同生-准同生期大气淡水溶蚀对微生物碳酸盐岩储层的控制作用:以塔里木盆地下寒武统为例

Meteoric water dissolution controls on microbial carbonate reservoir formation in the penecontemporaneous stage:Insight from the Lower Cambrian formation of the Tarim Basin

微生物碳酸盐岩是中国震旦—寒武纪古老地层的重要组成部分,勘探已经证实微生物碳酸盐岩可以形成储层,明确这类储层成因并进行有效预测对于古老碳酸盐岩勘探有重要意义。然而针对塔里木盆地寒武系微生物碳酸盐岩储层成因,尚未形成统一的认识,不同学者提出了诸如同生准同生期溶蚀、埋藏/热液溶蚀或多种流体综合作用等成因解释。本次研究选择塔里木盆地西北缘什艾日克和肖尔布拉克剖面寒武系肖尔布拉克组为重点研究对象,通过露头和薄片观察、阴极发光和碳氧同位素分析等手段,研究了早成岩期大气淡水溶蚀作用对微生物碳酸盐岩储层形成的影响。结果表明:(1)研究区寒武系肖尔布拉克组识别出4种微生物碳酸盐岩岩石类型,分别是叠层石白云岩、凝块石白云岩、泡沫绵层白云岩和与蓝细菌相关的(含砾)颗粒白云岩;微生物碳酸盐岩建造具有复杂的孔隙系统,常见的孔隙类型包括晶间孔、溶孔、不同尺度的溶洞以及裂缝等,但是溶蚀孔洞是最主要的孔隙类型。(2)同生准同生期大气淡水溶蚀是孔隙形成的主要原因,有三方面证据。一是微生物碳酸盐岩建造顶部发育小型溶蚀坑。二是新月形胶结物和胶结不整合现象表明经历了短期暴露。新月形胶结物通常被看作是渗流带成岩作用的产物。胶结物不整合是指在颗粒与埋藏期形成的粒状胶结物之间部分缺失了纤状等厚环边胶结物,这些缺失的纤状胶结物可能是准同生期被大气淡水溶解了。三是胶结物具有斑状中等亮度阴极发光特征,明显有别于表生岩溶与埋藏环境胶结物的阴极发光特征;此外大气淡水是导致微生物丘顶部样品氧同位素值较原岩负漂的原因。(3)储层具有非均质性,分布受微生物丘沉积结构和相对海平面变化控制。微生物丘可以分为丘基、丘核和丘盖三部分。其中丘基主要由凝块石白云岩组成,丘核以泡沫绵层白云岩为主,丘盖主体是含砾颗粒白云岩。储层物性丘核最好,其孔隙度平均为5.47%;丘盖次之,为4.27%;丘基较差,为2.01%。微生物建造岩石组成差异是造成储层非均质性的主要原因。

Microbial carbonates are an important part of Sinian and Cambrian formations in China.They are a significant reservoir type,and it is of great significance for carbonate propecting to understand their genesis and reservoir forming mechanisms.However,the formation mechanism of the Cambrian microbial carbonate reservoir in the Tarim Basin is still controversial with various explanations proposed,such as simultaneous and penecontemporaneous dissolution,burial hydrothermal dissolution or multiple fluid interactions,and so on.In this study,we investigated the Xiaoerbulake Formation of the Shiairike and Xiaoerbulake sections in the northwest margin of the Tarim Basin.We analyzed the effect of fresh water on the formation of microbial carbonate reservoir in the early diagenetic period through means of outcrop observation,thin section identification,cathode luminescence,carbon/oxygen isotope analysis and others.The results show that(1)four microbialite facies can be identified in the Xiaoerbulake Formation:stromatolite dolostone,thrombolite dolostone,foam spongy dolostone and microbial-related dolograinstones.The pore system in microbial carbonate rock is complex and the pore types in microbial mounds include intergranular pores,dissolution pores,various scale dissolution vugs/caves and fractures,among which the dissolved pores and vugs/caves are the main pore types.(2)Short-term subaerial exposure and penecontemporaneous dissolution of carbonates by atmospheric fresh water predominantly contributed to pore formation as evidenced by three observations.Firstly,there are some small-scale tracks formed by dissolution on the surface of microbial buildups.Secondly,meniscus cement and unconformity are found in slices,indicating short-term exposure.Meniscus calcite cement is a sign of vadose freshwater diagenesis;and unconformity means a partial absence of fibrous cementing materials between grains and drusy cement that generally formed in burial environment.Thirdly,luminescence of the cement is mottled medium bright,distinctive from that of cement formed in meteoric or burial environment as bright and orange-red or dark red rings in general.In addition,affected by atmospheric fresh water,carbon and oxygen isotopes in the outcrops are slightly lower than in seawater.(3)The microbial carbonate reservoir is heterogeneous and its distribution was controlled by the sedimentary architecture of microbial mounds and sea level fluctuations.It is clear that thrombolite dolostone,foam spongy dolostone and cyanobacteria related dolo-grainstone are developed successively from bottom to top,constituting a complete sedimentary assemblage of mound base,core and cap,respectively.The porosity of the mound core(average 5.47%)is much favorable than that of mound cap(average 3.51%)and mound base(average 2.01%).Difference in rock types in microbial mounds is the main cause of disparity in reservoir quality.