Characteristics and Genetic Distribution Model of Top Calcareous Cementation Layers within Zhujiang Formation in Panyu A Oilfield, Pearl River Mouth Basin
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摘要: “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.Abstract: 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.
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图 1 珠江口盆地构造分区(a)和西江凹陷构造分区及研究区位置(b)
据彭光荣等(2013)和吴智平等(2015)修改.1.韩江凹陷;2.陆丰凹陷;3.惠州凹陷;4.西江凹陷;5.恩平凹陷;6.阳江凹陷;7.琼海凹陷;8.文昌凹陷;9.顺德凹陷;10.开平凹陷;11.白云凹陷;Ⅰ.海丰隆起;Ⅱ.惠陆低凸起;Ⅲ.阳江低凸起;Ⅳ.琼海凸起;Ⅴ.神狐‒暗沙隆起;Ⅵ.番禺低隆起;Ⅶ.东沙隆起;Ⅷ.云开低凸起
Fig. 1. Tectonic subdivision of Pearl River Mouth basin (a) and the Xijiang sag, South China Sea showing the location of study area (b)
图 4 顶钙镜下特征及主量元素特征
a.碎屑组分及胶结物XRF扫描元素特征,胶结物普遍表现含铁的特征,但含量分布不均,P⁃2井,1 987.60 m;b.铁方解石胶结物呈基底-孔隙式胶结充填在粒间,和生物碎屑伴生,P⁃2井,1 987.30 m;c.铁方解石胶结物和生物碎屑伴生,可见少量粉晶状白云石,AP⁃5井,2 325.05 m;d.铁方解石和生物碎屑背散射特征,AP⁃5井,2 325.05 m;e.铁方解石、生物碎屑和黄铁矿背散射特征,黄铁矿对生物碎屑进行交代,P⁃2井,1 987.65 m;f.铁方解石和生物碎屑阴极发光特征,铁方解石发暗橙色光,生物碎屑发亮橙色-暗橙色光,P⁃2井,1 987.50 m;g.铁方解石和生物碎屑阴极发光特征,铁方解石发暗橙色光,生物碎屑发亮橙色光,P⁃2井,1 987.65 m;h和i分别为胶结物和生物碎屑主量元素特征
Fig. 4. Microscopic characteristics and major elements of top calcareous cementation layers
图 7 顶钙碳酸盐胶结物沉淀温度计算图版
据Friedman and O’Neal(1977)计算公式
Fig. 7. Plot of calculated precipitation temperature of carbonate cements of top calcareous cementation layers
表 1 研究区珠江组顶钙组分含量统计
Table 1. Component percentage statistics of top calcareous cementation layers in the Zhujiang Formation
组分 最大含量(%) 最小含量(%) 平均含量(%) 石英 58.5 42.5 54.2 钾长石 11.2 6.5 7.8 斜长石 3.1 0 0.7 岩屑 7.5 2.5 3.9 生物碎屑 13.6 1.05 5.4 铁方解石 29.3 22.5 26.9 白云石 3.5 0 0.6 黄铁矿 3.8 0 0.5 -
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