The origin and distribution of thick calcareous cementation of ultra-deep sandstone under the unconformity: A case study of the Qigu Formation in Yongjin area of Junggar Basin

Key words: 
• Zhungharian Basin /
• Yongjin area /
• Jurassic system /
• Qigu formation /
• thick calcareous cement /
• high-quality reservoir

Abstract: 【Objective】 Thick layers (8~30 m) of calcareous cement developed in deep sandstones (current burial depth 5500~6000 m) below the unconformity in the Yongjin area of the Junggar Basin show a significant spatial correlation with hydrocarbon distribution. However, their formation mechanisms and distribution patterns remain unclear, particularly lacking systematic understanding of the contribution of eogenetic diagenesis to calcareous cementation below the unconformity.【Methods】 Utilizing integrated analytical techniques including reservoir cast thin sections, fluid inclusion analysis, and cathodoluminescence microscopy, the stages of calcareous cementation were systematically classified. Physical simulation experiments were designed to analyze the genetic mechanisms and distribution patterns of eogenetic calcareous cements.【Results】 Three types of calcareous cement are developed within the Qigu Formation: Cement Type I: Dominated by poikilotopic cement of silt-sized calcite aggregates, exhibiting orange-red cathodoluminescence color. This CL color is consistent with that of micritic calcite within calcareous nodules in the paleosol layers of the Qigu Formation. Cement Type II: Primarily composed of pore-filling calcite, with fluid inclusion homogenization temperatures of 80~100°C. Cement Type III: Consists mainly of granular dolomite, incompletely filling pores, with fluid inclusion homogenization temperatures generally >100°C, formed subsequent to large-scale hydrocarbon charging. Cast thin section and X-ray diffraction analyses indicate that Calcareous Cements Type I and Type II are predominantly developed in the down-dip structural direction and in the middle-lower parts of sand bodies, exerting a significant impact on reservoir quality. Calcareous Cement Type III is developed in various locations but exhibits generally low abundance, exerting a weak impact on reservoir quality. The total calcareous cement content is low proximal to the denudation front and high distal to it. Physical simulation experiments demonstrate that eogenetic calcareous cement is generally located in the lower parts of sand bodies and in the down-dip direction of dipping strata. Concurrently, low-permeability fine-grained beds inhibit calcareous cement precipitation in underlying coarse-grained sand layers, consistent with observations in the study area.【Conclusion】 Thick calcareous cement layers primarily develop distal to the denudation front (down-dip), while calcareous cement content is low proximal to the denudation front. Their formation is primarily controlled by the superimposition of eogenetic calcareous cement (Type I) and Type II calcareous cement, influenced by dipping strata. The actual development of calcareous cement is also influenced by reservoir heterogeneity; calcareous cement is more prone to develop within fine-grained reservoir intervals and at interfaces between coarse- and fine-grained reservoir facies.