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Diagenetic product distributions and origins and their controls on reservoir quality in the Triassic Yanchang Formation reservoir, Ordos basin, China

 

JIANG Hong Xia¹ WU Ya Sheng¹ LUO Xiao Rong¹ YU Jian² MAO Ming Lu² YANG Yang²

 

 

1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

2. Exploration Institute of Changqing Oil Company, X’ian 710004, China

 

 

Abstract  The Bed Chang-8 of the Triassic Yanchang Formation in the Ordos Basin, west China, is a main oil reservoir composed of deltaic sandstone. Petrographic analysis has been used to assess the types of main mineral cements, their spatial distribution, and their controls on reservoir quality. The most important diagenesis included compaction, quartz overgrowths, the development of abundant grain-coating authigenic chlortie, secondary kaolinite occurring in discrete patches, copious ferroan and non-ferroan calcite cement, especially poikilotopic ferroan calcite, the dissolution of matrix and detrital grain edges with the derived secondary pores generally filled by asphaltenes, and dissolution of feldspar creating cleaning secondary pores. Abundance of quartz overgrowths was described as four orders: 0 meaning no overgrowth, 1 meaning narrow overgrowths, 2 meaning overgrowths completely filling pores in places, 3 meaning overgrowths completely filling pores in almost everywhere. Point-count content of the pores with grain-coating chlorite, and those of diagenetic calcite, secondary kaolinite, and feldspar dissolution pores were obtained during petrographic analysis. These data were shown in isographs using Sufer software. The similar distribution patterns of quartz overgrowths and diagnetic calcite indicate they are related in origin. Analysis shows their origins were related to alternation of K-feldspar to kaolinite and illite. Comparison of diagenetic product distributions to total point-count porosity indicates that besides compaction,  both quartz overgrowths and diagenetic calcite are the most important two detrimental controls to the reservoir quality, while grain-coating authigenic chlorite and feldspar dissolution are the most important two factors raising the reservoir quality.

 

Keywords: Changqing oilfield; Ordos; Yanchang Formation; diagenesis

 

 

 

In Chinese:

 

鄂尔多斯盆地中南部三叠纪储层的成岩产物分布、

成因及其对储层物性的控制作用

 

 

姜红霞¹ 吴亚生¹ 罗晓容¹ 喻建² 毛明陆² 杨飏²

 

 

 

1. 中国科学院地质与地球物理研究所，北京 100029

2. 长庆油田公司勘探开发研究院；西安710004

 

 

摘　要  鄂尔多斯盆地中南部三叠纪延长组长8段储层的主要成岩作用包括压实作用、石英次生加大、云母和杂基等的次生高岭石化、自生绿泥石膜在原生孔隙壁上的生长、连晶方解石交代粒间物质和颗粒边缘、颗粒边缘和粒间物质的无选择溶蚀和沥青对这种次生溶孔的充填、长石溶蚀。根据铸体薄片下的统计，确定了各种主要成岩产物的空间分布、成因。石英次生加大与方解石分布格局的相似，说明它们在成因上有关联。综合分析认为石英次生加大和连晶方解石的形成可能与钾长石的粘土化作用有关。分析了成岩产物分布与现今总面孔率的关系 ，从而确定长8段储层的物性主要受石英次生加大、连晶方解石、长石溶孔、具自生绿泥石膜的剩余原生孔隙分布的控制。石英次生加大和连晶方解石发育的地方，储层物性就差；具自生绿泥石膜的剩余原生孔隙和长石溶孔发育的地方，储层物性就好。

 

 

关键词　鄂尔多斯盆地; 三叠纪; 延长组; 成岩作用; 储层;长庆油田

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