Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs:A case study of Middle Permian Qixia Formation in Gaoshiti-Moxi area of Sichuan Basin,SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin,SW China,the controlling factors of reservoir development were analyzed,and the formation model of“intra-platform shoal thin-layer dolomite reservoir”was established.The Qixia Formation is a regressive cycle from bottom to top,in which the first member(Qi1 Member)develops low-energy open sea microfacies,and the second member(Qi2 Member)evolves into intra-platform shoal and inter-shoal sea with decreases in sea level.The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member.The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite,followed by vuggy dolomite.The semi-filled saddle dolomite is common in fracture-vug,and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network.Based on the coupling analysis of sedimentary and diagenesis characteristics,the reservoir formation model of“pre-depositional micro-paleogeomorphology controlling shoal,sedimentary shoal controlling dolomite,penecontemporaneous dolomite benefiting preservation of pores,and late hydrothermal action effectively improving reservoir quality”was systematically established.The“first-order high zone”micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member.Shoal facies is the basic condition of early dolomitization,and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent.The grain limestone of shoal facies is transformed by two stages of dolomitization.The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores.The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event,makes it recrystallized into medium–coarse crystal dolomite.With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures,the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed.The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Episodic hydrothermal alteration on Middle Permian carbonate reservoirs and its geological significance in southwestern Sichuan Basin,SW China

To analyze the episodic alteration of Middle Permian carbonate reservoirs by complex hydrothermal fluid in southwestern Sichuan Basin,petrology,geochemistry,fluid inclusion and U-Pb dating researches are conducted.The fractures and vugs of Middle Permian Qixia–Maokou formations are filled with multi-stage medium-coarse saddle dolomites and associated hydrothermal minerals,which indicates that the early limestone/dolomite episodic alteration was caused by the large-scale,high-temperature,deep magnesium-rich brine along flowing channels such as basement faults or associated fractures under the tectonic compression and napping during the Indosinian.The time of magnesium-rich hydrothermal activity was from the Middle Triassic to the Late Triassic.The siliceous and calcite fillings were triggered by hydrothermal alteration in the Middle and Late Yanshanian Movement and Himalayan Movement.Hydrothermal dolomitization is controlled by fault,hydrothermal property,flowing channel and surrounding rock lithology,which occur as equilibrium effect of porosity and permeability.The thick massive grainstone/dolomites were mainly altered by modification such as hydrothermal dolomitization/recrystallization,brecciation and fracture-vugs filling.Early thin-medium packstones were mainly altered by dissolution and infilling of fracturing,bedding dolomitization,dissolution and associated mineral fillings.The dissolved vugs and fractures are the main reservoir space under hydrothermal conditions,and the connection of dissolved vugs and network fractures is favorable for forming high-quality dolomite reservoir.Hydrothermal dolomite reservoirs are developed within a range of 1 km near faults,with a thickness of 30–60 m.Hydrothermal dolomite reservoirs with local connected pore/vugs and fractures have exploration potential.

Reconstruction and petroleum geological significance of lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in northwestern Sichuan Basin,SW China

Based on analysis of outcrop,drilling,logging and seismic data,and geotectonic background,the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed,and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed.The Maokou Formation is divided into 3 long-term cycles,namely LSCl,LSC2 and LSC3,which correspond to the Member 1,Member 2 and Member 3 of the Maokou Formation,respectively.Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting,the area had tectonic differentiation in NWW and NE,and sedimentary basement took on episodic settlement from north to south,as a result,the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf.According to the residual thickness,paleokarst geomorphologic units such as karst highland,karst slope and karst depression at different stages were reconstructed.The karst geomorphological units were developed successively on the basis of sedimentary geomorphology.Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation.The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations:high energy grain shoal and karst monadnock,platform margin slope and karst slope.Based on this understanding,the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology.The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.

Characteristics of “Guangyuan-Wangcang” trough during Late Middle Permian and its petroleum geological significance in northern Sichuan Basin, SW China

Based on outcrop characteristics, combined with regional tectonic background, drilling and geophysical data, the sedimentary characteristics of the Middle Permian Maokou Formation in the northern Sichuan Basin was studied by means of macroscopic and microscopic observation, geochemical element test, total organic carbon content and vitrinite reflectance measurement. There is a set of deep-water sediments rich in organic matter in the Guangyuan-Wangcang area of northern Sichuan during the late depositional period of the Middle Permian Maokou Formation. The strata are distributed from northwest to southeast, with thickness of 10–30 m, mainly composed of siliceous rocks and siliceous mudstones, intercalated with gravity flow deposits. Siliceous rocks and siliceous mudstones are characterized by thin single layer, flat bedding and rich siliceous radiolarians, calthrop and brachiopod with small body and thin shell, belonging to the typical sedimentary characteristics of deep-water trough facies. The contents of Cu, Co, Mo, Ni and the ratio of Ni to Co in the geochemical tests all indicate that the siliceous rocks are products of deep-water reducing environment. The TOC value ranges from 3.21% to 8.19%, with an average of 5.53%, indicating that the siliceous rocks have good hydrocarbon generation ability. The south side of the trough is in platform margin facies with high energy, and the sediments are mainly thick massive micritic-calcsparite biogenic(clastic) limestone, which is conducive to the formation and evolution of the reservoir. During the late sedimentary period of the Maokou Formation, the northward subduction and extension of the oceanic crust at the northwestern margin of the Yangtze Plate provided the internal dynamic conditions for the formation of the "Guangyuan-Wangcang" trough. According to the location, sedimentary characteristics and formation dynamics of the trough, it is similar to the "Kaijiang-Liangping" trough during Late Permian proposed by previous researchers. It is believed that the "Kaijiang-Liangping" trough already had its embryonic form during the Late Middle Permian.

Facies Interpretation, Depositional Environment and Sequence Stratigraphy of the Sartakht Formation in the Bakhshi Section, Located in Kalmard Block, East-Central Iran

Permian rocks in Kalmard block are recognized with Khan Group, enjoying various characteristics in different outcrops. This group is made up of three informal formations, namely Chili, Sartakht and Hermez. Middle Permian deposits (Sartakht formation) are composed chiefly of sandstone and carbonate rocks. This formation is composed of 58.6 m sandstone and dolomitic limestone in the Bakhshi section. Lower Permian carbonate deposits (Chili formation) unconformably underlie this formation while lateritic paleosols of upper Permian (Hermez formation) are depicted overlying an erosional unconformity above this formation. According to lithologic and microscopic investigations, the deposits of Sartakht formation can be divided into 2 siliciclastic petrofacies and 12 carbonate microfacies. Field observations, along with microscopic examinations, have resulted in identifying tidal flat, lagoon, shoal and open marine environments in the rocks of the studied formation. Vertical changes of microfacies and depth variation curve point to the high thickness of the microfacies of lagoon and shoal environments and low thickness of the microfacies of tidal flat and open marine environments. The carbonate-siliciclastic sequence of Sartakht formation is made up of a third-order depositional sequence, separated from carbonate depositions of lower Permian (Chili formation) and lateritic paleosols of upper Permian by type 1 sequence boundary (SB1). Siliciclastic deposits include LST system tract, and carbonate microfacies involve TST and HST system tracts, separated from each other by MFS. Sartakht formation rocks in Bakhshi section are deposited in a low-angle homoclinal ramp, mostly in the inner ramp, located in the south of Paleotethys Ocean. The depositional sequence identified in Sartakht formation points to the age of middle Permian, conforming to middle Absaroka II supersequence. The upper erosional boundary between Sartakht and Hermez formations conforms to the global-scale sea level fall.

四川盆地南部向斜区二叠系岩溶孔隙型石灰岩储层成因与天然气勘探新领域

随着油气勘探程度不断提高,勘探方向逐渐从构造高部位往向斜区拓展。但是,传统认识认为向斜区岩性一般较致密,寻找向斜区优质储层成为其油气获得勘探突破的关键因素之一。为此,以四川盆地南部向斜区中二叠统茅口组石灰岩为例,基于钻井岩心、薄片、测井以及地球化学等资料综合分析,开展了向斜区孔隙型石灰岩储层特征及成因研究,并建立了储层发育模式,指出了天然气勘探新方向。研究结果表明:(1)四川盆地南部向斜区茅口组发育岩溶孔隙型碳酸盐岩储层,储层岩性主要为泥晶—亮晶生屑石灰岩和亮晶生屑石灰岩,储集空间主要为生物体腔孔、生物铸模孔及粒内溶孔等组构选择性溶蚀形成的孔隙;(2)研究区茅口组储层平均孔隙度为4.07%,平均渗透率为0.273 mD,孔隙度与渗透率有明显的正相关关系,为典型的孔隙型储层;(3)茅口组储层段的铝、钪、锆及总稀土元素含量较低,锰和铁含量也较低,碳同位素值位于中二叠世海水沉淀方解石碳同位素值分布范围内,氧同位素呈现出一定的负偏;(4)茅口组岩溶孔隙型储层的形成与沉积期微地貌及高频层序控制的准同生期岩溶有关,溶蚀流体主要为大气淡水。结论认为,岩溶孔隙型储层在盆地内普遍发育,且该类储层远离不整合面,受沉积古地貌、沉积相带控制的准同生期岩溶改造,因此,天然气勘探思路需要向“有利沉积相带规模控储”方向转变,向斜区将是盆地内未来天然气勘探的重要新领域。

四川盆地东部地区二叠系茅口组孤立浅滩的发现及天然气勘探潜力

中二叠统茅口组长期以来一直是四川盆地天然气勘探的重点层系之一,茅口组天然气勘探开发先后以四川盆地南部地区岩溶缝洞型石灰岩储层和四川盆地中部地区滩相白云化储层为主要目的层系。为了寻找并落实茅口组下一个天然气勘探方向,在勘探程度相对较低的四川盆地东部(以下简称川东)地区开展了大量基础研究工作,在重新梳理地层格架的基础上,对茅口组沉积、储层及成藏条件进行了系统分析,指出了该区孤峰段硅质岩下茅二下亚段孤立浅滩的存在,最后明确了茅口组天然气的勘探潜力。研究结果表明:(1)川东地区在茅口组孤峰段事件性硅质岩沉积之前,宣汉—万州地区茅二下亚段发育碳酸盐岩孤立浅滩,面积可达6500 km^(2);(2)在宣汉—万州地区茅二下亚段发现了滩相基质孔储层,其中发育于茅二下亚段底部的孔隙型白云岩储层厚3~12 m,发育于茅二下亚段中上部的孔隙型石灰岩储层厚5~18 m;(3)川东地区茅口组滩相储层成藏条件优越,其中烃源岩主要为下伏上奥陶统五峰组—下志留统龙马溪组厚层页岩,且与烃源岩沟通的断裂发育,区域盖层主要为孤峰段硅质岩及吴家坪组海相泥岩、页岩,保存条件良好。结论认为,川东地区茅口组孤立浅滩的发现和良好的成藏组合条件,将进一步加快该区天然气勘探开发进程,茅口组大面积分布的高能浅滩有望成为下一个天然气勘探开发重要领域。

台内滩型薄层状白云岩储集层特征及主控因素——以四川盆地高石梯—磨溪地区中二叠统栖霞组为例

通过对四川盆地高石梯—磨溪地区中二叠统栖霞组台内滩分布及白云岩储集层特征的研究,分析储集层发育控制因素,建立“台内滩型薄层状白云岩储集层”成储模式。研究表明:栖霞组自下而上为海退旋回,栖一段发育低能开阔海亚相,栖二段海平面下降演化为台内滩及滩间海,台内滩主要分布于栖二段两个次级变浅旋回近顶部。栖霞组主要的储集层岩性为薄层状裂缝-孔洞型白云岩,其次为孔洞型白云岩,缝洞中常见鞍状白云石半充填,晶间孔、残余溶蚀孔洞叠加裂缝形成了有效的孔渗网络。基于沉积及成岩特征耦合分析,系统建立了“沉积前微古地貌控滩、沉积期滩体控制白云岩展布、准同生白云石化有利于孔渗网格保存,晚期热液作用有效改善储集物性”的成储模式。栖霞组沉积前的“一级高带”微古地貌控制栖二段沉积期高石梯地区台内滩的大面积发育;滩相是早期云化的基础条件,台内滩与白云岩储集层分布范围的吻合程度较高;滩相颗粒灰岩受两期白云石化叠加改造,准同生期白云石化有助于原生孔隙和次生溶孔保存,埋藏期热液沿峨眉山玄武岩事件伴生的裂缝进入早期白云岩体,使其重结晶为中—粗晶白云岩,晶间孔叠加顺缝热液溶蚀的残余孔洞,形成了台内滩型薄层状优质白云岩储集层。该成储模式的建立可为四川盆地二叠系气藏的可持续开发提供地质理论支撑。

川东红星地区中上二叠统页岩气勘探成果及方向展望

基于构造、岩心、测井等资料,采用电镜扫描、低温N_(2)吸附实验、高压压汞实验、盆地模拟等手段,系统分析了川东红星地区中上二叠统页岩的沉积演化、储层特征及页岩气富集模式,明确了勘探有利区,总结了勘探成果及意义。研究结果表明:(1)红星地区中上二叠统的沉积演化表现为:栖霞组—茅口组三段为开阔台地相;茅四段底部为台地-陆棚相,相带变化快,茅四段顶部为斜坡-陆棚相,地层被剥蚀、南厚北薄;吴家坪组一段(吴一段)底部为海陆过渡滨岸沼泽-潟湖相,吴一段顶部为台地-斜坡-陆棚相,斜坡-陆棚相带分布范围小、相变快;吴二段为斜坡-陆棚相沉积,从早至晚依次受控于古气候、火山活动和古气候、火山活动,沉积中—晚期古生产力较高,TOC均值大于8.00%。(2)研究区茅四段及吴二段陆棚相区发育2套优质页岩,具有“高有机碳-高灰质”的特征,富有机质页岩厚度分别为19 m和25 m,孔隙类型以有机孔为主,结构以微孔和介孔为主;富碳凝灰岩薄夹层混合质页岩岩相、高碳凝灰岩薄夹层硅质页岩为优质岩相,孔隙度分别为6.27%和6.43%,TOC值分别为10.11%和9.35%,含气饱和度分别为92.59%和91.81%,脆性指数分别为55.24%和61.19%,是地质和工程的双“甜点”段。(3)研究区二叠系广泛发育的层状藻为主要有机质来源;在侏罗纪主排烃期,其构造稳定、二叠系烃源岩排烃较少,中侏罗纪—早白垩纪早期为主生气期,构造活动较弱,页岩气的保存条件好,现今已完成生气过程,处于成熟—过成熟阶段,Ro值约2.1%,勘探潜力巨大。(4)建南、龙驹坝、三星区块为有利勘探区,其中建南区块潜力最大;茅四段(3)小层及吴二段(3)小层为优质层系的靶窗层段。

幕式热液作用对川西南部中二叠统碳酸盐岩储集层复合改造效应及地质意义

基于岩石学、地球化学、流体包裹体及同位素定年等研究,就热液对四川盆地西南部中二叠统碳酸盐岩储集层幕式改造进行分析。四川盆地西南部中二叠统栖霞组—茅口组白云岩缝洞中充填多期中—粗晶鞍状白云石及其他热液伴生矿物,主要属印支期构造挤压及推覆作用下规模性高温、深部富镁卤水沿基底断裂/伴生裂缝等渗滤通道对早期灰岩/白云岩幕式改造的结果。富镁热液作用时间主要为中—晚三叠世,其次为燕山运动中晚期及喜马拉雅期,热液改造导致了硅质及方解石充填。热液白云石化作用受断层、热液性质、流动路径及围岩岩性等因素复合控制,作用效果以物性“平均化”为主,热液对早期厚层块状颗粒灰岩/白云岩以热液白云石化/热液重结晶作用、破碎角砾化及缝洞充填等改造作用为主;对早期薄—中层泥粒灰岩以压裂、顺层白云石化、溶蚀及伴生矿物充填等产生的溶蚀-充填复合改造为主。热液作用下的破裂缝及溶蚀孔/洞为主要储集空间类型,溶蚀孔洞与网状裂缝的有效连通可形成优质白云岩储集层;典型热液白云岩储集层发育于断裂附近1 km范围内,厚30~60 m,部分孔/洞-缝连通体系发育的热液白云岩储集层具备勘探潜力。

龙门山中段北川通口剖面中二叠统栖霞组地层沉积特征

川西龙门山前带栖霞组是近期油气勘探的热点,龙门山中段栖霞组勘探程度低,明确其地层沉积特征,对于天然气勘探具有重要的指导意义。基于对龙门山中段北川通口剖面实测,结合薄片鉴定和主量、微量元素分析,对栖霞组层序地层及沉积特征进行了研究,明确了沉积环境。北川通口剖面梁山组—栖霞组识别出2个三级和1个四级层序界面,划分为1个三级层序和2个四级层序。梁山组—栖霞组发育滨岸—沼泽、开阔台地及台地边缘3种沉积相,开阔海、滩间海、台内滩和台缘滩4种亚相,台内生屑滩、台缘生屑滩和台缘砂屑、生屑滩3种微相。栖一段至栖二段下部整体处于开阔台地,栖二段中上部则处于台地边缘,建立了龙门山中段栖霞组镶边碳酸盐岩台地沉积模式。栖霞组沉积期整体处于温暖、湿润、富氧、古生物大量繁殖的海水—陆表海沉积环境,栖二段中上部高位体系域Sr/Ba比值整体最低,MgO/CaO比值最高,平均古水温高,V/(V+Ni)、V/Cr低值,古盐度m值明显较高,代表远离陆地的浅水、高能台地边缘沉积环境,厚大高能台缘砂屑、生屑滩发育,是规模优质滩相孔隙型白云岩储层发育有利部位。龙门山中段栖二段与北段、南段具有相似的沉积相带和储集条件,是栖霞组油气勘探的有利区带,有望成为实现规模增储的重要战略接替阵地。

川中龙女寺地区中二叠统茅口组储层特征及其主控因素

近年来,川中龙女寺地区JT1井和MX145井等多口钻井在中二叠统茅口组获得高产工业气流,展现出茅口组良好的勘探开发潜力,但由于储层特征及其发育主控因素不清楚,严重制约了龙女寺地区茅口组进一步的油气勘探。以川中龙女寺地区茅口组储层为对象,开展储层岩性、物性和孔喉结构等地质特征及地球化学特征研究,提出了川中龙女寺地区中二叠统茅口组优质储层主控因素。研究结果表明:(1)龙女寺地区茅口组储层岩性为白云岩及灰岩2类,晶粒白云岩、角砾白云岩和颗粒灰岩为主要储层岩石类型。白云岩储层可划分为3大类、6个亚类,灰岩储层可划分为4大类、5个亚类。(2)晶间孔、非选择性溶孔和溶洞为茅口组储层主要储集空间类型,裂缝-孔洞型储层的构造-溶蚀缝、洞体系储集空间大,分布广泛且连通性好,为有利储层。(3)茅口组储层具有“沉积作用-白云石化作用构建骨架,岩溶作用-构造作用叠加改造”的形成与发育模式。沉积作用、白云石化作用和构造作用是优质储层的主要控制因素,角砾白云岩为优质储集岩。

Guadalupian (Middle Permian) Radiolarian and Sponge Spicule Faunas from the Bancheng Formation of the Qinzhou Allochthon,South China

The Gujingling （古井岭） Section in Xiaodong （小董）, Guangxi （广西） Zhuang Autonomous Region, China, consists of cherts and siliceous siltstones, which belongs to the Bancheng （板城）Formation of the Qinzhou （钦州） allochthon. Three successive radiolarian assemblage-zones related to the Guadalupian （Middle Permian） appear in the section in the following ascending order： Pseudoalbaillella sp. aff. Pseudoaibailleila longicornis-Pseudoalbaillella fusiformis, Follicucuilus monacanthus, and Follicucuilus scholasticus-Foilicucullus porrectus. The radiolarian content in this section is generally greater than that of sponge spicules. The radiolarian fauna commonly consists of abundant Albaillellaria and spherical radiolaria with minor Latentifistularia and Entactinaria. The sponge fauna is composed mainly of hexactinellids with minor demosponges. These siliceous fossil faunal features, comparable with those in a deep basin of the western belt of the Phosphoria Basin in the western United States, indicate that the Bancheng Formation in the Gujingling Section was deposited in a basin deeper than 1 000 m. The siliceous siltstones in the section are characterized by inclusion of silt-sized quartz and no inclusion of sand-sized materials, suggesting that the Gujingling Section was located at least a few hundred kilometers from the South China Block in the Guadalupian.

川东南地区中二叠统茅口组三段白云岩成因机理

白云岩是四川盆地海相层系最为重要的天然气储层类型之一,也是深层碳酸盐岩油气勘探的重点,白云岩成因机理的研究是预测深层优质白云岩储层分布的基础。通过川东南地区茅口组三段白云岩露头、岩心、薄片等资料,结合碳氧同位素、锶同位素、稀土元素等研究了白云岩的成因机理。川东南地区茅三段白云岩可分为层状粉晶白云岩、层状细—中晶白云岩以及鞍形白云石胶结物。层状细—中晶白云岩是最主要的白云岩类型,未受热液影响的该类型白云岩δ^(18)O_(VPDB)值为-7.04‰~-6.08‰,与泥晶灰岩相似,^(87)Sr/^(86)Sr比值为0.707 550~0.707 740,处于同期海水范围值内,主要成因为准同生—埋藏期封闭局限海水渗透回流白云岩化;受到热液影响的该类型白云岩δ^(18)O_(VPDB)值为-7.33‰~-7.11‰,与泥晶灰岩相比略微偏负,^(87)Sr/^(86)Sr比值为0.707 950~0.708 171,高于同期海水。层状粉晶白云岩未明显受到热液影响改造,其成因与上述未受热液影响的层状细—中晶白云岩一致。而溶洞、裂缝中的鞍形白云石胶结物则是热液流体直接沉淀或重结晶所形成,具有较高的^(87)Sr/^(86)Sr比值(均值0.707 970)、明显偏负的δ^(18)O_(VPDB)值(均值-7.32‰)以及Eu的正异常。从层状粉晶白云岩、层状细—中晶白云岩到鞍形白云石胶结物,呈现出一种受控于封闭局限海水到热液流体的梯度性变化,研究区内的白云岩是由多期次流体、多次成岩作用的结果。

鄂尔多斯盆地及邻区中—晚二叠世构造-沉积环境与原型盆地演化

根据钻井、测井、野外露头及地球化学资料,参考盆地周缘构造环境、盆地构造沉降特征及沉积相展布,恢复了鄂尔多斯盆地中—晚二叠世不同时期构造-沉积环境,探讨了盆地性质及演化过程。研究结果表明:①中—晚二叠世鄂尔多斯盆地南缘和北缘经历了洋盆俯冲消减过程,以强烈的挤压构造环境为特征,上二叠统石千峰组沉积期盆地北缘古亚洲洋闭合进入陆陆碰撞阶段,盆地内构造沉降速度缓慢。②中—晚二叠世鄂尔多斯盆地为大型的克拉通内坳陷盆地,中二叠统石盒子组沉积期基准面呈下降趋势,盆地主体延续山西组沉积期南北高、中部低的古地理格局,南华北地区地势相对更低;周缘隆起区快速抬升且范围逐渐扩大,为坳陷盆地内提供物源,盆地内发育冲积扇、冲积平原、三角洲和浅湖相,向南发育残存潮坪—潟湖,呈南北向展布;晚二叠世基准面趋于稳定,湖相范围扩大,主要发育于盆地南部,潮坪—潟湖环境向南缩小。③研究区油气成藏受控于沉积环境,中二叠世发育的陆相三角洲沉积体系有利于形成储集层和盖层,且和下部煤系地层组成中二叠统良好的生-储-盖组合;上二叠统为氧化干旱的古气候,沉积水体较浅,形成的泥岩厚度小,生烃潜力有限,无法形成大规模气藏。

四川盆地南部中二叠统茅口组岩溶古地貌恢复及特征

岩溶古地貌是含油气盆地沉积微相类型、分布范围及储层发育的主控因素,恢复岩溶古地貌对指导油气勘探至关重要。基于钻井、测井及地震等资料,采用沉积速率法和地层厚度对比法对四川盆地南部地区中二叠统茅口组顶部不整合界面的剥蚀厚度进行了精确计算,并根据剥蚀厚度的地区差异及古地貌指示,对岩溶古地貌单元进行了进一步划分。研究结果表明:①四川盆地南部中二叠统茅口组顶部主要发育平行不整合,仅在局部地区形成角度不整合。早中二叠世,上扬子地台内部沉积稳定,构造运动较弱,沉积速率法与地层厚度对比法相对更适用于研究区的岩溶古地貌恢复。②研究区中二叠统茅口组顶部剥蚀厚度为0~120m,其中北部剥蚀最强烈,如LG1井区、W4井区代表了岩溶古地貌地势的最高位置,向东南方向剥蚀强度逐渐减弱,地势逐渐降低;剥蚀最薄弱地区位于LS1井东北部,代表了岩溶古地貌地势的最低位置,整体呈现北部高,南部低,东北部最低的地貌格局。③研究区茅口组古地貌特征控制了岩溶作用的发育强度,可划分为岩溶高地、岩溶斜坡、岩溶盆地等3个二级岩溶古地貌单元以及溶峰盆地、溶丘洼地、丘丛谷地、丘丛洼地、丘丛沟谷、峰林平原、残丘平原等7个三级古地貌单元,其中岩溶斜坡易形成良好的储集空间,为油气圈闭的形成奠定了良好的基础,是下一步有利勘探方向。

川西南地区中二叠统茅口组一段沉积微相特征及有机质富集模式

为了研究四川盆地中二叠统茅口组一段(茅一段)沉积微相和有机质富集规律,开展了岩石学、沉积微相分析。将川西南乐山沙湾剖面茅一段划分为生物碎屑含灰泥岩(MF1)、生物碎屑泥灰岩(MF2)、生物碎屑含泥灰岩(MF3)、灰泥灰岩(MF4)、生物碎屑粒泥灰岩(MF5)、钙藻粒泥灰岩(MF6)、生物碎屑泥粒灰岩(MF7)和钙藻泥粒灰岩(MF8)共8个沉积微相。结合沉积古环境及地球化学分析认为:①茅一段早、中期,主要发育MF2沉积微相,其次为MF7和MF8沉积微相。该期整体为海侵、缺氧环境,发育上升洋流,初级生产力较高。中、晚期主要发育MF6、MF7和MF8沉积微相,为相对海退环境,含氧量略有上升,上升洋流强度减弱,初级生产力下降。末期以MF2沉积微相为主,含氧量略有上升,初级生产力增强。②茅一段的有机质富集主要受海洋初级生产力与缺氧环境的调控,同时受上升洋流的间接影响。较高的初级生产力与较低的含氧量有利于有机质富集与保存。上升洋流可带来深层营养成分,从而增强海洋初级生产力。③MF2与MF7沉积微相在区域上稳定分布,且具有较高初级生产力和较低含氧量沉积环境,是茅一段有机质富集的有利沉积微相。

湘南地区东湘桥沉积型锰矿床地球化学特征及成矿机理

湘南地区广泛发育二叠纪海相沉积型锰矿,为揭示其成矿过程及机理,文章以湖南永州东湘桥锰矿床沉积剖面为研究对象,对其开展主、微量元素和碳-氧同位素分析。结果表明,富锰岩层富Cr、Ni、Co等元素的地球化学特征,其Co/Zn、Mn/Fe、Y/Ho比值和多判别图解共同指示该矿床为热液沉积,表明成矿物质Mn主要来源于海底热液。Mo和U元素富集因子(MoEF和UEF)、U/Th和V/(V+Ni)比值等沉积氧化还原指标分析结果表明,高品位锰矿石层产于氧化向缺氧转变的背景下,而低品位含锰岩层则主要产于氧化或强烈缺氧环境下。富锰岩层的δ^(13)C_(PDB)(-0.2‰~1.9‰)和δ^(18)O_(SOMW)(18.1‰~23.1‰)与海相碳酸盐岩相似,表明成矿物质的C和O主要来源于海水。基于上述结果,笔者认为东湘桥锰矿床成矿过程受海水CO_(3)^(2-)浓度和沉积氧化还原条件的双重控制,其成矿过程可以概况为2个阶段:①深水盆地的热液Mn在还原条件下以可溶Mn^(2+)向近岸CO_(2)-3过饱和的浅水台地迁移;②当沉积环境由氧化向缺氧转变时,锰氧化物在沉积/水界面还原释放Mn^(2+),显著提高水柱中溶解Mn浓度,促进富锰碳酸盐矿物析出,形成高品位锰矿石。

基于改进型随机森林算法的页岩岩性识别——以准噶尔盆地芦草沟组为例

在储集层岩性识别的应用中,特别是对页岩等非均质性较强的非常规储集层的岩性识别,机器学习算法的高效性、准确性和有效信息整合能力已经得到了充分验证。考虑到岩性识别的特征参数优选问题,优选自然伽马、T2几何平均值、结构指数、骨架密度指数、密度和深侧向电阻率,采用结合递归特征消除的随机森林算法,对准噶尔盆地中二叠统芦草沟组页岩储集层的主要岩性进行识别;利用传统的随机森林算法和支持向量机法,对同一套资料进行岩性预测,并与岩石薄片鉴定结果对比。结合递归特征消除的随机森林算法只需选择一半的测井参数,便能够达到更好的效果,而且通过优选特征参数,缩短了算法的运行时间。因此,结合递归特征消除的随机森林算法能够实现测井特征参数的优选,提高页岩岩性识别的准确率,缩短运行时间,为复杂岩性识别和多参数选择提供了新的思路。

下扬子地区早二叠世末期碳酸盐台地死亡事件的沉积学与碳同位素记录

华南板块在早二叠世时期位于赤道附近,发育了以栖霞组为代表的浅海碳酸盐岩,随后被孤峰组黑色页岩和硅质岩所取代,代表一次碳酸盐台地死亡事件。文章对下扬子区安徽巢湖平顶山剖面和江苏句容正盘山剖面上栖霞组与孤峰组进行了系统的沉积微相、碳同位素地层学等研究,试图探讨早二叠世末期栖霞组碳酸盐台地死亡的机制。分析表明,栖霞组顶部可划分为9种微相,属于碳酸盐内缓坡—中缓坡沉积环境;孤峰组底部可划分为5种岩相,属于深水盆地环境。栖霞组与孤峰组界处沉积环境和古水深发生突变,代表了下扬子地区早二叠世末期的碳酸盐台地死亡。栖霞组与孤峰组之间的碳酸钙含量突然降低、碳酸盐碳同位素快速负偏、有机质碳同位素明显正偏,指示碳酸盐台地死亡的突发性和事件性。观察显示,无机碳δ^(13)C_(carb)与有机质δ^(13)C_(org)存在镜像关系。文章认为,导致有机和无机碳解耦的原因可能与陆源有机质输入有关,而碳酸盐台地死亡可能为相对海平面上升所导致。