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Karst Breccias and Pseudobreccias in the Ordovician Carbonates in the Halahatang Area, Tarim Basin, Northwestern China


The Ordovician carbonate reservoirs in the Halahatang area (northern Tarim Basin) are highly productive. Previous studies suggested that the Yingshan, Yijianfang, and Lianglitage Formations were extensively karstified; however, no significant karst features were observed in the Yingshan and Yijianfang Formations based on current detailed study of 380 m cores from 16 wells. True breccias are only present in the Lianglitage Formation in two wells, including crackle and mosaic breccias as well as chaotic, clast-supported breccias. The breccias are interpreted as karst breccias, which formed in near surface environments during pre-Silurian subaerial exposures. Most “Karst features” identified in the cores by many previous studies are actually mottled fabric (pseudobreccias) caused by selectively diagenetic alteration and infiltration of bitumen in the deeper subsurface. The color difference between pseudoclasts and pseudomatrix does not reflects a lithological contras, but instead is characterized by distinct, often subtle, variations in the limestone fabric and whether bitumen is present or not. The objectives of this investigation are to (1) characterize paleokarst breccias and diagenetic pseudobreccias, (2) interpret the origin of paleokarst breccias, and (3) differentiate paleokarst breccias from pseudobreccias. One the basis of core and thin section observation, SEM (scanning electron microscope) and XRD (X-ray diffraction) analyses, this study proposes a series of criteria to distinguish between karst breccias and diagenetic pseudobreccias, which can be applied to other similar carbonate deposits. This study concludes that pseudobreccias concurrently occurred or postdated early stylolitization in burial settings, and the occurrence and quality of carbonate reservoirs in the Yijianfang Formation in the study area might not be related to epigenetic karstification.