Petroleum geology and ring-shaped distribution of the Paleogene-Neogene hydrocarbon resources in western Qaidam Depression,Qaidam Basin
The western Qaidam Depression features rich Paleogene-Neogene petroleum systems,with conventional and shale oil accounting for 82.4%and 100%of the counterparts of the whole basin,respectively.Based on the investigated sedimentary reservoirs and petroleum geology,we analyze the sedimentary facies and hydrocarbon accumulation characteristics of the Paleogene-Neogene in the western Qaidam Depression and propose a ring-shaped hydrocarbon distribution pattern therein.The results reveal that the Paleogene-Neogene sedimentary facies zones in the depression present a ring-shaped distribution pattern comprising outer,middle,and inner rings.The outer ring,featuring deltaic and beach-bar facies,is dominated by clastic sediment,with the presence of small quantities of carbonate sediment.This ring contains conglomerate,coarse-grained conglomeratic sandstone and medium-grained sandstone reservoirs.The middle ring is dominated by the limy dolomite flat and limy muddy flat microfacies of the shore-shallow lacustrine subfacies,including fine-grained sandstones,siltstones,limy dolomites,and algal limestones.Of these,algal limestones serve as the most distinctive lithofacies with the highest porosity observed in the middle ring.The inner ring is composed of fine-grained diamictites of the semi-deep to deep lacustrine facies,encompassing deep-and dark-gray fine-grained sedimentary rocks.This establishes the middle ring as the key area to source rock development in the Paleogene-Neogene system.Reservoir types differ across the different sedimentary facies rings.In the outer ring,which is distant from hydrocarbon kitchens,hydrocarbons,transported via faults,accumulate in clastics such as conglomerates and coarse-grained sandstones,resulting in structural hydrocarbon reservoirs.The middle ring,immediately adjacent to the major hydrocarbon kitchens,has hydrocarbons migrating through faults and accumulating in carbonate reservoirs like algal limestones,resulting in the structural-lithological hydrocarbon reservoirs.The inner ring sitting on major source rock sequence has hydrocarbons either undergo short-distance migration or reside in situ in fine-grained diamictites,forming shale oil reservoirs.The western Qaidam Depression exhibits overlapped structural-lithological hydrocarbon reservoirs and shale oil reservoirs vertically from external to intra source rocks,and manifests a ring-shaped distribution of structural hydrocarbon,structural-lithological hydrocarbon,and shale oil reservoirs in a plan view.
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