The exploration for reef-shoal reservoirs in the regions of Puguang, Longgang and Jiulongshan in recent years has shown a promising exploration future in the Kaijiang- Liangping trough area. Using pre-stack migration seismic information combined with drilling and logging results, technologies such as horizon-flattening, volume-curvature, sequence stratigraphy, and prestack inversion have been successfully used in the identification of bioherm in the Changxing formation and oolitic reservoir of the Feixianguan formation in the Longgangxi region. An assorted preliminary workflow for reef-shoal body identification is developed and may contribute an important reference for research in the similar areas.
Global production from carbonate reservoirs primarily comes from platform edges (56.2%) or inner platforms (35%). In China, many basins are made up of carbonate sedimentary rocks and have the potential for reef-shoal reservoir exploration. Carbonate reef-shoal gas reservoirs have been an important domain for gas exploration since the Puguang and Longgang reef-shoal gas reservoirs were found in the Sichuan basin, centered on the Kaijiang-Liangping trough. Many new breakthroughs in gas exploration have been made in the Sichuan Basin. It shows huge potential and good prospects as a carbonate reef-shoal area. Three resource assessments have shown that the total amount of reef-shoal gas resources is about 1.5 trillion m3 in the Sichuan Basin (40% of the whole basin), demonstrating reserves of 0.2 trillion m3 with a rate of less than 15%. With all of this potential, the development of techniques for reef-shoal reservoir identification is important. [1] The identification of carbonate reef-shoal reservoirs is a tough problem and key for directing the exploration targets in this kind of gas reservoir exploration. Predecessors have mainly used horizon-flattening and “bright point” to identify reef-shoal reservoirs; however, the interpretation is not high quality because the seismic reflection characteristics of reef-shoal reservoirs are variable.
The growth scale and distribution of reef-shoal reservoirs are mainly controlled by carbonate sedimentary facies. The analysis of sedimentary facies is an important research area and the confirmation of the carbonate platform edge facies is the key. Generally, a reef’s growth is controlled by the water environment and hydrodynamic force. It mainly grows insitu along the carbonate platform edge, and migrates to the inter-platform and basin with change in eustatic sea level. The relationship between the growth rate of bioherm and rate of change of sea level is a key factor for controlling the sedimentary structure of different platform edges (Figure 1). The reef growth rate is closely related to hydrodynamic force, which is concerned with the wind direction; the reef grows the most in the windward direction along the platform edge, and has an obvious nonsymmetry from windward edge to the leeside edge (Figure 2). Sedimentary rocks of carbonate oolitic shoals form from carbonate grains in high-energy environments. The scale of grains is related to the strength of the hydrodynamic force and the distribution of sediments and is relative to the tideway of the platform edge and wind direction.
