Geol 755: Basin Analysis Geophysics Week 4.5 John N. Louie, Presenter Seismic Stratigraphy Basics Reflector Terminations Sequence Boundary Identification Procedure Example Objective: Define the genetic reflection packages that envelope seismic sequences and systems tracts. These bounding discontinuities are identified on the basis of reflection termination patterns and their continuity. Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A.W. (ed.), Atlas of seismic stratigraphy: AAPG Studies in Geology No. 27, Vol. 1, p. 1-10. Boggs, S. Jr., 2001, Principles of Sedimentology and stratigraphy, 3rd Ed., Prentice-Hall, Inc., New Jersey, 726 p. Boundaries are defined on a seismic line by identifying the termination of seismic reflectors at the discontinuity surfaces. Terminations occur: Below a discontinuity and the definition of the upper sequence boundary. Examples of this include: 1. Toplap 2. Truncation Above a discontinuity and the definition of the lower sequence boundary: 1. Onlap 2. Downlap Toplap: Termination of strata against an overlying surface mainly as a result of nondeposition (sedimentary bypassing) with perhaps only minor erosion. Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A.W. (ed.), Atlas of seismic stratigraphy: AAPG Studies in Geology No. 27, Vol. 1, p. 1-10. Boggs, S. Jr., 2001, Principles of Sedimentology and stratigraphy, 3rd Ed., Prentice-Hall, Inc., New Jersey, 726 p. Mitchum Jr., R.M., 1977. Seismic stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic stratigraphy. In: Payton, C.E. (Ed.), Seismic Stratigraphy Applications to Hydrocarbon Exploration, vol. 26. A.A.P.G. Memoir, pp. 205 212. 212. 1
Truncation: Termination of strata or seismic reflections interpreted as strata along an unconformity surface due to post-depositional erosional or structural effects. This implies the deposition of strata and their subsequent tilting and removal along an unconformity surface. This termination is the most reliable top-discordant criterion of a sequence boundary. Such truncation can also be caused by termination against an erosional surface, as for instance a channel. Onlap: : A base-discordant relationship in which initially horizontal strata progressively terminate against an initially inclined surface, or in which initially inclined strata terminate progressively updip against a surface of greater initial inclination. Mitchum Jr., R.M., 1977. Seismic stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic stratigraphy. In: Payton, C.E. (Ed.), Seismic Stratigraphy Applications to Hydrocarbon Exploration, vol. 26. A.A.P.G. Memoir, pp. 205 212. 212. Mitchum Jr., R.M., 1977. Seismic stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic stratigraphy. In: Payton, C.E. (Ed.), Seismic Stratigraphy Applications Downlap: : a relationship in which seismic reflections of inclined strata terminate downdip against an inclined or horizontal surface. Examples of downlap surfaces include a top basin floor fan surface, a top slope fan surface, and a maximum flooding surface. Note: If onlap cannot be distinguished from downlap because of subsequence deformation, the term baselap is used. Mitchum Jr., R.M., 1977. Seismic stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic stratigraphy. In: Payton, C.E. (Ed.), Seismic Stratigraphy Applications Mitchum Jr., R.M., 1977. Seismic stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic stratigraphy. In: Payton, C.E. (Ed.), Seismic Stratigraphy Applications 2
Recommended procedures for performing seismic sequence analysis include: Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A.W. (ed.), Atlas of seismic stratigraphy stratigraphy:: AAPG Studies in Geology No. 27, Vol Vol.. 1, p. 1-10. Boggs, S. Jr Jr.,., 2001, Principles of Sedimentology and stratigraphy stratigraphy,, 3rd Ed., Prentice-Hall, Inc., New Jersey, 726 p. Identifying the unconformities in the area of interest. Unconformities are recognized as surfaces onto which reflectors converge. Mark these terminations with arrows. Draw the unconformity surface between the onlapping and downlapping reflections above; and the truncating and toplapping reflections below. Extend the unconformity surface over the complete section. If the boundary becomes conformable, trace its position across the section by visually correlating the reflections. Recommended procedures for performing seismic sequence analysis (cont.): Continue identifying the unconformities on all the remaining seismic sections for the basin. Make sure the interpretation ties correctly among all the lines. Identify the type of unconformity: Sequence boundary: this is characterized by regional onlap above and truncation below. Downlap surface: this is characterized by regional downlap. downlap. Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A.W. (ed.), Atlas of seismic stratigraphy:: AAPG Studies in Geology No. 27, Vol stratigraphy Vol.. 1, p. 1-10. Boggs, S. Jr Jr.,., 2001, Principles of Sedimentology and stratigraphy stratigraphy,, 3rd Ed., Prentice-Hall, Inc., New Jersey, 726 p. Recommended color codes: Red: Red: Reflection patterns and reflection terminations. Green: Green: Downlap surfaces Blue: Blue: Transgressive surfaces Other colors: colors: Sequence boundaries If using only black and white: Thin solid lines: Reflection pattens Thicker solid lines: Sequence boundaries Dashed lines: Downlap surfaces Dotted lines: Transgressive surfaces 3