Genesis of calcification in Neogene carbonate rocks of Liuhua oilfield, Pearl River Mouth Basin

The chalky texture carbonate in the Pearl River Mouth Basin is a kind of white, loose, and porous biomicrite formed in carbonate depositional environments, including reefs and banks. It has experienced diagenesis in a multiphase calcification environment and a "rinse" by underground water. The Liuhua oilfield reservoir of the Zhujiang Formation is a locally calcified reservoir. It is formed in an environment superimposed by the reef bank where it has experienced the process of calcification. In its discussion on the genesis of calcification, this study focuses on the distribution of calcified horizons in coring wells of the Liuhua oilfield, microscopic texture of chalkified rocks, carbon and stable isotope oxygen compositions, and the chemical action of underground oil-field water. The processes of calcification in different well intervals are divided into three categories: (1) weak calcification in stylolites; (2) moderate calcification of carbonate reservoir rocks along oil-well columns; and (3) strong calcification in oilfield bottom waters. The degree of calcification is positively correlated with the porosity and delta C-13 of reservoir rocks. The occurring process of calcification can be divided into three periods; (1) the chalky texture in stylolites is formed by dissolution and leaching in the fresh water vadose zone of the early diagenetic stage and by pressure solution in late diagenetic stage; (2) the moderate calcification process is controlled by selective dissolution and degrading neomorphism in the early diagenetic stage and controlled by the non-selective dissolution of the regenerated groundwater from the compaction of overlying argillite strata in the middle and late diagenetic stage; and (3) the strong calcification process occurs through the enhancement of formerly calcified rocks through "rinsing" by the bottom water of the underground oil reservoirs.