威远地区龙马溪组高频层序地层划分与纵向优质页岩段探讨——以龙一^(1)亚段上部气层为例

High-frequency sequence stratigraphic division and vertical quality shale intervals,Longmaxi Formation,Weiyuan area:An example from the upper Long1^(1) submember

四川盆地威远页岩气田下志留统龙马溪组下部地层海相黑色页岩广泛发育,已实现了规模效益开发。但是随着气田投产年限增长,部分气井产量递减明显,给气田稳产带来较大压力,迫切需要开辟新的勘探开发层系和领域。以威远地区龙一^(1)亚段上部地层为研究对象,综合利用野外剖面、岩心、岩石薄片、古生物化石、测井、地球化学资料,以岩石地层学、化学地层学和生物地层学理论为指导对目的层进行层序界面的识别和层序的划分。建立了区内龙一^(1)亚段上部地层的层序格架,并探讨层序格架内有机质富集和储层品质的特征。结果表明:①龙一^(1)亚段上部地层经历了3次中小规模的海侵—海退旋回,划分出3个四级层序和6个体系域;②层序格架对有机质富集和储层品质的控制作用,主要体现在层序Pss7较层序Pss8水体更深,且海侵规模更大,尤其在上升半旋回内TOC和孔隙度值明显优于Pss8,层序Pss7之后海侵规模减小,Pss8-Pss9水体逐渐变浅,TOC和孔隙度变差,③层序Pss7—Pss8海侵期—海退早期为上部地层优质页岩储层发育期,是有机质富集的优势层位,也是立体开发的重点开发单元。

There widely exists marine black shale in the Lower Silurian Longmaxi Formation,Weiyuan shale gas field,Sichuan Basin,which has been developed extensively and effectively.Shale gas production,however,is in decline as this field has put into production for many years,bringing tremendous pressure to its stable production.It is urgent to open up new exploration and development strata 1 or domains.So,taken the upper Long^(1) submember in Weiyuan shale gas field as an example,its sequence boundary was identified to divide stratigraphic sequence through outcrop survey,core analysis,thin-section identification,and paleontological fossil,as well as well logging and geochemical data and according to some theories of lithostratigraphy,chemical stratigraphy,and biostratigraphy.Moreover,the sequence framework was established for this submember.Finally,enrichment characteristics were summed up for organ-1 ic matter,and reservoir quality within this framework was discussed.Results show that(i)the upper Long^(1) submember experienced three transgressive to regressive cycles in small to medium scale,and can be divided into three fourth-order sequences and six system tracts;(ii)the sequence framework affecting organic-matter enrichment and reservoir quality mostly gives expression to deeper water-body and larger transgressive scale in Pss7 sequence compared to Pss8 one,in particular significantly better TOC and porosity within the rising half-cycle for the former.After Pss7,this scale decreases,and waterbody from Pss8 to Pss9 gradually becomes shallower,creating TOC and porosity getting worse;and(iii)the period from transgression to early regression,also corresponding to Pss7-Pss8 sequences,represents the development of quality shale reservoirs of the upper submember,in which is prevalent for organic-matter en-richment and can be conducted as the key unit for 3D development.