Chinese Science Bulletin © 2009
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High density methane inclusions in Puguang Gasfield: Discovery and a T-P genetic study LIU DeHan1†, DAI JinXing2,, XIAO XianMing1†, TIAN Hui1, YANG Chun3, HU AnPing3, MI JingKui3 & SONG ZhiGuang1 1
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2 PetroChina Institute of Petroleum Exploration, Development and Research, Beijing 100083, China; 3 Zhejiang University, Hangzhou 310027, China
Based on measurement of homogenization temperature of inclusions and Raman spectral analysis, high density methane inclusions were discovered in the Triassic reservoirs of Puguang Gasfield. The methane inclusions show a homogenization temperature Th = 117.5― 118.1℃, a corresponding density of 0.3455―0.3477 g/cm3, and a Raman scatter peak v1 shift varying between 2911―2910 cm1, which signifies a very high density of methane inclusions. The salt water inclusions paragenetic with methane inclusions show a homogenization temperature Th=170―180℃. Based on the composition of methane inclusions as determined by Raman spectra, PVTsim software was used to simulate the trapping pressure for high density methane inclusions in geologic history, and the trapping pressure was found to be as high as 153―160 MPa. Even though Puguang Gasfield is currently a gas pool of normal pressure, and the fluid pressure for the gas pool ranges between 56―65 MPa. However, data from this study indicates that remarkable overpressure may be generated at the stage of mass production of gas cracked from oils in Cretaceous, as high density methane inclusions constitute key evidence for overpressure in gas pool in geologic history. Meanwhile, discovery of small amounts of H2S, CO2 or heavy hydrocarbon in part of the high density methane inclusions indicates that the geochemical environment for trapping of inclusions may be related to formation of H2S. Therefore, the observation results can help to explore the thermochemical sulfate reduction (TSR) conditions for oil cracking and H2S formation. Puguang Gasfield, high density methane inclusions, supercritical fluid, Raman spectral analysis, gas cracked from oils
Methane occurs extensively in nature, as abundant methane hydrate is concentrated in oceans, biogenic methane continues to be generated from recent sediments, a great number of methane-bearing gas pools were preserved in old sedimentary rocks, and even hydrothermal ore deposits and minerals of metamorphic rocks and igneous rocks contain methane[1,2]. Minerals or rocks formed in different geological environments or in different geological times may trap methane inclusions of multiple genetic types. Particularly, in reservoirs of hydrocarbon-bearing pools, methane-bearing fluid inclusions and gas inclusions mainly consisting of methane
are extensively distributed, and have been used as key indicators for evaluation and prospecting of hydrocarbon resources. However, methane-bearing inclusions in ordinary sedimentary rocks and hydrocarbon reservoirs generally show low density[1,2], and few reports are available for high density methane inclusions, which Received March 23, 2009; accepted July 20, 2009; published online November 3, 2009 doi: 10.1007/s11434-009-0582-8 † Corresponding authors (email:
[email protected] or
[email protected]) Supported by the National Foundation for Outstanding Young Scientists (Grant No. 40625011), Knowledge Innovation Program of Chinese Academy of Sciences (Grant Nos. KZCX2-YW-114 and KZCX2-YW-Q05-03) and GIGCAS3rd Stage Innovation Program (Grant No. 5407341801)
Citation: Liu D H, Dai J X, Xiao X M, et al. High density methane inclusions in Puguang Gasfield: Discovery and a T-P genetic study. Chinese Sci Bull, 54: 4714―4723, doi: 10.1007/s11434-009-0582-8
1 Geological background The Puguang Gasfield, located in the northeast Sichuan Basin, is a large gasfield with beach and reef facies consisting of the Lower Triassic Feixianguan Formation and Upper Permian Changxing Formation as reservoirs. Based on available literature[6,7], the Puguang Gasfield is a large gas pool of both structural and lithologic type, and has a trap area of 50 km2, with the trap being finalized in the Himalayan stage. The reservoir rocks mainly consist of karst dolomite, crystalline dolomite and particulate dolomite, and shows a porosity of 6.3%―28.0% (averaging 8%) and a permeability of (0.65―4.23)× 103 μm2. The gas pool has an effective thickness up to 229 m and a current burial depth of 4923―5259 m, its maximum burial depth in geologic history is > 6500 m. The gas in the gas pool mainly consists of CH4 (74.46%― 77.91%), and trace C2H6 and C3H8, H2S (12.1%―16.89%, averaging 15.2%), and CO2 (7.89%―9.1%, averaging 8.3%), and shows a desiccation factor >0.99. In the reservoir, pyrobitumen of high thermal maturity and pyrobitumen of intermediate phase and with special structures are extensively developed. Previous results hold that the gas pool mainly results from gas cracked from oils[6,7]. Currently, the geothermal gradient of the
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stratigraphy is 2.18℃/100 m, and the gas pool shows a pressure factor of 1.09―1.2, indicating that the gas pool is of normal pressure and high gas saturation type. Samples for this study were mainly collected from Puguang Gasfield, specifically from the carbonate reservoirs of the Lower Triassic Feixianguan Formation, Upper Permian Changxing Formation at Puguang 2 well, Puguang 3 well, Puguang 5 well, Puguang 8 well, Puguang 9 well, Puguang 11 well and Maoba 3 well, Maoba 4 well, and from the overlying Xujiahe Formation. The drill holes for the samples generally have a depth of 3730―5850 m. Here the samples collected from Puguang 2 well, Puguang 3 well, Puguang 5 well, Puguang 8 well and Maoba 3 well, showing development of abundant bitumen solids and inclusions, will be the focus for our study.
2 Test instruments and methodology Fluid inclusions and bitumen solids will be observed with Leica DMR XP polarizing, reflection and fluorescence microscope, with 100 W power, and 20―100× objectives. The homogenization temperatures of inclusions will be determined using LINKAM THMS-Q600 microscope heating stage and a temperature testing software (Linkam Scientific LINKSYS 32) with heating rate or cooling rate being determined at 0.1―15℃/min. Bitumen reflectivity will be determined using 3Y-LEICA DMR XP microscopic photometer, with a wavelength of 514 nm, calibration glass NR1149 Ro = 1.24%, immersion oil refraction 1.515, objectives 50 / 0.85 Oil P or 125/1.25 Oil, grating d =0.6 mm. Methodology and major test processes for sample observation and parametric determination can be seen in refs. [1,2]. For Raman spectral analysis of mineral inclusions, RM-2000 microscopic laser Raman spectrometer manufactured by Renishan (R) Inc. in UK will be used. The spectrometer has an argon ion laser emitter, with a wavelength of 514 nm, a power of 30 mW, a line width of 4 GHz. Argon ion laser incident on sample surface will have a power of 2―5 mW, and the samples for laser testing will have a diameter of 3―5 μm (with a minimum of 1 μm). The Raman peak shift for silicon used for calibration of instrument wave number is 520 cm1, and the data collection time is 10―100 s.
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were mainly discovered in structures or hydrothermal fluids with very special geological conditions [3–5]. In hydrocarbon-bearing reservoirs in China, abundant water-bearing methane inclusions, CO 2-bearing methane inclusions and gas inclusions mainly consisting of methane were discovered, but no report is available on high density methane inclusions in hydrocarbon-bearing pools. For this study, a series of Triassic gas reservoir samples collected from Puguang Gasfield were analyzed with Raman spectrometer and microscopes, and many methane inclusions of high abundance, high density and single phase were since discovered. In this paper, the occurrence of high density methane inclusions in the samples will be described, and the P-T conditions for formation of the inclusions will be calculated with a combination of techniques. In this way, the authors aim to explore the origin of high density methane inclusions in the reservoirs, the origin of the gas pools and the TSR processes for H 2S formation in the gas pools.
3 Results and discussion 3.1 Description of the occurrence of high density methane inclusions Generally, methane-bearing gas inclusions appear to be gray-black and show poor transparency under the transmitting microscope. However, methane inclusions of high density and supercritical fluid[8] (SCF) will appear to be homogeneously semi-transparent to transparent and consist of single phase, and the inclusions need to be refrigerated or be subject to microscopic Raman spectral analysis for effective identification. Methane inclusions are distributed inhomogeneously
inside the samples, and the reservoir samples collected from Puguang 5 well at 5054―5298 m, Puguang 8 well at 5119 m, Puguang 3 well, Puguang 2 well, etc. contain abundant inclusions and bitumen solids, while the reservoir samples collected from Maoba 3 well at 4360― 4380 m contain abundant bitumen solids, but the inclusions in these samples are too tiny to be observed under the microscope. High density methane inclusions, confirmed with laser Raman spectra and microscopic observation, mainly occur in quartz veins or well-crystallized, transparent calcite blocks, and generally appear to be very tiny, even though some occurring in quartz crystals can be as long as 60 µm (Figure 1). Bitumen-
Figure 1 Methane inclusions and pyrobitumen in quartz and calcite from Puguang 3 well and Puguang 5 well. (a) Showing the close distribution of single-phase methane inclusions, bitumen-bearing inclusions and ordinary bitumen inclusions in quartz veins; (b), (c) showing the single phase methane inclusions under high magnification; (d) showing distribution of pyrobitumen with inhomogeneous structure in reservoir; (e), (f) showing the distribution of single-phase methane inclusions, bitumen-bearing inclusions and small number of two-phase salt water inclusions in calcite.
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3.2 Raman spectral analytical results of inclusions Microscopic Raman spectral analysis is an effective technique for identification of methane inclusions, particularly for well-preserved inclusions with regular shape. The Raman spectra for methane inclusions in the samples show that the Raman scatter peak v1 shift generally varies between 2911―2910 cm1, typically for methane inclusions of high concentrations and high density. In addition, the Raman spectra for the samples would show differences in peaks for trace components, aside from the principle peak for methane. For one class of samples, the Raman spectra show strong peaks for high purity of methane inclusions, but only weak peaks for trace concentrations of gas components like nonhy-
3.3 Determination of homogenization temperature for methane inclusions Homogenization temperature for methane inclusions is a key factor for determination of the types and density of
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drocarbon (Figure 2(a)). For another class of samples, the Raman spectra show remarkable peaks for trace amounts of components like H2S, CO2, etc., aside from the strong peaks for high concentration of methane inclusions (Figure 2(b), (c)). Shown in Figure 2 are laser Raman spectra for three representative methane inclusions selected from the samples. As can be seen from the micrographs for the tested inclusions in Figure 2, their shapes are well preserved, and their Raman spectra all show high intensity of Raman scatter peak characteristic of methane, as the methane Raman scatter peaks are remarkable in the range between 0―4000 cm1. For example, in Figure 2 (a), the tested methane inclusions all show well-preserved hexagonal negative crystal shape, and are represented by the very strong scatter peak 2910 cm1 in the corresponding Raman spectra, which indicate that the inclusions consist of simple composition. Besides strong scatter peaks for methane, the other scatter peaks are mainly for quartz inclusions, while peaks for nonhydrocarbons like CO2, and for heavy hydrocarbons are very weak. Figure 2(b), (c) are Raman spectra for methane inclusions occurring in calcite blocks, with the strong Raman scatter peak shift being 2911 cm1 and 2910 cm1 for methane, respectively. In contrast to Figure 2(a), the Raman spectra in Figure 2(b), (c) also show weak peaks of 2604―2603 cm1 for H2S and even weaker peaks of 1384 cm1 and 1280 cm1 for CO2. Additionally, the strong peak at 1085 cm1 in the spectra is the Raman scatter peak for calcite, the host mineral for methane inclusions. As demonstrated by the results of observation and analysis of a great number of inclusions, the conditions for trapping of high density methane inclusions in the samples may possibly be related to gas cracked from oils in the reservoir. Meanwhile, the geologic and geochemical environment for TSR processes which are responsible for H2S formation may possibly exist in reservoir fluids enriched in methane. As a result, the Raman spectra for some high density methane inclusions in the samples may show scatter peaks characteristic of trace amounts of H2S and CO2.
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bearing inclusions, bitumen inclusions and small amounts of two-phase salt water inclusions as well as broken bitumen-bearing inclusions are in close association with methane inclusions in the samples, so the inclusion combination appears to be rather complicated and the inclusion distribution seems to be very compact. Generally, methane inclusions occurring in quartz veins show bigger size and more regular shapes than those in calcite minerals, and some high density methane inclusions even take the shape of the negative crystal of quartz (Figure 1(b)―(c), Figure 2(a)). In calcite minerals, bitumen-bearing inclusions occur abundantly, with some inclusions showing breakage or leakage, some inclusions are tiny in size, while some other inclusions are well preserved and suitable for determination with Raman spectral analysis (Figure 1(e)―(f), Figure 2(b), (c)). A great amount of pyrobitumen of high thermal maturity and pyrobitumen of spherulitic structure and intermediate phase structure occurs in reservoirs showing development of high density methane inclusions (Figure 1(d)). In reservoir samples collected from Puguang 5 well, Maoba 3 well, Maoba 4 well and so on, pyrobitumen show apparent inhomogeneity and the bitumen reflectivity(BRo) ranges between 2.85%―4.2%. In regard to the two-phase salt water inclusions in the samples, the homogenization temperature falls approximately in four groups, i.e., the low temperature group (120―135℃), the intermediate temperature group (140― 155℃), the high temperature group (160―180℃) and the high temperature group B (185―200℃). Here high density methane inclusions are mainly paragenetic with salt water inclusions in the high temperature group.
Wave number (cm1)
Figure 2 Laser Raman spectra for high density methane inclusions. (a) Laser Raman spectrum for high density and high purity methane inclusions containing small amount C2H6, CO2, in quartz; (b) Laser Raman spectrum for high density methane inclusions containing small amount of other components like C2H6, H2S, CO2, in calcite blocks; (c) Laser Raman spectrum for high density methane inclusions containing small amount of other components like C2H6, H2S, CO2, in calcite blocks. The peaks at 2910 cm1, 2911 cm1 are Raman scatter peaks characteristic of methane. The peaks at 2948 cm1，2947 cm1 are Raman scatter peaks characteristic of ethane. The peaks at 1384 cm1 and 1281 cm1 are Raman scatter peaks characteristic of CO2, the peak at 2604 cm1 is Raman scatter peaks characteristic of H2S, while the peak at 3065 cm1 is for trace amount of heavy hydrocarbons (C6H6). Moreover, the peaks at 463 cm1, 206 cm1 and 126 cm1 are all for quartz hosting the inclusions, the peak at 1085 cm1 is for calcite hosting the inclusions. The micrographs for inclusions in the spectra are for Raman spectral analysis. 4718
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methane inclusions in the vision range when the temperature is increased quickly to 118℃―117.5℃, which signals that the high density methane inclusions have reached their homogenization temperature. Figure 3(d), (e), (f), (g) are local magnifications of Figure 3(b) and (c). As shown from Figure 3(d), a small bubble occurs when the temperature is quickly reduced to < 129℃. In Figure 3(e), when the temperature is increased slowly to 118℃, the bubble disappears, and liquid phase becomes homogenized. The phase transformation in methane inclusions during microscopic temperature determination indicates that methane inclusions in the samples were trapped in a high density supercritical system[1,8]. Table 1 lists 7 results of homogenization temperature for supercritical phase methane inclusions, and
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inclusions[8]. As the high density methane inclusions in the samples tend to break and leak, determination by cooling of the inclusions would be difficult. Therefore, methane inclusions which are well preserved, show high methane purity and bigger sizes will be picked out for microscopic observation (Figure 2(a)) and for cooling and heating. Shown in Figure 3(a)―(g) are actual micrographs of samples taken on the microscope heating stage during the processes of cooling and heating. Here Figure 3(a) shows distribution of groups of abundant single-phase supercritical methane inclusions under room temperature. Figure 3(b) shows bubbles generated from single phase methane inclusions when the temperature is quickly reduced to 129℃― 130℃. Figure 3(c) shows disappearance of the bubbles among the majority of the
Figure 3 Determination of homogenization temperatures for supercritical methane inclusions. (a) Micrograph showing abundant supercritical methane inclusions at 29.1℃ before temperature determination on microscope heating stage; (b) micrograph showing bubble formation in abundant supercritical methane inclusions when the temperature is cooled to 129℃; (c) micrograph showing disappearance of bubbles from the majority of the methane inclusions when the temperature is increased to 118.1℃; (d), (e), (f) and (g) micrographs showing individual supercritical methane inclusions for determination of homogenization temperatures via cooling, here (d) is a micrograph for bubble formation in a methane inclusion at 129℃, (e) is a micrograph for bubble disappearance from the same methane inclusion as in (d) at Th = 118.1℃, (f) is a micrograph for bubble formation in another methane inclusion at 130℃, (g) is a micrograph for bubble disappearance from the same methane inclusion as in (f) at Th = 117.5℃. Table 1
Test results of supercritical methane inclusions in the samples via cooling and heating Inclusion
1
2
3
4
5
6
7
Phase
supercritical
supercritical
supercritical
supercritical
supercritical
supercritical
supercritical
Morphology
round
spherulitic
triangular
strip-like
polygonal
triangular
strip-like
Size (µm)
20×30
8×7
20×35
3×11
20×33
15×26
8×12
Temperature for bubble generation (℃)
122
130
128
127
127
122
122
Homogenization temperature (℃, Th)
117
118
118
118
118
117
117
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shows that the homogenization temperatures determined for methane inclusions in the samples are rather close to one another. As the high density methane inclusions in the samples contain only very small amount of CO2, and are very tiny in sizes, the phase equilibrium between CH4 and CO2 needs to be further observed through cooling[9]. 3.4 Density of methane inclusions In regard to determination of homogenization temperature for methane inclusions, only inclusions containing very high concentration of methane and only trace amount of non-hydrocarbon components will be selected. Therefore, the density of methane inclusions for temperature determination can be derived from the figures or tables of thermodynamic parameters for gas-liquid homogenization of methane[810], or from the P-T phase diagram for methane. Based on the homogenization temperature determined for methane inclusions (mainly 117℃― 118℃), and from the thermodynamic parameter tables for homogenization temperature, density, pressure and molar volume for supercritical methane inclusions, as prepared by Liu et al. [10], it can be inferred that the density for methane inclusions in the test samples corresponding to the homogenization temperature (117℃― 118℃) in the table[10] varies from 0.3455 to 0.3477 g/cm3, which far exceeds the critical density for methane (0.162 g/cm3). Additionally, the density of methane inclusions in the samples can be approximately represented by the Raman spectral shift. According to the curve describing the relationship between methane Raman peak v1 shift and methane pressure experimentally determined for a glass capillary system under different pressure conditions by Lu et al.[11] and Lin et al.[12], when the methane Raman peak shift varies from 2918 to 2910 cm1, the corresponding pressure will be gradually increased from 65―66 MPa under room temperature, according to the figures and tables listed in Lu et al. [11]. However, as fewer experimental data are available for high pressure from literature, the relationship curve is slowly inclined, hence the extrapolated pressure would inevitably show 4720
remarkable errors. Therefore, the extrapolation can only be used for an approximate evaluation of the high density of methane inclusions under high pressure. In regard to methane inclusions for which homogenization temperature data are find to determine, pressure data corresponding to Raman shift may be derived, and PVTsim software was used to simulate and calculate the density and trapping pressure of methane inclusions in the rocks, which prove to be geologically significant for exploration and evaluation of hydrocarbon resources. 3.5 Temperature and pressure for formation of high density methane inclusions In a hydrocarbon reservoir, the trapping temperature and trapping pressure of fluid inclusions are key data for understanding the P-T conditions for reservoir fluid in an old hydrocarbon-bearing pool. However, simulation and calculation of trapping pressure for inclusions are subject to the composition of the inclusions. Since inclusions purely of single component are hard to find among fluid inclusions in naturally occurring minerals, and each component is hard to be determined in a multicomponent fluid inclusion, simulation and calculation of pressure for the inclusion would similarly not be precise. Based on the results of laser Raman detection and microscopic observation of inclusions in a sample, it is possible to discover some methane inclusions with high purity, and this will be favorable for simulation and calculation of trapping pressure of inclusions by using PVTX software. In order to extrapolate the trapping pressure for high density inclusions in geologic history, it is necessary to first determine the trapping temperature for methane inclusions in samples. Here the trapping temperature is not only related directly to the homogenization temperature of salt-water inclusions paragenetic with high density inclusions, but also related approximately to the reflectivity and optical structure of the associated bitumen solids. The bitumen solids from the reservoir rocks in Puguang and Maoba reservoir are distinct in not only reflectivity (BRo%), but also in texture, structure and inhomogeneity. For Puguang 5 well (with hole depth of 5060―5158 m) characterized by development of high density methane inclusions, the reservoir bitumen is pyrobitumen of high thermal maturity, showing remarkable inhomogeneity (Figure 1(d)) and reflectivity (BRo%) varying mainly between 2.5%―3.5% (averaging 2.95%), which indicates that the samples underwent high tem-
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Table 2
Homogenization temperature, Raman shift parameters, PVTsim-simulated pressure and density for methane inclusions Results of homogenization temperature for methane inclusions
Homogenization temperature (℃) 3
117
Calculation method 118
from test a)
Raman shift (cm1) and pressure for methane inclusion
Calculation method
Data for PVTsim simulation of pure methane inclusions
2911
2910
from test
117
118
Density (g/cm )
0.3455
0.3477
from table
0.3232
0.3251
PVTsim
0.3455
0.3477
Volume (cm3/mol)
49.0
48.7
PVTsim
52.39
52.09
PVTsim
46.43
46.13
25℃ pressure (MPa)
78.6
80.1
PVTsim
65.0
66.0
from figure b)
94.5
96.5
170℃ pressure (MPa)
153.0
155.6
PVTsim
129.1
131.0
PVTsim
178.6
182.0
180℃ pressure (MPa)
158.0
160.7
PVTsim
133.4
135.1
PVTsim
184.2
187.6
200℃ pressure (MPa)
167.9
170.7
PVTsim
141.9
143.9
PVTsim
195.5
199.0
Molar composition of methane inclusions simulated with PVTsim (extrapolated based on Raman spectral peaks for methane inclusions): CH4 95.97%, C2H6 0.897%, C6H6 trace, CO2 1.962%, H2S 1.194%
Molar composition simulated with PVTsim: CH4=100%
a) Liu and Shen[10]; b) Lu et al.[11]. Liu D H et al. Chinese Science Bulletin | December 2009 | vol. 54 | no. 24
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different temperatures as simulated with homogenization temperature data determined for methane inclusions in natural samples is slightly lower than the pressure simulated for pure methane under identical temperature, this is mainly because in actual simulation for methane inclusions, trace amount of components like CO2 and H2S as determined by Raman spectral analysis of methane inclusions was added, which leads to remarkably decreased pressure being simulated for methane inclusions under different temperatures. This demonstrates that a change in type or content of inclusion components to be simulated would result in significant change in pressure obtained; and (3) pressure simulated based on Raman scatter shift data for methane inclusions is lower than pressure simulated under different temperatures for identical methane inclusions, which are themselves simulated based on homogenization temperature data. This is mainly because pressure 65―66 MPa corresponding to Raman shift data 2911 cm1 and 2910 cm1 as selected under normal temperature for methane inclusions is underestimated (among the experimental data of Lu et al.[11], the maximum extreme pressure corresponding to Raman shift 2910.97 cm1 for methane is as low as 65.53 MPa, and the relationship curve is slowly inclined at high pressure stage, which may result in significant errors in pressure estimate). As a result, pressure simulated based on Raman scatter shift data for methane inclusions can only be used as a reference to the high pressure for methane inclusions as demonstrated with the other technique proposed in this paper. In regard to simulation of trapping pressure for high density methane inclusions in the samples with PVTsim software, density data 0.3455 g/cm3 and 0.3477 g/cm3 corresponding to homogenization temperature for methane inclusions
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perature for thermal evolution. On the other hand, based on the homogenization temperature data determined for salt-water inclusions paragenetic with high density methane inclusions in the samples, the inclusions fall mainly into the high temperature group (170―190℃). Therefore, PVTsim software can be used for pressure simulation under different temperature conditions [13]. To test the reliability of the technique for simulation and calculation of trapping pressure for inclusions by using PVTsim software, PVTsim software was first used to simulate pressure for pure methane inclusions under different temperatures, and the data, listed in the far right column of Table 2, are basically close to those in classical literature[10], demonstrating that the inclusion pressure results as simulated using PVTsim software in this paper are reliable. In actual simulation of trapping pressure for high density methane inclusions, pressure under different temperatures is not only simulated (in the far left column of Table 2) with density data 0.3455 g/m3 and 0.3477 g/m3, corresponding to homogenization temperature 117℃ and 118℃ for high purity methane inclusions, but also simulated (in the middle of Table 2) with approximate pressures corresponding to Raman scatter peak shifts 2911 cm1 and 2910 cm1 for methane inclusions, with the two groups of data being compared with each other. When the three groups of simulation data as listed in Table 2 are compared with one another, features of methane inclusions can be summarized as follows: (1) Pressure for 100% methane component under different temperatures as simulated with PVTsim is higher than the data listed in either the left column or the middle column of Table 2, but is basically close to the data for pure methane in classical literature; (2) pressure under
(as listed in Table 2, left column) and trace amount of non-hydrocarbon components will be adopted, and the results indicate that the trapping pressure is 153―155.6 MPa when the trapping temperature is 170℃, and is 158 ― 160.7 MPa when the trapping temperature is 180℃ (Table 2, left column). As demonstrated by the simulation results of trapping pressure for methane inclusions, even though the current gas pool pressure is 56.73―65.27 MPa, the trapping pressure for high density methane inclusions in the samples is as high as 153―160 MPa in geologic history, obviously this is an over pressure, which is consistent with the results of dynamic simulation for an oil-cracked gas pool studied by Tian et al.[14], as Tian et al.[14] proposed that remarkable overpressure is resulted at the stage of massive production of oil-cracked gas in Cretaceous. Additionally, the overpressure statement also conforms to the descriptions about a geologic structure model proposed by Zhao et al.[15].
4 Geological significance Occurrence of high density methane inclusions in geological bodies generally means that the geologic and geochemical environment is special. For example, Hurai et al.[3] discovered methane inclusions with density as high as 0.43 g/cm3 in an overthrust zone in the Carpathian Range, indicating that the zone has undergone a key structure-related geothermal event; Chen et al.[4] reported development of methane inclusions with a density of 0.309 g/cm3 in analcime crystals in Dongying depression, and deemed that the inclusions could be non-biogenic and related to hydrothermalism associated with basalt eruption; Beeskow et al.[5] investigated the phase equilibrium features of inclusions consisting of high density methane and ca. 15% CO2 in inclusions in euhedral quartz from South Wales coalfield in UK; Song et al.[16] treated high density methane inclusions as discovered in harzburgite in the Northern Qilian Mountains as evidence of old fluid associated with a plate attenuation zone[16]. However, methane-bearing gas inclusions occurring in hydrocarbon-bearing pool reservoir samples generally show low density. The density (up to 0.3477 g/cm3) of methane inclusions as discovered in Lower Triassic Feixianguan Formation in Puguang Gasfield far exceeds the critical density for methane (0.162 g/cm3), and the trapping pressure of high density methane inclusions in geologic history as simulated with PVTsim 4722
is >153―160 MPa, demonstrating that the inclusions are of the high temperature-overpressure type, and so the oil-cracked gas pool in the study area is characterized by formation and distribution of high density methane inclusions[14,15]. Besides, H2S-bearing gas pool in Puguang gasfield is suitable for “TSR” study[1720], as the P-T conditions for formation of high density methane inclusions and the trace amount of H2S, CO2 in methane inclusions reveal that overpressure resulted from high temperature cracking of deep-buried crudes charged in intermediate to early stages in the reservoir, and the process may possibly be accompanied by H2S formation. Therefore, further study of the laws governing occurrence and distribution of high density methane inclusions will yield more data for exploring the conditions for crude-cracked gas formation and the TSR processes for H2S formation in Puguang Gasfield.
5 Conclusions (1) Based on microscopic observation and Raman spectral analysis of inclusions, a group of very special, high density methane inclusions were discovered for the first time in Triassic reservoirs in Puguang Gasfield, and their homogenization temperatures (Th) were found to vary mainly between 117― 118℃, corresponding to a density of 0.3455―0.3477 g/cm3. The major Raman spectral peak v1 shift for methane inclusions varies from 2911―2910 cm1, also symbolizing high density of the inclusions. (2) Based on the density of methane inclusions and the homogenization temperature 170 ― 180 ℃ determined for the associated salt-water inclusions, PVTsim software was used to simulate the trapping pressure and give a result of 153―160.7 MPa, which demonstrates that the high density methane inclusions in the study area are mainly trapped in a geologic environment characterized by high temperature and over pressure at the stage of massive production of oil-cracked gas. (3) Based on the results of Raman spectral analysis of the inclusions, it was discovered that some high density methane inclusions contain trace amount of H2S, CO2, heavy hydrocarbons and bitumen, which indicates that the trapping conditions for high density methane inclusions are related not only to gas cracked from oils, but also possibly to the TSR processes for H2S formation. The authors thank Tan Dayong of GIGCAS for his help in performing the
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Liu D H et al. Chinese Science Bulletin | December 2009 | vol. 54 | no. 24
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ARTICLES
cal methane inclusions in geological bodies and comments for revision of the manuscript.
GEOLOGY
Raman spectral analysis of inclusions, and Prof. Lu Huazhang of Quebec University in Canada for his providing data on distribution of supercriti-
