Characteristics of porosity and permeability layer of fossil Halimeda reef mineral rock of Miocene in the Xisha Islands and its genetic model XU Hong 1 *, ZHU Yurui 2 , EBERLI G. P. 3 , LUO Wei 4 , ZHAO Xinwei 5 , CAI Ying 1 , LIU Xinyu 4 , YAN Guijing 1 , ZHANG Bolin 1 , WEI Kai 1 , CUI Ruyong 1 1 Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Qingdao Institute of Marine Geology, Ministry of Land and Resources, Qingdao 266071, China 2 Chengdu University of Technology, Chengdu 610059, China 3 Comparative Sedimentology Laboratory, University of Miami, Miami 33149, USA 4 Zhanjiang Branch of China National Offshore Oil Corporation (CNOOC) Limited, Zhanjiang 524057, China 5 China University of Geosciences, Beijing100083, China Received 21 June 2014; accepted 30 September 2014 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2015 Abstract Halimeda is one of the major reef-building algas in the middle Miocene of Xisha, and one of the significant reef- building algas in the algal reef oil and gas field of the South China Sea. However, there have been few reports regarding the characteristics of mineral rocks, reservoir porosity and permeability layers, and sedimentation- diagenetic-evolution of fossil Halimeda systems. The present paper briefly introduces the relevant studies on chlorophyta Halimeda and the research status of oil and gas exploration. Through the 1 043 m core of the Xichen- 1 well, we studied the characteristics of the mineral rocks and porosity and permeability of the middle Miocene Halimeda of the Yongle Atoll, identified and described the segments of fossil Halimeda, and pointed out that most of the segment slides are vertical sections in ovular, irregular or long strips. The overwhelming majority of these fossil Halimeda found and studied are vertical sections instead of cross sections. In this paper, knowledge regarding the cross sections of fossil Halimeda is reported and proven to be similar with the microscopic characteristics of modern living Halimeda; fossil Halimeda are buried in superposition; it is shown that there are different structures present, including typical bio-segment structure, and due to its feature of coexisting with red alga, tying structure, twining structure and encrusting structure are all present; and finally, it is suggested to classify the fossil Halimeda into segment algal reef dolomites. In addition, all of the studied intervals are moderately dolomitized. Secondary microcrystalline-dolosparite dominates the original aragonite raphide zones, and aphanitic-micrite dolomite plays the leading role in the cortexes and medullas; in the aragonite raphide zones between medulla and cysts, secondary dissolved pores and intercrystalline pores are formed inside the segments, and algal frame holes are formed between segments; therefore, a pore space network system (dissolved pores + intragranular dissolved pores—intercrystalline pores + algal frame holes) is established. Segment Halimeda dolomite has a porosity of 16.2%–46.1%, a permeability of 0.203×10 –3 –2 641×10 –3 μm 2 , and a throat radius of 23.42–90.43 μm, therefore it is shown to be a good oil and gas reservoir. For the reasons mentioned above, we suggest building the neogene organic reef-modern reef sedimentation-diagenetic-evolution models for the Xisha Islands. Key words: Xisha Islands, Miocene, fossil Halimeda, segment dolostone, reservoir evolution model Citation: Xu Hong, Zhu Yurui, Eberli G. P., Luo Wei, Zhao Xinwei, Cai Ying, Liu Xinyu, Yan Guijing, Zhang Bolin, Wei Kai, Cui Ruyong. 2015. Characteristics of porosity and permeability layer of fossil Halimeda reef mineral rock of Miocene in the Xisha Islands and its genetic model. Acta Oceanologica Sinica, 34(4): 74–83, doi: 10.1007/s13131-015-0638-3 1  Introduction As a type of plant, alga has been doubted, or even denied, to have a reef-building function (Zhang et al., 1989); however, since 1985, eight organic reef oil fields have been found in the Pearl River Mouth Basin of the northern South China Sea, such as Huizhou 33-1 and Liuhua 11-1. We found also that there were large numbers of reef-building coral red algas, rather than reef- building corals, in these organic reef oil fields (Chen and Hu, 1987; Xu, 1992), which reminded us that it was necessary to reex- amine the contribution of alga to the construction of organic reefs, and begin to pay attention to and study coral red alga and its reef-building characteristics (Wang et al., 1996; Cai et al., 1996; Zhu et al., 1997; Fan, 1996; Xu et al., 1999a, b, 2001; Wang, 2001; Wu et al., 2010, 2011; Hu et al., 2010); during this period, large amounts of Chlorophyta Halimeda coexisting or associated with reef-building red alga were found in middle-late Miocene inter- Acta Oceanol. Sin., 2015, Vol. 34, No. 4, P. 74–83 DOI: 10.1007/s13131-015-0638-3 http://www.hyxb.org.cn E-mail: hyxbe@263.net Foundation item: The National Science and Technology Major Project for Large Oil-Gas Fields and Coal-formed Gas Development under contract Nos 2008ZX05023 and 2011ZX05025-002; the National Natural Science Foundation of China under contract Nos 49206061 and 41106064; the National Basic Research Program (973 Program) of China under contract Nos 2012CB956004 and 2009CB219406. *Corresponding author, E-mail: qdxhong@163.com