Research paperCharacteristics and formation mechanism of multi-source mixed sedimentary rocks in a saline lake, a case study of the Permian Lucaogou Formation in the Jimusaer Sag, northwest China
Introduction
The term mixed sediment normally refers to mixtures of siliciclastic materials and carbonates. Such mixtures form during deposition, either in one layer or as interfingering layers (Holmes, 1983; Tirsgaard, 1996). When such mixtures were first studied, it was assumed that carbonate production is reduced by an influx of siliciclastic material and that, for the most part, the two sediments do not deposit together. Later studies refined this assumption and described more complex mixed systems (e.g. Bruckner, 1953; Button and Vos, 1977; McIIreath and Ginsburg, 1982; Zhang and Ye, 1989; Brooks et al., 2003). Mixed siliciclastic-carbonate systems frequently develop on shelves and in platform settings, in both modern and ancient deposits (Dorsey and Kidwell, 1999; Testa and Bosence, 1999; Purdy and Gischler, 2003; Page, 2006; Gischler et al., 2010). The distribution of siliciclastics and of carbonates is governed by different processes, and as a result, the mixtures of these vary across the depositional environments. The siliciclastic sediment is introduced into the system predominantly by rivers, while the carbonate sediment production takes place in situ in shallow marine or lacustrine environments (Caracciolo et al., 2013; Accordi and Carbone, 2016). The carbonate production is controlled by a variety of physical, chemical and biological processes, which vary throughout the shallow marine and lake environments (Choi and Ginsburg, 1982; Flood and Orme, 1988; Pilkey et al., 1988).
Mount (1984) identified four sedimentary mixing processes on rimmed carbonate platforms by studying mixed siliciclastic and carbonate sediments in shallow shelf environments of more than 150 modern and ancient examples. He divided the mixing processes into four general categories: punctuated mixing, facies mixing, in situ mixing, and source mixing. This classification has been widely used in subsequent research (e.g. Li, 2008; Zhao et al., 2013). Following this model, different mixing sedimentation types were proposed based on the characteristics in different areas (Dorsey and Kidwell, 1999; Zhang, 2000; Wang, 2001; Dong et al., 2007, 2009; Ding et al., 2013). Various depositional architecture models of mixed siliciclastic-carbonate rocks have been developed (Mack and James, 1986; Dorsey and Kidwell, 1999; Wright et al., 2005; GarcÍa-Hidalgo et al., 2007; Reis and Suss, 2016) that describe vertical and lateral facies assemblages and cyclic sequences of mixed sedimentation. These changes are caused by varying sedimentation and subsidence rates, eustatic sea level changes and climate changes (Yose and Heller, 1989; Einsele, 1996; Wright et al., 2005). Compared with shallow marine mixed siliciclastic-carbonate sediments, lacustrine mixed sedimentary systems have been studied less well, especially saline lacustrine environments. Existing studies mainly focus on the reservoir characteristics of lacustrine mixed rocks (Palermo et al., 2008; Feng et al., 2013), rarely on the mixing characteristics and formation mechanisms. In terms of lithology, the carbonate components in mixed sedimentary systems in previous studies (Orpin et al., 2004; Isaack et al., 2016; Lubeseder et al., 2009; Yose and Heller, 1989) are mainly limestone (bioclastic packstone/wackestone/grainstone, micrite, oolites, marl and marginal reef in modern sediments), while mixed siliciclastic-dolomitic sequences have been paid less attention before (Escalona and Abud, 1989; Wu et al., 2016).
The rocks of the Lucaogou Formation were deposited in a saline lacustrine siliciclastic-dolomitic mixed system (Wu et al., 2016), where in addition large amounts of volcanic material were deposited, making the mixed system more complicated. In this setting, the syngenetic and penecontemporaneous dolomicrite was produced in a saline lacustrine basin, mixed with terrigenous clastic sand, silt, and with transported tuffaceous materials, and thus formed a mixed system of three end-member components.
Previous studies indicate that the Lucaogou Formation was deposited mainly in shallow near-shore to semi-deep lacustrine environments and partially in a delta front environment (Cao et al., 2016; Wu et al., 2016; Qiu et al., 2016). The main lithologies are dark mudstones, fine sandstones, siltstones, dolomicrite, dolarenite, limestone, tuff and some transitional lithologies (dolomitic siltstone, dolomitic mudstone, calcareous dolomite, etc.) (Si et al., 2013; Jiang et al., 2015).
In this study, the petrological characteristics of the Lucaogou Formation mixed rocks are discussed systematically and a previously undocumented type of mixed sedimentary rocks (sliciclastic-carbonate-tuffaceous) is introduced by taking the Lucaogou Formation as an example. A combination of petrological, mineralogical and granulometry analysis was employed to systematically investigate the classification system, lithological assemblages, lateral and vertical distribution characteristics, mixing processes and mixed depositional models.
Section snippets
Geological setting
The Junggar Basin, located in northwestern China, is a large intracontinental superimposed basin. It has experienced multi-stage tectonic movements, including, from oldest to youngest, Hercynian, Indosinian, Yanshanian and Himalayan movements. In the southeast of the basin lies the Jimusaer Sag, covering an area of 1278 km2 (Fig. 1a). This sag is a dustpan-shaped depression, which is bounded by a steep fault in the west and more gently dipping faults around the rest of the basin (Fig. 2). The
Samples and experiments
A total of 89 core samples from nine wells in the Jimusaer Sag have been investigated in detail (Fig. 1), among which 48 samples cover the lower section of the Middle Permian Lucaogou Formation, and 41 samples cover the upper section. Petrological and mineralogical analyses of the samples were carried out at the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum. The analyses include core and thin section observation, X-ray diffraction mineralogical
Classification of lithologies
Core and thin-section observations show that the lithologies in the Lucaogou Formation vary frequently at millimeter scale or even at micrometer scale (Fig. 5, Fig. 6). Therefore, the lithological characteristics are very complicated. The mixed sedimentary rocks here have been classified into three categories: terrigenous-dominated mixed rocks, carbonate-dominated mixed rocks, and tuffaceous-dominated rocks. They are further divided into nine types according to the terminology above (Fig. 4).
Mixing processes of three-component mixed rocks
The four mixing processes defined by Mount (1984) for two-component systems can also be recognized in the three-component mixing system of the Lucaogou Formation, but they are expanded here to account for the addition of volcanic materials.
Conclusions
The Lucaogou Formation in the Jimusaer Sag, Junggar Basin is a set of siliciclastic-carbonate-tuffaceous mixed rocks deposited in a saline lake. This new type of mixed rocks has been classified into three categories based on core description, thin section description, and grain size measurements, with nine subtypes according to the proportion of the three end-member components of terrigenous clastics, carbonates, and volcanic material. More than 20 lithologies have been recognized. The main
Acknowledgments
This study was supported by the National Natural Science Foundation of China, China Project entitled “Effectiveness of micro-nano pore throat system to oil charging in tight sandstone and its control on oil accumulation” (Project No. 41472114), and the National Science and Technology Major Project, China “The Accumulation Law and Key Technologies for Exploration and Exploitation of Tight Oil” (Grant No:2016ZX05046). We also would like to thank Xinjiang Oilfield for providing the core samples
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