Seismic Geomorphology of Carbonate Reservoirs in the Lower Triassic Feixianguan Formation, Northern Sichuan Basin, Southwest China

During the late Permian to early Triassic period, a northwest-southeast-extending Deepwater trough was developed in northern Sichuan Basin, southwest China, which was named the ‘Kaijiang-Liangping Trough’ (Wang et al., 2006). To the southwest, the trough was surrounded by a rimmed carbonate platform. Our study area is located in the platform and trough junction, covering features of the platform, platform margin, slope and the Deepwater trough from southwest to northeast. The lower Triassic Feixianguan Formation shoal oolitic carbonates on the platform margin constituted the most excellent reservoirs in the area.

In the study, a 3-D seismic survey with an area of about 1050 km2 were interpreted by using an integrated seismic stratigraphic and geomorphological analysis procedure, aimed to investigate the lateral occurrence and vertical evolution of the sedimentary geomorphological elements. Well logs from two exploration wells were used for horizon calibration by well-to-seismic tie.

Through seismic sequence analysis, six sequence boundaries and nine maximum flooding surfaces were recognized, which classified the Feixianguan Formation into five sequences and nine systems tracts. The five sequences were named SQ1 to SQ5 upwards, respectively. Except for the lowermost sequence (SQ1), in which only transgressive systems tract was developed, each of the other four sequences consisted of a lower transgressive systems tract and a upper highstand systems tract.

Several methods were utilized to reconstruct the occurrence and evolution of the sedimentary geomorphological elements, including horizon flattening, horizon slicing, stratal slicing, and multi-attribute neural-network-based seismic facies classification. The results suggested that the study area was characterized with the rimmed platform to the southwest and the Deepwater trough to the northeast before the deposition of Feixianguan Formation. From SQ 1 to SQ 4, the carbonate platform was gradually prograded northeastwards to the trough as a response of an overall falling of relative sea level. As a result, a set of prograding clinoforms were developed. During the late stage of Feixianguan Formation deposition (SQ5), the trough was gradually filled, and the study area was evolved into a gentle northeast-dipping ramp, on which evaporative tidal flat environments developed. As the most potential reservoirs, the oolitic shoal facies on the platform margin were delineated.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California