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Origin of Botryoidal Dolostone of the Sinian Dengying Formation in Sichuan Basin, China


The Dengying Formation (Sinian) is a thick succession (up to 1000m), which is formed largely of finely crystalline dolostones and has high gas production in Sichuan Basin, China. In some parts of the formation (mainly in Member 2 and 4) there are large, hard, variably shaped botyroidal masses of dolomite, up to 75cm long, that are either parallel to bedding or cutting across bedding. Cross-sections reveal isopachous bands of various types of dolomite that developed as cements. Irregular-shaped cavities are still present in the central parts of some of the larger bodies. Although previously referred to as grapestone or prehnite, the botryoidal dolostone developed as various types of cements progressively filled cavities that formed as a result of subterranean dissolution that was associated with karst development. After the surrounding micritic dolostone originally deposited from seawater, the fact that each phase of cement is crystallographically and geochemically distinct indicates that each fabric probably formed at different times under different conditions: (1) the dolomitized automicrite in some samples was the 1st stage cement, which formed within the cavities due to various physiochemical and possible biological influences; (2) the 2nd generation fibrous dolomite had absolute ordering crystal structure and near-ideal crystal cell parameter, which may have originated from marine waters and adjusted to be ordered during the later diagenesis; (3) the 3rd generation fine-to medium crystal dolomite including traces of fibrous dolomite crystals formed near-surface meteoric environment after tectonic uplift; (4) the 4th generation medium-coarse crystal dolomite primarily precipitated under burial environment and partially filled the central parts of the cavities. It is apparent that the Dengying Formation was subject to multiple phases of diagenesis that included karst development, precipitation of various cements, and multiple episodes of dolomitization. Except the paleokarstification, all of the other processes critically reduced the porosity of the petroleum reservoir that developed in the Dengying Formation. The paleokarstification which related to botryoidal dolostone has crucial importance to the Dengying Formation reservoir. The study of botryoidal dolostone in Sinian Dengying Formation conduces to understand the reservoir origin and evolvement, the origin of Dengying Formation dolostone, and the Pre-Cambrian petroleum development of Sichuan Basin.