DEVELOPMENT OF SAND-BODY RESERVOIRS IN FAULT-BOUNDED LAKE BASINS: PLEISTOCENE-HOLOCENE POYANG LAKE, EASTERN CHINA

In fault-bounded lacustrine basins, the lake basement may be exposed due to tilting of crustal blocks, forming islands of varying size. Such islands are commonly associated with sandy facies that may serve as important reservoirs for oil and gas accumulation, as in the Eocene Boxing Sag of Bohai Bay Basin in northeastern China. The present study investigated large sand bodies of Pleistocene-Holocene age on Songmenshan Island in the center of Poyang Lake, Jiangxi Province, eastern China. The relationship of sedimentation on Songmenshan Island to its formation history was analyzed by means of Google satellite observations and field studies combined with SEM, grain size, and ¹⁴C dating analyses. The area of Songmenshan Island (39 km²) represents 8% of the total lake area (500 km²), and its height (81 m) is several times greater than that of the depth of the surrounding lake (13 m). The island has been uplifted since ~5 Ma at a rate of ~16 m Myr^-1 as a consequence of intrabasinal block faulting. The surficial deposits of the island consist of wave-controlled beach-bars in the lower part (< 36 m elevation) and wind-controlled eolian dunes in the upper part. Sedimentary characteristics were different between beach bars and sand dunes in terms of bedding, sedimentary structures, and grain size and texture. Whereas sand movement in the beach-bar facies was dominantly through traction, saltation was the major process in the eolian dune environment. The depositional history of Songmenshan Island can be divided into 3 stages: 1) deltaic sedimentation from the Ganjiang and Xiushui rivers, which are connected to the Yangtze River via a channel; 2) wave reworking of deltaic sediments in shoreline beach-bar facies after initial uplift; and 3) wind reworking of sands in eolian dune facies following further uplift. Later in its history, Songmenshan Island was reduced through wind and wave erosion into two subequal parts (19.6 and 19.8 km²), producing its present configuration. The large sand bodies comprising this island may eventually be buried in a cocoon of organic-rich lacustrine muds, yielding an exploration target with an estimated volume of 1.3 km³. Songmenshan Island may thus serve as a model for development of sand-rich reservoir facies in fault-bounded lacustrine basins.