Formation of low permeability reservoirs and gas accumulation process in the Daniudi Gas Field, Northeast Ordos Basin, China

Marine and Petroleum Geology 70 (2016) 222e236

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Research paper

Formation of low permeability reservoirs and gas accumulation process in the Daniudi Gas Field, Northeast Ordos Basin, China Zhi Yang a, *, Sheng He b, Xiaowen Guo b, **, Qiyan Li a, Zhaoyou Chen c, Yanchao Zhao b a

Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan, China c Exploration and Production Research Institute of Huabei Company, SINOPEC, Zhengzhou 450006, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 June 2015 Received in revised form 24 October 2015 Accepted 28 October 2015 Available online 11 November 2015

The Daniudi Gas Field is a typical large-scale coal-generated wet gas field located in the northeastern Ordos Basin that contains multiple Upper Paleozoic gas-bearing layers and considerable reserves of gas. Based on integrated analysis of reservoir petrology, carbonate cement CeO isotope, geochemistry of source rocks and HC gas and numerical basin modeling, a comprehensive study focusing on the formation of low permeability reservoirs and gas generation process uncovers a different gas accumulation scene in Daniudi Gas Field. The gas accumulation discovered was controlled by the reservoir permeability reduction and gas generation process, and can be divided into two distinct stages by the low permeability reservoir formation time: before the low permeability reservoir formation, the less matured gas was driven by buoyancy, migrated laterally towards NE and then accumulated in NE favorable traps during Late Triassic to early Early Cretaceous; after the low permeability reservoir formation, highly matured gas was driven by excessive pressure, migrated vertically and accumulated in-situ or near the gas-generating centers during early to late Early Cretaceous. The coupling relationship between reservoir diagenetic evolution and gas generation process controlled on gas accumulation of the Daniudi Gas Field. This study will aid in understanding the gas accumulation process and planning further E&D of the Upper Paleozoic super-imposed gas layers in the whole Ordos Basin and other similar super-imposed low permeability gas layer basins. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Low permeability sandstone gas Coal-generated gas Gas-generating center Unconventional oil and gas Super-imposed gas layers Ordos basin

1. Introduction In the northern Ordos Basin, a total of five Upper Paleozoic large-scale gas fields with reserves of more than 1
1011 m3 have been discovered: the Sulige, Yulin, Daniudi, Wushenqi and Zizhou (Dai et al., 2005; Zou et al., 2014). The Upper Paleozoic in the northern Ordos Basin is characterized by large prospective area, multiple gas-bearing layers, and gas-bearing universality, and forms widespread lithologic gas reservoirs with low reservoir porosity, permeability, formation pressure and gas-bearing abundance (Dai et al., 2005; Hao et al., 2007; Fu et al., 2008; Yang et al., 2008; Zhao et al., 2008; Zou et al., 2013, 2014). Various research efforts are focused on gas accumulation model of the Upper

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Yang), [email protected] (X. Guo). http://dx.doi.org/10.1016/j.marpetgeo.2015.10.021 0264-8172/© 2015 Elsevier Ltd. All rights reserved.

Paleozoic gas fields in the Ordos Basin with a variety of ideas, including the “deep basin gas or source-contacting gas”, “rich gas accumulation in coarse facies belt or near-source box-shaped accumulation under a widespread gas generation”, “complex pressure compartment accumulation with coexisted high pressure and low pressure facies”, and “reservoir permeability reduction followed by gas charging” (Min et al., 2000; Zhang et al., 2000; Yang, 2002; He et al., 2003; Fu, 2004; Dai et al., 2005; Hao et al., 2007; Li et al., 2007; Zhang et al., 2007; Zhao et al., 2008; Yang et al., 2008; Zhang, 2008; Zou et al., 2013, 2014; Liu et al., 2013; Ren et al., 2014; Li et al., 2014; Zhao et al., 2014; Yang and Liu, 2014; Yu et al., 2014). All of these ideas assume that sufficient gas sources (widespread gas generation), low permeability reservoirs (instead of buoyancy, pressure difference between source rocks and reservoirs serves as the driving force for gas migration) and effective caprocks (sealing provided by pressure and physical property of the Shiqianfeng (P2sh) and Upper Shihezi (P2s) formations) were available during the large-scale gas charging of the Upper Paleozoic gas fields in the northern Ordos Basin. Such assemblage allows

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substantial gas generated prior to regional uplift event since 95Ma to be effectively preserved. However, when studying the coupling relationship between diagenetic evolution and gas accumulation process for the Daniudi Gas Field, a different accumulation model has been proposed, based on integrated study of numerous data, including the gas geochemical data (particularly the methane carbon isotope data), reservoir petrological data, fluid inclusion references, carbon and oxygen isotopic data of carbonate cements, and numerical basin modeling data. A detailed discussion on this complex accumulation process will aid in planning further E&D of the Upper Paleozoic super-imposed low permeability gas layers in the whole Ordos Basin and other similar basins. 2. Geologic setting The Daniudi Gas Field, located within Sinopect's Tabamiao block, covers a prospective area of approximately 2004.8 km2. Structurally, it is a west trending mono-clinal structure situated in the northern part of the Yishan Slope in the Ordos Basin and characterized by down dip gradient of 3e6 m/km and few lowrelief structures with no fault (Fig. 1). In the northern Ordos Basin, the Late Carboniferous Taiyuan-age coastal swamp coal measure strata and the Early Permian Shanxiage alluvial plain coal measure strata are source rocks for the gas found in the Upper Paleozoic reservoirs. The channel sand bodies, delta plain distributary channel sand bodies, delta front underwater distributary channel sand bodies, river mouth sand bars,

223

transitional shoreland sand bars, and tidal channel sand bodies deposited between and above source rocks serve as the Carboniferous and Permian reservoirs. The widespread Late Permian-age flood plain and lacustrine mudstones of the Upper Shihezi (P2s) and Shiqianfeng formations (P2sh) provide the regional caprocks for the Upper Paleozoic gas reservoirs in the basin. The Daniudi Gas Field contains a total of seven gas-bearing layers: (Fig. 2), C3t1, C3t2, P1s1, P1s2, P1x1, P1x2 and P1x3 members in order of descending depth. The C3t1 and C3t2 members are transitional facies deposits; P1s1 and P1s2 members are braided river-meandering river facies deposits; and P1x1, P1x2 and P1x3 members are braided river-meandering river facies deposits (Hao et al., 2007). By year-end 2013, the Daniudi Gas Field has reported over 4.5
1011 m3 of proved gas-inplace, with reserves abundance as low as 2.3
108 m3/km2. The producing area includes the P1x2 and P1x3 braided river channel sand body gas reservoir present in the southwestern corner of study area, and the C3t2 barrier sand bar reservoir in the northeastern corner. Together these reservoirs account for about 90% of the gas production of the field. 3. Research results 3.1. Formation of low permeability reservoir 3.1.1. Coarse-grained sandstone distribution During the Late Paleozoic, extensive coal deposits were formed. The lower part of the coal measures, the Upper Carboniferous

Fig. 1. Locations of structural units and study area in Ordos Basin (up) and a westeeast profile (down).

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Fig. 2. Lithological comprehensive colum in the Daniudi Gas Field, Northeast Ordos Basin.

Taiyuan Formation (C3t), was lacustrine to tidal flat facies, transforming into littoral, deltaic and marshy shale interbedded with sandstone of the Lower Permian Shanxi Formation (P1s) (Hao et al., 2007; Yang et al., 2008). From the early to the late Permian, the sedimentary environment evolved from a humid climate to an arid climate, resulting in an inland facies of variegated sandstones interbedded with shale. The Lower Permian Xiashihezi Formation (P1x) comprises mainly fluvial and delta sandstones as important reservoirs. The P1x and P1s are characterized by multiple stages of superimposed inland fluvial and delta sand-bodies in Daniudi Gas Field, with a total coarse-grained sandstone thickness of 50e120 m in large dimensions (Fig. 3, Fig. 4-A-E), and the C3t is typical in barrier bar quartz sandstone in NEeSW direction, with coarsegrained sandstone 8e18 m thick, which was an important pathway in the early gas lateral migration (Fig. 3, Fig. 4-F).

3.1.2. Reservoir characteristics The Upper Paleozoic sandstone in the Daniudi Gas Field consists mainly of lithic sandstone, lithic quartz sandstone, and quartz sandstone (Fig. 5), with 5.3%e5.7% feldspar-sandstone in P1x, without feldspar-sandstone in P1s and C3t (Fig. 5). The P1x and P1s are composed largely of lithic sandstone, which accounts for 60% of samples obtained, and the C3t contains a high proportion of quartz sandstone. The clastic quartz is composed of monocrystalline quartz and polycrystalline quartz, with minor amounts of chert, and the lithic fragments are provided by epimetamorphic rocks, such as phyllite, schist, slate and metamorphic sandstone, and secondly by volcanic rocks. These lithic fragments can be plastic and have been strongly compacted, as shown in Fig. 6-A. The fillings are dominated by authigenic cements and terrigenous matrix. The authigenic cements mainly include siliceous matrix (Fig. 6-B and6-

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Fig. 3. Comprehensive column diagram of well A15 in the Daniudi Gas Field (Well location shown in Fig. 4), Northeast Ordos Basin.

C), ferriferous calcites (Fig. 6-C and 6-D), ferriferous dolomites, siderites, kaolinites and chlorites; and the terrigenous matrix consists mainly of tuffaceous matrix, hydromicas and chlorites. The grain size of gas-bearing sandstone ranges from 0.4 to 1.2 mm, suggesting the coarse sandstone, pebbly coarse sandstone and partially medium sandstone. The majority of grains are closely contacted, with linear, linear to concavo-convex and concavoconvex contact observed, and characterized by moderate sorting, being sub-angular and porous cementation (Fig. 6). The Upper Paleozoic sandstone in the Daniudi Gas Field has low porosity and low permeability, with 63.3% of samples having less than 8% porosity and 73.3% having less than 0.5
103 mm2 permeability (a total of 6853 samples), Fig. 7. The P1x3 and P1x2 members have the best physical properties, the C3t the second, and the P1x1 and P1s the worse. The pore type is dominated by intergranular secondary pores, followed by primary residual intergranular pores. The carbonate cements were less dissolved than the feldspar composition in feldspar minerals and lithic fragments. The water type in the sandstone reservoir are all CaCl2 type, implying the good seal condition (Table 1). The temperature gradient is about 2.9 C/100 m, and the formation pressure is negative pressure system, with pressure gradient range 0.6e1.0 MPa/100 m (Fig. 8). 3.1.3. Formation of low permeability reservoir The sandstones of the Daniudi Gas Field contain the highly ordered I/S mixed layer clay mineral, with the order degree in most samples ranging from 15% to 30%. Reservoirs commonly have a porosity of less than 10% and contain only minor amounts of secondary dissolved pores and fractures. The coal measure source rocks are thermally highly mature, with Ro values between 1.4% and 1.5% and Tmax values between 460  C and 480  C. The result of thermal evolution modeling suggests the maximum temperature of

130  Ce175  C for the Daniudi Gas Field over the geologic history. According to the data mentioned above, sandstones in the Daniudi Gas Field are considered to be within the B stage and A2 sub-stage of Late Diagenesis. The low permeability sandstone reservoirs in the Daniudi Gas Field have a complex diagenetic evolution history, and there are two major factors that result in the formation of the low permeability reservoir: (1) Mechanical compaction of numerous plastic lithic fragments Sandstone reservoirs in the Daniudi Gas Field contain a high proportion of plastic materials (e.g., the phyllite and argillite), which would be deformed and compressed to squeeze the pores under the long-term strong compaction caused by the overlying strata and hence form the low permeability reservoir characterized by micro-pores. (2) Siliceous and carbonate cementation Siliceous and carbonate cements between the clastic particles allow sandstone reservoirs with low-porosity and lowpermeability to form by reducing their porosity and permeability during the process of burial. Siliceous cements are produced in forms of secondary enlarged authigenic quartz, idiomorphic quartz and healing quartz that fill the intergranular or dissolved pores. As indicated by homogenization temperature determination of authigenic quartz inclusion, the authigenic quartz in this area is mainly formed at a temperature of 80  Ce130  C, with the exception of thoese formed at a temperature of 160  Ce170  C (Liu et al., 2007; Guo et al., 2007). The carbonate cements consist mainly of ferriferous calcites, ferriferous dolomites, calcites, and few siderites. The ferriferous calcites can observed in P1x and P1s, and the

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Fig. 4. Coarse grain-sized sandstone thickness of the main target formations in the Daniudi Gas Field, Northeast Ordos Basin. Note: A: P1x3; B: P1x2; C: P1x1; D: P1s2; E: P1s1; F: C3t2.

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Fig. 5. Ternary diagram of sandstone classification in the Daniudi Gas Field, Northeast Ordos Basin, based on the Folk's (1968) classification. Abbreviations in diagrams: Q for Quartz, F for feldspar, R for debris.

ferriferous dolomites are present in the basal C3t and P1x. The intergranular carbonate cements were formed following the secondary enlargement of quartz at a temperature of 85  Ce135  C, as indicated by homogenization temperature determination and oxygen isotope thermometer of authigenic carbonate inclusions (Liu et al., 2007; Guo et al., 2007). In addition, the intergranular filling of argillaceous matrix and precipitation of authigenic clay minerals are also considered the key factors that result in the formation of the low permeability reservoir (Table 2). Fifteen sandstone samples were selected from 9 wells in different formations for stable carbon and oxygen isotopic measurements. Fifteen samples of carbon dioxide were collected from the relevant samples by the method of McCrea (Epstein et al., 1953) for stable carbon and oxygen component analysis with MAT253 isotope ratio mass spectrometers (IRMS) from Firmigan Company of Germany. In addition, the inter-granular pores filled with shaly matters and the sedimentation of authigenic clay minerals are some other key facts that lead to the formation of the low permeability reservoir (Zhang et al., 2007). Various factors that result in the formation of the low permeability reservoir cause the sandstone reservoir in this field to have a special diagenetic evolution sequence, as shown in Fig. 9. The medium-coarse sandstones have formed the low permeability reservoirs at 140 Ma (Early Cretaceous), with a burial depth of 3000 m, geo-temperature of 130  C, and Ro of about 0.8%. The low permeability reservoirs are highly heterogeneous and have a porosity of less than 10% and a permeability of less than 0.5
103 mm2. High-quality reservoirs were segmented into several belts or blocks under a strong diagenesis context, and the buoyancy of gas column is less likely to serve as the driving force for gas migration. Mechanical compaction at the paleo-burial depth of 2100 m reduced the porosity by 15%e20%. The authigenic quartz was formed from the B stage of Early Diagenesis until the B stage of Late Diagenesis at a temperature of 80  Ce130  C, and reduced the porosity by 5%e10%. The carbonate cements were formed following the quartz enlargement at a temperature of 90  Ce135  C, and reduced the porosity by 2%e5%. The secondary dissolved pores were formed at the A1 and early A2 sub-stages of Late Diagenesis.

3.2. Reconstruction of gas generation process 3.2.1. Geochemical characteristics of source rock The Upper Paleozoic Carboniferous-Permian in the Daniudi Gas Field consists of coal measure strata deposited in shoreland swamp, flood plain and lacustrine swamp environments, with coal measure strata of the C3t1 and P1s1 members serving as the primary gas source rocks and dark mudstones of C3t and P1s the secondary (Table 3). The coal is rich in organic matter, with TOC values of 71.52%e85.5%, and S1 þ S2 values (gas generating potential of source rocks) of 101.62e185.18 mg/g. The dark mudstones in coal measure strata contain Type III kerogen, with TOC values of 4.63%e 28.17%, S1 þ S2 values of 6.23e49.61 mg/g and HI values of 73e202.85 mg/g (averaging 154.31 mg/g, mostly ranging from 120 to 170 mg/g). With Ro values between 0.95% and 1.53%, (mostly ranging from 1.4% to 1.5%) and Tmax values between 450 and 491  C (mostly ranging from 470 to 490  C), these mudstones are considered thermally highly mature. In the Daniudi Gas Field, coal bed is relatively thicker in the southwestern part, ranging from 20 to 28 m in total thickness. Similarly, dark mudstone is also thicker in this part, ranging from 38 to 56 m in thickness. In addition, a 36e40 m thick dark mudstone bed is distributed across the middle and southern parts of the field. Relatively thicker source rocks (coal bed and dark mudstone) of the C3t are present mainly in the southwestern, central-southern and northeastern parts, exceeding 46 m thick, and relatively thicker source rocks (coal bed and dark mudstone) of the C3t in the centralsouthern, eastern and northwestern parts, exceeding 32 m thick. Overall, thick source rocks are limited in the southwestern, centralsouthern and eastern parts of the Daniudi Gas Field, suggesting that the material basis for gas generation is highly variable across the field. 3.2.2. Geochemical characteristics of natural gas The majority of natural gas accumulated in seven net pays of the Upper Paleozoic reservoirs in the Daniudi Gas Field is hydrocarbon gas, ranging from 88.11% to 99.98% (Table 4). The gas composition is dominated by methane with a relative content between 70.91% and 95.03%, mostly ranging from 80% to 90%, a dry to wet ratio of 2.64e35.12, and a dry coefficient that ranges from 72.59% to 97.23%.

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Fig. 6. Micro-photomicrographs showing main diagenesis process in Daniudi Gas Field, Northeast Ordos Basin. A. A47, 2386 m, P1s1, fine-medium-grained sandstone, clastic particles were subject to a strong compaction and hence contacted closely, Crossed Nicol; B. A79, 2677.67 m, P1s1, moderate-coarse-grained quartz sandstone, quartz enlargement edge, Crossed Nicol; C. A47, 2353.55 m, C3t2, coarse-grained sandstone, carbonate cements were developed after quartz enlargement, filling the pores and throats, Crossed Nicol; D. A61, 2727.24 m, P1s2, coarse–grained sandstone, carbonate cements were developed after quartz enlargement, filling the pores and throats, with the porosity 4.1% in the whole rock, Crossed Nicol; E. A64, 2678.71 m, P1s2, medium-grained sandstone, Cathodoluminescence image; F. A79, 2654.26 m, P1s1, coarse-grained sandstone, showing carbonate cemetation

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Fig. 7. Porosity and permeability for the sandstone reservoirs in the Daniudi Gas Field, Northeast Ordos Basin.

Table 1 Water chemical composition in the Daniudi Gas Field (The well locations are shown in Fig. 4) Northeast Ordos Basin. Well

Fm.

Cation (mg/l) K

A1 A4 A22 A28 A46 A47 A48 A49 A52

P1s1 P1s1 C3t2 P1x1 P1s1 P1x3 P1x2 C3t2 P1s1

þ

15972.12 13356.33 13454.17 3924.40 22725.39 9301.70 5363.53 24029.57 18853.96

Na

þ

Antion (mg/l) 2þ

2þ

Ca

Mg

13412.76 6935.04 7743.14 1324.48 10897.00 7153.30 1558.30 11568.17 2341.60

384.12 677.74 626.87 30.89 680.00 70.50 63.30 1368.19 165.70

SO2 4

Cl

506.94 596.41 1354.64 878.73 550.19 282.46 447.52 614.92 1006.92

0.00 2.88 0.00 0.00 0.00 0.00 0.00 0.00 0.00

49161.62 34482.92 35693.77 7708.25 56614.50 26921.58 10659.37 63535.13 31100.29

Strata above and below the source rock of the P1s1 Member increase in methane content, dry to wet ratio and dry coefficient, reflecting a good fractionation effect and a high content of nearsource heavy hydrocarbon gas. The gas accumulated in the Upper Paleozoic across the Daniudi Gas Field has the methane carbon isotopic values between 39.02‰ and 33.28‰, mostly ranging from 37‰ to 34‰, the ethane carbon isotopic values between 26.5‰ and 23.54‰, and the L-shaped connection of alkane gas d13C values showing a positive order of carbon isotopes; that is, d13C1 < d13C2 < d13C3 < d13CnC4 < d13CiC4, illustrating its primary organic origin (Fig. 10). The Ro value is estimated at 0.5%e1.5% with

salinity (mg/l)

PH

Water type

79437.97 56051.32 61631.70 14041.28 92567.09 43729.54 18478.58 106457.56 53658.99

6.2 6.6 6.7 6.5 6.0 6.0 6.0 6.0 6.0

CaCL2 CaCL2 CaCL2 CaCL2 CaCL2 CaCL2 CaCL2 CaCL2 CaCL2



HCO 3

the d13C1 given by various formulas (Dai, 1992; Chen et al., 1993), suggesting that the source rocks are thermally mature to highly mature. 3.2.3. Reconstruction of gas generation process One-dimensional basin-modeling technology is sufficient to model petroleum generation and primary migration, defined as the movement of petroleum through and out of the fine grained source rocks (Tissot and Welte, 1984). The Statoil fluid flow porosity reduction method was applied and calibrated by the measured porosity values. A transient heat-flow model was employed to handle the heat transfer in the basin (Bethke, 1985; Lerche, 1990),

after quartz enlargement cemetation, backscattered electron image; G. A66, 2558.89 m, P1x3, medium-grained sandstone, showing the main reservoir space, Single Nicol; H. A79, 2680.25 m, P1x3, coarse-grained sandstone, showing the dissolved pores and throats, with the porosity 10.3% in the whole rock, Single Nicol. Abbreviation in Photomicrographs: Q for Quartz, QC for Quartz Cemetation, CC for Carbonate Cematation, POR for Porosity.

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Fig. 8. Formation temperature and pressure in the Daniudi Gas Field, Northeast Ordos Basin.

and temperatures of drillstem tests (DSTs) were used to do calibration. The thermal maturity of organic matter was calculated by the Easy %Ro model (Sweeney and Burnham, 1990), and the measured vitrinite reflectance (Ro) values were used to cope with calibration. In the Early Cretaceous, an episode of tectonic thermal event occurred in the Ordos Basin at ca. 140e100Ma, and remained active for 10 to 40 Ma (Ren et al., 2007). As a result of this event, the gas generation of the Upper Paleozoic source rocks in the Daniudi Gas Field can be divided into two distinct phases, as shown in Fig. 11. The first phase commenced in the Late Triassic until the end of Early Cretaceous (210Ma-144Ma-95Ma). During the Late Triassic to Early Jurassic, the quickly buried source rocks reached their hydrocarbon-generating threshold and mainly generated CO2 gas (Liu et al., 2007; Guo et al., 2007) that contains hydrocarbon; during the Middle-Late Jurassic, the slowly subsided source rocks were thermally mature and began to generate and expel hydrocarbon

Table 2 Carbon and oxygen isotopic of carbonate cements in sandstones in the Daniudi Gas Field, Northeast Ordos Basin (The well locations are shown in Fig. 4). Well A23 A23 A 23 A 23 A 24 A 26 A 26 A 33 A3 A 50 A 50 A 50 A 62 A 64 A 79

Fm. 1

P1s P1x1 P1s1 P1x1 C3t2 P1x1 P1s2 P1x2 P1x2 P1x2 P1x3 P1s2 C3t1 P1x1 P1x1

d13C‰ (PDB)

d18O‰ (PDB)

13.48 13.77 11.31 11.30 12.27 13.66 0.95 10.19 4.58 12.42 13.87 14.64 7.28 6.30 7.13

17.58 19.24 17.20 15.24 16.81 16.65 14.09 17.28 16.09 15.04 17.10 14.67 14.87 15.44 13.46

consisting of few liquid hydrocarbons and more gas; and during the Early Cretaceous that is considered a critical period for gas accumulation, the Yanshanian tectonic thermal event caused source rocks to be quickly thermally mature to highly mature, allowing substantial gas to be generated. The second phase commenced at the end of Early Cretaceous during which the basin was uplifted and continues to the present day (95Ma-present). As a result of basin uplift, the strata within the work area of the Daniudi Gas Field were eroded by 950e1350 m, with temperature and pressure of source rocks decreased, the impact of the Yanshanian tectonic thermal event vanished gradually, only small amounts of gas was generated by source rocks, and the hydrocarbon generation effect significantly decreased and tended to cease (Chen et al., 2006). A gas-generating center refers to a zone with maximum gasgenerating intensity, and is a comprehensive reflection of source rock thickness, organic matter abundance, organic matter type, and maturity (Dai et al., 2005). The Daniudi Gas Field contains two gasgenerating centers of the C3t and P1s, as indicated in Fig. 12. The gas-generating center of the C3t is located in the southeastern part of the field, with gas-generating intensity of 1000e1200 m3/m2, which is contributed by the C3t1 Member (Fig. 12-A); and the gasgenerating center of the P1s is situated in the central-western part, with gas-generating intensity of 1500e2350 m3/m2, which is contributed predominately by the P1s1 Member. Overall, the gasgenerating centers of the Daniudi Gasield are located in the southeastern and central-western parts, with gas-generating intensity of over 2500 m3/m2 (Fig. 12-B), and have direct control on the formation and distribution of the field.

4. Discussion As discussed earlier in this paper, gas found in the Daniudi Gas Field is methane-dominated and considered to be thermally mature to highly mature coal-generated gas, and the methane

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Fig. 9. Diagenetic evolution sequence of the Upper Paleozoic sandstone reservoirs in the Daniudi Gas Field, Northeast Ordos Basin.

Table 3 Geochemical parameters of the coal and dark mudstone in the Daniudi Gas Field, Northeast Ordos Basin (The well locations are shown in Fig. 4). Well

A23 A60 A63 A64 A66 A66 A66 A79 A17 A66 A26 A51 A51 A79 A79 A63

Fm.

P1s1 P1s1 P1s1 P1s1 P1s1 P1s1 P1s1 P1s1 P1s2 P1s2 C3t1 C3t1 C3t1 C3t1 C3t1 C3t2

Lithology

Coal Coal Coal Coal Coal Dark Coal Coal Dark Dark Coal Coal Dark Dark Coal Coal

Ro

TOC

Tmax

HI

S1

S2

S1 þ S2

%

%



mgHC/g TOC

mg/g

mg/g

mg/g

1.53

79.99 77.23 74.60 76.62 85.50 7.56 77.04 71.52 28.17 4.63 76.76 79.67 5.22 7.64 74.99 84.75

485 482 470 458 483 472 476 450 465 464 483 481 480 491 490 462

120.00 146.06 161.77 241.69 135.58 161.24 153.97 202.85 176.11 182.07 143.26 132.85 149.81 73.04 118.95 169.70

5.63 5.75 9.09 13.34 6.41 1.21 7.99 11.49 3.23 0.37 7.62 5.33 0.64 0.65 3.77 6.51

95.99 112.8 120.68 185.18 115.92 12.19 118.62 145.08 49.61 8.43 109.97 105.84 7.82 5.58 89.2 143.82

101.62 118.55 129.77 198.52 122.33 13.40 126.61 156.57 52.84 8.80 117.59 111.17 8.46 6.23 92.97 150.33

1.44 0.95 1.37 mudstone

mudstone mudstone

mudstone mudstone

1.5 1.19 1.11 1.19 1.51 1.45 1.37 1.53 1.53

C

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Table 4 Natural gas composition content in the Daniudi Gas Field, Northeast Ordos Basin (The well locations are shown in Fig. 4). Well

A2 A3 A5 A9 A10

A11 A20 A22 A23 A24 A25 A26 A28 A29 A30 A33 A34 A36 A37 A40 A43 A45 A47 A48 A49 A50 A51 A53 A54 A55 A56 A58 A59 A60 A62

Fm.

1

P1s P1s1 C3t2 P1s2 P1x3 P1x1 P1s2 P1x3 P1s1 C3t2 P1x1 C3t2 P1x1 P1x1 P1x1 P1s1 P1x1 P1s2 P1x2 P1x1 P1s2 P1s1 P1x1 P1x1 P1s2 P1x1 P1s1 C3t2 P1x2 C3t2 P1x1 C3t2 P1x2 C3t2 C3t2 C3t2 P1s2 P1x1 P1x2 P1s1

HC composition content (%)

Non-HC composition content (%)

C1

C2

C3

iC4

nC4

iC5

nC5

N2

CO2

H2

He

85.69 88.32 90.537 83.583 95.033 88.047 82.542 90.42 84.043 90.26 87.15 88.761 87.297 86.043 85.161 84.638 90.51 86.968 90.349 81.51 80.854 87.8842 80.074 81.348 81.268 81.318 77.77 87.009 80.627 84.072 78.732 75.68 81.641 82.25 80.317 86.188 89.105 83.04 83.432 70.91

8.543 7.66 3.331 8.431 2.316 6.798 11.577 5.251 9.192 4.16 7.31 4.538 6.462 6.912 10.334 9.595 2.78 6.729 4.607 4.815 8.504 8.0477 9.183 7.472 7.515 8.393 9.951 7.76 10.05 5.41 9.895 14.985 7.511 11.994 12.761 8.198 6.26 10.895 8.191 21.485

1.622 1.882 / 2.868 0.253 1.74 2.849 0.909 2.279 0.81 1.91 0.955 1.534 1.882 3.02 2.504 0.641 1.979 0.858 1.15 2.992 2.5506 2.614 2.562 2.42 2.727 3.29 1.785 1.354 1.121 2.424 3.361 1.62 2.776 3.198 2.101 1.69 2.624 2.411 4.018

0.262 0.237 0.404 0.514 0.07 0.243 0.304 0.179 0.308 0.13 0.25 0.138 0.235 0.256 0.444 0.279 0.184 0.335 0.135 0.25 0.718 0.457 0.574 0.68 0.423 0.482 0.509 0.345 0.206 0.2 0.421 0.604 0.279 0.435 0.528 0.394 0.271 0.432 0.554 0.387

0.394 0.322 / 0.695 0.067 0.365 0.409 0.203 0.548 0.14 0.38 0.165 0.345 0.366 0.642 0.453 0.215 0.407 0.18 0.218 0.808 0.6188 0.7 0.558 0.575 0.665 0.725 0.265 0.534 0.196 0.541 0.491 0.31 0.536 0.638 0.438 0.38 0.581 0.631 0.6

/ 0.101 0.101 0.348 / 0.143 0.093 0.088 0.01 0.06 0.12 0.066 0.142 0.148 0.23 0.119 0.156 0.163 0.076 0.061 0.374 0.2547 0.336 0.182 0.232 0.285 0.246 0.09 0.274 0.092 0.243 0.142 0.117 0.203 0.246 0.199 0.147 0.233 0.05 0.16

/ 0.057 0.095 0.195 / 0.079 0.064 0.609 0.151 0.03 0.08 0.036 0.101 0.125 0.133 0.087 0.1 0.108 0.059 0.107 0.194 0.1651 0.176 0.322 0.136 0.252 0.177 0.135 0.252 0.051 0.137 0.23 0.064 0.131 0.156 0.121 0.146 0.158 0.185 0.12

0.763 0.471 2.502 2.637 1.857 2.163 1.226 1.604 1.173 1.37 2.22 2.519 3.233 3.421 / 1.31 4.387 2.663 2.721 10.932 4.831 / 5.769 6.43 6.932 5.432 6.502 0.809 6.501 6.7 6.77 2.529 3.546 0.605 1.359 / 1.623 1.845 4.04 1.296

2.379 0.753 2.953 0.58 0.252 0.357 0.874 1.226 1.567 2.98 0.49 2.8 0.582 0.758 / 0.998 0.962 0.599 0.965 0.938 0.686 / 0.573 0.391 0.481 0.401 0.789 1.754 0.157 2.11 0.796 2.19 4.912 1.07 0.766 2.304 0.369 0.228 0.484 1.015

0.024 / / 0.115 / / 0.02 0.044 / / 0.04 / / 0.042 0.013 / / / / 0.008 / 0.011 / / / / / 0.047 0.045 / / / / / / / / / / /

/ / 0.024 0.035 0.087 0.065 0.043 / / 0.06 0.05 0.024 0.068 0.048 0.024 0.017 0.064 0.049 0.05 0.01 0.039 0.011 / 0.053 0.018 0.045 0.036 / 0.665 0.048 0.041 / / / 0.032 0.066 / / 0.021 0.01

Note: “/” indicates none data.

carbon isotope can therefore serve as the best index for reflecting the gas maturity in the study area (Dai et al., 2005). An analysis of the areal distribution of methane carbon isotope data of 27 gas samples obtained from the study area (Table 5 and Fig. 13) indicates that, gas samples from the northeastern side of the study area are mainly distributed at a NEeSW trending barrier sand bar of the C3t2 Member, have a relatively lower d13C1 (‰), ranging from 39.02‰

Fig. 10. HC gas composition isotope in Daniudi Gas Field, Northeast Ordos Basin (data from Dai et al., 2005; Fei, 2008).

to 36.58‰ (equivalent to Ro of 0.65%e0.86%), which suggests a relatively lower maturity, and were formed at a paleogeotemperature of 100  Ce128  C (equivalent to 210 to 140 Ma); and gas samples from the southwestern side of the study area are mainly distributed in the proximity of the gas-generating center of the P1s, have a relatively higher d13C1 (‰), ranging from 36.15‰ to 33.28‰ (equivalent to Ro of 0.89%e1.5%), which suggests a relatively higher maturity, and were formed at a paleogeotemperature of 133  Ce175  C (equivalent to 140 to 95Ma). As mentioned above, the analysis of the low permeability reservoir formation shares the same critical parameters with that of diagenetic phase: geotemperature of 130  C and geologic time of 140 Ma. This suggests an interesting coupling relationship between diagenetic evolution and gas charging and accumulation of the low permeability reservoirs in the Daniudi Gas Field. There are two stages of large-scale gas charging and accumulation in this field. The early stage commenced at Late Triassic (about 210 Ma) until early Early Cretaceous (about 140 Ma). During this stage, substantial gas and minor liquid hydrocarbons were generated prior to the low permeability sandstone reservoirs. Gas generated by the gasgenerating centers of the P1s and C3t is thermally less mature, with Ro values between 0.65% and 0.90%. Reservoirs are deemed to have favorable physical properties, especially the barrier sand bar

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233

Fig. 11. Burial history, geothermal history, and maturity history of the strata in the Daniudi Gas Field, Northeast Ordos Basin.

quartz sandstone of the C3t2 Member that allows for the presence of relatively higher and continuous gas columns. These columns provided the buoyancy that was significant enough to enable substantial gas to migrate laterally towards NE and then accumulate in favorable lithologic traps at structural high (Figs. 13 and 14A). In the northeastern region, gas accumulation and the low permeability reservoir formation occurred simultaneously in the sandstone reservoirs of the C3t and P1s. The late stage commenced at early Early Cretaceous (about 140 Ma) until late Early Cretaceous (about 95 Ma). After the low permeability reservoir formation, substantial thermally highly mature gas was generated, with Ro value between 0.9% and 1.5%. Reservoirs were highly heterogeneous, and the capillary pressure was too significant to form high gas columns. Therefore, buoyancy no longer serves as the primary driving force for migration and lateral migration is difficult. Natural gas migrated vertically from gas-generating center, driven by the excessive gas pressure, mostly into the P1s1 Member, through the well developed fracture systems of the P1s and P1x1 Member resulted from the gas-induced pressure rise and tectonic stress (Zhou et al., 2006). The low permeability reservoir formation of the C1t, P1s and P1x is followed by gas-charging. Favorable lithologic traps in the proximity of the gas-generating centers of the P1s and C3t are considered prospective (Fig. 14-B). Accordingly, the gas accumulation model of the Daniudi Gas Field can be summarized as “two gas-generating centers, two distinct stages of reservoir permeability reduction, lateral migration at early stage and vertical charging at later stage”.

5. Conclusion 1. Strong mechanical compaction and siliceous and carbonate cementation are two factors resulting in the low permeability

reservoir formation. The authigenic quartz was formed at a temperature of 80  Ce130  C, while carbonate cements were formed at a temperature of 85  Ce135  C, following the quartz enlargement. The primary residual pores and secondary dissolved pores formed in Late Diagenesis Stage are the main reservoir space. The medium to coarse-grained sandstones present across this field have become low permeability reservoirs by end of Early-Cretaceous (about 140 Ma ago). 2. The coal measures in C3t1 and P1s1 are the primary source rocks with VRE from 1.4% to 1.5% in the Daniudi Gas Field. The majority of the gas generated by them is typical coal-generated wet gas, being formed during the Late Triassic to the end of Early Cretaceous (about from 210 to 95Ma) and can be classified into two major types: the thermal degradation gas with a moderate maturity and the thermal cracking gas with a high maturity. The gas-generating centers of P1s1 and C3t1 are located in the central-western and southeastern parts of the study area, respectively, reaching a cumulative gas-generating intensity of 2500 m3/m2. 3. The gas accumulation discovered in the Daniudi Gas Field was controlled by the low permeability reservoir formation and gas generation process, and occurred in two different models. Before the low permeability reservoir formation, relatively thermally less mature gas, driven by buoyancy, migrated laterally by NEeSW conducting sand-body pathways and then accumulated in favorable traps at NE structural high point during Late Triassic to early Early Cretaceous (about from 210 to 140 Ma); after the low permeability reservoir formation, relatively thermally highly mature gas, driven by excessive pressure, mainly migrated vertically through fracture systems and accumulated in-situ or near the gas-generating centers during early to late Early Cretaceous (about 140 to 95Ma).

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Z. Yang et al. / Marine and Petroleum Geology 70 (2016) 222e236 Table 5 Table of gas maturity parameter of the Daniudi Gas Field, Northeast Ordos Basin (The well locations are shown in Fig. 12). Sample no.

Well

Fm. d13C1 (‰) R*o(%) T ( C) Methodology Data source

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

A47-3 A47-6 A47-7 A28-5 A35-4 A35-5 A25 A25 A10-2 AK22 A24 A24 A13 A11 A15 AK17 AK9 A16 AK15 AK4 A30 A1-414 AP1 A2-34 AK2 A4-4 AK25

P1x3 C3t1 P1s1 P1s1 P1s1 C3t2 P1x1 C3t2 C3t2 C3t2 P1x1 P1x1 P1s2 P1x3 P1x3 P1x2 P1x1 P1x2 P1x2 P1x3 P1x2 C3t2

37.82 37.73 37.66 37.1 37.78 37.08 38.13 38.47 39.02 38.06 37.12 37.22 36.58 34.49 35.99 35.98 34.98 35.1 35.63 34.49 34.03 33.28

0.71 0.78 0.73 0.78 0.72 0.86 0.68 0.7 0.65 0.75 0.78 0.77 0.84 1.1 0.91 0.91 1.03 1.02 0.95 1.1 1.17 1.5

110 128 115 120 110 130 105 110 100 115 120 120 125 150 135 135 145 145 140 150 155 175

② ① ② ② ② ① ② ① ① ① ② ② ② ② ② ② ② ② ② ② ② ①

Fei (2008) Fei (2008) Fei (2008) Fei (2008) Fei (2008) Fei (2008) Fei (2008) Dai et al. (2005) Fei (2008) Dai et al. (2005) Dai et al. (2005) Fei (2008) Dai et al. (2005) Dai et al. (2005) Fei (2008) Dai et al. (2005) Dai et al. (2005) Dai et al. (2005) Fei (2008) Dai et al. (2005) Fei (2008) Fei (2008)

P1s1 P1s2 P1x3 P1s1 P1s1

36.15 35.14 34.89 34.63 34.43

0.89 1.01 1.05 1.08 1.11

133 143 145 150 155

② ② ② ② ②

Dai et al. (2005) Fei (2008) Fei (2008) Fei (2008) Fei (2008)

23 24 25 26 27

① d13C1(‰) ¼ 15.84lgRo-36.06 (Coastal swamp coal measure, Chen et al., 1993), the formula for the samples in C3t uses. ② d13C1(‰) ¼ 17.64lgRo-35.23 (Flood plain swamp coal measure, Chen et al., 1993), the formula for the samples in P1s and P1x uses.

Fig. 12. Contour line of cumulative gas-generating intensity of the C3t1 and P1s1 members in the Daniudi Gas Field, Northeast Ordos Basin. A. Contour line of cumulative gas-generating intensity of the C3t1 Member, m3/m2; B. Contour line of cumulative gas-generating intensity of the P1s1 Member, m3/m2.

Fig. 13. The location of gas samples and the two gas-generating centers, and the earlystage migration direction of the relatively less matured gas in the Daniudi Gas Field, Northeast Ordos Basin.

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Fig. 14. Gas accumulation model of the Daniudi Gas Field, Northeast Ordos Basin (The well locations are shown in Fig. 4). Note: A. Large-scale lateral gas migration and accumulation occurred during 210 to 140 Ma (prior to the low permeability reservoir formation); B. Near-source vertical gas migration and accumulation occurred during 140 to 95Ma (after the low permeability reservoir formation).

Acknowledgments This work was supported by the National Key Basic Research and Development Program (973 Program), China (Grant 2014CB239000), and China National Science and Technology Major Project (Grant 2016ZX05046). This work could not have been achieved without the cooperation and support from SINOPEC Huabei Company, China University of Geosciences in Wuhan and PetroChina Research Institute of Exploration and Development. Thanks are also given to Jianhua Guo, Bingjie Wang, Shili Dai, Jiao Yang, Yang Luo, Hong Li and Weihua Zhang, who contributed to the Project. The authors appreciate both journal editors and anonymous reviewers for their precious time and useful suggestions. References Bethke, C.M., 1985. A numerical model of compaction driven groundwater flow and heat transfer and its application to the paleohydrology of intracratonic sedimentary basins[J]. J. Geophys. Res. 90 (8), 6817e6828. Chen, Anding, Zhang, Wenzheng, Xu, Yongchang, et al., 1993. Sedimentary rocks isotopic characteristics and application products of hydrocarbon thermal simulation experiment[J]. Sci. China, Ser. B 23 (2), 209e217. Chen, Ruiyin, Luo, Xiaorong, Chen, Zhankun, et al., 2006. Erosion thickness estimation of Mesozoic strata in Ordos basin and its geological significance[J]. Acta Geol. Sin. 80 (5), 685e693. Dai, Jin-xing, Li, Jian, Ding, Wei-wei, et al., 2005. Gas geochemical characteristics on

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