苏北盐城天然气储层成岩作用研究

盐城凹陷天然气储层为成分成熟度很低的砂砾岩、砾状砂岩、砂岩和少量粉砂岩,砂岩类型主要为长石岩屑砂岩和少量岩屑长石砂岩。成岩矿物主要类型有粘土矿物、石英、钠长石、碳酸盐、硬石膏和石膏等。储层以次生孔隙发育为特点,以粒间孔隙和骨架颗粒溶孔最为发育,溶解作用发育程度与泥质岩在成岩过程中粘土矿物和有机质的演化关系极其密切。盐城天然气进入储层发生在始新世———新近纪,主要成藏期发生在4.5~10Ma。天然气储层成岩演化阶段处于晚成岩A亚期。储层性质明显地受到沉积相和成岩作用的影响。沉积物粒度较粗、厚度较大的河道砂的储集物性明显优于各种粒度较细、厚度较薄的席状砂体。

Diagenesis of Gas Reservoirs in the Yancheng Depression

The gas reservoirs in the Yancheng Depression are mainly conglomeratic sandstone,sandstone,and minor siltstone.The sandstone reservoirs can be classified as compositionally immature feldspathic litharenite and lithic arkose.The main diagenetic minerals of the sandstones include: authigenetic clay minerals,calcite cement,anhydrite,quartz and feldspar overgrowth,albite and later ferroan carbonate.Authigenic quartz overgrowths and microcrystals contain a variety of organic fluid inclusions that indicate the petroleum filling history.Petroleum filling history shows that at least twice filling processes occurred and the later process was significant.Secondary porosity has created mainly by feldspar,rock fragment and clay matrix dissolution.The dissolution mechanisim is closely related to organic acids and transformation of clay minerals in adjacent source rocks.Based on diagenetic events related to temperature,the reservoir diagenetic stages can be determined to be in the A-substage of later diagenesis.Reservoir quality is controlled mostly by sedimentary mifacies and diagenesis.The channel sandbodies commonly have high porosity and permeability,whereas interdistributary and distal sandsheets may have low petrophisical properties.