Structural Features of Weathering Crust of Granitic Basement Rock and Its Petroleum Geological Significance: A Case Study of Basement Weathering Crust of Dongping Area in Qaidam Basin
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摘要: 近年来，不少学者开始关注基岩油藏，但有关基岩风化壳的研究尚未有报道.以柴达木盆地东坪地区基岩风化壳为研究对象，结合东坪地区的地质资料、测井资料及地球化学资料，对花岗质基岩风化壳的结构特征及其与油气储集的关系进行了系统的研究.结果表明：(1) 建立了以Al2O3、CaO、Na2O、K2O为主要判别依据的花岗质基岩风化指数，准确的识别出古土壤层、残积层、半风化层、未风化层；(2) 根据成像测井响应特征及常规测井曲线可以很好地识别古风化壳特征；(3) 东坪地区基岩风化壳的主要储集空间有基岩溶蚀孔洞、基质矿物溶孔和基岩裂缝，共3种，为致密储层，风化壳由风化基面向上，孔隙逐渐发育，至半风化岩石中部的孔隙度较大，且以大孔隙为主，由半风化岩石到风化粘土层，孔隙的规模变小，逐渐以微孔隙为主，纵向上，孔隙度与风化程度、构造缝发育情况、埋藏深度及其和应力释放中心的距离具有相关性.风化壳的存在大大的改善了储层物性，扩展了含气面积，增加了含气层系，对花岗质基岩地层油气藏和深层油气勘探具有重要的理论价值和科学意义.Abstract: In recent years, many scholars have begun to pay attention to the basement reservoir, but there is no report about the study of weathering crust of basement. In this paper, structural characteristics of bedrock weathering crust and its relationship with oil and gas accumulate are systematically studied in Dongping area in the Qaidam Basin, combining with the geological data, logging and geochemical data. It is found that establishment of granitic bedrock weathering indexes based on Al2O3, CaO, Na2O, and K2O can ensure accurate identification of the ancient soil layer, elurium, semi-weathered layer, and unweathered layer. According to the imaging logging response characteristics and logging curves can well identify paleocrust characteristics, there are three types of tight reservoir space in weathering crusts of Dongping area: rock erosion holes, matrix mineral dissolved pores, and cracks in bedrock. Weathering crust on the base up gradually develops porosity, large semi-weathered rock porosity to the middle, and large pores mainly from semi-weathered rock to weathering exclusion soil, the pore size becomes smaller, and gradually to micro pore-based. In the longitudinal direction, porosity and degree of weathering and tectonic development, the burial depth and distance from the stress relief center have some relevance. The presence of weathering crust, greatly improves the reservoir properties, extends the bearing area, increases the gas-bearing system, which has important theoretical values and scientific significance to the granitic bedrock stratigraphic reservoirs and deep oil and gas exploration.
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Key words:
- weathering crust /
- weathering crust structure /
- weathering index /
- petroleum geology
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图 4 东坪103井ECS元素及基岩风化指数特征
Fig. 4. ECS elements and bedrock weathering index feature of Dongping 103 wells
图 8 基岩风化壳理论结构模型及纵向结构剖面对比
Fig. 8. Theory and structure model of weathering crust and comparison of profile the vertical structure
图 12 东坪地区基岩岩性及储集空间类型
a.东坪H301，1 880.25 m，花岗岩，发育粒间孔，(-)100×；b.东坪H301，1 880.25 m，粒内缝、粒内孔、铸体孔，(-)100×；c.东坪H301，1 880.5 m，花岗岩，主要矿物石英、长石、黑云母次之，裂缝发育；d.东坪H301，1 880.5 m，高角度缝、水平缝，两组；e.东坪103井，3 153.82 m，片麻岩，(+)100×；f.东坪H301井，1 880.25 m，裂缝花岗岩，矿物成分为石英、长石、黑云母，(-)100×；g.东坪H101，3 211.99 m，裂缝花岗岩，矿物成分为英、长石，×100；h.东坪103，3 230.36 m，风化溶孔
Fig. 12. Bedrock lithology and reservoir space of Dongping area
图 13 东坪地区下干柴沟组E32储层孔隙结构分类
Fig. 13. Under Ganchaigou E32 pore structure classification of Dongping area
图 14 坪地区下干柴沟组E32储层孔隙结构分类
Fig. 14. Under Ganchaigou E32 pore structure classification of Dongping area
图 17 东坪3井区纵向⑷和平面(b)孔隙度变化
Fig. 17. Longitudinal porosity change (a) and plane porosity comparison (b) of Dongping Well-3
图 18 东坪1井区纵向含气性分布规律
Fig. 18. Longitudinal containing gas distribution of Dongping well-1
图 19 东坪地区气藏含气性分布规律
Fig. 19. Distribution of containing gas in Dongping regional gas reservoirs
图 20 东坪4-东坪104-东坪1-东坪103基岩风化壳气藏剖面
Fig. 20. Gas reservoir profile of bedrock weathering crust of Dongping well-4-Dongping104-Dongping1-Dongping well-103
表 1 基岩纵向结构在地质、测井上的特征
Table 1. Geological and well logging features of vertical structure on bedrock
纵向结构 地质特征 常规曲线特征 电成像特征 阵列声波特征 古土壤层 岩石结构完全破坏，遇水崩解，分化蚀变，次生矿物发育 GR高值、DEN及CNL中低值、电阻率低值 受粘土含量高的影响，成像整体呈暗色 纵波时差大，岩石抗压强度和弹性模量较小 残积层 岩石结构大部分破坏，岩石破碎 电阻率中高值 成像图颜色较暗，存在微裂缝，裂缝间距大 纵波时差小，有一定的各向异性 半风化层 岩石结构完整，风化裂缝较发育 电阻率高值、CNL低值、DEN高值、井眼扩径 裂缝发育，主要集中在中上部，裂缝间距小 纵波时差小，裂缝发育层段斯通利波衰减强烈 
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表 2 基岩风化壳纵向结构带的DRW划分标准
Table 2. DRW division standard weathering crust vertical structure
基岩纵向结构带 风化指数DRW 古土壤层 ＞0.55 残积层 0.37~0.55 半风化层 0.10~0.37 未风化层 ＜0.10
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