Discussion of the Mode and Mechanism of Oil and Gas Accumulation in the Nanbaxian Pool in the North of the Qaidam Basin

Because of the difference ofoil and gas accumulation condition between the hanging wall and the footwall of a fault, there is a peculiar accumulation mechanism that oil and gas mainly exists in the hanging wall of the basement fault, but in the footwall of the shallow detachment fault in the Nanbaxian pool. The oil and gas of the Nanbaxian pool came from the mature Jurassic hydrocarbon source rock of the Yibei depression located at the south of the Nanbaxian pool. Firstly, the oil and gas accumulated in the traps of the hanging wall of the basement fault by way of the unconformity and the basement faults, and turned into some primary deep pools; and then, the shallow detachment fault that formed in the later tectonic movement broke into the deep primary pools, which caused the oil and gas migration upwards along the basement faults and the shallow detachment faults and the evolvement into some secondary oil and gas pools later. The history of the Nanbaxian oil and gas accumulation can be summarized successively as the syndepositional upheaval controlled by faults; single hydrocarbon source rock; unconformities and faults as migration channels; buoyancy, overpressure and tectonic stress as dynamic forces; multistage migration and accumulation of oil and gas; and finally an overlapped double-floor pattern of oil and gas accumulation. The most important explorative targets in the north of the Qaidam Basin are traps connected with the primary pools in the footwall by shallow detachment faults.

GEOTHERMAL EVOLUTION AND TECTONOSEDIMENTATION OF THE NORTH CHINA CRUSTOBLOCK AND THEIR EFFECTS ON THE FORMATION OF OIL/GAS POOLS

The geothermal history of North China can be divided into at least four stages, i. c. Archaean (much hisher geothermal) stage, Paleoprotcrozoic (hish geothermal)stage, which resulted in four geotectonic stages (pregeosynehoe, geosyncline, platform and diwa stage) in the region. The geothermal field consists of three subgeothermal fields, theupper subgeothermal field with its depth of less than 2000 m, the middle subgeothermal field ranging from 2000 m to 5000 m in depth and the lower subgeothcrmal field locating at more than 5000 m in depth in North China. Sis thermostructural layers are recognised in North China, i. e. the mantle, the lower erust with its heat generation ratc of 0. 6 HGU. Oeothermal field is Corresponding to tectonosedimentary divisions in North China, controlling the tectonosedimentation, the evolution of souree rocks and the formation oF the oil/gas pools.

Conditions for the Formation of Oil and Gas Pools in Tertiary Volcanics in the Western Part of the Huimin Sag, Shandong and Their Distribution

Conditions for the Formation of oil and gas pools in Tertiary volcanics in the western part of the Huimin sag, Shandong and then (?)stribution have been studied based on the geological, seismic and well-logging information. In this paper, the types and lithofacies of the volcanic rocks in the western part of the Huimin sag are described; the relationship between rocks and electrical properties, the seismic reflection structures, the development and distribution of the volcanic rocks are expounded; and the fourfold role of the volcanic activities in the formation of the oil and gas pools is also dealt with. It is considered by the authors that the volcanic activities were not destructive to the formation of oil and gas pools but a factor favourable to the accumulation of organic matters and their conversion to hydrocarbon. The volcanic rocks might have served as reservoir rocks and cap rocks, or as a synsedimentary anticline. The prerequisites and important factors for the formation of oil and gas pools and their distribution are pointed out in the paper.

Formation conditions and exploration direction of large natural gas reservoirs in the oil-prone Bohai Bay Basin, East China

The Bohai Bay Basin is a typical oil-prone basin, in which natural gas geological reserves have a small proportion. In this basin, the gas source rock is largely medium-deep lake mudstone with oil-prone type Ⅱ2-Ⅱ1 kerogens, and natural gas preservation conditions are poor due to active late tectonic movements. The formation conditions of large natural gas fields in the Bohai Bay Basin have been elusive. Based on the exploration results of Bohai Bay Basin and comparison with large gas fields in China and abroad, the formation conditions of conventional large-scale natural gas reservoirs in the Bohai Bay Basin were examined from accumulation dynamics, structure and sedimentation. The results show that the formation conditions of conventional large natural gas reservoirs in Bohai Bay Basin mainly include one core element and two key elements. The core factor is the strong sealing of Paleogene "quilt-like" overpressure mudstone. The two key factors include the rapid maturation and high-intensity gas generation of source rock in the late stage and large scale reservoir. On this basis, large-scale nature gas accumulation models in the Bohai Bay Basin have been worked out, including regional overpressure mudstone enriching model, local overpressure mudstone depleting model, sand-rich sedimentary subsag depleting model and late strongly-developed fault depleting model. It is found that Bozhong sag, northern Liaozhong sag and Banqiao sag have favorable conditions for the formation of large-scale natural gas reservoirs, and are worth exploring. The study results have important guidance for exploration of large scale natural gas reservoirs in the Bohai Bay Basin.

Nanoscale mechanical property variations concerning mineral composition and contact of marine shale

Mechanical properties of shales are key parameters influencing hydrocarbon production – impacting borehole stability, hydraulic fracture extension and microscale variations in in situ stress. We use Ordovician shale(Sichuan Basin, China) as a type-example to characterize variations in mineral particle properties at microscale including particle morphology, form of contact and spatial distribution via mineral liberation analysis(MLA) and scanning electron microscopy(SEM). Deformation-based constitutive models are then built using finite element methods to define the impact of various architectures of fracture and mineral distributions at nanometer scale on the deformation characteristics at macroscale.Relative compositions of siliceous, calcareous and clay mineral particles are shown to be the key factors influencing brittleness. Shales with similar mineral composition show a spectrum of equivalent medium mechanical properties due to differing particle morphology and mineral heterogeneity. The predominance of small particles and/or point-point contacts are conducive to brittle failure, in general, and especially so when quartz-rich. Fracture morphology, length and extent of filling all influence shale deformability. High aspect-ratio fractures concentrate stress at fracture tips and are conducive to extension, as when part-filled by carbonate minerals. As fracture spacing increases, stress transfer between adjacent fractures weakens, stress concentrations are amplified and fracture extension is favored. The higher the fractal dimension of the fracture and heterogeneity of the host the more pervasive the fractures. Moreover, when fractures extend, their potential for intersection and interconnection contributes to a reduction in strength and the promotion of brittle failure. Thus, these results provide important theoretical insights into the role of heterogeneity on the deformability and strength of shale reservoirs with practical implications for their stimulation and in the recovery of hydrocarbons from them.

A Study of Thin Sandstone Reservoirs by High-resolution Seismic Inversion

In this paper seismic inversion was used as a key technique and the seismic wavelet most suitable to the actual underground situation was extracted with the higher-order statistics algorithm. The wavelets extracted in this way and the wavelets extracted with the seismic statistics techniques were used separately for inverting the seismic data of the southern part of Tahe oilfield, Tarim basin. The results showed that the resolution of the wavelet inversion with the higher-order statistics method was greatly improved, and the wavelet-inverted section could better distinguish the thin sandstone reservoirs of the upper and lower Carboniferous and their lateral distribution, providing a reliable basis of analysis for the study of thin sandstone reservoirs.

Foaming supramolecular surfactants for gas mobility control in naturally fractured carbonate reservoirs at high temperature,salinity,and hardness

Oil production and mainte nance are essential issues in naturally fractured reservoirs because they are the largest and most productive on earth.However,they present early water and gas channeling but could be remediated by using foaming agents to control these phenomena through blocking channeling areas.In Mexico these reservoirs have pressure up to 5,500 psi,high temperature up to 200℃,salinity up to400,000 ppm,and hardness up to 250,000 ppm;due to these thermodynamic conditions,there has been no available technology to form stable enough foams.In this work,a foaming supramolecular surfactant with the capability to chelate Ca^(2+)ions is examined.As a result,surfactant monomers are bridged by captured Ca^(2+)cations leading to the formation of high-molecular-weight oligomers,which significantly increment the viscosity of the solution improving the foam stability,and since at this manner the Ca2+cations are no longer available to precipitate as components of solid salts,the foaming supramolecular surfactant also performs as antiscalant.These observations are explained through quantum theoretical modeling.The foam is stable,effectively blocking the gas channels,whereas in presence of oil the foam is broken leading the oil to pass into the wellbore.The characteristic rheological properties of the foam allow its injection into the formation at a range of flow rates,foam qualities,and shear stress to achieve the flooding and the blocking of a variety of fractured carbonate formations,and the change of the wettability of the matrix,which is a desirable behavior in a huff and puff process,as reported in a previous publication about a successful pilot test of this foam.

Characteristics of China’s oil and gas pool formation in latest geological history

The structural activities took place extensively in the Asia continent during the Cenozoic era owing to the strong continent-to-continent collision and continuous compression between the India Plate and the Eurasia Plate. Huang Jiqing called such structural activities Himalayan movement. China’s sedimentary basins developed and took shape mainly during the Himalayan movement period. It is also the main period for formation and development of the oil and gas reservoirs. Of 366 large and medium-sized oil and gas fields currently found in China, 212 reservoirs were formed in the Neo- gene-Quaternary period. The proportion is as high as 68.2%. The oil and gas migration and accumu- lation in the latest geological period, which were controlled by the times, properties, styles and strength of the Himalayan movement, took place mainly in eight regions, such as the low uplift area of Bohai Sea, the onshore faulted sag area of Bohai Bay, anticlinorium zone in Daqing, the foreland fold-and-thrust belt in West China, the tilted structural zone in West China, the cratonic palaeohigh in the Tarim Basin, the zone of fault and fold belt in the East Sichuan Basin, and the biological gas zone in the East Qaidam Basin. The oil and gas pool formations in those regions have their own charac- teristics. With the great potential and broad prospect, those regions are the main exploration areas in China in the future.

Petroleum Exploration of Craton Basins in China

Craton basins are a significant petroliferous provenance. Having undergone multiple open- dose tectonic cycles and strong reworking of the late Cenozoic tectonic movement, the craton basins in China are highly broken. This has resulted in multi-source and multiphase hydrocarbon generation and later hydrocarbon accumulation so that a complicated spatial assemblage of primary, paraprimary and secondary oil-gas pools has been formed. The primary factors controlling hydrocarbon accumulation include hydrocarbon-generating depressions, paleouplifts, paleoslopes, unconformity surfaces, paleo-karst, faults and fissure systems as well as the later conservation conditions. In consequence, the strategy of exploration for China＇s craton basins is to identify the effective source rocks, pay attention to the different effects of paleohighs and late reworking, enhance studies of the secondary storage space, attach importance to the exploration of lithologic oil-gas reservoirs and natural gas pools, and approach consciously from the secondary oil pools to the targets near the source rocks. At the same time, a complete system of technologies and techniques must be built up.

Formation Models and Distribution of Oil and Gas Pools in Tarim Basin, China

This article reports the main formation models and distribution of the oil and gas pools in Tarim basin, China, including （1） occurrence of the found oil and gas pools, （2） main formation models of oil and gas pools, and （3） distribution law of oil/gas pools. Petroleum is distributed widely in the strata of Tarim basin from the Sinian at the bottom to the Neogene at the top. However, the found oil and gas fields are mainly distributed in Shaya （沙雅） uplift, Tazhong （塔中） uplift, and Kuche （库车） depression. This article presents 4 main formation models, namely, early formation and long-term preservation, early formation and late reformation, middle-late multiphase-multisource formation, late single-stage formation. Tarim basin is very rich in petroleum resources. Long-term inherited intrabasinal paleohighs and slope zones are the most favorable areas for accumulation of hydrocarbons, but the types of oil and gas pools are different from area to area. The control of unconformities and faults on hydrocarbon accumulating is prominent in Tarim basin. Preservation conditions are of utmost importance. Formation of some oil and gas pools is the result of reforming and re-accumulating of early accumulated hydrocarbons.

Geological constraints of giant and medium-sized gas fields in Kuqa Depression

There is a gas-rich and well-charged petroleumsystem in the Kuqa Depression where Triassic and Jurassicsource rocks play important roles. Distributed in an area ofmore than 10000 km and with a thickness of up to 1000 m,they are composed of dark mudstones, carbonaceous mud-stones and coal seams containing 6%, 40% and 90% of TOC,respectively, and are mainly the humic organic matter. Ashigh-quality regional cap rocks, the Neogene and Eogenegypsum rocks and gypseous mudstones matched well withthe underlying Neogene and Cretaceous-Eogene sandstones.They have formed the most favorable reservoir-seal assem-blages in the Kuqa Depression. Also the Jurassic sandstonesand mudstones formed another favorable reservoir-seal as-semblage. The traps are shaped late in the fold-thrust belt,mainly fixed in the Tertiary-Quaternary, where ten structurestyles have been distinguished. These traps spread as a zonein N-S, are scattered like a segmental line in W-E and showtier-styled vertically. The best traps are gypsum-salt coveredfault-bend anticlines related to the passive roof duplex. Thispetroleum system is characterized by late accumulation. Inthe early Himalayan Movement, mainly gas condensate andoil accumulated and were distributed in the outer circularregion of the kitchen; whereas in the middle and late Hima-layan the gas accumulations mainly formed and were dis-tributed in the inner circular region near the kitchen. Theoverpressure of gas pools is common and is formed by sealcapacity of thick gypsum layers, extensive tectonic compres-sion and large uplift. The well-preserved anticline traps un-derlying the high-quality regional cap rocks of the Tertiarygypsum rocks and gypseous mudstones are the main targetsfor the discovery of giant and medium-sized gas fields. Aboveconclusions are important for the petroleum geology theoryand the exploration of the fold-thrust belt in foreland basinsin central and western China.

Crude-oil hydrocarbon composition characteristics and oil viscosity prediction in the northern Songliao Basin

Crude oil hydrocarbon composition characteristics and oil viscosity prediction are important bases in petroleum exploration. A total of 54 oil/heavy-oil samples and 17 oil sands were analyzed and quantified using both comprehensive 2D gas chromatog raphy （GCxGC） and comprehensive 2D gas chromatography/time-of-flight mass spectrometry （GCxGC/TOFMS）. The results show that crude oil in the West slope is mainly heavy oil and its hydrocarbon composition is characterized overall by paraf fins 〉 mono-aromatics 〉 naphthenes 〉 non-hydrocarbons 〉 di-aromatics 〉 tri-aromatics 〉 tetra-aromatics. Aromatics are most abundant and non-hydrocarbons are least abundant, whilst content differences among paraffins, naphthenes, aromatics, and non-hydrocarbons are less than 15%. There are two types of heavy oil, secondary type and mixing type. Biodegradation is the main formation mechanism of heavy oil. Biodegradation levels cover light biodegradation, moderate biodegradation, and se- vere biodegradation. With increasing biodegradation, paraffin content decreases while contents of aromatics and non- hydrocarbons increase. In contrast, naphthene content increases first and then decreases with increasing biodegradation. In se- vere biodegradation stage, naphthenes decrease more quickly than aromatics and non-hydrocarbons. This provides a new method for studying oil/heavy-oil biodegradation mechanism and biodegradation resistance of different hydrocarbons at dif- ferent biodegradation stages. In the Longhupao-Daan terrace and Qijia-Gulong depression, most crude oil is conventional oil. Its composition is dominated by paraffins with the lowest content of aromatics. In some casual oil wells from the Long- hupao-Daan terrace, crude oil from Saertu oil reservoirs is moderately biodegraded whereas crude oil from Putaohua oil reser voir is lightly biodegraded. Chemical parameters using saturate hydrocarbons and aromatics are usually not suitable for deter mining organic type and thermal maturity of biodegraded oil, especially of moderately or severely biodegraded oil, whilst Ts/（Ts＋Tm） ratio can be used to determine thermal maturity of both conventional crude oil and heavy oil.

Hydrocarbon accumulation conditions and key exploration and development technologies for PL 19-3 oilfield

The PL 19e3 Oilfield is the only super-large monolithic oilfield with oil and gas reserves up to 1×10^(9) t in the Bohai Bay Basin,and it has been successfully developed.Exploration and development practices have provided abundant data for analyzing formation conditions of this super-large oilfield.On the basis of the exploration and development history,fundamental reservoir features,and with available geological,geophysical and test data,the hydrocarbon accumulation conditions and key exploration&development technologies of the PL 19e3 Oilfield were discussed.The key conditions for forming the super-large Neogene oilfield include four aspects.Firstly,the oilfield is located at the high position of the uplift that contacts the brachy-axis of the multi-ridge slope in the biggest hydrocarbon-rich sag in the Bohai Bay Basin,thus it has sufficient hydrocarbon source and extremely superior hydrocarbon migration condition.Secondly,the large-scale torsional anticlines which formed in the Neogene under the control of the Tanlu strike-slipping movement provide sufficient storage spaces for oil and gas preservation.Thirdly,the“multiple sets of composite reservoir-caprock assemblages”developing in the special shallow-water delta further contributes greatly to the effective storage space for oil and gas preservation.Fourthly,due to the coupling of the uplift and strike slip in the neotectonic period,extensive faulting activities constantly released the pressure while the late period massive hydrocarbon expulsion of the Bozhong took place at the same time,which assures the constant and intense charging of oil and gas.The super-large PL 19e3 Oilfield was controlled by the coupling effects of all those special geologic factors.In view of this oilfield's features(e.g.violently reformation caused by strike slip,and the special sedimentary environment of shallow-water delta),some key practical technologies for exploration and development have been developed.Such technologies include:the special prestack depth migration processing for gas cloud zones,the prediction of thin interbed reservoirs based on high-precision inversion of geologic model,the reservoir description for the shallow-water braided river delta,the quantitative description for remaining oil in the commingled oil reservoirs with wide well spacing and long well interval,and the well pattern adjustment for formations during high water cut period in the complex fluvial-facies oilfields.

库车坳陷的地质结构及其对大油气田的控制作用

库车坳陷是在晚二叠世之前的古生代褶皱基底上历经晚二叠世-三叠纪的前陆盆地、侏罗-古近纪的伸展坳陷盆地和新近纪-第四纪陆内前陆盆地的演化而形成的。基底中的软弱层、侏罗系煤层和古近系库姆格列木组与新近系吉迪克组膏盐(泥)岩构成了自山前向盆地内部逐渐抬升的滑脱面,与自山前向盆地内部逐渐趋缓的地表面构成楔形体。该楔形冲断体的内部结构具有"垂向分层、横向分带与纵向分段"特点,NW向的阿瓦特-喀拉玉尔滚和NE向的库车横向构造转换带将其分割为乌什、拜城与阳霞3个构造区段。构造层发育特点决定了库车坳陷发育三叠-侏罗系的区域展布的有效烃源岩和(侏罗系、)白垩系-第三系储盖组合;分层变形特点导致盐下层形成叠瓦冲断构造组合,冲断层成为油源断层;叠瓦式的冲断层相关褶皱背斜组合导致了复式天然气聚集区带的形成,即在大北-克拉苏式的构造带上每一冲断层相关褶皱背斜带独立成藏,复合连片形成复式油气聚集(区)带,目前拜城北、克深、克拉苏背斜带已呈现这种趋势;撕裂断层则决定着构造带上具体的油气富集区段。库车坳陷油气资源丰富,地质结构特点决定了不同类型油气田分布的分区性。

流体包裹体分子组成分析技术应用中存在的问题

随着包裹体分析技术的发展,其在油气勘探中的作用也越来越重要。国内外已经开始通过包裹体的组分分析方法来研究油气藏的充注成藏史;这种方法直接以古油气藏成分为研究对象,更有利于油气成藏史的恢复。针对这种分析方法在实施过程中可能出现的问题进行了讨论,提出了在分析过程中应该注意的事项,同时指出包裹体组分分析方法只有结合地质实际,才能在实际应用中进一步完善。

论油气分布的有序性

从准噶尔、塔里木和四川盆地及其主要含油气区石油地质基本条件的分析说明含油气盆地的形成和发展与区域构造背景及演化密切相关，盆内构造单元、烃源岩、储集层与盖层沉积条件以及油气藏类型等五个方面均有规律分布，决定了从前陆到克拉通盆地油气藏分布的有序性，并构成以复合油气藏为特征的有机统一整体。勘探实际中需要对含油气盆地整体考虑，实施三维立体交叉式综合勘探。

Classification of Hydrocarbon-Bearing Fine-Grained Sedimentary Rocks

Fine-grained sedimentary rocks are defined as rocks which mainly compose of fine grains（〈62.5 μm）. The detailed studies on these rocks have revealed the need of a more unified, comprehensive and inclusive classification. The study focuses on fine-grained rocks has turned from the differences of inorganic mineral components to the significance of organic matter and microorganisms. The proposed classification is based on mineral composition, and it is noted that organic matters have been taken as a very important parameter in this classification scheme. Thus, four parameters, the TOC content, silica（quartz plus feldspars）, clay minerals and carbonate minerals, are considered to divide the fine-grained sedimentary rocks into eight categories, and the further classification within every category is refined depending on subordinate mineral composition. The nomenclature consists of a root name preceded by a primary adjective. The root names reflect mineral constituent of the rock, including low organic（TOC〈2%）, middle organic（2%4%） claystone, siliceous mudstone, limestone, and mixed mudstone. Primary adjectives convey structure and organic content information, including massive or limanited. The lithofacies are closely related to the reservoir storage space, porosity, permeability, hydrocarbon potential and shale oil/gas sweet spot, and are the key factor for the shale oil and gas exploration. The classification helps to systematically and practicably describe variability within fine-grained sedimentary rocks, what＇s more, it helps to guide the hydrocarbon exploration.