Marine and Petroleum Geology 77 (2016) 1042e1055

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Marine and Petroleum Geology

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Research paper Lacustrine massive in the Eocene Jiyang Depression, Bohai Bay Basin, China: Nature, origin and significance

* Jianguo Zhang a, b, Zaixing Jiang a, b, , Chao Liang c, Jing Wu d, Benzhong Xian e, f, Qing Li e, f a College of Energy, China University of Geosciences, Beijing 100083, China b Institute of Earth Science, China University of Geosciences, Beijing 100083, China c School of Geosciences, China University of Petroleum (east China), Qingdao 266580, China d Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China e College of Geosciences, China University of Petroleum, Beijing 102249, China f State Key Laboratory of Petroleum Resources and Prospecting, Beijing 102249, China article info abstract

Article history: Massive mudrock refers to mudrock with internally homogeneous characteristics and an absence of Received 13 May 2016 laminae. Previous studies were primarily conducted in the marine environment, while notably few Accepted 6 August 2016 studies have investigated lacustrine massive mudrock. Based on core observation in the lacustrine Available online 8 August 2016 environment of the Jiyang Depression, Bohai Bay Basin, China, massive mudrock is a common deep water fine-grained . There are two types of massive mudrock. Both types are sharply delin- Keywords: eated at the bottom and top contacts, abundant in angular terrigenous debris, and associated with Massive mudrock oxygen-rich (higher than 2 ml O /L H O) but lower water salinities in comparison to adjacent black Muddy mass transportation deposit 2 2 Turbiditic mudrock . In addition, type 1 is laterally isolated and contains abundant sand injections and contorted layers Eocene formed in the depositional process, but type 2 exactly distributes in the distal part of deep water gravity- Bohai Bay Basin driven units, and shows scoured bases, high-angle mineral crytsals, and fining-upward trend. It is suggested that type 1 is a muddy mass transport deposit (MMTD) formed by slide, slump, and/or debris flow, and type 2 is a turbiditic mudrock deposited by settling from dilute turbidity currents. A warm and humid climate and high subsidence rate are two main triggering events. Because of its mass movement nature, MMTD preserves the mineralogic composition and organic matter characteristics of the source . By contrast, dilute turbidity currents are able to greatly entrain biochemically- formed micrite and planktonic organisms from the water column, and deposit them in the turbiditic mudrock. Because of their different ability to deposit organic matter, MMTD have poor or fair source rock potential, but the turbiditic mudrock is able to be a potentially effective source rock. The minerals in the massive mudrock are disorganized and chaotic, which cause fractures to develop in various directions, thereby, enhancing the vertical migration of oil and gas molecules to horizontal wellbore in reservoir exploitation. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction (e.g., Loucks and Ruppel, 2007; Abouilresh and Slatt, 2012). Massive mudrock (also named as uniform mudrock by some geologists; e.g., The deposition of fine-grained (<62.5 mm; Schieber Tripsanas et al., 2004) was originally discovered in the eastern and Southard, 2009; Jiang et al., 2013) is important in sedimento- Mediterranean (Stanley, 1977, 1981), and was also reported in the logic research because of the economic importance as source rocks western equatorial Atlantic and the northwest Gulf of Mexico in and shale reservoirs. Diverse types of (mainly composed subsequent studies (e.g., Behrens, 1985; Tripsanas et al., 2004). It of fine-grained sediments) were recognized in previous studies refers to the mudrock with internally homogeneous characteristics (e.g., similar color and mineralogic composition) and an absence of (i.e., non-laminated) (Behrens, 1985; Xian et al., 2013). Possible sedimentary characteristics of massive * Corresponding author. College of Energy, China University of Geosciences, fi Beijing 100083, China. mudrock include the following: 1) a subtle textural ning-upward E-mail address: [email protected] (Z. Jiang). trend with the basal components being siltier (Damuth, 1977; http://dx.doi.org/10.1016/j.marpetgeo.2016.08.008 0264-8172/© 2016 Elsevier Ltd. All rights reserved. J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055 1043

Blanpied and Stanley, 1981; Cita et al., 1996), 2) a narrow range of Es3. The subsidence rate was about 260 m/My, 520 m/My in the time interval according to radiocarbon dating, which indicates upper Es4 and lower Es3 strata, respectively (Fig. 2). The climate rapid deposition (Stanley, 1981), 3) often occurring in trench basins changed from arid to humid from the Es4 to Es3 strata (Li et al., with steep margins (Cita and Aloisi, 2000), and 4) disorganized and 2004)(Fig. 2). A series of fan-deltas and braid-deltas developed chaotic mineral assemblages (Bennett et al., 1991; Ochoa et al., along the lake margin during this time, and the center of the lake 2013). was filled with deep water mudrock that reaches 1,000 m in Several mechanisms have been proposed for the deposition of thickness in the Es4 and Es3 strata. The thick mudrock layers are the massive mudrock. Ryan (1977) indicated that massive mudrock was primary source rocks for the petroliferous depression. deposited by pelagic suspension-related processes in still water. The Paleozoic carbonate of the North China Craton was one of More recent studies suggest that the massive structure is attributed main provenance rocks for these Cenozoic strata in the Jiyang to the development of a stratification interface generated by the Depression (e.g., Wang et al., 2013). It makes the mudrock rich in continuous introduction of a turbulent cloud of the fine-grained carbonate minerals (4e92 wt%, avg. 43.06 wt%) (Fig. 3). In addition, nature formed by depositional segregation (Stow and quartz and content ranges from 6 to 82 wt% (avg. 31.59 wt Shanmugam, 1980; Tripsanas et al., 2004; Ochoa et al., 2013). In %), and content ranges from 2 to 65 wt% (avg. 25.35 wt%) in the addition, McCave and Jones (1988) found that high-density non- mudrock (Fig. 3). turbulent turbidity currents can also be responsible for massive mudrock deposition when the current “froze” in some way. Note 3. Samples and methods that later studies discovered that there are no high-density non- turbulent turbidity currents in nature, and the so-called high- Cores and cut samples of the deep water lacustrine mudrock U density non-turbulent turbidity currents deposits are actually from the upper member of Es4 (Es4 ) and the lower member of Es3 L debrites (e.g., Shanmugam, 1996, 2002). (Es3) were collected from five continual core sections (wells N1, To date, massive mudrock has been exclusively discovered in L69, F1, L1, and L75; Fig. 1B), with a total thickness of 1,093 m. The marine environments. Lacustrine mudrock is equally as important core lengths are 203 m, 223 m, 414 m, 218 m, and 35 m in the above as its marine counterpart (Katz and Lin, 2014). However, very few five wells, respectively. The basic analysis includes thin section and studies have investigated lacustrine massive mudrock, thus leaving spore and pollen samples from 508 samples, whole rock X-ray its sedimentary characteristics and depositional origin poorly un- diffraction (XRD) from 163 samples, major and trace element derstood. In addition, the relationship between massive mudrock geochemistry from 670 samples, total organic carbon (TOC) from 78 and hydrocarbon exploration and exploitation has received little samples, Rock-Eval from 78 samples, high-resolution field emission attention in previous investigations. scanning electron microscopy (FESEM) from 45 samples, and Lacustrine mudrock is widely distributed in the half-graben of separation from 45 samples. In addition, sedimentary the Eocene Jiyang Depression, Bohai Bay Basin, China (e.g., Jiang cross-sections from the lake margin to lake center are established et al., 2014; Zhang et al., 2016). Here massive mudrock is a com- using the wire-line logging data. mon component of the fine-grained sedimentary rock succession of In regards to XRD analysis, pretreatment of the samples are the Eocene Jiyang Depression. A systematic study of the massive necessary, in which the particles were dried at 40 C for 2 days and mudrock is conducted in this paper, including: dispersed by an agate mortar. D8 DISCOVER equipment was used for analysis of the minerals at a voltage of 40 kV and a current of 1) A study of the sedimentary characteristics (e.g., petrology, 25 mA. Major elements were measured from the fusion discs, and depositional setting) of the massive mudrock; trace elements from the pressed powder pellets, using the Philips 2) An analysis of the origin (e.g., depositional mechanism, trig- PW 1400 XRF spectrometer. TOC was determined by a LECO CS-200 gering events, and depositional process and model) of massive carbon analyzer, in which the measurement technique was based mudrock; and on the combustion of the samples in an oxygen atmosphere to 3) An investigation of the relationship between massive mudrock convert the organic carbon to CO2. The TOC value was determined and hydrocarbon exploration and exploitation. with the aid of a combustion calculation. Rock-Eval analysis was carried out on a Vinci Rock-Eval-6 instrument, and about 100 mg for each sample was pyrolized using helium as the carrier gas. The 2. Geologic setting samples were heated to 300 C to release its residual hydrocarbons (S1), and then pyrolized to 600 C to obtain hydrocarbons (S2) The formation of the Bohai Bay Basin was co-controlled by cracked from kerogen. The FESEM analysis was conducted on a extensional and transtensional tectonics (e.g., Allen et al., 1997, HITACHI S-4800 with a working current set at 10 kV and working 1998; Qi and Yang, 2010). The Jiyang Depression located in the distance of 8 mm. The sample for kerogen separation should be no southeast of the Bohai Bay Basin (Fig. 1A) is a sub-basin filled with less than 100 g. First, the sample was dipped in fresh water for three lacustrine successions. The depression or sub-basin occupies an days to make the mudrock loosen. Second, it was dipped in 36% HCl area of 26,000 square kilometers, which is bounded by the Luxi for 24 h to remove the alkaline minerals (i.e., carbonate). Third, it Uplift in the southeast, the Kendong-Qingtuozi Uplift in the east, was washed to pH ¼ 7 and then dipped in 60% HF for 48 h to and the Chengning Uplift in the northwest (Fig. 1B). The Dongying remove the acidic materials (i.e., siliceous minerals). Fourth, it was Sag is located in the southern part of the Jiyang Depression with an once again washed to pH ¼ 7 and dipped in 36% HCl to further area of 5,700 square kilometers, and the Zhanhua Sag is located in remove the alkaline minerals. Framboidal pyrites were then the northeastern part of the Jiyang Depression with an area of 2,800 removed by a solution of NaBH4. The remaining material was square kilometers (Fig. 1B). dominantly kerogen. The Cenozoic Jiyang Depression includes Paleogene, Neogene, and Quaternary strata (e.g., Jiang et al., 2011)(Figs. 1C and 2). The 4. Sedimentary characteristics of massive mudrock Paleogene strata are sub-divided into the Kongdian (Ek), Shahejie (Es), and Dongying (Ed) Formations, and the Es strata are sub- The 1,093 m of core sections for this study are composed of divided into four members (Es4 to Es1 from bottom to top) 998 m of black shale and 95 m of massive mudrock. Massive (Fig. 2). The strata targeted in this study are the upper Es4 and lower mudrock accounts for 8.69% in thickness. It includes two types 1044 J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055

Fig. 1. (A). Geologic setting of the Bohai Bay Basin during the Eocene period. The Zhanhua and Dongying Sags of the Jiyang Depression are in the red rectangle. (B) Sketch of the Zhanhua and Dongying sags of the Jiyang Depression. The locations of cored wells in the study are annotated by shuriken. (C) A N-S trending section in the study area (A0-B0, see the location in (B)). O: Ordovician; C: Carboniferous; P: Permian; J: Jurassic; K: Cretaceous; E: Eogene; N: Neogene; Q: Quaternary. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) based on sedimentary characteristics, named as type 1 and type 2 the organic matter in type 1 massive mudrock fall in the Type II-III, here. Type III-II, and Type III areas, indicating the predominance of terrigenous input (Fig. 6). The dominant terrigenous organic matter is also supported by the kerogen separation, which shows organic 4.1. Petrology characteristics matter is mainly lignocellulosic debris (a type of terrigenous organic matter; Peters et al., 2005)(Fig. 4F). 4.1.1. Type 1 massive mudrock Type 1 massive mudrock is present in wells N1 and L75 with a total length of 19 m (Table 1). It is blue-gray and blue-green in 4.1.2. Type 2 massive mudrock color (Fig. 4AeD) and interbedded with black shale (Fig. 4A). The Type 2 massive mudrock appears in wells L69, F1, and L1 with a bed thickness is 0.10e0.95 m (avg. 0.45 m) in well N1 and total length of 76 m (Table 1). It is dark-gray and black in color and 0.14e0.91 m (avg. 0.39 m) in well L75. The interface between interbedded with black shale (Fig. 7AeD). Different stages of massive mudrock and black shale is dominantly sharp (Fig. 4A). massive mudrock are mostly stacked (Fig. 7AeB), but can also be Sand injection, contorted layers are common in type 1 massive vertically isolated (Fig. 7CeD). The average single bed thickness is mudrock (Fig. 4AeD). less than 1 m (Table 1). The bottom and top contacts are always Based on XRD analysis, clay (avg. 46.7 wt%) and quartz (avg. sharp (Fig. 7AeF). Enriched Gastropoda fossils and coal debris are 26.6 wt%) are the dominant minerals, while the percentages of common in the cross section of cores (Fig. 7GeH), and Ostracoda carbonate, feldspar and pyrite are all lower than 10 wt% (Table 2). fragments are also common in thin section (Fig. 7I). There are some However, the adjacent black shale intervals have much higher thin layers or micro-scour surfaces at the bottom contacts, carbonate content (ca. 50 wt% in average), but much lower clay and showing the fining-upward trend (Fig. 7B, D, F, J, K). Minerals are quartz content (Fig. 5). All minerals are disorganized and chaotic in chaotic in the mudrock, even with some vertically-oriented min- thin section observation (Fig. 4E). Quartz grains are always angular, erals (Fig. 7I). The interbedded black shale commonly shows soft- and range from several to a dozen micrometers in size (Fig. 4E). deformation structures (Fig. 7C). The TOC value is low (0.15e1.23 wt%, avg. 0.54 wt%) (Fig. 5). The In comparison to type 1 massive mudrock, type 2 is also rich in plot of the hydrogen index (HI) versus the pyrolysis Tmax shows that quartz based on XRD analysis (Table 2), and the quartz grains also J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055 1045

Fig. 2. Tertiary stratigraphy of the Zhanhua and Dongying Sags in the Jiyang Depression. The strata for this study are annotated by a blue line in the column. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) have sharply angular shapes in thin section. However, calcite is the 4.2. Depositional settings primary mineral in type 2 massive mudrock (Table 2), of which micrite is common (Fig. 7L). Also note that the feldspar content in 4.2.1. Water redox condition and salinity level type 2 massive mudrock is only half of that in type 1 massive Type 1 massive mudrock has blue-gray and blue-green color mudrock (Table 2). All minerals are disorganized and chaotic in the (Fig. 4AeD), suggesting an oxic environment in comparison to the massive mudrock, even with some upright Ostracoda fragments adjacent black shale. However, both type 2 massive mudrock and (Fig. 7E, I). its adjacent shale are dark-colored (Fig. 7AeD, G, J), thus the The TOC values are high (1.57e4.61 wt%, avg. 2.86 wt%) (Fig. 8). geochemical data may aid in the discrimination of the redox level. The plot of the hydrogen index (HI) versus the pyrolysis Tmax shows The DOP (Degree of pyritization), Th/U, and V/(V þ Ni) values are all that the organic matter in type 2 massive mudrock are mostly Type effectively indicators of redox conditions (e.g., Potter et al., 2005). ItoII(Fig. 6). This is supported by the result of kerogen separation, The oxygen-rich water column is indicated by DOP with the value which demonstrates that the organic matter is mainly Di- lower than 0.45, V/(V þ Ni) with the value lower than 0.46, and/or noflagellates (a type of planktonic organism; Fensome et al., 1993) Th/U with the value higher than 1.33 (e.g., Jones and Manning, (Fig. 7M). 1994; Pattan et al., 2005; Lyons and Severmann, 2006). These 1046 J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055

geochemical data suggest type 2 massive mudrock was deposited associated with an oxygen-rich water column, while the adjacent black shale deposited in more oxygen-poor conditions as indicated by DOP, Th/U, and V/(V þ Ni) values (Figs. 5 and 8). The Sr/Ba value is a good indicator of water salinity level (e.g., Chivas et al., 1986). Higher Sr/Ba value reflects higher salinity level. Both types of massive mudrocks have lower Sr/Ba values in com- parison to the adjacent shale, reflecting a relatively low water salinity level (Figs. 5 and 8).

4.2.2. Climate and tectonic condition U The Es4 strata were deposited under arid climate condition, but L the Es3 strata under humid climate condition (Fig. 2). In addition, L U the subsidence rate of Es3 (ca. 520 m/My) is double that of the Es4 (ca. 260 m/My) (Fig. 2). Correspondingly, the percentage of L mudrock is also different in the two strata, with the Es3 strata U approximately have four times than that of the Es4 strata (Table 3), indicating that massive mudrock prefers to deposit in environment with humid climate condition and high subsidence rate. To more precisely define the relationship between climate evo- Fig. 3. Mineralogical ternary diagram of mudrock in the Zhanhua and Dongying Sags lution and deposition of massive mudrock, we systematically of the Jiyang Depression. analyzed the spore and pollen data, one of the most effective ways to

Table 1 Bed thickness of massive mudrock. The single bed thickness is presented in the form of “avg./min.~max.”.

Type 1 Type 2

Well L75 Well N1 Well L69 Well F1 Well L1

Single bed thickness (m) 0:39 0:45 0:74 0:34 0:52 0:140:91 0:100:95 0:021:75 0:041:10 0:081:25 Total massive mudrock length/core length (m) 7:0 12:0 50:0 20:0 6:0 35:0 203:0 223:0 414:0 218:0

Fig. 4. Petrology characteristics of type 1 massive mudrock. (A). 3467.30e3468.10 m, well N1. Sharp interface between massive mudrock and black shale (yellow full line), deformed folded layer and lenticular sand injection (yellow arrows) (B). 3467.10 m, well N1. Contorted layer (upper most yellow arrow), sand injection (lower two arrows). (C). 3472.13 m, well N1. Sand injection (yellow dotted line). (D). 3474.90 m, well N1. Horizontally lenticular sand injection (yellow arrow). (E). 3472.13 m, well N1. Disorganized and chaotic minerals. A great deal of angular quartzes. (F) 3472.13 m, well N1. Lignocellulosic debris. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055 1047

Table 2 Mineralogical composition of massive mudrock. The numbers are presented in the form of “avg./min.~max.”.

Mineralogy (wt %) Type 1 Type 2

Well N1 (Num. ¼ 19) Well L69 (Num. ¼ 79) Well F1 (Num. ¼ 58) Well L1 (Num. ¼ 7)

Calcite 5:8 44:6 30:0 27:8 0:016:0 29:060:0 18:046:0 18:036:0 Dolomite 7:4 4:3 3:7 2:6 3:019:0 2:010:0 1:025:0 1:04:0 Quartz 26:6 21:4 23:1 28:8 15:037:0 14:025:0 17:028:0 18:038:0 Feldspar 8:7 1:7 3:4 4:5 5:021:0 1:04:0 2:05:0 1:06:0 Clay 46:7 23:5 33:4 34:4 20:058:0 12:040:0 24:045:0 28:043:0 Pyrite 3:5 3:01 3:4 3:0 0:08:0 1:08:0 2:07:0 2:04:0

Fig. 5. Integrated diagram of type 1 massive mudrock in well N1. Depth intervals of type 1 massive mudrock are annotated in red in the lithology column. The vertical dotted black lines in the columns of Th/U and DOP separate the oxygen-rich and oxygen-poor environment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) reconstruct the paleoclimate (e.g., Behling et al., 2004), along the type/(eurthermal type þ thermophilic type) * spore and pollen core section (Fig. 8). The spore and pollen includes hythermophilic number in each thin-section, and the calculation equation for hu- types, thermophilic types, and eurthermal types based on the midity is RH (ration of humidity) ¼ humid type/(humid type þ arid adaptability of its parent-plants to temperature, and humid type, type) * spore and pollen number in each thin section. Higher RT arid type, and medium type based on the adaptability of its parent- values represent higher temperatures, and higher RH values plants to humidity (Wang, 2013). The calculation equation for represent higher humidity levels (Li et al., 2004). The results of spore temperature (Wang, 2013) is RT (ratio of temperature) ¼ eurthermal and pollen analysis show that the massive mudrock have much 1048 J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055

(Shanmugam, 2006). The type 1 massive mudrock is surrounded by underlying and overlying black shale, whereas the sand injection or deformation phenomenon is absolutely absent in the adjacent black shale. In a post-depositional origin, the sand injection migrated upward from buried sandstone to overlying mudrock (Shanmugam, 2012), thus it is unable to explain the sole occurrence in type 1 massive mudrock. In the depositional process, the deformed folded layer (Fig. 4A, B) and sand injection (Fig. 4AeD) is explained by the mass transport deposits (MTD) (e.g., Stow and Johansson, 2000; Jenner et al., 2007; Shanmugam, 2016b), because of the coherent massive nature of slide and slump and/or plastic rheology nature of debris flows. On the other side, the depositional segregation of turbidity current is not able to explain these structures. Therefore, type 1 massive mudrock is muddy mass transport deposits (MMTD) rather than turbidites. However, the discrimination of one MMTD type from the other is difficult, thus we call them generally MMTD. Type 1 massive mudrock is allochthonous as a result of mass transport deposition, thus explaining the phenomenon that its mineral composition is incompatible with the adjacent black shale (Fig. 5). Because the depositional process of MTD brings oxygenated freshwater to the lake bottom (Potter et al., 2005), it is also able to explain the phenomenon that type 1 massive mudrock deposited in the less reducing and lower water salinity environment (Fig. 5). Fig. 6. Plot of the hydrogen index (HI) versus the pyrolysis Tmax for the massive mudrock, showing the kerogen type (modified after Mukhopadhyay et al., 1995). 5.1.2. Type 2 massive mudrock Type 2 massive mudrock is interpreted as deposited by dilute higher RT and RH values than of the black shale, indicating a rela- turbidity currents after the deposition of deep water gravity-driven tively warm-humid climate condition (Fig. 8). sandstone, with the massive structure formed from the rapid settling of fine-grained particles. This interpretation is supported 4.2.3. Distribution characteristics by the following proofs. In regards to the vertical variation, both types of massive First, the features of turbiditic origin are obvious in core mudrock correspond to strong terrigenous debris input. In Fig. 9, observation. The sharp bottom and top contacts in the type 2 type 1 massive mudrock only occurs in 3460e3485 m of well N1, massive mudrock indicate episodic deposition (Fig. 7AeD). With and the shallow-water coarse-grained sediments are also concen- the 1,093 m of cores from the deep water lacustrine environment in trated in this corresponding section. In Fig. 10, the type 2 massive this study, no sandstone layers occur. Typical fining-upward mudrock of well L69 mainly occurs in 2945e3010 m, and the cor- sequence from sandstone to mudrock, one of the most believable responding intervals in shallow-water also have more coarse- evidence for turbidites (Shanmugam, 2006), is missing. Fortunately, grained sediments. silt layers, though their thickness is mostly less than 2 cm, occur at In regards to the lateral variation, type 1 massive mudrock is the bottom of some type 2 massive mudrock (Fig. 7B, D, J), which laterally isolated. However, type 2 massive mudrock distributes in also indicates a fining-upward trend (e.g., Macquaker et al., 2010; the distal areas of sandy debrites/turbiditic sandstone (the sand- Lazer et al., 2015). An erosive current is indicated by the wide L stones in wells Y99 and Y9 in Es3 are deeper water sandy debrites/ occurrence of micro-scour surfaces at the bottom contacts (Fig. 7F, turbiditic sandstone; Sun et al., 2013)(Fig. 10). K), suppoing the turbiditic origin (Shanmugam, 2006). Because of the turbulent nature of turbidity currents, the minerals can have 5. Origin of massive mudrock high angle to the bedding or even vertically oriented, thus explaining the vertically oriented minerals in the type 2 massive 5.1. Depositional mechanism mudrock (Ochoa et al., 2013)(Fig. 7E, I). On the other side, the process for MMTD deposition is non-turbulent, and the minerals by 5.1.1. Type 1 massive mudrock slow suspension setting in still water are horizontally oriented Type 1 massive mudrock, which is blue-gray and blue-green in (Potter et al., 2005). color, is surrounded by black shale (Figs. 4A and 5). There are two Second, type 2 massive mudrock is laterally distributed in the scenarios for this phenomenon: 1) the lake level greatly fluctuated proximal areas of sandy debrites and/or turbiditic sandstone that the massive mudrock deposited in shallow water environ- (Fig. 9). In an ideal model for gravity-driven process, the deposits ment; or 2) the massive mudrock was transported from shallow successively experience slides, slumps, debris flows, and turbidity water to deep water environment. Scenario 1 is unable to explain currents (Zou et al., 2012; Shanmugam, 2016a). The sandy debrites the sharp interfaces between massive mudrock and the inter- and turbiditic sandstone should deposit proximally in comparison bedded black shale (Figs. 4A and 5). to the corresponding turbidtic mudrock. In a lacustrine deep water environment, the mechanisms of Third, the turbiditic origin is further reinforced by the presence allochthonous deposition include mass transport deposition (MTD) of abundant exogenous debris (e.g., angular quartz minerals, Gas- and turbidity current (e.g., Kremer et al., 2015). The seismic data can tropoda fossils, coal debris, and Osracodas)(Fig. 4EeK). Turbidity hardly be used to reveal the origin, because the massive mudrock current can also bring oxygenated fresh water to the lake bottom has a similar reflection coefficient as compared to the adjacent (Potter et al., 2005). The DOP, Th/U, and V/(V þ Ni) values suggest black shale. Sand injection is common in the type 1 massive that type 2 massive mudrock was deposited in a oxygen-rich water mudrock, which is depositional or post-depositional in origin column, while the other shale intervals are generally oxygen-poor Fig. 7. Petrology characteristics of type 2 massive mudrock. (A) 2992.52e2993.52 m, well L69. Three continual stages of structureless massive mudrocks with sharp interfaces. (B) 3047.13e3048.13 m, well F1. Two continuous stages of massive mudrocks with sharp interfaces. Silt layer at the bottom interface (yellow arrow). (C) 3388.94e3389.94 m, well F1. Four isolated stages of massive mudrock with sharp interfaces. Soft deformation structures at adjacent black shale (yellow arrow). (D) 3340.20 m, well F1. Massive mudrock with sharp interfaces and a silt layer at the bottom interface (yellow arrow). (E) 2993.20 m, well L69. Sampled at the interface of massive mudrocks “2” and “3” of Fig. 7A. Sharp interface and vertical Ostracoda fragments (yellow arrow). (F) 3046.60 m, well F1. Sampled at the interface of massive mudrock “1” and “2” of Fig. 7B. Micro-scour surface and silt layer (yellow arrow). (G) 2992.52 m, well L69. Enriched Gastropoda fossils at the cross-section of the massive mudrock (yellow arrow). (H) 2983.60 m, well L69. Enriched coal debris layer in the massive mudrock. (I) 2993.05 m, well L69. Upright Ostracoda fragment (yellow arrow). (J) 2920.92 m, well L69. Two silt layers in the massive mudrock (yellow arrow). (K) 2920.92 m, well L69. Scoured interface at the upper silt layer of Fig. 7I. (L) 2993.05 m, well L69. High-resolution field emission scanning electron microscopy (FESEM) image (left) and composition (right). Euhedral micrite. (M) 2992.55 m, well L69. Dinoflagellates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 1050 J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055

Fig. 8. Integrated diagram of well L69. Depth intervals of type 2 massive mudrock is annotated in red in the lithology column. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2012). Because the massive mudrock is accompanied by some Table 3 deformed black shale layers (Fig. 7C), earthquakes are one of the Massive mudrock in the upper section of the fourth member of Shahejie Formation U L main short-term events for the deposition (Mazumder et al., 2006). (Es4 ) and the lower section of the third member of the Shahejie Formation (Es3) strata. In long-term, climate and tectonic events are the two main factors for lake (e.g., Gierlowski-Kordesch, 2010; Renaut Strata U L Es4 Es3 and Gierlowski-Kordesch, 2010; Pietras et al., 2003). Section 4.2.2 Total core length (m) 42 671 in this paper has shown that massive mudrock is prone to occur Total massive mudrock length (m) 12 76 in condition with warm-humid climate and high subsidence rate. Percentage of massive mudrock (3) 3.08% 11.33% The warm-humid climate is able to cause heavy rainfall and frequent and intense flood (Shanmugam, 2012), which facilitates (Fig. 8). In addition, type 2 massive mudrock has lower Sr/Ba values slope failure by damaging its stability. The case studies of MTD or fl than the adjacent shale (Fig. 8), suggesting lower salinity level. turbidity current induced by heavy rainfall and ood have been Hence, the paleo-redox and paleo-salinity levels also support that reported in the modern environment of central Chile (Sepulveda type 2 massive mudrock was deposited by turbidity current. and Padilla, 2008) and the Norway (Longva et al., 2003). The high subsidence rate is able to increase the depositional loading 5.2. Triggering events (Moscardelli et al., 2006), and sediment failures can easily occur in a high depositional loading condition (Tripsanas et al., 2008). The fl The initiation of deep water sediment failure is controlled by South Pass mud ow (Coleman and Garrison, 1977), the delta-front tandem short-term events (only few minutes to several days) to mass movement in the Mississippi River Delta area (Coleman and fl long-term events (thousands to millions of years) (Shanmugam, Prior, 1982), and the debris ow in the Kitimat Arm fjord of J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055 1051

Fig. 9. Sedimentary cross-section from Wells T7 to N11 along the north/south direction in the Dongying Sag. All the type 1 massive mudrocks occur in the strata labeled “②” and are accompanied by abundant terrigenous debris supply from fan-delta (See typical characteristics of fan-delta in McPherson et al., 1987).

Fig. 10. Sedimentary cross-section from Wells K71 to L69 along the east/west direction in the Zhanhua Sag. Almost all the type 2 massive mudrocks occur in the strata labeled “②” and are accompanied by abundant terrigenous debris supply from braid-delta (See typical characteristics of braid-delta in McPherson et al., 1987).

British Columbia (Nemec, 1990) are all attributed to excessive micrite formed by photosynthesis in the water column), suggesting depositional loading. For the above reasons, warm-humid climate that the transportation and deposition process does not change its and high subsidence rate are the two main triggering events for the mineral composition. Therefore, the MMTD maintains the sedi- massive mudrock deposition in long-term. mentary characteristics of the mud in the delta-front (gravity flows in lakes typically start on delta-front), which has high terrigenous 5.3. Depositional process and model quartz and clay calcite, low calcite content, and only a little terrigenous organic matter in the Jiyang Depression. Type 1 massive mudrock (MMTD) is interpreted as a muddy The depositional process of type 2 massive mudrock (turbiditic slide, muddy slump and/or debris flow (Fig. 11A). The MMTD is deposits) is more complex in comparison to type 1 massive massively moved, and then suddenly deposited (similar to a mudrock (Fig. 11B). As shown in Fig. 10, type 2 massive mudrock double-decker bus knocked down the wall; Shanmugam, 2012). In occurs in more distal areas of deep water gravity-driven sand- addition, the mass movement followed by sudden deposition does stone. The turbiditic mudrock began to deposit after the deposi- not allow the MMTD to entrain and deposit additional floating tion of relatively coarse-grained sandstone. In relatively proximal particles from the water column (e.g., planktonic organisms and areas, the turbiditic mudrock can be underlain by gravity-driven 1052 J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055

Fig. 11. (A). Model of MMTD (type 1 massive mudrock) deposition in the Jiyang Depression. (B). Model of turbiditic mudrock (type 2 massive mudrock) deposition in the Jiyang Depression.

sandstone, because the transformation of MMTD process to than the MMTD. The notably higher feldspar content in MMTD turbidity current or depositional segregation of the turbidity than that of turbiditic mudrock is also consistent with this current. However, the underlying sandstone disappears in rela- interpretation (Table 2). tively distal areas (e.g., only very thin silt layers may occur at the bottom interface of turbiditic mudrock in wells L69, L1, and F1) (Fig. 11B). The deposition of turbiditic mudrock is estimated to be long lasting (1.5e3 months) (Stow and Wetzel, 1990; Tripsanas et al., 2004), and the dilute turbidity current is able to entrain a lot of additional floating particles from the water column (Fig. 11B). In addition, planktonic algae blooms (i.e., Dinoflagellates in this paper) are common and intensive during the Es3 and Es4 periods, and the algae blooms massively induce micrite precipi- tation (e.g., Liu and Wang, 2013). Therefore, biochemically- formed micrite and planktonic organisms are able to be incor- porated the turbiditic mudrock (Fig. 11B). This depositional pro- cess explains why the turbiditic mudrock contains not only abundant terrigenous debris but also euhedral micrite. In addi- tion, it also explains the reason the turbiditic mudrock has abundant Type I and II kerogen. The ability of turbidity current to entrain the additional floating particles is supported by previous studies. For example, the turbiditic mudrock in the Hellenic Trench, eastern Mediterranean contains mixed terrigenous debris and planktonic biogenic component from the marine water (Blanpied and Stanley, 1981). Ochoa et al. (2013) also reported that the turbiditic mudrocks from the El Rosario Formation out- crops (Baja California, Mexico) and the Woodford Shale (Okla- homa) not only contains abundant terrigenous particles, but also many biogenic particles (mainly coccolith) entrained from the marine water. The longer depositional time of turbiditic mudrock Fig. 12. Pyrolysis S1þ S2 versus total organic carbon (TOC). The plot shows generative suggests that it should have more mature mineral composition source rock potential for massive mudrock (McGlue et al., 2015). J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055 1053

The different depositional processes of MMTD and turbiditic 2012). The assumption that persistent anoxia and low-energy mudrock governed the difference in their mineralogic composition water is the precondition of source rocks is overestimated, and and organic matter content. These factors can function together as notification should also be paid to turbidites in studying source criteria for distinguishing MMTD and turbiditic mudrock. rock.

6. Significance 6.2. Significance of massive mudrock for shale reservoirs exploitation 6.1. Significance for organic matter accumulation and source rock Because the episodic nature of massive mudrock deposition The MMTD only has a small amount of terrigenous organic causes the minerals to be disorganized and chaotic, the natural matter (Figs. 5e6), but the turbiditic mudrock contains abundant and artificial fractures occur in various directions (Fig. 13AeD) planktonic organisms (mainly Dinoflagellates in this paper) (Ochoa et al., 2013). However, the natural and artificial fractures in (Figs. 7e8). Besides this study, organic-rich turbidites were also black shale are dominated by horizontal interlayer fractures discovered in the Congo fan in Africa (avg. 3 wt% in TOC) (Baudin because of the horizontally arranged minerals and well-developed et al., 2010) and Ogooue deep sea turbidites (up to 14 wt% in laminae (Ochoa et al., 2013). Therefore, MMTD and turbiditic TOC) (Biscara et al., 2011). However, they are all composed of mudrock are expected to have higher vertical permeability than terrigenous organic matter, sourced from the river. Obviously, black shale. terrigenous organic matter has much lower potential to be effective Shale reservoirs are generally exploited by horizontal well source rock in comparison to planktonic organisms. drilling (Fig. 14), and the fractures control the migration effi- The plot of the S1þS2 versus total organic carbon (TOC) shows ciency of oil and gas molecules to the wellbore. As the natural that turbiditic mudrock is potential effective source rocks and artificial fractures in black shale are horizontally dominated (Fig. 12). In previous studies, effective source rock was usually (Fig. 13E), the exploitation of thick black shale successions is regarded as deposited in persistent bottom-water anoxia and possibility not successful because the horizontal fractures low-energy conditions in order to preserve large volumes of restrict the vertical migration of oil and gas molecule. However, organic matter (Pedersen and Calvert, 1990; Tyson, 2001, 2005; the interbedded black shale and massive mudrock (Fig. 13B) Bohacs et al., 2005). However, the turbiditic mudrock in the can enhance the vertical connectivity, thus allowing for more oil Jiyang Depression also acted as potential effective source rock. In and gas molecules to vertically migrate to the horizontal well- addition, organic-rich event beds (up to 14.2 in TOC) in mudrock bore (Fig. 14). Therefore, the distribution of massive mudrock was also recorded in the Whitby Formation (Toarcian, plays a significant role in controlling the shale reservoir Lower Jurassic), northeast England (Ghadeer and Macquaker, characteristics.

Fig. 13. Fracture characteristics of massive mudrock and black shale. (A) 3051.13e3051.78 m, well F1. The fractures occur in various directions in massive mudrock. (B) 3371.10 m, well F1. The fractures in massive mudrock occur in various directions (blue arrow), but horizontal fractures are dominant in black shale (yellow arrow). (C) 3111.61 m, well F1. Fractures occur in various directions in massive mudrock. (D) 3472.23 m, well N1. Sampled from the massive mudrock in Fig. 4C. The micro-fractures in high-resolution field emission scanning electron microscopy (FESEM) observation also occur in different directions (reddish arrow). (E) 3652.95e3653.95 m, well L1. Horizontally dominated fractures in black shale. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 1054 J. Zhang et al. / Marine and Petroleum Geology 77 (2016) 1042e1055

Fig. 14. The difference in vertical permeability between black shale succession and the alternative black shale and massive mudrock succession.

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