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The Shunnan area is a newly discovered hydrocarbon field in Shuntuoguole lower uplift, in which the host rocks are Lower–Middle Ordovician carbonates. The carbonate reservoirs are well affected by hydrothermal silicification with well‐preserved intercrystalline pore of replacement quartz and megaquartz cements. Based on the petrography, pore types, and diagenetic processes, this research evaluated the reservoir quality and re‐constructed the porosity evolution in different diagenetic stages. Samples are collected from 7‐cored wells distributed along NNE‐ or ENE‐trending sinistral strike‐slip faults. Nine types of lithofacies were identified in the study area, including mudstone, bioclast wackestone, peloidal bioclast packstone, boundstone, peloidal grainstone, ooid grainstone, slightly silicified limestone (silica less than 25%), strongly silicified limestone (silica around 50%), and chert. Chert has better porosity (average 21.3%) and permeability (average 44.7 mD) than other lithofacies. The main reservoir pores are mainly developed in forms of fractures, intercrystalline replacement quartz and dissolved vugs. The porosity evolution shows that the primary porosity was limited due to cementation and compaction, and the earlier secondary porosity was occlusive as a result of coarse calcite cementation. Therefore, the precursor limestones may be tight, and the reservoir quality of the Lower–Middle Ordovician carbonates was controlled by hydrothermal silicification. During the hydrothermal silicification event, the burial depth of the Lower–Middle Ordovician carbonates was more than 3,000 m and their stratum temperature was higher than 120°C, representing a deep burial condition. The statistical data of porosity‐depth/temperature trends for carbonates suggest that the early secondary pores would be significantly reduced when the burial temperature reaches 100°C (or depth of strata reaches 3,000 m). Therefore, the dissolved vugs in strongly silicified limestone or hydrothermal chert are caused by hydrothermal silicification and not inherited from precursor carbonate rocks. [ABSTRACT FROM AUTHOR]

In this paper, the authors have summarized and analyzed the petrology and isotope and trace element characteristics of host rocks and calcite cements in the Yingshan and Yijianfang formations of Lower-Middle Ordovician in the Shunbei area of the Tarim Basin. The result successfully reveals four types of limestones and four types of calcite cements, such as the sparry grainstone (SG), the microcrystalline grainstone (MG), the micrite (MC), the silicified limestone (SL), the calcite coexisting with silicon in SL (C1), the calcite which develops in fractures or between the broken siliceous breccias (C2), the calcite which fills between the broken particles (C3), and the calcite filling the fractures of all the host rocks and cements (C4). The high U/Th and Sr/Ba ratios of all the calcite cements indicate that the fluids that formed them were of comparatively higher salinity in burial environments, and the differentiation of their Sr-isotopes, Fe, and Mn contents clearly shows two types of diagenetic fluids: the fluid forming C2 and C3 is the inherited seawater closely related to the host rocks, and the fluid forming C1 and C4 is the silica-rich hydrothermal fluid from the lower strata and flowed through the possible clastic strata. [ABSTRACT FROM AUTHOR]