Open Geosciences 2020; 12: 1105–1115 Research Article Shutong Li*, Shixiang Li, Xinping Zhou, Xiaofeng Ma, Ruiliang Guo*, Jiaqiang Zhang, and Junlin Chen The genesis model of carbonate cementation in the tight oil reservoir: A case of Chang 6 oil layers of the Upper Triassic Yanchang Formation in the western Jiyuan area, Ordos Basin, China https://doi.org/10.1515/geo-2020-0123 The development degree of carbonate cementation affects received April 17, 2020; accepted September 21, 2020 the physical properties of reservoir. Abstract: Carbonate cementation is one of the significant Keywords: carbonate cementation, Yanchang formation, tightness factors in Chang 6 reservoir of the western genesis model, Ordos Basin, tight oil reservoir Jiyuan (WJY) area. Based on the observation of core and thin sections, connecting-well profile analysis as well as carbon and oxygen isotope analysis, it is found that fer- rocalcite is the main carbonate cements in the Chang 6 1 Introduction reservoir of the WJY area. The single sand body controls fi the development of carbonate cements macroscopically. Carbonate cementation is a signi cant diagenesis type in [ – ] - Both carbonate cements and calcite veins hold similar the clastic reservoir 1 3 . It is the product of the interac fl diagenetic conditions: the dissolution of plagioclase is tion between rock and geological uid in the diagenesis - the main calcium source and the de-acidification of or- process under the changes of physical and chemical con [ – ] - ganic acids is the main carbon source. The diagenetic ditions such as temperature and pressure 1,4 6 . The con stage is identified as the mesogenetic A stage. The sedi- tent, source, and occurrence state of carbonate cements mentary environment is of low salinity. Accordingly, the have a great impact on the migration and accumulation development model of carbonate cementation in Chang 6 of oil and gas, as well as the reservoir physical properties [ – ] reservoir is summarized into three types: “eggshell pat- 7 9 . In previous studies, it has been found that the - tern,”“cutting pattern,” and “favorable reservoir pattern.” carbonate cementation is generally intense in thin sand stone layers which are adjacent to mudstones, and in the sandstone reservoir near the sandstone/mudstone con- tacts, which is interpreted that mudstone and/or source * Corresponding author: Shutong Li, Key Laboratory of Petroleum rock interlayers as carbon and ion sources transporting Resources, Gansu Province. Northwest Institute of Eco-Environment Ca2+,Mg2+, and Fe2+ to the adjacent sandstone reservoir and Resources, Chinese Academy of Sciences, Lanzhou 730000, under the condition of compaction and hydrocarbon gen- - Gansu, China, e mail: [email protected] [ – ] * Corresponding author: Ruiliang Guo, Shaanxi Key Laboratory of eration overpressure 5,6,10 13 . However, the genetic Petroleum Accumulation Geology, Xi’an Shiyou University, Xi’an model of carbonate cementation in the tight sandstone 710065, China; School of Earth Sciences and Engineering, Xi’an reservoir behind this phenomenon is rarely studied and Shiyou University, Xi’an 710065, China; University of Chinese discussed thoroughly and deeply. Carbonate cementation Academy of Sciences, Beijing 100049, China, is widely developed in Chang 6 tight oil reservoir in the e-mail: [email protected] western Jiyuan area (WJY area hereafter), Ordos Basin, Shixiang Li, Xinping Zhou: PetroChina Changqing Oil Field Company, Xi’an 710018, China China, which is one of the important factors leading to Xiaofeng Ma: Key Laboratory of Petroleum Resources, Gansu the reservoir tightness [14–16]. Meanwhile, the high con- Province. Northwest Institute of Eco-Environment and Resources, tent of carbonate cements is also found in the sandstone Chinese Academy of Sciences, Lanzhou 730000, Gansu, China layer near the boundary of sandstone and mudstone [17]. Jiaqiang Zhang, Junlin Chen: Key Laboratory of Petroleum Previous relevant studies mainly focused on the distribu- Resources, Gansu Province. Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, tion and source of carbonate cements in tight sandstone Gansu, China; University of Chinese Academy of Sciences, Beijing reservoirs, but there is a deficiency in the genesis pattern 100049, China of carbonate cements and development model of a Open Access. © 2020 Shutong Li et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 1106 Shutong Li et al. favorable reservoir [5,6,11–13]. Also, the related studies 2 Geological setting mainly focused on the impact of diagenesis on the carbo- nate cementation, while the research on the influence of Ordos Basin is a typical multi-cycle craton basin with a sedimentation factors on the forming and development long-term stable sedimentation process located in central pattern of carbonate cements is insufficient. Hence, based and western China [21–23](Figure 1a). The inner struc- on the characteristics of sand body development, the ture of the basin is relatively simple, while faults and microscopic observation of carbonate cementation, the folds are relatively developed in the margin [20].TheWJY analysis of carbon and calcium source [18,19], the ana- area is located in the northwestern Ordos Basin, spanning lysis of paleo-temperature and paleo-salinity [14,20], the three first-level tectonic units of West margin thrust belt, exploratory studies have been conducted about the ge- Tianhuan Depression, and Shanbei Slop (Figure 1b).The netic model of carbonate cements in the WJY area to Upper Triassic Yanchang formation is a set of terrigenous provide a scientific basis for the next step exploration clastic rock depositional systems with fluvial and lacustrine work and reference for the relevant study with similar facies formed in the process of basin subsidence [24,25].It geological features. can be divided into ten reservoir groups, and Chang 6 is the Figure 1: (a) Location of the Ordos Basin in China. (b) Structural division of the Ordos Basin. (c) Lithology section of the Upper Triassic Yanchang Formation in the Ordos Basin, Ch-Chang. Modified after Guo et al. (2020). The genesis model of carbonate cementation in the tight oil reservoir 1107 first lacustrine recession period after Chang 7 reaches 4 Results the maximum lacustrine flooding period [26](Figure 1c). The sediments of Chang 6 in the WJY area are mainly 4.1 Characteristics of Chang 6 sand body interbedded deposition of fine sandstone, siltstone, pelitic siltstone, and mudstone, which belongs to the delta front sub-facies [14]. The sedimentary characteristics of sand body control the physical properties and diagenetic features of the reser- voir macroscopically. In this study, four connecting-well profiles perpendicular to the source supply direction and 3 Samples and methods three profiles parallel to the source supply direction were established to analyze the distribution characteristics of First, based on the observation of 115 cores and compre- Chang 6 sand body in the WJY area. The profiles show hensive interpretation of 78 single well logging curves, we that delta front subfacies are mainly developed in Chang have established four connecting-well profiles perpendi- 6 tight reservoir of the WJY area, and the delta plain fi - cular to the source supply direction and three pro les par subfacies can be partly found in the northwest of the - allel to the source supply direction to analyze the sedimen study area. The underwater distributary channel sand tary characteristics of Chang 6 sand body. The well logging body and the river distributary channel sand body are curves of Gamma ray and spontaneous potential were mainly developed in the delta front sub-facies and delta used in the profiles to identify reservoir lithology. plain sub-facies, respectively (Figure 2a and b). The river Second, utilizing the axioskop 40 polarizing micro- mouth bar is barely developed because of the near-source scope manufactured by Zeiss and the Quanta FEG 450 sedimentary environment [24], where the river carrier is scanning electron microscope (SEM), 62 samples from 43 regarded as the “hyper-pycnal flow” relative to lake water wells were collected for microscopic identification in the and continues to advance toward the lake surface with Key Laboratory of Petroleum Resources Research, Institute ff of Geology and Geophysics, Chinese Academy of Sciences. little lateral di usion. At the same time, the lake surface - By observing 152 thin sections, which were impregnated is prone to oscillation because of the strong hydrody [ ] with blue epoxy resin to highlight the pore spaces and namic force 24 , and the bottom of the lake basin in some of them were stained with Alizarin Red S and the WJY area is relatively gentle [14]. Therefore, the K-ferricyanide for carbonate mineral identification, we sand body is vertically discontinuous and is separated analyzed the reservoir mineral type and content, the by a large number of clay interlayers (Figure 2a and b). degree of sorting and roundness, pore types and diagen- The deposit thickness of the single-layer sand body is esis characteristics. The specific content of diagenetic relatively thin and heterogeneous (mainly 2–15 m).In minerals and cements was quantified by counting at least addition, the sand body has good plane continuity and 350 points of each thin section using an optical collection obvious progradation process characteristics (Figure 2a system. Also, the porosity and