Seismic response characteristics and reservoir distribution prediction of karst reservoirs in the Yingshan Formation of the Lunguxi block
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摘要: 碳酸盐岩岩溶储层的定性预测是岩溶地震解释技术的关键之一。针对轮古西区块奥陶系鹰山组岩溶储层地震预测难点，通过提取轮古西区块奥陶系鹰山组碳酸盐岩岩溶储层地震响应特征，基于均方根振幅、瞬时能量、微分振幅、最大曲率、瞬时振幅和方向加权增强相干等多地震属性对比分析研究，探讨适合该区块碳酸盐岩岩溶储层准确预测的方法。结果表明：鹰山组碳酸盐岩岩溶储层可划分为表层岩溶储层、暗河岩溶储层和断控岩溶储层；表层岩溶储层地震特征为以杂乱强反射为主，一般位于残丘高部位，在古潜山风化面0~40 m以内分布；暗河岩溶地震特征主要为地震剖面连续强反射，阻抗属性为低值异常体，横向展布范围较大，有典型的河道特征；断控岩溶地震特征为沿断裂展布的串珠反射、杂乱反射，纵向延伸大，平面沿断裂线性展布。开展轮古西区块奥陶系鹰山组岩溶储层地震响应特征研究，基于地震属性特征对表层岩溶、暗河岩溶和断控岩溶分布进行半定量预测，结果可为厘清奥陶系鹰山组优质储层和油气分布规律提供一定支持。Abstract:
Globally, 279 carbonate oil and gas reservoirs have been discovered across 39 countries and regions. Notably, large marine carbonate oil and gas reservoirs are primarily distributed in China, mainly featuring lithological-stratigraphic traps. The buried depth of oil and gas reservoirs is generally greater than 5,500 m, and they are mainly concentrated in large oil-and-gas-bearing basins, such as the Tarim Basin, Sichuan Basin, Ordos Basin, etc. The rapid development of China’s petroleum industry has resulted in a growing volume of crude oil imports each year. The increasingly complex international landscape has posed challenges to the stability of China’s petroleum supply. The exploration and development of carbonate karst oil and gas reservoirs is one of the ways to ensure a stable energy supply. The relevant areas have been transforming from strategic oil reserve zones to regions for oil resource replacement. Therefore, accurate prediction and precise evaluation of carbonate karst reservoirs are increasingly receiving attention from petroleum geologists. Lunnan ancient buried hill is located in the central area of the Lunnan low uplift within the Tabei uplift of the Tarim Basin, which is the main part of the Lunnan paleo-uplift in the Tarim Basin. The Lunguxi block is located in the west of the Lungu oil and gas field. In the Lunguxi block, especially in its northern area, many wells have been drilled; however, their exploitation has been inefficient and unsuccessful. The low drilling rate of the target layer may be the main reason for these phenomena. Therefore, it is urgent to address the challenges of accurate prediction and precise evaluation of carbonate karst reservoirs. According to the characteristics of karst fracture-cavity development and the karst hydrodynamic conditions, and in conjunction with modern karst geological theory, carbonate karst reservoirs can be classified into three main types, surface karst reservoir, underground river karst reservoir, and fault-controlled karst reservoir. High-angle structural fractures, dissolution pores, and small caves are mainly developed in surface karst reservoirs. The dissolution pores and fractures are primarily connected vertically, with the potential for lateral interconnection over short distances along the fractures. The fractures and small-scale caves are distributed in a planar arrangement and exhibit a strong capacity for adjustment. Underground river systems of karst reservoirs, including underground rivers, sinkholes, vertical shafts, skylights, hall-type and corridor-type caves, underground river outlets and inlets, etc., exhibit good connectivity along the direction of groundwater runoff. Existing wells for exploitation show that the underground river karst reservoirs are the main reservoir type for oil and gas production, and are also one of the most important reservoir types in the study area. The development of fault-controlled karst reservoirs is influenced by fault activities. These reservoirs are formed by the dissolution and transformation of diagenetic fluids along fault surfaces or structural fractures, resulting in a collection of fracture-cavity complexes. They are developed vertically along fault zones, with a large range of depths, good vertical connectivity, strong lateral heterogeneity, mainly characterized by dissolution fracture-cavities. The carbonate reservoirs in the Ordovician Yinshan Formation of the Lunguxi block exhibit a complex and diverse pore system, with multiple coexisting and interacting geological factors, resulting in strong reservoir heterogeneity. The seismic reflection anomaly of the small fracture-cavity is weak or not easily discernible, exhibiting high concealment, which complicates identification. The development characteristics of low-order faults within faults and ancient river channels, as well as their relationship with the small fracture-cavities, are challenging to quantify. Therefore, based on previous exploration and development data, this study aims to clarify the reservoir types in the study area and subsequently elucidate the seismic response characteristics of these reservoirs. The results show,(1)The seismic characteristics of surface karst primarily exhibit chaotic strong reflections, typically found in the high part of the residual hills and distributed within 0 to 40 meters of the weathering surface of the ancient buried hills. In contrast, the seismic characteristics of underground river karst are characterized by continuous strong reflections in the seismic profile, with the impedance attribute presenting as a low-value anomaly. This anomaly has a broad lateral distribution range and displays typical river channel characteristics. Additionally, the fault-controlled seismic characteristics are predominantly represented by beaded and chaotic reflections that are distributed along the fault. These reflections exhibit large longitudinal extension and linear distribution patterns along the fault line in a plane view. (2)The seismic response prediction of karst reservoirs in the study area has been conducted from various scales and directions. The research findings indicate that the combination of amplitude attributes and AFE attributes can well characterize the surface karst reservoirs developed in the high part of the buried hill. Additionally, attributes such as differential amplitude and root mean square can systematically identify underground river karst reservoirs. Furthermore, spectrum decomposition discontinuity detection can elucidate the internal details of the fault fracture zone, providing an optimal approach for predicting fault-controlled karst reservoirs based on seismic response. (3)The surface karst is generally developed in the Lunguxi block, particularly in the structurally elevated parts. The fault-controlled karst reservoir is mainly developed in the area of Lungu Wells 9–40. In the northwest and central regions, the thickness of surface karst in karst valleys and slopes is relatively small, whereas in the southeast region, the thickness of surface karst on the karst platforms is substantial. The identification of fault-controlled karst reservoirs can be accomplished through the integration of amplitude attributes and structural tensor attributes. -
图 1 塔里木盆地轮古西区块构造纲要图
Figure 1. Structural outline of the Lunguxi block in the Tarim Basin
图 2 LG9C井周原始地震剖面和测井解释综合柱状图
Figure 2. Original seismic section around Well LG9C well and its comprehensive logging interpretation histogram
图 4 表层岩溶储层地震响应特征
Figure 4. Seismic response characteristics of surface karst reservoirs
图 5 LG903钻井钻遇表层岩溶与地震剖面对比图
Figure 5. Comparison between the surface karst drilled in Well LG903 and its seismic profile
图 6 顺暗河管道岩溶储层地震响应特征
Figure 6. Seismic response characteristics of karst reservoirs along underground river conduits
图 7 轮古西区块奥陶系灰岩顶—鹰二段微分振幅平面图
Figure 7. Differential amplitude plan from the top of Ordovician limestone to the second member of Yinghe in the Lunguxi block
图 8 轮古西区块断控岩溶储层地震多属性响应特征
Figure 8. Multi-attribute seismic response characteristics of fault-controlled karst reservoirs in the Lunguxi block
图 9 轮古西区块奥陶系灰岩顶面AFE+振幅和属性（0-40 m）图
Figure 9. AFE+ amplitudes and properties (0-40 m) of the top face of Ordovician limestone in the Lunguxi block
图 10 轮古西区块奥陶系（鹰二段-鹰山底）结构张量+（40 m-鹰二底）均方根振幅平面图
Figure 10. Plan of structure tensor and root-mean-square amplitude of the Ordovician (Ying II–Yingshan Bottom) in the Lunguxi block
表 1 碳酸盐溶蚀孔隙、溶洞、裂缝级别划分表（据塔里木油田公司，2012）
Table 1. Classification of carbonate dissolution pores, caves and fractures (according to Tarim Oilfield, 2012)
溶蚀孔隙 溶洞 裂缝 类型 孔径/mm 类型 洞径/mm 类型 缝宽/mm 大孔 0.50~2.00 巨洞 > 1000 大缝 >10 中孔 0.25~0.50 大洞 500~ 1000 中缝 1~10 小孔 0.01~0.25 中洞 20~500 小缝 0.1~3 微孔 <0.01 小洞 2~20 微缝 <0.1 
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表 2 轮古西区块奥陶系鹰山组岩溶储层地震响应特征表
Table 2. Seismic response characteristics of karst reservior of Ordovician Yingshan Formationin the Lunguxi block
储层类型 表层岩溶型 暗河岩溶型 断控岩溶型 主控因素 顶部储层表层岩溶作用为主 暗河岩溶作用为主 断控岩溶作用为主 典型井 LG9C LG15-3 LG9-H8 储层剖面 
地震剖面 
平面特征 
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