Characteristics and origin of Cambrian dolomite, northern Tarim Basin

From the Northern Tarim Uplift to the southern margin of the Kuche Depression,the northern Tarim region is one of the largest hydrocarbon enrichment regions in the Tarim Basin.The Cambrian platform sediment is 1 000 m thick,and is an important exploration layer.Due to big thickness,various types and wide distribution,dolomite in the region is favorable for reservoir generation and is the main exploration target.Based on lithologic,isotopic and REE analyses of well cores and outcrops of Cambrian dolomites in the northern Tarim region,combined with temperature studies of fluid inclusions,it has been concluded that the Cambrian dolomites in the northern Tarim region have various structural types,including mud microcrystal,fine-medium crystal and coarse crystal.Besides penecontemporaneous and burial dolomitization effects,they were also influenced by tectonic and hydrothermal dolomitization effects,which were controlled by fault-fracture,unconformity and thermal flow,usually resulting in the coexistence of saddle dolomite and multiple hydrothermal minerals.The tectonic and hydrothermal dolomitization effects were important for favorable dolomite reservoir generation.Controlled by cycles of tectonic and diagenesis evolutions,ancient uplifts in the northern Tarim region were eroded for several times.Angular unconformity took place between Cambrian dolomite and Meso-zoic,and strong weathering crust karst effect generated.Deep and big faults worked as pathways for deep fluid to flow into dolomite.The dolomite was transformed into complex high-quality reservoir of different origin.