Abstract:

Abstract: Reefbank facies belts in the Ordovian Yijianfang and Lianglitage Formations are important target for hydrocarbon exploration in Tarim Basin. In Well S76 in Tahe area, the reefbank reservoirs of the Yijianfang Formation are developed in the intraplatform ramp and belong to intraplatform reefbank facies. In contrast, in Well TZ-62 in Tazhong area, the reefbank reservoirs of the Lianglitage Formation are developed on the margin of steep platform and are of platformmargin reefbank facies. By analyzing subsequence patterns, six types of subsequence can be identified in both wells, but occur at different structural locations. Compared with Well S-76, the subsequences of Well TZ-062 were more sensitive to fluctuation of sea level thus are smaller in thickness. Analysis of sequence structures reveals that the intraplatform reefbank facies in Well S-76 were mostly developed in highstand system tract, i.e. the falling semicycle of the base level. In contrast, the platform margin reefbank facies in Well TZ-62 were developed in transgressive system tract, i.e. the rising semicycle of the base level. Therefore, the reefbank facies of Well S-76 feature in progradational growth, small thickness and rapid change of sedimentary facies. On the contrary, the reefbank facies of Well TZ-62 are characterized by aggradational growth, larger thickness and slow change of sedimentary facies due to the relatively stable accommodation space. The targets in both wells are located in the rising cycle of the base level of a thirdorder sequence and are dominated by grainstone and marl respectively. Comparisons of lithological association, sedimentary facies and sequence stratigraphy of these two kinds of reefbank facies reveal that the development patterns of reefbank reservoirs in different facie belts are different due to the impacts of structural locations and sea level fluctuation, resulting in the differences of lithology, thickness and scale of different reefbank facies. 

Key words:  , Key words： reefbank, sequence stratigraphy, base level, Ordovician, carbonate rock, Tarim Basin