Dolomitization of Gas Reservoirs: The Upper Permian Changxing and Lower Triassic Feixianguan Formations, Northeast Sichuan Basin, China
Dolomitization of Gas Reservoirs: The Upper Permian Changxing and Lower Triassic Feixianguan Formations, Northeast Sichuan Basin, China
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DOI：
10.2110/jsr.2014.65
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发表时间：
2014-10
影响因子：
2
通讯作者：
Lei Jiang;R. Worden;C. Cai;Kaikai Li;L. Xiang;Liulu Cai;Xunyun He
中科院分区：
文献类型：
--
作者：
Lei Jiang;R. Worden;C. Cai;Kaikai Li;L. Xiang;Liulu Cai;Xunyun He
摘要：中国四川盆地开江—梁平海槽两侧的下三叠统飞仙关组以及在较小程度上的上二叠统长兴组，包含了十多个近期发现的大型白云岩气田。本研究通过对露头样品以及产气田岩芯样品进行岩石学、流体包裹体、微量元素地球化学以及稳定同位素和放射性同位素分析，记录了白云岩的成因。揭示了三个白云石化阶段。开江—梁平海槽东北侧飞仙关组泻湖相的泥晶白云岩以及长兴组礁相少量的保留原始组构的白云岩代表了初始白云岩阶段，是在30°C的成岩第一阶段由飞仙关卤水回流形成的，其盐度接近石膏饱和度（约18 wt%盐度）。第二阶段且是主要的白云岩阶段（保留原始组构的白云岩），在开江—梁平海槽两侧的飞仙关组和长兴组中，是在约35 - 40°C的早期埋藏成岩过程中，成岩流体回流的结果，其盐度范围从正常的飞仙关海水（3.5%盐度）到中盐度水（6.5%盐度），这由单相水溶液包裹体占主导、切割第二阶段白云岩的浅成缝合线以及早三叠世海相碳、氧、锶同位素以及稀土元素和钇（REE + Y）信号所表明。次要的第三白云岩阶段（破坏原始组构的白云岩和白云岩胶结物）是由于开江—梁平海槽两侧的飞仙关组和长兴组在80 - 140°C之间的埋藏成岩重结晶作用。第三阶段的白云石化涉及从开江—梁平海槽东北侧稍年轻的下三叠统嘉陵江组海相蒸发岩和碳酸盐岩中排出的水，这由在80 - 140°C均一的高盐度水溶液包裹体以及升高的锶同位素数据所揭示。第三阶段白云岩的碳同位素和REE + Y信号与早期白云岩胶结物相似，证实了没有外来输入。未来的勘探应聚焦于类似三叠纪特提斯古地理环境中与局限相相关的白云石化高能沉积物（例如礁、鲕粒滩和堤岸），但下伏的白云石化高能二叠系地层也可能是良好的勘探目标。
Abstract The Lower Triassic Feixianguan Formation, and to a lesser extent the Upper Permian Changxing Formation, contain more than ten giant, recently discovered dolomite gas fields on either side of a paleo-embayment (Kaijiang–Liangping Bay), Sichuan Basin, China. This study documents the origin of the dolomite using a combination of petrology, fluid-inclusion, trace-element geochemistry, and stable-isotope and radiogenic-isotope analysis of samples from outcrop and core samples from producing gas fields. Three stages of dolomitization are revealed. Dolomicrite from the lagoon facies in the Feixianguan Formation and minor fabric-retentive dolomite from reef facies in the Changxing Formation on the northeast side of Kaijiang–Liangping Bay represent the initial dolomite phase, caused by reflux of Feixianguan brine, with salinity close to gypsum saturation (∼ 18 wt % salinity), during the first stage of diagenesis at 30°C. The second and dominant dolomite phase (fabric-retentive dolomite), in both the Feixianguan and Changxing formations on the both sides of Kaijiang–Liangping Bay, is the result of reflux of diagenetic fluids with salinity ranging from normal Feixianguan seawater (3.5% salinity) up to mesohaline water (6.5% salinity) during early burial diagenesis at about 35–40°C, as shown by the dominance of single-phase aqueous inclusions, shallow-formed stylolites that cut the second phase of dolomite, and the Early Triassic marine carbon, oxygen, and strontium-isotope, and rare-earth-element and yttrium (REE + Y) signals. The subordinate third dolomite phase (fabric-obliterative dolomite and dolomite cement) was due to burial diagenetic recrystallization between 80 and 140°C in the Feixianguan and Changxing formations on both sides of Kaijiang–Liangping Bay. The third phase of dolomitization involved water expelled from the slightly younger Lower Triassic Jialingjiang Formation marine evaporites and carbonates on the northeast side of Kaijiang–Liangping Bay, revealed by the high-salinity of the aqueous inclusions that homogenize at 80 to 140°C and elevated strontium-isotope data. The carbon-isotope and REE + Y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input. Future exploration should be focused on dolomitized, high-energy sediments (e.g., reef and oolitic shoal and bank) in similar Triassic Tethyan-paleogeographic settings that are associated with restricted facies, but underlying dolomitized high-energy Permian strata may also be good exploration targets.