Seismic sequence analysis presented by Vail et al. (1977) is based on the identification of reflection terminations on regional seismic profiles. It is difficult to be used in small 3D surveys with reflection terminations not so straight to be identified. In this paper, we present a method, which labeled as ‘all-reflector tracking’, for identifying subtle seismic sequence stratigraphic boundaries in small 3D seismic surveys. It has been used in the late Triassic Xujiahe Formation of a 180 km2 3D seismic survey in middle Sichuan Basin, southwest China. As a result, five seismic sequence boundaries and three transgressive surfaces were recognized in Xujiahe Formation, which subdivided the formation into four seismic sequences and seven seismic systems tracts, and therefore the seismic sequence stratigraphic framework for the survey was successfully constructed. It is suggested that all-reflector tracking is an efficient approach to seismic sequence stratigraphy in small 3D seismic surveys.
Seismic stratigraphy emerged in the 1970s with the work of Vail et al. (1977) is a milestone in the development of seismic interpretation techniques. It is based on the concept that primary seismic reflectors parallel geologically synchronous bedding planes and chronostratigraphicsignificant unconformities. By recognizing characteristic reflection terminations (onlap, downlap, toplap, and truncation), seismic interpreters can subdivide a seismic profile into seismic stratigraphic units of chronostratigraphic significance, including seismic sequences and systems tracts. Applications of seismic sequence analysis permit 1) the recognition of unconformity surfaces and the subdivision of the stratigraphic sections into genetically meaningful units, 2) the reconstruction of paleogeography and paleoenvironments, 3) true chronostratigraphic correlations, and 4) the recognition of a stratigraphic trap (Raymer 1979). Therefore, it has found wide applications in analyzing various types of basins for both academic and industrial purposes. However, this method has mainly based on regional 2D seismic lines or large 3D seismic surveys, which have greater probability of being able to image key stratal terminations and thereby allow for identification of sequences and systems tracts (Hart et al., 2007). Hart et al. (2007) discussed and demonstrated with case studies the challenges faced by traditional seismic sequence stratigraphy when used in small 3D seismic surveys. In this paper, we present a method, which labeled as ‘all-reflector tracking’, for identifying subtle seismic sequence stratigraphic boundaries in small 3D seismic surveys. It has been used in the late Triassic Xujiahe Formation in a 180 km2 3D seismic survey in middle Sichuan Basin, southwest China, and got satisfactory results.
In traditional seismic sequence analysis, horizon tracking and interpretation is focused on the identification of key stratigraphic surfaces, including sequence and systems tract boundaries, by observing seismic reflection terminations (onlap, downlap, toplap, and truncation). Large amount of reflectors between the key boundaries are not tracked, mostly for efficiency of an interpretation task. Although the method is less time-consuming and thus has a high efficiency of interpretation, most of the stratigraphic information suggested in the seismic reflections between the key surfaces tracked is not utilized. Therefore, some subtle seismic sequence stratigraphic boundaries are possible to be missed.
