Growth Conditions and 3-D Seismic Delineation of Carboniferous Barrier Reefs in the Southwestern Tarim Basin

The Tarim Basin was a craton block during the Carboniferous period. Except for its open southwestern (SWW) deepening basin margin, the central part of the basin was surrounded by northern, eastern, and southern uplifts. In most parts of the basin, there are three layers of carbonate rocks called Biocalcarenite member, standard limestone member and Xiaohaizi Formation. The present-day Maigeti slope, located in the southwestern area of the basin, was ever a paleogeographic carbonate platform margin to clinoform. Boreholes in the area reveal over ten meters of high-energy biospararenite of beach deposition in Xiaohaizi Formation. This feature is quite different from the low-energy micritic limestone of equivalent horizon in the middle and northern basins. Such typical carbonate beach and platform margin backgrounds indicate an environment conducive for reef growth. According to an interpretation of 3-D seismic data on a 600 km² Xinbaza survey, a 25 km-long barrier reef orientated in an east-westward direction is delineated with a hummocky geometry, a 1–2 km wide south-dip reef front, and a 3–6 km wide north-dip backreef. The reef buttress and outlines are also shown on an isopach map of the Xiaohaizi Formation as well as horizon slices of coherency, amplitude, and frequency attributes. The reef extends beyond the range of the rectangular 3-D seismic survey. The thickness of the Xiaohaizi Formation is over 60 m at the central reef, and 12 m larger than the outside of the reef. The barrier reef consists of segments separated by narrow tidal inlets. The neighborhood of the reef is mainly beach deposition of bioclasticgrainstones. Due to Late Permier and Late Tertial tectonic movements, the barrier reef was tilted to dip in a SWW direction. Its updip part was broken and misplaced by a set of reverse faults. Nevertheless, potential stratigraphic pools of a large amount of hydrocarbon reserves related to the reef are expected to be found in the future based on the substantial reef scale and the recent borehole indications beside it.