Early-diagenetic Carbonate Nucleation Cementation in Cenozoic Sandstones,Northern Tarim Basin：Evidences from High-resolution Electric Borehole Images
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摘要: 早期碳酸盐胶结作用是库车坳陷新生界浅埋藏河流相砂岩内除机械压实外目前正在进行的主要成岩作用,高分辨率电阻率成像测井资料为研究其特征及成因提供了重要的地球物理证据。在电阻率成像测井图像上,早期碳酸盐胶结物呈圆形、椭圆形或不规则斑块状、葡萄状等形态出现,多数顺层理方向分布,其产状有孤立状或分散状斑块、断续胶结纹层和连续胶结纹层等。它们可以发育于砂岩层顶部或底部,也可以与疏松未胶结或弱胶结砂岩呈互层或夹层状交替分布。早期碳酸盐胶结物的沉淀采取核心式胶结模式,即胶结物首先围绕少数核心沉淀,呈斑块状逐渐向外增生,直至从各个核心生长出来的碳酸盐胶结斑块相互靠近和连生导致砂岩层被完全胶结为止。砂体或砂层被碳酸盐胶结的过程遵循由外向内 (由砂体边缘向砂体中心 )、由高渗透纹层向低渗透纹层、由局部胶结核心向周围孔隙增生的规律.Abstract: Carbonate ccmcntation is the main diagcnctic process except for compaction in shallow-burial(shallower than 700 metcrs) Ncogcnc-Quatcrnarv fluvial sandstoncs of northern Tarim Basin.High-resolution electric borcholc images provide valuable information on the gcomctrv and occurrence of carbonate concretions in these sandstoncs. Carbonate concretions in the sandstoncs are characterized with white high-resistive features in both static and dynamic images.The shape of these concretions is round clliptoid or irregular. They may occur as isolate patches scattered in poorcemented sandstones, or appear in group as spiccato or continuous carbonate-cemented laminae intcrbcddcd with poor-cemcntcd sandstone laminae. Strong selectivity of carbonate ccmcntation at different scales is observed on the images of these sandstoncs.On scale of sandstone beds, carbonate concretions preferentially developed at the upper and/or lower parts, where ion materials needed for carbonate cement precipitation are believed to be more easily available from adjacent clay sediments.On scale of beddings,carbonate concretions usually aligned parallel to bedding surfaces.In most cases, they intcrbcddcd with poor-ccmcnted sandstone laminae as spiccato or continuous carbonate-cemented laminae which may be resulted from the texture and hence permeability differences between laminae.On laminae scale, carbonate cements selectively precipitate around some nuclei then enlarge outwards as cemented patches.when the patches grow closer or merge together spiccato or continuous cemented laminae form. fragments, which are the The selectivity of ccmcntation some relation to the,listrihution of elastic carbonate common gram components in these sandstones. Based on interpertation of electric images,this paper concluded a growth of early-diagenetic carbonate cements in these sandstoncs.They were developed following the sequences from the top and/or the middle of the sandstone beds, from high-permeable laminae to low-permeable laminae, and from some local nuclei to their vicinity.
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Key words:
- sandstone /
- cementation /
- carbonate mineral /
- electric borehole imaging /
- Tarim basin
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