China companies have a certain amount of oil assets overseas. These oil assets belong to the resource countries, as well as Chinacompanies and joint ventures, and are subject to contract periods. How to manage these assets and make full use of these oil reserves has become a subject for the formulation of China company reserves standard. In order to meet the needs of international oil cooperation, the standard is based on SPE-PRMS 2018, and reserves are divided into Class P reserves and Class C reserves. Generally, the oil and gas reserves that have development plan and have been approved by the company or the government of resource country are classified as Class P reserves, and the rest are classified as Class C reserves. At the same time, in order to manage underground oil resources and bridge with China reserves standard, the company reserves standard gives more connotation to reserves. Geological reserves, technical recoverable reserves and remaining technical recoverable reserves can be divided into Class P and Class C reserves, and it can be subdivided into P1, P2, P3, C1, C2 and C3 categories. The company reserves standard gives consideration to both resource management and asset management, and adopts the complete reserves management sequence of geological reserves, technical recoverable reserves (contract period technical recoverable reserves, contract period interested technical recoverable reserves) and remaining technical recoverable reserves (contract period remaining technical recoverable reserves and contract period interested remaining technical recoverable reserves).

Tight sandstone reservoirs have smaller pore throats, complex structures, strong heterogeneity in the pore throat system, and significant differences in fluid distribution. To reveal the occurrence characteristics of movable fluids in tight sandstone reservoirs, the typical tight sandstone core samples from the Chang 4+5 reservoir of the Upper Triassic Yanchang Formation of the Ordos Basin were selected and the micro pore throat classification standard was established by using casting thin slices, field emission scanning electron microscopy (FE-SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR) technology. The test analysis results showed that the Chang 4+5 tight sandstone reservoir can be divided into Type I, II, and III reservoirs by the different pore throat structure. Type I reservoirs have well-developed pore throats, good connectivity, low threshold pressure (0.42 MPa on average), high movable fluid saturation (46.82% on average), and a minimum pore throat radius of movable fluid (0.056 μm on average). The pore throat structure and fluid production degree of the other two reservoir types gradually deteriorated. Note that the tight sandstone reservoirs of the Chang 4+5 reservoirs have smaller pore throats but high movable fluid content in smaller pore throats (17% on average). With the deterioration of the pore throat structure in reservoir, the degree of fluid utilization in larger pore throats decreases (74.33%–57.33%), whereas the degree of fluid utilization in smaller pore throats does not change significantly (25.67%–13.82%). Many factors affect movable fluid parameters. The movable fluid parameters of tight sandstone reservoirs in the Chang 4+5 reservoir have a good positive correlation with permeability (R2 = 0.85) and sorting coefficients (R2 = 0.88). The movable fluid parameters of smaller and larger pore throats have no obvious correlation with a single factor. Multiple factors affect the fluid occurrence characteristics of different scale pore throats.

The Chang7 Member of the Triassic Yanchang Formation in the Ordos Basin is a typical continental tight oil province. The geological conditions and the main controlling factors for the formation of tight oil province in Chang 7 Member were studied based on extensive core analysis data, laboratory simulation tests and practical work of tight oil exploration and development in the basin. The tight oil in the Chang 7 Member is characterized by wide distribution, excellent source rock conditions, tight sandstone reservoirs, complicated pore-throat structure, poor physical properties, high oil saturation, high quality oil, and low pressure coefficient. During the depositional period of Chang7 Member, the bottom shape of the basin was steep at southwest and gentle at northeast, the tectonic movements were active, favorable for the deposition of source rock and reservoir; the widespread high quality source rock can provide sufficient oil supply for the large tight oil province; the large scale sand bodies provide good reservoir condition for the large tight oil province; the abundant structural fractures in the tight reservoir act as pathways for tight oil migration; and the stable deposition and tectonic evolution of the basin provide good preservation conditions for the tight oil province. The main controlling factors of Chang7 Member tight oil enrichment are as follows: (1) good configuration of source rock and reservoir and constant charging are the key to the formation of the tight oil province; (2) abundant micro-scale pores are the premise of tight oil enrichment; (3) strong and sustained charging guarantees the enrichment and high yield of oil in the tight reservoirs.

Based on the rock physical analysis of tight gas reservoirs in He8 Member of Permian Lower Shihezi Formation of Ordos Basin, and considering that bulk modulus is sensitive to pore fluid, this study proposes a fluid property detection method by compression coefficient of tight sandstone reservoirs under the constraint of reservoir lithofacies. In this method, lithofacies is identified first by calculating distribution of tight sandstone facies with the cross plot of Vp and Vs obtained from pre-stack seismic inversion; secondly, the compression coefficient is calculated by P-wave impendance and velocity from stable pre-stack seismic inversion with the restriction of lithofacies (excluding the influence of clay content); and finally, pore fluid properties are determined using the differences of compression coefficients in gas and water layers. Its application in tight gas exploration and development in Sulige gas field of Ordos Basin shows that this pore fluid prediction method by calculating compression coefficient can effectively and efficiently delineate the distribution of gas-bearing and water-bearing sandstone.

Using core, well logging, geological analysis, production, and test data, this study characterizes a method of well logging for identifying lithology, oil or water layers, and thickness of oil layers of tight sandstone reservoir in the Chang 7 member of the Upper Triassic Yanchang Formation, southwestern Ordos Basin, China. This reservoir consists of two rock types: fine sandstone and siltstone. The fine sandstone has distinct oil traces and flecks, which strongly indicate the presence of oil, and this rock is therefore the superior reservoir. The siltstone exhibits essentially no oil shows and thus is an unproductive layer. A method of density and neutron curve normalization and overlay was used to identify the tight fine sandstone, tight siltstone, mudstone, and shale. The criteria for identifying the tight fine sandstone are a natural gamma value of less than 93 API (American Petroleum Institute units) and a difference between the normalized curves exceeding 0.05. A resistivity-porosity plot was used to identify oil or water layers relatively effectively. The criteria for identifying the tight oil layers are (1) a resistivity exceeding 28 Ω·m and a porosity exceeding 9.5% or (2) a resistivity between 20 and 28 Ω·m, a porosity exceeding 9.5%, and an oil saturation exceeding 65%. Based on the lithologic identification via normalization and overlay of the density and neutron curves and the criteria for distinguishing the tight oil layers, the thickness of the tight fine sandstone oil reservoir was accurately determined by overlaying the normalized difference curve on the resistivity curve.

Algae with siliceous structures are proposed to have originated following the Permian-Triassic extinction or even earlier, but there have been no robust fossil records to indicate their existence before the Jurassic-Cretaceous Periods. Recently, abundant chrysophyte (classes Chrysophyceae and Synurophyceae) cyst fossils were discovered in a Late Triassic lacustrine stratum deposited similar to 228-235 m.y. ago in the Ordos Basin, China. The high morphological variety of the discovered cyst fossils suggests that the chrysophytes were remarkably diversified, and that their common ancestors emerged even earlier. This discovery provided fossil records of the early evolution of algae with siliceous structures. The cyst fossils formed thin laminations in organic-rich shale, reflecting variable environmental conditions in the ancient lake basin.

Abstract: Efficient large-scale development of ultra-low-permeability reservoirs (0.3–1 mD) has been achieved in the Changqing Oilfield, Ordos Basin of China. According to unique features of petroleum exploration and development in this basin, tight oil herein refers to petroleum that occurs in oil-bearing shales and interbedded tight sandstone reservoirs adjacent to source rocks with ambient air permeability <0.3 mD. Tight oil in tight sandstone and shale have generally not yet experienced large-scale long-distance migration. In the Yanchang Formation, tight oil has mainly accumulated in the semi-deep to deep lacustrine facies, typically in oil-bearing shales and tight sandstones of the 7th member oil-bearing formation and tight sandstones of the 6th member oil-bearing formation in the center of the basin. Tight oil resource in the Ordos Basin is characterized by wide spatial distribution, excellent source rocks, extremely tight sandstone reservoirs, complex pore throat structures, poor physical properties, high oil saturation, good crude-oil properties, and low reservoir pressure. A fundamental feature of the continuous oil and gas accumulation in tight oil reservoirs is the widespread development of nano-scale pore-throat systems. In the Yanchang Formation, most of connected pore throats in tight sandstone reservoirs have diameters greater than critical pore throat diameter, allowing oil and gas migration in the tight reservoirs. According to contact relationship between tight reservoirs and source rocks, three types of tight oil reservoirs are identified in the Yanchang Formation, i.e., tight massive sandstone reservoir, sand - shale interbed reservoir, and oil-bearing shale reservoir. In the Ordos Basin, tight oil is widely distributed in the 6th and 7th members of the Yanchang Formation, with total resources estimated to be 3×109 t. These include > 1×109 t of oil resources in shale in the 7th member of the Yanchang Formation and approximately 0.9×109 t and 1.1×109 t of tight sandstone oil resource in the 6th and 7th members of the Yanchang Formation, respectively. These tight oil resources are the realistic resources addition for the oilfield, which can ensure an annual production of 50×106 t of oil and gas equivalent and maintain long-term stable oil production in the Changqing Oilfield, Ordos Basin, China. Key words: tight oil, tight sandstone reservoir, shale reservoir, resource potential, Yanchang Formation, Ordos Basin

PetroChina Changqing Oilfield Company (hereinafter referred to as Changqing Oilfield) built the largest oil and gas production base of China in the Ordos Basin in 2013, achieving the yearly natural gas production of 375 × 108 m3 in 2015. For the further sustainable and stable production and quality and benefit improvement, such great achievements made in the 12th Five-Year Plan were first summarized, and the relevant favorable conditions for natural gas development were also analyzed as follows: abundant natural gas resources; increasingly mature E&P technologies; continuously improved delicacy management; and a surging demand for natural gas. Then, challenges to natural gas development were also discussed, including obvious deterioration of natural gas resources; diminished capacity of stable production of produced gasfields; increasing number of low-yield wells; and prominent supply–demand contradiction in present natural gas market. Finally, its prospect in the 13th Five-Year Plan was studied: to carry out natural gas exploration with focus on Upper Paleozoic tight gas, Lower Paleozoic carbonate rocks, and new areas and new domains, so as to achieve rapid growth of natural gas reserves and orderly replacement in exploration domains; to make rational development planning with focus on the stable production of mature gasfields; and to enhance tight gas recovery and strengthen capacity building in new areas, so as to achieve a steady rise of annual gas production in the giant gas province. It is expected that by the end of 13th Five-Year Plan period, Changqing Oilfield will achieve its annual gas production of up to 400 × 108 m3.

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editorial Board, the journal has decided to retract this article, since the authors are unable to provide a report of the original experimental data of the relevant study to prove the authenticity of the research results.

The geological conditions and exploration potential of shale oil in Chang7 Member, Upper Triassic Yanchang Formation, Ordos Basin, were studied from various aspects, including petrographic characteristics, storage ability, geochemical features, friability and mobility of hydrocarbon in the source rock, etc. A classification criterion of lithofacies for Upper Triassic Chang 7 source rock in Ordos Basin were established based on the correlation between lithology, organic carbon content and logging parameters, from which, the spatial distribution and development scale of two types of shale, black shale and dark massive mudstone, have been predicted. Qualitative and quantitative characterization of the micro-structures of Chang7 source rock using state-of-the-art microscopic facilities including argon ion milling – field emission scanning electron microscopy (FESEM), focused ion beam scanning electron microscopy (FIB-SEM) and Nano-CT reveal that the dominant pore types in Chang7 source rock are intra-granular pores and inter-granular pores; the pores and throats in the two kinds of lithofacies are both nano-scale, and the dark massive mudstone has better physical properties than the black shale. The Chang7 shale oil resources and mineability were evaluated based on the parameters from geochemical experiments on the source rock, including pyrolysis S1, chloroform bitumen ‘A’, TOC and thermal maturity, free hydrocarbon content, as well as geo-mechanical properties such as brittle mineral content and development of fractures. With large scale of favorable lithofacies, good storage ability and abundant hydrocarbon, Chang7 Member has the material basis for shale oil occurrence and accumulation, in addition, the shale oil there has accumulated greatly and has favorable properties for flowing in nano-scale pores and throats. All these show that Chang7 Member has high potential for shale oil exploration, in which, the dark massive mudstone is a more favorable target for shale oil exploration under the present technical conditions.

The origin of formation water salinity variation, in the bottom of upper Triassic, the northwest of Ordos basin, China, is studied. 91 formation water samples show that water salinity is characterized by a wide range (5g/L-95g/L) and a complex plane distribution. In order to find out the main cause of such distribution complexity, core data, scanning electron microscope(SEM) images, chemical analysis results of formation water and log data are deeply analyzed from perspectives of diagenesis and tectonism. And then, their characteristics are presented as the followings. In high salinity area, tuff is found growing through SEM images and well log response characteristics. Meanwhile, when total salinity is higher than 40g/L, the condition of Ca2+ is opposite to that of Na++K+, for its increasing rate of concentration increases with total salinity accumulating. It means that besides albitization of plagioclase, some other factors should contribute to such abnormal phenomenon. In low salinity area, while, with fracture and faults developing, water type changes from CaCl2 to MgCl2, NaHCO3 or Na2SO4 type. Thus, the main causes are proposed to be composed of two aspects. One covers tuff alteration and later diagenesis for the high salinity. To be specific, montmorillonite, developed from tuff alteration, absorbs Ca2+ from formation water selectively and then ions migrate, during which more Na++K+ get lost, while more Ca2+ reserved. Afterwards, those reserved Ca2+ get released with montmorillonite transforming to illite, which results in a loss of Na++K+ and accumulation of Ca2+. Lots of ions are released into formation water during that process and later diagenetic process, which leads to the high water salinity. The other aspect is the development of faults and fractures, through which, formation water becomes the mixture of connate water and upper low salinity water. And that is the main cause of low salinity. The results reveal that in any other analogous clastic reservoirs, formation water salinity probably varies significantly because of complex diagenesis and tectonism. It is necessary to use variable formation water salinity in formation evaluation.

By the end of 2014, the giant Shenmu Gas Field had been found in the Ordos Basin with an explored gas-bearing area of 4069 km2 and the proved geological gas reserves of 333.4 billion m3. This paper aims to review the exploration history of this field and discusses its reservoir-forming mechanism and geological characteristics, which may guide the further discovery and exploration of such similar gas fields in this basin and other basins. The following research findings were concluded. (1) There are typical tight sand gas reservoirs in this field primarily with the pay zones of the Upper Paleozoic Taiyuan Fm, and secondly with those of the Shanxi and Shihezi Fms. (2) Gas types are dominated by coal gas with an average methane content of 88% and no H2S content. (3) The gas reservoirs were buried 1700–2800 m deep underneath with multiple pressure systems and an average pressure coefficient of 0.87. (4) The reservoir strata are composed of fluvial delta facies sandstones with an average porosity of 7.8% and permeability of 0.63 mD, having high pressure sensibility and a strong water-locking effect because the pore throat radius are mostly less than 1 μm. (5) There are different dynamics at various stages in the gas reservoir-forming process. The abnormal well-developed strata pressure was the main reservoir-forming force at the Early Cretaceous setting stage while the fluid expansibility became the main gas-migrating force at the uplift and denudation stage after the Early Cretaceous period. (6) Gas reservoirs with ultra-low water saturation are mainly controlled by many factors such as changes of high temperature and high pressure fields in the Late Jurassic and Early Cretaceous periods, the charging of dry gas at the highly-mature stage, and the gas escape and dissipation at the post-reservoir-forming periods. (7) Natural gas migrated and accumulated vertically in a shortcutting path to form gas reservoirs. At such areas near the source rocks, large-scale gas reservoirs were easily found with plenty of gas sources and high gas saturation; but at those far from the source rocks, relatively small-scale and mostly secondary gas reservoirs were discovered.

页岩油是重要的非常规油气资源之一.利用地球化学、测井、X射线衍射、扫描电镜等资料对鄂尔多斯盆地延长组长7页岩油的地质特征展开了研究.研究结果表明,鄂尔多斯盆地延长组长7湖相页岩层十分发育,根据岩性组合、地球化学等特征可以将长7页岩划分为"砂岩-页岩互层"、"厚层状Ⅰ类页岩"与"厚层状Ⅱ类页岩"等三种类型.长7湖相页岩层分布范围广且厚度较大,提供了页岩油成藏的基本地质条件;岩石中脆性矿物含量高,为页岩油勘探开发的必要条件;有机质丰度高、有机质类型好且有机质热演化程度适中,奠定了页岩油聚集成藏的物质基础;虽然长7页岩十分致密,但在扫描电镜下可见微米级裂隙,并且湖盆中部等地区发育物性相对较好的薄砂岩夹层,提供了页岩油的储集空间.分析测试资料显示,长7页岩含油性较好,并具有一定的含气量;薄砂岩夹层油源充足,紫外光下具有较强的荧光显示.综合地质条件及含油气性研究,"砂岩-页岩互层"分布区被评价为最有利的页岩油勘探目标区,"Ⅰ类页岩"较"Ⅱ类页岩"生烃条件更好,脆性更强,微米级裂缝更为发育,含油气性较好,为第二类有利勘探目标区.

致密砂岩目前已成为中国油气勘探开发的重点领域.以鄂尔多斯盆地陇东地区长7段为代表的典型致密砂岩主要发育辫状河三角洲和重力流沉积砂体,其中重力流沉积砂体具有很大的勘探开发潜力.在区域构造、古气候、古地貌和沉积体系等研究基础上,应用沉积模拟技术,再现了研究区长7段重力流沉积砂体形成过程及其主要控制因素,在实验条件下,半深湖-深湖区的沉积微相类型主要包括砂质碎屑流、浊积岩、深湖泥等.实验研究表明,影响重力流沉积砂体的形成及其演化的主要控制因素有:(1)泥砂含量决定了重力流的沉积类型;(2)湖盆底形控制重力流的砂体展布;(3)湖水位及流速决定重力流的沉积厚度及规模;(4)多物源交汇决定重力流的横向连通.并通过实验结果对比分析,预测了有利储集层分布区域.综合陇东地区原始地质模型和实验模拟成果,建立了沉积模拟实验条件下陇东地区三叠系长7段沉积模式.

Three tephra layers in the middle part of the Yahe section have been discovered in Liushan Basin in Nanzhao County, Henan Province. This paper presents zircon U–Pb ages measured on the sensitive high-resolution ion microprobe (SHRIMP)-II from zircons separated from one of these tephra layers. Thirteen analyses yielded a 206Pb/238U weighted mean age of 237 ± 2.5 Ma (95 % confidence, Mean Square of Weighted Deviates = 0.72). This age suggests that the Yahe section strata were deposited as early as the Middle Triassic. The new SHRIMP U–Pb data on the tephra from the Yahe section provides strong age constraints on the Triassic stratigraphic subdivision in Liushan Basin. By comparing the reported ages for similar tephra exposed around Liushan Basin, we infer that the tephra layers may be the sedimentary response to the first episode of the Indosinian tectonism and can be used as a marker bed for regional stratigraphic correlation.

Based on practices of gas exploration and development in the Ordos Basin, this paper analyzes the controlling factors of large-scale accumulation conditions and distribution characteristics of coal-derived gas, and makes conclusions on the exploration progress of coal-derived gas based on present exploration situations. The approach has proven coal-derived gas reserves of 5.24×1012 m3 in the Ordos Basin. Twelve coal-derived gas fields have been found so far, mainly distributed in Upper Paleozoic Carboniferous-Permian clastic rocks and Lower Paleozoic Ordovician marine carbonate reservoirs. Tight sandstone gas reservoirs are developed in the Upper Paleozoic. The features of widely-distributed hydrocarbon-generation, interlayered with large-scale distribution of sandstones, where the reservoirs have become tight and followed by natural gas accumulation later, close range migration and efficient hydrocarbon accumulation, has resulted in a large area of distribution of tight sandstone gas zones. The Sulige large gas field has been found with proven coal-derived gas reserves of 3.49×1012 m3, and two large-scale gas reserve replacement fields have been developed and implemented in the eastern part of the basin and in the Longdong area. Marine carbonate gas reservoirs are developed in the Lower Paleozoic. Gas generated from overlying source rocks have migrate downwards and accumulated in Ordovician weathering crust karst and dolomite reservoirs. The Jingbian gas field was discovered, with proven coal-derived gas reserves of 7000×108 m3, meanwhile, several gas-rich regions have been found in the eastern side of the dolomite.

By analyzing evolution principles and features of lands' deformation in vertical direction and monitoring data about underground water and layerwise marks,mechanism of land subsidence in Xi'an is studied in this paper. As confined water over- exploitation and tectonic activity are the main cause of land subsidence,the paper puts forward four precautionary measures——strengthen research development on urban geology and mechanism of geological disasters,pay attention to the comprehensive effects of nature and mankind,make city development plan with the consideration of local geology and development of disaster,and finally take proper utilization of groundwater.

Taking the Yanchang Formation in the southwestern Ordos Basin as an example, based on the studies of petrology, sedimentary characteristics, measurements of paleocurrent directions shown by outcrops, as well as framework grains and minerals in sandstones and their distribution characteristics, the paper studied the provenance and depositional characteristics of the research area. The study shown that early sediment source was mainly from the southwest of the basin with some contribution from the south. This was followed by subsequent deposition of sediments sourced from the southwestern and southern areas with some contribution from the northeast of the basin. Debris of the Chang 8 sandstone is dominated by metamorphic rocks, small amount of sedimentary and volcanic rocks. Debris of the Chang 6 sandstone is dominated by sedimentary rocks, and a small amount of metamorphic rock and volcanic rocks. The presence of more felsic materials of the late Archean in the Chang 6 sandstone suggested that the western margin of the basin uplift had undergone intensive erosion, resulting in a large number of younger strata eroded and added to the source rocks. The distribution of the Yanchang Lake basin was along northwest - southeast. It mainly developed braided fluvial and braided delta depositional systems in the southwestern slope, meandering river delta depositional system in northeastern terrain-relief and turbidite fan deposits in the deep lake systems.

Extended abstract Tight sands gas, coalbed methane and shale gas are three kinds of typical unconventional natural gas. With the decrease of conventional oil and gas reserves, unconventional reservoirs, especially unconventional gas-bearing reservoirs have become the most important supplement and alternative resources. At present, studies of the tight sands gas, coalbed methane and shale gas have become the hotspot of energy research all over the world. As the unconventional gas-bearing reservoirs, they all characterize as ultra-low porosity and permeability, the pore sizes are nanoscale. For tight gas sands, the porosity is main lower than 10.0%, and the permeability is always lower than 0.5 mD, the pore size ranges from 300 to 900 nm, the proportion of nanoscale pore is nearly 85.0%, and the critical pore radius of the lower limit is 20 to 60nm (Zou et al., 2011). For coalbed methane reservoir, the porosity is always lower than 6.0%, and the permeability is lower than 0.002mD, the pore size is lower than 130nm (Yan et al., 2008). In the shale gas formation, the porosity is lower than 10.0%, and permeability is even lower than 0.002mD, the pore size ranges from 80 to 200nm (Erik et al. 2013). The information of pore structure is very important in unconventional gas-bearing reservoirs evaluation. The coalbed methane and shale gas are authigenic reservoirs, and the gas contents are dominant by the adsorbed gas (Sun et al., 2011). Hence, the developmental degree of micropore is of great importance in effective unconventional coalbed and shale gas reservoirs evaluation. Zhao et al. (2012) pointed out that the more development of micropore volumen, the higher content of gas in coalbed methane reservoir. However, in tight gas sands, the micropore content is inversely proportional to the gas production. In this paper, to understand the pore structure of the effective unconventional gas bearing reservoirs, we review the microstructure of these three kinds of unconventional gas bearing formation, and compare the NMR T2 spectra at laboratory and field conditions. The results illustrate that in tight gas sands, the lab NMR T2 spectra in the Xujiahe tight gas sands of Sichuan basin are dominant as unimodal distribution; the T2 relaxation time of the main peak is lower than 100.0 ms. The Bound fluid volume (BFV) ranges from 35.0% to 60.0%, and the average value is 44.3%, and the NMR T2 distributions of field condition are also main unimodal, the T2 distribution are wide in formations with high gas production, and on the contrary, for formations with low gas production, the T2 distributions are narrow and unimodal, and the largest T2 transverse relaxation time is relative lower. In coalbed methane and shale gas reservoirs, the lab NMR T2 spectra are also main unimodal, and the largest T2 transverse relaxation times are far less than that of the tight gas sands. In field condition, the T2 spectra in coalbed methane formations are narrow, and the largest T2 transverse relaxation times are lower than those of the adjacent strata (Adrian et al., 2010). At present, we cannot collect the field NMR logs, hence, the NMR T2 distribution for shale gas reservoir in field condition cannot be compared. In the meanwhile, comparisons of lab NMR spectra for core samples drilled from coalbed methane and shale gas reservoirs illustrate that there are the rapid relaxation components, and a insulated peak with ultra-short relaxation time exist in the NMR T2 spectra under fully water saturated, and there are T2 spectra in dry core samples. For core samples drilled from coalbed methane reservoirs, the relaxation time of insulated peak is lower than 2.0 ms, and for core samples drilled from shale gas reservoirs, the relaxation time of insulated peak is lower than 1.0 ms. With the lab NMR experimental measurements of braise, Guo et al. (2007) and Zhao et al. (2011) pointed that the signals on NMR spectra with rapid relaxation time are contributed by the blind pore. Odusina et al. (2011) and Sulucarnain et al. (2011) also obtained the same conclusion that the signals on NMR spectra with rapid relaxation time for core samples drilled from shale gas reservoirs are contributed by the disconnected blind pore.

A large area of deep-water sandstone was formed under the combined action of delta and gravity flow in the central Ordos Basin during the depositional period of the Ch6 and Ch7 oil-bearing formations of the Yanchang Formation. According to U-Pb dating of zircons by the method of LA-ICP-MS, the ages of the tuff layers at the base and upper parts of the Ch7 oil-bearing formation are 228.2±6.0 Ma and 222.2±2.0 Ma, respectively. Based on the ages of the tuff layers, it is suggested that the gravity flow complexes at the middle-upper Ch7 and lower-middle Ch6 oil-bearing formations were formed 215 to 224 Ma. The timing of deposition of the tuff and gravity flow is roughly the same as the period when tectonics are most active in the Qinling area, i.e. middle Indosinian period. Therefore, the even, widespread tuff at the bottom of the Ch7 oil-bearing formation and the thick deep-water sandstone of the Ch6 oil-bearing formation deposited during an early lake regression period, are the direct outcomes of the tectonic event. Tuff layers are deposited under the influence of the middle Indosinian tectonic events, and the clastic composition of the top and base tuff layers vary greatly. The events control the formation of large-area deep-water depressions, change of depositional system, migration of the depo-center, and occurrence of gravity flows.