Characteristics and control factors of feldspar dissolution in gravity flow sandstone of Chang 7 Member,Triassic Yanchang Formation,Ordos Basin,NW China

To clarify the formation and distribution of feldspar dissolution pores and predict the distribution of high-quality reservoir in gravity flow sandstone of the 7^(th) member of Triassic Yanchang Formation(Chang 7 Member)in the Ordos Basin,thin sections,scanning electron microscopy,energy spectrum analysis,X-ray diffraction whole rock analysis,and dissolution experiments are employed in this study to investigate the characteristics and control factors of feldspar dissolution pores.The results show that:(1)Three types of diagenetic processes are observed in the feldspar of Chang 7 sandstone in the study area:secondary overgrowth of feldspar,replacement by clay and calcite,and dissolution of detrital feldspar.(2)The feldspar dissolution of Chang 7 tight sandstone is caused by organic acid,and is further affected by the type of feldspar,the degree of early feldspar alteration,and the buffering effect of mica debris on organic acid.(3)Feldspars have varying degrees of dissolution.Potassium feldspar is more susceptible to dissolution than plagioclase.Among potassium feldspar,orthoclase is more soluble than microcline,and unaltered feldspar is more soluble than early kaolinized or sericitized feldspar.(4)The dissolution experiment demonstrated that the presence of mica can hinder the dissolution of feldspar.Mica of the same mass has a significantly stronger capacity to consume organic acids than feldspar.(5)Dissolution pores in feldspar of Chang 7 Member are more abundant in areas with low mica content,and they improve the reservoir physical properties,while in areas with high mica content,the number of feldspar dissolution pores decreases significantly.

Types,characteristics and geological significance of event deposits of Chang 9 Member of Triassic Yanchang Formation in southwestern Ordos Basin,NW China

Through core observation,thin section identification,and logging and testing data analysis,the types and characteristics of event deposits in the ninth member of Yanchang Formation of Triassic(Chang 9 Member)in southwestern Ordos Basin,China,are examined.There are 4 types and 9 subtypes of event deposits,i.e.earthquake,gravity flow,volcanic and anoxic deposits,in the Chang 9 Member in the study area.Based on the analysis of the characteristics and distribution of such events deposits,it is proposed that the event deposits are generally symbiotic or associated,with intrinsic genetic relations and distribution laws.Five kinds of sedimentary microfacies with relatively developed event deposits are identified,and the genetic model of event deposits is discussed.Seismites are mainly developed in the lake transgression stage when the basin expands episodically,and commonly affected by liquefaction flow,gravity action and brittle shear deformation.Gravity flow,mainly distributed in the high water level period,sandwiched in the fine-grained sediments of prodelta or semi-deep lake,or creates banded or lobate slump turbidite fan.It is relatively developed above the seismites strata.The volcanic event deposits are only seen in the lower part of the Chang 9 Member,showing abrupt contact at the top and bottom,which reflects the volcanic activity at the same time.Anoxic deposits are mostly formed in the late stage of lake transgression to the highstand stage.Very thick organic-rich shales are developed in the highstand stage of Chang 9 Member,and the event deposits in the depositional period of these shales are conducive to potential reservoirs.

Characteristics and exploration targets of Chang 7 shale oil in Triassic Yanchang Formation, Ordos Basin, NW China

The geological characteristics and enrichment laws of the shale oil in the third submember of the seventh member of Triassic Yanchang Formation(Chang 7_(3)) in the Ordos Basin were analyzed by using the information of core observations, experiments and logging, and then the exploration potential and orientation of the Chang 7_(3) shale oil were discussed. The research findings are obtained in three aspects. First, two types of shale oil, i.e. migratory-retained and retained, are recognized in Chang 7_(3). The former is slightly better than the latter in quality. The migratory-retained shale oil reservoir is featured with the frequent interbedding and overlapping of silty-sandy laminae caused by sandy debris flow and low-density turbidity current and semi-deep-deep lacustrine organic-rich shale laminae. The retained shale oil reservoir is composed of black shale with frequent occurrence of bedding and micro-laminae. Second, high-quality source rocks provide a large quantity of hydrocarbon-rich high-quality fluids with high potential energy. The source-reservoir pressure difference provides power for oil accumulation in thin interbeds of organic-poor sandstones with good seepage conditions and in felsic lamina, tuffaceous lamina and bedding fractures in shales. Hydrocarbon generation-induced fractures, bedding fractures and microfractures provide high-speed pathways for oil micro-migration. Frequent sandstone interlayers and felsic laminae provide a good space for large-scale hydrocarbon accumulation, and also effectively improve the hydrocarbon movability. Third, sand-rich areas around the depression are the main targets for exploring migratory-retained shale oil. Mature deep depression areas are the main targets for exploring retained oil with medium to high maturity. Theoretical research and field application of in-situ conversion in low-mature deep depression areas are the main technical orientations for exploring retained shale oil with low to medium maturity.

The role of provenance in the diagenesis of siliciclastic reservoirs in the Upper Triassic Yanchang Formation,Ordos Basin,China

A better understanding of the controls on reservoir quality has become essential in the petroleum exploration in recent years. Determining the original composition of tile sediment framework is important not only for paleogeographic reconstructions, but it is also vital tbr predicting the nature of physical and chemical diagenesis of the potential reservoirs. Depositional setting and diagenesis are important factors in controlling the type and quality of most siliciclastic reservoirs. We studied the Upper Triassic Chang 8 and 6 members, where the relationship between sediment provenance and diagenesis was examined. The study attempts to clarify sediment provenance and post-depositional diagenetic modifications of the sandstones through systematic analytical methods including petrographic macro- and microscopic analysis of grain and heavy mineral types, and measurements of the palaeocurrent direction of the Yanchang Formation sediments in the outcrops in order to determine the provenance of the studied sediments. Furthermore, the relationship between framework grains, pore types and diagenesis of the sediments was analyzed by thin section petrographic characterization using a polarizing microscope. Additionally, a JEOL JSM-T330 scanning electron microscope （SEM） equipped with a digital imaging system was used to investigate the habits and textural relationships of diagenetic minerals. On the basis of our results, we believe that sediment provenance is a significant factor which controls the type and degree of diagenesis which may be expected in sandstones. In the Chang 8 and 6 members, tile formation of chlorite rims and laumontite cement was observed where volcanic rock fragments constitute a large part of the framework grains. Furthermore, high biotite content provides abundant iron and magnesium and enables the tbnnation of chlorite rims due to biotite hydrolysis. In addition, ductile deformation of biotite leads to strong mechanical compaction of the sediments. Conversely, high feldspar content diminishes the degree of mechanical compaction, however the dissolution of feldspar minerals in sandstones is commonly observed. Apart from feldspars, quartz and other rigid fi＇amework grains highly control the degree of mechanical compaction during the initial stage of burial （0-2 km）.

Diagenesis of the Triassic Yanchang Formation Tight Sandstone Reservoir in the Xifeng–Ansai Area of Ordos Basin and its Porosity Evolution

This work aims to reveal the evolution of the porosity in the Triassic Yanchang Formation tight sandstone reservoir of the Xifeng-Ansai area of Ordos Basin. Based on destructive diagenesis （compaction and cementation） and constructive diagenesis （dissolution） of sandstone reservoirs, this study analyzed the diagenesis characteristics of the tight sandstone reservoirs in this area, and discussed the relationship between sandstone diagenesis and porosity evolution in combination with present porosity profile characteristics of sandstone reservoir. The effect simulation principle was employed for the mathematical derivation and simulation of the evolution of porosity in the Yanchang Formation tight sandstone reservoirs. The result shows that compaction always occurs in tight sandstone reservoirs in the Yanchang Formation, and cementation occurs when the burial depth increases to a certain value and remains ever since. Dissolution occurs only at a certain stage of the evolution with window features. In the corresponding present porosity profile, diagenesis is characterized by segmentation. From the shallow to the deep, compaction, compaction, cementation and dissolution, compaction and cementation occur successively. Therefore, the evolution of sandstone porosity can be divided into normal compaction section, acidification and incremental porosity section, and normal compaction section after dissolution. The results show that the evolution of sandstone porosity can be decomposed into porosity reduction model and porosity increase model. The superposition of the two models at the same depth in the three stages or in the same geological time can constitute the evolution simulation of the total porosity in sandstone reservoirs. By simulating the evolution of sandstone reservoir porosity of the eighth member in Xifeng area and the sixth member in Ansai area, it shows that they are similar in the evolution process and trend. The difference is caused by the regional uplift or subsidence and burial depth.

Features of Sandy Debris Flows of the Yanchang Formation in the Ordos Basin and Its Oil and Gas Exploration Significance

Sandy debris flow is a new genetic type of sand bodies,which has gained much attention in recent years and its corresponding theory is proved to be a significant improvement and even partial denial to the 'Bouma Sequence' and 'turbidite fan' deep-water sedimentary theories to some point. Oil exploration researchers are highly concerned with sandy debris flows for its key role in controlling oil and gas accumulation processes.In this article,by applying sandy debris flows theory and combining a lot work of core,outcrop observation and analysis plus seismic profile interpretation,we recognized three types of sedimentary gravity flows that are sandy debris flows,classic turbidites and slumping rocks in chang-6 member of Yanchang Formation in the deep-water area of central Ordos Basin.Among the three types,the sandy debris flows are the most prominent and possesses the best oil bearing conditions.On the contrary,the classic turbidites formed by turbidity currents are limited in distribution;therefore,previous Yanchang Formation deep-water sedimentary studies have exaggerated the importance of turbidite currents deposition.Further study showed that the area distribution of deep water gravity flow sand bodies in Yanchang Formation were controlled by the slope of the deep-water deposits and the flows had vast distribution,huge depth and prevalent advantages for oil forming,which make it one of the most favorable new areas for Ordos Basin prospecting.

Insights into the Tectonic Fractures in the Yanchang Formation Interbedded Sandstone-Mudstone of the Ordos Basin Based on Core Data and Geomechanical Models

The distribution and intensity of tectonic fractures within geologic units are important to hydrocarbon exploration and development. Taken the Upper Triassic Yanchang Formation interbedded sandstone-mudstone in the Ordos Basin as an example, this study used the finite element method（FEM） based on geomechanical models to study the development of tectonic fractures. The results show that the sandstones tend to generate tectonic fractures more easily than mudstones with the same layer thickness, and the highest degree of tectonic fractures will be developed when the sandstone-mudstone thickness ratio is about 5.0. A possible explanation is proposed for the tectonic fracture development based on two important factors of rock brittleness and mechanical layer thickness. Generally, larger rock brittleness and thinner layer thickness will generate more tectonic fractures. In interbedded sandstone-mudstone formations, the rock brittleness increases with the increasing mechanical layer thickness, hence, these two factors will achieve a balance for the development of tectonic fractures when the sandstone-mudstone thickness ratio reaches a specific value, and the development degree of tectonic fractures is the highest at this value.

Characteristics and Origin of Tight Oil Accumulations in the Upper Triassic Yanchang Formation of the Ordos Basin,North-Central China

The Upper Triassic oil accumulations in the Ordos Basin is the most successful tight oil play in China,with average porosity values of less than 10% and permeability values below 1.0 mD.This study investigated the geological characteristics and origin of the tight oil accumulations in the Chang 6 member of the Upper Triassic Yanchang Formation in the Shanbei area based on over 50,000 petrological,source-rock analysis,well logging and production data.The tight oil accumulation of the Chang 6 member is distributed continuously in the basin slope and the centre of the basin.The oilwater relationships are complex.Laumontite dissolution pores are the most important storage spaces,constituting 30%-60% of total porosity and showing a strong positive relationship with oil production.The pore-throat diameter is less than 1 μm,and the calculated critical height of the oil column is much larger than the tight sand thickness,suggesting that the buoyancy was probably of limited importance for oil migration.The pressure difference between the source rocks and sandstone reservoirs is inferred to have provided driving force for hydrocarbon migration.Two factors of source-reservoir configuration and laumontite dissolution contributed to the formation of the Chang 6 tight oil accumulations.Intense hydrocarbon generation and continuous sand bodies close to the hydrocarbon kitchen are the foundation for the large-scale oil distribution.Dissolution of feldspar-laumontite during the process of organic matter evolution generated abundant secondary pores and improved the reservoir quality.

Genesis of the high gamma sandstone of the Yanchang Formation in the Ordos Basin,China

Recently, more attention has been paid on the high gamma sandstone reservoirs of the Yanchang Formation in the Ordos Basin, China. These high gamma sandstones have logging characteristics different from conventional sandstones, which influences the identification of sandstone reservoirs. Zhang et al （2010） proposed that the high gamma sandstones of the Yanchang Formation might be the result of re-deposition of homochronous sedimentary tufts or previous tufts as a part of the sandstone. However, we present a different viewpoint： 1） few tufts or tuff debris have been found in the high gamma sandstones of the Yanchang Formation; 2） high gamma （or high Th content） sandstones of Yanchang Formation are not related to either clay minerals or feldspar; 3） the heavy minerals in the sandstone reservoirs of the Yanchang Formation are dominated by zircon, which is characterized by abnormally high Th and U contents, up to 2,163 ppm and 1,362 ppm, respectively. This is sufficient to explain the high gamma anomaly. The conclusion is that the high gamma value of the Yanchang Formation sandstones might be caused by zircon with high Th and U contents in sandstones rather than from the tuff components.

Nanoscale pore morphology and distribution of lacustrine shale reservoirs:Examples from the Upper Triassic Yanchang Formation,Ordos Basin

Pore structure plays an important role in the gas storage and flow capacity of shale gas reservoirs. Fieldemission environmental scanning electron microscopy（FE-SEM） in combination with low-pressure carbon dioxide gas adsorption（CO2GA）,nitrogen gas adsorption（N2GA）,and high-pressure mercury intrusion（HPMI） were used to study the nanostructure pore morphology and pore-size distributions（PSDs） of lacustrine shale from the Upper Triassic Yanchang Formation,Ordos Basin. Results show that the pores in the shale reservoirs are generally nanoscale and can be classified into four types： organic,interparticle,intraparticle,and microfracture. The interparticle pores between clay particles and organic-matter pores develop most often,l with pore sizes that vary from several to more than 100 nm. Mercury porosimetry analysis shows total porosities ranging between 1.93 and 7.68%,with a mean value of 5.27%. The BET surface areas as determined by N2 adsorption in the nine samples range from 10 to 20 m2/g and the CO2 equivalent surface areas（2 nm）vary from 18 to 71 m2/g. Together,the HPMI,N2 GA,and CO2 GA curves indicate that the pore volumes are mainly due to pores 100 nm in size. In contrast,however,most of the specific surface areas are provided by the micropores. The total organic carbon（TOC） and clay minerals are the primary controls of the structures of nanoscale pores（especially micropores and mesopores）. Micropores are predominantly determined by the content of the TOC,and mesopores are possibly related to the content of clay minerals,particularly the illite-montmorillonite mixed-layer content.

Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin,northern China

It is important to predict the fracture distribution in the tight reservoirs of the Ordos Basin because fracturing is very crucial for the reconstruction of the low-permeability reservoirs. Three-dimensional finite element models are used to predict the fracture orientation and distribution of the Triassic Yanchang Formation in the Longdong area, southern Ordos Basin. The numerical modeling is based on the distribution of sand bodies in the Chang 7a and 72 members, and the different forces that have been exerted along each boundary of the basin in the Late Mesozoic and the Cenozoic. The calculated results demonstrate that the fracture orientations in the Late Mesozoic and the Ceno- zoic are NW-EW and NNE-ENE, respectively. In this paper, the two-factor method is applied to analyze the distribution of fracture density. The distribution maps of predicted fracture density in the Chang 71 and 72 members are obtained, indicating that the tectonic movement in the Late Mesozoic has a greater influence on the fracture development than that in the Cenozoic. The average fracture densities in the Chang 71 and 72 members are similar, but there are differences in their distributions. Compared with other geological elements, the lithology and the layer thickness are the primary factors that control the stress distribution in the study area, which further determine the fracture distribution in the stable Ordos Basin. The predicted fracture density and the two-factor method can be utilized to guide future exploration in the tight-sand reservoirs.

Genesis of the high gamma sandstone of the Yanchang Formation in the Ordos Basin,China:A reply

The authors of ＇Genesis of the high gamma sandstone of the Yanchang Formation in the Ordos Basin, China＇ questioned the viewpoint that high-gamma-ray sandstone might be caused by homochronous sedimentary volcano tuff ash or previous tuff. The authors argued that the main reason for the high-gamma-ray sandstone should be from high Th and U contents in zircon. In reply, we discuss the problems with the authors from the category of high-gamma-ray sandstones, rock characteristics, and possible sources of radioactivity. The results still indicate that the high gamma ray characteristics might be caused by homochronous sedimentary volcano tuff ash or reworked previous turfs.

Predicting the present-day in situ stress distribution within the Yanchang Formation Chang 7 shale oil reservoir of Ordos Basin, central China

The Yanchang Formation Chang 7 oil-bearing layer of the Ordos Basin is important in China for producing shale oil.The present-day in situ stress state is of practical implications for the exploration and development of shale oil;however,few studies are focused on stress distributions within the Chang 7 reservoir.In this study,the present-day in situ stress distribution within the Chang 7 reservoir was predicted using the combined spring model based on well logs and measured stress data.The results indicate that stress magnitudes increase with burial depth within the Chang 7 reservoir.Overall,the horizontal maximum principal stress(SHmax),horizontal minimum principal stress(Shmin) and vertical stress(Sv) follow the relationship of Sv≥SHmax>Shmin,indicating a dominant normal faulting stress regime within the Chang 7 reservoir of Ordos Basin.Laterally,high stress values are mainly distributed in the northwestern parts of the studied region,while low stress values are found in the southeastern parts.Factors influencing stress distributions are also analyzed.Stress magnitudes within the Chang 7 reservoir show a positive linear relationship with burial depth.A larger value of Young's modulus results in higher stress magnitudes,and the differential horizontal stress becomes higher when the rock Young's modulus grows larger.

Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation(Late Triassic),Binchang area,Ordos Basin,China

Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accu- mulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sand- stone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. How- ever, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.

Oderly accumulation theory of shale system petroleum resource and its prospecting significance-A case study of Chang 7 Member of Yanchang Formation in Ordos Basin

1 Introduction Shale formations bear abundant mineral resource and*unconventional petroleum resource,and the unconventional petroleum resource that contain in the shale formation should be integrated and researched.

Sedimentary characteristics and model of gravity flow channel-lobe complex in a depression lake basin:A case study of Chang 7 Member of Triassic Yanchang Formation in southwestern Ordos Basin,NW China

To reveal the development characteristics and distribution of gravity flow sedimentary system under micro-paleogeomorphologic units of the Chang 7 Member of Triassic Yanchang Formation in the southwestern Ordos Basin,on the basis of the restoration of the paleogeomorphological form of the Chang 7 depositional period by the impression method,each micro-paleogeomorphologic unit was depicted in-depth,and the characteristics and development models of gravity flow deposits in the study area were studied in combination with outcrop,core,mud logging and log data.The results show that:(1)The paleogeomorphology in the Chang 7 depositional period was an asymmetrical depression,wide and gentle in the northeast and steep and narrow in the southwest.Three sub-paleogeomorphologic units were developed in the basin,including gentle paleo-slope,paleo-slope and paleo-depression,and they can be further subdivided into eight micro-paleogeomorphologic units:bulge,groove,slope break belt,plain of lake bottom,deep depression of lake bottom,paleo-channel,paleo-ridge of lake bottom,and paleo-uplift of lake bottom.(2)There are 9 types of lithofacies and 4 types of lithofacies assemblages of Chang 7 Member.According to lithofacies composition and lithofacies vertical combination,the gravity flow deposit is further divided into 5 types of microfacies:restricted channel,unrestricted channel,natural levee,inter-channel,lobe.(3)Paleogeomorphology plays an important role in controlling sediment source direction,type and spatial distribution of sedimentary microfacies,genetic types and distribution of sand bodies in Chang 7 Member.

Laminae combination and shale oil enrichment patterns of Chang 73 sub-member organic-rich shales in the Triassic Yanchang Formation, Ordos Basin, NW China

The Chang 73 sub-member of Triassic Yanchang Formation in the Ordos Basin was taken as an example and the lamina types and combinations,reservoir space features and shale oil enrichment patterns in organic-rich shale strata were investigated using core observation,thin section analysis,XRF element measurement,XRD analysis,SEM,high solution laser Raman spectroscopy analysis,and micro-FTIR spectroscopy analysis,etc.According to the mineral composition and thickness of the laminae,the Chang 73 organic-rich shales have four major types of laminae,tuff-rich lamina,organic-rich lamina,silt-grade feldspar-quartz lamina and clay lamina.They have two kinds of shale oil-bearing layers,"organic-rich lamina+silt-grade feldspar-quartz lamina"and"organic-rich lamina+tuff-rich lamina"layers.In the"organic-rich+silt-grade feldspar-quartz"laminae combination shale strata,oil was characterized by relative high maturation,and always filled in K-feldspar dissolution pores in the silt-grade feldspar-quartz laminae,forming oil generation,migration and accumulation process between laminae inside the organic shales.In the"organic-rich+tuff-rich lamina"binary laminae combination shale strata,however,the reservoir properties were poor in organic-rich shales,the oil maturation was relatively lower,and mainly accumulated in the intergranular pores of interbedded thin-layered sandstones.The oil generation,migration and accumulation mainly occurred between organic-rich shales and interbedded thin-layered sandstones.

Geological characteristics and exploration of shale oil in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin, NW China

A set of shale-dominated source rocks series were deposited during the heyday of lake basin development in the Member 7 of Triassic Yanchang Formation,Ordos Basin,and the thickness is about 110 m.Aimed at whether this layer can form large-scale oil enrichment of industrial value,comprehensive geological research and exploration practice have been carried out for years and obtained the following important geologic findings.Firstly,widely distributed black shale and dark mudstone with an average organic matter abundance of 13.81%and 3.74%,respectively,lay solid material foundation for the formation of shale oil.Secondly,sandy rocks sandwiched in thick organic-rich shale formations constitute an oil-rich"sweet spot",the average thickness of thin sandstone is 3.5 m.Thirdly,fine-grained sandstone and siltstone reservoirs have mainly small pores of 2–8μm and throats of 20–150 nm in radius,but with a large number of micro-pores and nano-throats,through fracturing,the reservoirs can provide good conductivity for the fluid in it.Fourthly,continued high-intensity hydrocarbon generation led to a pressure difference between the source rock and thin-layer reservoir of up to 8–16 MPa during geological history,driven by the high pressure,the oil charged into the reservoirs in large area,with oil saturation reaching more than 70%.Under the guidance of the above theory,in 2019,the Qingcheng Oilfield with geologic oil reserves of billion ton order was proved in the classⅠmulti-stage superimposed sandstone shale reservoir of Chang 7 Member by the Changqing Oilfield Branch through implementation of overall exploration and horizontal well volume fracturing.Two risk exploration horizontal wells were deployed for the classⅡthick layer mud shale interbedded with thin layers of silt-and fine-sandstones reservoir in the Chang 73 submember,and they were tested high yield oil flows of more than 100 tons per day,marking major breakthroughs in petroleum exploration in classⅠshale reservoirs.The new discoveries have expanded the domain of unconventional petroleum exploration.

Distribution of shallow water delta sand bodies and the genesis of thick layer sand bodies of the Triassic Yanchang Formation, Longdong Area, Ordos Basin

Based on the cores from 47 wells and logging data of 130 wells,the main types of sand bodies in the shallow water delta of the Chang 8_(2) sub-member in the Triassic Yanchang Formation,Longdong Area,Ordos Basin were comprehensively analyzed,the distribution characteristics of sand bodies more than 20 m thick were identified,and the genetic types of the thick sand bodies were sorted out.In this region,thick-bed sand bodies can be divided into two types according to the shape and position:type 1 is the sand body in wide strip and is distributed between the average high water mark and the average low water mark;type 2 is distributed on both sides of the average low water mark and is in irregular lump shape.Based on the principle of the volume distribution of sediments and the change rule of accommodation space,the genetic models of two types of thick-bed sand bodies in the Chang 8_(2) sub-member are superimposed distributary channel sand bodies in high accommodation space and superimposed composite sand bodies in low accommodation space.

Imaging-Based Characterization of Perthite in the Upper Triassic Yanchang Formation Tight Sandstone of the Ordos Basin,China

This work investigated the element distribution of perthite from the Upper Triassic Yanchang Formation tight sandstone in the Ordos Basin of northern China by field emission scanning electron microscopy(FE-SEM) and energy dispersive spectrometer(EDS). FE-SEM results indicate significant differences in the morphology of Na-rich feldspar when K-rich feldspar is the main component of the perthite. EDS results show that different types of perthite have clearly defined differences on different element indexes. Additionally, indexes such as average-weight-K(K-rich)/Na(Na-rich), maximumweight-K(Na-rich)/Na(Na-rich) and average-atomic-K(K-rich)/Na(Na-rich) might be the most effective ones to identify perthite types. Perthite is divided into six main types, i.e., perthite with thick parallel stripe distribution, with thin parallel stripe distribution, with lumpy stripe distribution, with dendritic stripe distribution, with encircling stripe distribution, and with mixed stripe distribution.