Sedimentary characteristics and origin of lacustrine organic-rich shales in the salinized Eocene Dongying Depression
Lacustrine organic-rich shales are well developed within the Eocene Dongying Depression in the Bohai Bay Basin in eastern China and across Southeast Asia. Understanding the sedimentation of these shales is essential to the study of depositional processes, paleoenviron ment, and paleoclimate reconstruction. This study investigates the sedimentary characteristics and formation mechanisms of lacustrine shales in the upper fourth member of the Eocene Shahejie Formation (Es4s) within the Dongying Depression based on thin sections and field emission scanning electron microscope (FESEM) observations of well cores combined with X-ray diffraction and geochemical indicators. Six litho facies were identified: (1) laminated calcareous mudstone, (2) laminated dolomitic mudstone, (3) laminated clay mudstone, (4) laminated gypsum mudstone, (5) massive mudstone, and (6) siltstone. The organic matter in the Es4s shale is mainly type I and type II kerogens, as well as a small proportion of type III kerogen. On the basis of lithofacies associations, paleosalinity values, redox properties, and terrigenous inputs, the lower Es4s shale can be divided into six intervals from bottom to top, numbered I, II, III, IV, V, and VI. The thickness of each interval ranges from several meters to more than 10  m, reflecting highfrequency oscillations in the environment of the lake basin, markedly different from a relatively stable marine environment. The laminated mudstones are characterized by fine grain sizes, scarce large terrigenous debris (quartz and feldspar), and compositions that are rich in pyrite and sapropelic organic matter. These features indicate that these lithofacies were deposited out of suspension in a quiet water body characterized by a relatively low rate of deposition. The characteristic laminae of these litho facies indicate subtle differences in depositional processes. The laminated gypsum mudstone was likely deposited in an evaporative environment, because its formation would have consumed Ca2+ and SO4, promoting the depo si tion of a laminated dolomitic mudstone. In contrast, laminated clay mudstone was deposited in a manner that increased the volume of small terrigenous materials. Deposi tion of this lithofacies was controlled by the nature of the water body, paleoclimate, and terrigenous inputs. Laminated mudstones are dominant in the lower Es4s shale, suggesting that suspension was the main depositional process leading to formation of the lower Es4s shale. In contrast, the massive mudstones were likely rapidly deposited asso ciated with siltstone as the result of finegrained turbidites. The lower Es4s shale was formed in a depositional environment composed of a saline, medium-depth lake under anoxic conditions, with limited terrigenous inputs. The depositional process included suspension and turbidity currents. The high salinity is suggested to be related to a marine transgression, which may have been facilitated by a rise in sea level caused by global warming in the early Eocene, together with the large-scale tectonic activity of East Asia. Seawater input affected the lithofacies, influenced lake water body conditions, triggered turbidity currents, and prompted the accumulation of organic matter. The deposition of the Es4s shale in the Dongying Depression may help us to understand the deposition of lacustrine shale, paleoclimate reconstructions for the Eocene, and the tectonic activity of East Asia. INTRODUCTION Fine‐grained sedimentary rocks mostly contain grains that are smaller than 62.5  mm and comprise approximately two thirds of the stratigraphic record (Aplin et  al., 1999; Stow and Mayall, 2000; Tucker, 2001; Aplin and Macquaker, 2011). Of these rocks, mudstones consist of a variable mixture of clay minerals, quartz, feldspars, carbonates, sulfides, amor‐ phous material, and organic matter (Macquaker and Adams, 2003; Potter et al., 2005; Milliken, 2014). Organic‐rich mudstones, in particular, act as important petroleum sources, reservoirs for shale oil and gas, and seals in conventional reservoirs (Schieber, 1999; Andersson and Wor‐ den, 2004; Bowker, 2007; Jarvie et  al., 2007; Abouelresh and Slatt, 2012). Because of its ap‐ parent homogeneity and the limitations of ultra‐ microscopic experimental equipment, shale has been often overlooked in sedimentological stud‐ ies over recent decades (Arthur and Sageman, 1994; Schieber, 1999; Potter et al., 2005; Jiang et al., 2013). However, because of extensive and successful deep‐water hydrocarbon explora‐ tion, especially for shale oil and gas (Hill et al., 2007; Loucks and Ruppel, 2007; Slatt, 2007; Kuang et  al., 2012), deep‐water sedimentation has become the focus of considerable atten‐ tion, including the depositional environments, transport, and depositional processes associ‐ ated with organic matter preservation in black shales ( Macaquaker et  al., 2007, 2010b; Piper and Calvert, 2009; Aplin and Macquaker, 2011; Konitzer et al., 2014; Liang et al., 2016). The depositional processes leading to black shales mostly involve pelagic and hemi pelagic settings, turbidity currents, debris flows, slides, and wave‐enhanced sediment gravity flows (Stow and Bowen, 1980; Stow and Mayall, 2000; Soyinka and Slatt, 2008; Bouma and Stone, 2000; Macquaker et al., 2010a; Ghadeer GSA Bulletin; January/February 2018; v. 130; no. 1/2; p. 154–174; https://doi.org/10.1130/B31584.1; 16 figures; 3 tables; published online 29 August 2017. liangchao0318@163 .com For permission to copy, contact editing@geosociety.org © 2017 Geological Society of America Sedimentary characteristics and origin of lacustrine organic-rich shale Geological Society of America Bulletin, v. 130, no. 1/2 155 and Macquaker, 2011; Ochoa et al., 2013; Plint, 2014). One or more of these processes may con‐ tribute to the deposition of a black shale, sug‐ gesting that mud may not always be the product of “low‐energy” or “quiet‐water” deposition (Cheel, 1990; Konitzer et  al., 2014; Frébourg et  al., 2016). Indeed, flocculation processes contribute to the long‐distance transport of large amounts of muddy marine sediments (Schieber et al., 2007; Macquaker et al., 2010a; Aplin and Macquaker, 2011), and flume experiments have shown that flocs can be present as a hydraulic equivalent in coarse clastics, transported by high‐density flows and turbidity currents on the seabed (Schieber and Southard, 2009; Schieber et al., 2010). Most studies to date, however, have focused on marine shales, and many of these are siliciclastic. In contrast, research on lacustrine organic‐rich shales has been rarely reported, particularly carbonate‐rich examples (Burton et al., 2014; Liu et al., 2015; Roop et al., 2015). While the deposition of marine siliciclastic shale mainly results from physical and chemi‐ cal processes, carbonate‐rich shales are instead predominantly biochemical in origin (Schieber et al., 2007; Macquaker et al., 2010b; Aplin and Macquaker, 2011; Jiang, 2011). Such lacustrine carbonate‐rich shales are widely developed in Mesozoic and Cenozoic terrestrial strata across Southeast Asia (Butterworth and Wain, 1995; Tänavsuu‐Milkeviciene and Sarg, 2012; Xu et  al., 2014), but obvious differences between them and marine siliciclastic shales include the structural backgrounds of the basins, material compositions and sources, sedimentary environ‐ ments, and depositional processes (Jiang et al., 2013; Burton et al., 2014). Recently, due to increasing demand for un‐ conventional oil and gas exploration, especially shale oil and gas in China (Song et  al., 2013; Wang et al., 2013; Zou et al., 2013), the Sinopec Shengli Oilfield company initiated research aimed at the exploration of lacustrine shale oil in the Dongying Depression of the Bohai Bay Ba‐ sin. The study intervals targeted in this research were 200  m sequences within the Paleogene lower Es4s black shale, which is predominantly composed of carbonate and clay minerals. The sedimentary characteristics and mechanisms of formation of the black shale are discussed in detail in this paper. Here, we characterize the lithofacies of lacustrine carbonate‐rich black shales within the Es4s unit, analyze the deposi‐ tional environments and processes, and discuss the salinization of lake water and accumula‐ tion of organic matter. This study will help in understanding lacustrine shale deposition and comparisons with marine shale, paleoclimate reconstructions, tectonic activity, and deposi‐ tional responses in Eocene East Asia. GEOLOGICAL SETTING Bohai Bay Basin, a fault depression basin developed in the Paleozoic North China craton basement (Fig. 1A), is located in eastern China and is filled by Mesozoic–Cenozoic strata. The formation of the Bohai Bay Basin was con‐ trolled by the evolution of the tectonic frame‐ work in East Asia. During the later Cretaceous– Paleocene, the Indian plate and the Pacific plate subducted under the Eurasian plate, from south to north, and from east to west, respectively (Li et  al., 2013; Ding et  al., 2014). At ca. 55 Ma, the rate of Pacific plate movement decreased and resulted in strong retreating extension in the eastern part of East Asia, forming a series of sedimentary basins (Zhang et al., 2014; Zhao and Li, 2016). Bohai Bay Basin was formed under this framework, during which a series of half‐graben lake basins formed, including the Dongying Depression. The Dongying Depression is located in the southeastern Bohai Bay Basin (Fig.  1B). This depression is 5700 km2 in total area, bordered by the Luxi and Guangrao uplifts to the south, the Chenjiazhuang uplift to the north, the Linfanjia and Gaoqing uplifts to the west, the Qingtuozi uplift to the northeast, and the Wangjiagang fault belt to the southeast. The Dongying De‐ pression includes four sags, the Lijin, Boxing, Niuzhuang, and Minfeng (Fig. 1C). The Dongying Depression is a typical half‐ graben lake basin, with a steep slope in the north and west controlled by faults and a gentle slope in the sou