Article pubs.acs.org/EF
Geochemical Characteristics and Genesis of Oil-Derived Gas in the Jingbian Gas Field, Ordos Basin, China Wenxue Han,† Shizhen Tao,*,† Lianhua Hou,† and Jingli Yao‡ †
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, People’s Republic of China Exploration and Development Research Institute, PetroChina Changqing Oilfield Company, Xi’an, Shaanxi 710021, People’s Republic of China
‡
ABSTRACT: The source rocks of natural gas of the Jingbian gas field in Ordos Basin, China, are unclear, and the origin of oilderived gas is controversial. On the basis of a series of experiments on source rock evaluation, including total organic carbon, Rock-Eval pyrolysis, carbon isotope composition of kerogen, chloroform bitumen A, maceral compositions, and gas chromatograms, we evaluated the hydrocarbon generation potential of the possible source rocks. We also conducted the components and stable carbon isotopes of natural gas and compared geochemical characteristics of natural gas and source rockadsorbed gas. The results indicate that source rocks from the Lower Permian Taiyuan Formation have some hydrocarbongenerating capability but far less than its proximate Carboniferous and Permian coal measures. Oil-derived gas originated from Lower Permian Taiyuan Formation limestone was expulsed to the Majiagou Formation by coal-derived gas generated from the Carboniferous and Permian coal measures, as indicated by source rock evaluation, adsorbed gas, and geochemical characteristics of gas collected from the limestone-developed area. The former two gases mixed in the Majiagou Formation above the gypsum− salt bed, which corresponds to the geochemical characteristics of gas from the above gypsum−salt bed. Source rocks from the Majiagou Formation above the gypsum−salt bed experienced much more weathering, leaching, and oxidation compared to those under the gypsum−salt bed, suggesting that the latter are comparatively better than the former. As a result of the barrier of the gypsum−salt bed, source rocks of the Majiagou Formation under the gypsum−salt bed can generate a self-stored natural gas reservoir, as indicated by adsorbed gas and geochemical characteristics of gas from under the gypsum−salt bed. Natural gas of the Jingbian gas field is mainly from the Carboniferous and Permian coal measures, with limited oil-derived gas from the Taiyuan Formation.
1. INTRODUCTION In China, we defined coal-derived gas as natural gas derived from humic organic matter (mainly in coal measures) and oilderived gas as natural gas derived from sapropelic organic matter (including secondary cracking of oil). At present, the Chinese researchers commonly agree that gas in giant accumulations in the Upper Paleozoic clastic rocks of the Ordos Basin are coal-derived gas originated from the Carboniferous and Permian coal measures.1−4 However, no agreement has been reached on gas sources of the Jingbian gas field (also known as the Central gas field). Since the discovery of the Jingbian gas field, its gas sources have been controversial.5−10 Some scholars hold that it is dominated by coal-derived gas originated from Carboniferous and Permian coal measures; however, others deem that it is dominated by oil-derived gas. In addition, the source rocks of oil-derived gas are still unclear. The hydrocarbon generation potential of possible source rocks of both the Lower Permian Taiyuan Formation and the Ordovician of Majiagou Formation is still controversial. In this respect, there are mainly four views: (i) Primarily oilderived gas: The gas mainly comes from the carbonate rocks of Majiagou Formation, which is self-generation and self-storage. The pure carbonate rocks with about 0.2% total organic carbon content are considered to have potential to generate natural gas.8,9 Different from the common standard, these scholars insist that carbonate rocks with advanced maturity and huge © XXXX American Chemical Society
thickness may compensate for the low total organic carbon (TOC) values. (ii) Primarily coal-derived gas, together with a small amount of oil-derived gas: The coal-derived gas mainly originated from the Carboniferous and Permian coal measures, while the small amount of oil-derived gas comes from the Permian Taiyuan Formation marine−terrigenous limestone.5,7 (iii) Distal mixed type gas: It is mainly the mixture of oilcracked gas generated in the Ordovician marl and shale in the western−southwestern basin and coal-derived gas derived from the Carboniferous and Permian coal measures.6,10 (iv) Primarily coal-derived gas, followed by oil-derived gas: This view is similar to view ii above; namely, natural gas mainly comes from the Carboniferous and Permian coal measures but different in the source of oil-derived gas. Researchers holding this view believe that the oil-derived gas comes from the Ordovician. That is, the Upper Paleozoic coal-derived gas migrated into the Majiagou Formation and mixed with the selfgenerated oil-derived gas.4 It is inferred from the above views that all researchers agree on the existence of oil-derived gas in the Jingbian gas field but argue about the source of the oil-derived gas. As mentioned above, there are two possible source rocks (Lower Permian Taiyuan Formation and the Ordovician of Majiagou ForReceived: April 26, 2017 Revised: August 22, 2017 Published: August 29, 2017 A
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Regional geological map of (a) tectonic units and distribution of Central gas fields in the Ordos Basin and (b) generalized lithologic column of Upper Paleozoic strata (modified with permission from ref 5. Copyright 2005 Elsevier).
widely distributed karst reservoirs.13 The reservoirs are lithologically dominated by anhydrite-bearing muddy micrite dolomite and situated in the development zone of karst terrace, with abundant pores, cavities, and fractures existing because of interlayer karstification, weathering crust karstification, and compaction released water karstification successively.14,15 The Jingbian gas field is located in a zone with relatively weak tectonic activity and covered by high-quality cap rocks. Specifically, the 240−350 m thick lacustrine argillaceous rocks of the Upper Shihezi Formation, the bauxitic mudstone, and the calcareous mudstone at the bottom of the Benxi Formation serve as the cap rocks, and the overlying coal measures could form a hydrocarbon seal.16,17 The Jingbian gas field has three sets of possible source rocks, i.e., the Carboniferous and Permian coal measures, the Lower Permian Taiyuan Formation marine−terrigenous limestone, and the Ordovician Majiagou Formation marine carbonate rocks. The Carboniferous and Permian coal measures, with their hydrocarbon generation potential universally recognized and widely studied, are considered as the main gas source rocks in the Ordos Basin,5,18,19 and they are not discussed in this paper. The other Lower Permian Taiyuan Formation and Ordovician Majiagou Formation possible source rocks are controversial. The Ordovician Majiagou Formation set can be divided into two parts by the gypsum rocks. Both the Lower Permian Taiyuan Formation and Ordovician Majiagou Formation source rocks reach certain thickness in the central
mation). In recent years, more and more wells were drilled below the Majiagou Formation gypsum rocks (in this paper, we divided Majiagou Formation into two parts: strata that are under the gypsum rocks, we call “pre-salt”, and strata that are above the gypsum rocks, we call “post-salt”), and the testing of the existence of effective pre-salt rocks becomes one of objectives of this study. In addition, some researchers pointed out that the Lower Permian Taiyuan Formation marine− terrigenous transitional limestone might generate some oilderived gas, but they have not dealt with hydrocarbon generation potential.4,5,7 This paper presents a systematic evaluation on three sets of possible gas source rocks in the Jingbian gas field. The natural gas and adsorbed gas in source rocks are compared with respect to their geochemical features, to determine the source of the oil-derived gas and the hydrocarbon generation capabilities of the possible source rocks. The study results will have direct significance to the further exploration and development of natural gas in the Jingbian gas field.
2. GEOLOGIC SETTING The Jingbian gas field, in the central Ordos Basin of China, resides in the middle of the Yishan slope structurally, with an area of about 6000 km2 (Figure 1). The paleogeomorphic and lithologic combination traps formed in the Hercynian period.11,12 The extensive microfacies belts of dolomite flat containing anhydrite provide an ideal basis for the formation of B
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
8 cylinder gas samples from Lower Paleozoic post-salt, and 6 cylinder gas samples from Lower Paleozoic pre-salt. TOC and Rock-Eval pyrolysis analysis data were carried out using a LECO CS-230 carbon sulfur analyzer and Rock-Eval 6 pyrolysis, respectively. During the experiments, the samples were analyzed at a constant temperature of 300 °C for 3 min to obtain S1 (free hydrocarbon) and heated at a heating rate of 25 °C/min between 300 and 650 °C to obtain S2 (pyrolysis hydrocarbon). Chloroform bitumen “A” was obtained using an ASE300 solvent extraction instrument. During the experiments, the samples were analyzed at a constant pressure of 10 MPa, temperature of 100 °C, time span of 5 min, and cycle times of 2. The stable carbon isotopes of kerogen were measured by the FlashEA-ConFlo.-irmMS method, using a Finngan MAT-252 mass spectrometer. Other experimental conditions included the reactor temperature of 980 °C, chromatographic column temperature of 50 °C, flow rate of carrier gas of 300 mL/min, purge gas flow of 200 mL/min, and oxygen injection rate of 175 mL/min. Maceral compositions were measured by the Axiophot microscope. Gas chromatograms of saturated hydrocarbon were obtained using Agilent 7890GC. As a result of the inexistence of vitrinite, we use vitrinite-like maceral to approximately substitute vitrinite reflectance. The composition of natural gas was analyzed using a HP6890 chromatograph with an inlet temperature at 300 °C, N2 as the carrier gas, a split ratio of 50:1, flow rate of 1 mL/min, and heating rate of 3 °C/min with the starting temperature at 30 °C. Stable carbon isotopes were analyzed using a Finnigan MAT-252 mass spectrometer. The reported carbon isotope values were compared to the GBW04405 standard to determine the relative Pee Dee Belemnite (PDB) values.
and eastern Ordos Basin, and they are in direct contact with the Ordovician weathering crust reservoirs, with favorable source− reservoir assemblages.
3. SAMPLING AND METHODS 3.1. Adsorbed Gas Experiments. Samples used in the adsorbed gas experiment include four samples taken from the Permian Taiyuan Formation limestone and four samples from the Ordovician Majiagou Formation pre-salt. All of these samples are effective source rocks and allow for sufficient amounts of adsorbed gas to be obtained with a TOC value greater than or equal to 0.5%. The Ordovician Majiagou Formation samples were collected under the gypsum−salt rocks to rule out the possible intrusion of gas derived from the Upper Paleozoic coal measures. Details of the samples are shown in Table 1.
Table 1. Details of Samples well
strata
depth (m)
S137 Y19 Y60 Z5 S36 T17 T38 J1
P1t P1t P1t P1t O1m O1m O1m O1m
3478 2428 2357 2400 2279 3803 3628 3723
lithology argillaceous argillaceous argillaceous argillaceous argillaceous argillaceous argillaceous argillaceous
limestone limestone limestone limestone dolomite dolomite dolomite dolomite
TOC (%)
Rb (%)
0.9 1.3 1.0 1.3 0.6 0.5 0.6 0.5
1.3 1.5 1.7 1.4 1.4 1.7 1.6 1.7
4. RESULTS 4.1. Source Rock Evaluation. 4.1.1. Organic Matter Abundance. We collected 60 source rock samples from Permian Taiyuan Formation marine−terrigenous limestone to evaluate the hydrocarbon generation potential. The TOC was measured as 0.03−4.72%, with an average of 0.61%, and the percentage of TOC over 0.5% is 36%. The TOC of dark argillaceous limestone reaches 1.0−5.0%, with an average of around 1.2%. The potential hydrocarbon generation amount (S1 + S2) is 0.02−1.78 mg/g, with an average of 0.18 mg/g. The chloroform bitumen “A” content ranges from 0.02 to 0.11%, with an average of 0.08%, and the average total hydrocarbon content is 493.2 μg/g. When all of these parameters are taken
The adsorbed gas used in this experiment was prepared by the following procedure: (1) Core samples were roughly crushed and then loaded into a closed stainless-steel tank with three impact beads. Then, the tank was placed under vacuum for 2 min. (2) After evacuation, the tank was put into the holder and dealt with 10 min of fine crushing in a pulverizer. (3) The tank was dried for 1 h at 80 °C in a drying case to desorb the adsorbed gas from the core powder. (4) The collected gas was analyzed for stable carbon isotopes and components. 3.2. Source Rock Evaluation and Natural Gas Experiments. Furthermore, a series of source rock screening analyses were conducted on cores of the possible source rocks. Moreover, the components and stable carbon isotope values of the cylinder gas samples (gas collected from the well) were measured. We collected 60 source rock samples from Permian Taiyuan Formation marine− terrigenous limestone, 13 cylinder gas samples from Upper Paleozoic,
Figure 2. Ternary diagram of sapropel−vitrinite−inertinite showing the organic matter types (data of Lower Paleozoic from refs 21 and 23). C
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 3. Gas chromatogram of saturated hydrocarbon of (a) source rocks of the Permian Taiyuan Formation and (b) source rocks of the Ordovician Majiagou Formation pre-salt.
Table 2. Geochemical Features of Source-Rock-Absorbed Gas in the Ordos Basin, China carbon isotope (‰, VPDB) well
strata
depth (m)
C1
C2
S137 Y19 Y60 Z5 J1 T38 S36 T17
P1t P1t P1t P1t O1m O1m O1m O1m
3478 2428 2357 2400 3723 3628 2279 3803
−34.1 −38.4 −23.0 −33.2 −38.8 −38.5 −36.2 −30.6
−23.8 −25.0 −24.0 −25.7 −32.3 −33.8 −30.2 −31.5
C3
component (%) C4
C1
C2
C3
iC4
nC4
89.0 97.5 97.8 95.9 97.1 98.6 89.7 95.1
1.6 2.0 2.1 3.4 2.3 1.2 8.0 2.8
7.3 0.3 0.2 0.6 0.5 0.2 1.2 2.1
1.5 0.1
0.7 0.1
0.1 0.1
0.1 0.1
0.8
0.3
the ternary diagram of sapropel−vitrinite−inertinite (Figure 2). The organic matter of the Ordovician Majiagou Formation source rocks is of type I (sapropel). Gas chromatograms of saturated hydrocarbon were used to study the source rocks (Figure 3). The maximum peak carbon numbers of the Permian Taiyuan Formation and the Ordovician Majiagou Formation pre-salt of source rocks are C17−18 and C24−25, respectively, indicating that they are from different parent material.24,25 The original parent material of the source rocks from the Permian Taiyuan Formation is mainly green algae, with those of the Ordovician Majiagou Formation pre-salt being mainly planktonic algae and acritarchs. 4.2. Geochemical Features of Adsorbed Gas. Sourcerock-adsorbed gas is generated from a single source rock, retained in situ, slightly affected by migration differentiation, and therefore considered a good indicator for investigating gas generation. As a result of the above reasons, adsorbed gas can reflect the geochemical characteristics of primary natural gas generated from source rocks. The experimental data are shown in Table 2. 4.3. Geochemical Features of Cylinder Gas. 4.3.1. Geochemical Features of Gas from the Upper Paleozoic. To verify the gas generation potential of the Permian Taiyuan Formation limestone, we collected gas samples from 13 wells located in the limestone-developed area to analyze the components and stable carbon isotopes. In this area, the limestone is the thickest in the whole Ordos Basin, with the thickness of 30−35 m. In addition, the wells are located in the same area as source rocks analyzed in the adsorbed gas experiment to verify the accuracy of adsorbed gas experimental results. It was found that methane dominates the gas generated in the zone, and the dryness coefficient (C1/C1−4, %) averages 97.2%, indicating the typical dry gas. Non-hydrocarbon
into account, the Permian Taiyuan Formation limestone reached a moderate level. The TOC values of Ordovician Majiagou Formation were collected from references. The TOC measured by Xia Xinyu in 1999 on 702 Ordovician samples in the Ordos Basin showed that only 12.9% of samples had TOC > 0.3% and 2.8% with TOC > 0.5%, while the average TOC of all of the samples was as low as 0.20%.20 His study mainly focused on the Majiagou Formation post-salt bed. Along with further exploration and development, some wells were drilled below the gypsum−salt layer, providing more Majiagou Formation material under the gypsum−salt bed. The study by Liu et al. revealed that the average TOC of 163 under the gypsum−salt bed samples was 0.30% and 28.2% of the samples had TOC > 0.5%.21 4.1.2. Organic Matter Types. Because maturity of source rocks in the Ordos Basin is high, the pyrolysis indices are limited. The stable carbon isotope value of kerogen, which is less influenced by the maturity, serves as an important indicator for defining the types of organic matter and is widely used in China.22 The δ13C values of typical (algae-type) type I kerogens range between −28 and −30‰; those of type II kerogens range between −25 and −28‰; and those of type III kerogens sourced from terrestrial higher plants are the heaviest, in a range from −20 to −25‰, usually more than −25‰. The Permian Taiyuan Formation limestone source rocks have δ13C values ranging between −27.87 and −23.76‰ (average of −25.72‰). The organic matter is dominated by type II with a high content of sapropelic components, being humic− sapropelic type. In addition, we put the maceral compositions of source rocks into the ternary diagram of sapropel−vitrinite− inertinite (Figure 2). We can draw a similar conclusion as the stable carbon isotope based on Figure 2. Organic matter in the Ordovician Majiagou Formation is mainly planktonic algae and acritarchs,23 which are plotted into D
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 3. Component and Carbon Isotope Properties of the Gas from Upper Paleozoic , Ordos Basin, China δ13C (‰, VPDB)
component (%) well
strata
C1
C2
C3
iC4
nC4
CO2
N2
C1
C2
C3
P1 QC1 M1 ZC1 Z4 Z5 Y12 Y15 S132 S193 S10 F5 S3 S19 S26 Y30 Y45 Y69 Y80 P2
P1t P1t P1t P1t P1t P1t P1t P1t P1t P1x P1t P1t P1x C2b P1t P1s P1s P1s P1s P1s
96.3 95.1 97.2 92.0 95.0 94.2 95.3 92.3 94.8 94.2 96.9 94.4 91.5 95.0 87.2 94.1 94.2 94.9 93.8 95.4
1.0 1.4 1.0 4.4 2.8 2.9 1.2 3.7 0.5 0.8 1.5 2.6 4.3 1.9 1.8 3.1 3.1 2.9 2.8 2.8
0.2 0.4 0.1 0.9 0.4 0.5 0.7 0.2 0.0 0.1 0.3 0.7 1.3 0.2 0.2 0.5 0.5 0.4 0.4 0.4
0.02 0.01 0.01 0.21 0.03 0.09 0.14 0.11 0.01 0.01 0.03 0.13 0.04 0.04 0.02 0.07 0.08 0.06 0.06 0.05
0.01 0.01 0.01 0.19 0.03 0.08 0.16 0.10 0.01 0.01 0.04 0.12 0.03 0.03 0.02 0.08 0.08 0.06 0.06 0.04
0.8 1.1 0.9 1.5 1.4 1.0 1.1 3.2 4.4 4.7 0.6 1.2 0.6 2.7 7.1 1.6 1.6 1.3 1.2 0.7
1.7 2.0 0.7 0.7 0.3 1.3 1.3 0.5 0.2 0.2 0.7 0.9 2.2 0.1 3.0 0.4 0.4 0.4 1.5 0.5
−31.7 −32.6 −31.8 −34.7 −32.8 −32.5 −36.0 −31.3 −32.2 −31.5 −37.1 −33.4 −30.9 −31.3 −35.4 −33.1 −33.2 −32.8 −32.7 −32.3
−25.3 −24.5 −26.9 −28.7 −23.5 −24.6 −23.1 −31.4 −33.7 −32.9 −24.5 −32.2 −36.4 −25.3 −25.8 −23.0 −25.2 −26.3 −25.3 −25.1
−23.9 −22.7 −21.5 −24.6 −23.6 −23.2 −30.7 −27.4 −29.3 −22.6 −27.6 −35.4 −25.8 −24.9 −23.4 −23.1 −24.1 −23.5 −23.1
C4
data sources this paper
−23.8 −23.2 −21.7 −22.5 −21.7 −22.7 −23.2
5 26
Figure 4. Radar plot for natural-gas-type identification based on carbon isotopes.
some generation potential, however, with lower hydrocarbon generation intensity than the Carboniferous and Permian coal measures. Even the gas generated in the limestone-developed area shows obvious features of coal-derived gas. Nevertheless, the Lower Permian Taiyuan Formation limestone also has certain hydrocarbon generation potential and may generate gas (dominantly oil-derived gas) in some areas. This explains why some Upper Paleozoic wells show the features of oil-derived gas and indicates why the gas in the post-salt karst reservoirs within the Jingbian gas field shows the features of source-mixed gas.
contents, mainly CO2 and N2, are generally low, which are consistent with test results of the Permian Taiyuan Formation adsorbed gas (Table 3). According to the stable carbon isotope values (Table 3), few wells drilled in the Upper Paleozoic limestone zone have δ13C2 values less than −28‰, showing typical features of oil-derived gas.27,28 However, δ13C2 values in most wells are greater than −28‰, revealing obvious features of coal-derived gas. According to the stable carbon isotope features of the adsorbed gas (generally the features of coal-derived gas), it is concluded that the Lower Permian Taiyuan Formation limestone has E
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 4. Component and Carbon Isotope Properties of the Gas from Lower Paleozoic Post-salt , Ordos Basin, China δ13C (‰, VPDB)
component (%) well
strata
C1
C2
C3
iC4
nC4
CO2
N2
C1
C2
C3
S193 S27 S5 S6 S62 S24 S18 S25 S155 S190 S84 S61 S20 S74 S45 S58
O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5 O1m5
94.3 94.7 94.1 92.7 96.4 98.8 96.8 97.9 92.9 92.9 92.4 97.5 93.1 94.3 94.9 94.1
0.7 1.1 0.5 0.3 0.6 0.3 0.3 0.3 0.7 0.6 0.8 0.8 0.3 1.0 0.2 1.1
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1
0.01 0.02 0.01 0.00 0.01 0.00 0.00
0.01 0.02 0.01 0.00 0.01 0.00 0.01
4.45 2.79 3.63
0.01 0.01 0.01 0.01 0.01 0.02 0.00 0.02
0.01 0.01 0.01 0.01 0.02 0.01 0.00 0.02
2.62 0.13 5.95 5.40 5.09
0.23 1.26 1.60 2.30 0.64 0.70 0.02 1.35 0.22 0.20 0.99
0.55 4.43 4.44 4.43
1.02 0.10 0.25 0.12
−33.1 −34.6 −32.3 −33.8 −32.5 −32.1 −33.4 −33.6 −32.7 −33.0 −31.8 −34.0 −34.6 −33.4 −33.5 −33.9
−30.6 −30.1 −30.2 −30.1 −33.6 −29.7 −31.3 −32.2 −30.2 −29.6 −28.5 −27.7 −31.0 −27.4 −30.6 −26.9
−27.4 −27.0 −27.7 −26.8 −28.9 −26.3 −26.1 −27.5 −27.8 −27.1 −24.2 −28.4 −27.5 −25.9 −22.9 −27.3
C4
data sources this paper
−22.1 26 −23.3 −20.9 −24.8 −22.1 −22.1 −22.5 −23.0
Table 5. Component and Carbon Isotope Properties of Gas from Lower Paleozoic Pre-salt, Ordos Basin, China δ13C (‰, VPDB)
component (%) well
strata
C1
C2
C3
iC4
nC4
CO2
N2
C1
C2
C3
S322 S379 T39 L30 L12 L19 T36 T37 S222 LT1
O1m57 O1m56 O1m56 O1m57 O1m57 O1m57 O1m3 O1m510 O1m4 O1m57
95.7 96.8 96.5 93.7 98.4 95.1 82.2 88.1 90.9 96.9
0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.1 0.1 1.8
0.03 0.01 0.02 0.02 0.01 0.01 0.00 0.01 0.10 0.30
0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00
1.04 1.08 1.01 0.96
1.79 1.75 2.33 3.40
0.00 0.00 0.00 0.10
0.87 9.49 6.01 4.82 0.10
0.94 6.22 5.67 4.04 0.63
−36.3 −38.3 −31.1 −30.2 −28.7 −30.5 −33.0 −30.7 −34.9 −23.8
−33.2
0.00 0.01 0.00 0.10
−36.1 −36.5 −38.1 −36.7 −37.2 −36.9 −37.3 −38.2 −33.1 −39.3
C4
data sources this paper
−21.9 −29.9 −30.8 −25.8 −20.0
−17.4
−19.7
−19.3
28
7
their generally low TOC values. In contrast, the limestone of the Permian Taiyuan Formation has certain hydrocarbon generation capability, and the Carboniferous and Permian coal measures are featured by extensive hydrocarbon generation, so that the generated hydrocarbons can drive the gases derived from the Permian Taiyuan Formation into favorable Ordovician post-salt karst reservoirs. The inference is consistent with the experimental results above. Thus, the oilderived gas in the Ordovician Majiagou Formation post-salt gas comes from the limestone of the Permian Taiyuan Formation, but the coal-derived gas generated from the Carboniferous and Permian coal measures dominates. 4.3.3. Geochemical Features of Gas from the Lower Paleozoic Pre-salt. The geochemical features show that the pre-salt gas has features of oil-derived gas based on the δ13C2 values (Table 5). The exception is gas from the Longtan-1 well, with δ13C2 > −28‰, which shows the features of coal-derived gas. It appears that intensive thermochemical sulfate reduction (TSR) resulted in heavier ethane carbon isotopes.7 This is consistent with the experimental results of the adsorbed gas, indicating that the Ordovician Majiagou Formation has certain hydrocarbon generation capability and may form small gas pools dominated by oil-derived gas if the appropriate source− reservoir−cap rock assemblages exit below salt. However, presalt gas wells yield low production, which is attributed to two aspects. First, the hydrocarbon generation capability of the Majiagou Formation pre-salt is limited, making it less probable
On the basis of the stable carbon isotope features of natural gas, a radar plot with end members of stable carbon isotopes was established (Figure 4). The four ends are CH4, C2H6, C3H8, and C4H10, which are assigned to −35, −28, −27, and −24‰, respectively, and the inner area formed by the generated envelope corresponds to the coal-derived gas. 4.3.2. Geochemical Features of Gas from the Lower Paleozoic Post-salt. After the analysis on stable carbon isotopes of the Ordovician Majiagou Formation post-salt gas in the Ordos Basin (Table 4), it was found that most of δ13C1 values are higher than −35‰ and δ13C2 values are less than −28‰. As judged by the methane carbon isotopes, the gas is obviously coal-derived gas; however, by the ethane carbon isotopes, it is typical of oil-derived gas.27,28 The δ13C1 values are almost in the same range as those of the coal-derived gas formed in the Upper Paleozoic, while the δ13C2 values are greatly different from those of the coal-derived gas generated in the Upper Paleozoic (panels a and b of Figure 8). The results from δ13C1 and δ13C2 appear contradictory, which eventually resulted in completely different conclusions. The post-salt gas is a mixture of coal- and oil-derived gases, but the question lies on the source of the oil-derived gas. Therefore, the gas source cannot be determined just geochemically but analyzed comprehensively by combining the geological setting and the accumulation conditions. As described above, the post-salt source rocks of Majiagou Formation cannot serve as effective source rocks because of F
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 5. Gas migration and accumulation cross-section of Ordovician Majiagou Formation post-salt (modified with permission from ref 26. Copyright 2016 Science Press).
Figure 6. Gas accumulation model of Ordovician Majiagou Formation pre-salt.
to form large commercial gas fields. Second, the Changqing Oilfield Company did not pay enough attention to Ordovician Majiagou Formation pre-salt compared to post-salt, which may miss the high production gas field.
Formation to migrate to and accumulate in the Ordovician Majiagou Formation post-salt bed (Figure 5). For this reason, part of the Ordovician Majiagou Formation post-salt bed gas has the characteristics of mixed gas, where the oil-derived gas is supposed to come from the Permian Taiyuan Formation limestone. Although source rocks of post-salt and pre-salt belong to the same Majiagou Formation, the source rocks of post-salt experienced much more weathering, leaching, and oxidation compared to those of pre-salt, suggesting that source rocks of pre-salt are comparatively better than those of post-salt. As mentioned above, the data of Majiagou Formation pre-salt does not seem likely to generate large gas fields, but with favorable source−reservoir−cap rock assemblages (that is, the gypsum can provide good cap rocks, and the Majiagou Formation dolomite can provide both source rocks and reservoirs) (Figure
5. DISCUSSION 5.1. Analysis of the Possible Source Rocks. The Lower Permian Taiyuan Formation marine carbonates, which are dominated by sapropelic kerogen, were supposed to generate oil-derived gas, but their adsorbed gas presents as coal-derived gas. The limestone from Lower Permian Taiyuan Formation has limited hydrocarbon generation potential, which, however, is inferior to that of the adjacent Carboniferous and Permian coal measures. The gas generated in the Carboniferous and Permian coal measures drove the gas generated in the Taiyuan G
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 7. Relationship between δ13C1 and C1/C2 + 3 (modified with permission from refs 31 and 32. Copyrights 1978 John Wiley & Sons, Inc. and 1999 Elsevier, respectively; partial data from refs 5,7,26,28).
Figure 8. Relationship between δ13Cn and carbon number (1/Cn) of natural gas for (a) Upper Paleozoic, (b) Lower Paleozoic post-salt bed, and (c) Lower Paleozoic pre-salt bed (partial data from refs 5,7,26,28).
6), the pre-salt Majiagou Formation is considered to be selfgeneration and self-storage gas reservoirs, which has been proven by Tao 38 well, Tong 74 well, and Longtan-1 well.7,29,30 This is also confirmed by the adsorbed gas experiments; as seen from Table 2, both the Ordovician Majiagou Formation pre-salt dolomite and the Permian Taiyuan Formation limestone can generate gas dominated by methane, of which the average content is 95%. The Ordovician Majiagou Formation pre-salt contains effective source rocks and presents good source rock−reservoir collocation, facilitating the formation of self-generation and self-storage reservoirs. 5.2. Natural Gas Origins. The relationship between δ13C1 and C1/(C2 + C3) is widely used to identify kerogen types for gas generation and genetic types of gas.31,32 We put samples from the Upper Paleozoic and Lower Paleozoic into the plot (Figure 7). Most of the gases from the Upper Paleozoic plot to coal-derived gas from type III kerogen, suggesting that they are derived predominantly from Carboniferous and Permian humic
source rocks. The gases from the Lower Paleozoic pre-salt plot to oil-derived gas from type I kerogen, suggesting that they are derived from Ordovician Majiagou Formation sapropelic source rocks. The gases from the Lower Paleozoic post-salt are between them, suggesting that they are mixed with coal- and oil-derived gases. In addition, the natural gas from the Upper Paleozoic and Lower Paleozoic post-salt have some coincident parts, suggesting that they had a genetic relationship and the mixed gases of Lower Paleozoic post-salt may mainly originate from Upper Paleozoic source rocks. Alkane gases from a single source tend to show the same trends between δ13Cn and 1/Cn (carbon number), and the trends can be used to identify whether the gases are mixed or homogeneous (Figure 8). Different source rocks can generate gases with different carbon isotope values.33−36 As seen from Figure 8, the trends of gases from Upper Paleozoic and Lower Paleozoic post-salt and Lower Paleozoic pre-salt are obviously different, suggesting that they are different genetic types and H
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
gas originated from Lower Permian Taiyuan Formation to migrate to and accumulate in the Lower Paleozoic Majiagou Formation post-salt. For this reason, part of the Ordovician Majiagou Formation post-salt gas has the features of mixed gas, of which the oil-derived gas is supposed to have come from the Permian Taiyuan Formation limestone. (2) The Majiagou Formation post-salt has experienced much more weathering, leaching, and oxidation and, thus, is not the source for oilderived gas in the Jingbian gas field. Gases in the Majiagou Formation post-salt karst reservoirs in the Jingbian gas field are mainly coal-derived gases derived from the Upper Paleozoic Carboniferous and Permian coal measures, with the oil-derived gases from the Permian Taiyuan Formation but with a small proportion, which corresponds to the geochemical characteristics of gas from the above gypsum−salt bed. (3) The source rocks of pre-salt are comparatively better than those of the post-salt, which have experienced much more secondary change. In addition, natural gas derived from Carboniferous and Permian coal measures cannot migrate to Majiagou Formation that is under the gypsum−salt bed because of the barrier of the gypsum−salt bed. The natural gas in the Majiagou Formation pre-salt is self-generation and self-storage reservoirs, as indicated by adsorbed gas and geochemical characteristics of gas from the pre-salt. The pre-salt source rocks, with limited hydrocarbon generation capability and a low level of exploitation, can form self-generation and self-storage gas reservoirs, and they may form some gas reservoirs in certain areas.
originated from different source rocks. However, the trends of gases from the Upper Paleozoic and Lower Paleozoic post-salt are closer than those of gases from Lower Paleozoic post-salt and Lower Paleozoic pre-salt, suggesting that gases from the Upper Paleozoic and Lower Paleozoic post-salt have a genetic relationship and the mixed gases of Lower Paleozoic post-salt may mainly originate from Upper Paleozoic source rocks. We find that the generation type of natural gas can also be judged from the relationship between δ13C1 and δ13C1−δ13C2 (Figure 9). Different sources of gas plotted into different parts
■
Figure 9. Correlation chart of δ13C1 and δ13C1−δ13C2 (partial data from refs 5,7,26,28).
AUTHOR INFORMATION
Corresponding Author
of the figure, and if they have a genetic relationship, then they will have coincident points. The methane carbon isotope composition of coal-derived gas from the Upper Paleozoic with δ13C1 > −35‰ is relatively heavier than that of oil-derived gas from the Lower Paleozoic pre-salt with δ13C1 < −35‰. The mixed gas from Lower Paleozoic post-salt is between them. As seen from the above analysis and Figure 9, the natural gas from the Upper Paleozoic and Lower Paleozoic pre-salt have no coincident parts, suggesting that they had no genetic relationship and generated from different source rocks. The natural gas from the Upper Paleozoic and Lower Paleozoic post-salt have some coincident parts, suggesting that they had a genetic relationship and the mixed gases of Lower Paleozoic post-salt may mainly originate from Upper Paleozoic source rocks.
*Telephone: +86-10-83592209. E-mail:
[email protected]. ORCID
Wenxue Han: 0000-0003-2313-1516 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We appreciate Chinese Academy of Sciences academician Jinxing Dai for his guidance and support of this work. We express our sincere thanks to the associate editor, Dr. Patrick Hatcher for his hard work. We are also grateful to the invaluable comments by Professor Barry J. Katz. This work was financially supported by the Chinese National Special Plan Project “Formation Conditions, Enrichment Regularity and Exploration Potential of Tight Oil” (2016ZX05046-001).
6. CONCLUSION On the basis of a series of experiments on source rock evaluation and determination of components and carbon isotopes of natural gas and adsorbed gas, we evaluated the hydrocarbon generation potential of the possible source rocks and analyzed the source rocks of oil-derived gas. We can draw the following conclusions: (1) As a result of source rock evaluation and a comparison between gases collected from a limestone-developed area and adsorbed gas, the Permian Taiyuan Formation contains type II organic matter and sapropel components, presenting as moderate rocks with advanced maturity. It can generate gases dominated by oilderived gas in some certain areas. However, the Permian Taiyuan Formation has limited hydrocarbon generation capability, and it is much less than the adjacent Carboniferous and Permian coal measures. The coal-derived gas generated from the Carboniferous and Permian coal measures drove the
■
REFERENCES
(1) Li, Y.; Tang, D. Z.; Elsworth, D.; Xu, H. Characterization of coalbed methane reservoirs at multiple length scales: A cross-section from Southeastern Ordos Basin, China. Energy Fuels 2014, 28 (9), 5587−5595. (2) Dai, J. X.; Xia, X. Y.; Qin, S. F.; Zhao, J. Z. Origins of partially reversed alkane δ13C1 values for biogenic gases in China. Org. Geochem. 2004, 35 (4), 405−411. (3) Zhao, W. Z.; Wang, Z. C.; Li, J. M.; Li, J.; Xie, Z. Y.; Wang, Z. Y. Natural gas resources of the sedimentary basins in China. Mar. Pet. Geol. 2008, 25 (4−5), 309−319. (4) Zhao, J. Z.; Zhang, W. Z.; Li, J.; Cao, Q.; Fan, Y. F. Genesis of tight sand gas in the Ordos Basin, China. Org. Geochem. 2014, 74, 76− 84. (5) Dai, J. X.; Li, J.; Luo, X.; Zhang, W. Z.; Hu, G. Y.; Ma, C. H.; Guo, J. M.; Ge, S. G. Stable carbon isotope compositions and source I
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
(24) Clark, R. C., Jr.; Blumer, M. Distribution of n-paraffins in marine organisms and sediment. Limnol. Oceanogr. 1967, 12 (1), 79−87. (25) Ventura, G. T.; Kenig, F.; Reddy, C. M.; Frysinger, G. S.; Nelson, R. K.; Mooy, B. V.; Gaines, R. B. Analysis of unresolved complex mixtures of hydrocarbons extracted from Late Archean sediments by comprehensive two-dimensional gas chromatography (GC × GC). Org. Geochem. 2008, 39 (7), 846−867. (26) Dai, J. X. Giant Coal-Derived Gas Fields and Their Sources in China; Science Press: Beijing, China, 2016. (27) Liang, D. G.; Zhang, S. C.; Zhao, M. J.; Wang, F. Y. Hydrocarbon sources and stages of reservoir formation in Kuqa depression, Tarim Basin. Chin. Sci. Bull. 2002, 47 (1), 62−70. (28) Dai, J. X.; Zou, C. N.; Li, J.; Ni, Y. Y.; Hu, G. Y.; Zhang, X. B.; Liu, Q. Y.; Yang, C.; Hu, A. P. Carbon isotopes of Middle−Lower Jurassic coal-derived alkane gases from the major basins of northwestern China. Int. J. Coal Geol. 2009, 80 (2), 124−134. (29) Yang, H.; Liu, X. S. Progress in Paleozoic coal-derived gas exploration in the Ordos Basin, West China. Pet. Explor. Dev. 2014, 41 (2), 144−152. (30) Yao, J. L.; Bao, H. P.; Ren, J. F.; Sun, L. Y.; Ma, Z. R. Exploration of Ordovician Subsalt Natural Gas Reservoirs in Ordos Basin. China Pet. Explor. 2015, 20 (3), 1−12. (31) Bernard, B. B.; Brooks, J. M.; Sackett, W. M. Light hydrocarbons in recent Texas continental shelf and slope sediments. J. Geophys. Res. 1978, 83 (C8), 4053−4061. (32) Whiticar, M. J. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chem. Geol. 1999, 161 (1−3), 291−314. (33) Chung, H. M.; Gormly, J. R.; Squires, R. M. Origin of gaseous hydrocarbons in subsurface environments: The oretical considerations of carbon isotope distribution. Chem. Geol. 1988, 71 (1), 97−104. (34) Rooney, M. A.; Claypool, G. E.; Chung, H. M. Modeling thermogenic gas generation using carbon isotopic ratios of natural gas hydrocarbons. Chem. Geol. 1995, 126 (3−4), 219−232. (35) Du, J. G.; Jin, Z. J.; Xie, H. S.; Bai, L. P.; Liu, W. Stable carbon isotope compositions of gaseous hydrocarbons produced from high pressure and high temperature pyrolysis of lignite. Org. Geochem. 2003, 34 (1), 97−104. (36) Zou, Y. R.; Cai, Y. L.; Zhang, C. C.; Zhang, X.; Peng, P. A. Variations of natural gas carbon isotope-type curves and their interpretation: A case study. Org. Geochem. 2007, 38 (8), 1398−1415.
rock geochemistry of the giant gas accumulations in the Ordos Basin, China. Org. Geochem. 2005, 36 (12), 1617−1635. (6) Wang, C. G.; Wang, Y.; Xu, H. Z.; Sun, Y. P.; Yang, W. L.; Wu, T. H. Discussion on evolution of source rocks in Lower Paleozoic of Ordos Basin. Acta Pet. Sin. 2009, 30 (1), 38−45. (7) Yang, H.; Zhang, W. Z.; Zan, C. L.; Ma, J. Geochemical characteristics of Ordovician subsalt gas reservoir and their significance for re-understanding the gas source of Jingbian gas field, East Ordos Basin. Nat. Gas Geosci. 2009, 20 (1), 8−14. (8) Chen, A. D.; Dai, J. Y.; Wang, W. Y. Characteristics and origin of gas reservoirs and the favorable geological conditions in Jingbian gas field, Ordos Basin. Mar. Origin Pet. Geol. 2010, 15 (2), 45−55. (9) Zhao, Z. Y.; Guo, Y. R.; Xu, W. L.; Zhang, Y. L.; Gao, J. R.; Zhang, Y. Q. Significance of three reservoir profiles for the risk exploration in Ordos Basin. Pet. Explor. Dev. 2011, 38 (1), 16−22. (10) Liu, Q. Y.; Chen, M. J.; Liu, W. H.; Li, J.; Han, P. L.; Guo, Y. R. Origin of natural gas from the Ordovician paleo-weathering crust and gas-filling model in Jingbian gas field, Ordos Basin, China. J. Asian Earth Sci. 2009, 35 (1), 74−88. (11) Xu, H.; Cao, D. Y.; Li, Y.; Liu, J. C.; Niu, X. L.; Zhang, Y.; Qin, G. H. Geochemical and Preliminary Reservoir Characteristics of the Carboniferous−Permian Coal-Bearing Strata in the Junger Area, Northeastern Ordos Basin, China: Source Implications for Unconventional Gas. Energy Fuels 2016, 30 (9), 6947−6957. (12) He, X. Y.; Shou, J. F.; Shen, A. J.; Wu, X. N.; Wang, Y. S.; Hu, Y. Y.; Zhu, Y.; Wei, D. X. Geochemical characteristics and origin of dolomite: A case study from the middle assemblage of Ordovician Majiagou Formation Member 5 of the west of Jingbian Gas Field, Ordos Basin, North China. Petroleum Exploration and Development 2014, 41 (3), 417−427. (13) Li, J.; Zhang, W. Z.; Luo, X.; Hu, G. Y. Paleokarst reservoirs and gas accumulation in the Jingbian field, Ordos Basin. Mar. Pet. Geol. 2008, 25 (4−5), 401−415. (14) Zheng, W. B.; Hu, X. Y.; Chen, S. W.; Liu, J. D.; Jia, C. The discovery and significance of a sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China: Discussion. AAPG Bull. 2016, 100 (6), 1061−1066. (15) Wang, G. W.; Chang, X. C.; Yin, W.; Li, Y.; Song, T. T. Impact of diagenesis on reservoir quality and heterogeneity of the Upper Triassic Chang 8 tight oil sandstones in the Zhenjing area, Ordos Basin, China. Mar. Pet. Geol. 2017, 83, 84−96. (16) Yang, H.; Fu, J. H.; Wei, X. S.; Liu, X. S. Sulige field in the Ordos Basin: Geological setting, field discovery and tight gas reservoirs. Mar. Pet. Geol. 2008, 25 (4−5), 387−400. (17) Jiang, F. J.; Chen, D.; Chen, J.; Li, Q. W.; Liu, Y.; Shao, X. H.; Hu, T.; Dai, J. X. Fractal Analysis of Shale Pore Structure of Continental Gas Shale Reservoir in the Ordos Basin, NW China. Energy Fuels 2016, 30 (6), 4676−4689. (18) Dai, J. X.; Zou, C. N.; Qin, S. F.; Tao, S. Z.; Ding, W. W.; Liu, Q. Y.; Hu, A. P. Geology of giant gas fields in China. Mar. Pet. Geol. 2008, 25 (4−5), 320−334. (19) Yao, Y. B.; Liu, D. M.; Qiu, Y. K. Variable gas content, saturation, and accumulation characteristics of Weibei coalbed methane pilot-production field in the southeastern Ordos Basin, China. AAPG Bull. 2013, 97 (9), 1371−1393. (20) Xia, X. Y.; Hong, F.; Zhao, L.; Zhang, W. Z. Organic facies type and hydrocarbon potential of carbonates in Majiagou Formation Lower Ordovician in Ordos Basin. Acta Sedimentol. Sin. 1999, 17 (4), 638−643. (21) Liu, D.; Zhang, W. Z.; Kong, Q. F.; Feng, Z. Q.; Fang, C. C.; Peng, W. L. Lower Paleozoic source rocks and natural gas origins in Ordos Basin, NW China. Pet. Explor. Dev. 2016, 43 (4), 591−601. (22) Huang, D. F.; Li, J. C.; Zhang, D. J. Kerogen types and study on effectiveness, limitation and interrelation of their identification parameters. Acta Sedimentol. Sin. 1984, 2 (3), 29−31. (23) Tu, J. Q.; Dong, Y. G.; Zhang, B.; Nan, H. L.; Li, C. J.; Wang, X. M.; Fei, X. D.; Zhou, W. B. Discovery of effective scale source rocks of the Ordovician Majiagou Formation in the Ordos Basin and its geological significance. Nat. Gas Ind. B 2016, 3 (4), 330−338. J
DOI: 10.1021/acs.energyfuels.7b01179 Energy Fuels XXXX, XXX, XXX−XXX
