Research On Oil Enrichment Characteristics Of Chang 6 Member In Zhangjiagou District, Ordos Basin

The Triassic is the most important oil producing strata in the Ordos Basin.Although the reservoirs of the Triassic Chang 6 and the strata below it are tight,they have a good petroleum potential.In order to explore the enrichment of Triassic resorevoirs and its controlling factors,the Chang 6 member in Zhangjiagou area was taken as an example.This paper is aimed at fine classification in strata,great depiction on low amplitude structures,better understanding of sedimentary and reservoirs and more discussion on occurrence and controlling factors of petroleum.These works are based on the fundamental theories and methods of sedimentology,tectonic geology,logging geology,reservoir geology,and petroleum geology,associated with the data of drilling,well logging,mud logging and well tests.Chang 6 member was divided into Chang 611,Chang 612,Chang 613,Chang 621,Chang 622,Chang 623,Chang 631,Chang 632 and Chang 633,nine layers were steadily distributed in the studied area.In terms of tectonic structures,oil reservoir in Chang 6 member developed 3 southwest-northeast low amplitude nose-like structures under the background of monocline.These low amplitude nose-like structures were steadily distributed in the 9 layers and the inheritance of them are good.According to sedimentary analysis,oil reservoir in Chang 6 member belongs to delta front subfacies,where subaqueous distributary channel,subaqueous interdistributary bay and subaqueous natural levee can be found;and horizontal distribution of sand bodies is mainly controlled by sedimentary facies and sand body of subaqueous distributary channel is northeast-southwest spread.The thickness of sand body in the Chang 6 member is in the range of 4?6m,and the distribution of sand body is controlled obviously by the sedimentary facies.The results of a large number of thin slices show that the main reservoir space of Chang 6 reservoir is intergranular pore and dissolution pore within feldspar.The pore structure and laryngeal tract is dominated by mesoporous fine larynx and small pore throat.It belongs to low porosity and permeability reservoir.The GC-MS spectra of the saturated hydrocarbon in the Yan 9 reservoir extractives in the study area is compared with the Chang 7 and the Chang 9 source rocks in the Ordos Basin.The oil source of Chang 6 reservoir in the study area mainly comes from Chang 7 source rocks of Yanchang Formation.Sand of subaqueous distributary channel can be used as the main reservoir of oil,argillaceous sediments in the lower part of the sandstone reservoir of the Chang 4+5.The low porosity and ultra-low permeability reservoirs and sediments in the subaqueous interdistributary bay can serve as a good seal for Chang 6 structure-lithology reservoirs in the study area.The Chang 6 reservoir had a handing distribution.The sand bodies of the subaqueous had a good physical property and a high oil saturation.It is considered that the subaqueous distributary channel is the most favorable place for the accumulation of oil in the area,the reason is that sedimentation controls the variation of physical properties of reservoir and affects the accumulation of oil.It is found that the oil-bearing ability of the thick sand bodies in the subaqueous distributary channel are generally better than the thin sand bodies developed in the microfacies such as the subaqueous distributary Bay.In addition,the oil accumulation area coincides well with the higher parts of low amplitude nose-like structures,which indicates that the low amplitude nose-like structures play an important role in oil accumulation.Based on the influence of sedimentary facies,reservoir physical property,sand body thickness and low amplitude structure on oil enrichment,it is predicted that the oil may be enriched in three regions of A,B and C.