Relationship between depositional textures and seismic velocities of the Yijianfang Ordovician reef complexes in the Bachu area, west Tarim Basin, China

The Yijianfang Ordovician reef complexes in the Bachu area, western Tarim Basin contain eight facies elements: reef cores, fore-reef breccias, reef bases, tidal channels, fore-reef inner shoals, fore-reef outer shoals, back-reef inner shoals, and back-reef outer shoals. The reef cores and fore-reef breccias are characterized by Calathium bafflestones and rudstones, respectively. The reef bases are composed of skeletal packstones with moderately sorted grains of 0.1-2.5 mm diameter, whereas the tidal channels are dominated by cross-bedded packstones in which the grains are moderately to poorly sorted with a diameter of 0.3-6 mm. The fore-reef and back-reef inner shoals are composed of grainstones, while the fore-reef and back-reef outer shoals are dominated by packstones. From inner to outer part of the shoals, the grain content becomes gradually less, the size of grains becomes gradually finer, and the grains become worse sorted. The fore-reef and back-reef inner shoals tend to be potential reservoirs for hydrocarbon. Studies on predictive relationship between depositional textures and seismic velocities show that the seismic velocities decrease with an increase of grain content or median grain diameter, and a decrease of standard deviation. With these correlations, acoustic data could provide useful information about recognizing the depositional textures of reef complexes, which is crucial to predict facies of the reef complexes based on seismic data. Seismic simulated experiment reveals that the mound seismic reflection configurations of the reef cores are pronounced, surrounding which the potential reservoirs can be recognized in the synthetic seismic profiles.