Microstructure And Sweet-Spot Area Evaluation Of Type Tight Oil Reservoir

Tight reservoirs with great resource potential have become the focus of oil exploration and development in recent years.Affected by the changeable sedimentary environment,the source-reservoir relationship of tight oil reservoir is diverse on the macro scale,mostly manifested as "source-reservoir neighbor","source-reservoir symbiosis",etc.On the micro scale,the pore throat structure of tight oil reservoir also has great differences and heterogeneity.Therefore,the reservoir quality and evaluation parameters of sweet-spot area of tight oil reservoir are very different from those of conventional oil reservoir.The Triassic Yanchang Formation in Ordos Basin contains abundant tight oil resources.The Chang 7 oil member develops a typical "source-reservoir symbiosis" tight reservoir.At present,there are relatively few studies on evaluation standards of sweet-spot area,and it is still in the exploratory stage.The evaluation parameters and standards of sweet-spot area are not clear.Based on the investigation of regional exploration background and core observation,this paper comprehensively applies a series of experiments,such as cathodoluminescence experiment,particle size analysis,X-ray diffraction(whole rock and clay minerals),constant-pressure mercury injection,constant-rate mercury injection,nuclear magnetic resonance experiment,3D CT scanning and oil-water relative seepage.The reservoir characteristics,diagenesis type,reservoir microstructure,seepage characteristics and overpressure of Chang 7 member in the study area are studied systematically,and the sweet-spot area evaluation of tight reservoir is carried out at the same time.The purpose of this paper is to explore and deepen a new method for sweet-spot area evaluation of "source-reservoir symbiosis" type tight reservoir.There are delta front and pre-delta subfacies deposits in Chang 7 oil member in the study area.The subaqueous distributary channel extend in NE-SW direction,with superimposed,which are the main reservoirs.The average porosity is 7.9 % and the average permeability is0.37 m D,which is a typical tight sandstone reservoir.The lithology of the reservoir is mainly fine-grained feldspathic sandstone,with a small amount of lithic feldspathic sandstone and feldspathic lithic sandstone.The lithic is mainly metamorphic lithic.The cements mainly include clay minerals,carbonate minerals and secondary quartz;the content of chlorite is the highest in clay minerals,followed by kaolinite and illite.Carbonates are mainly calcite,mostly filled in pores in the form of crystals.The main pore types of the reservoir are dissolution intergranular pores and intragranular dissolution pores,with few residual intergranular pores,intercrystalline pores and microfractures.There are five kinds of diagenetic facies belt are developed.The chlorite film-feldspar dissolution facies is developed in the subaqueous distributary channel,with dissolution intergranular pores and intragranular dissolution pores;Feldspar dissolution-clay mineral weakly cemented facies is the most widely developed in the study area.It is mainly distributed in the subaqueous distributary channel.The pore types are mainly intragranular dissolved pores and dissolution intergranular pores;The strongly clay minerals cementation facies are distributed zonally along the edge of channel sand body;Calcite cemented facies are intermittently and sporadically developed in subaqueous distributary channel;Argillaceous compaction facies are mainly developed in sedimentary environments such as subaqueous distributary bay and edge of channel.The average throat radius and main throat radius of the reservoir in the study area have a high positive correlation with permeability.The movable fluid saturation is obviously affected by the maximum pore throat radius,maximum throat radius and pore throat allocation,meanwhile the distribution of movable fluid has a high positive correlation with the maximum throat radius and separation coefficient,indicating that the percolation ability of reservoir is mainly controlled by the development of throat and good configuration of pore throat.It is a comprehensive reflection of the internal throat network of the reservoir.According to the microstructure and the response of seepage characteristics,the reservoirs in the study area can be divided into four types.Type I is “fine pore – fine throat” type reservoir,which diagenetic facies is chlorite film-feldspar dissolution facies;dissolution intergranular pore and intragranular dissolution pore are developed;pore-throat relationship is good,with high movable fluid saturation,and the degree of oil-water two-phase interference is weak.Type Ⅱ is “fine-micro pore – fine throat” type reservoir,which diagenetic facies is feldspar dissolution-clay mineral weak cemented facies;The main pore types are mainly intragranular dissolution pore and dissolution intergranular pore,medium movable fluid saturation,and the interference between oil-water two-phase is weak.Type Ⅲ is “micro-fine pore – micro-fine throat” type reservoir,which diagenetic facies is clay mineral strong cementation facies;Pore type is mainly intragranular dissolution pore and intercrystalline pore;movable fluid saturation is low,and the interference between the oil-water two-phase is strong.Type IV is“micro pore – micro throat” type reservoir,and the main types of diagenetic facies are calcite cemented facies and argillaceous compaction facies,with the worst reservoir microstructure,lower movable fluid saturation and stronger interference between the oil-water two-phase.Overpressure is generally developed in Chang 7 oil member in the study area,which is mainly caused by mudstone undercompaction.The overpressure is basically greater than 4MPa,with an average of 9.2 MPa and 14.3 MPa in some areas;the overpressure difference between source and reservoir is the main driving force of oil migration.When the pressure difference between source and reservoir is greater than the capillary resistance of reservoir by3.96 MPa,the generated oil can break through the source reservoir interface and preferentially enter the reservoir with low pressure.Under the background of the development of high-quality source rocks,energy storage coefficient,oil layer thickness,and effective source reservoir pressure difference are important criteria for the evaluation of tight reservoir sweet-spot area.High-quality reservoir and effective source reservoir pressure difference co-control the distribution of tight oil sweet-spot area.