苏北高邮凹陷戴一段砂岩矿物和孔隙成岩机理的探讨

DIAGENETIC MECHANISM OF MINERALS AND POROSITY IN SANDSTONE OF THE LOWER DAINAN FORMATION, IN THE GAOYOU SUB-DEPRESSION, THE NORTH JIANGSU BASIN

苏北盆地高邮凹陷下第三系戴一段岩层是主要的产油层系.通过对不同埋深的砂岩、泥岩和有机质的分析,表明砂岩的颗粒、胶结物、孔隙的性质发生了一系列的成岩变化,长石矿物的溶蚀所形成的大量次生孔隙是本区砂岩储集层的明显特征.在两个不同深度上出现的次生孔隙密集带,具有不同的成因机制(有机质的脱羧基作用和矿物的交代作用),并分别控制着低成熟、高成熟石油及天然气的分布.

The Lower Daman Formation, Early Tertiary , of the Gaoyou Sub- Depression in the north Jiangsu Basin is a main oi-Ibearing reservoirs, with the maximum thickness of more 800m in the central part of the basin. Core sample was systematically made from 22 wells in different depths from the west margin accross the studies area eastwards. Based on detailed analysis of minerology, geochemistry, and rock physical properties, using the methods of thin section, SEM, pore casts, kerogen, and vitrinite reflectane, etc., the burial diagenetic chacteristics of sandstones are discussed. With burial deeping, the grains of quartz and feldspar and minor heavy minerals were dissolved, overgrown, pressure-dissolved and replaced or altered. Carbonate cements occurred dissolution and replacment, and formed two different diagenetic mineral series in the shallower and deeper depths. According to constituent, aggregation, series of minerals in grain and cement, and properties and types of porosity, four distinct diagenetic zones can be divided: immature, low mature, mature Ⅰ and mature Ⅱ . Sandstone reservoirs have primary and secondary porosities, which control the distribution of physical parameters in the shallower and deeper reservoirs, respectively. In the shallower reservoir predomnantes primary porosity where occurred less dissolved secondary porosity relatively. With the increase of machanical compaction and chemical precipitation, the total porosity decrease gradually. The peak of carbonate precipitation occurred at 2800 m and form a high carbonatizicd sandstone zone and sametime the porosity tended to diminish. Below the carbonatizied sandstone zone the secondary porosity became predominance with total porosity increase. The dissolution of silicates, carbonates and other salts is a major factor in generation of secondary porosity. Two important conditions of mineral dissolution are pH of pore water and temperature. Twenty patterns of secondary porosity show aregular evolution of dissolution in various diagenetic environments. In the different depths two intensive secondary porosity zones occurred caused by different origins. The first intensive secondary porosity /one ranges from 2950 to 3150 m in depth, and the face pore density can reach about 14%, which just coincides with the high vale ranges of ratio of pristane over phytane. hydrocarbon over organic carbon. It suggests that occurrence and development of the first intensive secondary porosity zone is related to dccarboxylation and organic acids leicase of kerogen from associated mudstones during the early organic mature stage. By theorctical calculation, about 1.7-4.6% of ncw porosity could form by the dissolution of feldspar and carbonate resulting only from decarboxylation. This zone is a principal place of hydrocarbon primary migration and accumulates mainly low mature oil. The second intensive secondary porosity zone occurred from 3300 to 3650 m, with face pore density reaching about 16%. Its origin is closely related to the specific replacement reaction between feldspars, silica and dolomites. By theoretical calculation. These two reactions could produced CO268034g and 54427g and caused new dissolved porosity about 7.7% by dissolution of feldspars and calcites. The second intensive secondary porosity zone caused by inorganic reactions within the sandstones could accumulate high mature oil or condensates, which should be a potential site for oil and gas exploration in the deep depth.