Reservoir space type and storage capacity of Oligocene lacustrine carbonate in the Yingxi area of western Qaidam Basin, China

• Qingsong Xia ^[1] ; Chenggang Huang ^[2] ; Jun Cui ^[3] ; Yafeng Li ^[3]
  1. [1] Southwest Petroleum University,Chengdu
  2. [2] Key Laboratory of Reservoir Description of the CNPC
  3. [3] Qinghai Oilfield Research Institute of Exploration & Development, Dunhuang

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• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 34, Nº. 3, 2019, págs. 1077-1094
• Idioma: inglés
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  • Over the past 2 years, a great progress in petroleum and gas exploration has been made in tight lacustrine carbonate rocks of E 3 2 in the Yingxi area. Many high-yield commercial oil flow wells with more than 1000 tons of oil production per day have been found, but their reservoir space type and storage capacity are the key issues highly concerned and disputed by many explorationists. According to the study on systemic petrology and mineralogy, analysis of physical property, study on capillary pressure curve, analysis of ultra-micro crystal morphology, analysis of micro-area geochemical composition, and other comprehensive analysis, and pore types of the E 3 2 reservoir are mainly intercrystalline pores of dolomite, and a small number of dissolution phenomena have been found. Therefore, large-scale oil and gas reservoirs can be formed. The analysis involves these key results and evidences: (1) under the microscope and scanning electron microscope, many intercrystalline pores are developed in dolomite rocks, presenting a “high-wide platform” phenomenon on the capillary pressure curves to imply “small bore—large quantity”. (2) The electron probe shows that the molecular formula of dolomite is Ca(Mg0.96Fe0.04)(CO3)2. In the penecontemporaneous dolomitization process, intercrystalline pores come from volume shrinkage when Mg2+ ions replace Ca2+ ions, and a small number of Fe2+ ions are involved in this chemical reaction. The 13.36% plane porosity can be formed in ideal conditions. (3) According to the statistical data, intercrystalline pores are enlarged by organic acids in several samples, no evidence is available for leaching dissolution of atmospheric water because of exposure, and many types of feldspar do not dissolve. The reservoir rocks are formed due to a high deposition rate. (4) According to the measured pore sizes and the values based on the chemical theory, the pore size is in the range of 0.4–1.07 μm. These pores can store all oil, gas, and symbiosis substances, which can flow or migrate in the pores because of the pore sizes. The pictures of fluorescence thin sections and the backscattered electron images of SEM show that these micro-pores are full of hydrocarbons, creating the oil and gas accumulation pattern of “containing oil for the whole sag” and “producing oil for each well” in the Yingxi area. Based on the oil and gas accumulation model, the Yingxi area is an integrated oil and gas field with the most potential and industrial value in Qaidam Basin. The research results are crucial to the petroleum and gas exploration of carbonates in a saline lacustrine basin.