Diagenesis and its effect on reservoir quality in Chang-2 reservoir of Ansai Oil Field

Chang-2 reservoir sandstones of Ansai Oil Field mainly consis of fine-middle grained arkoses. Sandstones contain a diagenetic mineral assemblage that record major diagenetic events during burial history. The main diagenetic minerals of the sandstones include chlorite, calcite, quartz and feldspar overgrowth, illite, and albite. Chlotite grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital grains. Authigenic quartz overgrowths contain a variety of organic fluid inclusions that indicate the petroleum filling history. The fluid inclusion temperatures measured in Chang-2 reservoir samples are 48.1~76.5(~oC). Secondary porosity has created mainly by feldspar, rock fragment and clay matrix dissolution. The mean porosity for the whole population of 165 samples is 11.6%. Based on diagenetic events related to temperature, reservoir diagenetic stages can be divided into early diagenesis ,later diagenesis and epidiagenesis. Reservoir quality is controlled mostly by sedimentary microfacies and diagenesis. Distributary channel sandbodies commonly have high porosity and permeability, whereas interdistributary and distal sandsheets may have low petrophisical properties.