GRAVITY OVERTURNS, EXTENSION AND BASEMENT FAULT ACTIVATION: REPLY

HOSPERS (pp. 359–361, this issue ) raises two questions on the application of model results in my 1991 paper on the Norwegian-Danish Basin: (1) unlike the Norwegian-Danish Basin, the buoyant layer in the models is resting on a flat, horizontal base; and (2) sedementation is not simulated in the models. These two points are not justified because their answers are already in the paper. I am surprised that Hospers did not comment on a horizontal and flat base in the models in earlier correspondence (Hospers, pers. comm., 1990, Koyi, 1991). Here, I will clarify parts of my paper, and draw a general conclusion on the effects of basement faults on diapirism and basin floor slope.     

渤海海域新生代褶皱作用

基于地震资料解释,总结了渤海海域新生代褶皱作用的类型、控制因素及分布.从力学成因上看,渤海海域褶皱作用可以划分为4种类型,即与拉张作用有关的、与扭动作用有关的、与重力滑动诱导的挤压作用有关的和与压实、底辟等重力作用有关的褶皱作用.与拉张和挤压有关的褶皱作用发生在古近纪,分别沿长期活动的控制性断裂和斜坡带分布.与扭动有关的褶皱作用发生在新近纪晚期,主要分布在盆地东部郯庐断裂经过的地方.重力褶皱中的披覆褶皱作用贯穿于整个新生代,分布在凸起上;而盐底辟褶皱作用分布局限,目前仅在盆地南部的莱州湾凹陷有所发现.从目前渤海海域油气勘探实践看,沿断裂带分布的与新近纪晚期扭动褶皱相关的圈闭是有利的储油气构造.     

EXTENSIONAL EVOLUTION OF THE GULF OF MEXICO BASIN AND THE DEPOSITION OF TERTIARY EVAPORITES

Current interpretations of the evolution of the Gulf of Mexico Basin are guided by the critical assumption that all the salt in this huge basin is Jurassic and time‐equivalent to the Louann salt of the interior evaporite basins of Texas, Louisiana and Mississippi. This assumption forces the interpretation that extensive salt sheets that now reside within Eocene to Plio‐Pleistocene stratigraphic levels are allochthonous, having been emplaced by several episodes of Jurassic salt piercement and horizontal flow as shallow salt sills or sub‐aqueous salt glaciers during Tertiary sedimentation. This interpretation is contested from several geological viewpoints. The uncontaminated Louann salt differs substantially from salt of the coastal, offshore northern Gulf and Mexican Saline Basin, in all of which the salt carries multiple shaley layers and exotic material. The intra‐salt deformation of offshore sheets appears from cores and seismic profiles to be much less severe than would be expected if it had been emplaced via several generations of deep piercement diapirs. The intra‐salt shale laminations in the Belle Isle Salt Mine, Southern Louisiana, carry Oligocene fauna and are so closely conformable with primary salt layering that their contemporaneity with the salt is a valid conclusion. Because the Belle Isle Shale Zone is so similar to shaley zones encountered in offshore sheets, a contemporaneous origin for them is also implied. The mixed Tertiary fauna found within, and below salt sheets is so typical of sedimentary reworking that its origin from evaporite basin margins is a logical conclusion. The Gulf of Mexico Basin is unique as the presumed depository of multiple allochthonous salt sheets. Salt piercements in the remote Kavir Basin of Iran have been proposed as an analogue for the emplacement of salt canopies, but there is no sub‐surface control in the Kavir Basin and the canopy concept is not unanimously accepted. Furthuremore, the popular view that salt sheets were emplaced by submarine salt glaciers during successive episodes of piercement diapirism is doubted because of the complete absence of glaciers in todays halokinetically active basin. The preservation from solution of purported submarine salt extrusions lacks both experimental and credible geological support. Angular sub‐salt contacts of many offshore salt sheets in the northern continental slope have been interpreted as sheer or thrust contacts. However, thrusting seems unlikely in an overall extensional environment and it is here concluded that the angular contacts represent sub‐salt unconformities and that the Cretaceous and older exotic blocks found in some piercement domes have been plucked from a base‐salt unconformity surface. There is much circum‐Gulf evidence for extensional fracturing of the basement during the Tertiary period which, it is thought, supports the expectation of basement block faults, now hidden from view below thick Tertiary sediments; these faults governed sedimentation into depocenters as the basin expanded. It is concluded that salt sheets at Eocene, Oligocene, Mio‐Pliocene and Plio‐Pleistocene levels are autochthonous, a conclusion that has evolved from critical geological observations in the Belle Isle Salt Mine.     

PALAEOZOIC STRUCTURES AND OIL EXPLORATION PROSPECTS IN THE EASTERN TARIM BASIN, NW CHINA

The Tabei Uplift in the northern Tarim Basin, the Tadong Depression in the eastern Tarim Basin, and the Tarim Central Uplift were formed as a result of mantle diapirism. Four stages in the geological development of this area can be recognized: an initial "embryonic" stage in the late Proterozoic; a "formational" stage in the Sinian; a "developmental" stage in the Early to mid-Palaeozoic; and a "completion" stage in the late Palaeozoic.
With long-term Palaeozoic subsidence and well-developed marine source rocks, the Tadong area is thought to be a promising source region for hydrocarbons. The Tabei and Central Uplifts, located respectively north and south of the Tadong Depression, are ancient, well-preserved structures in which hydrocarbons may have accumulated, and could therefore constitute prospective areas for the location of "giant" oil- and gasfields in Palaeozoic sequences. Palaeozoic oil prospects have been confirmed in the Tabei area by the highly-productive Sachan 2 well. It is expected that intensified exploration will result in the location of both oil- and gasfields in the Central Uplift.     

THE EFFECT OF BASEMENT FAULTING ON DIAPIRISM

Experimental and natural examples illustrate the influence of sub-salt horizon basement faults on diapirism. In a series of experimental models, viscous diapirs were observed to form above or close to basement faults. In all the models, basement faults initiated a half-graben, where thicker overburden units enhanced differential loading on an underlying buoyant layer. The buoyant material flowed updip to the low-pressure zones in the uplifted block, and updip along the tilted upper boundary of the hanging-wall. Basement faulting extended the overburden, and provided the space through which the buoyant layer could rise. Subsidence and faulting of overburden layers allowed diapirism along the faulted zones. In all cases, the deformation in the overburden was accommodated within a wider zone of faulting than the discrete basement fault which initiated the deformation. Differential compaction enhances differential loading and accumulation of thicker overburden on the downthrown sides of basement faults.
Seismic profiles from the Danish Basin, Dutch Central Graben, Gulf of Mexico and North Sea show that diapirs are spatially associated with basement faults. However, model results show that diapirs triggered by basement faults are not necessarily located directly above the faults.
Basement faults extend the cover sequences. If detached from the cover by a ductile layer, thick-skinned extension is accompanied by thin-skinned extension and decoupling of the cover. The influence of basement faults on diapirs depends on: the thickness and effective viscosity of the ductile layer (e.g. rock salt); the thickness ratio between the buoyant layer and the overburden; the mechanical properties of the brittle cover in the case of clastic sediments; the rate of sedimentation; the displacement rate throw and the dip of the basement faults.     

SOME CONSEQUENCES OF FAULTING IN THE PRESENCE OF A SALT ROCK INTERVAL

Movement (and diapirism) in salt rock is frequently associated with faulting. In some cases, as in the Southern North Sea Zechstein, the association is clearly demonstrable. In other cases, more subtle effects occur. There is a variety of phenomena which may be observed under different geological circumstances in the salt-fault association, and this paper discusses some of the more frequently-encountered of these, offering some rather speculative suggestions as to the mechanisms which may be involved.
Comment is also made upon apparent inconsistencies existing in currently-held views on deformation in salt rock, and on the initiatory causes of diapirism.
The ideas expressed and the geological interpretations made, are based on the study of current seismic data which has, where possible, been related to released well information.     

初论中国东部含油气盆地的底辟构造

近年来,底辟型油气构造圈闭在我国东部各含油气盆地的研究中,已开始引起重视。盐丘构造的存在,一般已不再有很大争论。但是,对底辟构造存在的广泛性,特别是对底辟作用在我国东部中、新生代含油气盆地的构造演化中,究竟有多大影响,我国底辟型构造圈闭的形态、结构以及形成机制上有什么特点,对油气聚集的控制有那些规律可循等等问题,为文论证者还不多见。     

川西北部地区侏罗系气藏成藏特征——与川西南部侏罗系气藏比较

通过对比分析川西南部和川西北部地区侏罗系气藏在烃源、储集、圈闭、保存条件及异常高压等方面的成藏特征的差异，总结出川西北部地区侏罗系气藏可能的成藏模式。结果认为：作为气源的上三叠统的生烃强度在川西南部明显优于川西北部，南部拥有丰富的气源断裂，有利于油气的垂向运移，北部有可能形成侏罗系自生自储的油气藏；川西的南部地区和北部地区储集层厚度相差不大，沙溪庙组和千佛崖组在北部地区储集条件更为优越；构造圈闭条件南部更优；保存条件南部有白垩系的膏盐层作为区域盖层、北部有异常高压封存箱。地层异常高压在川西北部对侏罗系成藏起着关键作用，有助于压力封存箱内部形成油气藏。钻探情况证实，川西北部地区所有侏罗系产层和显示层段都集中在中侏罗统沙溪庙组及其以下层位。因此在今后的勘探实践中，应该重点注意该层段。     

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地球化学特征显示,松辽盆地在多处发现的高含量CO2气藏属于无机成因天然气,徐家围子地区发现的具重碳同位素和负碳同位素系列的烃类气体说明盆地不内不仅存在无机成因的CO2气藏,而且也极有可能形成无机成因的烃类气藏。盆地深部地质研究表明,地幔上隆、地壳减薄、地壳中发育的“网状”结构以及部分深大断裂的发育,使松辽盆地具备了形成无机成因气的条件,也说明盆地内现在发现的无机成因天然气可能来源于地球深部。 地球内部不同圈层内发育的流体是无机成因天然气 形成的基础。松辽盆地古中央隆起带两侧、西部凹陷区是无机成因天然气发育的有利地区;沿嫩江大断裂带和幔源岩浆底劈带是寻找无机烃类气体的有利地区。     

巴西圣埃斯皮里图盆地油气地质特征及勘探方向

近几年来,桑托斯（Santos）、坎波斯（Campos）2个盆地取得了较大的油气发现,而紧邻这2个盆地的圣埃斯皮里图（Espirito Santo）盆地油气发现较少,勘探程度较低。为了揭示该盆地的油气成藏规律,指明勘探方向,对该盆地的沉积特征、构造演化、烃源岩、圈闭和储集层等进行了深入研究和剖析,初步摸清了该盆地的油气分布规律、成藏主控因素和模式。该盆地位于巴西东海岸被动大陆边缘盆地群,先后经历了裂谷期、过渡期、漂移期3个演化阶段,相应地发育了盐下、盐岩和盐上3套层序。受盐岩底辟作用影响,形成特征迥异的盐下和盐上双层构造,其中盐下受裂谷体系控制,总体上表现为倾向分带的“斜向棋盘”格局,盐上为重力滑覆体系,发育了较多的盐构造类型。阿普第阶Cricare组Neocomian湖相页岩为最好的烃源岩,油气主要分布在上白垩统-新近系的盐上浊积砂中,以岩性圈闭为主。分析认为,浊积砂的分布和运移通道是油气成藏主要控制因素,盆地目前存在6个勘探方向,其中靠近陆岸的斜坡区是油气运移和聚集的最有利区。      

四川盆地侏罗系沙溪庙组天然气地球化学特征及地质意义

四川盆地侏罗系沙溪庙组天然气具有埋藏浅、成本低、周期短、见效快等优点，受到广泛的重视，是目前低油价下效益开发的重要现实领域之一。通过对侏罗系沙溪庙组天然气组分、天然气碳同位素等地球化学实验数据的分析，揭示了川西地区侏罗系沙溪庙组天然气来自于下伏的须家河组煤系烃源岩，川中北地区沙溪庙组天然气来自于下伏侏罗系湖相烃源岩，川东北地区侏罗系沙溪庙组天然气来自于下伏的须家河组煤系烃源岩和侏罗系湖相烃源岩。结合气源分析结果、构造背景和目前盆地致密气勘探开发程度，明确了侏罗系沙溪庙组浅层致密气向川西北、川中北和川东北地区进行"深化、甩开勘探开发部署"的总体方针，其中川西地区龙门山前缘北段的梓潼凹陷，川中北地区的金华、秋林、公山庙、营山地区以及川东北大巴山前缘的五宝场、渡口河地区具备良好的成藏条件，是下一步浅层致密气的重点勘探地区。     

莺歌海盆地流体底辟构造及其对天然气成藏的贡献

莺歌海盆地是一个独特的沉积盆地 ,其以强烈的底辟构造、巨厚的沉积盖层、快速的沉降沉积活动和异常高温高压等地质现象而著称。在调研大量文献的基础上 ,对莺歌海盆地底辟构造及其对天然气成藏的影响进行了分析。认为盆地内的底辟构造属于流体底辟构造 ;阐述了底辟构造的发育特征 ;从底辟产生的物质基础 (流体源 )和外界条件 (热能、退覆层系、外力 )等方面探讨了流体底辟的成因机制 ,总结出流体底辟构造形成的理论模式 ;并从圈闭、运移、保存等方面阐述了底辟构造对天然气成藏的贡献。     

BURIAL AND THERMAL HISTORY MODELLING OF THE MANNAR BASIN,OFFSHORE SRI LANKA

The Mannar Basin is a Late Jurassic – Neogene rift basin located in the Gulf of Mannar between India and Sri Lanka which developed during the break‐up of Gondwana. Water depths in the Gulf of Mannar are up to about 3000 m. The stratigraphy is about 4 km thick in the north of the Mannar Basin and more than 6 km thick in the south. The occurrence of an active petroleum system in the basin was confirmed in 2011 by two natural gas discoveries following the drilling of the Dorado and Barracuda wells, located in the Sri Lankan part of the Gulf. However potential hydrocarbon source rocks have not been recorded by any of the wells so far drilled, and the petroleum system is poorly known. In this study, basin modelling techniques and measured vitrinite reflectance data were used to reconstruct the thermal and burial history of the northern part of the Mannar Basin along a 2D profile. Bottom‐hole temperature measurements indicate that the present‐day geothermal gradient in the northern Mannar Basin is around 24.4 ^oC/km. Optimised present‐day heat flows in the northern part of the Mannar Basin are 30–40 mW/m². The heat flow histories at the Pearl‐1 and Dorado‐North well locations were modelled using SIGMA‐2D software, assuming similar patterns of heat flow history. Maximum heat flows at the end of rifting (Maastrichtian) were estimated to be about 68–71 mW/m². Maturity modelling places the Jurassic and/or Lower Cretaceous interval in the oil and gas generation windows, and source rocks of this age therefore probably generated the thermogenic gas found at the Dorado and Barracuda wells. If the source rocks are organic‐rich and oil‐ and gas‐prone, they may have generated economic volumes of hydrocarbons.     

文留盐构造成因与掩埋机制

文留盐构造位于东濮坳陷中央隆起带上,是中原油田重要油气圈闭构造之一.该构造形成于早第三纪沙三段早期,其演化完全受区域拉张、局部构造及同构造沉积的控制.在厚皮拉张过程中,塑性流动盐层使上覆层与基底的形变发生部分拆离,形成文留构造的三层式结构：下部基底地垒,中部盐核及上覆层强制褶皱背斜与拱顶地堑.区域拉张形成中部隆起垒块,同构造沉积将盐体从两凹挤向中隆,使早期的盐枕演化成次动盐刺穿.晚第三纪时,区域拉张减弱,盐构造运动随之减弱,文留构造被上第三系和第四系沉积物深埋.尺度物理模拟采用干石英砂及硅树脂胶为实验材料,模拟了文留构造的动力演化过程.     

PETROLEUM PROSPECTS OF CRETACEOUS SEDIMENTS OF THE CAMBAY BASIN, GUJARAT, INDIA

The Cambay Basin in the NW part of the Indian Peninsula is a commercial petroleum producer from Tertiary sediments resting on the basic lava flows known as Deccan Traps. Cretaceous sediments which underlie the Deccan Traps are exposed in the eastern and western margins of the basin and are also encountered in the subsurface. The paleogeographic reconstruction of the Cambay Basin suggests that the northern part, possessing a thin cover of Cretaceous sediments, has meagre petroleum prospects. However, the southern part of the basin, where more than 1,000 m of Cretaceous sediments were deposited by two independent drainage systems, may be more lucrative. Suitable facies for generation and accumulation of petroleum are expected in this part of the basin.     

构造演化对莺歌海盆地天然气成藏差异性的控制作用

尽管两个重要的天然气勘探区"东方区"和"乐东区"均位于莺歌海盆地莺歌海凹陷中央底辟带,但二者天然气的成藏机制却存在一定差异。为此,分析了构造演化对烃源岩发育和底辟活动的控制作用以及对两区天然气成藏差异性的影响:①中新世以来的构造演化导致沉降、沉积中心由东方区向乐东区迁移,这是控制该区中新统烃源岩发育和底辟活动的主要因素,对天然气的生成、成藏和分布产生重要影响;②东方区烃源岩生气时间较早、持续时间较长,而乐东区烃源岩的生气时间则相对较晚、生气更集中,更有利于天然气聚集成藏;③晚中新世以来的底辟活动对天然气的运移、成藏和分布起决定性作用;④东方区底辟构造较少、活动能量较弱,而乐东区底辟构造更发育且活动更为复杂;⑤二者天然气的成藏机制也有共同之处,即均处于高温高压带,天然气成藏均受控于底辟活动。结论指出,该盆地底辟波及区能量相对较低,中深层天然气的成藏时间较早,保存条件较好,CO_2风险较低,是有利的优质天然气分布区域。     

塔里木盆地北缘沙雅隆起两阶段走滑变形的证据

沙雅隆起(塔北隆起)是塔里木盆地北缘极为重要的近东西向构造带。前人研究认为,沙雅隆起以逆冲变形为主,但是,这一认识既与地震剖面上的花状构造样式相矛盾,又与平面图上断层呈扫帚状和拖尾状分布的特征相矛盾。该文基于高分辨率的三维地震剖面,提出塔里木盆地北缘沙雅隆起经历了2个阶段的走滑构造变形:第一阶段为加里东晚期—印支期的压扭构造变形,形成了深部构造层中的大型正花状构造,同时地层被大幅抬升并遭受剥蚀;第二阶段为喜马拉雅期的负反转—张扭构造变形,形成了浅部构造层中的负花状构造。这一认识表明,塔里木盆地北缘可能并非统一的构造—沉积环境,沙雅走滑构造带为一重要的构造—沉积分隔带,而加里东晚期与喜马拉雅早期是塔里木盆地北缘非常重要的2个构造变革期。     

东营凹陷底辟型构造圈闭的形成机制

“底辟构造”又称“挤入构造”,是一种重要的含油气构造圈闭类型,在世界油田中占有相当大的比重。关于底辟构造的形成机制,国外文献中早已有大量的,相当深入的论述。但是在我国东部各新生代含油气盆地中,究竟存不存在这种类型的油气藏,却一直有争议。近年来,随着石油勘探工作的迅速发展,深井钻探、数字地震成果不断增加,对深层构造也不断提出新的认识。就目前取得的资料看,东营凹陷、潜江凹陷、东濮凹陷中底辟构造的显示是比较明显的。     

柴达木盆地盐类沉积的形成及与油气的关系

柴达木盆地的盐类沉积相当发育,不仅现代盐湖星罗棋布,晚第三纪—早第四纪盐类沉积也比比皆是,发育着石盐、石膏、芒硝、白钠镁矾、泻利盐、水氯镁石、钾盐、硼酸盐、天青石等。     

塔里木盆地北缘温宿凸起压扭隆升的证据

温宿凸起是塔里木盆地北缘极为重要的近东西向构造带。前人研究认为,温宿凸起以逆冲变形为主。但是,这一认识既与地震剖面上背冲的2个方向断层同等发育矛盾,又与主干断层产状突变矛盾。基于高分辨率二维地震剖面的精细地质解释,提出塔里木北缘温宿凸起的构造变形以走滑为主,而非前人认为的以逆冲为主。主要证据是:（1）在地震剖面上,表现为2个正花状构造的变形特征;（2）形成花状构造的主干断层（沙井子断裂与古木别兹断裂）在东段向南强烈逆掩,在西段则向北强烈逆掩,顺断层走向产状多变;（3）温宿凸起的前中生界为古老的震旦系—寒武系,向南、向北分别逆冲于阿瓦提凹陷与乌什凹陷中—新生界之上。走滑构造变形发生于加里东中期（晚奥陶世末）、加里东晚期（志留纪末）、海西早期（早—中泥盆世末）、海西晚期（晚二叠世）、印支—燕山期（三叠纪末期和晚白垩世）和喜马拉雅期（古近纪、新近纪和第四纪）。该走滑构造带的厘定表明,古生代时期塔里木盆地北缘可能并非统一的构造—沉积环境,温宿凸起为一重要的构造—沉积分隔带。