PT
Wang Z.C, Zhao W.Z., Zhang L., et al.,2002. The structural sequence of the Sichuan basin and the natural gas exploration[M]. Beijing: Geological Publishing House, 27-94.
SC
Xu K.D.,1997. The origin of Kangshan bituminous lode. Marine Origin Petroleum Geology, 2;54-59.
Xu S.L.，Chen H.D.，Chen A.Q.，Lin L.B.，Li J.W.， Yang J.B.,2011. Source rock
M AN U
characteristics of marine strata, Sichuan Basin. Journal of Jilin University( Earth Science Edition). 41,343-350+358.
Yang Y.Q., Feng Z.Z, 2000. Permian depositional systems in south China. Journal Of Palaeogeography, 2, 11-18.
Yin C.H., Wang T.D., Wang S.Y., Lin F.,2001. Differences between Kerogen-and
TE D
Oil-cracked Gases in Sinian Reservoirs of Weiyuan and Ziyang Area. Acta Sedimentologica Sinica. 19, 156-160.
Yuan Q.D.，Li B.L.，Liu H.T.，Liu S.J.，Wang L., 2010. The tectonics evolution and lithofacies palaeogeography in the northwest of the Sichuan Basin. Journal of
EP
Daqing Petroleum Institute, 34, 42-52. Zhai G.M., 1992. Petroleum geology of China [M].Beijing: Petroleum Industry Press.
AC C
Zhu G.Y, Zhang S.C., Liang Y.B., et al., 2006. The characteristics of natural gas in Sichuan basin and its sources. Earth Science Frontiers, 13, 234-248.
18
ACCEPTED MANUSCRIPT
muddy limestone
OM Abundance
Organic Matter Type 13
TOC/%
δ C（‰）
0.2-0.7
-28.11～-27.40
0.38(47)
-27.69(23)
P 0.11-0.51 0.25(10) 0.1-2.33 S
S1l
shale 0.85(12) 0.6-6.2
∈1q
mudstone 2.7(10)
-28.25（9）
-32.04～-28.78
AC C
∈
-28.82～-27.36
TE D
muddy limestone
EP
P1q
TI
SC
P1m
Sample Type
M AN U
Formation
RI PT
Tab.1. Parameters of three sets potential source rocks in the Northwest Sichuan basin, SW China.
Ro/% Type
43-87.25
1.79-2.45 Ⅱ1
57.12(18)
2.0(13)
41.5-80.25
1.83-2.17 Ⅱ1
58.47(10)
2.05(5)
74.5-85.5
2.18-2.83 Ⅰ-Ⅱ1
-30.23(8)
80.1(5)
-34.96～-31.59
87.75-97.3
2.37(6) 2.7-3.83 Ⅰ
-33.06（9）
92.3(7)
3.32(5)
ACCEPTED MANUSCRIPT
Kuangshanliang Shejianhe Jiulongshan Shuangyushi
N2
0.03 0.05 0.04 0.05 0.06 0.07 — — — — — 0.032 0.023 0.035 0.04 0.027 0.03
0.14 0.13 0.16 — 0.23 0.1 0.5 0.1 — 3.5 0.05 — 0.005 0.005 0.001 0.001 0
1.27 1.43 1.38 1.18 0.37 0.97 1.69 0.51 0.48 6.47 1.06 1.01 0.92 1.38 1.34 0.69 0.87
1
H2S
CO2
C1
C2+
— — 0.05 0.05 — 0.2 — — 0.3 — — 0.72 0.81 0.36 0.74 0.017 0.34
0.06 0.21 0.06 0.33 0.07 0 5.93 7.31 3.79 0.1 1.33 0.02 0.68 0.96 0.041 0.81 2
97.21 96.01 97.67 97.82 98.28 90.92 91.68 91.79 95.22 88.67 97.35 97.5 97.42 97.16 96.94 96.94 96.65
1.32 0.688 0.564 0.971 7.82 0.11 0.317 0.19 1.573 0.22 0.15 0.14 0.1 0.14 0.131 0.1
SC
H2
M AN U
P1m P1m P1m P1m P1m P1m P1q P1m P1m P1m P1m P1m P1m P1q P1m P1m P1q
He
TE D
Wujiaba
He2 He2 He2 He3 He6 WuJia1 WuJia1 Kuang1 Kuang1 Kuang3 She1 Long4 Long16 Long17 Long004-X1 ShuangTan1 ShuangTan1
Formation
EP
Hewanchang
Well
hydrocarbon gases/%
non-hydrocarbon gases/%
AC C
Area
RI PT
Tab2. Chemical compositions of lower Permian natural gases in the Northwest Sichuan basin, SW China.
ACCEPTED MANUSCRIPT Tab3. Carbon isotopes of lower Permian natural gases in the Northwest Sichuan basin, SW China.
Shuangyushi
Jiulongshan
δ13C1（‰）
δ13C2（‰）
He1 He2 He2 He2 He3 Heshen1 Shuangtan1 Shuangtan1 Long16 Long16 Long17 Wujia1 Wujia1 Kuang1
P1m P1m P1m P1m P1m P1m P1m P1q P1m P1m P1q P1m P1m P1m
-35.4 -35.52 -33.7 -34.5 -35.5 -35.65 -29.7 -30.1 -27.66 -28.8 -28.31 -28.49 -29.77 -31.55
-35.35 -33.35 — -34.7 — -36.34 -29.9 -28.2 -26.27 -28.4 -25.23 -34.5 -34.83 -29.82
M AN U
Wujiaba
Formation
RI PT
Hewanchang
Well
SC
Area
AC C
EP
TE D
Kuangshanliang
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig.1. Sketch map showing structural units and gas distribution in the Northwest Sichuan basin, SW China.( ① Hewanchang,② Kuangshanliang,③ Shejianhe,④ Shuangyushi,⑤ Wujiaba,⑥ Zhangjiabian,⑦
TE D
Jiulongshan， ，⑧Changjianggou， ，⑨Wulongzhai; ①-⑦are gas fields, ⑧、⑨ are outcrops; Hewanchang-
AC C
EP
Shuangyushi-Jiulongshan Section is showing in Fig.3)
b
c
d
SC
a
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
Fig10. Pictures show the bitumen in the lower Permian reservoirs are rarely existed.( Pic. a & b are core from well He1 and well Bian1 and no evident bitumen in them. Pic. c & d are fluorescence photographs and a little bitumen in fissure and the larger pore )
c
d
e
f
SC
b
M AN U
a
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig11. Pictures show large amount of bitumen in the Cambrian reservoirs.(Pic.a & b are the larger scale bitumen in the outcrop; c & d are the cores that filled by bitumen in the fissures and pores; e & f are the fluorescence photographs and bitumen are obvious)
ACCEPTED MANUSCRIPT
N2 in the study area is not as much as Weiyuan gas feild.
The abundance of N2 is high in Weiyuan gas feild, which is the typical Kerogen creaking gas.
6.26
RI PT
WELLS
0.87 0.69 1.34 0.92 1.01 1.06 0.48 0.51 1.69 0.97 0.37 1.18 1.38 1.43 1.27
7.08 7.55
0
2
4
6
8
M AN U
N2(%)
SC
8.33
10
AC C
EP
TE D
Fig12. The gas in study area is different from Weiyuan gas field in abundance of N2, which means the gas origins are diverse.（ （The Nitrogen content of Well Wei 106, 39, 30, 2 respectively are 6.26%,7.08%,7.55% and 8.33%,the dates are courtesy of the Yin, et al.,2000 ）.
ACCEPTED MANUSCRIPT 30
20
15
10
5
0 90-100
100-110
110-120
120-130
RI PT
Samples(n)
25
130-140
SC
Homogenization temperatures(℃)
140-150
AC C
EP
TE D
M AN U
Fig.13. The distribution of homogenization temperatures of hydrocarbon inclusion showing two temperature intervals: 110-120 ℃ and 130-140 ℃, and showing the possibility that there are two key periods of massive oil migration in this area.
SC
RI PT
ACCEPTED MANUSCRIPT
Fig.14. Model of natural gas accumulation in the Northwest Sichuan basin, SW China. The section is
AC C
EP
TE D
M AN U
Hewanchang-Shuangyushi-Jiulongshan.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
Fig.2. Generalized stratigraphic column for the Northwest Sichuan basin, SW China.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig.3. Schematic cross section of HWC-JLS at different geological times showing tectonic evolution of HWC-JLS from the Sinian to the present.( HWC is Hewanchang area, and SYS and JLS represent Shuangyushi area and Jiulongshan area respectively).
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Fig.4.Distribution of source rocks in the Northwest Sichuan basin, SW China. (a)The Lower Cambrian Qiongzhusi Formation source rock; (b)The Lower Silurian Longmaxi Formation source rock; (c)The Lower Permian marine carbonate source rocks( Guangyuan , Jiange, Changjianggou, Tianjingshan, Shuangyushi, Kuangshanliang, Hewanchang, ⑧ Shejianhe, ⑨ Wujiaba, ⑩ Zhangjiabian, ⑪Jiulongshan).
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 5. Representative geochemistry of source rocks in the Northwest Sichuan basin, SW China. ( TOC = total organic carbon; Ro = vitrinite reflectance;δ δ 13C=Kerogen carbon isotopes； ； TI is type index ， [TI=Liptinite+Exinite×0.5-Vitrinite×0.75-inertinite].
The
figure
shows
that
there
are
significant
geochemistry differences in three set of source rocks in the study area. The Qiongzhusi formation source rock, with high abundance total organic carbon and typical organic matter type Ⅰ, is superior to the
AC C
EP
TE D
source rock in the Lower Permian formation.
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Fig.6. Thermal evolution during the burial history of the Northwest Sichuan basin, SW China. The figure
AC C
EP
TE D
shows that three set of source rocks are all in highly or over maturity.
SC
RI PT
ACCEPTED MANUSCRIPT
Fig7. Representative m/z191 and 217 mass fragments showing the correlation in the potential source
AC C
EP
TE D
M AN U
rocks(CJG= Changjianggou; WLZ=Wulongzhai; Wj1= well Wujia1; Hs1=Heshen1).
SC
RI PT
ACCEPTED MANUSCRIPT
Fig8. Representative m/z191 and 217 mass fragments showing the correlation in reservoir extracts and
AC C
EP
TE D
M AN U
bitumens(Wj1= well Wujia1; CJG= Changjianggou).
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Fig9. Cross plot showing δ13C1 vs. δ13C2 values of natural gases in the Northwest Sichuan basin, SW China. The gas carbon isotopes of several wells were reversed or close to reversed.(HWC=Hewanchang, WJB= Wujiaba, SYS= Shuangyushi, KSL= Kuangshanliang, JLS = Jiulongshan)
ACCEPTED MANUSCRIPT 1. Three sets of source rock contribute to the gas accumulation and the deep formation source rock provided more. 2. The geochemistry of different gas field is diverse and the gas accumulation mechanisms are complex.
RI PT
3. Crude oil creaked gases from deep formation are proved that mixed in the lower Permian gas reservoirs. And the amounts and scales of fractures affect the amounts of
AC C
EP
TE D
M AN U
SC
gas from deep formation in different gas fields.