Geological Features and Reservoiring Mode of Shale Gas Reservoirs in Longmaxi Formation of the Jiaoshiba Area

Abstract: This study is based on the sedimentation conditions, organic geochemistry, storage spaces, physical properties, lithology and gas content of the shale gas reservoirs in Longmaxi Formation of the Jiaoshiba area and the gas accumulation mode is summarized and then compared with that in northern America. The shale gas reservoirs in the Longmaxi Formation in Jiaoshiba have good geological conditions, great thickness of quality shales, high organic content, high gas content, good physical properties, suitable depth, good preservation conditions and good reservoir types. The quality shales at the bottom of the deep shelf are the main target interval for shale gas exploration and development. Shale gas in the Longmaxi Formation has undergone three main reservoiring stages: the early stage of hydrocarbon generation and compaction when shale gas reservoirs were first formed; the middle stage of deep burial and large-scale hydrocarbon generation, which caused the enrichment of reservoirs with shale gas; the late stage of uplift, erosion and fracture development when shale gas reservoirs were finally formed.     

涪陵气田焦石坝区块页岩气储层储集空间特征及其定量评价

四川盆地涪陵气田焦石坝区块上奥陶统五峰组—下志留统龙马溪组一段海相页岩气储层是由孔隙和裂隙组成的双孔隙岩层。为确定储层的含气量和产出能力,应用页岩气测井评价技术对储层的孔隙度和渗透率进行了定性认识,并提出了定量评价方法。认为用"经验统计法"可以有效计算出页岩气储层的孔隙度,但由目前的测井评价技术所得到的渗透率仅能反映储层渗透率的变化趋势。     

雪峰山前陆盆地下志留统页岩气勘探潜力

雪峰山志留系前陆盆地分布在湘中-湘西北-四川盆地,是上扬子板块主要的盆地类型,目前针对该前陆盆地沉积特征和页岩气勘探潜力的研究少见.根据野外资料、测井资料和分析测试资料分析了湘西北和湘中地区前陆盆地沉积建造过程和页岩气勘探潜力.结果表明湘中前陆盆地前渊带以浊积扇和浅海陆棚相沉积为主,而湘西北隆后盆地沉积相类型丰富,包括潮坪相、浅海陆棚相、滨岸相、三角洲相和碳酸盐岩台地相等.虽然前渊带和隆后盆地沉积相类型和沉积厚度差异较大,但沉积旋回的划分却是一致的,因此前渊带和隆后盆地相互连通,雪峰山对其两侧盆地的遮挡作用有限.地球化学测试分析表明研究区隆后盆地页岩的有机碳含量、有机质类型均较四川盆地的焦石坝差.湖南地区志留系页岩各项地化指标较差的主要原因是其缺少龙马溪组第一旋回的黑色碳质页岩.      

“Source-Diagenesis-Accumulation” enrichment and accumulation regularity of marine shale gas in southern China

After the breakthrough of shale gas exploration and development in the Ordovician Wufeng Formation (Fm.) and Silurian Longmaxi Fm. of Chongqing Jiaoshiba area, Changning-Weiyuan area, etc. in Sichuan basin, a series of discovery and breakthrough were obtained by China Geological Survey in the Cambrian Niutitang Fm. and Sinian Doushantuo Fm. shale of the areas with complicated structure outside Sichuan basin. Based on the understanding of the law of shale gas enrichment in Longmaxi Fm. in the basin, this paper puts forward three elements of the formation and enrichment of shale gas, which are the “Source”, the “Diagenesis” and the “Accumulation”, after deeply studying the law shale gas enrichment and accumulation in Sinian-Cambrian reservoir of the complex structure area outside the basin. The “Source” means the sedimentary environment and petrological characteristics of organic shale. The “Diagenesis” means the basin tectonic subsidence and hydrocarbon generation and expulsion process of organic matter. The “Accumulation” means the tectonic uplift and shale gas preservation. It is proposed that the Sinian-Cambrian and Ordovician-Silurian black shale series in the middle and upper Yangtze region of southern China were both formed in the deep-water shelf environment of rift trough and foreland basin respectively. The dessert intervals were formed in the strong reduction environment under transgressive system tract. The shale lithology belongs to calcium-siliceous and charcoal-siliceous respectively. Based on the summary of structural evolution in Yangtze area, the correlation of structural burial depth with shale diagenesis and the coupling evolution of organic matter with pore structure are discussed. Combining with structural styles, the preservation conditions of shale gas are discussed. Five types of shale gas reservoir control models are further described. Two types of future exploration directions, which are reverse fault syncline and paleo-uplift margin in complex structural area outside the basin, are proposed.     

四川盆地及其周缘奥陶系五峰组志留系龙马溪组页岩气富集主控因素分析

四川盆地及周缘地区奥陶系五峰组志留系龙马溪组页岩气富集主控因素包括：(1)特殊的古环境所形成的优质页岩是富集的基础。四川盆地及其周缘在五峰组和龙马溪组沉积时期,在弱挤压背景下发生了陆内拗陷沉降,奥陶纪末期冰期后全球性海平面快速上升,形成了较大规模的深水陆棚环境,为低等生物的大规模繁殖提供了有利条件,闭塞的海湾背景,为有机质的保存提供了有利的地球化学环境,欠补偿状态导致了地层中的高有机质含量。特殊生物类型所形成的大量有机硅,使成岩早期就形成了具有较强抗压实能力的岩石骨架,为早期生成原油的滞留提供了良好的空间,也为后期油向气转化时形成并保持大量的有机质孔奠定了物质基础。(2)有机质孔和特殊裂缝是页岩气富集的重要保障。有机碳含量高的优质页岩,有机质孔也发育较多,页岩的含气量大,游离气所占的比例更高。特殊裂缝主要指页岩层理缝和小尺度裂缝。在层理面堆积的笔石和藻类等成烃生物促进了这类层理缝的发育。小尺度裂缝提高了页岩自身的储集空间和渗流能力,有利于页岩气的聚集和成藏。(3)适度抬升状态下的有效保存是富集的关键。构造变形和抬升剥蚀是一把双刃剑。如果变形与抬升剥蚀作用太强,地层的封闭保存系统会破坏,导致不论是常规还是非常规的油气系统完全或者部分失效。因抬升产生微裂缝但没有出现大的穿层裂缝或断裂,即表现为一种“裂而不破”的状态是最理想的。远离断裂尤其是通天断裂、具有一定埋深且地层平缓的地区页岩气保存条件良好。四川盆地内与周缘地区保存条件的差异,主要是由于燕山中期运动所控制的构造变形、沉降沉积与抬升剥蚀作用的差异性所决定的。四川盆地及周缘地区奥陶系五峰组志留系龙马溪组页岩气有利富集区是位于富有机质黑色页岩厚度较大的深水陆棚沉积中心区、有机孔和微裂缝或页理发育的层段以及燕山期抬升时间晚且喜山期抬升幅度小区带所叠合的区域。     

Characteristics of black shale in the Upper Ordovician Wufeng and lower Silurian Longmaxi formations in the Sichuan Basin and its periphery,China

Geochemical and mineralogical analyses, in addition to isothermal adsorption experiments on field samples, are used to characterise the sedimentary environments, reservoirs and adsorbed gas of the Upper Ordovician Wufeng–lower Silurian Longmaxi formations in the Sichuan Basin and its peripheral areas. The sedimentary environment of the Wufeng and the lower part of Longmaxi formations is a deep-water shelf with five different lithologies identified: siliceous shale, black shale, siltstone, biolithite limestone and bentonite. The black shale in the Wufeng and the lower part of Longmaxi formations is 50 m thick, with an average organic carbon content (TOC) of 3.81 wt% and a maturity (Ro) of 1.62%. Quartz comprises 54.94 vol% of the shale and positively correlates with the TOC. Micropores in the black shale include intergranular pores, intragranular pores, organic matter pores and microfractures. Among these pores, spaces between clay sheets and organic molecules represent a favourable storage space for the accumulation and preservation of oil and gas. The Langmuir volume parameter ranges between 1.52 and 3.01 cm³/g, with an average value of 2.33 cm³/g. The presence of organic matter pores and pores between clay sheets in the black shale is the main and controlling factor for accumulated gas.     

鄂尔多斯盆地东部上古生界煤系页岩气藏特征及勘探方向

煤系页岩气是未来非常规天然气的勘探方向。以鄂尔多斯盆地东部为例,对区内上古生界煤系太原组和山西组页岩气成藏的构造条件、储层特征和资源潜力进行了分析。研究认为,区内构造简单,泥页岩储层埋藏浅,厚度大,泥页岩有机质演化程度较高,有机碳含量较高,页岩气含量较高,页岩气成藏条件较优越,页岩气勘探开发的有利区位于府谷-神木-临县一带。鄂尔多斯盆地东部上古生界煤层发育,煤层气含量较高,局部层段存在砂岩气,页岩气可与煤层气、煤系砂岩气等综合勘查、共同开发。      

川东南地区志留系龙马溪组页岩气有利区评价优选

通过对川东南地区志留系龙马溪组页岩有机质丰度、热成熟度、有机质类型、脆性矿物含量、页岩层厚度、埋藏深度、构造形态等七大地质因素的综合分析,认定该区分布广泛的页岩有机质含量较高,有机质类型较好,热演化程度高,生烃潜力大,具备形成页岩气藏的气源条件。页岩厚度较大,埋深较浅,脆性矿物含量较大,页岩气的保存条件良好,有利于页岩气的压裂与开采。     

湖北当阳复向斜北部页岩气地质评价与有利区优选

以露头、钻井、地震等资料为基础,对湖北当阳复向斜北部上奥陶统五峰组—下志留统龙马溪组页岩气成藏条件和勘探潜力展开评价。研究表明,当阳复向斜五峰组—龙马溪组具备良好的页岩气成藏地质条件:五峰组—龙马溪组沉积期以深水陆棚沉积环境为主,页岩广泛发育,优质页岩层厚度约50 m;有机质类型为Ⅱ_2型,有机碳含量一般大于2%,有机质成熟度R_o在3.0%左右,已进入过成熟阶段;页岩中的矿物以石英和黏土为主,有利于后期压裂改造和吸附气的大量富集;储层具有低孔特低渗特征;当阳复向斜构造简单,区域上盖层连续分布,保存条件较好。通过地质评价和资源量计算,当阳复向斜页岩气资源量较为丰富,优选巡检—溪前向斜带和龙坪—肖堰—栗溪背斜带为页岩气勘探有利区。     

滇黔北昭通示范区龙马溪组页岩气资源潜力评价

研究区位于滇黔北坳陷威信凹陷的中西部区域——川南低陡褶皱带南缘带,本文利用钻井、野外露头、区域地质等资料,对研究区龙马溪组页岩气基本地质特征进行了分析,并采用体积法对页岩气资源量进行了评价。研究成果表明,区内龙马溪组下段暗色页岩有机质含量及成熟度较高,有机质类型偏腐泥型;岩石脆性矿物含量高,物性一般,总含气量中等,总体与长宁、威远等邻区同层位产气页岩较为相似,并与北美主要产气页岩具有一定可比性。初步估算研究区龙马溪组页岩气资源量24871.8×10~8m~3,展现出良好的勘探前景,但受储层发育、保存条件等影响,页岩气资源分布严重不均,纵向上质量较好的页岩气资源主要集中在龙马溪组底部第1小层,平面上主要分布于研究区北部的川南低陡褶皱带内,该区内保存条件好的宽缓复向斜带和箱状背斜构造带是下步勘探的有利区。     

Controlling Factors of Organic Nanopore Development: A Case Study on Marine Shale in the Middle and Upper Yangtze Region,South China

The Upper Ordovician Wufeng-Lower Silurian Longmaxi and the Lower Cambrian Qiongzhusi shales are the major targets for shale gas exploration and development in China. Although the two organic-rich shales share similar distribution ranges and thicknesses, they exhibit substantially different exploration and development results. This work analyzed the nanopore structures of the shale reservoirs in this region. Pore development of 51 shale samples collected from various formations and locations was compared using the petromineralogical, geochemical, structural geological and reservoir geological methods. The results indicate that the reservoir space in these shales is dominated by organic pores and the total pore volume of micropores, mesopores, macropores in different tectonic areas and formations show different trends with the increase of TOC. It is suggested that organic pores of shale can be well preserved in areas with simple structure and suitable preservation conditions, and the shale with smaller maximum ancient burial depth and later hydrocarbon-generation-end-time is also more conducive to pore preservation. Organic pore evolution models are established, and they are as follows: ① Organic matter pore development stage, ② Early stage of organic matter pore destruction, and ③ late stage of organic matter pore destruction. The areas conducive to pore development are favorable for shale gas development. Research results can effectively guide the optimization and evaluation of favorable areas of shale gas.     

四川盆地东部龙马溪组页岩气成藏地质条件对比分析

为了揭示四川盆地东部页岩气成藏地质规律,指导页岩气勘探部署,选取四川盆地东部焦石坝地区、彭水地区及湖南保靖地区进行页岩气成藏地质条件对比分析。结果表明: 在龙马溪沉积时期,四川盆地东部形成了前陆盆地,沉积了富含有机质的碳质页岩,为页岩气生成提供了物质条件; 前陆盆地沉积中心位于重庆一带,其碳质页岩厚度从中心向周围减薄,碳质页岩中有机质丰度由中心向周边减小,有机质成熟度由中心向周边降低,地层压力由中心向周边减小,生烃时间从西向东逐渐缩短; 渝东焦石坝地区页岩气储集空间以孔隙为主,湖南保靖地区储集空间以裂缝为主; 从西向东,储层中孔隙空间缩小,裂缝发育程度加大; 构造运动对页岩气产生破坏作用,由东向西逐渐减弱。综合分析认为,四川盆地东部从渝东焦石坝地区向湖南保靖地区,页岩气成藏条件逐渐变差,勘探难度增加。     

A Combination of N2 and CO2 Adsorption to Characterize Nanopore Structure of Organic-Rich Lower Silurian Shale in the Upper Yangtze Platform, South China: Implications for Shale Gas Sorption Capacity

The pores in shales are mainly of nanometer-scale, and their pore size distribution is very important for the preservation and exploitation of shale gas. This study focused on the organic-rich Lower Silurian black shale from four wells in the Upper Yangtze Platform, and their TOC, mineralogical composition and pore characterization were investigated. Low pressure N2 and CO2 adsorption were conducted at 77.35 K and 273.15 K, respectively, and the pore structures were characterized by modified Brunauer-Emmett-Teller (BET), Dubinin-Radushkevich (DR), t-plot, Barrett-Joyner-Halenda (BJH) and density functional theory (DFT) methods and then the relationship between pore structure and shale gas sorption capacity was discussed. The results indicate that (1) The Lower Silurian shale has high TOC content of 0.92%–4.96%, high quartz content of 30.6%–69.5%, and high clays content of 24.1%–51.2%. The total specific surface area varies from 7.56 m2/g to 25.86 m2/g. Both the total specific surface area and quartz content are positively associated with the TOC content. (2) Shale samples with higher TOC content have more micropores, which results in more complex nanopore structure. Micropore volumes/surface areas and non-micropore surface areas all increase with the increasing TOC content. (3) A combination of N2 and CO2 adsorption provides the most suitable detection range (~0.3–60 nm) and has high reliability and accuracy for nanopore structure characterization. (4) The TOC content is the key factor to control the gas sorption capacity of the Lower Silurian shale in the Upper Yangtze Platform.     

蜀南地区龙马溪组页岩气工业建产区游离气和孔隙度下限讨论

游离气作为页岩气重要赋存形式之一,决定了页岩气的开发潜力及经济效益。以蜀南地区龙马溪组页岩为例,结合X-射线衍射、岩石有机碳、储层物性、测井解释及生产测试等资料,对比分析了四川盆地及周缘页岩游离气含量与测试日产气量的关系,初步提出了工业建产区页岩游离气含量下限,并从烃源、储集及保存等方面对页岩游离气含量主控因素进行了系统研究,在此基础上探讨了工业建产区页岩孔隙度下限。结果表明:页岩游离气含量大于2.5 m³/t时,更易获得工业气流;蜀南地区龙马溪组页岩游离气含量与TOC、孔隙度、硅质矿物含量、地层压力系数、裂缝发育整体呈一定正相关,而与含水饱和度、碳酸盐矿物含量具有一定负相关;裂缝发育、含水饱和度以及硅质矿物、碳酸盐矿物等能通过关联孔隙度而间接影响游离气含量,均不作为页岩游离气含量主控因素,页岩游离气含量主控因素为TOC、孔隙度及地层压力系数;利用容积法对不同孔隙度下页岩游离气含量进行了蒙特卡罗模拟计算,发现当页岩孔隙度大于4%时,游离气含量大于下限值2.5 m³/t的概率超过50%,建议以4%作为工业建产区选择的孔隙度下限,以此控制页岩气开发的地质风险。页岩储层含气量评价建议以游离气评价为中心,重点从TOC、孔隙度、地层压力系数等3个方面开展系统评价。     

湘西北下志留统龙马溪组页岩气成藏条件及有利区分析

湘西北地区下志留统龙马溪组发育富含有机质页岩。通过野外露头地质剖面、钻井及有机地球化学等资料分析,认为泥页岩厚度较大,有机质类型较好、有机碳含量较高、热演化程度适中、储层物性好、埋深适宜及保存条件相对较好,满足页岩气成藏要求。根据页岩气资源潜力评价方法与有利区优选标准,选定桑植-石门复向斜一带为页岩气的有利勘探区域。     

Pore structure and adsorption behavior of shale gas reservoir with influence of maturity: a case study of Lower Silurian Longmaxi formation in China

Knowledge of pore structure and adsorption capacity provides guidance for better studying the origin, hydrocarbon distribution, and productivity of shale gas reservoir. In this study, pore structure characteristics of six shale core plugs with different maturity from the Lower Silurian Longmaxi formation in south China were investigated using the Rock-eval analysis, X-ray diffraction, total organic carbon (TOC) content test, and scanning electron microscope (SEM) observation. To further investigate the influence of maturity, the adsorption behavior of gas shale was experimentally measured, with the maximal pressure being 20 MPa. Rock-eval analysis indicates that Ro is 0.67~1.34%. SEM results show that organic matter (OM) pores are abundant in high-maturity shale sample. The OM pores are mainly irregular to elliptical in shape, the size is 8~100 nm. The TOC content is 0.16~4.21% and shows a positive correlation with the BET surface area. A negative relationship exists between TOC content and average pore diameter, which indicates that abundant nanometer pores are related to the OM. A noticeable characteristic in the pore size distribution curve is that the content of micropores (pore width <?2 nm) increases with the increasing TOC content. Additionally, the thermal maturity results in significant difference in methane adsorption capacity. Maximal adsorption capacity of shale samples is also lineally correlated with TOC content, which increases with maturity. This study provides a quantitative understanding of how maturity affects pore structure and adsorption behavior of shale gas reservoir.     

川南地区龙马溪组有机质特征及其对页岩气富集规律的影响研究

四川盆地南部（简称“川南”） 广泛发育多套下古生界海相泥页岩层系,其中龙马溪组为该区一套优良的烃源岩 层,具备优越的页岩气形成条件。根据长宁和威远页岩气探区的钻井岩心、露头样品分析资料,对川南地区下志留统龙马 溪组页岩有机质特征及其对页岩气富集规律的影响进行研究后表明,川南龙马溪组页岩的有机质类型为Ⅰ～Ⅱ1型,有机 碳含量较高（平均为2.52%）,热演化程度为2.0%～3.8%,达到了高-过成熟阶段;研究区的有机质特征对页岩气的富集具 有重要的影响,干酪根的类型决定着甲烷吸附能力强弱,有机质丰度和有机演化程度决定了页岩的生气量和含气量,同时 高TOC和Ro也能促进有机质微孔隙的发育,为页岩气的富集提供更多有利空间。因此,研究认为川南地区龙马溪组有机质 特征较好,有利于页岩气的富集成藏。     

四川盆地东南缘丁山地区页岩气成藏富集的关键控制因素

川东南丁山地区上奥陶统五峰组—下志留统龙马溪组实施的两口页岩气探井,优质页岩段(一般TOC≥2.0%)实测总含气量,DY1井平均3.07 m~3/t,DY2井平均6.79 m~3/t。对比发现,优质页岩段烃源条件相似:丁山地区五峰组—龙马溪组沉积早期处于深水陆棚沉积环境,优质页岩段总体厚度较大,一般在30~35.5 m之间;平均TOC基本相当,在3.42%~3.95%之间;热演化程度适中,R_o值一般大于2.0%。研究认为保存条件差异是造成丁山地区页岩含气性差异的关键因素:距离齐岳山断裂较近的DY1井区,页岩高角度缝发育,加上埋藏浅,页岩气侧向逸散严重,页岩含气性相对变差;远离齐岳山断裂的深埋藏平缓带是有利的页岩气藏发育带。     

Geological Factors Controlling the Accumulation and High Yield of Marine-Facies Shale Gas: Case Study of the Wufeng-Longmaxi Formation in the Dingshan Area of Southeast Sichuan,China

The main geological factors controlling the accumulation and yield of marine-facies shale gas reservoirs are the focus of the current shale gas exploration and development research. In this study, the Wufeng-Longmaxi Formation in the Dingshan area of southeast Sichuan was investigated. Shale cores underwent laboratory testing, which included the evaluation of total organic carbon(TOC), vitrinite reflectance(Ro), whole-rock X-ray diffraction(XRD), pore permeability,and imaging through field emission scanning electron microscopy(FE-SEM). Based on the results of natural gamma ray spectrum logging, conventional logging, imaging logging, and seismic coherence properties, the exploration and development potential of shale gas in the Dingshan area have been discussed comprehensively. The results showed that(1)layer No. 4(WF2-LM4) of the Wufeng-Longmaxi Formation has a Th/U ratio 2 and a Th/K ratio of 3.5–12. Graptolites and pyrite are relatively abundant in the shale core, indicating sub-high-energy and low-energy marine-facies anoxic reducing environments.(2) The organic matter is mainly I-type kerogen with a small amount of II1-type kerogen. There is a good correlation among TOC, Ro, gas content, and brittle minerals; the fracturing property(brittleness) is 57.3%. Organic and inorganic pores are moderately developed. A higher pressure coefficient is correlated with the increase in porosity and the decrease in permeability.(3) The DY1 well of the shale gas reservoir was affected by natural defects and important latestage double destructive effects, and it is poorly preserved. The DY2 well is located far from the Qiyueshan Fault. Large faults are absent, and upward fractures in the Longmaxi Formation are poorly developed. The well is affected by low tectonic deformation intensity, and it is well preserved.(4) The Dingshan area is located at the junction of the two sedimentary centers of Jiaoshiba and Changning. The thickness of the high-quality shale interval(WF2-LM4) is relatively small, which may be an important reason for the unstable production of shale gas thus far. Based on the systematic analysis of the geological factors controlling high-yield shale gas enrichment in the Dingshan area, and the comparative analysis with the surrounding typical exploration areas, the geological understanding of marine shale gas enrichment in southern China has been improved. Therefore, this study can provide a useful reference for shale gas exploration and further development.     

湘西北下古生界海相页岩储层特征与含气性分析

基于野外地质和钻井资料,结合相关实验测试结果,对湘西北地区下古生界海相页岩储层特征进行了深入研究,并探讨了页岩甲烷含气性及影响因素。结果表明：牛蹄塘组黑色页岩以深水陆棚斜坡相沉积为主,厚度范围为50~250 m;龙马溪组为闭塞海湾沉积,底部黑色页岩发育。两组页岩有机质类型均属于Ⅰ型,有机碳含量平均为3.57%和1.16%,热演化程度较高,平均达2.61%和2.08%。受沉积环境和成岩作用影响,两组页岩均具有高石英、低黏土、少量碳酸盐矿物的组成特征。页岩储集空间可划分为3大类：矿物基质孔、有机质孔、微裂缝。受有机质和黏土矿物等因素影响,页岩内部孔隙结构参数各不同,但主体上孔径小于50 nm的微孔和中孔提供了大部分比表面积和孔体积,为气体存储主要场所。牛蹄塘组页岩甲烷最大吸附量平均为1.98 cm3/g;龙马溪组页岩甲烷最大吸附量较低,为1.16 cm3/g。其中有机质与黏土矿物对页岩甲烷吸附量均有一定的贡献,而过高的成熟度和含水量可导致页岩吸附能力下降。