海相黑色页岩甜点类型、特征及页岩气勘探意义——以四川盆地南部五峰组—龙马溪组为例

Types and characteristics of sweet spots of marine black shale and significance for shale gas exploration:A case study of Wufeng-Longmaxi in southern Sichuan BasinWANG

黑色页岩甜点类型、识别标准及分布规律直接影响勘探层位优选及钻井靶点设计。为此,以四川盆地南部(以下简称川南地区)上奥陶统五峰组—下志留统龙马溪组海相黑色页岩为例,结合岩石学、层序地层学等理论,综合分析了相对海平面变化、裂缝类型与页岩储层品质之间的关系。研究结果表明:①川南地区五峰组—龙马溪组黑色页岩可划分出沉积型和裂缝型2类甜点,其中,沉积型甜点细分为早期海进型、快速海进型、晚期海进型和近滨海进型4类,裂缝型甜点分为网状微裂缝型和网状宏观裂缝型2类。②早期海进型和近滨海进型以半远洋沉积为主,粉砂和黏土混杂堆积,TOC较低;快速海进型以远洋沉积为主,微晶石英发育,TOC高,有机孔发育;晚期海进型以等深流沉积为主,TOC偏低,无机孔相对发育。③裂缝型甜点储层宏观裂缝和微裂缝发育,但基质孔隙度和渗透率不一定高。④早期海进型甜点分布于笔石带WF2—WF3,快速海进型甜点分布于笔石带LM1,晚期海进型甜点分布于笔石带LM2—LM4及LM5,近滨海进型甜点分布于笔石带LM1—LM4,网状微裂缝型甜点主要分布于笔石带LM1,网状宏观裂缝型甜点的分布受断层规模和构造转换带变形程度控制。⑤笔石带LM6—LM8黏土矿物含量高、成岩收缩缝发育的位置可形成网状微裂缝型甜点;构造转换带与调节带具有弱变形与弱改造特征,网状宏观裂缝发育,可形成网状宏观裂缝型甜点。结论认为,川南地区快速海进型和近滨海进型甜点勘探已取得重大突破,早期海进型和晚期海进型是下一步勘探的重要目标;以沉积学及裂缝特征为判识依据的甜点类型划分为页岩气深化勘探和效益开发提供了理论支撑。

The types,identification standards,and distribution rules of sweet spots of black shale directly affect the optimization of exploration intervals and the design of drilling targets.Therefore,taking the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation(Wufeng-Longmaxi)in southern Sichuan Basin as an example,according to the theories of petrology and sequence stratigraphy,the relationships between relative sea level changes or fracture types and shale reservoir quality were analyzed.The results are obtained in five aspects.First,the sweet spots of Wufeng-Longmaxi black shales can be divided into two categories:sedimentary and fractured.The sedimentary sweet spots are subdivided into four types:early transgression type,rapid transgression type,late transgression type,and nearshore transgression type.The fractured sweet spots are divided into two types:network microscopic fracture type and network macroscopic fracture type.Second,the sweet spots of early transgression and nearshore transgression types are dominated by hemipelagic sediments,mixed sediments of siltstone and clay,and TOC is lower.The sweet spots of rapid transgression type are mainly characterized by pelagic sediments,with well-developed microcrystal quartz,high TOC,and well-developed organic pores.The sweet spots of late transgressive type are dominated by contourites,with lower TOC and relatively developed inorganic pores.Third,the reservoirs of fractured sweet spots have well-developed macroscopic and microscopic fractures,but the matrix porosity and permeability may be lower.Fourth,the sweet spots of early transgressive type are distributed in the graptolite zones WF2-WF3,those of rapid transgressive type are distributed in the graptolite zone LM1,those of late transgressive type are distributed in the graptolite zones LM2-LM5,those of nearshore type are distributed in the graptolite zones LM1-LM4,and those of network microscopic fracture type are mainly distributed in the graptolite zone LM1.The distribution of network macroscopic fractured sweet spots is controlled by the scale of faults and the deformation degree of structural transition zones.Fifth,in the graptolite zones LM6-LM8,where clay mineral contents are high and diagenetic contraction joints are well developed,sweet spots of network microscopic fracture type can be formed.The structural transition zones and adjustment zones have the characteristics of weak deformation and weak transformation,with well-developed network macroscopic fractures,which can form sweet spots of network macroscopic fracture type.The conclusion is that significant breakthroughs have been made in the exploration of sweet spots of rapid transgression type and nearshore transgression type in southern Sichuan Basin,and those of early and late transgression types are important targets for future exploration.The sweet spot classification based on sedimentology and fracture characteristics provides technical support for the in-depth exploration and benefit development of shale gas.