Forming Condition Of Late-Cenozoic Reef Facies Carbonate Rocks In Xisha Sea Area And Their Oil And Gas Exploration Potential

Located at the northwestern continent slope of the South China Sea, based on the characteristics of basement structure and the covering sedimentary strata, Xisha sea area consists of eastern reef island area and western basin area. The eastern reef island area, with shallow seawater, is higher parts of Xisha apophysis, where the basement is directly covered with reef facies carbonate rocks. The western basin area, stretching into Qiongdongnan basin and with deep seawater, is the western slope of Xisha apophysis, where reefs developed in geological history, but were covered with clastic rocks or seawater now.By means of 2D seismic data in western basin area and data, in well Xichen 1 of eastern reef island area, of rock flake, major element, trace element, isotope, liquid enclosure, X-ray diffraction, electron probe, back dispersion, SEM, image analysis, capillary pressure curve analysis, porosity, permeability, and so on, reef facies carbonate rocks were studied, and the following understanding and conclusions were acquired.1. All basement rocks in eastern reef island area is covered with pure reef facies carbonate rocks in the thickness of 1250m. The carbonate rock area ascertained by 2D seismic data in western basin area is up to 2549km2 with the thickness of 1183 to 1438m. The reef facies stratum sequence include 6 stages, those are, Oligocene, early-Miocene, middle-Miocene, late-Miocene, Pliocene, and Quaternary.2. The Late-Cenozoic reef in Xisha sea area is mainly plant algae one, and secondly coral one, and the latter are mostly found in Quaternary. Reef facies include reef core (framework or skeleton part), reef front, and reef back (lagoon), and the like. Rock component types include limestone and dolostone. According to Well Xichen 1 in Xisha sea area, 7 lithology segments are confirmed in reef facies stratum sequence, including 3 dolostone segments and 4 limestone segments.3. Petrochemical analysis results of reef facies carbonate rocks in Xisha sea area indicate that the major element, trace element, and isotope in the rocks represent obvious subsection characteristics in stratum profile. These manifest that, (a) CaO, with 4 high value sects and 3 low value sects, and MgO, with 3 high value sects and 4 low value sects, compensate each other. (b) Sr content, varying largely from 139×10^-6 to 2180×10^-6, can be marked off 7 sects in strata. The variations of Na, K, P are similar, with high value in the upper part of, and low value in the lower part of, this set of strata in well Xichen 1. Fe, V, B have similar variation, but the value varies the other way round with Na, K, P. (c) Carbon and Oxygen isotopes can be divided into 7 sects in the stratum profile of Well Xichen 1.⁸⁷Sr/⁸⁶Sr value, increasing by degrees from bottom to top, can be carved up to 4 stages. These subsection characteristics are the responses or track records, in different aspect, of palaeoclimate, Qinghai-Tibet Plateau upheaving, volcano activities, and palaeo-ocean events. Thereinto, South Pole icecap formation in Miocene, mantle magma activity caused by lithosphere thinning, and continent upheaving brought byIndian-Eurasian plate collision influence the ancient South China Sea evidently. The middle- to late-Miocene South China Sea might be occlusive continent edge sea which communicates with the Philippine sea and Pacific ocean only by Bashi Channel, which sometimes opened and sometimes closed, and when, apart from deep sea in central South China Sea basin, all Dongsha, Zhong-Xisha, and Nansha were shallow sea, bank, or lagoon.4. The formation, development, and distribution of reef facies carbonate rocks were controlled in extensity by the South China Sea evolution tectonic setting and great deltas coming from continents. The reef types and dimensions were determined by sea level rising and falling.5. Rock types and original reservoir capability were controlled by palaeo-ocean environment, while the final reservoir performance was determined by mineral cementation, recrystaline, eluviation by freshwater, and dolomitization. The middle- to upper-Miocene is the most favorable reservoir based on reservoir character parameters and the controlling factors mentioned above.6. There exists no enclosure forming condition in eastern reef island area of Xisha sea area. No oil and gas reservoirs there. However, with fecund source rocks, advantaged reservoirs, favorable cap rocks, good trap condition, good oil and gas motion condition, and preservation condition, western basin area of Xisha sea area has a wide oil and gas exploration potential.