Diagenesis of the Paleogene Sandstones in the DN2 Gas Field,Kuqa Foreland Basin and its Link to Tectonics

We investigated diagenesis of the sandstones from the DN2 Gas Field of the Kuqa Foreland Basin(KFB),in order to infer the timing of fluid migration and discuss the linkage between fluids and tectonics.The textures and chemical composition of authigenic minerals,fluid evidence from fluid inclusions and formation water measurements were all used to fulfill this aim.Eodiagenesis occurred with the participation of meteoric water and connate water.Mesodiagenesis is related to high salinity fluids,which were attributed as originating from the overlying Neogene Jidike Formation evaporite(principal minerals including halite,anhydrite,glauberite,carnallite and thenardite).The onset of high salinity fluid migration is inferred to occur during the late Miocene(12.4-9.2 Ma)through the use of homogenization temperatures measured in the present study and K-Ar dating of authigenetic illites from previous work.This period is consistent with the crucial phase(13-10 Ma)that witnessed the rapid uplift of the southern Tianshan Mts and the stage when calcite and anhydrite veins formed in the studied strata.We thus argue that diagenesis related to high salinity fluids occurred as a response to the Tianshan Mts'rapid uplift and related tectonic processes.The flow of high salinity fluids was probably driven by a density gradient and channeled and focused by fractures formed contemporaneously.

Control of neotectonic movement on hydrocarbon accumulation in the Kuqa Foreland Basin,west China

Neotectonic movement refers to the tectonic movement that has happened since the Cenozoic, which is the latest movement. It has the most important influence on the basins in west China, especially on the hydrocarbon accumulation in the western foreland basins. We determined the time of neotectonic movement in the Kuqa Foreland Basin, which began from the Neogene, and analyzed the patterns of movement, which were continuous and fast subsidence in the vertical direction and intense lateral compression. The structure styles are that the faulting is weakened and the folding is strengthened gradually from north to south. We studied the control of neotectonic movement on the hydrocarbon accumulation process and model in the Kuqa Foreland Basin with basin simulation technique. The largest subsidence rate of the Kuqa Foreland Basin reached 1,200 m/Ma during the neotectonic movement, leading to rapid maturing of source rock within 5 Ma and a large quantity of hydrocarbon being generated and expelled. The thick neotectonic strata can form high quality reservoirs with the proved gas and oil reserves accounting for 5% and 27% of the total reserves, respectively. 86% of the structural traps were formed in the neotectonic movement period. The faults formed during the neotectonic movement serve as important migration pathways and they exist in the region where the hydrocarbon reservoirs are distributed. Abnormally high pressure caused by the intense lateral compression, thick neotectonic strata deposition and rapid hydrocarbon generation provide driving force for hydrocarbon migration. The accumulation elements match each other well over a short period, leading to many large gas fields formed later in the Kuqa Foreland Basin.

Sedimentary Characteristics and Reservoir Prediction of Paleogene in the East Part of Kuqa Foreland Basin

Most of the Mesozoic and Cenozoic large-scale hydrocarbon-bearing basins in western China were formed in a similar foreland setting. Hydrocarbon exploration of the Kuqa foreland basin requires research into the sedimentary characteristics and filling evolution of the depositional sequences and their response to the basin process. Based on an analysis of outcrops, well logs and high resolution seismic data, the sedimentary system types and distribution characteristics of the Paleogene in the east part of Kuqa foreland basin were systematically studied. The results show that： （ 1 ） Three types of sedimentary systems are developed in the area： an oxidative salty wide shallow lacustrine system, a fan delta system and an evaporitic bordersea system. （2） The configuration and evolution of the depositional systems of the Paleogene in the Kuqa foreland basin were predominantly determined by foreland tectonism. Vertically, the Paleogene sedimentary sequence can be divided into three parts： the lower, middle and upper depositional system tracts. The lower and upper tracts commonly consist of progradational or aggradational sequences, while the middle part is usually comprised of a set of aggradational to transgressive third-order sequences. Laterally, the sedimentary systems in the east part of the Kuqa foreland basin spread from east to west as a whole, and the sedimentary facies obviously vary from south to north. The sand bodies of the delta front facies are excellent gas reservoirs, characterized by rather thick, extensive and continuous distribution, high porosity and permeability, and just a few barrier beds.

Comparison of Kuqa foreland basin with Persian Gulf Basin in salt tectonics

Compared Kuqa foreland basin with Persian Gulf Basin in development of salt layers, salt tectonics, and the relation between salt tectonics and hydrocarbon, it is concluded that the salt diapirs are relative to hydrocarbon. Searching salt diapirs and related traps in Kuqa foreland basin is important. The forming mechanism of salt tectonic in Kuqa foreland basin is different from that of Hormuz Series, but similar to that of Lower Fars Series/Gachsaran Formation. Inspired by the role of salt tectonics of Lower Fars Series/Gachsaran Formation in hydrocarbon accumulation, the authors considered that the exploration below salt layer should be enforced, and the traps below salt layer in the southern part of the Kuqa foreland basin would be found where salt layer is thicker. On the contrary, the traps should be found both above and below the salt layer in front of the northern mountain where salt layer is thin. The Triassic and Jurassic source rocks are rich in this area with great exploration prospective.

Geochemical Characteristics and Origin of Crude Oil and Natural Gas in the Southern Slope Zone,Kuqa Foreland Basin,NW China

Continuous exploration has triggered a heated debate on hydrocarbon resource potential in the southern slope zone of the Kuqa foreland basin,and sources of the Mesozoic-Cenozoic oil and gas have become a key problem to be solved in this region.Composition and organic geochemical parameters of crude oil and natural gas from the southern slope zone of the Kuqa foreland basin were illustrated in order to reveal their origin by using a combination of gas chromatograph(GC),gas chromatogram-mass spectrum(GC-MS)and carbon isotope analyses.The characteristics of crude oil,such as low density,viscosity,solidification point and sulfur content,and high wax content,indicate that source of the crude oil is continental.The biomarker compositions of crude oil are characterized by low to medium molecular weight compounds(n-C_(12) to n-C_(20)),high Pr/Ph ratios(>1.0),low phytane/n-C18 ratios(0.06-0.54),and predominant regular sterane C_(29).All biomarker parameters clearly indicate that the crude oil was derived mainly from algae and aquatic plankton and deposited under weak reduction-oxidation environment,and has the characteristics of mixed kerogens.The Cretaceous crude oil was mainly derived from the Triassic lacustrine source rocks,which also contributed to the Paleogene crude oil together with Jurassic coal source rocks.Natural gas is characterized by moderate methane content,high heavy hydrocarbon and nitrogen content,and no hydrogen sulfide.The methane and ethane in Paleogene natural gas are relatively rich in ^(13)C withδ^(13)C_(1) andδ^(13)C_(2) values ranging from-37.3‰to-31.2‰(mean=-34.25‰)and from-25‰to-21.3‰(mean=-23.09‰),respectively,indicating the coal-derived gas from the Middle and Lower Jurassic strata.Hydrocarbon products in the southern slope zone of the Kuqa foreland basin are primarily generated from source rocks in the mature stage.The low-amplitude structural and lithologic traps with the updip pinch-out sand bodies or plugging secondary fault at relatively high tectonic positions are the most favorable areas for discovery and breakthrough in the study area.Results of this study will provide useful information for controlling factors of reservoirs and oil and gas exploration deployment in the southern slope zone of the Kuqa foreland basin.

Formation condition of deep gas reservoirs in tight sandstones in Kuqa Foreland Basin

Due to the high expense of deep oil and gas exploration,prediction of gas-bearing properties before drilling is crucial for deep gas reservoir of tight sandstone.Deep tight sandstone gas fields in Kuqa Foreland Basin are characterized by high abundance,high gas saturation,high pressure,high and stable yield,which belong to high-efficiency tight gas reservoir.Based on theoretical analysis of controlling factors and mechanisms of gas-bearing properties for tight sandstone gas reservoir,and taking tight sandstone gas fields with high effectiveness such as Dibei,Keshen and Dibei gas fields in Kuqa Foreland Basin as examples,formation condition and mechanism of high-efficiency tight sandstone gas reservoir in Kuqa area are studied through a comparative analysis of typical tight sandstone gas reservoir in Sichuan Basin and Ordos Basin.The results show that the formation condition of deep gas reservoir of tight sandstone in Kuqa foreland basin includes four factors:i.e.,overpressure gas charging,fracture development,“early-oil and late-gas”accumulation process and favorable preservation condition.The overpressure gas charging and fracture development are the most important factors for formation of high-efficiency tight gas reservoirs in Kuqa Foreland Basin.High-quality source rocks,high sourcereservoir pressure difference,and overpressure filling induced thereby are preconditions for formation of tight sandstone with high gas saturation.The fracture development controls gas migration,accumulation,and high yield of tight sandstone gas reservoir.The reservoir wettability changed by the early oil charging is beneficial to late natural gas charging,and the preservation condition of high-quality gypsum cap rocks is the key factor for gas reservoirs to maintain overpressure and high gas saturation.Matching of above four favorable factors leads to the tight sandstone gas reservoir with high abundance,high gas saturation and high gas production in Kuqa Foreland Basin,which is very different from other basins.Under the condition of little difference in physical property of tight sandstone reservoir,excessive source-reservoir pressure difference,facture development,preservation condition and current formation overpressure are the most significant factors to be considered in exploration and evaluation of deep tight sandstone gas.

Origin,migration and mixing of oilfield brines: Stable iso-topic evidence from Kuqa Foreland Basin

87Sr/86Sr, 8 D and δ180 together with chemistry of oilfield brines, were measured from Cambrian to Neogene strata in the Kuqa Foreland Basin, northwest China. The brines have 87Sr/86Sr ratios ranging from 0.70944 to 0.71716, 8D values from -33‰ to -67‰ and δ18O from -9.0‰ to 3.9‰. The most radiogenic 87Sr brines have relatively heavy δ18O in the Cambro- Ordo-vician carbonates, and are suggested to originate from crystalline basement. 8 D and δ18O sys-tematics show that all the brines slope to Global Meteoric Water Line, and intersect at 8 D = -60‰ and δ18O = -8.8‰, close to that of present-day local meteoric water, suggesting that meteoric water has mixed with evaporated ancient meteoric water. More saline samples have the lowest δD and δ180 contents and molar Na/CI ratios of more than 0.9, resulting from salt dissolution.

A Large-scale Tertiary Salt Nappe Complex in the Leading Edge of the Kuqa Foreland Fold-Thrust Belt, the Tarim Basin, Northwest China

The tectono-stratigraphic sequences of the Kuqa foreland fold-thrust belt in the northern Tarim basin, northwest China, can be divided into the Mesozoic sub-salt sequence, the Paleocene-Eocene salt sequence and the Oligocene-Quaternary supra-salt sequence. The salt sequence is composed mainly of light grey halite, gypsum, marl and brown elastics. A variety of salt-related structures have developed in the Kuqa foreland fold belt, in which the most fascinating structures are salt nappe complex. Based on field observation, seismic interpretation and drilling data, a large-scale salt nappe complex has been identified. It trends approximately east-west for over 200 km and occurs along the west Qiulitag Mountains. Its thrusting displacement is over 30 km. The salt nappe complex appears as an arcuate zone projecting southwestwards along the leading edge of the Kuqa foreland fold belt. The major thrust fault is developed along the Paleocene-Eocene salt beds. The allochthonous nappes comprise large north-dipping faulting monoclines which are made up of Paleocene-Pliocene sediments. Geological analysis and cross-section restoration revealed that the salt nappes were mainly formed at the late Himalayan stage (c.a. 1.64 Ma BP) and have been active until the present day. Because of inhomogeneous thrusting, a great difference may exist in thrust displacement, thrust occurrence, superimposition of allochthonous and autochthonous sequences and the development of the salt-related structures, which indicates the segmentation along the salt nappes. Regional compression, gravitational gliding and spreading controlled the formation and evolution of the salt nappe complex in the Kuqa foreland fold belt.

Three-dimensional discrete element numerical simulation of Paleogene salt structures in the western Kuqa foreland thrust belt

Taking the Paleogene salt strata in the west of Kuqa foreland thrust belt as study object, the deformation features of salt structure in the compression direction and perpendicular to the compression direction were examined to find out the control factors and formation mechanisms of the salt structures. By using the three-dimensional discrete element numerical simulation method, the formation mechanisms of typical salt structures of western Kuqa foreland thrust belt in Keshen and Dabei work areas were comprehensively analyzed. The simulation results show that the salt deformation in Keshen and Dabei work areas is of forward spread type, with deformation concentrated in the piedmont zone;the salt deformation is affected by the early uplift near the compression end, pre-existing basement faults, synsedimentary process and the initial salt depocenter;in the direction perpendicular to the compression direction, salt rocks near the compression end have strong lateral mobility with the velocity component moving towards the middle part, and the closer to the middle, the larger the velocity will be, so that salt rocks will aggregate towards the middle and deform intensely, forming complex folds and separation of salt structures from salt source, and local outcrop with thrust faults. Compared with 2 D simulation, 3 D simulation can analyze salt structures in the principal stress direction and direction perpendicular to the principal stress, give us a full view of the formation mechanisms of salt structures, and guide the exploration of oil and gas reservoirs related to salt structures.

Influence of Salt Beds on the Segmentation of Structure and Hydrocarbon Accumulation in Qiulitag Structural Belt,Tarim Basin,China

Seismic information and balanced profile technology were used to reveal the influence of the salt bed in segmentation of structure and hydrocarbon accumulation in Qiulitag structural belt in Tarim basin. From west to east, the shortening of strata above the salt beds gradually decreases, while, the shortening below the salt beds gradually increases, which shows that the segmentation of structure integrated the seismic profile. There is great difference of the deformation of strata below and above the salt beds between the west segment and the east segment. The analysis of the distribution of oil/gas fields and the hydrocarbon properties indicates the similar segmentation to the structure segmentation. The salt beds in relatively shallow layers change the stress condition from basement of Kuqa foreland basin, which leads to the segmentation of Qiulitag structural belt. Because the salt beds in the west segment came into being earlier than those in the east segment, the west segment captures hydrocarbon from two sets of source rock, while the east segment can only capture hydrocarbons from one set of source rock. So, the salt beds play an important role in the segmentation of structure and hydrocarbon accumulation.

库车前陆盆地天然气成藏过程及聚集特征

天然气的组分和碳同位素主要受母质类型和成熟度双重因素的控制,这是我们研究天然气的基础。但是越来越多的证据显示成藏过程对天然气组分和碳同位素也具有明显的控制作用。尽管库车前陆盆地不同构造单元的天然气来源一致,即主要来源于侏罗系烃源岩、其次为三叠系烃源岩,但是库车前陆盆地冲断构造带、斜坡区和前缘隆起区在天然气组分、干湿程度和碳同位素组成上具有明显的差异。库车前陆盆地经历了早期油气的聚集、破坏和晚期天然气聚集、调整的过程,研究认为成藏过程对库车前陆盆地不同构造单元的天然气组分和碳同位素特征具有明显的控制作用。

库车前陆盆地古近纪—新近纪盐湖环境变迁及其成钾效应探讨

库车前陆盆地古近纪—新近纪发育巨厚的蒸发岩沉积。古新世,盐湖沉积主要集中在库车盆地西部拜城凹陷的中东部;始新世,盐湖沉积扩展至整个拜城凹陷,沉积中心和蒸发浓缩中心位于其北部;渐新世,盐湖沉积仍然分布于拜城凹陷,但沉积和浓缩中心已从北部迁移到南部,改变了此前"南浅北深"的箕状盆地构造格局;中新世,盆地沉积中心已从西部的拜城凹陷转移到东部的阳霞凹陷,盐湖沉积区也随之迁移到东部,浓缩中心呈串珠状分布。库车前陆盆地古近纪—新近纪环境变迁显示,盆地次级凹陷的发育、迁移明显受控于盆地构造演化;而盆地构造活动在制约盐湖岩相古地理面貌(沉积中心、物质来源与浓缩中心等)变迁的同时,亦对盐湖晚期富钾卤水的再汇集过程具有明显的控制作用,进而对随后可能的钾盐沉积起到决定性作用。进一步分析表明,库车前陆盆地古近系—新近系蒸发岩序列中钾离子在横向上的富集势亦明显受控于这一时期盆地岩相古地理的变迁,即在盐岩沉积面积、厚度大的凹地出现钾离子相对富集。而西部拜城凹陷,钻孔(始新统)盐岩岩屑的钾离子含量从下向上逐渐增高的趋势,峰值分别为1.07%、1.43%、3.05%;东部凹陷钻孔(中新统)盐岩岩屑钾离子含量由下向上亦呈增高趋势,最低值0.008%,最高值为0.152%等证据表明,地层中钾离子纵向上的富集趋势,则正是始于对岩相古地理面貌变迁的地球化学响应。综上所述,认为库车前陆盆地具有钾盐成矿远景:始新世时期,库车盆地西部凹陷的蒸发浓缩中心区成钾远景相对较好;中新世时期,有利的成钾区可能转移到东部凹陷的浓缩中心区。

库车前陆盆地油气成藏特征与分布规律

库车前陆盆地是中国目前油气勘探程度和研究程度最高的前陆盆地。在深入认识库车前陆盆地油气成藏条件和油气聚集过程的基础上,总结了库车前陆盆地的油气分布规律:1平面上,油气相态在克拉苏构造带中部以干气为主,以此为中心向南、北两侧及东、西两端渐变为凝析气、油藏;2垂向上,以第三系含盐地层和侏罗系煤系泥岩2套区域盖层划分为盐上、盐下和深层3套成藏组合,盐下白垩系成藏组合是天然气勘探主体;3从南向北,不同构造带的油气藏类型、油气相态、油气成藏期次表现为时空上的有序聚集;4从东向西,不同构造段构造特征、油气藏类型、成藏特征和资源潜力具有差异性,具有中段油气富集的特征。通过对油气成藏规律和油气分布特征的认识,整体建立了库车前陆盆地从非常规致密油气到常规油气的有序聚集模式,深入认识了油气分布规律和有利勘探领域。

塔里木盆地北部构造与油气分布规律

塔里木盆地北部的构造演化和构造格局直接受控于南天山碰撞造山带的形成与演化过程。在此构造演化过程中形成了中生代的库车前陆盆地和新生代的北塔里木前陆盆地。塔里木盆地北部的构造演化和构造格局又控制了该地区的油气分布规律 :秋里塔格带以北的前陆褶皱冲断带 ,即库车坳陷的主体部分是天然气富集区 ;塔北隆起的南部至中央隆起的北部是黑油富集区 ;

库车前陆盆地构造压实作用及其对储集层的影响

库车地区存在强烈的构造压实作用,对储集层分布及品质产生了重大影响。在分析构造压实测井响应基础上,建立了古构造应力定量评价模型,然后依据测井信息确定古地应力分布,分析构造压实作用对储集层的影响。构造压实作用对储集层物性的影响具有两面性:一方面随着构造应力增大,构造压实作用和构造成岩作用加强,储集层孔隙度、渗透率降低,尤其是渗透率下降显著;另一方面随着构造挤压作用增强,砂岩致密化程度增强,砂岩中易于发育裂缝,从而形成渗滤性能良好的裂缝性储集层。此外,封闭构造压实作用形成了本区高压—超高压流体压力,这种高异常流体压力的存在抵消了部分压实作用对储集层的影响,使储集层承载的有效应力降低,从而为储集层保持较高孔渗性创造了有利条件。

塔里木盆地北缘库车前陆盆地地温梯度分布特征

结合塔里木库车前陆盆地的 10 9口井的地温数据 ,给出了库车盆地的地温场分布特征 .库车盆地各构造单元的平均地温梯度值为 18～ 2 8℃ km ,与我国其他大、中型沉积盆地相比 ,仍然偏低 .库车前陆盆地整体上是属于低温冷盆 ,山前带部位 ,依奇克里克、克拉苏和大宛齐等构造分区是低温冷盆中的高地温梯度带 ;从山前带由北向南 ,地温梯度逐渐降低 .在盆地的南部 ,地温梯度又存在明显差异 .总体上 ,地温梯度随深度的增加而降低 ,趋于一致 ,但在不同地质构造条件下其变化的速率不同 .在此基础上 ,进一步探讨地温梯度与盆地形成和构造演化的关系以及油气勘探意义 .

膏盐岩对地层温度及烃源岩热演化的影响定量分析——以塔里木库车前陆盆地为例

库车前陆盆地发育两套巨厚的膏盐岩，依据盆地东西向的二维地震剖面和边界条件，运用瞬态热流法进行模拟，定量分析膏盐岩厚度变化对地温及烃源岩热演化的影响。模拟结果表明：①当盐体总厚度一定时，盐体层数的变化不会对地温场造成影响；②地温随膏盐岩厚度的变化为：西部盐上地温增加0．3～0．6℃／100m，盐下地温减小0．6～1．0℃／100m；东部盐上地温增加1．9～2．3℃／100m，盐下地温减小0．2～2．6℃／100m；⑨镜质体反射率R0随膏盐岩厚度的变化为：西部平均滞后约0．02％／100m，东部平均滞后约0．05％／100m。膏盐岩本身的热导率与温度呈负相关关系，东部盐体埋藏较浅，地温相对较低，其总体热导率较高，导致地温和尺。值变化率比西部高。相对于泥岩，膏盐岩使下伏烃源岩热演化滞后，导致库车东部迪那2凝析气田油气充注时间滞后7．5～9．0Ma，使得其排烃充注时期与圈闭形成时期相匹配，有利于该地区的晚期成藏。

库车新生代构造性质和变形时间

库车构造位于南天山古生代碰撞造山带之南，为塔里木盆地最北的一个构造带。它自北而南可分为边缘逆冲（ 隐伏构造楔） 、斯的克背斜带、北部线性背斜带、拜城盆地、南部背斜带。每个背斜带又包含有若干逆冲断层相关褶皱，它们是断层转折褶皱、断层传播褶皱、滑脱褶皱、断层传播 滑脱混生褶皱、双重逆冲构造、突发构造、三角带构造。底部逆冲断层向南变浅，堆叠逆冲岩席向南变薄，总体上形成一个向南的逆冲构造楔。逆冲断层在斯的克背斜带侵位最早（２５ Ｍａ） ，在北部线性背斜带为１６９ Ｍａ，拜城盆地中的大宛其背斜为３６ Ｍａ，南部背斜带为５３ Ｍａ（ 北部） 和１８ Ｍａ（ 南部） ，变形作用向南变新。库车构造是印 藏板块碰撞的内陆构造响应。

新疆库车盆地东秋里塔格构造带新生代的构造演化

东秋里塔格构造带位于库车前陆盆地的南缘,新生代经历了强烈的构造挤压和构造沉降。平衡剖面分析、生长地层识别和重点井沉降史的数值模拟表明:古新-始新世库姆格列木群沉积期间构造活动微弱并沉降缓慢。渐新世苏维依组沉积期间,构造活动开始加强,沉降速度加快,并形成了一些小断距的逆断层。中新世构造活动进一步加强,沉降加速,沉积了厚层的吉迪克组膏盐层;康村组沉积时期,构造挤压使得膏盐层发生塑性流动,形成盐枕,康村组发育生长地层。随后的上新世库车组沉积期间,研究区先发生快速沉降,然后,随着南天山急剧隆升,冲断作用迅速向南扩展。约在早更新世,库车褶皱冲断带前锋到达东秋里塔格构造带,并最终定型,使得该区发生强烈的构造变形,形成大量的逆冲断裂构造带,膏盐层表现出明显的塑性流动,形成盐推覆构造。

超深层裂缝-孔隙型致密砂岩储集层表征与评价——以库车前陆盆地克拉苏构造带白垩系巴什基奇克组为例

以库车前陆盆地克拉苏构造带深层白垩系巴什基奇克组砂岩储集层为例，研究成岩压实和构造挤压双重作用下裂缝-孔隙型（裂缝-原生孔隙型和裂缝-溶蚀孔隙型两类）超深层储集层的表征和评价方法。巴什基奇克组砂岩储集层埋深超过6000m，属超深层储集层，针对性构建了集宏观微相．岩相识别、厘米一微米级裂缝描述、微米级孔隙刻画、纳米级喉道表征为一体的超深层裂缝性致密砂岩储集层表征与评价技术。研究认为有效储集层的储集空间由构造裂缝及微米级孔隙、纳米级孔喉组成，基质孔隙半径主要为2～100gm，基质喉道半径主要为10-500nm，裂缝开启度主值区为100-300μm；有效储集层发育主要受控于微相一岩相、构造挤压和溶蚀作用。相对优质储集层的储集空间由裂缝、残余粒间孔和溶蚀孔隙组成，发育于弱构造挤压带与水下分流河道叠合区。7000m以深有利储集层可成带连片分布，8000m以深仍发育有效储集层。