Accumulation Machanism Research On Ultra-low-Permeability And Large Scale Lithogical Reserviors Of Triassic Yanchag Formation In Ordos Basin

Large oil reserviors with low permeability are well developed in Ordos basin. Oil exploration and major breakthroughs have been achieved. Large oil pools, such as Xifeng, Jiyuan and Huaqing oil pools and some oil riched regions as Taerwan and Heshui have been found in the last ten years. So far, four large-scale hydrocarbon riched accumulations have taken shape, located in the northeast, the southwest, the northwest and the depocenter in Ordos basin respectively. The physical property of large-sized lithogical reservior is poor in the depositional center. It belongs to ultra-low permeability reservior. Some oil-riched area with submited proved-geological-oil-reserves exceeding 100 millions tons, have been discovered. Complex accumulation process makes it difficult to study. The research of accumulation machanism on ultra-low-permeability reserviors in the depositional center of Ordos basin will provide scientific basis for production and reserve increasing in the basin, and will perform enlightenment for similar investigation in other hydrocarbon riched bearing basins in future. Aiming at exploring the key parameters of reservior and sumerizing hydrocarbon migration and accumulation rules, this research emphasizes on the sedimental type, diagenesis and densification of reservior, hydrocarbon transporting systems and petroleum migration and accumulation dynamic. Some new views are promoted as follow:It is presented that early Indosinian movement dominant the evolution of Ordos basin and the characteristics of multi-cycle sedimentary evolution is the consequence of polycyclic tectonic movement. The tectonic epochs is identified on the base of comprehensive analysis of apatite fission track and a LA-ICP-MS U-Pb chronological study of zircons, sampled from magmatic rock around the basin and the tuff rock at the bottom of chang7 member in Yanchang formation in the basin. A new option is advanced that affacted by west Qingling orogenesis, a intense tectonic event happened at the end of sedimental phase of Chang8 strata. It made graet change on basin framwork, depositional feature and biological character.New modle of thick deep-water sandstone is built. Stretching along the facies lines in depositional centre of Ordos basin, the thick deep-water sandstone of Chang6 and Chang7 oil bearing formation consists of deep-water delta, debrite, slide and turbidite, and can be named as large delta-gravity complex. The compound sandbody, origining from gravity-flow, parallels the faces borderlines, and deep-water delta sandbody takes on the shape of lobe and belt, distributing at Baibao and its northern areas.The reservior porosity at premium stage of hydrocarbon generation is uncovered. It is suggested that hydrocarbon inclusions cathched in quartz overgrowths stand for the oil injection occurred at premium stage of hydrocarbon generation and expulsion. Probing into the computational method of reservior porosity at premium stage of hydrocarbon generation and expulsion, it shows that the average porosity of Chang6-Chang8 was mainly arranged from 12%to 20%, and the permeability should exceed 10x10-3μm2 at the stage. Ironed carbonate cement made the reservior compacted eventually and the sandstone peroperty at that time was raletively better than that at present.The role of different transporting passages in oil migeration is argued. Interconnected and uncompletely densified sandbodies palyed as the main passages for migration and accumulation at the phase of oil-pools forming. Fractures and joints with calcium intense filling, which extend along the direction of NNE-SSW, were formed at the Himalayan period or at the end of Yanshanian period. Some of them remain the trail of petroleum transit. They connected the premium oil source of Chang7 member and the reserviors of upper Yanchang formation or Yan'an formation. Early Jurrasic river channel did as one of the important transport passage for oil accumulation at upper Yanchang formation and Yan'an formation.The hydrocarbon dynamic system is divided and fluid migeration feature in different systems and sub-systems is discussed. Middle and lower Yanchang formation was a excess pressure hydrodynamic system during the deepes-depth period. According to lithology, property and overpressure, it can be divided into two subsystems, i.e. the southwest dynamic subsystem and the northeast dynamic subsystem. As to the southwest hydrodynamic subsystem, the higher the altitude, the more seriously compacted the reservoir is. Vertical hydrocarbon migeration played a dominant role during the process of pool forming. The areas with low overpressure-difference or the areas with relative low overpressure-difference in high overpressure-difference region are the preferential places for oil migeration. As to the northeast hydrodynamic subsystem, oil migerated not only vertically but also transversely through the layered carrier bed. The areas with low overpressure are the faverable places for oil accumulation in the northeast dynamic subsystem.Accumulation sequence of Mesozoic Yanchang formation and Yan'an formation in Ordos basin is set up. The sequence is:①reserviors above the source rock, drivern by buoyancy and excess pressure;②eserviors underneath the source rock, drivern by excess pressure;③eservior of Chang3 and its upper strata, connected by fractures and joints and oil tranporting in Jurrasic river channels. The enrichment regularity of petroleum in Yanchang formation is summarized. It is shown that the delta-front and gravity-flow sediment are the main areas for the ultra-low-permeability reserviors. The raletive higher permeability reserviors located in the delta front and it domimants the distribution of oil-riched area and high yield area:the places with relative low overpressure or relative low overpressure-difference are the preferential places for oil migeration and accumulation at the middle and low part of Yanchang formation. Pre-Jurrassic palaeo-geomorphology controls the reservior distribution of upper Yanchang formation and Jurrassic.