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

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.