四川盆地高石梯-磨溪地区震旦系灯影组热液白云岩证据

川中高石梯-磨溪地区震旦系灯影组为当前四川盆地天然气勘探热点层系之一,因其早期沉积组构遭受成岩作用影响严重,白云岩化成因机理不明确,结合已钻井取心的岩石学、地球化学特征等开展系统研究。岩石学特征分析表明,川中高石梯-磨溪地区灯影组见构造角砾岩及晶洞、裂缝中充填的鞍状白云石,含少量闪锌矿、方铅矿、硬石膏、黄铁矿、方解石、萤石、石英及长石等热液矿物。流体包裹体测温结果显示鞍状白云石胶结物均一化温度为132.6~218.7℃,平均为175.6℃,显著高于基质白云石包裹体温度(平均125℃)及地层正常埋深所经历的最高温度。鞍状白云石δ¹³C值为-2.16‰~2.94‰,平均值为0.63‰;δ¹⁸O值为-12.43‰~-6.8‰,平均值为-9.63‰,碳、氧同位素的负异常明显指示受热液作用影响。微量元素分析表明鞍状白云石及粗晶白云岩相对基质白云岩更富集Fe、Mn元素。在拉张性区域构造背景及上覆筇竹寺组泥页岩封闭层条件下,底部热流体沿基底断裂向上运移至灯影组,并在高石梯-磨溪地区断层附近产生热液交代作用形成热液白云岩。

Evidences for hydrothermal dolomite of Sinian Dengying Formation in Gaoshiti-Moxi area,Sichuan Basin

Sinian Dengying Formation in Gaoshiti-Moxi area, Central Sichuan Basin is one of natural gas exploration hotspots in Sichuan Basin at present. Due to the fact that early sedimentary fabrics were severely influenced by diagenesis, the genetic mechanism of dolomitization is still unclear. A systematical research was conducted based on the petrologic and geochemical characteristics of drilled cores in this study. The analysis of petrologic characteristics shows that tectonic breccias and the saddle dolomites filled in geodes and fissures are generally found in Gaoshiti-Moxi area, Central Sichuan Basin, containing a small number of sphalerite, galena, anhydrite, pyrite, calcite, fluorite, quartz, feldspar and other hydrothermal minerals. The temperature measurements of fluid inclusion indicate that the homogenization temperature of saddle dolomite cement is 132.6-218.7℃ with an average of 175.6℃, significantly higher than the temperature of matrix dolomite inclusion (averaged at 125℃) and the maximum temperature of formation at a normal buried depth. The δ¹³C value of saddle dolomite is ranging from -2.16‰ to 2.94‰, averaged at 0. 63‰; δ¹⁸O value is ranging from -12.43‰ to -6.8‰, averaged at -9.63‰. It is clear that the negative anomaly of carbon and oxygen isotopes is significantly influenced by hydrothermalism. The trace-element analysis shows that Fe and Mn are more enriched in saddle dolomites and coarse-grain dolomites than matrix dolomites. Under the tectonic settings of tensile areas and the condition of the overlying confined shale horizons in Qiongzhusi Formation, bottom hydrotherm migrates upwards to Dengying Formation along basement rift, and hydrothermal dolomites are formed due to the hydrothermal metasomatism near the fault in Gaoshiti-Moxi area.