BURIAL DISSOLUTION OF ZHUJIANG REEF LIMESTONE ON THE LIUHUA CARBONATE PLATFORM, SOUTH CHINA SEA

A number of sinkholes have been discovered in the Zhujiang reef limestone on the Liuhua carbonate platform, South China Sea, by high resolution 3D seismic survey. Most of the sinkholes occur along the faults in NWW direction. The high heterogeneity of reservoir caused by sinkholes has much increased the complexity of hydrocarbon distribution, and thus the difficulties and challenges in exploration. In this paper, we discussed the origin of the sinkholes in the chalk-like limestone and the source of acid fluids after burial, studied the burial dissolution based upon the GC, GC/MS extracts from the Zhujiang reef limestone as well as the regional tectonics and the dynamic characteristics of formation water. It is concluded that the acid fluids are mainly from:①pyrolysis of organic matter and interactions of organic-inorganic matters with hydrocarbon in source rocks; ②oxidation of oil compounds and generation of organic acids in the process of migration; ③biodegradation of oil in reservoirs, and the concentration of organic acids generated by biodegradation may reach the value of 4 300 μg/g oil. The chalky limestone is formed by the continuous corrosion, soaking and washing by organic acid. Tight limestone layers found in the oil-bearing zone are caused by partial solution of the external part by acidic fluid with the inner part remaining unaffected. The acid fluids migrated together with active formation water along complex fracture networks. Therefore, along the faults and fractures and in the areas of broken strata, there is more space and highest acid-rock interaction. With the collapse of carbonate strata, sinkholes are formed.