Characteristics of limestone buried-hills and their distribution in the Weixinan Depression of the Beibu Gulf Basin

After years of exploration in the Weixinan Depression of the Beibu Gulf Basin, the oil fields in conventional clastic rocks become rare with time. However, the degree of exploration in the limestone buried-hill is low, and the remaining resources are great in amount. A number of buried-hill oil fields and/or oil-bearing structures have been found in the surrounding areas of the fault zones 1and 2, and show good prospects for exploration. By means of core observation, thin section determination, well logging analysis and other laboratory testings, combined with a thorough review of regional tectonics and sea level fluctuations, the controlling factors and the distribution of high-quality reservoirs are studied in this paper. The following understandings are obtained. The types of pore and fracture-cave reservoir space are widely distributed in grain limestone which is well developed in the border of carbonate beaches of carbonate platforms, and may provide a high quality material basis for the later fracturing and karstification. Strong dolomitization obviously improves the pore type reservoir. Besides, The fault zone 1, which is the extension of the Lianjiang-Xinyi fault offshore have experienced complex tectonic evolution from sinistral rotation and compressive shearing to the dextral transtensional later in the Mesozoic Era. The later transtensional fault may provide percolation path for karst structures, which accelerated the process of weathering and eluviation to form holes and pores of good communication. It is further pointed out that the Weizhou E is a favorable exploration target similar to the Weizhou A, located in the zone of strong stress where structural fractures accelerated dolomitization and dissolution. The research results provide a geological guidance for the exploration of limestone buried-hill in the Weixinan Depression.