The nature, type, and origin of diagenetic fluids and their control on the evolving porosity of the Lower Cambrian Xiaoerbulak Formation dolostone, northwestern Tarim Basin, China

Petroleum Science (2020) 17:873–895 https://doi.org/10.1007/s12182-020-00434-0 ORIGINAL PAPER The nature, type, and origin of diagenetic fluids and their control on the evolving porosity of the Lower Cambrian Xiaoerbulak Formation dolostone, northwestern Tarim Basin, China Pei‑Xian Liu1 · Shi‑Biao Deng1 · Ping Guan1 · Yi‑Qiu Jin2 · Kai Wang1 · Yong‑Quan Chen3 Received: 16 March 2019 / Published online: 6 June 2020 © The Author(s) 2020 Abstract The study on Lower Cambrian dolostones in Tarim Basin can improve our understanding of ancient and deeply buried carbonate reservoirs. In this research, diagenetic fluid characteristics and their control on porosity evolution have been revealed by studying the petrography and in situ geochemistry of different dolomites. Three types of diagenetic fluids were identified: (1) Replacive dolomites were deviated from shallow burial dolomitizing fluids, which might probably be concentrated ancient seawater at early stage. (2) Fine-to-medium crystalline, planar-e diamond pore-filling dolomites (Fd1) were likely slowly and sufficiently crystallized from deep-circulating crustal hydrothermal fluids during Devonian. (3) Coarse crystalline, non-planar-a saddle pore-filling dolomites (Fd2) might rapidly and insufficiently crystallize from magmatic hydrothermal fluids during Permian. Early dolomitizing fluids did not increase the porosity, but transformed the primary pores to dissolution pores through dolomitization. Deep-circulating crustal hydrothermal fluids significantly increased porosity in the early stages by dissolving and then slightly decreased the porosity in the late stage due to Fd1 precipitation. Magmatic hydrothermal fluids only precipitated the Fd2 dolomites and slightly decreased the porosity. In summary, Devonian deep-circulating crustal hydrothermal fluids dominated the porosity evolution of the Lower Cambrian dolostone reservoir in the Tarim Basin. Keywords Lower Cambrian · Dolostone reservoir · In situ geochemistry · Diagenetic fluids · Porosity evolution 1 Introduction Dolostone reservoirs are important components of carbonate hydrocarbon reservoirs in many petroliferous basins worldwide (Ehrenberg et al. 2006; Li et al. 2011a, b; Sonnenberg and Pramudito 2009; Sun 1995; Warren 2000; Zhao et al. 2005), but dolomite genesis and the mechanism for generating dolostone reservoirs remain intensely debated (Hardie Edited by Jie Hao * Ping Guan 1 Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China 2 Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China 3 Research Institute of Petroleum Exploration and Development, Tarim Oilfield Branch, PetroChina, Korla 841000, Xinjiang, China 1987; Kirmaci and Akdag 2005; Machel 2004; Morrow 1998; Warren 2000; You et al. 2015). Recent geophysical research found the Lower Cambrian platform margin facies dolostones in the subsurface of the Tabei Uplift (Ni et al. 2015), which made the Lower Cambrian Xiaoerbulak Formation dolostones become potential exploration targets for hydrocarbon reservoirs (Du and Pan 2016; Liu et al. 2017). Limited by scarce well samples, researches on the Lower Cambrian dolostones were mainly conducted in the Sugetbulak outcrop area of the northwestern Tarim Basin, where abundant pores, bitumen, and plentiful pore-filling dolomites were found (Li et al. 2011a, b, 2015; Song et al. 2014). Recent researches show that high-quality reservoirs in the Xiaoerbulak Formation are mainly distributed in the platform margin facies dolograinstones (Li et al. 2015; Song et al. 2014) and a few microbial dolostones (Li et al. 2015; Song et al. 2014). The Lower Cambrian dolostone reservoirs are regarded as one of the most ancient and deeply buried carbonate reservoirs in the world (Li et al. 2016; Pan et al. 2012; Zhang et al. 2014). However, the formation mechanism of the 13 Vol.:(0123456789) 874 high-quality dolostone reservoirs remains unknown. Due to the lack of systematically petrographic and geochemical studies, the origin and nature of different diagenetic fluids remain debated. Previous studies concluded that the genesis of the Lower Cambrian dolostones was controlled by one or a mixture of three types of diagenetic fluids: meteoric water, concentrated seawater or heated formation water, and hydrothermal fluids (Cai et al. 2008; Ji et al. 2013; Li et al. 2011a, b; Pan et al. 2012; Zhang et al. 2011, 2014; Zhu et al. 2010). However, the correlation between different types of dolomites and their forming fluids remain unclear, as well as the origin and nature of these different diagenetic fluids (Zhang et al. 2014). Some studies concluded that the saddle dolomite and recrystallized dolomites were precipitated from magmatic hydrothermal fluid (Chen et al. 2009a; Dong et al. 2013; Pan et al. 2009; Zhao et al. 2012; Zhu et al. 2010). Other studies proposed that these dolomites were precipitated from stratigraphic hydrothermal fluids (Pan et al. 2012), such as the heated formation water from the Cambrian dolomite strata (Qian et al. 2012; Zhang et al. 2009, 2011). Moreover, the influence of the diagenetic fluids on the formation of such a reservoir is also unclear. For example, previous studies (Li et al. 2011a, b; Li et ai. 2016) concluded that primary pores were strongly cemented, and the high-quality dolostone reservoirs were mainly resulted from the corrosion of deep hydrothermal fluids based on the geochemistry property of pore-filling dolomites. However, recent studies have also identified abundant interparticle and intercrystalline pores without any pore-filling dolomites, thus concluding that the primary pores were only slightly cemented and further dissolution occurred during epidiagenesis (Li et al. 2015; Shen et al. 2016). Based on the basin’s sedimentary, tectonic evolution and systematically petrographic and in situ geochemical research on different types of dolomites and reservoir spaces in the dolostones of the Lower Cambrian Xiaoerbulak Formation, this study (1) identified the types, nature, and origin of diagenetic fluids for different dolomites; (2) clarified the influence of different diagenetic fluids on the formation of reservoir spaces; and (3) explained the porosity evolution of the high-quality dolostone reservoir controlled by multiplestage diagenetic fluids. The results of this study can improve our understanding of very ancient and deeply buried carbonate reservoirs. 2 Geological setting The Tarim Basin is the largest basin in China with an area of nearly 560,000 km2 (Wang et al. 2009). The basin is located in northwestern China (Fig. 1a), surrounded by the Tian Shan Mountains and West Kunlun–Altun Mountains 13 Petroleum Science (2020) 17:873–895 in the north and south, respectively (Fig. 1b). The basin has undergone a multiple-stage history of tectonic evolutionary processes, e.g., the Caledonian, Hercynian, Indosinian (...truncated)