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Inversion and propagation of the Late Paleozoic Porjianghaizi fault (North Ordos Basin, China): Controls on sedimentation and gas accumulations
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Research paper
Inversion and propagation of the Late Paleozoic Porjianghaizi fault (North Ordos Basin, China): Controls on sedimentation and gas accumulations Qinghai Xua,b,c, Wanzhong Shib,∗, Xiangyang Xiec,∗∗, Arthur B. Busbeyc, Litao Xub, Rui Wub, Kai Liub a
College of Geosciences, Yangtze University, Wuhan, 430100, China Key Laboratory of Tectonics and Petroleum of Ministry of Education, China University of Geosciences, Wuhan, 430074, China c School of Geology, Energy, and Environment, Texas Christian University, Fort Worth, TX, 76129, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Porjianghaizi fault Fault evolution Sediment-transport pathways Fault seal Hangjinqi area Ordos Basin
In contrast to typical intracratonic basins, the Ordos Basin comprises marginal deformation belts and complex fault systems. This study uses 3D seismic data tied to sonic and stratigraphic data from exploration wells, to document the geographic extend of the Porjianghaizi fault and its control on sedimentation and gas accumulation in the northern Ordos Basin. Our results show that the geometry of large Porjianghaizi fault is controlled by five small fault segments. The growth history of the Porjianghaizi fault composes four stages: (1) an initiation stage (Late Carboniferous to Early Permian); (2) a reactivation stage during the Late Triassic; (3) an inversion stage during the Middle Jurassic; and (4) an interaction and linkage stage during the Late Jurassic. Detailed studies show that the Porjianghaizi fault had significant control on sedimentation and gas accumulation. During the initiation stage (Late Carboniferous to Early Permian), five fault segments and associated relay ramps controlled the sedimentation in the study area. The relay ramps formed transport pathways in which sediment extended from north basin margin to the basin center. In contrast, sedimentation along the fault was more localized. Based on Shale Gouge Ratios (SGR) and formation water salinity, the fault sealing capacity of the Porjianghaizi fault is characterised by “horizontal segmentation”, which means the fault composes laterally sealed areas and laterally connected areas along the fault strike. As a corollary, we propose that gas resources were mainly generated from the south where there are thick source rocks, migrating vertically through interbedded open fractures first, and then laterally through the junction zone of fault (relay ramps). It eventually accumulated in structural traps found in the northern areas. Understanding the evolution of the Porjianghaizi fault is important for predicting the distribution of sedimentary facies in Upper Carboniferous and Lower Permian units. As well as understanding the migration and distribution of gas resources in the north Ordos Basin, these results can further help in hydrocarbon exploration in the north Ordos Basin.
1. Introduction The Ordos Basin is a typical intracratonic basin with a basin fill gently dipping (< 2°) to the west (e.g., Darby et al., 2001; Darby and Ritts, 2002; Xiao et al., 2005; Zhang et al., 2007; Ritts et al., 2004; Xie and Heller, 2013, Fig. 1). As one of the main petroliferous basins in China (Yang et al., 2005; Hanson et al., 2007), several Ordovician and lower-middle Permian gas fields, including the Changqing, Jinbian, Sulige, and Yulin gas fields, have been discovered in the north and central parts of basin since gas exploration in the Ordos Basin began in the 1980s (Duan et al., 2008; Tang et al., 2012; Zou et al., 2012; Yang et al., 2015a,b; Fig. 1B). Recent exploratory wells in the northern Ordos
∗
Basin (i.e., the Hangjinqi area) found significant volumes of gas in upper Paleozoic sandstones (Wang et al., 2011, Figs. 1 and 2). Gas systems in this area are characterised by upper Carboniferous (Taiyuan Formation) and lower Permian (Shanxi Formation) deltaic and swamp coal source rocks, upper Carboniferous to middle Permian fluvial and deltaic sandstone reservoirs, and upper Permian deltaic and fluvial mudstone cap rocks (Zhang et al., 2009; Ji et al., 2013). Recent studies based on aeromagnetic data, and newly collected 2D seismic lines, suggest that the northern Ordos Basin developed several major fault systems (Yao and Zhang, 2003). Yang et al. (2013) propose a minimum of two phases of fault movement, and that fault systems are composed of multiple, linked fault strands. Sedimentation and gas
Corresponding author. Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, China. Corresponding author. School of Geology, Energy, and Environment, Texas Christian University, Fort Worth, TX, 76129, USA. E-mail addresses: [email protected] (W. Shi), [email protected] (X. Xie).
∗∗
https://doi.org/10.1016/j.marpetgeo.2018.02.003 Received 15 February 2017; Received in revised form 18 December 2017; Accepted 2 February 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Xu, Q., Marine and Petroleum Geology (2018), https://doi.org/10.1016/j.marpetgeo.2018.02.003
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Fig. 1. A: Major tectonic units of the Ordos Basin showing the study area and 3D seismic survey location (modified after Yang et al., 2013). B. 3D seismic survey and 2D seismic sections (grey) and all wells included in this study. Abbreviations: SF: Sanyanjing Fault; WF: Wulanjilinmiao Fault; PF: Porjianghaizi Fault. Section 1 to Section 3 are the locations of seismic profiles used in Fig. 5. Wells in blue color are the location of wells in Fig. 16. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
converge until, at least, the Late Triassic (Lin et al., 1985). In the Jurassic, local rifting and tectonic uplift took place on the Ordos Basin's margins (Yang et al., 2005; Xie and Heller, 2013). In the Middle Jurassic and Cenozoic, large scale strike-slip movements and intraplate deformation resulted in tectonic inversion of parts of the Ordos Basin (Zhang et al., 1998; Yang et al., 2005), which eventually separated the Ordos Basin from the North China block. Currently, the Ordos Basin is surrounded by the Lang and Daqing Mountains to the north, the Luliang Mountains to the east, the Qinling Mountain ranges to the south and the Liupan and Helan mountain ranges to the west (Fig. 1). The basin can be further divided into six tectonic units: the Yimeng Uplift, Western Thrust Belt, Tianhuan Depression, Yishan Slope, Jinxi Flexure Belt and the Weibei Uplift (Fig. 1B). The Hangjinqi area is located in the Yimeng Uplift of the northern basin (Fig. 1B). The Ordos Basin is filled by sedimentary units of Paleozoic to Cenozoic ages (Fig. 2). Its basement is composed of Archean–Proterozoic metamorphic units. In the Hangjinqi area no Cambrian to Devonian strata are present, and the basement is overlain by Late Carboniferous coal-bearing deposits (Yang et al., 2015a,b; Fig. 2). Permian to Triassic strata comprise alluvial fan, deltaic and lacustrine deposits (Zhu et al., 2010; Zou et al., 2010; Yang et al., 2013). Subduction along the eastern margin of Asia in the Early Jurassic caused erosion of Late Triassic deposits and the relative absence of Early
accumulations show differences across major fault systems from south to north, and along the fault strike from west to east (Jia et al., 1997; Zheng et al., 2006; Zheng and Yan, 2006; Xue et al., 2009). However, the geometries and growth history of major fault systems in the Ordos Basin remain poorly understood. In this study, sediment core, well-log, and 3D/2D seismic data were used to document fault geometry, growth, and their control on sedimentation and gas accumulation. Our results show that the current Porjianghaizi fault is a single fault, but its development can be subdivided into four stages. Our results are important for gas exploration that predict the distribution of reservoir and source rocks and may apply to other similar intracratonic basins in China. 2. Geological setting The Ordos Basin is situated in north-central China (Fig. 1). Prior to the Permian, the basin evolved as part of the North China block (Xie and Heller, 2013). By the end of the Paleozoic, the composite TarimNorth China block was sutured to the Mongolian arc terranes and, probably, the Siberian craton to the north (Yin and Nie, 1996; Ritts et al., 2004, Fig. 3). To the south, collision between the South China block and North China block took place progressively from east to west (Yin and Nie, 1996; Webb et al., 1999) and the two blocks did not fully
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Fig. 2. Simplified regional stratigraphic succession and tectonic history of the Hanjinqi area, Ordos Basin (modified from Yang et al., 2015a,b; Xie, 2016).
3. Data and methods
Jurassic deposits in the Hangjinqi area (Yang et al., 2015a,b). Middle Jurassic strata are mainly composed of fluvio-lacustrine clastic deposits (Yang and Liu, 2006); and Early Cretaceous strata are characterised by fluvial and eolian red beds overlain by Quaternary loess deposits (Zheng and Yan, 2006; Xue et al., 2009). This region lacks late Cretaceous-Neogene deposition (Ding et al., 2001; Yang et al., 2015a,b).
The data used in this study include ten 2D seismic lines, a 4322 km2 3D seismic survey and 30 wells in the east Hangjinqi area (Fig. 1C). First, we calibrated and tied the seismic data to exploration wells using a synthetic seismogram to facilitate the recognition of stratigraphic boundaries. A total of three high-amplitude unconformity events and six other stratigraphic boundaries were identified and interpreted.
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Fig. 3. Simplified tectonic map of China (modified from Yang et al., 2005).
the slipped interval (Yielding et al., 1997), and it usually requires 0.25 of SGR to seal a fault (Yielding et al., 1997). The equation is as follows:
Acoustic logs from thirty wells were used to create synthetic seismograms for time-depth conversion of 3D seismic data. An isopach map of upper Paleozoic strata was accurately computed. Additionally, we mapped the distribution of the Porjianghaizi -fault in the Hangjinqi area. Expansion Index (EI), a ratio derived by dividing the hanging wall stratal thickness by the adjacent footwall stratal thickness for a given stratigraphic unit, has been used to identify periods of significant fault growth (Thorsen, 1963; Cartwright et al., 1998; McDonnell et al., 2007; Ze and Alves, 2016). The thicknesses of the hanging wall and adjacent footwall were based on the time-depth conversion and stratigraphic interpretation. In general, the EI of reverse faults is < 1, the EI of normal faults is > 1, and where EI is = 1, the fault is interpreted to be dormant (Lewis et al., 2013; Reeve et al., 2015). The changes of EI along fault strike, in different stratigraphic units, can be used to characterise fault development through time, particularly if sedimentation rates keep up with (or are moderately outpaced) by hanging-wall subsidence (i.e. Basins are not sediment starved) fault-related subsidence is of large scale(s) (Ze and Alves, 2017). In this study, the hanging wall stratal thickness and the footwall stratal thickness were measured based on the depth converted seismic profile, as we calibrated and tied the seismic data to exploration wells using a synthetic seismogram. The fault sealing ability of the Porjianghaizi fault was estimated based on formation water salinity and shale gouge ratio (SGR). Core observations and thin sections were also used to further determine and document the small fault sealing properties of the faults of Lower Paleozoic strata. The SGR is simply the percentage of shale or clay in
SGR =
∑ VSH × ΔZ Fault throw
Vsh is the clay volume of fraction in the sequence. ΔZ is the thickness of each reservoir zone. In this study, ten seismic profiles of 3D seismic surveys and three 2D seismic profiles along the Porjianghaizi fault strike were used to calculate the SGR so that we could analyse the fault sealing capacity. The vertical throw was measured on seismic data, and the clay volume (Vsh) was acquired from nearby wells. Two core photographs and two thin section micrographs were used to analyse the gas vertical migration from the source to reservoir. 4. Results 4.1. Seismic stratigraphy Three major unconformities (T3, T5, and T9) were interpreted on seismic data (Fig. 4). Additionally, there is a minor unconformity between T5 and T3 (Fig. 4). The strata between T9 and T5 are composed of Upper Carboniferous (Taiyuan Formation), Permian (Shanxi, Lower Shihezi, Upper Shihezi, and Shiqianfeng Formations), and Triassic (Liujiagou, Heshanggou, Ermaying, Yanchang Formations) strata
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Fig. 4. Uninterpreted (top) and interpreted (bottom) seismic profile A-A′ across the Hangjinqi area (see Fig. 1B for the location).
Porjianghaizi fault is a single north-trending fault which divided the eastern Hangjinqi area into southern and northern parts (Figs. 5 and 6). From west to northeast, the orientation of the fault changes from eastwest (95°) to northeast to southwest (65°) (Fig. 6B). Based on fault orientations and dips, the Porjianghaizi fault can be divided into five different parts (Figs. 6B, 7A and 8A). The Expansion Index (EI) of the Porjianghaizi fault shows three different types of patterns (Table 1; Fig. 9). Type 1 is characterised by an EI value less than 1.0 (∼0.96–1.0) from the Late Carboniferous to Triassic, but over 1.0 (∼1.04–1.38) after the Late Jurassic (Table 1). Type 2 is characterised by an EI value remaining at 1.0 from the Late Carboniferous to the Late Permian, and starting at the Triassic, an EI value less than 1.0 (ca. 0.9), then changing to over 1.0 (∼1.03–1.69) in the Middle Jurassic (Table 1). Type 3 has the smallest EI value (∼0.79) from the Late Carboniferous to the Early Permian, and its value increases 0.95 to 0.99 during the Middle Permian to Late Permian. In the Triassic, its EI value falls to 0.9, then starting at the Middle Jurassic, it increases to over 1.0 (1.22–1.38) (Table 1; Fig. 9). Based on the expansion index (Table 1), the evolution of Porjianghaizi fault was composed of four stages: (1) an initiation stage during Late Carboniferous to
(Figs. 2 and 4). The strata between T9 and T5 show a series of progressive onlapping strata over on Archean–Proterozoic carbonate rocks and metamorphic basement (T9) as it progresses northward, and is bounded by a high-amplitude reflection above (Fig. 4). The Taiyuan Formation shows significant thickness changes across the Porjianghaizi fault from south to north, and in most areas the sediments terminate to the north against the fault (Fig. 4). In some areas, Late Triassic strata are truncated by a minor angular unconformity (T5) located between the Yan'an and Yanchang Formations (Fig. 4). The strata between T5 and T3 are concordant with the unconformity (T5) between the Yanchang and Yan'an Formations (Figs. 2 and 4). Cretaceous strata (Zhidan Formation) lie over the unconformity T3. 4.2. Fault characteristics Overall, in the eastern Hangjinqi area we identified normal, reverse, and reactivated faults (Figs. 4 and 5), and the majority of faults are SWNE and NW-SE in their orientation (Fig. 6A and B). The normal faults mainly offset Upper Triassic to Lower Cretaceous strata, and reverse faults are mainly developed in Permian strata (Fig. 5). The current
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Fig. 5. Interpreted line of seismic cross sections along the north-south direction showing major faults See Fig. 1B for the location of the seismic sections.
ramp areas from the south to the north. The type 2 patterns of expansion index represent there show no faults growth until Late Triassic (Table 1). Based on the changes of expansion indexes along the fault strike and seismic interpretation, five separate reverse faults were identified from the Late Carboniferous to Early Permian (Fig. 9A). The five faults were linked into the current Porjianghaizi fault in the Late Jurassic (Fig. 9B).
Early Permian; (2) a reactivation stage during the Late Triassic; (3) an inversion stage during the Middle Jurassic; and (4) an interaction and linkage stage during the Late Jurassic. At the initiation stage, the Porjianghaizi fault was composed of five separate reverse faults that controlled the sedimentation of the Taiyuan and Shanxi Formations. Relay ramps among faults were the main sediment-transport pathways to the south. The Porjianghaizi fault was divided into lateral migration fault areas and laterally sealed fault areas, which means gas can be migrated laterally through the migration fault areas but not the sealed fault areas. The junction zones (relay ramp areas) of the Porjianghaizi fault have the highest connectivity as these have the lowest vertical throw and sand-sand juxtaposition. Gas mainly migrated through these relay
4.3. Fault seal competence In general, higher Shale Gouge Ratio (SGR) numbers mean that faults are likely sealed (Harris et al., 2002). Results of SGR calculations
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Fig. 6. A: Seismic coherence slice show the distribution of Porjianghaizi fault; B: Simplified distribution of the faults in the east Hangjinqi area; C: Vertical fault throw (T9) of the Porjianghaizi Fault.
zones are likely lateral migration paths. Based on the SGR results, the Porjianghaizi fault can be divided into a lateral migration fault area (SGR < 0.25) and a lateral sealed fault area (SGR > 0.25) (Fig. 10). Because of the small vertical throw, generally there is sand-sand juxtaposition in the junction zones (Fig. 7A) which have poor fault sealing ability (Fig. 10). This result is further confirmed by the formation water salinity of the Taiyuan Formation which shows that areas near the Porjianghaizi fault have similar salinity, except in the circle area where
show that the Porjianghaizi fault is horizontally segmented with the SGR values alternating between large and small from west to east along the fault strike (Fig. 10B). The SGR values of the fault are mostly less than 0.25. Only near lines XLN6990 and XLN7290 is the SGR higher than 0.25 (Fig. 10B). Additionally, the SGR of the junction zone (the relay ramp area during the Late Carboniferous to Early Permian, Fig. 9A) of the fault is generally smaller. For example, the SGR calculated for line XLN6470 is less than 0.1, which suggests that the junction
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Fig. 7. A. Fault-plane between the hanging wall and footwall of the Porjianghaizi fault from the Taiyuan Formation to the upper Shihezi Formation. B: Fault dip of the Porjianghaiz fault along the fault strike. T9c: bottom surface of the Taiyuan Formation; T9d: top surface of the Shanxi Formation; T9e: bottom surface of upper lower Shihezi Formation; T9f: top surface of upper lower Shihezi Formation.
thin from south to north and, crossing the Porjianghaizi fault, part of the Taiyuan Formation is absent as indicated by wells J65 (Fig. 12), J38, and J39 (Fig. 13). The coal of the Taiyuan Formation is mainly developed on the uppermost succession as seen in wells J55 to J48 (Fig. 12), and wells J76 to J36 (Fig. 13), and its thickness diminishes rapidly from south to north, from 17 m to 5 m after comparing wells J55 and J76 with wells J48 and J36 (Figs. 12 and 13). Furthermore, the sections show that the strata thin from south to north (wells J55 to J70). Cross section 2 shows that the thickness of the Taiyuan Formation remains stable at 32 m, but the Shan1 sublayer thickens from 30 m to 55 m from south to north (well J76 to J74). Both cross sections show that coal thins from south to north. In summary, thickness shows significant changes northwards across faults, indicating that the Porjianghaizi fault controlled sedimentation during the deposition of the Taiyuan and Shanxi Formations. The thickness of other potential reservoir units (the Shihezi and Shiqianfeng Formations) remain constant, suggesting that the Porjianghaizi fault did not actively affect the deposition of those units (Figs. 4 and 10).
the water salinity of the northern Porjianghaizi fault (well 35, 76700 mg/L; well 12, 83140.75 mg/L) is much higher than the southern Porjianghaizi fault (well 52, 39825.13 mg/L) (Fig. 10A). This suggests that the Porjianghaizi fault is laterally connected except in the circular area of the Taiyuan Formation. That is consistent with the SGR results where only in the XLN7390 area is the SGR over 0.25. 4.4. Sedimentation of the eastern Hangjinqi area The thickness of the Taiyuan and Shanxi Formations in eastern Hangjinqi area increases across the relay ramp between two fault strands (Fig. 11). The thickest deposits are distributed along the relay ramp (100–140 m) and at the lower tip of the fault (40–110 m) (Fig. 11). Two cross-sections were used to show the details of the depositional differences between the via faults and relay ramps during Upper Carboniferous (Taiyuan Formation) to the Lower Permian (Shanxi Formation) from south to north (Figs. 12 and 13). The Taiyuan and Shanxi Formations show significant thickness changes across the eastern Hangjinqi area, and the Taiyuan Formation terminates to the north against the Porjianghaizi Fault (Figs. 4, 12 and 13). Overall, strata
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Fig. 8. Seismic sections showing the characteristics of the Porjianghaizi fault along fault strike.
Table 1 Three types of EI of PF from Late Carboniferous to Jurassic. C3t: Carboniferous Taiyuan Formaiton; P1s: Permian Shanxi Formation; P2x: Permian Lower Shihezi Formation; P2s: Permian Upper Shihezi Formation; P2sh: Permian Shiqianfeng Formation; T: Triassic; J2y: Jurassic Yan'an Formation; J2z: Jurassic Zhidan Formation; J2a: Jurassic Anding Formation. Strata
J2z + J2a J2y T P2s + P2sh P1x C3t + P1s
Type 1
Type 2
Type 3
Hangingwall (m)
Footwall (m)
EI
Hangingwall (m)
Footwall (m)
EI
Hangingwall (m)
Footwall (m)
EI
289 250 1102 375 135 124
209 239 1157 384 141 128
1.38 1.04 0.95 0.98 0.96 0.97
316 199 1084 362 123.7 103.5
186 194 1200 362 123.7 103.5
1.69 1.03 0.90 1.0 1.0 1.0
352 248 977 418 109 66
256 223 1086 418.7 114.4 83.4
1.38 1.11 0.90 0.99 0.95 0.79
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Fig. 9. Maps of faults in the study area during Late Carboniferous to Early Permian (A) and Late Jurassic (B). Type 1 to Type 3 are the types of EI that indicate different fault evolution.
Fig. 10. A: Formation water salinity (mg/L) distribution of the lower Shihezi Formation. B: SGR distribution diagram of the Porjianghaizi fault in the Taiyuan to lower Shihezi Formations along the fault strike. STZ, Structural transfer zone.
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Fig. 11. Isopach map of strata thickness of the Taiyuan and Shanxi Formations based on 3D seismic interpretation. And the locations of cross sections of Figs. 12 and 13.
Fig. 12. Cross section B-B'showing stratigraphic section and lithology from south to north, and the GR signatures of the Taiyuan Formation and the Shanxi Formations (See location in Fig. 11).
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Fig. 13. Cross section C-C′ showing stratigraphic section and lithology from south to north, and the GR signatures of the Taiyuan Formation and Shanxi Formations (See location in Fig. 11).
5. Discussion
of existing faults and the orientation of stress were similar (Yang et al., 2013). Affected by the extensional strike-slip, the five reverse faults were reactivated to normal offsets (Fig. 14C), and started interacting on the relay ramp areas. During the Late Jurassic, the five separate faults finally linked into the current Porjianghaizu fault (Figs. 9A and 14D).
5.1. Controlling factors on fault inversion and propagational model of the Porjianghaizi fault The growth and propagational model of faults are often controlled by the tectonic activity and reactivated basement structures (Pinheiro and Holdsworth, 1997; Escalona and Mann, 2011; Yang et al., 2015a,b). During the Late Carboniferous to Early Permian, the North China block began colliding with the Mongolian arcs, and the South China block began subducting beneath the North China block (Watson et al., 1987; Meng and Zhang, 1999; Yang et al., 2005; Yin and Nie, 1996, Fig. 3). These collisions caused a series of reverse faults in the eastern Hangjinqi Area (Fig. 14A). In the study area, tectonic convergence caused five separate reverse faults (Type 1 and Type 3; Figs. 9A and 14A). During the Middle Permian to Late Triassic, the South China block also subducted beneath the Qiantang block and had slight convergence with the North China block (Yin and Nie, 1996). Therefore, the previously formed faults either stopped or minimally developed with values of EI at or near 1.0 (Table 1). During the Late Triassic, these early reverse faults were reactivated by compression between the North China and South China blocks (Enkin et al., 1992; Webb et al., 1999; Yang et al., 2015a,b; Fig. 14B). During the Middle Jurassic, there was an extensional strike-slip event. and during this same period, the strike direction
5.2. Porjianghaizi fault controls on sedimentation and gas accumulation The progression of fault propagation, growth, linkage, and death are major tectonic controls on basin sedimentation, and may control gas migration and accumulation (Gawthorpe and Leeder, 2000; Peng, 2000; Alves et al., 2002; Liesa et al., 2006). In the study area, the Upper Paleozoic strata have relatively good source-reservoir-cap combinations of gas systems in comparision to the other units (Xiao et al., 2005; Xue et al., 2009). The Taiyuan Formation is composed of mudstone, carbonaceous mudstone, sandstones of all grain sizes, and coal (Figs. 12 and 13). The lower Permian Shanxi Formation is composed of coal, mudstone, carbonaceous mudstone, fine-to coarse-grained sandstones with gravel. Previous gas-system analyses suggest that the coal seams of the Upper Carboniferous Taiyuan and Lower Permian Shanxi Formations are main source rocks in the north Ordos Basin (Xue et al., 2009). Sandstones of the Shanxi Formation and the Upper Shihezi Formation are main reservoirs, and the mudstone of the Shiqianfeng Formation serves as the cap rock (Sun et al., 2007). Therefore, the Porjianghaizi
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Fig. 14. Schematic diagram showing the development of the inversion and propagation of Porjianghaizi fault. A: Initiation stage with five reverse faults from Late Carboniferous to Early Permian. B: Fault reactivation during Triassic. C: Inversion stage affected by strike-slip event, thrust faults inverted to normal fault starting from the Middle Jurassic. D: Fault interaction and linkage to a through-going fault.
their depocenters at the foot of the fault and were bounded by a relatively high-angle fault (Fig. 15). Mapping suggests that sediments crossing over relay ramps can extent far away from the north in the direction of the basin center (Figs. 13 and 15). In contrast, sediments directly crossing over faults deposited locally and not far away from the fault (Fig. 15). The seismic stratigraphy results show the thickness of Lower Shihezi to Shiqianfeng Formations is constant beyond the Porjianghaizi fault from south to north. This indicates the Porjianghaizi fault had less control on sedimentation during late Permian. The coal source rocks of the Taiyuan and Shanxi Formations began generating gas in the Early Cretaceous time (Ji et al., 2013). The Hangjinqi area experienced two main periods of gas migration, including one from the Middle-Late Jurassic and the other from Early Cretaceous to present (Xue et al., 2009). Many small faults and fractures
fault control on sedimentation and gas accumulation are mainly recorded in Upper Paleozoic units. The Yin Mountains to the north, formed by collision between the North China block and the Mongolia Arc during the Late Carboniferous to Lower Permian, are the main sediment sources for the northern Ordos Basin (Yang et al., 2005). During the deposition of the Taiyuan and Shanxi Formations, sediment derived from north passed through the Porjianghaizi fault first, and then continued to the south. As mentioned before, five isolated faults developed during Late Carboniferous to Early Permian (Fig. 9A). During the fault initiation stage (Late Carboniferous to Early Permian), there were four relay ramps (Fig. 9A). The thickness maps of the Taiyuan and Shanxi Formations indicate that deposition in the southern Porjianghaizi fault areas was mainly via relay ramps (Figs. 11 and 16). Sediments crossing isolated faults had
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Fig. 15. Tectono-sedimentary model of the eastern Hangjinqi area of north Ordos Basin. Cross the relay ramps, sediments were transported far away and deposited in the basin center. But cross the faults, sediments just deposited near the fault.
6. Conclusions
were developed in Late Paleozoic strata (Figs. 4–6) and these faults and fractures are not sealed (Fig. 16). Thus, gas mainly generated from coal of the southern Porjianghaizi fault area could vertically migrate from source rock to upper sand reservoirs through open fractures (Fig. 17). As indicated by the SGR results the fault seal of the Porjianghaizi fault was characterised by ‘horizontal segmentation’, and the fault junction zones of the Porjianghaizi fault (relay ramp areas during Late Carboniferous to Early Permian) have the lowest fault sealing capacity (lowest value of SGR). Therefore, the gas generated in the Taiyuan and Shanxi Formations of the southern Hangjinqi area could migrate vertically to upper sand reservoirs via small faults and fractures. Gas then migrates laterally through the junction zone of the Porjianghaizi fault (relay ramp area), and eventually accumulates in those structural traps found in the northern areas (Fig. 17).
The detailed study of the Porjianghaizi fault provides a good example of the evolution of inversion and reactivated faults and how they controlled the sedimentation and gas accumulation in the northern Ordos Basin. The main conclusions from this study include: 1. Three major unconformities (T9, T5, T3) and six stratigraphic boundaries were marked. The thickness of the lower part of the strata between T9 and T5 (the Taiyuan and Shanxi Formations) generally thins and pinches out from south to north beyond the Porjianghaizi fault. 2. The current Porjianghaizi fault is a continuous single fault, but different parts of the Porjianghaizi fault show different features. Based on vertical throw, orientation, and fault dips, the Porjianghaizi fault can be divided into five segments. 3. The growth history of the Porjianghaizi fault, from Late
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Fig. 16. Core photos and thin sections showing small faults and fractures in Lower Paleozoic sandstones. A: Core photo of well J35 at ca. 2128.04 showing vertical fractures; B: Core photo of well J39 at ca. 2183.1 m showing vertical fractures; C: Thin section micrographs of well J69 at 2964.77 m showing vertical fractures; D: Thin section micrographs of well J67 at 2502.9 m showing vertical fractures. The blue lines in C and D are fractures (See well locations in Fig. 1B). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
northern area via relay ramps tend to extend further. In contrast, sediments directly across isolated faults were accumulated at the foot of relatively high-angle bounding faults. 6. The sealing capacity of the Porjianghaizi fault is characterised by “horizontal segmentation” which mains the Porjianghaizi fault was divided into lateral migration fault areas and lateral sealed fault areas. The junction zones (relay ramp area) of the Porjianghaizi fault have the highest connectivity. Gas generated in the Taiyuan and Shanxi Formations of the southern Hangjinqi area migrated vertically to upper sand reservoirs via open small faults and fractures, and then chiefly migrated laterally northwards beyond the Porjianghaizi fault junction zone (lateral migration fault areas) to the northern Hangjinqi area. Gas accumulated in structural traps found in the north Porjianghaizi fault area.
Carboniferous to Late Jurassic, is composes four stages: an initiation stage during Late Carboniferous to Early Permian, a reactivation stage at Late Triassic, an inversion stage at Middle Jurassic, an interaction and linkage stage at Late Jurassic. 4. The Porjianghaizi fault was composed of five reverse fault segments during the Late Paleozoic caused by the North China Block collision with the Mongolian arcs and the South China Block subduction beneath the North China block. During the Middle Jurassic, the five faults began inverting into a normal fault. During the Late Jurassic, the five normal faults interacted and linked to the current single Porjianghaizi fault. 5. During the Late Carboniferous to Early Permian, there were four relay ramps among the five fault segments. The relay ramps were the main sediment-transport pathways. Sediment delivered from the
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Acknowledgement All data used in this study are provided by North China Bureau of Petroleum, SINOPEC. We wish to thank the masters for assistance with preliminary work including core analysis and seismic stratigraphy interpretation. This study was jointly supported by the program of introducing talents of scientific disciplines to universities (No. B14031), the 13th “Five-year” plan of the Ministry of Science and Technology of China (2016ZX05034002-003) and the shale gas survey of National Geological Survey (12120114055801). We are grateful to the journal editor Dr. Alejandro Escalona, Dr. Tiago Alves, and an anonymous reviewer, for their critical, constructive, helpful comments and suggestions that considerable improved the quality of this manuscript. References
Fig. 17. Vertical and lateral migration and accumulation of gas in the Hangjinqi area, Ordos Basin.
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
Inversion and propagation of the Late Paleozoic Porjianghaizi fault (North Ordos Basin, China): Controls on sedimentation and gas accumulations Qinghai Xua,b,c, Wanzhong Shib,∗, Xiangyang Xiec,∗∗, Arthur B. Busbeyc, Litao Xub, Rui Wub, Kai Liub a
College of Geosciences, Yangtze University, Wuhan, 430100, China Key Laboratory of Tectonics and Petroleum of Ministry of Education, China University of Geosciences, Wuhan, 430074, China c School of Geology, Energy, and Environment, Texas Christian University, Fort Worth, TX, 76129, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Porjianghaizi fault Fault evolution Sediment-transport pathways Fault seal Hangjinqi area Ordos Basin
In contrast to typical intracratonic basins, the Ordos Basin comprises marginal deformation belts and complex fault systems. This study uses 3D seismic data tied to sonic and stratigraphic data from exploration wells, to document the geographic extend of the Porjianghaizi fault and its control on sedimentation and gas accumulation in the northern Ordos Basin. Our results show that the geometry of large Porjianghaizi fault is controlled by five small fault segments. The growth history of the Porjianghaizi fault composes four stages: (1) an initiation stage (Late Carboniferous to Early Permian); (2) a reactivation stage during the Late Triassic; (3) an inversion stage during the Middle Jurassic; and (4) an interaction and linkage stage during the Late Jurassic. Detailed studies show that the Porjianghaizi fault had significant control on sedimentation and gas accumulation. During the initiation stage (Late Carboniferous to Early Permian), five fault segments and associated relay ramps controlled the sedimentation in the study area. The relay ramps formed transport pathways in which sediment extended from north basin margin to the basin center. In contrast, sedimentation along the fault was more localized. Based on Shale Gouge Ratios (SGR) and formation water salinity, the fault sealing capacity of the Porjianghaizi fault is characterised by “horizontal segmentation”, which means the fault composes laterally sealed areas and laterally connected areas along the fault strike. As a corollary, we propose that gas resources were mainly generated from the south where there are thick source rocks, migrating vertically through interbedded open fractures first, and then laterally through the junction zone of fault (relay ramps). It eventually accumulated in structural traps found in the northern areas. Understanding the evolution of the Porjianghaizi fault is important for predicting the distribution of sedimentary facies in Upper Carboniferous and Lower Permian units. As well as understanding the migration and distribution of gas resources in the north Ordos Basin, these results can further help in hydrocarbon exploration in the north Ordos Basin.
1. Introduction The Ordos Basin is a typical intracratonic basin with a basin fill gently dipping (< 2°) to the west (e.g., Darby et al., 2001; Darby and Ritts, 2002; Xiao et al., 2005; Zhang et al., 2007; Ritts et al., 2004; Xie and Heller, 2013, Fig. 1). As one of the main petroliferous basins in China (Yang et al., 2005; Hanson et al., 2007), several Ordovician and lower-middle Permian gas fields, including the Changqing, Jinbian, Sulige, and Yulin gas fields, have been discovered in the north and central parts of basin since gas exploration in the Ordos Basin began in the 1980s (Duan et al., 2008; Tang et al., 2012; Zou et al., 2012; Yang et al., 2015a,b; Fig. 1B). Recent exploratory wells in the northern Ordos
∗
Basin (i.e., the Hangjinqi area) found significant volumes of gas in upper Paleozoic sandstones (Wang et al., 2011, Figs. 1 and 2). Gas systems in this area are characterised by upper Carboniferous (Taiyuan Formation) and lower Permian (Shanxi Formation) deltaic and swamp coal source rocks, upper Carboniferous to middle Permian fluvial and deltaic sandstone reservoirs, and upper Permian deltaic and fluvial mudstone cap rocks (Zhang et al., 2009; Ji et al., 2013). Recent studies based on aeromagnetic data, and newly collected 2D seismic lines, suggest that the northern Ordos Basin developed several major fault systems (Yao and Zhang, 2003). Yang et al. (2013) propose a minimum of two phases of fault movement, and that fault systems are composed of multiple, linked fault strands. Sedimentation and gas
Corresponding author. Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, China. Corresponding author. School of Geology, Energy, and Environment, Texas Christian University, Fort Worth, TX, 76129, USA. E-mail addresses: [email protected] (W. Shi), [email protected] (X. Xie).
∗∗
https://doi.org/10.1016/j.marpetgeo.2018.02.003 Received 15 February 2017; Received in revised form 18 December 2017; Accepted 2 February 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Xu, Q., Marine and Petroleum Geology (2018), https://doi.org/10.1016/j.marpetgeo.2018.02.003
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Fig. 1. A: Major tectonic units of the Ordos Basin showing the study area and 3D seismic survey location (modified after Yang et al., 2013). B. 3D seismic survey and 2D seismic sections (grey) and all wells included in this study. Abbreviations: SF: Sanyanjing Fault; WF: Wulanjilinmiao Fault; PF: Porjianghaizi Fault. Section 1 to Section 3 are the locations of seismic profiles used in Fig. 5. Wells in blue color are the location of wells in Fig. 16. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
converge until, at least, the Late Triassic (Lin et al., 1985). In the Jurassic, local rifting and tectonic uplift took place on the Ordos Basin's margins (Yang et al., 2005; Xie and Heller, 2013). In the Middle Jurassic and Cenozoic, large scale strike-slip movements and intraplate deformation resulted in tectonic inversion of parts of the Ordos Basin (Zhang et al., 1998; Yang et al., 2005), which eventually separated the Ordos Basin from the North China block. Currently, the Ordos Basin is surrounded by the Lang and Daqing Mountains to the north, the Luliang Mountains to the east, the Qinling Mountain ranges to the south and the Liupan and Helan mountain ranges to the west (Fig. 1). The basin can be further divided into six tectonic units: the Yimeng Uplift, Western Thrust Belt, Tianhuan Depression, Yishan Slope, Jinxi Flexure Belt and the Weibei Uplift (Fig. 1B). The Hangjinqi area is located in the Yimeng Uplift of the northern basin (Fig. 1B). The Ordos Basin is filled by sedimentary units of Paleozoic to Cenozoic ages (Fig. 2). Its basement is composed of Archean–Proterozoic metamorphic units. In the Hangjinqi area no Cambrian to Devonian strata are present, and the basement is overlain by Late Carboniferous coal-bearing deposits (Yang et al., 2015a,b; Fig. 2). Permian to Triassic strata comprise alluvial fan, deltaic and lacustrine deposits (Zhu et al., 2010; Zou et al., 2010; Yang et al., 2013). Subduction along the eastern margin of Asia in the Early Jurassic caused erosion of Late Triassic deposits and the relative absence of Early
accumulations show differences across major fault systems from south to north, and along the fault strike from west to east (Jia et al., 1997; Zheng et al., 2006; Zheng and Yan, 2006; Xue et al., 2009). However, the geometries and growth history of major fault systems in the Ordos Basin remain poorly understood. In this study, sediment core, well-log, and 3D/2D seismic data were used to document fault geometry, growth, and their control on sedimentation and gas accumulation. Our results show that the current Porjianghaizi fault is a single fault, but its development can be subdivided into four stages. Our results are important for gas exploration that predict the distribution of reservoir and source rocks and may apply to other similar intracratonic basins in China. 2. Geological setting The Ordos Basin is situated in north-central China (Fig. 1). Prior to the Permian, the basin evolved as part of the North China block (Xie and Heller, 2013). By the end of the Paleozoic, the composite TarimNorth China block was sutured to the Mongolian arc terranes and, probably, the Siberian craton to the north (Yin and Nie, 1996; Ritts et al., 2004, Fig. 3). To the south, collision between the South China block and North China block took place progressively from east to west (Yin and Nie, 1996; Webb et al., 1999) and the two blocks did not fully
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Fig. 2. Simplified regional stratigraphic succession and tectonic history of the Hanjinqi area, Ordos Basin (modified from Yang et al., 2015a,b; Xie, 2016).
3. Data and methods
Jurassic deposits in the Hangjinqi area (Yang et al., 2015a,b). Middle Jurassic strata are mainly composed of fluvio-lacustrine clastic deposits (Yang and Liu, 2006); and Early Cretaceous strata are characterised by fluvial and eolian red beds overlain by Quaternary loess deposits (Zheng and Yan, 2006; Xue et al., 2009). This region lacks late Cretaceous-Neogene deposition (Ding et al., 2001; Yang et al., 2015a,b).
The data used in this study include ten 2D seismic lines, a 4322 km2 3D seismic survey and 30 wells in the east Hangjinqi area (Fig. 1C). First, we calibrated and tied the seismic data to exploration wells using a synthetic seismogram to facilitate the recognition of stratigraphic boundaries. A total of three high-amplitude unconformity events and six other stratigraphic boundaries were identified and interpreted.
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Fig. 3. Simplified tectonic map of China (modified from Yang et al., 2005).
the slipped interval (Yielding et al., 1997), and it usually requires 0.25 of SGR to seal a fault (Yielding et al., 1997). The equation is as follows:
Acoustic logs from thirty wells were used to create synthetic seismograms for time-depth conversion of 3D seismic data. An isopach map of upper Paleozoic strata was accurately computed. Additionally, we mapped the distribution of the Porjianghaizi -fault in the Hangjinqi area. Expansion Index (EI), a ratio derived by dividing the hanging wall stratal thickness by the adjacent footwall stratal thickness for a given stratigraphic unit, has been used to identify periods of significant fault growth (Thorsen, 1963; Cartwright et al., 1998; McDonnell et al., 2007; Ze and Alves, 2016). The thicknesses of the hanging wall and adjacent footwall were based on the time-depth conversion and stratigraphic interpretation. In general, the EI of reverse faults is < 1, the EI of normal faults is > 1, and where EI is = 1, the fault is interpreted to be dormant (Lewis et al., 2013; Reeve et al., 2015). The changes of EI along fault strike, in different stratigraphic units, can be used to characterise fault development through time, particularly if sedimentation rates keep up with (or are moderately outpaced) by hanging-wall subsidence (i.e. Basins are not sediment starved) fault-related subsidence is of large scale(s) (Ze and Alves, 2017). In this study, the hanging wall stratal thickness and the footwall stratal thickness were measured based on the depth converted seismic profile, as we calibrated and tied the seismic data to exploration wells using a synthetic seismogram. The fault sealing ability of the Porjianghaizi fault was estimated based on formation water salinity and shale gouge ratio (SGR). Core observations and thin sections were also used to further determine and document the small fault sealing properties of the faults of Lower Paleozoic strata. The SGR is simply the percentage of shale or clay in
SGR =
∑ VSH × ΔZ Fault throw
Vsh is the clay volume of fraction in the sequence. ΔZ is the thickness of each reservoir zone. In this study, ten seismic profiles of 3D seismic surveys and three 2D seismic profiles along the Porjianghaizi fault strike were used to calculate the SGR so that we could analyse the fault sealing capacity. The vertical throw was measured on seismic data, and the clay volume (Vsh) was acquired from nearby wells. Two core photographs and two thin section micrographs were used to analyse the gas vertical migration from the source to reservoir. 4. Results 4.1. Seismic stratigraphy Three major unconformities (T3, T5, and T9) were interpreted on seismic data (Fig. 4). Additionally, there is a minor unconformity between T5 and T3 (Fig. 4). The strata between T9 and T5 are composed of Upper Carboniferous (Taiyuan Formation), Permian (Shanxi, Lower Shihezi, Upper Shihezi, and Shiqianfeng Formations), and Triassic (Liujiagou, Heshanggou, Ermaying, Yanchang Formations) strata
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Fig. 4. Uninterpreted (top) and interpreted (bottom) seismic profile A-A′ across the Hangjinqi area (see Fig. 1B for the location).
Porjianghaizi fault is a single north-trending fault which divided the eastern Hangjinqi area into southern and northern parts (Figs. 5 and 6). From west to northeast, the orientation of the fault changes from eastwest (95°) to northeast to southwest (65°) (Fig. 6B). Based on fault orientations and dips, the Porjianghaizi fault can be divided into five different parts (Figs. 6B, 7A and 8A). The Expansion Index (EI) of the Porjianghaizi fault shows three different types of patterns (Table 1; Fig. 9). Type 1 is characterised by an EI value less than 1.0 (∼0.96–1.0) from the Late Carboniferous to Triassic, but over 1.0 (∼1.04–1.38) after the Late Jurassic (Table 1). Type 2 is characterised by an EI value remaining at 1.0 from the Late Carboniferous to the Late Permian, and starting at the Triassic, an EI value less than 1.0 (ca. 0.9), then changing to over 1.0 (∼1.03–1.69) in the Middle Jurassic (Table 1). Type 3 has the smallest EI value (∼0.79) from the Late Carboniferous to the Early Permian, and its value increases 0.95 to 0.99 during the Middle Permian to Late Permian. In the Triassic, its EI value falls to 0.9, then starting at the Middle Jurassic, it increases to over 1.0 (1.22–1.38) (Table 1; Fig. 9). Based on the expansion index (Table 1), the evolution of Porjianghaizi fault was composed of four stages: (1) an initiation stage during Late Carboniferous to
(Figs. 2 and 4). The strata between T9 and T5 show a series of progressive onlapping strata over on Archean–Proterozoic carbonate rocks and metamorphic basement (T9) as it progresses northward, and is bounded by a high-amplitude reflection above (Fig. 4). The Taiyuan Formation shows significant thickness changes across the Porjianghaizi fault from south to north, and in most areas the sediments terminate to the north against the fault (Fig. 4). In some areas, Late Triassic strata are truncated by a minor angular unconformity (T5) located between the Yan'an and Yanchang Formations (Fig. 4). The strata between T5 and T3 are concordant with the unconformity (T5) between the Yanchang and Yan'an Formations (Figs. 2 and 4). Cretaceous strata (Zhidan Formation) lie over the unconformity T3. 4.2. Fault characteristics Overall, in the eastern Hangjinqi area we identified normal, reverse, and reactivated faults (Figs. 4 and 5), and the majority of faults are SWNE and NW-SE in their orientation (Fig. 6A and B). The normal faults mainly offset Upper Triassic to Lower Cretaceous strata, and reverse faults are mainly developed in Permian strata (Fig. 5). The current
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Fig. 5. Interpreted line of seismic cross sections along the north-south direction showing major faults See Fig. 1B for the location of the seismic sections.
ramp areas from the south to the north. The type 2 patterns of expansion index represent there show no faults growth until Late Triassic (Table 1). Based on the changes of expansion indexes along the fault strike and seismic interpretation, five separate reverse faults were identified from the Late Carboniferous to Early Permian (Fig. 9A). The five faults were linked into the current Porjianghaizi fault in the Late Jurassic (Fig. 9B).
Early Permian; (2) a reactivation stage during the Late Triassic; (3) an inversion stage during the Middle Jurassic; and (4) an interaction and linkage stage during the Late Jurassic. At the initiation stage, the Porjianghaizi fault was composed of five separate reverse faults that controlled the sedimentation of the Taiyuan and Shanxi Formations. Relay ramps among faults were the main sediment-transport pathways to the south. The Porjianghaizi fault was divided into lateral migration fault areas and laterally sealed fault areas, which means gas can be migrated laterally through the migration fault areas but not the sealed fault areas. The junction zones (relay ramp areas) of the Porjianghaizi fault have the highest connectivity as these have the lowest vertical throw and sand-sand juxtaposition. Gas mainly migrated through these relay
4.3. Fault seal competence In general, higher Shale Gouge Ratio (SGR) numbers mean that faults are likely sealed (Harris et al., 2002). Results of SGR calculations
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Fig. 6. A: Seismic coherence slice show the distribution of Porjianghaizi fault; B: Simplified distribution of the faults in the east Hangjinqi area; C: Vertical fault throw (T9) of the Porjianghaizi Fault.
zones are likely lateral migration paths. Based on the SGR results, the Porjianghaizi fault can be divided into a lateral migration fault area (SGR < 0.25) and a lateral sealed fault area (SGR > 0.25) (Fig. 10). Because of the small vertical throw, generally there is sand-sand juxtaposition in the junction zones (Fig. 7A) which have poor fault sealing ability (Fig. 10). This result is further confirmed by the formation water salinity of the Taiyuan Formation which shows that areas near the Porjianghaizi fault have similar salinity, except in the circle area where
show that the Porjianghaizi fault is horizontally segmented with the SGR values alternating between large and small from west to east along the fault strike (Fig. 10B). The SGR values of the fault are mostly less than 0.25. Only near lines XLN6990 and XLN7290 is the SGR higher than 0.25 (Fig. 10B). Additionally, the SGR of the junction zone (the relay ramp area during the Late Carboniferous to Early Permian, Fig. 9A) of the fault is generally smaller. For example, the SGR calculated for line XLN6470 is less than 0.1, which suggests that the junction
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Fig. 7. A. Fault-plane between the hanging wall and footwall of the Porjianghaizi fault from the Taiyuan Formation to the upper Shihezi Formation. B: Fault dip of the Porjianghaiz fault along the fault strike. T9c: bottom surface of the Taiyuan Formation; T9d: top surface of the Shanxi Formation; T9e: bottom surface of upper lower Shihezi Formation; T9f: top surface of upper lower Shihezi Formation.
thin from south to north and, crossing the Porjianghaizi fault, part of the Taiyuan Formation is absent as indicated by wells J65 (Fig. 12), J38, and J39 (Fig. 13). The coal of the Taiyuan Formation is mainly developed on the uppermost succession as seen in wells J55 to J48 (Fig. 12), and wells J76 to J36 (Fig. 13), and its thickness diminishes rapidly from south to north, from 17 m to 5 m after comparing wells J55 and J76 with wells J48 and J36 (Figs. 12 and 13). Furthermore, the sections show that the strata thin from south to north (wells J55 to J70). Cross section 2 shows that the thickness of the Taiyuan Formation remains stable at 32 m, but the Shan1 sublayer thickens from 30 m to 55 m from south to north (well J76 to J74). Both cross sections show that coal thins from south to north. In summary, thickness shows significant changes northwards across faults, indicating that the Porjianghaizi fault controlled sedimentation during the deposition of the Taiyuan and Shanxi Formations. The thickness of other potential reservoir units (the Shihezi and Shiqianfeng Formations) remain constant, suggesting that the Porjianghaizi fault did not actively affect the deposition of those units (Figs. 4 and 10).
the water salinity of the northern Porjianghaizi fault (well 35, 76700 mg/L; well 12, 83140.75 mg/L) is much higher than the southern Porjianghaizi fault (well 52, 39825.13 mg/L) (Fig. 10A). This suggests that the Porjianghaizi fault is laterally connected except in the circular area of the Taiyuan Formation. That is consistent with the SGR results where only in the XLN7390 area is the SGR over 0.25. 4.4. Sedimentation of the eastern Hangjinqi area The thickness of the Taiyuan and Shanxi Formations in eastern Hangjinqi area increases across the relay ramp between two fault strands (Fig. 11). The thickest deposits are distributed along the relay ramp (100–140 m) and at the lower tip of the fault (40–110 m) (Fig. 11). Two cross-sections were used to show the details of the depositional differences between the via faults and relay ramps during Upper Carboniferous (Taiyuan Formation) to the Lower Permian (Shanxi Formation) from south to north (Figs. 12 and 13). The Taiyuan and Shanxi Formations show significant thickness changes across the eastern Hangjinqi area, and the Taiyuan Formation terminates to the north against the Porjianghaizi Fault (Figs. 4, 12 and 13). Overall, strata
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Fig. 8. Seismic sections showing the characteristics of the Porjianghaizi fault along fault strike.
Table 1 Three types of EI of PF from Late Carboniferous to Jurassic. C3t: Carboniferous Taiyuan Formaiton; P1s: Permian Shanxi Formation; P2x: Permian Lower Shihezi Formation; P2s: Permian Upper Shihezi Formation; P2sh: Permian Shiqianfeng Formation; T: Triassic; J2y: Jurassic Yan'an Formation; J2z: Jurassic Zhidan Formation; J2a: Jurassic Anding Formation. Strata
J2z + J2a J2y T P2s + P2sh P1x C3t + P1s
Type 1
Type 2
Type 3
Hangingwall (m)
Footwall (m)
EI
Hangingwall (m)
Footwall (m)
EI
Hangingwall (m)
Footwall (m)
EI
289 250 1102 375 135 124
209 239 1157 384 141 128
1.38 1.04 0.95 0.98 0.96 0.97
316 199 1084 362 123.7 103.5
186 194 1200 362 123.7 103.5
1.69 1.03 0.90 1.0 1.0 1.0
352 248 977 418 109 66
256 223 1086 418.7 114.4 83.4
1.38 1.11 0.90 0.99 0.95 0.79
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Fig. 9. Maps of faults in the study area during Late Carboniferous to Early Permian (A) and Late Jurassic (B). Type 1 to Type 3 are the types of EI that indicate different fault evolution.
Fig. 10. A: Formation water salinity (mg/L) distribution of the lower Shihezi Formation. B: SGR distribution diagram of the Porjianghaizi fault in the Taiyuan to lower Shihezi Formations along the fault strike. STZ, Structural transfer zone.
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Fig. 11. Isopach map of strata thickness of the Taiyuan and Shanxi Formations based on 3D seismic interpretation. And the locations of cross sections of Figs. 12 and 13.
Fig. 12. Cross section B-B'showing stratigraphic section and lithology from south to north, and the GR signatures of the Taiyuan Formation and the Shanxi Formations (See location in Fig. 11).
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Fig. 13. Cross section C-C′ showing stratigraphic section and lithology from south to north, and the GR signatures of the Taiyuan Formation and Shanxi Formations (See location in Fig. 11).
5. Discussion
of existing faults and the orientation of stress were similar (Yang et al., 2013). Affected by the extensional strike-slip, the five reverse faults were reactivated to normal offsets (Fig. 14C), and started interacting on the relay ramp areas. During the Late Jurassic, the five separate faults finally linked into the current Porjianghaizu fault (Figs. 9A and 14D).
5.1. Controlling factors on fault inversion and propagational model of the Porjianghaizi fault The growth and propagational model of faults are often controlled by the tectonic activity and reactivated basement structures (Pinheiro and Holdsworth, 1997; Escalona and Mann, 2011; Yang et al., 2015a,b). During the Late Carboniferous to Early Permian, the North China block began colliding with the Mongolian arcs, and the South China block began subducting beneath the North China block (Watson et al., 1987; Meng and Zhang, 1999; Yang et al., 2005; Yin and Nie, 1996, Fig. 3). These collisions caused a series of reverse faults in the eastern Hangjinqi Area (Fig. 14A). In the study area, tectonic convergence caused five separate reverse faults (Type 1 and Type 3; Figs. 9A and 14A). During the Middle Permian to Late Triassic, the South China block also subducted beneath the Qiantang block and had slight convergence with the North China block (Yin and Nie, 1996). Therefore, the previously formed faults either stopped or minimally developed with values of EI at or near 1.0 (Table 1). During the Late Triassic, these early reverse faults were reactivated by compression between the North China and South China blocks (Enkin et al., 1992; Webb et al., 1999; Yang et al., 2015a,b; Fig. 14B). During the Middle Jurassic, there was an extensional strike-slip event. and during this same period, the strike direction
5.2. Porjianghaizi fault controls on sedimentation and gas accumulation The progression of fault propagation, growth, linkage, and death are major tectonic controls on basin sedimentation, and may control gas migration and accumulation (Gawthorpe and Leeder, 2000; Peng, 2000; Alves et al., 2002; Liesa et al., 2006). In the study area, the Upper Paleozoic strata have relatively good source-reservoir-cap combinations of gas systems in comparision to the other units (Xiao et al., 2005; Xue et al., 2009). The Taiyuan Formation is composed of mudstone, carbonaceous mudstone, sandstones of all grain sizes, and coal (Figs. 12 and 13). The lower Permian Shanxi Formation is composed of coal, mudstone, carbonaceous mudstone, fine-to coarse-grained sandstones with gravel. Previous gas-system analyses suggest that the coal seams of the Upper Carboniferous Taiyuan and Lower Permian Shanxi Formations are main source rocks in the north Ordos Basin (Xue et al., 2009). Sandstones of the Shanxi Formation and the Upper Shihezi Formation are main reservoirs, and the mudstone of the Shiqianfeng Formation serves as the cap rock (Sun et al., 2007). Therefore, the Porjianghaizi
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Fig. 14. Schematic diagram showing the development of the inversion and propagation of Porjianghaizi fault. A: Initiation stage with five reverse faults from Late Carboniferous to Early Permian. B: Fault reactivation during Triassic. C: Inversion stage affected by strike-slip event, thrust faults inverted to normal fault starting from the Middle Jurassic. D: Fault interaction and linkage to a through-going fault.
their depocenters at the foot of the fault and were bounded by a relatively high-angle fault (Fig. 15). Mapping suggests that sediments crossing over relay ramps can extent far away from the north in the direction of the basin center (Figs. 13 and 15). In contrast, sediments directly crossing over faults deposited locally and not far away from the fault (Fig. 15). The seismic stratigraphy results show the thickness of Lower Shihezi to Shiqianfeng Formations is constant beyond the Porjianghaizi fault from south to north. This indicates the Porjianghaizi fault had less control on sedimentation during late Permian. The coal source rocks of the Taiyuan and Shanxi Formations began generating gas in the Early Cretaceous time (Ji et al., 2013). The Hangjinqi area experienced two main periods of gas migration, including one from the Middle-Late Jurassic and the other from Early Cretaceous to present (Xue et al., 2009). Many small faults and fractures
fault control on sedimentation and gas accumulation are mainly recorded in Upper Paleozoic units. The Yin Mountains to the north, formed by collision between the North China block and the Mongolia Arc during the Late Carboniferous to Lower Permian, are the main sediment sources for the northern Ordos Basin (Yang et al., 2005). During the deposition of the Taiyuan and Shanxi Formations, sediment derived from north passed through the Porjianghaizi fault first, and then continued to the south. As mentioned before, five isolated faults developed during Late Carboniferous to Early Permian (Fig. 9A). During the fault initiation stage (Late Carboniferous to Early Permian), there were four relay ramps (Fig. 9A). The thickness maps of the Taiyuan and Shanxi Formations indicate that deposition in the southern Porjianghaizi fault areas was mainly via relay ramps (Figs. 11 and 16). Sediments crossing isolated faults had
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Fig. 15. Tectono-sedimentary model of the eastern Hangjinqi area of north Ordos Basin. Cross the relay ramps, sediments were transported far away and deposited in the basin center. But cross the faults, sediments just deposited near the fault.
6. Conclusions
were developed in Late Paleozoic strata (Figs. 4–6) and these faults and fractures are not sealed (Fig. 16). Thus, gas mainly generated from coal of the southern Porjianghaizi fault area could vertically migrate from source rock to upper sand reservoirs through open fractures (Fig. 17). As indicated by the SGR results the fault seal of the Porjianghaizi fault was characterised by ‘horizontal segmentation’, and the fault junction zones of the Porjianghaizi fault (relay ramp areas during Late Carboniferous to Early Permian) have the lowest fault sealing capacity (lowest value of SGR). Therefore, the gas generated in the Taiyuan and Shanxi Formations of the southern Hangjinqi area could migrate vertically to upper sand reservoirs via small faults and fractures. Gas then migrates laterally through the junction zone of the Porjianghaizi fault (relay ramp area), and eventually accumulates in those structural traps found in the northern areas (Fig. 17).
The detailed study of the Porjianghaizi fault provides a good example of the evolution of inversion and reactivated faults and how they controlled the sedimentation and gas accumulation in the northern Ordos Basin. The main conclusions from this study include: 1. Three major unconformities (T9, T5, T3) and six stratigraphic boundaries were marked. The thickness of the lower part of the strata between T9 and T5 (the Taiyuan and Shanxi Formations) generally thins and pinches out from south to north beyond the Porjianghaizi fault. 2. The current Porjianghaizi fault is a continuous single fault, but different parts of the Porjianghaizi fault show different features. Based on vertical throw, orientation, and fault dips, the Porjianghaizi fault can be divided into five segments. 3. The growth history of the Porjianghaizi fault, from Late
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Fig. 16. Core photos and thin sections showing small faults and fractures in Lower Paleozoic sandstones. A: Core photo of well J35 at ca. 2128.04 showing vertical fractures; B: Core photo of well J39 at ca. 2183.1 m showing vertical fractures; C: Thin section micrographs of well J69 at 2964.77 m showing vertical fractures; D: Thin section micrographs of well J67 at 2502.9 m showing vertical fractures. The blue lines in C and D are fractures (See well locations in Fig. 1B). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
northern area via relay ramps tend to extend further. In contrast, sediments directly across isolated faults were accumulated at the foot of relatively high-angle bounding faults. 6. The sealing capacity of the Porjianghaizi fault is characterised by “horizontal segmentation” which mains the Porjianghaizi fault was divided into lateral migration fault areas and lateral sealed fault areas. The junction zones (relay ramp area) of the Porjianghaizi fault have the highest connectivity. Gas generated in the Taiyuan and Shanxi Formations of the southern Hangjinqi area migrated vertically to upper sand reservoirs via open small faults and fractures, and then chiefly migrated laterally northwards beyond the Porjianghaizi fault junction zone (lateral migration fault areas) to the northern Hangjinqi area. Gas accumulated in structural traps found in the north Porjianghaizi fault area.
Carboniferous to Late Jurassic, is composes four stages: an initiation stage during Late Carboniferous to Early Permian, a reactivation stage at Late Triassic, an inversion stage at Middle Jurassic, an interaction and linkage stage at Late Jurassic. 4. The Porjianghaizi fault was composed of five reverse fault segments during the Late Paleozoic caused by the North China Block collision with the Mongolian arcs and the South China Block subduction beneath the North China block. During the Middle Jurassic, the five faults began inverting into a normal fault. During the Late Jurassic, the five normal faults interacted and linked to the current single Porjianghaizi fault. 5. During the Late Carboniferous to Early Permian, there were four relay ramps among the five fault segments. The relay ramps were the main sediment-transport pathways. Sediment delivered from the
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Acknowledgement All data used in this study are provided by North China Bureau of Petroleum, SINOPEC. We wish to thank the masters for assistance with preliminary work including core analysis and seismic stratigraphy interpretation. This study was jointly supported by the program of introducing talents of scientific disciplines to universities (No. B14031), the 13th “Five-year” plan of the Ministry of Science and Technology of China (2016ZX05034002-003) and the shale gas survey of National Geological Survey (12120114055801). We are grateful to the journal editor Dr. Alejandro Escalona, Dr. Tiago Alves, and an anonymous reviewer, for their critical, constructive, helpful comments and suggestions that considerable improved the quality of this manuscript. References
Fig. 17. Vertical and lateral migration and accumulation of gas in the Hangjinqi area, Ordos Basin.
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