川北地区下寒武统龙王庙组沉积相及与储层的关系

Sedimentary facies and the reservoir of the Lower Cambrian Longwangmiao Formation in northern Sichuan Basin, China

以四川盆地北部下寒武统龙王庙组为研究对象,利用野外露头、测井及岩心等资料,探讨了川北地区下寒武统龙王庙组的沉积相类型及特征,并分析了其对储层的影响。研究认为:川北地区在早寒武世龙王庙期的沉积环境为碳酸盐台地,发育潮坪、局限台地和开阔台地等3种相带,这些相带又进一步细分为混积潮坪、台内点滩、潟湖和滩间海等4种亚相。其中,混积潮坪亚相主要发育砂质白云岩坪、白云岩坪等,部分地区可见潮道;台内点滩亚相以砂屑白云岩最为常见,沉积厚度不大;潟湖亚相以白云质潟湖沉积和白云质灰质潟湖沉积为主,分布面积大;滩间海亚相沉积物以深灰色块状石灰岩为主,局部夹白云质豹斑灰岩。纵向上,大部分地区龙王庙组沉积由2次海退旋回构成,潟湖主要发育在单个沉积旋回的中下部;台内点滩主要发育在沉积旋回的中、上部,单个滩体厚度较薄;混积潮坪发育在旋回顶部,即旋回末期。平面上,由于受古陆影响,研究区自西向东具有物源供应减少、沉积水体逐渐变深,以及沉积相从潮坪—局限台地—开阔台地过渡的特征,台内点滩主要围绕局限潟湖分布。结合不同沉积相带储层物性的分析,认为台内点滩的颗粒白云岩储集性能最好,晶粒白云岩次之,膏盐岩最差。龙王庙组储层发育的最有利沉积环境为台内点滩。

Based on the analysis of field outcrops, well logging and core data, the sedimentary facies types and features of the Lower Cambrian Longwangmiao Formation in northern Sichuan Basin are discussed, and their influences on reservoirs are analyzed. During Early Cambrian Longwangmiao period, the sedimentary environment in northern Sichuan Basin was carbonate platform, which developed three types of facies including tidal flat, restricted platform and open platform, which were further subdivided into four types of subfacies: mixed tidal flat, intra-platform shoal, lagoon and interbank sea. The mixed tidal flat was mainly composed of sandy dolomite and dolomite, and tidal channels were visible in some areas. The intra-platform shoal was mainly composed of doloarenite with small deposition thickness. The lagoon mainly consisted of dolomite lagoon and dolomiticlimy lagoon with large distribution area. The deposition of interbank sea was mainly composed of dark gray massive limestone with intercalated dolomitic mottled limestone. The Longwangmiao period is composed of two regressive cycles in most areas,and lagoon mainly developed in the middle and lower part of a single sedimentary cycle, and intra-platform shoal mainly developed in the middle and upper part of a sedimentary cycle with thin single layer, and the mixed tidal flat mainly developed at the top of a sedimentary cycle. Due to the influence of paleocontinents, the sediment source supply decreased and the water depth gradually deepened, and the sedimentary facies translated from tidal flat to restricted platform to open platform successively from west to east in the study area. The intra-platform shoal mainly distributed around the restricted lagoon. Combined with the analysis of reservoir property of different sedimentary faces, it is concluded that grain dolomite depositing on the intraplatform shoal have the best reservoir property, followed by the crystalline dolomite, and the gypsum was the worst. Therefore,the most favorable sedimentary environment for reservoir development in Longwangmiao Formation is intra-platform shoal.