张 杰1,2, Brian Jones3, 潘立银1,2, 周进高1, 秦玉娟1,2, 郝 毅1, 武明德4
1 中国石油杭州地质研究院,浙江杭州 310023 2 中国石油集团公司碳酸盐岩储层重点实验室,浙江杭州 310023 3 Department of Earth and Atmospheric Sciences,University of Alberta,Edmonton,Alberta T6G 2E3,Canada 4 青海省环境地质勘查局,青海西宁 810007
Origin of botryoidal dolostone of the Sinian Dengying Formationin Sichuan Basin
Zhang Jie1,2, Brian Jones3, Pan Liyin1,,2, Zhou Jin′gao1, Qin Yujuan1,2, Hao Yi1, Wu Mingde4
1 PetroChina Hangzhou Research Institute of Geology,Hangzhou.310023,Zhejiang 2 Key Laboratory of Carbonate Reservoir,CNPC,Hangzhou.310023,Zhejiang 3 Department of Earth and Atmospheric Sciences,University of Alberta,Edmonton,Alberta T6G 2E3,Canada 4 Qinghai Exploration Bureau of Environment Geology,Xining.810007,Qinghai
Abstract：Sinian Dengying Formation (up to 1000m) in Sichuan Basin is composed of fine crystalline dolostones. In some parts of the formation(mainly in Members 2 and 4) distributed various shaped botyroidal masses of dolomite,up to 75cm long in maximum. They are either parallel to bedding or cutting across bedding. Those cross sections reveal that isopachous bands of various types of dolomite are developed as cements. Large amount of irregular shaped cavities are still present in the central parts of some larger bodies, which formed as a result of subterranean dissolution that was associated with karst development. The surrounding micritic dolostone was originally deposited from seawater protocrystallization, and the subsequent cements can be divided into four stages:(1)the dolomitized automicrite in some samples was the 1st stage cement,which formed close to the surrounding micritic dolostone due to various physiochemical and possible biological influences;(2)the 2nd stage fibrous dolomite had an absolute ordering crystal structure and nearly ideal crystal cell parameter,which may have originated from marine waters and adjusted to be ordered during the late diagenesis;(3)the 3rd stage fine- to medium-crystal dolomite including residual of fibrous dolomite crystals dissolved in a near-surface meteoric environment after tectonic uplift;(4)the 4th stage medium-coarse crystal dolomite primarily precipitated under a burial environment and partially filled the central parts of the cavities, with some unfilled cavities. It is apparent that the Dengying Formation was subject to multiple phases of diagenesis including 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 evolution of the Dengying Formation dolostone,and guide the Precambrian petroleum development of Sichuan Basin
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