Introduction
Event deposition refers to a sudden, instantaneous sedimentary process, and represents a spatially extensible stratigraphic record [1]. It is a short-term, rapid, and abrupt deposition process occurring during a relatively long- term, slow, and gradual deposition process. Event deposits commonly include seismite, tsunamite, gravity flow deposit, anoxic event deposit, and boundary clay layers. As a branch of sedimentology, event deposits have always been a hotspot of research. On one hand, event deposits are of great significance to study global environmental change, climate change, paleoenvironmental reconstruction, and tectonic movement. On the other hand, they have important practical significance and application value for the studies on the formation and preservation conditions of sedimentary mineral resources such as oil and gas, including analyzing the sedimentary origin and identifying the types and distributions of reservoirs and source rocks.
The Triassic Yanchang Formation in the Ordos Basin is a set of fluvial-deltaic-lacustrine deposits in a large depressed lake basin, and acts as the key hydrocarbon source rock system and major exploration target in the basin. More and more research has found that although the basin had a relatively stable internal structure during the deposition of the Yanchang Formation without distinct sedimentary discontinuity, event deposits were developed frequently and diversely, including earthquake [2], gravity flow [3], volcanic [4] and anoxic [5] deposits. These deposits have important impacts on the sequence stratigraphic structure and sedimentary filling sequence [6], and the development of large-scale source rocks and reservoir rocks [7], which have been studied and discussed from multiple perspectives. The existing studies on event deposits in the Yanchang Formation mainly focused on the 8th Member of Yanchang Formation ("Chang 8 Member" for short) and above horizons (especially the Chang 7 and Chang 6 members formed in the maximum lake transgression period. The Chang 9 Member corresponded to the rapid expansion stage of the lake basin when the tectonic setting allowed for event deposition. Its event deposits have rarely been reported, and the scarce studies mostly focused on a single type of deposit but did not probe into their distribution and genetic relations. Through core observation, thin section identification, logging data, XRD (X-ray diffraction), XRF (X-ray fluorescence), and organic geochemical analysis of the core samples of Chang 9 Member continuously collected from wells C22 and Y84 in the Ordos Basin, this paper dissects the types and characteristics of event deposits of the Chang 9 Member, and explains the origins of event deposits during the early depression stage of the basin and the first lake transgression process. On this basis, the sedimentary model is established, and the scale and distribution of the deposits are clarified. The study results provide references for petroleum exploration of Chang 9 Member in the basin.
1. Geologic setting
The Ordos Basin in the western part of the North China Plate was formed initially during the deposition of the Middle Triassic Zhifang Formation. It is a residual intracratonic basin superimposed on the Paleozoic North China Craton [8]. It is bounded by the Yinshan Mountain to the north, the Qinling Mountain to the south, the Helan and Liupan Mountains to the west, and the Lüliang Mountain to the east covering an area of approximately 25×104 km2. According to the present-day structural form and basement nature, the basin is divided into six first-order structural units: Yimeng uplift, Weibei uplift, West margin thrust belt, Tianhuan depression, Western Shanxi flexure fold belt, and Yishan slope (Fig. 1 ).
The Indosinian Movement that took place in the Middle-Late Triassic period resulted in the collision of the Yangtze Plate along the Mianlüe suture zone with the Qinling Block and the North China Plate, forming the Ordos Basin, a large inland depression lake basin by the basin-mountain coupling. The Triassic strata in this basin include Liujiagou Formation, Heshanggou Formation, Zhifang Formation and Yanchang Formation, from bottom to top. The Yanchang Formation mainly consists of clastic rocks of fluvial-lacustrine facies dominantly, with a thickness of 1000-1400 m. Based on sedimentary cycles, marker beds, oil-bearing properties and electrical properties, the Yanchang Formation is divided into Chang 10-Chang 1 members from bottom to top. These members corresponded to the initial subsidence stage (Chang 10 Member), rapid expansion stage (Chang 9-Chang 8 members), intense depression stage (Chang 7 Member), gradual shrinkage stage (Chang 6-Chang 3 members), and dying out stage (Chang 2-Chang 1 members), which together formed a complete evolution cycle of the lake basin vertically. During the deposition of Chang 9 Member, the tectonic activity of the Indosinian Movement was intensified, and the fault at the southwestern margin of the basin and the indented secondary faults that obliquely intersected with it became more active, which led to a larger basin subsidence rate. The remarkable difference in lithologic features between Chang 9 Member and Chang 10 Member also suggests that the expansion of the basin was a sudden change process. At the same time, the paleoclimate gradually changed to humid, the lake area rapidly expanded during the deposition of Chang 9 Member, and the first widely-distributed set of organic-rich shales (Lijiapan Shale) of the Yanchang Formation was developed. Wells C22 and Y84 are located in the southwestern Ordos Basin, which is structurally a part of the Yishan slope. The lithologic assemblage, sedimentary facies, and sequence of Chang 9 Member in the two wells reflect the sediments of braided river delta and lake deposited in lake transgression period, where the sediments of slump turbidite fan were also developed.
2. Types and characteristics of event deposits
Based on the observation results of nearly 500 m cores in wells C22 and Y84 with the precision of centimeter level, combined with the analysis and testing data as well as previous studies, 4 types and 9 subtypes of event deposits were found in Chang 9 Member, such as earthquake, gravity flow, volcanic and anoxic (Table 1 ).
Table 1. Types and characteristics of event deposits of Chang 9 Member, Ordos Basin |
| Type | Subtype | Common lithology | Sedimentary structure characteristics | Genetic mechanism |
|---|---|---|---|---|
| Earthquake event deposit | Liquefaction flow deformation | Siltstone, sandstone | Dish structure, liquefied vein, liquefied breccia, liquefied crinkle, liquefied diapir | Seismic vibration formed the excess pore water pressure in sedimentary layers that allow for sediments liquefying and flowing |
| Gravity deformation | Sandstone and mudstone with different densities | Loop bedding, sandstone load and mudstone diapir, ball- and-pillow structure | Gravity-driven vertical migration and deformation of sediments | |
| Brittle shear deformation | Various rocks | Fault-graded beds, mirco-fault | Seismic vibration moved and broke or liquefied the sedimentary layers to form differential compaction | |
| Gravity flow event deposit | Block slump | Various rocks | Slide surface, slump fold, abrupt structure | Instability of sediments |
| Sandy debris flow | Thick massive sandstone, mud pebble- bearing sandstone | Thick massive, abrupt contact at top and bottom, floating or bedded mud and gravel | Transition of slump block | |
| Turbidity current | Fine sandstone to mudstone | Normal graded bedding, bottom scour, gradual contact at top, and parallel, ripple, horizontal bedding | Transition of debris flow | |
| Volcanic event deposit | Volcanic ash deposition | Tuff | Unconspicuous horizontal bedding | Volcanic eruption |
| Anoxic event deposit | Anaerobic deposition | Black shale | Lamellar | Anaerobic environment |
| Dysaerobic deposition | Black gray mudstone | Layered, massive | Dysaerobic environment |
2.1. Earthquake event deposits and seismites
The term "seismites" was first proposed by Seilacher when studying fault-graded beds in the shale outcrop of the Miocene Monterey Formation, Central California, USA, and defined as the soft-sediment deformational layers formed by earthquakes [9]. It was initially cited and translated as "Zhenjiyan" in China by Gong [1]. Once an earthquake occurs, in-situ sediments are transformed first, and a part of the sediments are then relocated in accompany by the formation of deformation structures, if a tsunami or gravity flow is triggered [10].
In this paper, seismites specifically refer to the in-situ sediment deformational layers that are formed by seismic events, without seismic unconformity inside or erosion surface at the bottom, excluding the parautochthonous and allogenic sediments (e.g. seismoturbidite). At present, the recognized identification markers of seismites are fault-graded beds and liquefied homogeneous layers. In addition, other sedimentary structures such as a liquefied vein, liquefied crinkled deformation, autoclasitc breccia, and concertina fold are commonly found in seismites. Although these structures might be originated from different processes individually, their combinations are believed to be significant for seismites identification. Through core observation, three types of contemporaneous deformation structures related to seismic events were identified and classified into liquefaction flow, gravity, and brittle shear deformation according to formation mechanism.
2.1.1. Liquefaction flow deformation
Under the effect of seismic vibration, especially the seismic shear stress, sediments become mechanically unstable, forming excess pore water pressure in saturated sedimentary layers. This allows the sediments to be liquefied and flow towards adjacent layers, which facilitates the fluids in the pores to be expelled [11]. Liquefaction is determined as the main cause of earthquake damage and deformation of sediments [12].
The features of liquefaction flow deformation observed from cores of wells C22 and Y84 were obvious and diverse, including dish structure (Fig. 2a ), liquefied vein (Fig. 2b -2e ), liquefied breccia (Fig. 2f -2h ), liquefied crinkle (Fig. 2i ) and liquefied diapir (Fig. 2j -2k ).
Fig. 2. Typical characteristics of seismites in Chang 9 Member, Ordos Basin. (a) Well Y84, 2289.63 m, showing curve upward dish-shaped laminas on both sides of liquefied vein; (b) Well C22, 2323.36 m, liquefaction flow and emplacement of sand layer into mud layer; (c) the other side of Fig. b; (d) Well Y84, 2306.00 m, liquefaction emplacement of sand layer and micro-faulting of the overlying mud layer; (e) Well Y84, 2312.56 m, liquefaction emplacement of sand layer, and micro-faulting and tractive bending of the underlying siltstone; (f) Well C22, 2362.50 m, micro-faulting of the mud layer, appearing as gravels; (g) Well Y84, 2205.72 m, liquefied sand layer impaling the overlying mud layer, forming mud gravels; (h) Well Y84, 2226.45 m, showing mud gravels formed by liquefaction with short transport distance, varied size, and are generally angular with spiny edges, part of them can be pieced together; (i) Well C22, 2342.28 m, liquefaction flow of sand layer to form crinkled deformation; (j) Well Y84, 2299.80 m, diapir structure; (k) the other side of Fig. j; (l) Well Y84, 2207.18 m, loop bedding; (m) Well C22, 2414.78 m, gray fine sandstone separated from parent rock and then sank into the underlying dark gray muddy siltstone; (n) Well C22, 2411.31 m, gray siltstone load and dark gray muddy siltstone diaper; (o) Well C22, 2409.71 m, ball-and-pillow structure, showing broken sandstone ball-and-pillow, which partially appear as gravels, due to multiple earthquakes or an intensive earthquake; (p) Well C22, 2366.42 m, fault-graded beds with throw less than 10 mm; (q) Well C22, 2380.38 m, liquefied sand flowing and filling along fractures, with throw of about 10 mm. |
2.1.2. Gravity deformation
Gravity deformation refers to vertical migration and deformation of sediments driven by gravity during an earthquake process. Its common types include loop bedding, load structure, and ball-and-pillow structure. In Chang 9 Member, loop bedding is usually associated with liquefaction flow deformation, which is formed by the collapse of the overlying thin sand-mud interbeds into the underlying deformation layer, and preserves the curl deformation well (Fig. 2l ). Load structure, which was proposed by Kuenen [15], describes that the overlying coarse sediments sink into the underlying fine sediments under the seismic vibration and gravity action. Ball-and-pillow structure is an expression of further migration and deformation of load structure, where the load is completely separated from the overlying layer and sinks into the underlying layer. The morphologies of load structure (Fig. 2m and 2n) and ball-and-pillow structure (Fig. 2o ) indicate that the overlying sediments have higher density and the underlying layer has undergone flow deformation.
2.1.3. Brittle shear deformation
Brittle shear deformation refers to fractures and joints in consolidated rocks or unconsolidated sediments at the top of the sedimentary layer caused by an earthquake [11]. The brittle deformation observed in the core samples of Chang 9 Member mainly includes fault-graded beds and micro-faults, which are commonly developed in thin-layered argillaceous rocks.
Fault-graded beds are a series of nearly parallel faults in a ladder-like arrangement (Fig. 2p ), most of which are normal micro-faults with throws smaller than 15 mm and high dip angle. Micro-faults generally appear alone in the core, with a throw greater than 10 mm (Fig. 2q ), and they are dominant normal faults. Fault-graded beds and micro-faults are intraformational deformation structures without any influence on the overlying and underlying layers, which is an important feature to distinguish them from the later tectonic faults.
2.2. Gravity flow deposits
Unconsolidated sediments in the delta front are prone to slide and slump, and change into sand debris flows and turbidity currents deposited on a slope or deep water zone if transported further and diluted continuously [16]. This process has been confirmed by Yan et al. [17] through a laboratory simulation of seismoturbidite in the water tank. For example, cable breakage was the result of turbidity current and slump induced by the Grand Bank earthquake in 1929, Canada [18].
2.2.1. Slump
Slump blocks are formed by unstabilized sediments sliding downhill along the shear direction, when the additional stresses from earthquakes, volcanoes, or the component parallel to the slope of gravity force exceeds the shear strength [19]. Resulted of the rotational shear force, synsedimentary deformation structures are commonly developed within the blocks, such as slump fold. Slides and slumps generally do not influence the underlying layers, with no striation structure or scour structure. Slump blocks can be identified by sliding surfaces (Fig. 3a and 3b) and slump folds (Fig. 3c and 3d).
Fig. 3. Typical characteristics of gravity flow deposits in Chang 9 Member, Ordos Basin. (a) Well Y84, 2201.50 m, slide surface formed by block slump; (b) Well Y84, 2311.05 m, slide surface; (c) Well C22, 2385.72 m, slump fold; (d) Well C22, 2328.60 m, slide surface and slump fold; (e) Well C22, 2324.57 m, sandy debris flow deposits, generally massive; (f) Well C22, 2325.61 m, generally massive; (g) Well Y84, 2245.74 m, generally massive; (h) Well C22, 2368.02 m, abrupt sand-mud contact at the top of sandy debris flow deposits; (i) Well C22, 2368.31 m, abrupt sand-mud contact at the bottom; (j) Well Y84, 2243.98 m, floating mud gravels in sandy debris flow deposits, which are generally angular with spiny edges and partially can be spiced together; (k) the other side of Fig. j; (l) Well Y84, 2288.72 m, coarsening-upward mud gravels in sandy debris flow deposits; (m) Well Y84, 2225.90 m, fining-upward mud gravels in debris flow deposits; (n) Well Y84, 2245.43 m, part of mud gravels with original sedimentary structures; (o) Well Y84, 2259.23 m, bottom scour of turbidity current deposits; (p) Well C22, 2371.52 m, mud gravels at the bottom of turbidite current deposits; (q) Well Y84, 2263.45 m, normal graded bedding in turbidity current deposits; (r) Well C22, 2367.47 m, normal graded bedding in turbidity current deposits; (s) Well Y84, 2258.02 m, parallel bedding in turbidity current deposits; (t) Well C22, 2389.87 m, ripple bedding in the middle and lower parts, and parallel lamination in the upper part; (u) Well Y84, 2265.42 m, rhythmic layer composed of gray silts and dark gray muddy silts, laminated horizontal bedding; (v) Well C22, 2379.83 m, horizontal bedding in the lower part and ripple bedding in the upper part. |
2.2.2. Sandy debris flow
According to the flowing grain layer and fine detrital grain flow at the bottom of the turbidity current, a sandy debris flow was defined by Shanmugam [20]. It is a gravity flow with plastic rheology and laminar state, and is mainly supported by matrix strength, dispersion pressure, and buoyancy [20-21]. Along the sedimentary source direction, sandy debris flow moves and deposits as a whole in a “freezing pattern”, but does not subside by grain size. Gravels differing greatly in shape, size, and roundness are dispersedly "floating" in the sand layer or distributed parallel to bedding. The deposits mainly develop massive bedding, show no obvious graded bedding, and have abrupt contact at top and bottom. The sandy debris flow deposits of Chang 9 Member mainly consist of sandstone or mud gravel-bearing sandstone. The thickness of a single sand layer is generally greater than 0.5 m, and the thickness of overlapped multiple layers reaches over 10 m. They are generally massive (Fig. 3e -3g ), have abrupt and nearly straight contact surfaces at top and bottom (Fig. 3h , 3i), where poorly sorted and rounded, dispersedly floating mud gravels are commonly observed with spiny edges that can be pieced together (Fig. 3j , 3k), indicating brecciation as well as aggregated deposition and transportation. The bedding-parallel mud gravels show an indistinct imbricate arrangement, which is probably a result of the transition from sandy debris flow to turbidity current. The reverse and normal graded mud gravels within layers (Fig. 3l , 3m) are probably the deposits of strong and weak sandy debris flows. The mud gravels that remained in original sedimentary structures are occasionally observed (Fig. 3n ), with similar lithology to the overlying and underlying mud layers, and are identified as endogenous mud gravels that underwent short-distance transport.
2.2.3. Turbidity current
A sandy debris flow is continuously diluted in the flowing process, which reduces its strength and increases its flowability, and thus changes into a weak sandy debris flow or even turbidity current [22]. Turbidity current is a gravity flow with Newtonian rheology and turbulent state and is supported by the upward component force of turbulent flow [20-21]. Turbidity current deposits are formed by step-by-step deposition of suspended sediments with gradually decreasing flow velocity; thus, abrupt contact at bottom (e.g. erosion structure and bottom gravel), normal graded bedding, and gradual contact at the top are commonly observed, and parallel bedding, ripple bedding and horizontal bedding are generally developed. The turbidites of Chang 9 Member, which are mainly fine sandstone to siltstone or mudstone, are generally finer than the sandy debris flow deposits. The thickness of a single layer is generally less than 0.3 m, while the thickness of overlapped multiple layers can exceed 5 m. Scour structure (Fig. 3o ) and bottom gravel (Fig. 3p ) are observed at the bottom, indicating an erosion of the underlying mud sediments in turbulent state. They are fining-upward generally or locally (Fig. 3q , 3r), and develop parallel bedding (Fig. 3s ), ripple bedding (Fig. 3t ), and horizontal bedding (Fig. 3u , 3v), reflecting a deposition of sediments in a decreasing order of grain size with the decreasing flow velocity, and the traction flow characteristics were observed in the middle and upper parts. Bouma sequences are generally incomplete and appear as superpositions of multiple intervals in, for example, ABAB and CDCDE arrangement.
2.3. Volcanic event deposits and tuff
Tuff is formed by volcanic eruption materials with a particle size less than 2 mm which are deposited in a relatively standing water environment through transport by air and water. It is used as an effective reflector of the tectonic setting. The tuffs of Chang 9 Member are light greenish-gray in color (Fig. 4a ) and are thinly sandwiched in thick black shales, with abrupt contact at top and bottom. The observation results of thin sections reveal crystal pyroclasts, vitric pyroclasts, and glasses (Fig. 4b ). The vitric pyroclasts are lacerated, angular, and harbor-shaped. The crystal pyroclasts include striped biotite and broken angular quartz. The matrix is composed mainly of glasses, with alteration to illite observed. XRD analysis of whole rock and clay minerals indicates that the main components are quartz (56.8%, which is mass fraction, the same as below), plagioclase (6.3%), and clay minerals (35.2%), as well as a small amount of K-feldspar. The clay minerals are mainly illite/smectite (I/S) and illite. The tuff layers are thin but have distinct logging responses, including high GR, high AC, and low DEN. Spotted sandstone is detected in the lower part of Chang 9 Member in Well Y84 (Fig. 4c ). On the thin section, it is observed as poorly rounded medium-grained feldspathic lithic sandstone, where laumontites are embedded in as crystal stocks filling the pores (Fig. 4d ). Microcline and perthite are observed, and gypsum cements appear occasionally. XRD analysis of whole rock reveals quartz (18.2%), feldspar (29.0%), clay minerals (26.5%), and laumontite (26.3%). XRD analysis of clay minerals shows the dominance of chlorite. Similar to tuff, it exhibits a high GR value. Laumontites are believed to originate from several mechanisms, including alteration of volcanic materials [23], alteration of plagioclase [23], and reaction between calcite and kaolinite [24]. Considering that the spotted sandstone is distributed in the lower part of Chang 9 Member (a horizon equivalent to the tuff in Well C22), and its formation temperature is far lower than the thermodynamic reaction between calcite and kaolinite, it is tentatively determined that laumontite is originated from hydration and alteration of volcanic materials.
Fig. 4. Characteristics of tuff, spotted sandstone and shale in Chang 9 Member, Ordos Basin. (a) Well C22, 2382.25 m, light greenish gray tuff; (b) Well C22, 2382.25 m, tuff, under plane polarized light; (c) Well Y84, 2288.63 m, spotted sandstone, with bedding-parallel muddy rip-up clasts; (d) Well Y84, 2288.63 m, spotted sandstone, under plane polarized light (e) Well Y84, 2196.50 m, black gray mudstone; (f) Well Y84, 2194.48 m, black shale; (g) Well Y84, 2196.50 m, massive mudstone, under plane polarized light; (h) Well Y84, 2194.48 m, laminated shale, under plane polarized light; (i) Well C22, 2382.30 m, laminated shale, under plane polarized light; (j) Well C22, 2316.05 m, crinkled laminas, under plane polarized light; (k) Well C22, 2349.45 m, laminated and shaly sand-mud interbeds, with thin layers of silts, containing multiple wormholes with diameter of about 1 cm and inclination of 40°-50°. |
2.4. Anoxic event deposits and source rocks
Compared with the fine-grained dark argillaceous rocks of Chang 10 Member, which are sandwiched as thin layers in deltaic sandstones, abundant medium-thick layers of shales are observed in the cores of Chang 9 Member from wells C22 and Y84 (Fig. 4e , 4f). Microscopic analysis of the shale samples of Chang 9 Member reveals layered or massive mudstones and laminated shales (Fig. 4g -4j ). XRD analysis of whole rock and clay minerals shows that they are dominated by quartz, plagioclase, and clay minerals, with a small amount of pyrite. The content of clay minerals generally ranges between 52% and 58%, with an average of 54.4%. I/S and illite are the predominant components, accounting for 78%-85% (avg. 81.6%). Rock pyrolysis analysis indicates that the shales have the TOC mainly ranging between 3% and 6%, with an average of 4.8%, the hydrocarbon generation potential ranging from 4 mg/g to 8 mg/g, with an average of 8.6 mg/g, and the kerogen type index mostly in the range of 30-80. The relatively high organic matter abundance, high hydrocarbon generation potential, and favorable kerogen type index (Table 2 ) suggest an oxygen-deficient environment during the deposition of Chang 9 Member.
Table 2. Redox and pyrolysis parameters of shales of Chang 9 Member, Ordos Basin |
| Well | Depth/m | Lithology | V/(V+Ni) | TOC/% | (S1+S2)/(mg•g-1) | S2/S3 | HI/(mg•g-1) | Tmax /°C |
|---|---|---|---|---|---|---|---|---|
| C22 | 2316.01 | Black shale | 0.75 | 4.76 | 8.03 | 45 | 151 | 447 |
| C22 | 2323.96 | Black shale | 0.69 | 9.55 | 26.01 | 134 | 252 | 442 |
| C22 | 2331.18 | Black gray mudstone | 0.64 | 2.63 | 4.12 | 29 | 141 | 442 |
| C22 | 2339.23 | Black shale | 0.68 | 4.93 | 7.85 | 45 | 145 | 446 |
| C22 | 2344.06 | Black shale | 0.64 | 8.03 | 18.40 | 114 | 213 | 443 |
| C22 | 2348.92 | Black gray mudstone | 0.64 | 3.22 | 5.88 | 54 | 168 | 442 |
| C22 | 2351.80 | Black gray mudstone | 0.71 | 2.61 | 4.45 | 34 | 155 | 442 |
| C22 | 2359.36 | Black shale | 0.84 | 8.06 | 20.44 | 104 | 232 | 441 |
| C22 | 2361.25 | Black gray mudstone | 0.69 | 2.17 | 3.07 | 21 | 127 | 444 |
| C22 | 2382.30 | Black shale | 0.80 | 3.98 | 9.66 | 90 | 226 | 443 |
| C22 | 2384.81 | Black shale | 0.73 | 4.10 | 6.20 | 52 | 139 | 445 |
| C22 | 2396.50 | Silty mudstone | 0.55 | 0.26 | 0.15 | 1 | 51 | 459 |
| C22 | 2400.97 | Black gray mudstone | 0.61 | 3.40 | 3.75 | 20 | 101 | 447 |
| C22 | 2401.78 | Black gray mudstone | 0.71 | 1.09 | 0.74 | 5 | 60 | 453 |
| C22 | 2412.26 | Black gray mudstone | 0.68 | 3.64 | 6.12 | 37 | 152 | 443 |
| Y84 | 2191.05 | Black shale | 0.90 | 14.14 | 31.48 | 145 | 205 | 458 |
| Y84 | 2194.48 | Black shale | 0.65 | 4.16 | 5.95 | 32 | 124 | 459 |
| Y84 | 2196.50 | Black gray mudstone | 0.51 | 2.99 | 3.19 | 16 | 92 | 463 |
| Y84 | 2198.66 | Black shale | 0.82 | 4.69 | 5.67 | 30 | 107 | 462 |
| Y84 | 2214.95 | Black gray mudstone | 0.63 | 2.20 | 2.65 | 13 | 92 | 462 |
| Y84 | 2216.60 | Black shale | 0.61 | 5.59 | 7.88 | 33 | 123 | 458 |
| Y84 | 2226.55 | Black gray mudstone | 0.61 | 2.54 | 2.59 | 13 | 89 | 462 |
| Y84 | 2249.02 | Silty mudstone | 0.68 | 2.29 | 2.16 | 6 | 81 | 463 |
| Y84 | 2260.40 | Black gray mudstone | 0.66 | 2.19 | 2.20 | 13 | 82 | 462 |
| Y84 | 2301.72 | Black gray mudstone | 0.65 | 4.03 | 4.11 | 23 | 91 | 462 |
| Y84 | 2305.02 | Black gray mudstone | 0.65 | 3.83 | 4.13 | 31 | 97 | 462 |
Water environments can be divided into anaerobic (oxygen concentration less than 0.1 mL/L), dysaerobic (oxygen concentration of 0.1-1.0 mL/L), and aerobic (oxygen concentration higher than 1.0 mL/L) environments according to the concentration of dissolved oxygen in water [25]. The concentration of dissolved oxygen is closely related to water depth. From shallow to deep water, aerobic zone, dysaerobic zone, and anaerobic zone occur successively. The dysaerobic and anaerobic zones of Chang 9 Member in the study area are mainly distributed in the prodelta/overflow and the semi-deep lake/outer fan. Due to weak hydrodynamics, the prodelta/overflow was deposited with fine-grained sediments that are mainly layered or massive black-gray mudstone, sandwiched with thin layers of siltstones or interbedded thinly with siltstones. The semi-deep lake/outer fan, corresponding to weaker hydrodynamics, mainly develops laminated and shaly black shales, and muddy siltstone or silty shale locally. Bioturbation intensity declines with the decreasing oxygen content. The sedimentary structures in oxygen-enriched waters are prone to be destroyed in the short term by benthos, but those in oxygen-deficient waters (e.g. laminae) are generally well preserved due to limited biological activity. Some mudstone of Chang 9 Member contains wormholes with recognizable beddings (Fig. 4k ). This indicates weak biological activity and suggests a transition from oxygen-deficient to an aerobic environment. Certain mudstone and most shales are detected with no trace of biological activity, reflecting an oxygen-deficient environment. As organic matters are difficult to preserve in an oxidation environment, a high TOC value can reflect the oxygen-deficient environment to some extent. The V/(V+Ni) value (ratio of V content to V and Ni contents) is usually used to restore the redox conditions. V/(V+Ni) greater than 0.54 indicates strongly stratified anaerobic waters and a value of 0.46-0.60 indicates weakly stratified dysaerobic waters [26]. The V/(V+Ni) value of Chang 9 Member shales is generally higher than 0.60, verifying the existence of anoxic event deposits. As shown in Fig. 5 , the samples with TOC less than 4.0% generally have V/(V+Ni) values ranging between 0.55 and 0.75, and those with TOC greater than 4.0% have V/(V+ Ni) values greater than 0.65. In other words, the dysaerobic environment has a TOC of 1.0%-4.0% and V/(V+Ni) of 0.55-0.75, and the anaerobic environment has a TOC greater than 4.0% and V/(V+Ni) greater than 0.65. Moreover, the anaerobic and dysaerobic depositional environments are recognized for Chang 9 Member according to the cross-plot of S2/S3 and TOC (Table 3 ).
Fig. 5. Discrimination diagrams of anoxic event deposits of Chang 9 Member, Ordos Basin. |
Table 3. Types and characteristics of anoxic event deposits of Chang 9 Member, Ordos Basin |
| Type of event deposits | Oxygen concentration/ (mL•L−1) | Depositional environment | Degree of stratification | Fine-grained component | Sedimentary structure | Bioturbation | TOC/ % | V/ (V+Ni) | S2/S3 | Organic matter type | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Anoxic | Anaerobic | <0.1 | Semi-deep lake/outer fan | Strongly stratified | Black shale | Laminated, shaly | None | ˃4.0 | ˃0.65 | ˃30 | I and II1 |
| Dysaerobic | 0.1-1.0 | Prodelta/ overflow | Weakly stratified | Black gray mudstone | Thin layered, massive | Visible | 1.0- 4.0 | 0.55- 0.75 | 10-40 | II1 and II2 | |
| Aerobic | ˃1.0 | Delta front- plain | Not stratified | Argillaceous rock | Strong | <1.0 | <0.55 | <10 | |||
3. Geological significance of event deposits
3.1. Paleoenvironment and sedimentary facies
Previous studies have reported that the Chang 9 Member in the southwestern Ordos Basin was mainly deposited in braided river delta and lake environments. This study discovers multiple types of event deposits (e.g. deep-water gravity flow deposits) in such sedimentary environments. These event deposits frequently alternate with the normal deposits vertically and form certain banded or lobate sedimentary bodies laterally. Based on the centimeter-level core description, logging data, and geochemical analysis of samples from wells C22 and Y84, five sedimentary microfacies of event deposits are identified in the study area (Fig. 6 ).
Fig. 6. Vertical sequences for event deposits of Chang 9 Member, Ordos Basin. |
(1) Inner-fan main channel. It is dominated by sandy debris flow, with the thickness of a single layer of 1.0-3.0 m and the thickness of overlapped multiple layers exceeding 10 m (Fig. 6a ). It is mainly composed of thick massive sandstones, with a small number of plant charcoals occasionally observed. It also contains mud gravel-bearing sandstone, where mud gravels have diverse sizes (grain diameter of 0-80 mm), varying lithology (from black gray mudstone to dark gray siltstone), and different degrees of roundness (from sub-rounded to angular with spiny edges). The mud gravel-bearing layers show no graded bedding as a whole, but contain multiple beds with coarsening-upward gravels (Fig. 6a ). As mentioned above, the gravity flow can be triggered by earthquakes, and seismic event deposits (e.g. liquefied sandstone veins and liquefied breccias) are commonly observed several meters below the main channel sediments.
(2) Mid-fan braided channel and inter-channel. It is dominated by turbidity current, where fining-upward sequences are common. Compared with the classic turbidite sequences, it shows larger grain size and thickness, and additionally develops sandy debris flow and overlapped scour sandstone intervals. The sandy debris flow interval at the bottom is composed of sandstone without graded bedding, where several gravel-bearing sandstone layers with the thickness varying from dozen to tens of centimeters (the intra-layer gravels have diverse grain sizes, poor roundness, and varied lithology) are developed. The high-density turbidity current interval in the lower part shows greatly varying lithology (from medium sandstone to siltstone), a rapid transition to Bouma A interval, and the presence of overlapped scour sandstones, with the thickness of a single layer ranging from several to dozen of centimeters (finning-upward grain size and massive to parallel bedding, similar to Bouma A-B interval). The Bouma C interval in the middle part is mainly composed of silts to fine sands, with ripple bedding developed commonly. The Bouma D interval in the upper part is dominated by silts and possibly a large amount of mud, with horizontal bedding observed commonly (Fig. 6b ). Similar to the main channel deposits, below some braided channel sequences are seismites and sliding deformations.
(3) Mid-fan inter-channel-overflow. It is mainly composed of silts, muddy silts, and silty muds, with abundant shaly or thin layers of silts to fine sands. Some sandstone interlayers contain bedding-parallel angular mud gravels, with soft sediment deformation locally and ripple bedding and horizontal bedding, corresponding to Bouma C-D interval sandwiched with Bouma A-B interval (Fig. 6c ).
(4) Outer fan/semi-deep lake and overflow. It is mainly composed of black gray shale and dark gray argillaceou siltstone, containing plant charcoals, in laminated, shaly, and stratified distribution, with silty-fine sandstones locally (in abrupt contact or unobvious normal graded sequence), corresponding to Bouma D-E interval sandwiched with Bouma C interval (Fig. 6d ).
(5) Delta front and prodelta. The underwater distributary channel is mainly composed of light gray and gray medium-fine sandstones. A single layer is 0.3-1.0 m thick, with fining-upward grain size, a massive structure in dominance, and parallel or cross-bedding locally. The thickness of overlapped multiple layers reaches 1.5-5.0 m. As a whole, the sandstones are generally coarsening-upward. Extending to the prodelta, the flow velocity of the underwater distributary channel reduces, leading to finer grain size and thinner layers. The prodelta is dominated by muddy deposits, and has only a few coarse terrigenous clasts. It is often observed with reverse graded sequences with a thickness of several to dozen of centimeters, as well as horizontal and wavy beddings, and lenticular silts locally in the areas with coarse grain size (Fig. 6e ). During an earthquake, soft sediment deformation structures are formed, especially near the sand-mud interface, such as liquefied veins, liquefied crinkles, load, and diapir structures.
3.2. Tectonic activity intensity and period
In the Middle-Late Triassic, with the closure of the Qinqi trough at the southern margin of the basin and the enhanced S-N thrusting of the western margin [27], the Ordos lake basin was formed by the basin-mountain coupling. The basin rapidly expanded during the deposition of Chang 9 Member, when the strong episodic activities at the basin margin provided tectonic conditions for event deposition (e.g. seismites). The intensity and period of earthquakes are directly related to tectonic activity, and the study of seismites can reveal the tectonic subsidence history of the basin. Currently, the determination of earthquake magnitude and distance to epicenter is based on the type of seismites by using data statistics methods, such as deformation type method [28] and maximum liquefaction distance method [29]. Corresponding relations between seismite type and earthquake magnitude have been detected by Rodriguez-Pascua et al. [28]: with the ascending earthquake magnitude, liquefied diapir (M5.0-6.5), liquefied crinkle (M5.5-6.5), liquefied vein (M5.0-8.5), slump fold (M6.0-8.0), ball-and-pillow structure (M6.0-8.0) and liquefied breccia (M7.0-8.0) occur successively. Obvious liquefaction and deformation of unconsolidated sediments require an earthquake magnitude greater than 5, while only disturbance bedding and vibration crinkle are generally formed when the magnitude is lower than 5 [30-31]. According to the relations between the liquefaction and distance to the epicenter and the earthquake magnitude summarized by Liu and Xie [29], the liquefaction range increases dramatically with the increase of earthquake magnitude, reaching 10-50 km at M6 and 200 km at M8. Liquefied sandstone veins, liquefied breccias, ball-and-pillow structures, and fault-graded beds are commonly observed in the cores of Chang 9 Member. This indicates that earthquakes of M5.5-8.0 occurred frequently during the deposition of Chang 9 Member and that the records of the same period of earthquake probably exist within 10-200 km of the epicenter. In wells, C22 and Y84 which are about 50 km apart, five seismite-developed intervals are identified in Chang 9 Member (Fig. 7 ), and show the characteristics of clustering by separation distance (i.e., earthquake episode as described by Qiao et al. [32]). In the tectonic setting the southwest marginal fault of the basin and the jagged secondary faults that obliquely intersected were intensively active during the deposition of Chang 9 Member, the earthquake periodicity is believed to be the result of the periodic "activation" of the boundary and internal faults (mechanically explained as the periodic change of interaction between adjacent faults [33]), reflecting the intensity and period of tectonic activity in the basin.
Fig. 7. Vertical distribution of event deposits and sedimentary facies column of Chang 9 Member, Ordos Basin. |
3.3. Event deposits stratigraphic correlation
With the variable sediment source supply and accommodation space, similar sedimentary environments may generate completely different lithological associations, which cause uncertainty in stratigraphic correlation. Event deposition is a short-term and sudden component in the long-term gradual deposition process (with isochronism). It is very different from normal deposition with respect to sedimentary structures and geochemical parameters, for example. Moreover, event deposits distribute widely in lateral direction (for example, the 2-4 m thick seismites in the Upper Triassic Cotham Formation in the United Kingdom are traceable within a range of 25× 104 km2 [34]; the micro-folds in the anhydrite-calcite layers in the Permian Castile Formation of the Delaware Basin, USA, are comparable within a length up to 113 km [35]). Thus, event deposits can provide an isochronous basis for stratigraphic correlation.
The vertical distributions of seismites of Chang 9 Member are basically consistent in wells C22 and Y84, so these event deposits can be used for stratigraphic correlation. For example, thick overlapped sandstones are developed in the middle-lower part of Chang 9 Member in both wells, which are 8 m and 25 m thick respectively. It is impossible to determine whether and how these two sets of sandstones can be correlated directly based on lithology. In the absence of other evidences, the period and intensity of earthquakes have confirmed that the sandstones at depth of 2370-2378 m in Well C22 can be correlated to the sandstones at depth of 2265-2277 m in Well Y84.
In the lower part of Chang 9 Member in Well C22, thin layers of light greenish-gray tuffs are sandwiched in thick shales with abrupt contact at top and bottom. In the lower part of Chang 9 Member in Well Y84, thin layers of spotted sandstones are observed, and show abrupt lithologic variation with the overlying and underlying sandstones. The laumontite cements are the hydration and alteration products of volcanic materials, as described above, which represent a period of volcanic activity. Since no rocks related to volcanic eruption have been observed in other layers of Chang 9 Member except for the roughly same horizon with the laumontite cements in the lower part, it is concluded that they indicate the same volcanic event and can be used for stratigraphic correlation.
Anoxic event deposit is closely related to water depth and formed during the rapid lake transgression and highstand stage, corresponding to the interval with most developed (semi-) deep-lake fine-grained sediments in a sequence, which can be used to assist in stratigraphic correlation. Although wells C22 and Y84 show dramatic differences in lithology of Chang 9 Member (fine-grained sediments are widely distributed throughout the Chang 9 Member, accounting for about 70% in Well C22, but only locally distributed in the upper part, accounting for about 45%, in Well Y84), their sequences and lake-level changes are basically synchronous. The Chang 9 Member corresponds to the lower part of a third-order sequence and can be subdivided into 4.5 fourth-order sequences (Fig. 7 ). Multiple secondary lake transgressions occurring during the overall lake transgression process are distributed in the lower, middle, and upper parts. In the shale at the top of Chang 9 Member, for example, the anoxic event deposits in highstand stage are characterized by high GR, high AC and low DEN values, making it an important marker bed for the Yanchang Formation in the basin.
3.4. Source-reservoir assemblage
The source rocks of the Yanchang Formation in the basin are lacustrine organic-rich shales, the formation and distribution of which are controlled by multiple factors, such as anoxic conditions, high productivity, and appropriate sedimentation rate. The paleoclimate inherited a dry and hot state at the end of the Late Permian to the Early Triassic and changed to humid in the Middle Triassic [36]. Several global humidification events occurred during the Middle-Late Triassic period, including the Carnian pluvial episode (CPE) [37-38], corresponding to the red beds of the Liujiagou and Heshanggou formations, the aubergine mudstone locally intercalated with black gray mudstone in the Zhifang Formation, and the multiple sets of black shales in the Yanchang Formation, Ordos Basin. According to the recent conclusion of Li et al. [39] that the black shales of Chang 9 Member roughly correspond to the CPE (Late Julianian to Tuvalian), and the sedimentologic and palaeoolimatologic study of Stefani et al. for the Triassic strata in Dolomites, Italy [38], it is speculated that the sedimentary period of Chang 9 Member corresponds to the Latin-Carnian humid interval (L-CHI). The humid paleoclimate and tectonic activity in episodic manner in the expansion period of the basin resulted in the occurrence of first lake flooding in the Chang 9 Member and the formation of an under-compensated reduced sedimentary environment, which provided favorable conditions for preservation of organic matters. From the late Permian extinction to the middle Anisian recovery and radiation (about 245 Ma) during the Middle Triassic [40], the constant input of P, Fe, and S into the basin from chemical weathering and volcanic eruptions [41-42] increased nutrients in the lake water during the deposition of Chang 9 Member, which contributed to the blooming of organisms and increased the primary productivity. At the same time, in the gravity flow deposits dominated by mud sediments, such as the Baumah D-E interval, an accelerated sedimentation rate probably led to local anomaly of organic matters.
After analyzing tectonic setting as well as types, characteristics and distribution of event deposits, an event deposition model for Chang 9 Member in southwestern Ordos Basin was established (Fig. 8 ). The strong collisional uplifting of Qinling orogenic belt led to a huge subsidence of the southern part of the basin, making the basin basement wide and gentle in the northeast and steeply inclined in the southwest. With abundant sediment supply, the southwestern basin developed braided river and delta deposits. Under the effects of earthquake, volcano and overloading, the unconsolidated sediments of delta front might slide and slump, forming slump-liquefaction deformation structures (e.g. slump folds, liquefied veins and fault-graded beds) near the foot of the slope. The slide-slump bodies changed into sandy debris flows and turbidity currents after transported further, forming the delta front-type turbidite fan mainly consisting of overlapped lobate clasts and outer turbidites. The gravity-flow sandstone reservoirs in the study area are relatively tight. The physical property analysis results show that the debris-flow sandstones have the porosity of 8%-12% (avg. 9.8%) and the permeability of (0.1-1.5)×10−3 μm2 (avg. 0.8×10−3 μm2), and the turbidity-current sandstones have the porosity of 4%-10% (avg. 7.6%) and the permeability of (0.05-1.50)×10−3 μm2 (avg. 0.4×10−3 μm2). Regardless of inferior reservoir properties to the sandy gravity-flow reservoir of delta front, the gravity-flow sandstones correspond to favorable source-reservoir-caprock assemblages due to their connection with the organic-rich shales in deep lake basin. The thick massive debris-flow sandstones are probably tight oil reservoirs, while the turbidity-current sandstones with small thickness of a single layer act as shale oil reservoirs.
Fig. 8. Event deposition model of Chang 9 Member, southwestern Ordos Basin. |
4. Conclusions
Through the observation and analysis of the Chang 9 Member in the Ordos Basin, including seismites, gravity flow deposits, tuffs, and organic-rich shales, 4 types and 9 subtypes of event deposits, such as earthquake, gravity flow, volcanic and anoxic deposits, were identified. These deposits reflect the frequent tectonic activity in the basin. Moreover, in the setting of the initial lake flooding, a series of event deposit combinations with genetic relations and regular distribution were formed from the delta to the semi-deep lake.
Five comparable seismic episodes reveal that the basin experienced at least five extensive earthquakes during the deposition of Chang 9 Member, all of which have the characteristics of episodic activity. The seismites and gravity flow deposits alternated with normal sediments are interconnected as a whole and are the products of debris flow and turbidity current from the slide and slump of delta front sediments triggered by earthquakes. The thick black-gray shales are closely related to the continuous lake transgression and can be used for stratigraphic correlation with the isochronism similar to seismic episodes.
The banded and lobate delta front-type slump turbidite fan entered the lake basin as debris flow and turbidity current deposits, forming potential tight oil and gas reservoirs and shale oil and gas reservoirs. The black-gray shales formed in the late stage of lake transgression to the highstand stage have a high organic matter abundance and high hydrocarbon generation potential, and they act as important source rocks in Chang 9 Member. Their favorable source-reservoir-caprock assemblages in the vertical direction allow for the formation of large unconventional oil and gas reservoirs.
Nomenclature
GR—natural gamma ray, API;
HI—hydrogen index, mg/g;
Rt—resistivity, Ω•m;
S2/S3—korogen type index, dimensionless;
SP—spontaneous potential, mV;
Tmax—maximum pyrolysis temperature, °C;
TOC—content of total organic carbon, %;
(S1+S2)—hydrocarbon generation potential, mg/g;
Δt—acoustic time difference, μs/m.