Table1_Geochemistry of Formation Water and Its Implications for Petroleum Source Rocks in the Fengcheng Formation, Mahu Depression, Xinjiang, China.xlsx

Geochemical properties of formation water reflect the sedimentary environments and the sealing conditions of the formation, which are of great significance for oil and gas exploration. In this research, the formation waters in the deep Permian Fengcheng Formation in the Mahu Depression were studied by analysis of chemical composition, stable isotopes, and the homogenization temperatures and salinities of fluid inclusions. The results show that the formation water is NaHCO3-type with high salinity, high HCO3-, low concentrations of Ca2+ and Mg2+ and high pH. The overall sealing of the formation is well maintained. Na2SO4-type water was found in some places with poor preservation of oil and gas. The fluid geochemistry and alkaline mineral distribution in the strata, which are dominated by deep fine-grain mixed deposits, indicate that the main body of the Fengcheng Formation was deposited in an alkaline lake sedimentary environment. The isotopic signatures are characterized by high δ13C, negative δ18O, and light strontium and heavy boron isotopes in the strata near the deep fault zone, which indicate that the Fengcheng formation may have been affected by deep hydrothermal fluids. From the first member to the third member of the formation, the salinity first increases and then decreases. This low-high-low trend corresponds to the initial stage of alkaline lake development followed by strong alkali formation and a final stage of water retreat. The coincidence between the spatial distribution of alkaline minerals, high-salinity formation water and source rocks indicates that the alkaline lake environment can form high-quality source rocks with good petroleum generation capacity.

地层水（formation water）的地球化学特征可表征沉积环境与地层封闭条件，对油气勘探具有重要指导意义。本研究以玛湖凹陷（Mahu Depression）深部二叠系（Permian）风城组（Fengcheng Formation）地层水为研究对象，通过化学成分分析、稳定同位素（stable isotopes）分析以及流体包裹体（fluid inclusions）均一温度与盐度测定开展相关研究。研究结果表明，研究区地层水以高矿化度、高HCO₃⁻、低Ca²⁺与Mg²⁺含量及高pH值的NaHCO₃型水（NaHCO3-type）为主；整体地层封闭性良好，但局部区域发育Na₂SO₄型水（Na2SO4-type），该类区域油气保存条件较差。研究区地层以深部细粒混合沉积为主，结合流体地球化学特征与碱性矿物分布规律，可判定风城组主体沉积于碱性湖盆沉积环境。深大断裂带（deep fault zone）附近地层的同位素特征表现为高δ¹³C值、负δ¹⁸O值，且锶同位素偏轻、硼同位素偏重，指示风城组可能受到深部热液流体（hydrothermal fluids）的影响。从风城组一段至三段，地层水矿化度呈现先升高后降低的变化趋势，即“低-高-低”的演化特征，对应碱性湖盆发育初期、强碱性阶段及后期湖退消亡三个演化阶段。碱性矿物、高矿化度地层水与烃源岩（source rocks）的空间分布耦合性表明，碱性湖盆环境可形成具备优良生烃能力（petroleum generation capacity）的优质烃源岩。