珠江口盆地番禺A油田珠江组“顶钙”发育特征、成因与分布模式

Characteristics and Genetic Distribution Model of Top Calcareous Cementation Layers within Zhujiang Formation in Panyu A Oilfield, Pearl River Mouth Basin

"顶钙"是砂岩储层中常见的一种碳酸盐胶结类型,其形成机理存在分歧,阻碍了对其分布预测,进而制约油田勘探开发.综合利用常规测井、岩心、XRF扫描、铸体薄片、扫描电镜、阴极发光、电子探针、碳氧稳定同位素等资料,对珠江口盆地番禺A油田珠江组"顶钙"的测井响应、岩矿学特征、物质来源及成因、分布规律、成因分布模式进行系统研究.研究区顶钙岩性主要为含砾砂岩和中粗砂岩,钙质成分主要为铁方解石,呈基底-孔隙式胶结,常与生物碎屑伴生,计算其沉淀温度在55.9~72℃;顶钙厚度主要集中在0.4~1.2 m,但不同油层其厚度及顶钙发育率差异较大.研究区顶钙形成于早成岩阶段,主要来自于内源的生物碎屑,其分布主要受到沉积微相和高频层序界面的控制,河口坝为其提供了重要物质来源,其相互叠置可促使顶钙连片和增厚;而高频海泛面可通过延长沉积物保留时间进一步促进顶钙的发育.研究区顶钙成因与分布主要受高能粗粒生物碎屑富集程度和分布的控制,因此通过对层序和沉积作用的分析,可对其分布进行预测.

Top calcareous cementation layers(TCCLs) are common type of carbonate cement that caps reservoir sandstone units.There is no consensus on the genetic mechanism of TCCLs at present and thus hindering our understanding of its distribution prediction and restricting the oilfield exploration and development. In this paper a suite of analytical techniques were employed to investigate the genesis and distribution of TCCLs within the Zhujiang Formation in the Panyu A oilfield, Pearl River Mouth basin,including core and log analysis, XRF scanning, casting thin section, scanning electron microscopy, cathodoluminescence, electron microprobe, stable carbon and oxygen isotopes. This enabled us to systematically study the log responses, petrologic characteristics,material source and genesis, distribution patterns, and genetic distribution model of TCCLs. TCCLs in the study area comprise mainly gravel-bearing sandstone and medium-coarse sandstone, and the carbonate cements consist mostly of ferrocalcite in basement-pore contacts accompanied by biological debris, and its precipitation temperature is between 55.9 and 72 ℃. The thickness of TCCLs is mainly in the range of 0.4-1.2 m, but there exist big differences between the thickness and development percentage of TCCLs among different oil-bearing reservoir beds. The analysis indicates that TCCLs were formed in the early diagenetic stage and their material sources are mainly composed of endogenous biological debris. The distribution of TCCLs is mainly controlled by sedimentary microfacies and high-frequency sequence stratigraphy surfaces. The estuary bars provide an important material source, and the superposition of each other contributes to the contiguous thickening of TCCLs, while the high-frequency marine-flooding surface can further promote the development of TCCLs by extending the retention time of sediments. The genesis and distribution of TCCLs in the study area are primarily controlled by the enrichment degree and distribution of bio-debris. Therefore, the spatial distribution of TCCLs can be predicted through the analysis of high-frequency sequence stratigraphy and detailed microfacies characterization.