| Author's Name: ZHANG Yingzhao, LI Xushen, XU Xinde, GAN Jun, YANG Xibing, LIANG Gang, HE Xiaohu, LI Xing |
| L25 gas field, locating in the western deepwater area, Qiongdongnan Basin, consists of vertically superimposed gas pools associated with two phases of confined gravity flow deposit in Upper Miocene Huangliu Formation. The early phase Ⅲ and Ⅳ gas units are strong overpressure gas pools with pore pressure coefficients from 1.806 to 1.840. The late phase Ⅰ and Ⅱ gas units are normal pressure gas pools with pore pressure coefficients from 1.170 to 1.188. The characteristics of natural gas component and carbon isotope of L25 gas field show that the natural gas was coal-type organic origin gas. The integrated study of the relationship between carbon isotope of methane and vitrinite reflection ratio of source rock, the biomarker of condensate oil, the burial history and thermal evolution history of source rock, indicates that the mature and high-mature gas dominantly come from marine source rock of Lower Oligocene Yacheng Formation in Ledong Sag. Analysis of natural gas accumulation process reveals that L25 gas field experience two stages of gas filling in the early and late periods. About 2.0Ma, mature and high-mature gas filled the lithological traps associated with the late phase confined gravity flow deposit, forming Ⅰ and Ⅱ gas units with normal pressures. About present, high-mature gas filled the lithological traps associated with the early phase confined gravity flow deposit, forming Ⅲ and Ⅳ gas units with strong overpressures. The comparative research on the features of deposit and reservoir, pressure evolution and accumulation, shows that the Neogene large-scale reservoir filled with gas in the early stage is more economical accumulation for exploration in the deepwater area, Qiongdongnan Basin. |
