莺歌海盆地乐东区储层碳酸盐胶结物成因机理及与流体活动的关系

乐东A区中新统黄流组是南海北部莺歌海盆地高温超高压天然气勘探的主力层系,发育重力流水道-海底扇沉积的细、中、粗粒砂岩,中、低、特低渗物性特征,多期次、多类型碳酸盐胶结。研究碳酸盐胶结物类型、期次、成因机理及其与流体活动的关系,对于储层评价预测有重要意义。研究结果表明,碳酸盐胶结物表现为早、中、晚三期方解石与早、晚两期白云石,由早期到晚期,方解石及白云石的δ¹³C、δ¹⁸O值负偏增大,MnO、BaO、SrO等含量增高,早—中期方解石的形成与生物碎屑、碳酸盐矿物溶解再沉淀有关;晚期铁方解石、(铁)白云石的形成与深部热流体、无机CO₂、有机酸脱羧有关。方解石的溶解-沉淀受地层压力及CO₂含量共同控制,地层压力与天然气组分中CO₂含量相对较高,溶解作用较强,(铁)白云石发育;反之则溶解作用较弱,(铁)方解石发育。

Genesis of carbonate cement and its relationship with fluid activity in the Ledong area, Yinggehai basin

The Miocene Huangliu Formation in the Ledong area is the main stratum of high temperature and ultra-high pressure natural gas exploration in the Yinggehai Basin in the north of South China Sea.Fine,medium and coarse-grained sandstones deposited by gravity water channel submarine fan are developed,with physical characteristics of medium,low and ultra-low permeability,and multi-stage and multi-type carbonate cementation.For reservoir evaluation and prediction,it is important to study the type,stage,genesis of carbonate cement and its relationship with fluid activity.The results show that carbonate cement was represented by calcite in the early,middle and late stages,and dolomite in earlyand late stages.From early to late stage,the values of δ¹³C and δ¹⁸O of calcite and dolomite decrease,and the contents of MnO,BaO and SrO increase.The formation of calcite in the early to middle stage was related to the dissolution and re-precipitation of bioclasts and carbonate minerals.The formation of calcite and dolomite in late stage was related to deep thermal fluid,inorganic CO₂ and organic acid decarboxylation.The dissolution and precipitation of calcite was controlled by the formation pressure and CO₂ content.The formation pressure and CO₂ content in the natural gas component are relatively high,the dissolution is strong,(iron)dolomite are developed;otherwise,the dissolution is weaker,(iron)calcite is developed.