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.