川西中二叠统栖霞组白云岩特征与成因

近期在川西地区中二叠统栖霞组斩获的多个白云岩油气发现彰显了白云岩储层的油气潜力,但该组白云岩储层非均质性极强,白云岩化机制不明确,优质白云岩储层分布预测困难。研究综合分析了该组不同类型白云岩的岩石学和地球化学特征,结合沉积相背景和峨眉山大火成岩省热事件,系统总结了不同类型白云岩化作用的期次和成因。结果显示：川西地区栖霞组白云岩多为斑状或层状分布,白云石晶体以细晶为主,中-粗晶、泥-粉晶次之,伴少量鞍状白云石胶结物。泥-粉晶白云岩主要发育在栖霞组底部,白云石晶体多为半自形-他形晶,常与草莓状黄铁矿伴生,部分泥晶白云石呈球粒状、哑铃状和花椰菜状;泥-粉晶白云岩的δ¹³C(VPDB)为负值,δ¹⁸O(VPDB)处于正常海水数值范围内,这些特征表明泥-粉晶白云岩可能为微生物介导作用的产物。细晶白云岩在栖一段和栖二段都有发育,其白云石晶体主要为自形-半自形晶,多具有雾心亮边结构,在阴极发光下晶体内部呈棕色-暗红色,向外过渡为亮红色后再变暗;细晶白云岩具有明显的残余粒屑结构,表明前驱岩岩性为颗粒灰岩;细晶白云岩的δ¹⁸O(VPDB)...     

THE ORIGIN OF DOLOMITE IN THE ASMARI FORMATION (OLIGOCENE-LOWER MIOCENE), DEZFUL EMBAYMENT, SW IRAN

Dolomitisation is an important factor controlling reservoir quality in the Asmari Formation in many producing fields in SW Iran. Dolostones have higher average porosities than limestones. Petrographic and geochemical studies have been used to determine the causes of Asmari dolomitisation at the Bibi Hakimeh and Marun fields and at the Khaviz anticline. The formation is generally characterized by a large‐scale trend of upward‐decreasing accommodation. Basal strata were deposited under relatively open‐marine, high‐energy conditions, whereas the Middle to Upper Asmari succession was deposited in relatively protected settings with more frequent evidence of exposure and evaporitic conditions. There is a general upward increase in the abundance of both anhydrite (occurring as nodules and cement) and dolomite. Two main types of dolomite fabric are recognised, reflecting the textures of the precursor limestones: (1) finely crystalline pervasive dolomite (commonly <20μ) replacing mud‐rich facies; and (2) combinations of finely crystalline replacive dolomite and surrounding areas of coarser dolomite cement (crystals up to 100μ) in grain‐supported facies. Fluid inclusion data indicate that finely crystalline dolomites formed at low temperatures (ca. <50°C), while the coarser dolomite formed at higher temperatures (50–;140°C). Whole rock‐carbonate oxygen and carbon isotope analyses of pure dolostone samples show no apparent correlation with either depositional or diagenetic textures: δ¹⁸O is generally 0 to 2.7‰ PDB, and δ¹³C is ?1 to 4‰ PDB. The importance of evaporated seawater to Asmari dolomitisation is indicated by the ubiquitous occurrence of felty‐textured anhydrite nodules in dolostone beds and the presence of high‐salinity fluid inclusions in dolomite. The derivation of dolomitising fluids from contemporaneous seawater is supported by the general correspondence between age estimates derived from the strontium isotope composition of anhydrites and dolomites and those derived from stratigraphic considerations. This suggested synsedimentary dolomitisation. Dolomitisation of the upper half of the Asmari Formation may have occurred as a result of two syn‐sedimentary mechanisms: (1) by the reflux of evaporative brines concentrated in shallow lagoons or sabkhas, through immediately underlying strata (mainly during highstands); and (2) by the flushing of platform‐top carbonates by basinal evaporated waters during lowstand/early transgression. Continued dolomitisation during deeper burial is supported by the presence of high‐temperature fluid inclusions and iron‐rich crystal rims. Dolomite within the lower part of the Asmari Formation probably mostly formed during burial as a result of compaction of, and fluid exclusion from, the underlying Pabdeh marls and shales.     

DOLOMITIZATION AND ANHYDRITE PRECIPITATION IN PERMO-TRIASSIC CARBONATES AT THE SOUTH PARS GASFIELD, OFFSHORE IRAN: CONTROLS ON RESERVOIR QUALITY

Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo-Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in a shallow-marine ramp or epeiric platform and, according to previous studies, underwent intense near-surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage-reflux models. The early dolomites then re-equilibrated or re-crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are significantly decreased. As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability. However, porosity in fabric-retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment. Low-temperature dolomitization is commonly fabric-selective and partially fabric-retentive. Whole rock stable isotope thermometry indicates that fabric-destructive dolomites in the reservoir rocks formed at temperatures above 22°C, whereas fabric-retentive dolomites and associated anhydrites formed in surface and near-surface conditions. Fabric-destructive dolomite or dolomite neomorphism post-date fabric-retentive dolomite and continued to form in deep burial conditions (∼1400m). These observations may explain why fabric-retentive dolomite and anhydrite fabrics are traversed by stylolites.     

川西南地区中二叠统栖霞组白云岩储层特征及成因

通过岩心观察、薄片鉴定、阴极发光和地球化学分析等方法,对川西南地区中二叠统栖霞组白云岩特征及成因进行了研究。研究结果表明：①研究区栖霞组基质白云岩主要是细—中晶白云岩,其次为粉—细晶白云岩,并伴生少量粗晶鞍形白云石。②研究区细—中晶白云岩与同期颗粒灰岩方解石胶结物的稀土配型模式一致,具有现代海水典型稀土配分特征,为半局限水体动荡环境下沉积的颗粒灰岩交代而成;粉—细晶白云岩具有∑REE含量较高、Ce微弱正异常、LREE整体抬升的平坦稀土配型形态特征,为半局限水体安静环境下沉积的泥晶灰岩或粒泥灰岩交代而成。不同类型白云石Fe,Mn元素含量随云化程度加深表现出协同性升高,表明其具有相同成因。③研究区沿坡折带发育的生屑滩滩间存在局部蒸发受限环境,形成浓缩咸化海水,发生准同生期回流渗透白云石化作用,主要形成粉—细晶白云岩;早成岩—浅埋藏期峨眉山大火成岩省岩浆活动提供异常高热,高温孔隙水与先期白云岩进一步进行水岩作用,形成细—中晶白云岩,具“雾心亮边”特征;埋藏期深部热液沿裂缝运移,在裂缝和围岩中沉淀出鞍形白云石。生屑滩导致的半局限环境与热源叠加是研究区白云岩形成的关键。     

塔里木盆地西北部下寒武统白云岩特征及多重白云石化模式

塔里木盆地下寒武统白云岩储层研究尚处于起步阶段,开展白云石化模式系统研究对于明确白云岩储层特征具有重要意义。基于露头观察、岩石薄片鉴定、主量元素、微量元素、稀土元素和碳、氧、锶、镁同位素等资料,对塔里木盆地西北部下寒武统白云岩特征和成因模式进行了系统分析。塔里木盆地西北部阿克苏露头区下寒武统发育微生物白云岩、泥晶白云岩、粉晶—细晶白云岩和中—粗晶白云岩4类白云岩。微生物白云岩属于典型微生物白云石化成因模式,白云石化流体以海水为主。泥晶白云岩、部分粉晶—细晶白云岩和少量中—粗晶白云岩属于典型蒸发泵与回流渗透白云石化成因模式,白云石化流体以回流卤水和海水为主。绝大多数中—粗晶白云岩和部分粉晶—细晶白云岩属于埋藏热液白云石化成因模式,白云石化流体为与烃类充注相关的地层卤水及陆源水。早成岩阶段微生物白云石化作用和蒸发泵与回流渗透白云石化作用是塔里木盆地西北部阿克苏露头区下寒武统微生物白云岩和粉晶—细晶白云岩孔隙发育的基础。     

ORIGIN AND RESERVOIR ROCK CHARACTERISTICS OF DOLOSTONES IN THE EARLY TRIASSIC FEIXIANGUAN FORMATION, NE SICHUAN BASIN, CHINA: SIGNIFICANCE FOR FUTURE GAS EXPLORATION

Major discoveries of natural gas have recently been made in the oolitic dolostones of the Early Triassic Feixianguan Formation in NE Sichuan Province, Southern China. These dolostones were formed by three facies‐controlled dolomitization processes: (i) meteoric mixing zone dolomitization with dolomites having a relatively high degree of crystalline order (δ¹³C:?1.0 to 2.5%_PDB; δ¹⁸O:?6.5 to ?2.5%_PDB); (ii) seepage‐reflux dolomitization associated with evaporative brines; the corresponding dolomite crystals are relatively ordered and were formed in tidal flat environments and platform‐margin oolitic shoals adjacent to lagoons; (iii) burial dolomitization (shallow to moderate burial depths, ca. 1,000 to 4,000m), whereby seawater‐derived brines were present in the host rock and the resultant water/rock reactions played a major role in dolomitization. The three dolomitization processes were controlled by the arid climate prevailing during the Early Triassic, and also by fourth‐order relative sea‐level changes, especially with respect to the reflux dolomitization. Burial dolomitization, which is of second‐order of importance for porosity development, was strongly dependant on the presence of sufficient original porosity to facilitate water‐rock reactions within the carbonates. The best reservoir rocks formed as oolitic banks and bars in the vicinity of evaporative lagoonal‐tidal complexes which experienced optimal conditions for dolomitization. Dolostones with a dolomite content of 80% to 90% form good vuggy reservoir rocks at the present day, indicating that the intensity of dolomitization influences the quality of reservoir rocks. According to our results, future gas exploration in the Feixianguan Formation dolostone reservoirs should focus on locating oolitic banks associated with evaporative lagoon and tidal flat complexes and delineating the best structural/lithological traps.     

玉北地区下奥陶统白云岩岩石学、地球化学特征及成因

通过岩心、薄片、扫描电镜、阴极发光、微量元素、同位素和包裹体测温等分析手段,对玉北地区下奥陶统白云岩岩石学特征、地球化学特征及成因进行研究。结果表明,该区白云岩主要由细晶、自形-半自形白云岩和细-粗晶、它形白云岩以及鞍形白云石充填物构成。白云岩的晶粒大小并不反映成因,反而是晶体结构特征（曲面-它形晶和平面-自形、半自形晶）与成因关系密切。细晶、自形-半自形白云岩具有高Sr、低Fe、Mn含量的特征以及与早奥陶世海水相似的O和C同位素组成,总体属于浅埋藏早期与海水有关的低温白云石化作用的产物。细-粗晶、它形白云岩的Sr含量较低而Fe和Mn含量相对较高,O同位素值明显偏负,包裹体测温结果显示部分样品具有较高的均一温度和盐度值,结合区域构造演化背景,认为该类白云岩以浅埋藏晚期到中埋藏阶段的高温白云石化为主,局部受热液改造,云化流体主要来自于地层中封存的海源卤水。鞍形白云石充填物有两期,一期与热液成因的基质白云岩形成时期相近,另一期则可能与二叠纪的岩浆期后热液有关。     

Sedimentological and petrophysical characteristics of Raha Formation at Wadi Tubia,Northern Gulf of Aqaba,Sinai, Egypt

The main aim of this study is to shed some light on the microfacies association, geochemistry and petrophysical properties of the rock types: dolomite, sandstone, shale and limestone forming the Raha Formation.The microfacies association includes: ferroan dolomite, sublithic arenite, ferruginous sublithic arenite, dolomitic lithic arenite, sandy bioclastic wackstone, bioclastic packstone, calcareous ferruginous lithic arenite and fossiliferous bioclastic grainstone.The depositional environments of these sediments are interpreted as restricted, shelf lagoon and open circulation.The dominant diagenetic processes presented in the studied samples are cementation, compaction, replacement, dissolution and dolomitization in sandstone rocks, while neomorphism, micritization, cementation, dolomitization, compaction and dissolution are present in carbonate facies. Geochemically, the ferruginous sublithic arenite microfacies are characterized by phosphate bands and high chemical weathering. Calcareous ferruginous lithic arenite microfacies are characterized by high concentration of iron oxides and fossiliferous bioclastic grainstone is characterized by high concentration of barium, as indicated from the chemical index of alteration (CIA).Statistical analysis of the measured petrophysical data shows that, the dolomitic lithic arenite and fossiliferous bioclastic grainstone microfacies have a good storage capacity in the Raha Formation of Wadi Tubia, Northern Gulf of Aqaba, Sinai, Egypt.     

A NEW MODEL FOR THE FORMATION OF DOLOMITE IN THE TRIASSIC DOLOMITES,NORTHERN ITALY

The Pale di San Martino and Pale di San Lucano (referred to together as the “Pale”) are remnants of an originally more extensive carbonate platform in the Dolomite Mountains of northern Italy. The platforms are composed of Middle Triassic dolomites and limestones up to 1.6km thick. Limestones comprise 2–3% of the platform carbonates and are restricted to narrow corridors (tens to a few hundred metres wide, hundreds of metres long and high) within the dolomite. The mainly sucrosic dolomites of the Pale are interpreted as the result of recrystallization of a depositional, nearly stoichiometric Mg calcite under burial temperatures of ca. 40–70°C. The principal arguments are:

• The quantitative composition indicates that all platform carbonates are composed mainly of micritic crusts (45%; boundstone fabric prevails) and early cement (35%; microcrystalline, fibrous). The platform carbonates were probably mainly bacterial precipitates and tight at the sediment‐water interface (porosities <5%, permeabilities in the micro‐Darcy range).
• The limestone‐dolomite transitions (centimetres to decimetres wide) lack dolomite gradients. The lack of evidence for flowing fluids causing dolomitization suggests stagnant pore waters.
• The δ¹³C of average dolomite is 1.3‰ heavier than that of coeval limestone (666 analyses). The difference corresponds to a primary difference of 50mol% MgCO₃ and is interpreted as the result of fractionation. It suggests a dolomite precursor of very high Mg calcite, whereas present‐day limestone of the Pale was probably deposited as a basically Mg‐free polymorph (aragonite and/or calcite).
• The dolomite δ¹⁸O (+1 to ?11‰ VPDB) values show a scatter over the platform thickness and preserve randomly distributed values around 0‰. The scatter is probably due to selective re‐setting of δ¹⁸O near pore spaces and is mainly a sampling effect.
• The observation that ⁸⁷Sr/⁸⁶Sr ratios (77 analyses) of limestone and dolomite are either slightly higher or lower than Middle Triassic seawater, but almost never “normal marine”, suggests that the platform carbonates of the Pale were deposited from seawater contaminated with artesian freshwater. The limestone corridors are probably caused by artesian springs of somewhat higher than ambient depositional temperature, with low Mg calcite and/or aragonite deposited in or near fracture zones. The volumetrically subordinate cycle‐cap dolomite is possibly a primary precipitate.

     

塔里木盆地东部上寒武统白云岩沉积特征

通过岩心观察和薄片鉴定,确定塔东寒武系白云岩以次生白云岩为主,分为交代白云岩和胶结白云岩两类,发育在上寒武统的突尔沙克塔格组,分布在塔东东部和西部的构造带上。地球化学分析表明,塔东寒武系白云岩经历了超盐度、埋藏和热液 3 种白云岩化环境,揭示了研究区的渗透回流白云岩化、埋藏白云岩化和热液白云岩化 3 种模式;塔东白云岩优质储集层主要受到碳酸盐岩沉积相带展布和白云岩成因叠加模式的控制。地震相分析和沉积相研究表明：塔东西部为台地边缘-台地边缘斜坡,中部为深海盆地,东部为中缓坡-外缓坡;西部的台地边缘和东部的中缓坡白云岩的分布厚度大;储集性能好的白云岩主要为渗透回流白云岩化叠加埋藏白云岩化或者热液白云岩化作用而形成。     

四川盆地东部中二叠统茅口组热液白云岩发育模式及对储层的改造

针对四川盆地东部中二叠统茅口组异常白云岩分布，利用岩心岩石学特征描述、地化测试、地震和测井资料综合解释等方法，分析了热液白云岩化作用的识别标志和发育时间，探讨了其成因模式及对储层改造。研究结果表明：不规则白云岩分布、中-粗晶基质白云岩、早期张性裂缝和溶洞中以充填鞍形白云石和石英-鞍形白云石是主要识别标志，白云石流体包裹体均一温度（115~145℃）高于地层温度，偏负的氧同位素值[δ¹⁸O（VPDB）为-5.4‰~-7.1‰]，高锶同位素比值（⁸⁷Sr/⁸⁶Sr为0.707 947~0.708 458），是识别热液的主要地化指标。热液白云岩化作用主要发生在张性或扭张性断裂系统附近，其成层性受原始沉积相控制，在断裂破碎带附近灰岩地层也被热液改造。热液改造主要时期是在茅口组经历浅埋藏、抬升暴露岩溶作用和吴家坪组沉积之后的晚二叠世-中三叠世。峨眉山玄武岩喷发为热液提供了热源，断裂、沉积相和热液侵入时间等因素在时间和空间上组合，造就了热液白云岩形成的条件和不同的分布形式。热液白云岩化作用对储层的改造具有复杂性，低渗透性灰岩层热液云化后新增孔隙空间不明显，具有初始空隙的高能滩体受热液云化适度改造后可能存在一定孔渗性。     

不同埋藏深度交代白云石晶体结构及其对白云岩储层研究的意义

交代白云石化作用是除了原生沉淀外白云石的主要成因,对于不同埋藏深度的交代白云石晶体结构的研究有助于分析白云石被交代的过程及主要影响因素。选取加勒比海Cayman岛中新统Cayman组、南海中新统宣德组和四川盆地上震旦统灯影组3类保留了原岩颗粒结构的交代白云石进行研究,3类白云石均未受到陆源物质影响。研究手段为:X衍射分析成分、有序度、晶胞参数;透射电镜观察晶体微形貌特征。随着埋藏深度从几米至几百米再至几千米依次加深,晶体自形程度逐渐变差,反映了埋藏成岩作用对交代白云石自由生长具有抑制作用;白云石有序度逐渐升高,晶胞参数逐渐向理想晶胞参数值接近,反映了后期稳定的埋藏成岩作用有利于白云石向有序转化。白云石的晶形与有序度之间不具有必然联系。通过X衍射发现,Cayman组白云岩样品中含有高镁方解石(HMC)和高钙白云石(HCD)、低钙白云石(LCD),其岩石学特征和晶体结构特征反映了交代白云石的形成是从最初的高镁方解石转化为高钙的白云石,再转化为低钙的白云石,交代白云石化可以被认为是镁离子逐渐进入方解石晶格的过程。经历足够长时间的稳定演化后,逐渐向理想白云石接近。3类交代白云石的镁离子来源为海水。表生岩溶作用不仅对于交代白云石化作用具有一定的催化作用,且经历白云石化后的岩石有利于保证原有的孔隙不易被破坏,形成了良好的储集空间,有利于后期油气的保存和运移。     

PETROGRAPHY AND DIAGENETIC CHARACTERISTICS OF THE UPPER OLIGOCENE – LOWER MIOCENE GHAR FORMATION IN SE IRAQ

This paper investigates the petrography, sedimentology and diagenetic evolution of the upper Oligocene – lower Miocene Ghar Formation in the subsurface of SE Iraq based on data from well logs, cuttings and core samples from boreholes in the Zubair, Majnoon and nearby oilfields. Lateral facies variations permit the Ghar Formation to be divided into two principal facies, a pebbly sandstone (facies A) and carbonate (dolomite) and calcareous sandstone (facies B), each of which can be divided into subfacies. Facies A rocks, interpreted as alluvial fan deposits, pass into facies B, paralic and shallow-marine sediments in a number of fining-upward successions. This facies change from mainly clastics (facies A) to mainly carbonates (facies B) suggests that the Ghar Formation may have some potential as a stratigraphic trap in SE Iraq.
Diagenetic processes include compaction, authigenesis, dolomitization and dedolomitization. In facies A, authigenic minerals were quartz, palygorskite, haematite, Fe-rich dolomite and smectite. In facies B, the main authigenic minerals were kaolinite, illite and illite-smectite. This difference in diagenetic mineralization may indicate variable interstitial waters ranging from fresh to marine. Dolomite was the main carbonate mineral encountered and could be divided into early matrix and pervasive types. The first type was represented by smooth crystals less than 20 microns in size which were associated with illite and illite-smectite mixed layer clays. Pervasive, fabric-destructive dolomite was represented by crystals 30-300 microns in size which were found as void fillings. Dedolomitization was observed where high-Mg calcite filled cavities near the contact with the overlying Fat'ha (Lower Fars) Formation. These diagenetic processes may enhance the reservoir characteristics of the Ghar Formation.     

四川盆地南江栖霞组白云岩岩石学、地球化学特征及成因

四川盆地南江桥亭剖面中二叠统栖霞组二段灰岩局部白云岩化,基质白云岩主要由非平直晶面他形晶白云石构成,孔洞缝中充填鞍形白云石和结晶方解石,溶解作用主要发生在白云石中。在岩石学研究基础上,综合运用碳氧同位素、元素、包裹体均一化温度、阴极发光测试手段,结合埋藏史、热史以及峨眉山玄武岩喷发非正常地热增温事件,研究了基质白云石和鞍形白云石的特征和成因。研究表明:基质白云石、鞍形白云石和结晶方解石的包裹体均一化温度分别为100~110,130~230,130~230℃;富含基质白云石、鞍形白云石和结晶方解石样品的δ18O_PDB值分别为-4.27‰~-6.28‰,-4.87‰~-5.80‰,-5.16‰~-6.50‰,δ13C_PDB值分别为3.97‰~5.22‰,4.88‰~5.35‰,2.73‰~4.29‰;与基质白云岩相比较,鞍形白云石和结晶方解石的δ18O值略负,并显示三者碳源为同时期海水。反演的基质白云岩形成流体的δ18O_SMOW值分布在3‰~6‰之间,鞍形白云石δ18O_SMOW主要分布在7‰~14‰之间,盐度均显著高于同期海水,因而两者都是在高温和高盐度的流体中形成的。三者都受到了峨眉山玄武岩喷发活动所引起的热事件影响,高温和高盐度克服了白云石沉淀的动力学屏障,形成交代灰岩的基质白云岩和孔洞缝中充填的鞍形白云石,流体的持续高温和Mg2+的消耗,导致了高温方解石的沉淀,显示热液对碳酸盐岩的改造作用。      

塔里木柯坪地区下丘里塔格群白云岩成因

根据白云岩的岩石学特征、化学元素分析和X衍射分析以及包裹体测温等多项资料,对塔里木盆地柯坪地区下丘里塔格群白云岩的成因进行了讨论。认为不同类型的白云岩形成于不同的沉积环境,其白云石化模式也不相同。潮坪—泻湖相的藻屑白云岩和泥晶白云岩,主要由泥晶白云石组成,白云石晶粒细小,晶形差,微量元索含量较高。元素Sr,Ba大于100ppm,B大于1ppm,白云石有序度较高(δ=0.84),为库龙白云石化的产物:浅滩相的颗粒白云岩、白云石晶粒变化较大,白云石有序度低(δ=0.56～0.76),为混合白云石化的产物;潮坪相结晶自云岩,由细—中晶白云石组成,有少量的粗晶白云石,白云石呈半自形、自形或他形,白云石有序度高(δ=0.88～0.95),为深埋藏白云石化的产物。     

塔里木盆地奥陶系孔隙型白云岩储层成因

塔里木盆地下奥陶统存在大量基质孔隙发育的深埋白云岩,是一类优质储层。岩心、薄片观察和阴极发光分析表明该类白云岩的发育具有明显的相控特征,以滩相颗粒灰岩为原岩。岩石学特征和微量元素、有序度及包裹体均一温度特征研究表明,该类白云岩形成于埋藏环境中,属埋藏白云石化作用的产物。由微量元素、稀土元素及锶同位素特征判断白云石化流体为受改造的海水,很可能与深部封存的蒸发台地相海水有关。推断塔里木盆地二叠纪大规模火山活动是台地相区白云石化作用的主要驱动机制,寒武系蒸发台地中富含镁离子的海水在广泛的热作用驱动下交代其上覆灰岩而形成白云岩。      

The effects of diagenesis on the petrophysical and geochemical attributes of the Asmari Formation,Marun oil field,southwest Iran

The distribution of good reservoir quality and its causes is the main challenges in carbonate reservoir characterization.This study investigates the effects of diagenetic processes on the reservoir quality of the carbonate successions of the Asmari Formation,in the Marun oil field,southwest Iran.The study applies an integrated approach including core petrography,petrophysical rock typing,stable carbon and oxygen isotopes as well as major and trace elements analyses.Petrographic studies and geochemical analyses express that the Asmari limestones have been affected mainly by compaction,dissolution,recrystallization,calcite and anhydrite cementation and dolomitization.Among those diagenetic overprints,dolomitization and dissolution played an important role to enhance the reservoir quality of the formation.Moreover,four types of dolomites were recognized and the rate of dolomitization increases toward the top of the Asmari carbonate successions.Possible models for dolomitization include mixing zone,brine reflux,seepage reflux and tidal pumping of seawater.Employing Flow Zone Index and Discrete Rock Type concepts led to classification of the Asmari reservoir into seven reservoir rock types.Integrating reservoir rock typing with petrographic studies and geochemical analyses also confirms that reservoir quality of the Asmari Fm.would have been mainly controlled by diagenetic processes.Moreover,stable isotopes,trace elements and facies analyses support the idea that carbonate intervals of the Asmari Formation were deposited in a warm,shallow-water environment under a saline condition.     

DOLOMITIZATION OF THE EARLY EOCENE JIRANI DOLOMITE FORMATION, GABES-TRIPOLI BASIN, WESTERN OFFSHORE, LIBYA

Dolomitization in the early Eocene Jirani Formation in the Gabes‐Tripoli Basin (offshore western Libya) occurred in two stages. Stage I dolomites are composed of two types, one associated with anhydrite (Type I) the other anhydrite free (Type II,). The stratigraphic and sedimentological settings together with petrographic and geochemical criteria suggest that dolomitization was effected by refluxed evaporative seawater. Stable isotope and trace element analyses suggest dolomitization of both Types from a fluid of near‐surface seawater composition under oxidising conditions modified by evaporation. Non‐luminescence and lack ofzonation of all the dolomite indicate that the dolomitizing fluids maintained a relatively constant composition. The geologic setting during the early Eocene, interpreted as hypersaline lagoon, supports an evaporative reflux origin for the anhydritic dolomite Type I. Type II developed under less saline conditions in the transition zone between lagoon and open marine shelf. Stage II dolomitization is recorded by negative isotope values in both Types I and II indicating their dissolution and recrystallization (neomorphism) by dilute solutions. A period of exposure of the overlying Jdeir Formation following a relative sea‐level fall allowed ingress of meteoric waters into both the Jdeir and the underlying Jirani Formations. Flushing by meteoric waters also resulted in development of excellent secondaly porosity and caused major dissolution of anhydrite to form the anhydritic‐free dolomite facies typical of Type II. Following, and possibly during, both Stages I and II, low temperature dolomites (Type IIIa) precipitated in pore spaces from residual jluids at shallow burial depths, partially occluding porosity. In the late stage of basin evolution, medium clystalline, pore‐filling saddle dolomite precipitated, causing some filling of mouldic and vuggy porosity (Type IIIb). Very light oxygen isotopic signatures confirm that it developed from high temperature fluids during deep burial diagenesis. Calculation of temperatures and timings of the dolomitization and cement phases show that the main dolomitization phases and Type IIIa cements occurred in the early Eocene, and that the saddle dolomite precipitated in the Miocene; these results are consistent with age relationships established from stratigraphic, petrographic and geochemical signatures. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity is both pre‐dolomitization and syn‐dolomitization in origin, but the latter is the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics.     

中国南方地区灯影组白云岩储层流体溶蚀改造机制

对南方地区震旦系灯影组未遭受溶蚀改造作用的泥粉晶白云岩、遭受溶蚀改造的富含溶蚀孔隙白云岩储层(包括粗晶砂糖状白云岩)以及孔洞中充填的粗晶白云石开展了碳、氧和锶同位素以及稀土元素分析,以判别白云岩储层发育经历的流体改造作用类型、过程和机制。未溶蚀白云岩的δ¹³C、δ¹⁸O和⁸⁷Sr/⁸⁶Sr比值平均分别为3.0‰、-3.5‰ 和0.708 779;稀土元素组成上无显著的Ce和Eu异常,属于正常的海水成因白云岩。溶蚀改造后的砂糖状白云岩的δ¹³C、δ¹⁸O和⁸⁷Sr/⁸⁶Sr比值平均分别为1.7‰、-7.7‰ 和0.709 579,与未溶蚀改造白云岩相比具有略偏轻的碳、氧同位素组成和较高的⁸⁷Sr/⁸⁶Sr比值;稀土元素组成上,具有较高的REE含量和显著的Ce负异常,δCe平均值为0.5,表明多孔的砂糖状白云岩储层是大气降水和含油气流体溶蚀改造的产物。孔洞中充填的粗晶白云石的δ¹³C、δ¹⁸O和⁸⁷Sr/⁸⁶Sr比值平均分别为0.3‰、-11.3‰ 和0.710 334,具有显著偏轻的碳、氧同位素组成和显著高的⁸⁷Sr/⁸⁶Sr比值;稀土元素组成上,具有显著的Eu正异常,δEu平均值为3.0,最大达到9.8,表明孔隙充填白云石是从热液流体中沉淀出来的,并受地层中有机成因流体的影响。根据所确定流体作用类型并结合构造地质演化背景,探讨了灯影组白云岩储层流体改造过程和机制。南方地区在震旦纪灯影组沉积时期发育海水台地相泥粉晶白云岩沉积,局部见藻纹层结构。震旦纪末期遭受大气降水溶蚀改造,下渗至地下深处,促使白云岩溶蚀和重结晶作用;在此后的埋藏过程中,进一步遭受含油气流体的溶蚀改造作用,最终形成富含溶蚀孔隙和沥青的中粗晶白云岩储层。埋藏过程中,受盆地深部热液的改造作用,在孔隙中形成白云石充填,使白云岩储层发生了一定程度的致密化作用。     

多参数综合识别塔里木盆地下古生界白云岩成因

塔里木盆地下古生界广泛发育白云岩,其成因主要可以用萨布哈白云石化、渗透回流白云石化、埋藏白云石化和热液白云石化4种模式来解释。萨布哈白云岩发育于潮间-潮上坪蒸发环境,以含膏泥晶白云岩为特征;渗透回流白云岩发育于蒸发台地(或潟湖)环境,以保留原岩结构的粉晶白云岩为特征;埋藏白云岩发育于埋藏环境的各个阶段,以细-中晶白云岩为特征;热液白云岩以伴生热液矿物的中-粗晶白云岩为特征。不同成因类型的白云岩形成于不同的成岩环境和阶段,具有不同的岩石学、地球化学和地球物理响应特征。通过大量岩心、薄片和主量元素、碳氧同位素、锶同位素、稀土元素等地球化学数据的细致研究,再结合测井、地震响应特征、古地理、古气候、构造背景等资料,系统总结了塔里木盆地下古生界4种成因白云岩的多参数识别特征,通过综合分析能较好地克服成因多解性问题。