The Study On The Sedimentary Systems And Enrichment Laws Of Oil Reservoirs During The Depositional Period Of Interval 6 Of Late-Triassic In Southwestern Ordos Basin

Ordos basin is one of the largest inland basin which has abundant energy and mineral source of Mesozoic in the east of china.And it was preserved completely.The Yanchang Formation of Triassic in the basin is not only researched early,but also the first obtain oil gas strata of the basin.For many years,it is the primary target of oil and gas exploration. The Interval 6 of Yanchang Formation which as important exploration object has extensive lithologic reservoirs,and achieves great benefit.Base on researching of a great deal of field and indoor work,analyzing the provenance, sedimentary systems, depositional model, event deposits, sequence stratigraphy, the sequence-based lithofacies paleogeography,sandbody distribution,characteristics of reservoirs and enrichment laws of oil reservoirs.From the view of characteristics of particles, heavy mineral, distribution of cuttings, REE geochemistry, characteristics of seismic prospecting and so on,analyzed the sedimentary of the depositional period of interval 6 in southwestern Ordos Basin. The result shows that the geotectonic background of source area is recycled orogen. And there are three source areas in study area which are the south, the west and the south-west, however, the north-east is mixed source area.Based on the information concluded from the field outcrop,well cores, logging data and palaeontologic marker,Comprehensively analyzing sedimentary facies and systems.There are lacustrine deposits,delta deposits and turbidites deposits in the depositional period of interval 6 of study area,but have extensive turbidites.The major reservoir is sand bodies of delta front and turbidite.There exist deepwater steep-slope braided delta front and turbidite deposits in study area.The mainly reservoirs are delta front sandbodies and Semi-deep lake turbidite sandbodies.Proposed the concept of event deposits of Yanchang Formation in study area for the first time.There were turbidity currents,volcanic,anoxic and earthquake events in Yanchang Formation.All are related to reservoirs and source rocks of Chang7-Chang6.These kinds of depositional events were regional and local,combining with many factors such as climate which making essential contribution to the oil reservoir of Yanchang Formation.The turbidites in Ordos Basin have the typical turbidity current characteristics,and they are the products of origin of reduction environment.The results of origin research of turbidites indicates that they are low-density flow in lacustrine facies and the collapse of braided river delta front in the south-west parts of the basin.The provenance analysis of turbidite indicates that the deposits of turbidite had mainly originated.With all this in mind,The developmental pattern of turbidites was set up.Using the base-level cycle principle,consequently,four long-term sequence cycles was indentified in Yangchang Formation.And then,subdivided into three middle-term sequence cycles,six short-and middle-term sequence cycles in the Interval 6 of Yanchang Formation.Furthermore,studying the developing characteristic and boundary of sequence,the types and genesis of different levels of sequence cycles.In the meantime,it went to analyze different kinds of sequence architectures and stacking patterns,and summarizing the distribution pattern of short-term sequence cycles.It was taking middle-term sequence cycles as units,drawing the maps of sequenee-based lithofacies paleogeography of Chang 6 oil member in study area,and building the sedimentary models.Based on the research of the distribution feature of high resistance mudstone and effective hydrocarbon source rocks,combine with sedimentary facies,lithofacies palaeogeography framework and seismic profile of Member 7 of Yanchang Formation,discussing the bottom shape of the lake basin in Ordos.The results show that the basin is wider in the north and steeper in the south,steeper in the west and wider in the east of Ordos Basin.And at the same time,the northeast delta belongs to a gentle-slope lake delta system,but the south and southwest delta mainly belong to a steep-slope fan-delta and there exists turbidite fan sediment on the end of the delta front.On the basis of genetic type of the reservoir sandstones,there have five sand bodies types,such as the subaqueous distributary channel sandstones,the superposition of subaqueous distributary channel sandstones,the channel-mouth bar sandstones,the components of subaqueous distributary channel sandstones with channel-mouth bar sandstones,turbidite sandstones.The major reservoir sandstones are the components of subaqueous distributary channel sandstones with channel-mouth bar sandstones and turbidite sandstones.The major oil-Bearing formation is Chang 6-3 oil member,but the secondary is Chang 6-2 oil member;In the same way,the major reservoir developed in Chang 6-3 oil member,but the secondary developed in Chang 6-2 oil member.The distribution of sandstone is controlled of distribution of sedimentary facies.The characteristics of reservoir of Chang 6 oil member in study area show the low maturity of sandstone components and textural maturity of clastic component.The reservoir are fine sandstones.The principal types are arkosic sandstone,lithic arkosic sandstone,arkosic lithic sandstone,but the minority are lithic sandstone and feldspathic quartz sandstone.The major types of porosity are original inter-granular pore and feldspar-dissolved pore;the secondary are remainder intergranular pore,intracrystalline pore,cuttings dissolved pore,carbonate dissolved pore and filling ingredient dissolution pore;but microcrack is seldom.Porosity has good correlation with permeability.The good porosity and permeability wells are distributed in the middle of the study area.The degree of agreement between porosity and sandbody distribution is better than permeability,and diversification of permeability is controlled by porosity.It shows the characteristic of pore reservoir.The research of factors and distribution of Oil Reservoirs,shows that there have five factors controlling the hydrocarbon accumulation of study area.This area have good condition of hydrocarbon generating and reservoirs,and have good lithologic trap condition which is relevant to paleostructure and neotectonics.The fluid potential have developed the low-potential areas which is located in XiFeng area by transverse movement at the time of expulsion.This low-potential areas is pointing of oil-gas migration.It has been found that the good reservoir is necessary condition of high production.