Study on characteristics and genesis of botryoidal dolostone of the Upper Sinian Dengying Formation: a case study from Hanyuan region, Sichuan, China

Abstract

Botryoidal dolostone (BD) developed extensively within the Neoproterozoic Upper Sinian Dengying Formation in the Hanyuan region, which is an important gas reservoir. Based on literature review, a study of their outcrop characteristics, petrology, microfabric, degree of order, and genesis reveal that BD mainly comprises botryoidal lumps (BL) originated from depositional-penecontemporaneous seawater and botryoidal and ctenoid cements or fillings. BL occurred in parallel beds during the depositional-syngenetic stage, including core and coating. The core consists of grapestone and thrombolite, which is dominated by muddy and microcrystalline high-magnesium calcite (HMC) resulting from marine deposition and micro-organism capturing and binding effects. The coating, classified as either thin or thick, is mainly isopachous fibrous aragonite (IFA) that formed during the depositional-syngenetic stage. Thin coatings formed in the turbulent sea bottom, while thicker ones formed in a relatively quiet sea bottom. Botryoidal and ctenoid cements and fillings compose IFA and HMC. IFA that includes marine phreatic zone deposition from the syngenetic-penecontemporaneous stage mainly occurs in the spaces between grains of BL and intraclast or on the top of them. The HMC that is the product from meteoric leaching in the penecontemporaneous stage and marine deposition occurs in the dissolved pores and fractures. During the process when BL and the botryoidal, ctenoid cements, or fillings formed, either abiogenetic or microbiogenic aragonite and HMC were mimetically dolomitized and preserved the primary texture and structure. BD was also reworked by the meteoric leaching in the penecontemporaneous and supergenous stages.