中国深层和超深层碳酸盐岩油气藏形成分布的基本特征与动力机制及发展方向

随着油气资源对外依赖度加大,中国的油气勘探已经拓展到深层和超深层领域,并相继在中西部盆地发现了塔河、普光、安岳、靖边、顺北等一批大型油气田,展示出广阔的勘探前景。中国已探明的深层和超深层碳酸盐岩油气藏特征与全球的有很大差异,经典的油气地质理论指导这类油气田勘探遇到了前所未有的重大挑战,需要完善和发展。通过调研和比较全球已探明的碳酸盐岩和砂岩油气藏地质特征,发现它们的油气来源条件、油气藏形成条件、成藏动力、演化过程特征等类同;同时,发现碳酸盐岩和砂岩油气藏的矿物组成、孔隙度和渗透率随埋深变化特征、孔渗结构特征、储层物性下限、油气藏类型等有着很大不同。中国深层和超深层碳酸盐岩油气藏与全球的相比较具有五方面差异:地层年代更老、埋藏深度更大、白云岩储层比率更大、天然气资源比率更高、储层孔渗关系更乱。中国已经发现的深层碳酸盐岩油气藏成因类型可以归为五种:沉积型高孔高渗油气藏、压实成岩型低孔低渗油气藏、结晶成岩型低孔低渗油气藏、流体改造型高孔低渗油气藏、应力改造型低孔高渗油气藏;它们形成的动力学机制分别与地层沉积和浮力主导的油气运移作用、地层压实和非浮力主导的油气运移作用、成岩结晶和非浮力主导的油气运移作用、流体改造介质和浮力主导的油气运移作用、应力改造和浮力主导的油气运移作用等密切相关。中国深层和超深层碳酸盐岩油气藏勘探发展的有利领域和油气藏类型主要有三个:一是低热流盆地浮力成藏下限之上自由动力场形成的高孔高渗常规油气藏;二是构造变动频繁的叠合盆地内外应力和内部流体活动改造而形成的缝洞复合型油气藏;三是构造稳定盆地内局限动力场形成的广泛致密连续型非常规油气藏。改造类非常规致密碳酸盐岩油气藏是中国含油气盆地深层和超深层油气资源的主要类型:它们叠加了早期形成的常规油气藏特征,又具有自身广泛连续分布的非常规特征,还经受了后期构造变动的改造;复杂的分布特征,致密的介质条件和高温高压环境使得这类油气资源勘探开发难度大、成本高。

川北中、下侏罗统烃源岩重排藿烷组成变化与油源对比

利用色谱—质谱定量分析技术,对40余个取自四川盆地北部石龙场和元坝地区不同成熟度的中、下侏罗统湖相烃源岩和原油样品进行分析,以此揭示其重排藿烷组成和分布的变化特征,并进行精细的油源对比。研究结果表明,不同成熟度的烃源岩中重排藿烷的丰度相差悬殊。石龙场地区Ro值为0.8%~1.0%的中侏罗统千佛崖组和下侏罗统自流井组大安寨段泥岩中,各类重排藿烷异常丰富,检出了17α(H)-重排藿烷、18α(H)-新藿烷和早洗脱重排藿烷3个完整碳数系列。它们的相对含量大都数倍于规则藿烷,是该层系烃源识别的分子标志。而在相邻的元坝地区相带、岩性相近的这两层段高成熟(Ro值主要在1.4%~1.9%)烃源岩中,藿烷类化合物出现异常变化,重排藿烷的相对含量很低,可能与干酪根在高热演化阶段生成的规则藿烷比例较高有关。烃源岩中高丰度重排藿烷并不取决于其绝对含量的高低,而在于它们相对于规则藿烷的富集。弱氧化的沉积环境是导致重排藿烷相对富集的一个重要原因,而有机质生源中的细菌组成可能更是一个关键因素。当烃源岩达到高成熟阶段时,藿烷类化合物的组成和分布不再受控于热化学动力学机制,各类重排藿烷均按一定比例分布,基本失去了其地球化学属性。区内中、下侏罗统原油中重排藿烷的组成和分布也随热演化程度而变化。它们在成熟原油中极丰富,而在高成熟原油中则很少,与烃源岩存在对应关系。经油—岩对比,认为这些原油来源于所在层位的烃源岩。

团簇同位素在白云岩化流体恢复中的应用与局限性

碳酸盐的团簇同位素(Δ47)可以直接确定矿物的形成温度,结合矿物的氧同位素,还可以根据传统的氧同位素温度计确定矿物生长的流体(水)的氧同位素(δ18 O),提供了一种潜在的确定白云化流体的δ18 O、分析白云岩化流体来源的新方法。受白云岩化之后的重结晶作用和高温固态重置作用的影响,白云岩的团簇同位素温度(TΔ47)会比白云化作用实际发生时期的温度偏高,给直接利用TΔ47来恢复白云岩化流体的δ18 O造成了一定的局限性。为了分析四川盆地东北部地区长兴组-飞仙关组白云岩化流体的来源,开展了普光气田和元坝气田的白云岩团簇同位素研究。普光气田和元坝气田长兴组-飞仙关组白云岩的TΔ47在70~130℃,是重结晶作用和固态重置之后的白云岩视平衡温度。根据地层温度演化史和白云石的固态重置模型,固态重置作用导致川东北地区白云岩的平衡温度增高了30~35℃。经固态重置恢复之后确定的白云化流体的δ18 O(SMOW)在0~4‰,表明白云岩化流体来源于高盐度的蒸发流体。

川东北须家河组致密砂岩断缝系统Ⅱ:断层破碎带的结构特征

断层破碎带结构是逐渐而非某次剧烈变化形成的,这也直接导致了破碎带结构几何空间特征的复杂性。通常断层破碎带主要由断层核和破碎带这两个部分组成,但大规模或者构造变形强烈的断层中,断层破碎带可能会由单一断层核演化为具有多个核部的复杂结构。如何获得这样的结构特征,对研究断层破碎带控制下的油气运移规律有重要意义。基于三维地震方差属性,本文提出对川东北马路背地区须家河组二段致密砂岩的断层破碎带构造形态进行刻画,并对断层破碎带的宽度进行量化分析。结果显示如下:(1)地震方差属性结果展示了地下存在单核断层破碎带和多核复合断层破碎带。本文利用累计方差值的斜率变化点,结合川东北致密砂岩具体情况,选取方差值0.2作为边界值,确定该断层破碎带宽度在700~1200m之间。(2)数值模拟显示,断层破碎带的形变程度呈指数衰减,即随着离核距离的增加,变形强度减少,这些特征和地表认识一致。(3)通过破碎带宽度量化分析,认为研究区目的层断裂带的宽度表现出极强的不均质性。致密砂岩不仅各断层破碎带之间宽度差异大,断层破碎带宽度在深度上也有明显变化,其结构显示出明显的"层控效应"。虽然确认断层破碎带宽度仍然依赖于经验取值,但该研究为致密砂岩断层破碎带内部结构解析提供了新的方法和工具,为进一步建立川东北致密砂岩断缝成藏系统提供基础,其断层破碎带构造特征对分析断缝成藏具有重要意义。

关于富油盆地深层天然气来源与勘探前景的几点思考——以渤海海域渤中地区为例

渤海湾盆地烃源岩有机质类型以Ⅰ-Ⅱ型为主,是典型的富油裂陷湖盆。以渤海海域(渤海湾盆地海域部分)渤中地区为例,从天然气来源与成因判识剖析油型盆地干酪根的生气特征以及深层天然气勘探前景。结果表明,对于相同热演化程度的干酪根而言,其热解气的量与烃源岩的氢指数呈正相关关系,而且大部分干酪根热解气在R_(o)=1.5%之前已经生成。生气强度表明,即使对于热演化程度相对偏低的富油盆地,以干酪根热解为主的天然气生气量仍满足大中型气田形成的物质基础。高生产率主导的倾油型母质具有相对重的沉积有机碳同位素组成,因此乙烷碳同位素较重的天然气不一定是来源于偏腐殖型或者煤系烃源岩,也可能来自较高成熟度的倾油型母质。以R_(o)=1.5%为界,早期生成原油深埋裂解整体可分为两个阶段:第一阶段原油裂解轻质化,该阶段油裂解生成的天然气量相对少,第二个阶段轻质油大量裂解生成天然气,表明生油高峰期(浅层)充注的原油在深埋过程具有较高的热稳定性,深层仍可能富集规模轻质油或凝析油气。

川东北须家河组致密砂岩断缝系统Ⅰ:断层破碎带的平面分布特征

断层破碎带通常伴生有复杂的裂缝系统,为地下流体渗流提供了重要通道,影响着油气、矿产、地下水等的储存。迄今为止,地下断层破碎带特征的研究因缺乏数据而相对较少。依托三维地震勘探资料,本文提取了振幅剖面、极大和极小曲率属性、方差等地震属对通南巴马路背地区须家河组致密砂岩断层破碎带的平面分布特征进行研究。结果显示通南巴背斜在不同时期受到来自不同方向不同强度应力的综合作用,形成现今的北东向的背斜主体,并叠加有一系列北西向的次生褶皱和断层破碎带。工区范围内主要有6条NNW向逆断层破碎带,包括3条延伸超过10 km的长断层破碎带,和3条延伸3~6 km的较短断层破碎。这些断层破碎带在走向上宽度差异明显,破碎范围在270~1250 m,是该地区的断缝系统主要组成部分。高产井Ma101和Ma201处于F2、F3断层破碎带之间的构造高部位,断缝成藏优势明显。其中F2断层为通源断裂,可以沟通下部海相气源;而其他区域虽也有处于断块两背斜之间的区域,但处于构造低部位,并且离大断层较远,断缝成藏优势较不明显。本文为地下断层破碎带平面分布特征的描述提供了地震多属性分析方法,对建立致密砂岩地下断缝系统和相应的油气富集成藏模式有重要作用。

Source Rocks for the Giant Puguang Gas Field,Sichuan Basin:Implication for Petroleum Exploration in Marine Sequences in South China

Detailed geochemistry studies were conducted to investigate the origin of solid bitumens and hydrocarbon gases in the giant Puguang gas field. Two types of solid bitumens were recognized： low sulfur content, low reflectance （LSLR） solid bitumens in sandstone reservoirs in the Xujiahe Formation and high sulfur content, high reflectance （HSHR） solid bitumens in the carbonate reservoirs in the Lower Triassic Feixianguan and Upper Permian Changxing formations. Solid bitumens in the Upper Triassic Xujiahe Formation correlate well with extracts from the Upper Triassic to Jurassic nonmarine source rocks in isotopic composition of the saturated and aromatic fractions and biomarker distribution. Solid bitumens in the Feixianguan and Changxing formations are distinctly different from extracts from the Cambrian and Silurian rocks but display reasonable correlation with extracts from the Upper Permian source rocks both in isotopic composition of the saturated and aromatic fractions and in biomarker distribution, suggesting that the Permian especially the Upper Permian Longtan Formation was the main source of solid bitumens in the carbonate reservoirs in the Feixianguan and Changxing formations in the Puguang gas field. Chemical and isotopic composition of natural gases indicates that the majority of hydrocarbon gases originated from sapropelic organic matter and was the products of thermal cracking of accumulated oils. This study indicates that source rock dominated by sapropelic organic matter existed in the Upper Permian and had made major contribution to the giant Puguang gas field, which has important implication for petroleum exploration in marine sequences in South China.

Preferential petroleum migration pathways and prediction of petroleum occurrence in sedimentary basins:A review

The aim of this paper is to review the major points of contention concerning secondary petroleum migration, to discuss the nature and primary controls of the positions of petroleum migration pathways in sedimentary basins, and to illustrate the importance of preferential petroleum migration pathways for the formation of large oil/gas fields away from generative kitchens. There is competition between the driving force （buoyancy） and the restraining force （capillary pressure controlled largely by permeability contrast） in controlling the positions of petroleum migration pathways in heterogeneous carrier beds. At a large scale, there is a critical angle of dip of the carrier beds which determines the relative importance of structural morphology or permeability contrasts in controlling the positions of petroleum migration pathways in heterogeneous carrier beds. Maximum-driving-force-controlled migration pathways occur in carrier beds with an angle of dip greater than the critical dip and the positions of petroleum migration pathways are controlled mainly by the morphology of the sealing surfaces. Minimum-restraining-force-determined migration pathways occur in carrier beds with an angle of dip smaller than the critical angle where permeability contrasts would exert a more important influence on the positions of petroleum migration pathways. Preferential petroleum migration pathways （PPMP）, defined as very restricted portions of carrier-beds that focus or concentrate petroleum from a large area of the generative kitchen, determine the distribution of oil and gas in sedimentary basins. The focusing of petroleum originating from a large area of the generative kitchens into restricted channels seems to be essential not only for long-range petroleum migration in hydrostatic conditions, but also for the formation of large oil or gas fields. Regions may miss out on petroleum migration because of its three-dimensional behavior, and two-dimensional migration modeling may be misleading in predicting petroleum occurrences in certain circumstances.

Accumulation Mechanisms and Evolution History of the Giant Puguang Gas Field,Sichuan Basin,China

Solid bitumens were found throughout the carbonate reservoirs in the Puguang gas field, the largest gas field so far found in marine carbonates in China, confirming that the Puguang gas field evolved from a paleo-oil reservoir. The fluid conduit system at the time of intensive oil accumulation in the field was reconstructed, and petroleum migration pathways were modeled using a 3-D model and traced by geochemical parameters. The forward modeling and inversion tracing coincided with each other and both indicated that oils accumulated in the Puguang-Dongyuezhai structure originated from a generative kitchen to the northwest of the Puguang gas field. The deposition of organic-rich Upper Permian source rocks dominated by sapropelic organic matter in the Northeast Sichuan Basin, the development of fluid conduit system that was vertically near-source rock and laterally near-generative kitchen, and the focusing of oils originated from a large area of the generative kitchen, were the three requirements for the formation of the giant paleo-oil reservoir from which the giant Puguang gas field evolved. The Puguang gas field had experienced a three-stage evolution. The post-accumulation processes, especially the organic-inorganic interaction in the hydrocarbon-water-rock system, had not only profoundly altered the composition and characteristics of the petroleum fluids, but also obviously changed the physicochemical conditions in the reservoir and resulted in complicated precipitation and solution of carbonate minerals.

Diagenesis and Restructuring Mechanism of Oil and Gas Reservoir in the Marine Carbonate Formation,Northeastern Sichuan:A Case Study of the Puguang Gas Reservoir

Based on the technology of balanced cross-section and physical simulation experiments associated with natural gas geochemical characteristic analyses, core and thin section observations, it has been proven that the Puguang gas reservoir has experienced two periods of diagenesis and restructuring since the Late Indo-Chinese epoch. One is the fluid transfer controlled by the tectonic movement and the other is geochemical reconstruction controlled by thermochemical sulfate reduction （TSR）. The middle Yanshan epoch was the main period that the Puguang gas reservoir experienced the geochemical reaction of TSR. TSR can recreate the fluid in the gas reservoir, which makes the gas drying index higher and carbon isotope heavier because C2＋ （ethane and heavy hydrocarbon） and 12C （carbon 12 isotope） is first consumed relative to CH4 and 13C （carbon 13 isotope）. However, the reciprocity between fluid regarding TSR （hydrocarbon, sulfureted hydrogen （H2S）, and water） and reservoir rock results in reservoir rock erosion and anhydrite alteration, which increases porosity in reservoir, thereby improving the petrophysical properties. Superimposed by later tectonic movement, the fluid in Puguang reservoir has twice experienced adjustment, one in the late Yanshan epoch to the early Himalayan epoch and the other time in late Himalayan epoch, after which Puguang gas reservoir is finally developed.

川北寒武系碳酸盐岩压溶作用的影响因素

前人对缝合线的研究多倾向于定性描述,缺乏缝合线特征(密度和幅度)与矿物成分含量、结构组分和岩相的定量分析。本文明确缝合线密度和幅度与岩石不同矿物含量、不同结构组分和不同岩相之间的定量关系,阐明研究区缝合线的形成的控制因素。综合四川盆地北部寒武系碳酸盐岩野外露头、岩心及薄片等资料,对川北寒武系碳酸盐岩缝合线特征、矿物成分含量、结构组分和岩相进行定量统计。结果表明:(1)碳酸盐岩中混入陆源碎屑,导致岩石非均质性增强,陆源碎屑含量为10%~15%,有助于缝合线的形成;(2)当岩石不含基质,颗粒含量在50%~70%、胶结物含量在25%~30%时,具有较强的抗压溶能力;(3)同一井不同岩相的抗压溶的能力存在差异,颗粒白云岩、混积岩、晶粒白云岩和泥晶白云岩缝合线密度和幅度逐渐增加,表明它们的抗压溶能力逐渐减弱。

Composition and Origin of Shallow Biogenetic Gases in the Baise Basin, South China

Based on the analytical data of over 30 gas samples, combined with geochemical and geological backgrounds, the composition and distribution characteristics of shallow biogenetic gases in the Baise Basin, a Tertiary residual basin in southern China, were extensively investigated, and the origin and formation mechanism tentatively approached. The shallow gases are primarily composed of gaseous hydrocarbons, generally accounting for over 90%. The abundances of methane and C2＋ homologues show a relatively wide range of variation, mainly 50%-100% and 0%-50%, respectively, depending on the mixing proportions between biogenetic and thermogenic gases. A highly negative carbon isotope is the significant signature for the shallow gases with δ^13C1 values of -55‰ to -75‰. According to molecular and isotopic compositions and light hydrocarbon parameters, the shallow gases in the basin can be classified into three types of origins： biogenetic gas, biogenetic/thermogenic mixed gas, and oii-biodegraded gas. They exhibit regular distribution both spatially and temporally, and are believed to be associated with the maturity of adjoining gas source rocks and biodegraded oil accumulation. The Baigang and Nadu source rocks can be considered to have experienced early and late gas generation during early burial and after basin uplift respectively. A late accumulation mechanism of multiple gas sources is put forward for the formation of the shallow gas reservoirs, which is responsible for the variations in chemical and isotopic composition of the gases in depth profile.

川东北元坝地区须家河组钙质交代—胶结致密层分布与成因

方解石是须家河组砂岩中常见的成岩矿物之一,可在局部层段富集形成钙质交代—胶结致密层,对砂岩的物性有明显的破坏作用。基于岩心观察和薄片鉴定,对川东北元坝地区钙质交代—胶结致密砂岩的岩石学特征开展了分析,刻画了致密砂岩的分布规律;结合全岩X射线衍射、稳定碳氧同位素和微量元素等测试资料,分析了钙质交代—胶结致密砂岩的形成期和物质来源,阐明了钙质胶结物和交代物的形成机制。钙质交代—胶结致密层主要发育在元坝地区西部与须家河组三段(须三段)源岩层相邻的须家河组二段和须家河组四段的砂岩中,在东部须三段源岩层附近的砂岩中则较少;通常出现在厚层砂体的中部,厚度为1~2 m;长石含量或白云岩岩屑含量较高,杂基含量较低;经历的主要成岩作用包括压实作用、方解石的胶结作用和交代作用。方解石的胶结物和交代物主要形成于早成岩阶段B期,此时的地层温度为60~80℃,烃源岩处于有机酸生成的高峰期。由于元坝地区西部须三段烃源岩中含有较高的碳酸盐矿物组分,其排出的富有机酸流体可携带较多的Ca2+和HCO3-离子进入砂岩层,当砂岩富含长石和白云岩岩屑等碱性矿物时,富Ca2+的有机酸流体能被有效缓冲,形成有利于方解石胶结物和交代物沉淀的微环境,从而导致方解石大量沉淀。

冀中坳陷饶阳凹陷北部烃源岩生物标志物特征与油源对比

饶阳凹陷北部油气资源丰富、圈闭类型多样,但其原油成因类型和来源尚不明确.利用烃源岩和原油生物标志物参数特征,有效区分3套烃源岩并通过聚类分析法划分原油成因类型,建立油岩关系并探讨各类型原油分布特征.研究表明:3套烃源岩在8组生标参数上表现出明显差异.饶阳北部地区发现的原油可划分为4种类型,包括沙三型、沙三和沙一下混合型、沙一下和沙三上混合型、沙一下型.3套烃源岩的生物标志物差异反映了它们具有不同的母质类型和沉积环境.沙三型分布在任丘潜山北部任北斜坡;沙三和沙一下混合型原油分布在马西洼槽中南部;沙一下和沙三上混合型分布在马西洼槽南边的八里庄油田;沙一下型分布在研究区西部蠡县斜坡上的高阳油田.这种分布特征主要是受到源岩分布和热演化程度的控制.

饶阳凹陷任丘潜山油藏油源对比及有效烃源岩成藏贡献

为进一步研究饶阳凹陷任丘潜山油藏的原油来源及烃源岩的成藏贡献,根据原油和烃源岩的饱和烃气相色谱-质谱分析数据以及生物标志物特征,对潜山原油进行了详细分析,确定了其与烃源岩的亲缘关系,并通过地质条件分析解释了混源成藏的原因,最终确定了混源油的混源比例。C29甾烷20S/(20S+20R)参数反映任丘潜山油藏原油基本为成熟原油,Pr/Ph、G/H、S/H、C19/C23等生物标志物参数表明任丘潜山油藏存在3种类型原油,分别为A类、B类和C类。3类原油生物标志物参数呈现规律性变化,并且空间分布亦存在一定规律性。A类为马西洼槽沙河街组三段中-下亚段(沙三段中-下亚段)烃源岩单一来源的原油,主要分布于任丘潜山北部偏东地区的奥陶系储层中。B类和C类原油为任西洼槽沙河街组一段下亚段(沙一段下亚段)和马西洼槽沙三段中-下亚段烃源岩混合来源的原油,其中B类原油中来自沙三段中-下亚段烃源岩的原油占比为60%~70%,其次为来自沙一段下亚段烃源岩的原油,主要分布于任丘潜山北部偏西地区的元古界雾迷山组和寒武系府君山组储层中。C类原油则以沙一段下亚段烃源岩贡献为主,来自沙三段中-下亚段烃源岩的原油占比较低,为30%~40%,集中分布于任丘潜山油藏南部地区的元古界雾迷山组储层中。整体上,沙一段下亚段烃源岩对任丘潜山油藏原油的贡献北低南高,而沙三段中-下亚段烃源岩则为北高南低。

Control of differential tectonic evolution on petroleum occurrence in Bohai Bay Basin

The Bohai Bay Basin(BBB)is the most petroliferous Cenozoic basin in the east of China.It consists of seven depressions.Each depression has been subjected to different stress states and then has experienced varying faulting processes since the Neogene,especially during the Neotectonism(from the Pliocene to the present).On the basis of the investigation of fault patterns,fault densities and fault activity rates(FARs)for each depression,this paper demonstrates the discrepancy of faulting development and evolution across the BBB.The dynamic mechanism for the differences in faulting is also discussed by the analysis of the regional stress state.The Bozhong Depression is just situated in the transtensional zone induced by the two active strike-slip faults,namely Yingkou-Weifang and Beijing-Penglai.In this depression,the major faults which cut through the Paleogene or the Cenozoic have had higher than 10 m/Ma FARs since the Neogene,and the highest FARs have reached or exceeded 25 m/Ma during the Neotectonism.As a result,most of the petroleum has migrated along these major faults and accumulated within the Neogene.In contrast,in the other depressions of the BBB away from the Bozhong Depression,the FARs of the major faults were decreased to lower than 10 m/Ma since the Neogene,and tended to be zero during the Neotectonism.Therefore,the major faults could not serve as vertical conduits for petroleum migration,and the petroleum was entrapped in the Paleogene.Consequently,the faulting since the Neogene,especially during the Neotectonism,controlled the petroleum richness in vertical strata.

Process and mechanism for oil and gas accumulation, adjustment and reconstruction in Puguang Gas Field, Northeast Sichuan Basin, China

With the discoveries of a series of large gas fields in the northeast of Sichuan Basin, such as Puguang and Longgang gas fields, the formation mechanism of the gas reservoir containing high H2S in the ancient marine carbonate formation in superposition-basin becomes a hot topic in the field of petroleum geology. Based on the structure inversion, numerical simulation, and geochemical research, we show at least two intervals of fluid transfer in Puguang paleo-oil reservoir, one in the forepart of late Indo-Chinese epoch to early Yanshan epoch and the other in the metaphase of early Yanshan epoch. Oil and gas accumulation occurred at Puguang structure through Puguang-Dongyuezhai faults and dolomite beds in reef and shoal facies in Changxing Formation (P2ch) - Feixianguan Formation (T1f) in the northwest and southwest directions along three main migration pathways, to form Puguang paleo-oil reservoir. Since crude oil is pyrolysised in the early stage of middle Yanshan epoch, Puguang gas reservoir has experienced fluid adjusting process controlled by tectonic movement and geochemical reconstruction process controlled by thermochemical sulfate reduction (TSR). Middle Yan-shan epoch is the main period during which the Puguang gas reservoir experienced the geochemical reaction of TSR. On one hand, TSR can recreate the fluid in gas reservoir, which makes the gas drying index larger and carbon isotope heavier. On the other hand, the reciprocity between fluid regarding TSR (hydrocarbon, H2S, and water) and reservoir rock induces erosion of the reservoir rocks and anhydrite alteration, which improves reservoir petrophysical properties. Superimposed by later tectonic movement, the fluid in Puguang reservoir has twice experienced adjustment, one in the late Yanshan epoch to the early Himalayan epoch and the other time in late Himalayan epoch, after which Puguang gas reservoir is finally developed.

Late-stage rapid accumulation of the PL19-3 giant oilfield in an active fault zone during Neotectonism in the Bozhong depression,Bohai Bay

Late-stage is defined here as the period when Neotectonism occurred since 5.1 Ma. Most petroliferous basins in China lie in the areas where Neotectonism occurred intensively. In recent years, Chinese petroleum geologists have paid much attention to late-stage petroleum accumulation. The PL19-3 giant oilfield is situated where faulting activities occurred violently during Neotectonism. To understand the mechanism of lat-stage rapid accumulation, we discussed the most important aspects responsible for the formation of the giant oilfield, including oil generation, active oil-source rock occurrence, fault activity and fault conduits, late-stage rapid oil injection as well as the distinguishing indicators. This study shows that: (1) sufficient oil was supplied to the PL19-3 field since 5.1 Ma because the PL19-3 structure was surrounded by four sags in which three intervals of high-quality source rocks remained active during Neotectonism; (2) densely distributed faults and high porosity/permeability sandstone carrier beds comprised the effective conduit system for oil migration and injection; (3) oil migrated along the faults and charged the PL19-3 structure rapidly by means of seismic pumping which was triggered by frequent earthquakes during Neotetonism. It is documented that elevated reservoir temperature, abnormal geothermal gradients and abnormally high homogenization temperatures of the fluid inclusions are the indicators for late-stage rapid oil accumulation.

Rapid hydrocarbon accumulation mechanism in later period in Kelasu thrust belt in Kuqa depression

Based on the analyses of generation, migration and accumulation of oil and gas in the structures of Kela 1, Kela 2 and Kela 3 in Kasangtuokai anticlinal belt using a series of geological and geochemical evidence, this paper proposes that the rapid rate of hydrocarbon generation, main drain path for over-pressured fluid flow and converging conduit system are indispensable conditions for the rapid, late-stage gas accumulation in the Kelasu thrust belt in the Kuqa depression. Due to structural over-lapping and the resultant rapid burial, the maturity of the source rocks had been increased rapidly from 1.3 to 2.5% Ro within 2.3 Ma, with an average rate of Ro increase up to 0.539% Ro/Ma. The rapid matura-tion of the source rocks had provided sufficient gases for late-stage gas accumulation. The kelasu structural belt has a variety of faults, but only the fault that related with fault propagation fold and cut through the gypsiferous mudstone cap could act as the main path for overpressured fluid release and then for fast gas accumulation in low fluid potential area. All the evidence from surface structure map, seismic profile explanation, authigenic kaolinite and reservoir property demonstrates that the main drain path related with faults for overpressured fluid and the converging conduit system are the key point for the formation of the giant Kela 2 gas field. By contrast, the Kela 1 and Kela 3 structures lo-cated on both sides of Kela 2 structure, are not favourable for gas accumulation due to lacking con-verging conduit system.

Thermal evolution and applications of aromatic hydrocarbons in highly mature coal-bearing source rocks of the Upper Triassic Xujiahe Formation in the northern Sichuan Basin

Based on the GC-MS analytical data of aromatic fractions of over forty highly mature coal-bearing source rock samples collected from the Upper Triassic Xujiahe Formation in the northern Sichuan Basin, the thermal evolution of aromatic hydrocarbons during late-mature to over-mature stage （R0=1.13%-2.85%） was characterized, and aromatic indicators suitable for recognizing the organic source and sedimentary environment of high maturity source rocks were discussed. The results indicated that the concentrations of low carbon-cycle naphthalene as well phenanthrene series reduce gradually with increasing Ro at the highly mature levels. However, some high-cyclic components such as chrysene, benzofluoranthene, and benzo[e]pyrene are relatively enriched, in companying an enhancement of parent aromatic compounds. The variations are attributed to thermal cracking and polymerization reactions due to continuous dehydrogenation under enhanced burial temperature. As thermal maturity rises, MPI1 （Methylphenanthrene Index） values display a two-modal varying trend, namely, increasing when Ro is below 1.80% and decreasing above 1.8% Ro. The relationships between Ro and MPI1 are Ro=0.98MPI1＋0.37 for R0〈1.80% and R0=-0.90MPI1＋3.02 at R0〉1.8%, being different from the previous research. The amount of dibenzofurans declines sharply at Ro higher than 1.1%, leading to a significant change of relative composition among dibenzothiophenes, dibenzofurans and fluorenes （referred as three-fluorenes series composition）. Thus, this parameter appears to be unsuitable for identifying the sedimentary environment of the highly matured source rocks. 4-/1-MDBT （methyldibenzothiophene） ratio could be served as an effective indicator for organic facies, and can distinguish coals from mudstones at over-maturity in this case. The ratios of 2,6-/2,10-DMP （dimethylphenanthrene） and 1,7-/1,9-DMP and relative abundance of triaromatic steroids in these highly mature rocks could be considered as biological source parameters for relative input of terrigenous versus aquatic organic matter.