东海盆地丽水凹陷古近系储层砂岩中碳酸盐胶结物及形成机制

Carbonate cements and their formation mechanism in Palaeogene sandstones of Lishui sag,East China Sea basin

碳酸盐胶结物是东海盆地丽水凹陷古近系储层砂岩中最为重要的自生矿物,主要类型为铁白云石和方解石。铁白云石分布于古新统顶部不整合面附近的相对浅部地层中,其1δ3C值在-2.1‰到-2.5‰之间,相对接近海水,1δ8O值在-3.7‰到-4.1‰之间,沉淀温度较低(37℃到39℃)。方解石主要分布于远离不整合面的相对深部地层中,其1δ3C值在-2.4‰到17.3‰之间(大多数样品都在-10‰以下),显示了有机碳的影响;1δ8O值在-4.1‰到-15.3‰之间,多数样品小于-14‰,沉淀温度较高(多数样品大于90℃)。研究结果表明:铁白云石是在瓯江运动暴露时间间隔中、岩石有效压实之前由大气淡水(包括与煤层有关的酸性水)溶解海相碳酸盐内源沉积物形成的,较早的铁白云石胶结作用增加了岩石的机械强度和抗压实能力,改变了砂岩压实曲线的斜率,对砂岩储集空间的保存具有显著的积极意义;方解石胶结物是在较晚成岩阶段的相对深埋藏条件下沉淀的,其物质来源与粘土矿物转化、有机酸对长石等铝硅酸盐的溶解有关,在储层演化中具有负面作用。

Carbonate cements are the most abundant authigenic minerals in the Palaeogene sandstones of Lishui sag, East China Sea basin. They mainly consist of ankerite and calcite and have distinctive stratigraphic distribution and geometry. The ankerite cement just occurs beneath the unconformity of Ojian movement and nearby coal beds and the calcite occurs in a relatively deep bed far from the unconformity. The stable-isotope data for ankerite are δ^13 C （VPDB） = -2. 1‰- 2. 5‰, δ^18 O （VPDB） =-3.7‰-4. 1‰（precipitation temperature is 37℃ to 39℃） and those for calcite are δ^13 C（VPDB） = --2.4‰- 17.3‰ （most of the samples are less than -- 10‰）, δ^18 O（VPDB） =--6.8‰-15.3‰ （precipitation temperature for most of samples is more than 90℃）. Based on the results, the authors have the following suggestions. （1） the precipitation of ankerite is from meteoric fluids and some kind of acids from the coal bed during early diagenesis, input into sediments during times of relative sea-level fall; （2） the source of ankerite cement is the dissolution of coeval carbonate matierals; （3） the precipitation of calcite cement is from buried fluids during later diagenesis; （4） the source of calcite cement is transformation of clay minerals and dissolution of feldspar and other aluminosilicates; and （5） the earlier cementation by ankerite has enhanced the mechanical strength of rocks and resistance ability to compaction. This is of positive significance for the preservation of porosity in under deeply buried conditions.