Oil & Gas Geology ›› 2025, Vol. 46 ›› Issue (1): 178-191.doi: 10.11743/ogg20250113

• Petroleum Geology • Previous Articles     Next Articles

Origin of calcium-rich formation water and its implications for natural gas migration and accumulation in the 2nd member of the Jurassic Shaximiao Formation, central Sichuan Basin

Yueming YANG1(), Maoyun WANG2,3(), Changjiang WU1, Jianhui ZENG2,3, Ke PAN1, Huanle ZHANG4, Xiaojuan WANG1, Dongxia CHEN2,3, Huwang CUI2,3   

  1. 1.Research Institute of Exploration and Development,Southwest Oil & Gasfield Company,PetroChina,Chengdu,Sichuan 610041,China
    2.State Key Laboratory of Petroleum Resources and Engineering,China University of Petroleum (Beijing),Beijing 102249,China
    3.College of Geosciences,China University of Petroleum (Beijing),Beijing 102249,China
    4.Research Institute of Exploration and Development,Xinjiang Oilfield Company,PetroChina,Karamay,Xinjiang 834000,China
  • Received:2024-09-09 Revised:2024-12-06 Online:2025-02-28 Published:2025-03-03
  • Contact: Maoyun WANG E-mail:yangym@petrochina.com.cn;wmy950904@163.com

Abstract:

Using data of the geochemistry, gas saturation, and rock and mineral compositions of formation water extracted from 34 wells, we discuss the origin of calcium-rich formation water in the 2nd member of the Jurassic Shaximiao Formation (hereafter referred to as the Sha 2 Member) in the central Sichuan Basin, and accordingly explore the implications of the formation water for natural gas migration and accumulation of tight-sand gas reservoirs with source-reservoir separated. The results indicate that the formation water in the Sha 2 Member exhibits pronounced calcium enrichment and sodium depletion compared to that in other tight-sand gas reservoirs in China. The formation water in the member displays total dissolved solids (TDS) ranging from 3.3 to 45.6 g/L (CaCl2 type) and equivalent concentration proportions of Ca2+γ(Ca2+)] ranging from 24.7 % to 69.5 %, based on which the formation water can be categorized into three types: high-calcium [γ(Ca2+) ≥ 60 %], medium-calcium [40 % ≤ γ(Ca2+) < 60 %], and low-calcium [γ(Ca2+) < 40 %] formation water. These three types of formation water differ greatly in chemical characteristics and distribution. Specifically, the high-calcium formation water is concentrated in channel sand bodies that are near and connected to Jiao ① fault. This type of formation water, among others, exhibits a high average TDS of 34.1 g/L, cations dominated by Ca2+, and the lowest Na+/Cl- ratio, desulfurization coefficient, and carbonate equilibrium coefficient. The low-calcium formation water principally occurs in channel sand bodies that are distant from or not directly connected to the Jiao ① fault. This type of formation water manifests an average TDS of 3.9 g/L, cations dominated by Na+, and the highest Na+/Cl- ratio, desulfurization coefficient, and carbonate equilibrium coefficient. The medium-calcium formation water exhibits an average TDS of 17.7 g/L and slightly different Na+ and Ca2+ concentrations, with Na+/Cl- ratios, desulfurization coefficients, and carbonate equilibrium coefficients falling between those of the high- and low-calcium formation water. The calcium enrichment in formation water of the Sha 2 Member is primarily attributed to the mixing of formation water with a high Ca2+ concentration from the Xujiahe Formation. Additionally, subsequent water-rock interactions, especially the albitization of plagioclase, further contribute to the calcium enrichment and sodium depletion in the formation water of Sha 2 Member. The carbonate equilibrium coefficients, TDS, and γ(Ca2+) of formation water in the Sha 2 Member decrease gradually along the lateral migration pathway of natural gas. This indicates that the chemical indices of calcium-rich formation water can serve as reliable tracers for tight-sand gas reservoirs of separated source-reservoir type. For the Sha 2 Member, higher calcium enrichment in formation water is associated with more favorable gas-bearing properties of reservoirs, suggesting that the calcium-rich formation water is indicative of natural gas accumulation.

Key words: water-rock interaction, gas reservoir of separated source-reservoir type, calcium-rich formation water, natural gas migration and accumulation, Shaximiao Formation, central Sichuan Basin

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