Fluid Charging and Paleo-pressure Evolution in the Ledong Slope Zone of the Yinggehai Basin,South China Sea

Large numbers of gas reservoirs have been discovered in overpressure basins.Fluid charging has a close relationship with paleo-pressure evolution,affecting the migration of gas reservoirs.To study fluid charging and the related pressure system,we analyzed burial histories and fluid inclusion(PVTx)simulations and conducted basin modeling of the Ledong Slope Zone in the Yinggehai Basin as an example.On the basis of fluid-inclusion assemblages(FIAs),homogenization temperature(Th),final melting temperature(Tm,ice)and Raman spectroscopy in fluid inclusions,there are three stages of fluid charging:during the first and second stage,methane-dominated fluid was charged at 2.2–1.7 Ma and 1.7–0.9 Ma,respectively.In the third stage,CO_(2)-rich hydrothermal fluid was charged since 0.9 Ma.It could be concluded from the well-logging data that the disequilibrium compaction in the Yinggehai Fm.,along with the fluid expansion and clay diagenesis in the Huangliu and Meishan formations,resulted in the overpressure in the Ledong slope zone.The evolution of paleo-pressure was affected by the sedimentation rate of the Yinggehai Fm.,as well as the hydrocarbon generation rate.Additionally,the Ledong Slope Zone is less affected by diapir activity than the nearby diapir area.Based on fluid inclusions,paleo-pressure,basin modeling and geological background,the gas migration history of the Ledong Slope Zone can be divided into four stages:in the first stage,excess pressure was formed around 5 Ma;from 2.2 to 1.7 Ma,there was a reduction in the charging of hydrocarbon fluid and steadily increasing excess pressure;during the 1.7–0.9 Ma period a large amount of hydrocarbon was generated,excess pressure increasing significantly and hydraulic fractures forming in mudstones,With gas reservoirs developing in structural highs;since 0.9 Ma,CO_(2)-rich hydrothermal fluid accumulated in reservoirs adjacent to faults and the pressure coefficient remained stable.The research results are helpful in the study of fluid migration and accumulation mechanisms in overpressure basins.

Effects of Overpressured Fluid Flow on Petroleum Accumulation in the Yinggehai Basin

The Yinggehai Basin is a strongly overpressured Cenozoic basin developed in the northern continental shelf of the South China Sea. The flow of overpressured fluids in this basin has given rise to strong effects on petroleum accumulation. (1) The overpressured fluid flow has enhanced the maturation of shallow-buried source rocks, which has caused the source rocks that would have remained immature under the conduction background to be mature for hydrocarbon generation. As a result, the overpressured fluid flow has increased the volume and interval of mature source rocks. (2) The overpressured fluid flow has strong extraction effects on the immature or low-mature source rocks in the shallow parts. This has increased, to some extent, the expulsion efficiency of the source rocks. More importantly, the extraction effects have strongly limited the effectiveness of biomarker parameters from oil and condensate in reflecting the source and maturity of the oil and gas. (3) The flow has caused the sandstones in the shallow parts to get into the late diagenesis stage, and significantly reduced the porosity and permeability of the sandstones. This study confirms that even in sedimentary basins in which no topography-driven groundwater flow systems have ever developed, the cross-formation migration of overpressured fluids and the resultant energy conduction and material exchange can significantly affect the thermal regime, source rock maturation and sandstone diagenesis. As a result, the effects of overpressured fluid flow must be taken into account in analyzing the mechanism of petroleum accumulation.

Episodic Fluid Injections in Overpressured Basins:A Case Study from Yinggehai Basin

The characteristics, origin and injection history of the Dongfang gas field, the largest gas field found in the Yinggehai basin, are studied by integrated geological, geothermal and geochemical data. The gas field shows considerable variation in hydrocarbon gas, nitrogen and carbon dioxide content, aud has been filled by at least four stage fluids. A strong thermal anomaIy caused by hydrothermal fluid flows occurs in the gas field, as evidenced rrom drill-stem test and fluid iuclusion homogenization temperatures, rock-eval tmax,vitrinite reflectance as well as clay-mineral transformation profiles. Such a thermal anomaly suggests focused, rapid flow of deeply-sourced hydrothermal rluids. The inter-reservoir heterogeneities, the strong migotion rractionation related to abrupt changes in pressure and temperature and tbe short-lived, transient nature of the thermal effect of fluid flow are evidences of episodic rluid injections from the overpressured systems into the reservoirs.

Characteristics of Overpressure Systems and Their Significance in Hydrocarbon Accumulation in the Yinggehai and Qiongdongnan Basins,China

Overpressure systems are widely developed in the central depression and paleo-uplift in the Yinggehai and Qiongdongnan basins. They can be divided into three types according to the origin of abnormally high formation pressure in the reservoirs, i.e. the autochthonous, vertically-transmitted and laterally-transmitted types. The autochthonous overpressure system results from rapid disequilibrium sediment loading and compaction. In the allochthonous overpressure system, the increase of fluid pressure in sandstone originates from the invasion of overpressured fluid flowing vertically or laterally through the conduit units. The autochthonous overpressure system occurs in the deep-lying strata of Neogene age in the central depression of the Yinggehai and Qiongdongnan basins. The vertically transmitted overpressure system is developed in the shallow strata of Late Miocene and Pliocene ages in the diapiric zone of the central Yinggehai basin, and the laterally transmitted overpressure system occurs in the Oligocene strata of paleo-uplifts, such as the structure of Ya-211 in the Qiongdongnan basin. The results indicate that the autochthonous overpressure system is generally a closed one, which is unfavorable for the migration and accumulation of hydrocarbons. In the allochthonous overpressure system, hydrocarbon accumulation depends on the relationship between the formation of overpressure systems and the spatial location and duration of hydrocarbon migration. The interval overlying the overpressure system is usually a favorable hydrocarbon accumulation zone if the duration of fluid expulsion coincides with that of hydrocarbon accumulation.

Active Depths of Main Faults in the Ying-Qiong Basin Investigated by Multi-Scale Wavelet Decomposition of Bouguer Gravity Anomalies and Power Spectral Methods

The Ying-Qiong Basin is located on the northwestern margin of the South China Sea and at the junction of the South China Block and the Indochina Block.It is characterized by complex geological structures.The existing seismic data in the study area is sparse due to the lack of earthquake activities.Because of the limited source energy and poor coverage of seismic data,the knowledge of deep structures in the area,including the spatial distribution of deep faults,is incomplete.Contrarily,satellite gravity data cover the entire study area and can reveal the spatial distribution of faults.Based on the wavelet multi-scale decomposition method,the Bouguer gravity field in the Ying-Qiong Basin was decomposed and reconstructed to obtain the detailed images of the first-to sixth-order gravitational fields.By incorporating the known geological features,the gravitational field responses of the main faults in the Ying-Qiong Basin were identified in the detailed fields,and the power spectrum analysis yielded the depths of 1.4,8,15,26.5,and 39 km for the average burial depths of the bottom surfaces from the first-to fifth-order detailed fields,respectively.The four main faults in the Yinggehai Basin all have a large active depth range:fault A(No.1)is between 5 and 39 km,fault B is between 26.5 and 39 km,and faults C and D are between 15 and 39 km.However,the depth of active faults in the Qiongdongnan Basin is relatively shallow,mainly between 8 and 26.5 km.

琼东南盆地Ⅱ号断裂带新生界多期热流体活动与天然气运聚特征

运用流体包裹体、有机地球化学分析和压力模拟手段,综合研究了琼东南盆地东部Ⅱ号断裂带新生界多期热流体活动及天然气运聚特征。研究结果表明:(1)琼东南盆地东部Ⅱ号断裂带BX19-2构造天然气主要为烃气和CO_(2),烃气为煤型气和油型气的混合气,但不同气层中天然气组分含量差异较大,浅部三亚组气层为相对高含量的烃气(体积分数为83.93%)和低含量的有机成因CO_(2)(体积分数为7.11%);较深部的陵水组气层为含量相对较低的烃气(体积分数为16.10%~76.63%)和含量相对较高的幔源CO_(2)(体积分数为18.70%~81.56%)。(2)流体包裹体和岩石地球化学参数显示烃气运移与3期热流体活动相关,每期热流体的活动深度及引起的热异常程度均存在一定差异。3期含烃热流体活动的时间分别为中新世晚期(约8.8 Ma)、上新世(约4.5~4.1 Ma)和第四纪(约1.1~0.1 Ma)。(3)热流体是以断裂为主要通道发生垂向高效快速充注。来自宝岛凹陷深部的幔源CO_(2)在上新世晚期—第四纪(约2.2~0.5 Ma)发生充注,并驱替了陵水组储层的烃气。因此,宝岛凹陷Ⅱ号断裂带附近的圈闭可能具有钻遇高CO_(2)含量的风险。

莺歌海盆地中央坳陷带成藏体系的盖层评价及控藏作用

莺歌海盆地是我国南海重要的天然气探区。通过对莺歌海盆地中央坳陷带不同成藏体系典型气藏和含气构造的精细解剖,评价了盖层的有效性及控制因素,并分析了盖层对天然气成藏的控制作用。研究结果表明:(1)莺歌海盆地中央坳陷带浅层和超浅层成藏体系中,盖层的封盖有效性受到盖层、底辟构造及底辟构造活动伴生断裂的共同控制;中深层成藏体系中普遍存在异常高流体压力,且断裂不太发育,盖层水力封闭是控制油气差异富集的关键因素。(2)浅层和超浅层成藏体系内,脆性盖层保持封盖有效性的临界断接厚度为86~98 m;中深层成藏体系内,盖层的封盖有效性可以通过水力破裂压力系数进行评价,当系数大于1时,意味着盖层已发生破裂或具有极强的水力破裂的风险。(3)整体上中央坳陷带超压诱发的水力破裂是中深层盖层封闭失效的根本原因,深层天然气通过水力破裂通道运移至浅层,最终经断裂调整至超浅层成藏,气源充足的条件下,超浅层及浅层气藏主要集中分布在底辟的顶部,而中深层气藏主要集中分布于底辟翼部及斜坡区。

Mechanism of diapirism and episodic fluid injections in the Yinggehai Basin

The diapirism in the Yinggehai Basin is a combined result of strong overpressure caused by rapid sedimentation of fine-grain sediments and the tensile stress field resulting from right-lateral slip of the boundary-fault. The diapirism showed multiple-stage, episodic nature, and caused intermittent counter-direction onlaps and changes in the thickness of strata. The shallow gas reservoirs in the diapir structural zone displayed obvious inter-reservoir compositional heterogeneities, and their filling history could be divided into 4 stages, with gases injected during different stages having different hydrocarbon gas, CO2 and N2 contents and different stable isotope compositions. The multiple-episode, intermittent activities of the diapirs, multiple-stage, non-continuous injections of fluids, and the transient thermal effect of fluid flow as well as the strong migration fractionation reflected episodic fluid injection and natural gas accumulation.

莺歌海盆地东方区块地层超压对成岩作用的影响

莺歌海盆地东方区块莺歌海组下部和黄流组发育地层超压体系,超压顶界面深度从盆地中心向边缘逐渐变深。在综合前人研究基础上,笔者应用热解分析、生物标志化合物研究及岩石显微镜和扫描电镜等技术,并与邻区的常压环境下的相关特征进行对比,深入分析了地层超压对成岩作用的影响。结果表明:1超压对反映有机质热演化特征的指标(镜质体反射率、热解峰温、生产指数等)未产生可识别的影响,但对黏土矿物转化和烃类结构演化这类产物浓度变化速率高、体积膨胀效应强的反应有明显的抑制作用;2超压可降低砂岩的机械压实速率,使孔隙度得以有效保存;3超压可以通过抑制黏土矿物的转化来减少胶结物生成的物质来源,同时通过增大碳酸盐在流体介质中的溶解度抑制碳酸盐胶结物的生成;4研究区储层普遍高含CO2,在超压环境下CO2在流体中溶解度增大,会大量生成H+,促进溶蚀作用的产生;5超压环境促进了烃源岩生烃作用生成的酸性流体的排出,增强了相邻砂岩储层的溶蚀作用,有利于次生孔隙生成,从而发育了异常高孔带。

莺歌海盆地汇聚型超压流体流动及天然气晚期快速成藏

对莺歌海盆地地层压力结构、底辟分布与发育机理及超压流体流动效应的综合研究揭示并证明了一种新的超压流体流动样式,即晚期汇聚型超压流体流动。该模式与超压流体从浅埋藏(<1000～1500m)开始、在盆地范围内普遍排放的Cartwright模式明显不同,莺歌海盆地超压流体早期被封闭,晚期通过底辟集中排放,超压流体的汇聚导致天然气的晚期幕式快速成藏。对东方1-1气田的圈闭构造分析表明,该气田形成于第四纪,其最低平均充注速率达140m^3/d。成藏流体的层间组分非均质性、流体流动的瞬态热效应及强烈的运移分异性反映了流体流动的不连续性、流体运移过程中温度、压力条件的快速变化及多期流体的相互作用,可成为幕式流体活动和油气幕式快速成藏的有效识别标志。

断-压双控流体流动与油气幕式快速成藏

通过对超压的成藏物质效应和能量效应分析 ,系统论述了超压盆地油气幕式快速成藏机理。超压对有机质热演化的抑制作用使深部源岩和时代较老的源岩与较年青的源岩 (准 )同时保持在有利的生排烃阶段 ,而超压伴生的流体排放延迟使深部超压源岩在不同温度、成熟度条件下生成的烃类在晚期集中排放 ,两者的共同作用增加了有效源岩的层位和体积 ,进而增大了晚期源岩的累积排烃速率 ,为油气晚期快速成藏提供了重要的物质基础。超压可以引起未变形地层的水力破裂和超压流体的初次排放 ;超压流体的二次排放受超压和断裂特征的共同控制 ,即断 压双控流体流动。超压的成藏物质效应和能量效应决定了新构造运动 (或晚期构造运动 )控制下的油气快速成藏 :①油气成藏由多幕次流体充注完成 ,每一幕次包括稳态汇聚期和瞬态充注期 ;②油气成藏速率异常高 ,大型油气田可在 0 1Ma内形成 ;③构造活动在油气成藏过程中具有建设性作用 ,晚期构造 (或新构造 )控制油气分布。

莺歌海盆地超压流体活动与断裂系统的相互关系

莺歌海盆地内超压流体活动十分强烈 ,主要活动形式有侧向对流型和垂向对流型 ,将超压流体的演化与区域构造演化结合起来 ,对断裂与超压流体的成因关系提出了新见解 ,揭示出断裂对超压流体的输导及超压流体对断裂的开启的相互作用。张裂隙作为超压流体活动的诱导 ,超压的幕式突破就主要起因于上覆负载层中的断裂、破碎带 ,发生压力释放 ;而普遍发生的幕式流体释放又进一步开启垂直裂隙 ,造成上下地层间的连通 。

延怀盆地上地壳热流体特征及其与地震活动的关系

系统研究了延怀盆地上地壳热流体的温度、水化学、气体化学与同位素化学特征 ,确定热水成因与大气降水渗入有关 ,并计算了热储温度与水循环深度 ,认为热水活动对中小地震活动有一定的控制作用并影响断裂活动强度。

红河活动断裂带在南海西北部的反映

红河断裂带是一条走滑的活动断裂带 ,它控制着南海西北部的构造活动 ,也控制着莺歌海盆地的形成和演化。根据南海西北部中穿过莺歌海盆地的地震剖面和历史资料进行解释 ,结果表明 ,莺歌海盆地的形成可分 3个阶段 :自 50MaB .P .开始 ,沿红河断裂带的左旋错动和在印支地块的顺时针旋转的应力作用下 ,形成了莺歌海盆地的雏形 ;2 4MaB .P .之后在左旋压扭应力场作用下 ,形成了盆地西北部的反转构造 ;5MaB .P .之后发生了右旋错动 ,盆地内快速沉降 ,发育巨厚沉积层。根据盆地内最老和最新的沉积中心之间的距离 ,推测沿红河断裂带的左旋位错约 2 0 0km。该断裂带发展到现代 ,其活动性大为减弱 ,曾发生 1 0次小于 5级地震。

莺歌海盆地LD区块地层超压对泥岩成岩作用的影响及其地质意义

在综合前人研究基础上,应用X衍射、薄片鉴定、热解分析、生物标志化合物研究等技术,并与邻区的常压环境下的相关特征进行了对比,深入分析了莺歌海盆地LD区块地层超压的分布状况及其对黏土矿物转化和有机质热演化的影响。结果表明,1研究区内,超压顶界面深度从盆地中心向边缘逐渐变深;2研究区内,超压顶界面之上,随埋深和地温的增大,蒙脱石、伊蒙混层中蒙脱石和高岭石的含量逐渐降低,而伊利石和绿泥石含量逐渐增加,但超压顶界面附近及往下时,黏土矿物各组分的含量几乎不再减少,受到了地层超压作用的抑制;3Pr/nC17和Ph/nC18随深度增大而降低,但在强超压顶界面以下,变化趋势发生逆转,出现微弱增大的趋势,超压顶界面以下的实测Ro值即有机质的成熟度也出现了明显的低异常。地层超压通过抑制黏土矿物转化,间接减弱了储层的胶结作用,有利于深层优质储层的形成。研究区地层超压抑制了有机质的热演化,地层超压对生烃的抑制作用扩大了研究区有效源岩的层位和体积,并使主力源岩的生、排烃期后移,从而有利于油气的晚期快速聚集成藏。

准噶尔盆地西北地区断层内流体活动过程及对断层启闭性的影响

利用野外、钻井、测井和地球化学分析等资料,对准噶尔盆地西北地区断层内流体活动过程及对断层启闭性的影响进行了研究。准西北地区断层发生过4期流体活动:断层形成初期,滑动破碎带受力最为集中,破碎较为严重,一期幔源流体首先进入滑动破碎带,并不断析出矿物充填其中,物性变差;二期幔源流体在滑动破碎带物性较好的部位穿插运移,胶结形成的矿物与一期形成的矿物脉体将滑动破碎带封堵;三期盆源流体主要在物性较好的主动盘裂缝带内运移,少部分进入被动盘诱导裂缝带;四期大气降水在断层物性较好的主动盘诱导裂缝带和被动盘诱导裂缝带内活动,初期主要以溶蚀作用为主,后期胶结作用较为强烈。由于第三期盆源流体活动期间,往往也是盆内大规模油气排烃时期,流体中往往含有大量油气,因此油气的运移主要在主动盘诱导裂缝带内进行,其次为被动盘诱导裂缝带,滑动破碎带实际上起封闭作用,不输导油气。

松辽盆地北部嫩一段泥岩超压形成与演化

利用声波时差随埋深的变化及等效深度法,对松辽盆地北部嫩一段泥岩超压大小、分布特征及形成时期进行了研究。结果表明:目前普遍的超压高值区分布在齐家—古龙凹陷,最大值可达到9 MPa以上;主要形成于明二段沉积时期,之后超压值逐渐增大,在明水组沉积末期和古近系沉积末期曾因T1断裂活动释放,超压值降低。根据超压形成演化过程和源岩生排烃史研究得到:①嫩一段源岩主要排烃期为古近系沉积末期;②嫩一段源岩排出的油气向下"倒灌"运移距离最大可达190 m左右,在大庆长垣、三肇凹陷、齐家—古龙凹陷、龙虎泡阶地和明水阶地等大部分地区均可向下伏萨尔图油层中进行"倒灌"运移,仅在局部地区油气可"倒灌"运移进入葡萄花油层;③超压形成时期早于大量排烃期,对于封闭其油气是有利的;④嫩一段泥岩超压自形成后,虽在明水末期和古近系末期曾发生释放超压降低,但封闭能力逐渐增强至今。这些条件应是松辽盆地北部萨尔图、葡萄花油层油气富集与分布的重要原因。

断陷湖盆超压封存箱形成机理与油气成藏机制——以渤海湾盆地东营凹陷为例

东营凹陷的超压分布于沙河街组三段中-下亚段大套泥岩和沙四段灰色膏盐岩段中,常压—低压系统发育于上部层段,包括沙二段上亚段、东营组、馆陶组、明化镇组和更新统,形成二元结构。这些发育程度不同的超压体系可以构成不同级别的超压封存箱系统,控制着油气的运移和聚集。超压封存箱的形成是一个地球化学自组织过程,与系统内的流体对流运动及成岩作用密切相关。超压体系构成了准封闭的超压封存箱型自源油气成藏动力学系统,同时也可以通过幕式排放为浅层的新近系及古近系的沙三段上亚段至东营组常压开放性它源油气成藏动力学系统提供油源及成藏动力。超压不但是油气运聚的主要动力,而且在其产生与演化的过程中能形成微裂缝型油气运聚隐蔽输导体系,是形成岩性等隐蔽油气藏的重要条件。超压和断裂形成陆相断陷湖盆断—压双控流体流动机制。断裂是超压体系卸压的重要渠道,也是幕式排放的主要途径;与断裂沟通的砂岩体及构造背斜等是有利的勘探目标,超压泥岩体周围的透镜状砂体是隐蔽油气藏的潜在目标。

断陷湖盆超压分布特征及其与油气成藏的关系

渤海湾盆地沾化凹陷古近系的超压在渤南洼陷和五号桩洼陷的沙三段中—下亚段大套泥岩和沙四段灰色膏盐岩段中分布,但在不同层位超压分布略有差别;常压—低压系统发育于上部层段,剖面上形成二元结构。从埋深上看,2 500m以上地层基本为常压或低压,2 500m以下地层开始出现较明显的压力异常。沾化凹陷超压系统对应主力烃源岩系沙三中、下亚段及沙四上亚段,生烃作用是超压的重要成因,但超压对生烃的抑制作用不明显。超压体系构成了准封闭的超压封存箱型自源油气成藏动力学系统,同时也通过幕式排放为浅层的新近系及古近系的沙三段上亚段—东营组常压开放性它源油气成藏动力学系统提供油源及成藏动力。断裂是超压体系泻压的重要渠道,也是油气幕式排放的主要途径,与断裂沟通的砂岩体及背斜构造是有利的勘探目标,超压泥岩体周围的透镜状砂体是潜在的目标。

塔中和巴楚地区深部流体活动控制因素及有利区预测

塔里木盆地深部流体活动的2个必要因素是火山岩浆作用和深大断裂发育。前者是深部流体形成的基础,后者为深部流体从深部向浅部运移提供通道。塔里木盆地塔中和巴楚地区二叠系火山岩广泛发育,在塔中的中1井附近区域和巴楚的吐木休克断裂、色力布亚断裂、康塔库木断裂以及玛扎塔格断裂区域是火山岩浆活动的中心区域。塔中的塔中Ⅰ号、塔中10井、塔中Ⅱ号等断裂和巴楚地区的吐木休克断裂、色力布亚断裂、康塔库木断裂、玛扎塔格断裂等在海西晚期是活动的,可作为深部流体活动的有效断裂。塔中和巴楚地区具有二叠纪火山岩浆活跃区域并且存在有效断裂发育区域,因此,既是深部流体活跃的区域,也是深部流体对下古生界碳酸盐岩储层溶蚀改造的有利区域。