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.

Developmental characteristics and geological significance of the bentonite in the Upper Ordovician Wufeng – Lower Silurian Longmaxi Formation in eastern Sichuan Basin, SW China

Based on the Qiliao section of the Upper Ordovician Wufeng Formation – Lower Silurian Longmaxi Formation in Shizhu, Chongqing city, the development characteristics of bentonite in eastern Sichuan Basin was examined systematically, and its geological significance and scientific value were analyzed. The main understandings are as follows:(1) Six bentonite dense layers were found in the Qiliao section, mainly occurring in 6 graptolitic belts of the Katian, Rhuddanian and Aeronian. Most of the bentonite dense layers showed obvious increase in clay, peak response of GR curve, and indistinct relationship between volcanic ash and total organic carbon(TOC).(2) The bentonite dense layers of Longmaxi Formation were widely distributed in eastern Sichuan Basin and its periphery, and generally showed GR peak, which can be an important reference interface for dividing the bottom boundary of the Coronograptus cyphus belt and the top boundary of the Rhuddanian in eastern Sichuan Basin and western Hubei province.(3) Taking the bentonite dense layers as the stratification basis of the Rhuddanian and Aeronian, it was determined that the sediment thickness of the Rhuddanian in the eastern Sichuan depression was generally 10–40 m, but only the upper part of the Coronograptus cyphus belt was deposited in the hinderland of Yichang Uplift, and the sedimentary thickness was only 3–7 m.(4) In the hinderland of the Yichang Uplift, at least five and a half graptolitic belts were missing in Wufeng Formation – Rhuddanian, and the deposition time of Rhuddanian was less than 0.4 Ma.(5) The bentonite dense layers were important sedimentary responses to the strong deflection of the Yangtze basin at the turn of the Ordovician–Silurian, which suggested that four tectonic activity periods existed in the eastern Sichuan depression, including the early stage of the depression, the middle-late stage of the depression, the early stage of the foreland flexure and the development stage of the foreland flexure. The high-quality shale was mainly developed from the early stage to the middle-late stage in the depression.

Quantitative prediction of shale gas sweet spots based on seismic data in Lower Silurian Longmaxi Formation,Weiyuan area,Sichuan Basin,SW China

Sweet spots in the shale reservoirs of the Lower Silurian Longmaxi Formation in Weiyuan 201 Block of Sichuan Basin were predicted quantitatively using seismic data and fuzzy optimization method. First, based on seismic and rock physics analysis, the rock physics characteristics of the reservoirs were determined, and elastic parameters sensitive to shale reservoirs with high gas content were selected. Second, data volumes with high precision of the elastic parameters were obtained from pre-stack simultaneous inversion. The horizontal distribution of key parameters for shale gas evaluation were calculated based on the results of rock physics analysis. Then, the fuzzy evaluation equation was established by fuzzy optimization method with test and logging data of horizontal wells with similar operation conditions. key parameters affecting the productivity of horizontal wells were sorted out and the weights of them in the sweet spots quantitative prediction were worked out by fuzzy optimization to set up a sweet spots evaluation system. Three classes of shale gas reservoirs which including two kinds of sweet spots were predicted with the above procedure, and the sweet spots have been predicted quantitatively by combining the above prediction results with the testing production. The testing results of 7 verification wells proved the reliability of the prediction results.

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.

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.

Geological characteristics and high production control factors of shale gas reservoirs in Silurian Longmaxi Formation, southern Sichuan Basin, SW China

Marine shale gas resources have great potential in the south of the Sichuan Basin in China.At present,the high-quality shale gas resources at depth of 2000–3500 m are under effective development,and strategic breakthroughs have been made in deeper shale gas resources at depth of 3500–4500 m.To promote the effective production of shale gas in this area,this study examines key factors controlling high shale gas production and presents the next exploration direction in the southern Sichuan Basin based on summarizing the geological understandings from the Lower Silurian Longmaxi Formation shale gas exploration combined with the latest results of geological evaluation.The results show that:(1)The relative sea depth in marine shelf sedimentary environment controls the development and distribution of reservoirs.In the relatively deep water area in deep-water shelf,grade-I reservoirs with a larger continuous thickness develop.The relative depth of sea in marine shelf sedimentary environment can be determined by redox conditions.The research shows that the uranium to thorium mass ratio greater than 1.25 indicates relatively deep water in anoxic reduction environment,and the uranium to thorium mass ratio of 0.75–1.25 indicates semi-deep water in weak reduction and weak oxidation environment,and the uranium to thorium mass ratio less than 0.75 indicates relatively shallow water in strong oxidation environment.(2)The propped fractures in shale reservoirs subject to fracturing treatment are generally 10–12 m high,if grade-I reservoirs are more than 10 m in continuous thickness,then all the propped section would be high-quality reserves;in this case,the longer the continuous thickness of penetrated grade-I reservoirs,the higher the production will be.(3)The shale gas reservoirs at 3500–4500 m depth in southern Sichuan are characterized by high formation pressure,high pressure coefficient,well preserved pores,good pore structure and high proportion of free gas,making them the most favorable new field for shale gas exploration;and the pressure coefficient greater than 1.2 is a necessary condition for shale gas wells to obtain high production.(4)High production wells in the deep shale gas reservoirs are those in areas where Long11-Long13 sub-beds are more than 10 m thick,with 1500 m long horizontal section,grade-I reservoirs penetration rate of over 90%,and fractured by dense cutting+high intensity sand injection+large displacement+large liquid volume.(5)The relatively deep-water area in the deep-water shelf and the area at depth of 3500–4500 m well overlap in the southern Sichuan,and the overlapping area is the most favorable shale gas exploration and development zones in the southern Sichuan in the future.With advancement in theory and technology,annual shale gas production in the southern Sichuan is expected to reach 450×108 m3.

Reservoir characteristics and genetic mechanisms of gas-bearing shales with different laminae and laminae combinations: A case study of Member 1 of the Lower Silurian Longmaxi shale in Sichuan Basin, SW China

Based on thin-section,argon-ion polished large-area imaging and nano-CT scanning data,the reservoir characteristics and genetic mechanisms of the Lower Silurian Longmaxi shale layers with different laminae and laminae combinations in the Sichuan Basin were examined.It is found that the shale has two kinds of laminae,clayey lamina and silty lamina,which are different in single lamina thickness,composition,pore type and structure,plane porosity and pore size distribution.The clayey laminae are about 100μm thick each,over 15%in organic matter content,over 70%in quartz content,and higher in organic pore ratio and plane porosity.They have abundant bedding fractures and organic matter and organic pores connecting with each other to form a network.In contrast,the silty laminae are about 50μm thick each,5%to 15%in organic matter content,over 50%in carbonate content,higher in inorganic pore ratio,undeveloped in bedding fracture,and have organic matter and organic pores disconnected from each other.The formation of mud lamina and silt lamina may be related to the flourish of silicon-rich organisms.The mud lamina is formed during the intermittent period,and silt lamina is formed during the bloom period of silicon-rich organisms.The mud laminae and silt laminae can combine into three types of assemblages:strip-shaped silt,gradating sand-mud and sand-mud thin interlayers.The strip-shaped silt assemblage has the highest porosity and horizontal/vertical permeability ratio,followed by the gradating sand-mud assemblage and sand-mud thin interlayer assemblage.The difference in the content ratio of the mud laminae to silt laminae results in the difference in the horizontal/vertical permeability ratio.

黔北正安地区安场向斜奥陶系五峰组—志留系龙马溪组页岩气富集规律

四川盆地外缘黔北地区具有良好的页岩气勘探开发前景。基于测井、录井及分析化验等资料,对黔北正安地区安场向斜奥陶系五峰组—志留系龙马溪组页岩气储层特征及富集规律进行了分析。研究结果表明:①黔北正安地区安场向斜奥陶系五峰组—志留系龙马溪组岩性主要为硅质页岩与硅质岩,孔隙类型主要为有机孔隙及粒间孔、粒内溶孔、晶间孔等无机孔隙与页理缝和构造缝及微裂缝等。岩石中总有机碳(TOC)为0.34%~6.65%,平均为3.69%,纵向上自下而上总体呈先增大后减小的趋势,平面上在向斜核部的TOC含量较高,呈环带状向四周降低。岩心含气的质量体积为1.6~6.2 m^(3)/t,平均为3.9 m^(3)/t,龙马溪组总含气量垂向上随着深度的增加而增加,平面上从向斜中部向南北两端逐渐降低,从向斜核部向两翼逐渐降低。矿物脆性指数总体较高,具有很好的可压裂性。②研究区五峰组—龙马溪组为“陆棚边缘、改造残留、向斜浅埋”的独特地质背景,其有利的沉积环境及古生产力为页岩气富集成藏奠定了基础,良好的保存条件避免了页岩储层中天然气的大量逸散,上覆的新滩组盖层与下伏的宝塔组底板形成了“上盖下堵”的封堵组合。③盆外残余向斜页岩气在向斜中心明显富集,核部是页岩气聚集的有利位置,两翼低角度逆断层遮挡处也是页岩气的有利富集部位,微裂缝的发育有利于页岩气的聚集与稳产,因此中浅层残余向斜的井位部署应重点考虑“宽、缓、深”的核心部位。

Lower limit of thermal maturity for the carbonization of organic matter in marine shale and its exploration risk

Based on the drilling data of the Silurian Longmaxi Formation in the Sichuan Basin and periphery, SW China, the Ro lower limits and essential features of the carbonization of organic matter in over-high maturity marine shale were examined using laser Raman, electrical and physical property characterization techniques. Three preliminary conclusions are drawn:(1) The lower limit of Ro for the carbonization of Type I-II1 organic matter in marine shale is 3.5%; when the Ro is less than 3.4%, carbonization of organic matter won't happen in general; when the Ro ranges from 3.4% to 3.5%, non-carbonization and weak carbonization of organic matter may coexist; when the Ro is higher than 3.5%, the carbonization of organic matter is highly likely to take place.(2) Organic-rich shale entering carbonization phase have three basic characteristics: log resistivity curve showing a general "slender neck" with low-ultralow resistance response, Raman spectra showing a higher graphite peak, and poor physical property(with matrix porosity of only less than 1/2 of the normal level).(3) The quality damage of shale reservoir caused by the carbonization of organic matter is almost fatal, which primarily manifests in depletion of hydrocarbon generation capacity, reduction or disappearance of organic pores and intercrystalline pores of clay minerals, and drop of adsorption capacity to natural gas. Therefore, the lower limit of Ro for the carbonization of Type I-II1 organic matter should be regarded as the theoretically impassable red line of shale gas exploration in the ancient marine shale formations. The organic-rich shale with low-ultralow resistance should be evaluated effectively in area selection to exclude the high risk areas caused by the carbonization of organic matter. The target organic-rich shale layers with low-ultralow resistance drilled during exploration and development should be evaluated on carbonization level of organic matter, and the deployment plan should be adjusted according to the evaluation results in time.

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。

四川盆地深层页岩气钻井关键技术新进展及发展展望

四川盆地页岩气资源丰富,目前深层页岩气已成为该盆地天然气增储上产的重点领域,但随着埋深增加和构造背景变化,地质工程条件将更加复杂,钻井过程中将面临井漏风险高、井下工具高温易失效、水平井轨迹控制难度大等技术难题。为此,在系统分析已完钻井实钻数据的基础上,依据深层页岩气区块地质工程特征,系统梳理了影响安全优快钻井的关键技术难点,形成了以地质工程一体化导向技术、钻井提速技术、防漏治漏与复杂防治技术为主体的深层页岩气安全优快钻井关键技术系列。研究结果表明:(1)以精细地质建模优选地质工程“双甜点”、实时靶体追踪为主的地质工程一体化导向技术,实现了地质目标的精准优选和精确追踪;(2)以“高效PDC钻头选型+个性化优化+大扭矩螺杆”高效破岩技术、“MSE+CCS”参数实时优化技术、油基钻井液地面降温+高温旋转导向技术为主的钻井提速技术,实现了页岩气钻井提速提效;(3)以井壁稳定性评价、裂缝性漏层识别、井漏与复杂防治为主的复杂防治技术,从源头降低了井下漏失和卡钻风险。结论认为:(1)形成的深层页岩气安全优快钻井技术,显著提高了机械钻速和铂金靶体钻遇率,在现场规模化推广应用200余口井,单井平均钻井周期降低42.7%,钻井提速效果显著,有力支撑了深层页岩气效益规模开发;(2)深层页岩气钻井将聚焦“地质工程一体化、水平段一趟钻、防漏治漏及智能钻井决策”等方面的技术攻关。

深层富有机质页岩孔隙结构分形特征及其地质意义——以四川盆地威远地区下志留统龙马溪组为例

与中、浅层页岩不同,四川盆地南部地区上奥陶统五峰组—下志留统龙马溪组深层富有机质页岩经历了复杂的成岩作用,发育不同的孔隙结构,影响页岩气的赋存状态。为了探讨深层富有机质页岩的孔隙结构、分形特征及其对页岩气富集的影响,以该盆地南部威远地区龙马溪组页岩为研究对象,综合运用有机碳分析、X射线衍射、高分辨率扫描电镜和低温氮气吸附等测试手段,定量表征研究区深层富有机质页岩储层的基础参数及其孔隙结构特征;并运用FHH分形理论,结合低温氮气吸附实验,计算页岩孔隙分形维数,讨论孔隙结构的影响因素及其地质意义。研究结果表明:(1)研究区龙马溪组深层页岩有机质含量(TOC)高,平均值为2.04%,石英等脆性矿物含量占38.00%~73.00%;(2)龙马溪组深层页岩储层普遍发育墨水瓶状有机孔、狭缝型或平板状无机孔等,BET比表面积介于1.252~28.676 m^(2)/g,BJH孔体积介于0.013 192~0.063 874 cm^(3)/g;(3)深层页岩孔隙分形具有明显的分段性,分形维数D_(2)>D_(1),孔隙结构受页岩组成控制,分形维数受控于比表面积和孔径,且与TOC密切相关;(4) LM1—LM3笔石带沉积的富有机质硅质页岩TOC高、分形维数D_(1)和D_(2)值大,介孔BET比表面积和BJH孔体积最大,既有较多的气体吸附位点赋存吸附气,又有良好的孔隙空间贮存游离气,是深层页岩气富集的“甜点段”。结论认为,综合页岩品质参数和分形维数特征可有效评价页岩气富集的“甜点段”,并对实现深层页岩气的增储上产具有现实的指导意义。

页岩储层渗吸过程微观孔缝演变特征及影响因素——以四川盆地渝西地区龙马溪组龙一1亚段为例

水力压裂已成为页岩气开采的重要手段,明确渗吸过程页岩储层孔隙、微裂缝的演变特征与影响因素,对指导页岩气井压后增产措施优化具有重要意义。为此,选取四川盆地渝西地区大足区块主力产层龙马溪组龙一1亚段底部黑色页岩为研究对象,开展渗吸水过程的氩离子抛光场发射扫描电镜(FE-SEM)定点观察实验,明确了渗吸水不同时间页岩储层微观孔缝演变规律。研究表明:①页岩储层渗吸水7 d后,有机质边缘有机孔出现不同程度的减小,而内部孔隙形态、大小基本不变;②粒内溶蚀孔和粒间孔会出现明显的扩溶现象,引起矿物颗粒溶蚀、脱落,增大页岩气泄气面积;③页岩储层渗吸水后不会大量萌生新的微裂缝,仅在原有微裂缝的基础上进行扩展,在吸水14 d后缝宽扩展为原来的5~10倍;④页岩储层面孔率在渗吸水后7 d达到最大值,大于7 d后微裂缝缝宽受黏土矿物持续膨胀影响出现不同程度的减小;⑤页岩储层增孔扩缝强度主要受矿物组成与孔渗性质影响,不稳定矿物与脆性矿物含量越高、粒径越大,增孔现象越明显,越有利于压后页岩气的渗流。

川南地区下志留统龙马溪组有机质热演化及其主控因素

为研究川南地区下志留统龙马溪组有机质热演化特征,根据钻井、岩性和温度实验数据,研究了现今地温场特征,利用激光拉曼光谱测定计算了龙马溪组页岩有机质镜质体反射率,以镜质体反射率(R_(o))为约束,用BasinMod盆地数值模拟技术重建了该区热流史及龙马溪组有机质热演化史。研究结果表明,川南地区龙马溪组储层沥青激光拉曼镜质体反射率(RmcR_(o))为2.7%~3.9%,有机质现均已到过成熟阶段,平面上从威远—泸州—长宁区块成熟度逐渐增加。加里东期大部分有机质尚未进入生烃门限。东吴期—印支期威远北部及渝西北部有机质处于低成熟-成熟阶段,同期长宁—宁西及泸州南部有机质分别进入高成熟和过成熟阶段。燕山期川南龙马溪组有机质普遍达到过成熟阶段。川南地区龙马溪组有机质热演化受古地貌、古埋深、峨眉山玄武岩喷发及构造隆升等地质因素的共同影响,其中古埋深及峨眉山玄武岩喷发是有机质热演化最主要的控制因素。

四川盆地天宫堂地区下志留统龙马溪组天然裂缝特征及形成期次

四川盆地西南缘下志留统龙马溪组页岩气储量巨大,裂缝发育特征与形成期次对页岩气富集及产能具有重要影响。以天宫堂地区龙马溪组为研究对象,采用岩心分析、FMI成像测井、岩石声发射实验、裂缝充填物碳氧同位素测试、包裹体均一温度测试和埋藏—热演化史分析等分析技术和综合地质方法,研究页岩中天然裂缝的发育特征和形成时期。结果显示,研究区龙马溪组天然裂缝以构造成因的直立缝与低角度缝共同发育为特征,岩心裂缝具有发育程度高、延伸短、充填程度高等特征。成像测井裂缝倾角比对、岩心裂缝交切关系、裂缝充填物流体包裹体测试及岩石声发射实验等结果表明,天宫堂地区龙马溪组裂缝形成与3期构造运动有关。结合埋藏—热演化史分析可证实:第一期为NW向、NNE向平面剪切缝和NEE向剖面剪切缝,形成于燕山中期—晚期(130~62 Ma),构造应力方位为近SN向(345°±5°),包裹体均一温度为185~206℃;第二期为NE向、NWW向平面剪切缝和NNW向剖面剪切缝,形成于燕山晚期—喜马拉雅中期(62~34 Ma),构造应力方位为EW向(80°±5°),包裹体均一温度为165~184℃;第三期为近SN向、NEE向平面剪切缝和NE向剖面剪切缝,形成于喜马拉雅中期—现今(34 Ma至今),构造应力方位为近NW向(315°±5°),包裹体均一温度为125~162℃。结合地质力学背景,建立了天宫堂地区龙马溪组3期构造裂缝演化模式。

威远地区龙马溪组天然裂缝特征、分布规律及形成期次

为了深入研究四川盆地非常规油气勘探开发的关键层位,以川南威远地区龙马溪组天然裂缝为研究对象,通过岩心观察、薄片分析和地球化学测试等多种方法分析了该区龙马溪组天然裂缝发育特征和形成时期,指出了该区龙马溪组天然裂缝发育类型及其形成期次。研究结果表明:①威远地区天然裂缝主要分为构造裂缝、生烃超压缝和层理缝3种类型;②威远龙马溪组存在多期天然裂缝。结论认为:①该区天然裂缝具有宽度细、水平层理缝占比高、偶见高角度构造缝、有效性低的特点,并且随着龙马溪组地层埋深的增加,裂缝发育密度逐渐增大;②根据温度计算和构造埋藏演化史,这些天然裂缝主要形成于印支期、燕山期、喜马拉雅晚期的3个时期,其中碳氧同位素和包裹体实验测试证实了3个时期的天然裂缝存在;③研究区印支期发育少量的NW剪切缝及砂泥互层层理缝,燕山期在断层附近、背斜高部位发育NW剪切缝和生烃超压缝,喜马拉雅晚期在断层附近、背斜高部位发育近NW和NWW组系剪切缝,背斜核部发育近S-N组系张性缝。结合研究区地质力学环境,查明了威远地区龙马溪组天然裂缝特征,明确了裂缝的形成期次及分布规律,为威远龙马溪组储层评价、有利区优选、优质储层甜点区评价及勘探开发提供了技术支撑。

丁山区块常压页岩气压裂认识及实践

针对四川盆地盆缘丁山常压气藏压裂裂缝复杂度低且缝控储量不足、压后排液难产能释放慢、可采储量低效益开发难三项技术挑战,以改造区储量充分控制、减轻排液负担及降低改造成本为目标,研究形成了以极限限流射孔促均衡技术、压-排一体化综合降负技术为主的技术措施和“减阻剂+助排剂”简易配方为降本措施的丁山盆缘常压页岩气高效低成本改造工艺。研究结果表明:①采用“极限限流射孔”保证多簇均匀起裂,单段覆盖率由75%提高至90%,改造体积较前期提高15%;②通过“压-排一体化综合降负技术”降排液负担,优化用液强度15~20m^(3)/m,加砂强度1.5~2.0m^(3)/m,配套压后不闷井快速开井排液、初期即下油管、气举助排等手段能够保障改造体积同时实现储层产能释放;③采用“低伤害低成本压裂液体系”,能够在储层改造需要的前提下有效控制成本。实践证明,2022-2023年新一代常压气藏低成本高效压裂技术已推广实施6井次,较前期实施井改造效果提高60.2%,压裂成本降低15.3%,为后续低品位常压储层规模效益开发提供了经验。

Micro-pore Structure and Gas Accumulation Characteristics of Shale in the Longmaxi Formation, Northwest Guizhou

For further study on characteristics of micro-pores in the marine shale reservoirs in Northwest Guizhou,shale samples in the Lower Silurian Longmaxi Formation were selected to investigate effect of micro-pores on the characteristics of gas accumulation in shales,through methods of the argon broad ion beam-field emission scanning electron microscope and the nitrogen adsorption/desorption,analysis on the characteristics of micro-pores and related geochemical analysis.Results show that there are seven types of pores in the Longmaxi Formation shale,including interparticle pores,intraparticle pores,intercrystalline pores,dissolution pores,fossil pores,organic pores,and microfractures,among which the interparticle pores and organic pores are best developed.According to the nitrogen adsorption/desorption curves,the pore structures can be classified into three types,including the cylindrical pores with both opening ends,narrow parallel-plate pores and tapered parallel-plate pores with four opening sides.Diameter of micro-pores ranges from 2 to 64 nm,mainly at 2-6 nm.The diameter of micro-pores(less than 2 nm)ranges from 0.4 to 1.8 nm,The micro-pores with a diameter of 0.4-1.0 nm contribute most to the pore volume.Pore volume is dominated by meso-pores(2-50 nm)with a proportion of 83.1%.The micro-pores and meso-pores make major contribution to the specific surface area of pores with proportions of 20.1%and 79.3%,respectively.The total organic carbon(TOC)is the major factor controlling development of nanopores.Different pore types have different characteristics of gas occurrence and migration,indicating that nanopores provide favorable conditions for the occurrence and microscopic migration of gas in shales.

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。

四川盆地东部地区下志留统龙马溪组页岩储层特征

四川盆地是中国西南部重要的含油气盆地,在东部和南部地区下志留统龙马溪组页岩广泛发育。在川东南、鄂西渝东地区的勘探井中志留系具有良好的气显示。研究区龙马溪组厚65～516m,底部为一套海侵沉积的富含笔石的黑色页岩,龙马溪组向上和向东砂质和钙质含量增加,演变为浅水陆棚沉积。龙马溪组主要由层状-非层状泥/页岩、白云质粉砂岩、层状钙质泥/页岩、泥质粉砂岩、层状-非层状粉砂质泥/页岩、粉-细粒砂岩、钙质结核、富含有机质非层状页岩8种岩相组成。总有机碳含量(TOC)为0.2%～6.7%。有机质以II型干酪根为主,Ro为2.4%～3.6%。页岩中石英矿物含量在2%～93%,主要呈纹层状或分散状分布,主要为陆源碎屑外源成因。龙马溪组页岩岩心孔隙度为0.58%～0.67%,渗透率为0.01×10-3μm2～0.93×10-3μm2。扫描电镜下龙马溪组页岩微孔隙度为2%左右,主要包括晶间孔和粒内孔,孔隙直径为100nm～50μm。页岩储层的形成机理主要为有利矿物组合、成岩作用和有机质热裂解作用。龙马溪组与美国Barnett页岩具有一定差异,主要表现在龙马溪组页岩埋藏较深、热演化程度较高、含气量较低、储层较致密、以陆源成因石英为主。对于评价下志留统龙马溪组页岩气勘探前景而言,今后须重点加强针对龙马溪组底部黑色硅质岩系石英成因、成熟度、埋藏史、含气量等方面的研究,以及进行详细的古地貌和古环境恢复。