岩溶背景下的优质天然气储层形成机理及主控因素——以鄂尔多斯盆地奥陶系马家沟组马五41储层为例

根据野外地质剖面、岩心和铸体薄片观察,结合阴极发光、X-射线衍射、扫描电镜等分析,认为鄂尔多斯盆地奥陶系岩溶古地貌自西向东依次发育岩溶高地、岩溶斜坡和岩溶盆地,为优质储层的形成奠定了基础。研究区内经历了同生期层间岩溶、表生期风化岩溶和埋藏期埋藏岩溶3类古岩溶,形成了大量原生孔隙和次生孔隙,为后期成岩作用的改造提供了条件。马五₄¹储层主要发育泥粉晶白云岩、（含）硬石膏结核泥粉晶白云岩、含膏白云岩/膏质云岩、膏岩及岩溶角砾岩,主要发育溶蚀膏模孔、晶间孔、晶间溶孔与微裂缝4种储集空间,溶蚀膏模孔是最主要的储集空间。研究区优质储层发育主要受2个方面因素控制,其中:岩溶古地貌影响储层分布、物性差异、气水分布,是优质储层形成的基础;在岩溶改造后形成的孔隙系统基础上又经历了3期溶蚀作用,2种作用相互叠合是形成优质储层的关键。     

塔河地区断裂对奥陶系古岩溶的控制作用

塔河地区奥陶系发育加里东中期、加里东晚期一海西早期和海西晚期的逆断裂,断裂走向总体为NW向、NE向、EW向和SN向为主。断裂对奥陶系古岩溶地貌的形成和发育具有明显的控制作用,岩溶残丘、沟谷和古水系常沿NW向、NE向和SN向断裂发育。断裂带附近古岩溶储层尤为发育,缝洞型储层发育优势方向展布受控于断裂展布方向。     

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通过岩心观察、薄片鉴定、阴极发光显微镜观察、岩心CT图像分析、测井解释等手段，以揭示山东埕岛油田埕北30区块潜山油气藏下古生界碳酸盐岩储层储集空间类型、裂缝发育特征、古岩溶等为目的开展工作，得到以下认识：储层具有裂缝—孔隙复合储集空间类型，发育溶孔、溶洞、晶簇孔洞、晶间孔隙、构造缝、风化破裂缝、缝合线等。下古生界发育5个裂缝频数高值带，风化壳以下150～250 m范围内为最有利的储集层段，地层总孔隙度可达15％以上，裂缝孔隙度一般大于0.5％。碳酸盐岩古岩溶作用是控制储层发育的主要因素，它分为加里东早期同生—近地表成岩早期溶蚀作用、加里东晚期—印支期风化壳溶蚀作用、燕山期浅埋藏溶蚀作用和喜山期深埋藏溶蚀作用等四期，受其影响储层具有强烈角砾化和非均质性严重等特点。应用该成果为油气田新增石油地质储量228×104 t、溶解气地质储量4.31×108 m3。     

古岩溶地貌与古岩溶储层岩溶效应分析——以鄂尔多斯盆地东部奥陶系风化壳为例

以鄂尔多斯盆地东部奥陶系风化壳为对象,以大量地质录井及实验分析资料为依据,密切结合古岩溶储层岩性和物性分析,综合探讨了古岩溶地貌与古岩溶储层的岩溶效应特征。研究结果表明,盆地东部奥陶系风化壳储层岩石类型可归纳为白云岩类、次生灰岩类、石灰岩类、云化灰岩类等四大成因组合。古岩溶地貌不同,则与储层的岩性效应不同,石灰岩及云化灰岩类是斜坡台地及残丘和盆地残台及残丘发育的标志性岩类;白云岩及次生灰岩类则是斜坡台缘、斜坡阶坪及盆地洼地发育的基础岩类。古岩溶地貌不同,则与储层的物性效应不同,斜坡台缘效应最好,斜坡阶坪中等,斜坡台地及残丘中下,盆地残台及残丘较差,盆地洼地最差。斜坡台缘与斜坡阶坪分界为盆地东部古岩溶效应的重要转折点,居其两侧的小层渗透率随古岩溶地貌的变迁而逆向增减,发育红色和蓝色2种不同的效应模式。小层物性效应则与白云岩类和次生灰岩类的含量密切相关,其效应模式呈现出此消彼长同时又互为补充的特点。如果储层岩类组成过于单一、或者某类组分含量过低或过高,都将对古岩溶地貌发育产生重要的影响;适度比率的岩类组合或者混合岩化对古岩溶储层的物性效应更为有利。     

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基于电成像测井解释，结合岩心标定和常规测井约束，建立了塔北隆起下奥陶统溶洞及其充填物的电成像测井识别模式。在电成像测井图像上，未充填溶洞一般呈黑色，而充填溶洞则可呈现出黑、棕、黄、白等各种颜色，这取决于充填物的类型和特征。溶洞充填物主要有角砾岩、砂泥岩和结晶碳酸盐岩三种类型：角砾岩呈斑状，其中浅色斑块解释为从围岩跨塌下来的角砾，斑块间深色部分为砂、泥质填隙物；洞穴砂、泥岩一般呈黑色—棕色高导显示，其中常见图像纹理，解释为各种层理构造；洞穴结晶碳酸盐岩因电阻率最高，图像呈白色，并可见纹层状生长构造。洞穴及其充填物与碳酸盐围岩浅棕—亮黄色中高阻电成像测井特征易于区别。研究表明，利用高分辨率电成像测井图像，结合岩心标定和常规测井资料，能够有效地识别碳酸盐岩地层中的古洞穴，并判断其充填与否及充填物的性质。     

CHARACTERIZATION AND DFN MODELLING OF THE FRACTURE NETWORK IN A MESOZOIC KARST RESERVOIR: GOMBA OILFIELD,PALEOGENE BASIN,CENTRAL HUNGARY

Reservoir rocks at Gomba oilfield, located in the Hungarian Paleogene Basin, include fractured and karstified Triassic carbonates with significant fluid storage potential. However little information is available about the fracture network in these carbonates which has led to production problems at Gomba. The purpose of this study is to investigate the relationship between the spatial distribution of high porosity zones in the carbonates and the microfracture system. For this evaluation, individual fractures were studied and used in a discrete fracture network (DFN) modelling exercise at two wells, Gomba‐1 and Gomba‐3. The investigation took place at two different scales. Fracture length and aperture distributions were derived at the micro‐scale (mm to cm), whereas fracture density and fracture orientation data were investigated at the reservoir scale. The results of both investigations were taken into consideration in the modelling. The results of the fracture network models were compared to reservoir lithologies which ranged from fractured carbonates to collapse breccias. Based on the results, it appears that the porosity associated with a significant proportion of the fractures was increased as a result of dissolution. Fracture distribution was not uniform along the studied well paths, and fracture orientation was chaotic at particular depth intervals. On the basis of the DFN models, three different fractured zones are predicted to occur in the reservoir, but only two of the zones have significant fracture porosity. Comparing these results with petrographic observations, an epigene karst phase is proposed which is older than (or the same age as) the time of hydrocarbon migration; karst‐related voids were therefore important pathways for fluid migration. The void system and karst caverns partly collapsed during subsequent burial, resulting in karst‐related traps. By analogy with modern cave systems, the karst cavern zones at Gomba probably extend horizontally rather than vertically, and are oriented NE‐SW parallel to major structural lineaments.