Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (1): 132-143, 200.doi: 10.11743/ogg20200112

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Pattern of hydrothermal dolomitization in the Middle Permian Maokou Formation, eastern Sichuan Basin, and its alteration on reservoirs herein

Tao Zhang1(), Juanhua Lin1, Yueqing Han1, Zeyu Wang2, Jun Qin3, Rongqiang Zhang1   

  1. 1. Petroleum Exploration & Production Research Institute, SINOPEC, Beijing 100083, China
    2. School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou, Guangdong 519000, China
    3. Jianghan Oilfield Branch Company, SINOPEC, Wuhan, Hubei 430223, China
  • Received:2019-10-12 Online:2020-02-01 Published:2020-01-19
  • Supported by:
    国家自然科学基金项目(U1663209);中国科学院A类战略性先导项目(XDA14010201)

Abstract:

Based on the distribution of abnormal dolomites in the Maokou Formation, eastern Sichuan Basin, we analyzed the identification markers and timing of hydrothermal dolomitization (HTD), and discussed its genetic model and reservoir modification by means of petrological description of cores, geochemical testing, as well as comprehensive interpretation of seismic and logging data. The results indicate that the main identification markers include irregular dolomite distribution, medium-to-coarse crystalline matrix dolomite, saddle dolomite and quartz-saddle dolomite in the earlier tensile fractures and cavens. The homogenization temperature of dolomite fluid inclusions(115-145℃)being higher than the host formations, more negative oxygen isotope (δ18O(VPDB)-5.4‰-7.1‰), and increased strontium isotope (87Sr/86Sr 0.707 947-0.708 458) are the main geochemical indicators of hydrothermal activities. HTD mainly occurred near tensional or transtentional fault systems, and its stratification is controlled by original sedimentary facies. The limestone formations near the fracture/fault zones were also altered by hydrothermal fluids. The main stage of HTD alteration is the Late Permian-Middle Triassic after the shallower burial and karstification via uplifting and explosion of the Maokou Formation and the deposition of the Wujiaping Formation. The eruption of Emei basalt served as heat source for hydrothermal fluids, and the factors such as faults, sedimentary facies and the intrusion of hydrothermal fluids are coupled in time and space, resulting in the development and various distribution patterns of hydrothermal dolomites. The effect of HTD on reservoir modification is complex.The newly-generated pore space is not remarkable in lower-permeability limestones under HTD, while the high-energy beach facies with initial pores may preserve certain porosity and permeability under HTD.

Key words: hydrothermal dolomitization(HTD), identification marker, reservoir modification, Maokou Formation, Sichuan Basin

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