轮南地区碳酸盐岩油气藏类型对地层水特征的控制作用

The control of reservoir types in Lunnan oilfield on characters of formation water

轮南油田下古生界中奥陶统碳酸盐岩油气藏地层水的产出较为普遍,但地层水的性质及分布规律均有明显的差异。其中轮古7、轮古2井区距天窗区较近,上奥陶统剥蚀殆尽,属风化壳油气藏;而轮古东井区中奥陶统储层上覆多套上奥陶统碳酸盐岩,为埋藏型油气藏。研究区地层水的差异主要体现为风化壳油气藏与埋藏型油气藏中地层水的差异,同时随油气藏埋深加大、上覆盖层增多变厚而呈现规律性的变化。相对来讲,风化壳油气藏中的地层水具有更高的矿化度、氯离子浓度、氯溴比及锶同位素值,同时具有明显偏轻的氢、氧同位素值。不同类型油气藏,尤其是盖层差异所导致的地层水溶解过程、混合作用及水岩作用等方面的差异是导致地层水特征变化的影响因素。

Formation water is often accompanied with oils and gases in the Lunnan Oilfield,Tarim Basin,while reservoirs with different kinds of caprocks show apparent variations in characters and distributions of formation water.In the study area,reservoirs in the Lungu 7 and 2 Lungu blocks have little Upper Ordovician deposits due to the adjacence to upheaval and severe denudation,while Middle Ordovician reservoirs in the eastern Lungu block are overlain with multiple suites of Upper Ordovician carbonates.Therefore,they are characterized by a weathering-crust reservoir and deeply-buried reservoir,respectively,and have different formation water that varies regularly with the increase of the burial depth and overlying caprock thickness of reservoirs.Comparatively,formation water in the weathering-crust reservoir shows higher salinity,chlorine content,Cl/Br ratio and 87Sr/86Sr ratio,but lighter δ18O and δD values than that in the deeply-buried reservoir.These variations of formation water in different reservoirs are probably attributed to the dissolution,admixture and water-rock interaction of formation water,which result from different kinds of reservoirs,particularly the difference in caprock thickness.