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%~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 m^2/g to 25.86 m^2/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.

Source Rock and Cap Rock Controls on the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation Shale Gas Accumulation in the Sichuan Basin and its Peripheral Areas

Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of ＂source rock and cap rock controls on hydrocarbon accumulation＂, of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.

Effects of astronomical orbital cycle and volcanic activity on organic carbon accumulation during Late Ordovician–Early Silurian in the Upper Yangtze area, South China

Based on field outcrop data,the effects of cyclic change of astronomical orbit and volcanic activity on organic carbon accumulation during the Late Ordovician-Early Silurian in the Upper Yangtze area were studied using cyclostratigraphic and geochemical methods.d13 C and chemical index of alteration(CIA)were used to filter the astronomical orbit parameters recorded in sediments.It is found that the climate change driven by orbital cycle controls the fluctuations of sea level at different scales,obliquity forcing climate changes drive thermohaline circulation(THC)of the ocean,and THC-induced bottom currents transport nutrient-laden water from high latitude regions to the surface water of low-latitude area.Hence,THC is the main dynamic mechanism of organic-carbon supply.The marine productivity indexes of Ba/Al and Ni/Al indicate that volcanic activities had limited effect on marine productivity but had great influences on organic carbon preservation efficiency in late Hirnantian(E4).Paleo-ocean redox environmental indicators Th/U,V/Cr and V/(V+Ni)show that there is a significant correlation between volcanism and oxygen content in Paleo-ocean,so it is inferred that volcanisms controlled the organic carbon preservation efficiency by regulating oxygen content in Paleo-ocean,and the difference in volcanism intensity in different areas is an important factor for the differential preservation efficiency of organic carbon.The organic carbon input driven by orbital cycle and the preservation efficiency affected by volcanisms worked together to control the enrichment of organic carbon in the Middle–Upper Yangtze region.

Basic characteristics of key interfaces in Upper Ordovician Wufeng Formation – Lower Silurian Longmaxi Formation in Sichuan Basin and its periphery,SW China

Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation-Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation-Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces(i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.

不同构造单元页岩孔隙结构差异及其油气地质意义——以四川盆地泸州地区深层页岩为例

不同构造单元页岩储层品质、含气性差异明显,构造改造作用对页岩气勘探开发具有控制作用,但关于深层页岩气的构造控制作用机理研究较少,相关认识尚不明确,制约了深层页岩气的勘探开发。为此,通过“岩心—薄片—扫描电镜”多尺度观察、全岩矿物X射线衍射分析、核磁共振等技术手段,对比分析了四川盆地南部泸州地区不同构造单元上奥陶统五峰组—下志留统龙马溪组深层页岩孔隙结构和储层特征的差异,探讨了不同构造单元页岩孔隙结构差异与储层品质的耦合关系,明确了页岩气产量差异的内在地质原因,落实了构造改造作用下的页岩气勘探开发有利区。研究结果表明:①向斜区页岩主要以有机质孔隙、非构造裂缝为主,孔径大;背斜区页岩孔隙结构被强烈改造,以矿物粒间孔、构造裂缝为主,矿物粒间孔狭长且定向排列,孔径小。②向斜区构造相对稳定,有机质孔隙、矿物粒间孔以及成岩裂缝保存较好,宏孔占比高,储集性能好,含气量和产量高;背斜区页岩储层发育大量构造裂缝与断层,孔隙和成岩裂缝被压实,孔径减小,储集性能变差,含气量和产量低。③距盆缘剥蚀区越远,构造越稳定的单元,保存条件越好,该类构造单元页岩储层越发育,含气量和产量越高,页岩气勘探开发潜力越大。结论认为,构造运动对于压力系统的影响是形成不同构造单元深层页岩孔隙结构特征差异的重要因素,并控制了页岩储层的含气性与产量;向斜区深层保存条件好,远离剥蚀区,为勘探开发的最有利区,该认识可为深层页岩气勘探开发提供技术支撑。

Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales,southwest Yangtze Plate,China:Insights from geochemical and petrological evidence

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formation is the most developed strata of shale gas in southern China.Due to the complex sedimentary environment adjacent to the Kangdian Uplift,the favorable area for organic-rich shale development is still undetermined.The authors,therefore,focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales to have a more complete understanding and new discovering of organic matter enrichment and favorable area in the marginal region around Sichuan Basin.Multiple methods were applied in this study,including thin section identification,scanning electron microscopy(SEM)observations and X-ray diffraction(XRD),and elemental analysis on outcrop samples.Five lithofacies have been defined according to the mineralogical and petrological analyses,including mudstone,bioclastic limestone,silty shale,dolomitic shale,and carbonaceous siliceous shale.The paleo-environments have been reconstructed and the organic enrichment mechanism has been identified as a reduced environment and high productivity.The Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment based on geochemical characterization.High dolomite content in the study area is accompanied by high TOC,which may potentially indicate the restricted anoxic environment formed by biological flourishing in shallower water.And for the area close to the Kangdian Uplift,the shale gas generation capability is comparatively favorable.The geochemical parameters implied that new favorable areas for shale gas exploration could be targeted,and more shale gas resources in the mountain-basin transitional zone might be identified in the future.

Hydrocarbon generation and storage mechanisms of deepwater shelf shales of Ordovician Wufeng Formation–Silurian Longmaxi Formation in Sichuan Basin, China

As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental modelling of shale gas formation, the shale gas generation and accumulation mechanisms as well as their coupling relationships of deep-water shelf shales in Wufeng–Longmaxi Formation of Sichuan Basin were analyzed from petrology, mineralogy, and geochemistry. The high quality shales of Wufeng–Longmaxi Formation in Sichuan Basin are characterized by high thermal evolution, high hydrocarbon generation intensity, good material base, and good roof and floor conditions;the high quality deep-water shelf shale not only has high biogenic silicon content and organic carbon content, but also high porosity coupling. It is concluded that:(1) The shales had good preservation conditions and high retainment of crude oil in the early times, and the shale gas was mainly from cracking of crude oil.(2) The biogenic silicon(opal A) turned into crystal quartz in early times of burial diagenesis, lots of micro-size intergranular pores were produced in the same time;moreover, the biogenic silicon frame had high resistance to compaction, thus it provided the conditions not only for oil charge in the early stage, but also for formation and preservation of nanometer cellular-like pores, and was the key factor enabling the preservation of organic pores.(3) The high quality shale of Wufeng–Longmaxi Formation had high brittleness, strong homogeneity, siliceous intergranular micro-pores and nanometer organic pores, which were conducive to the formation of complicated fissure network connecting the siliceous intergranular nano-pores, and thus high and stable production of shale gas.

Biostratigraphy and reservoir characteristics of the Ordovician Wufeng Formation-Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding areas,China

Through graptolite identification in profiles,graptolite zone division,contour map compilation,and analysis of mineral composition,TOC content,lamina distribution features of shale samples,the biostratigraphic and reservoir characteristics of Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin and its peripheral are sorted out.There are 4 graptolite zones(WF1 to WF4)in Wufeng Formation and 9(LM1 to LM9)in Longmaxi Formation,and the different graptolite zones can be calibrated by lithology and electrical property.The shale layers of these graptolite zones have two depocenters in the southwest and northeast,and differ in mineral composition,TOC,and lamina types.Among them,the graptolite zones of lower WF2 and WF4 are organic matter-poor massive hybrid shale,the upper part of WF1-WF2 and WF3 have horizontal bedding hybrid shale with organic matter,the LM1-LM4 mainly consist of organic-rich siliceous shale with horizontal bedding,and the LM5-LM9 graptolite zones consist of organic-lean hybrid shale with horizontal bedding.The mineral composition,TOC and lamina types of shale depend on the paleo-climate,paleo-water oxidation-reduction conditions,and paleo-sedimentation rate during its deposition.Deposited in oxygen-rich warm water,the lower parts of WF1 and WF2 graptolite zones have massive bedding,low TOC and silicon content.Deposited in cooler and oxygen-rich water,the WF4 has massive bedding,high calcium content and low TOC.Deposited in anoxic water with low rate,the upper part of WF2,WF3,and LM1-LM4 are composed of organic rich siliceous shale with horizontal bedding and high proportion of silt laminae.Deposited in oxygen rich water at a high rate,the graptolite zones LM5-LM9 have low contents of organic matter and siliceous content and high proportions of silt lamina.

川东南五峰组-龙马溪组深层、超深层页岩储层特征及其页岩气勘探意义

四川盆地东南地区上奥陶统五峰组—下志留统龙马溪组中深层领域页岩气勘探及页岩储层特征与主控因素的研究已经取得了丰硕成果,而作为当前页岩气勘探重点的深层、超深层领域,由于受钻井等因素的制约,对于其储层发育特征及与中深层页岩储层的差异等研究还不够深入。为了明确五峰组—龙马溪组深层、超深层页岩储层特征,为勘探部署提供依据,选取川东南地区4口埋深在2000~6000 m的典型页岩气钻井,系统开展了深层、超深层页岩储层发育特征及差异对比研究,探讨了储层孔隙发育的成因。研究表明:(1)埋深在6000 m以内,五峰组—龙马溪组页岩储层依然能够发育高孔有效储层,且随埋深的增大孔隙度无明显变化,但是有机质孔的形态、孔隙结构及连通性存在一定差异,即随着埋深的增大,有机质孔孔径相对变小,孔隙连通性变差;(2)明确了生物成因的硅质是孔隙发育的基础,流体超压是储层孔隙保持的关键,在两者联合作用下,深层、超深层页岩高孔隙优质储层得以发育和保持;(3)基于储层发育的研究,将页岩气勘探拓深至6000 m,明确了下一步页岩气勘探方向。初步评价四川盆地及周缘五峰组—龙马溪组深层、超深层页岩气(埋深为4000~5000 m)资源量超2×10^(12)m^(3)。

四川盆地大安区块五峰组—龙马溪组深层页岩地质特征与勘探有利区

近年来,四川盆地渝西地区大安区块上奥陶统五峰组—下志留统龙一1亚段连续获得多口高产工业气井,展示了该区块页岩气良好的勘探开发前景。但由于该区块页岩埋藏深,勘探程度低,深层优质页岩段沉积微相类型、储层特征及其空间展布规律不清,页岩气有利区优选依据不充分,制约了该区页岩气的深化勘探与开发。为此,通过岩心观察、扫描电镜、地球化学与物性测试、含气量测定和核磁共振等实验手段分析,结合钻测井资料,研究了大安区块五峰组—龙一1亚段各小层地质特征及储层展布规律,建立了深层页岩气有利区评价标准,优选了深层页岩气勘探有利区。研究结果表明:①高碳硅质页岩微相和高碳含黏土硅质页岩微相是最优质页岩气储层发育的沉积微相,主要发育于五峰组—龙一13小层。②龙一11—龙一14小层孔隙度平均超过4%,储集性好;五峰组—龙一13小层超大孔和裂缝较发育,所占比例分别为66%和25%。③五峰组—龙一13小层脆性矿物平均含量为55%,可压性好;含气量平均值不低于4.0 m^(3)/t,含气条件较好;龙一11—龙一13小层各层平均总有机碳含量超过3.0%,生烃条件好;地层压力系数普遍大于1.90。结论认为:①五峰组—龙一13小层页岩储层品质最优;②Ⅰ类、Ⅱ类和Ⅲ类页岩气有利区资源量分别为3775.78×10^(8) m^(3)、2970.05×10^(8) m^(3)、599.83×10^(8) m^(3);③该区深层页岩气勘探潜力大,是四川盆地重要的天然气产能接替区,其中临江向斜为下一步页岩气勘探的最有利区。

羌北地块上志留统龙木措上组岩石磁学特征

开展羌北地块早古生代古地磁学研究,定量约束出其古生代以来的古地理位置,可为研究青藏高原古构造格局、显生宙特提斯演化和古地理重建等提供重要基础和关键制约。在进行古地磁研究之前,首先进行岩石磁学特征的研究,明确岩石中载磁矿物的组合类型和特征,为后续退磁实验方案的选择以及剩磁的原生性的讨论提供岩石磁学基础。对羌北地块上志留统龙木措上组灰岩和砂岩进行岩石磁学特征研究,包括等温剩磁获得曲线、磁化率随温度变化(χ-T)曲线、三轴等温系统热退磁实验、低温磁学性质测试以及扫描电镜(SEM)和能谱分析(EDS)等。实验结果表明,龙木措上组灰岩样品中的磁性矿物以磁铁矿为主,另外还有少量的磁赤铁矿和磁黄铁矿,砂岩样品中的磁性矿物较为复杂,主要为磁铁矿,可能还含有磁黄铁矿等其他磁性矿物。研究结果表明龙木措上组地层中灰岩样品可以分离出稳定的高温剩磁分量,适宜开展进一步古地磁学研究。

速度-压力岩石物理模型及其在页岩孔隙压力预测中的应用

针对现有基于声波速度的经验公式进行地层压力预测时影响因素复杂的问题,应用弹性岩石物理模型来定量分析研究速度-压力关系的各种控制因素,提出孔构参数(γ)与压差之间的解析关系,并揭示压力对岩石体积模量和纵波速度等弹性性质的主控作用,进而准确预测孔隙压力。含气页岩中微裂缝及断裂发育时,异常高压下(高γ值)细小裂缝及扁平孔隙张开,岩石体积模量远低于其在静水压力下(低γ值)平均孔隙结构偏圆时的状态。结合岩心、测井及地震数据,利用此岩石物理压力关系刻画四川盆地丁山地区上奥陶统五峰组—下志留统龙马溪组页岩的孔隙压力三维空间分布变化,压力系数与含气量呈正相关,该预测结果与现场实测数据吻合较好。岩石物理压力关系的分析和方法的提出对非常规和常规油气田的勘探、生产和钻井安全都具有重要借鉴价值。

Rock physics model for velocity–pressure relations and its application to shale pore pressure estimation

Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here propose to use elastic rock physics models to understand and analyze quantitatively the various contributions from these different factors affecting wave velocity. We report a closed-form relationship between the frame flexibility factor(γ) in a rock physics model and differential pressure, which presents the major control of pressure on elastic properties such as bulk modulus and compressional wave velocity. For a gas-bearing shale with abundant micro-cracks and fractures, its bulk modulus is much lower at abnormally high pore pressure(high γ values) where thin cracks and flat pores are open than that at normal hydrostatic pressure(low γ values) where pores are more rounded on average. The developed relations between bulk modulus and differential pressure have been successfully applied to the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations in the Dingshan area of the Sichuan Basin to map the three-dimensional spatial distribution of pore pressure in the shale, integrating core, log and seismic data. The estimated results agree well with field measurements. Pressure coefficient is positively correlated to gas content. The relations and methods reported here could be useful for hydrocarbon exploration, production, and drilling safety in both unconventional and conventional fields.

四川盆地焦石坝地区页岩气储层特征及控制因素

页岩气的生成和聚集具有不同于常规油气藏的独特规律,页岩气储层的研究是页岩气勘探与开发的核心问题。目前,对焦石坝地区页岩气储层的认识是相对有限的,需要对本区页岩气储层做进一步研究。基于大量实验室测试数据的统计分析显示:上奥陶统五峰组和下志留统龙马溪组目的层段总有机碳(TOC)含量介于0.55%~5.89%,平均为2.54%,且具有自上而下有机碳含量逐渐增加的趋势;基于全岩X-射线衍射分析方法,页岩中黏土矿物含量介于16.6%~62.8%,平均为40.9%,自上而下逐渐减少,脆性矿物含量自上而下逐渐增加,总量介于37.2%~83.4%,平均为59.1%;基于氦气注入法检测了目的层段的孔隙度,实测氦气孔隙度介于1.17%~7.98%,平均为4.61%,目的层段孔隙度呈现出'两高夹一低'的三分性特征;稳态法水平渗透率介于0.002~335.209 m D,平均为23.785 m D;通过高压压汞法对储层孔隙结构进行了研究,大量的测试数据表明,介孔级别的孔隙发育,且介孔提供了主要的孔比表面积,而介孔和大孔对渗透率起主要的贡献;将氩离子剖光技术和扫描电镜(SEM)相结合对储层的孔隙类型进行了观察,总体表现为自上而下有机孔隙增加、无机孔隙减少;由解吸法测得总含气量介于0.44~5.19 m3/t,平均为1.97 m3/t,从上到下呈现出逐渐增大的趋势。研究表明,焦页1井海相页岩气储层发育的控制因素有矿物组成和有机质发育特征等。TOC是控制下部储层段的主要内在因素,也是提供页岩气储存空间的重要物质;成岩阶段晚期,黏土矿物组合发生变化,蒙脱石向伊利石转变,形成新的微孔隙,增加了储层的孔隙度,对上部储层段有较大影响;脆性矿物含量大于50%,易于形成裂缝,可造成地层渗透性能的显著增强。总体来看,五峰组和龙马溪组的底部层段是优质储层,也是主要的产气层段。

四川盆地及其周缘上奥陶统—下志留统页岩气聚集条件

通过对25处野外露头的观测、采样以及吸附气含量、地化参数和物性参数等测试分析,研究了四川盆地及其周缘上奥陶统—下志留统黑色页岩的沉积相、厚度及分布、有机质类型及含量、成熟度、孔隙度和含气量等页岩气成藏条件。与美国主要产气页岩相比,研究区发育的页岩具有沉积相有利、厚度大、有机碳含量高、成熟度高、孔隙度高和含气量高等特点,具备页岩气藏发育的良好地质条件。此外,建立了预测页岩气藏发育有利区的参数模型,采用综合信息叠合法预测四川盆地及其周缘上奥陶统—下志留统页岩气藏的有利发育区位于川南-黔北、鄂西-渝东和川(渝)东北-鄂西北等地。在各个有利区的具体指标中,川南-黔北有利区具有较高页岩含气量,是目前勘探较为现实的地区。根据实测含气量,采用体积法计算研究区上奥陶统—下志留统页岩气资源量为(2.2～8.5)×1012m3,中值为5.3×1012m3。

四川盆地五峰组—龙马溪组页岩气勘探进展、挑战与前景

四川盆地黑色页岩层系多、页岩气资源丰富,是迄今为止中国主要的页岩气勘探开发盆地。为了给今后四川盆地乃至整个中国南方地区页岩气勘探开发提供指导和参考,以该盆地上奥陶统五峰组—下志留统龙马溪组页岩气勘探开发实践为对象,总结了其发展阶段与进展,明确了页岩气富集成藏的有利条件,梳理了制约页岩气发展的理论与技术挑战,进而探讨了未来的勘探开发前景。研究结果表明:(1)深水陆棚相形成高富含有机质页岩且连续段厚度大、品质好,复背(向)斜宽缓区构造相对稳定是该盆地五峰组—龙马溪组页岩气持续聚集的有利条件,保存条件是页岩气富集的关键因素,页理(纹理)、微裂缝发育是页岩气高产的重要因素,超压是页岩气高产的重要条件;(2)制约该盆地五峰组—龙马溪组页岩气工业化发展的技术瓶颈,主要包括富有机质页岩沉积相与沉积模式,页岩储层成岩过程与评价体系,页岩气形成与聚集机理,页岩气层地球物理识别与预测,资源动用率低、不确定性大,3500 m以深页岩气勘探开发技术尚未突破。结论认为,四川盆地五峰组—龙马溪组仍是今后相当长一段时间内中国页岩气勘探开发的重点层系。

四川盆地及其周缘上奥陶统—下志留统页岩气成藏体系研究

通过对页岩层系发育的时代、沉积特征、演化历史、现今所处的大地构造位置、构造特征、页岩残留状况、分布特征、埋深和含气量等因素的综合考虑,根据"单源一位"页岩气成藏体系划分的一般性原则,将四川盆地及其周缘上奥陶统—下志留统页岩气成藏体系划分为川南—川东南、黔北、渝东、湘鄂西、龙门山、米仓山—大巴山和川中7个成藏体系,描述了不同成藏体系中页岩层系的厚度、有机碳含量、成熟度和含气量等特征,并按战略展开区、战略突破区、战略准备区和战略远景区4个层次对成藏体系进行了评价,明确了勘探层次。

南方海相上奥陶统－下志留统优质烃源岩发育的控制因素

从全球古气候和古海洋特征出发,结合中国南方晚古生代古地理演化特征,并利用中上扬子地区典型剖面系统的地球化学数据,探讨了南方海相上奥陶统—下志留统优质烃源岩发育的控制因素。研究表明V/(V+Ni)、Ni/Co、Ceanom、δEu和δ13Corg等地球化学指标与有机碳含量均有较好的相关性,这些指标的异常匀表明上奥陶统—下志留统优质烃源岩发育于缺氧环境。但是,从古气候和古海洋洋流的研究结果看,上奥陶统五峰组烃源岩与下志留统龙马溪组烃源岩的发育因素有明显的不同,五峰组沉积处于全球冰期的高峰阶段,在扬子地区发育强劲的上升洋流,提高了该地区的有机质的生产率,现今广泛分布在扬子地区的高有机质丰度的薄层硅质岩,是上升洋流的最好标志。龙马溪组沉积处于全球冰期过后的暖期,不利于上升洋流的大规模形成,上升洋流的影响范围仅局限于当时扬子克拉通盆地周缘深水斜坡部位。龙马溪组优质烃源岩主要发育在海侵的初期,主要是因为海侵初期海平面的快速上升,限制了陆源碎屑的大规模注入,海底的缺氧环境得以保存。而海侵的后期由于深层海水和表层海水有充足的时间混合,致使底部缺氧环境遭受破坏,有机质保存条件变差。

川南地区五峰组—龙马溪组页岩层序地层及其对储层的控制

随着四川盆地勘探程度的深入,页岩气藏已经成为勘探重点,有必要系统地开展页岩层序地层研究。在岩石地层、生物地层、年代地层研究的基础上,运用露头-钻井-地震、全球海平面变化曲线、地球化学、古生物等资料,在川南地区五峰组—龙马溪组页岩地层中识别出4个三级层序界面,划分了3个三级层序:SQ1相当于五峰组,沉积速率低(1.18~2.19 m/Ma);SQ2相当于龙马溪组下段及上段底部,沉积速率较低(约为15 m/Ma);SQ3相当于龙马溪组上段中上部,沉积速率较高(216.25~340 m/Ma)。建立了该区的层序地层格架。分析了层序对储层的控制作用,主要体现在黑色页岩层序SQ1—SQ2中相对海平面升降与页岩脆性指数呈负相关、与TOC及孔隙度呈正相关,渗透率高值多位于脆性段,渗透率低值多位于塑性段。灰色页岩层序SQ3以塑性段为主,孔、渗均低于SQ1、SQ2凝缩段,其相对海平面与有机质没有明显的相关性。相对海平面变化影响了优质页岩储层发育,SQ1、SQ2海侵期—海退早期为优质页岩储层发育期。综合本次研究成果,明确了层序地层对优质页岩储层的控制作用,对于在层序格架内预测优质页岩储层段具有指导意义。

中上扬子地区上奥陶统—下志留统烃源岩发育的古环境恢复

上奥陶统—下志留统烃源岩是中国南方海相4套主力区域烃源岩之一,有关它形成与发育环境的研究是南方海相油气成藏理论的基础性问题。本文选择中上扬子南缘2条代表性剖面,对上奥陶统—下志留统沉积岩进行了详细的地球化学分析,提取其中对沉积古环境有指示意义的地化指标,对该段地层的古环境进行了恢复。研究表明,优质烃源岩段主要集中在上奥陶统五峰组和下志留统龙马溪组底部,V/(V+Ni)、Ni/Co、Ceanom、δEu和δ13Corg等地球化学指标与有机碳含量均有较好的相关性,这些指标的异常均表明优质烃源岩发育于缺氧环境。而龙马溪组上部地层的有机碳含量较低,各种地球化学指标也显示,此时缺氧环境遭到了破坏,这与当时的沉积环境多为高能环境、不利于烃源岩发育是一致的。