Seismic Stratigraphy, a Fundamental Exploration Tool (1979)

ABSTRACT The basic principle of seismic stratigraphy is that primary seismic reflections parallel stratal surfaces (bedding planes) rather than gross lithostratigraphic boundaries. This concept coupled with the Law of Superposition can be used to answer a number of fundamental stratigraphic questions which are critical to exploration. For example, unconformity surfaces can be located on the seismic section by the identification of reflection terminations. This permits the interpreter to recognize the major shifts and changes in deposition which have occurred during a basin's evolution and provides a geologically meaningful basis for the subdivision of the geologic section into genetically significant stratal units (depositional sequences). Because reflectors follow bedding planes, reflection geometry parallels depositional geometry. Therefore after palinspastic reconstruction, the internal reflection patterns of a sequence and their areal distribution provide valuable information for paleoenvironmental reconstruction. Once the distribution of depositional environments within an area is determined, facies predictions and estimates of possible reservoir rock content can be made. Finally, the seismic section can be used as a true chronostratigraphic correlation tool. This attribute is particularly helpful in areas where reliable well log correlations are difficult due to rapid lateral facies changes, sparse well control and/or insufficient paleontologic control. Ideally these correlations are made using synthetic seismograms carefully tied to the seismic section. However, in the absence of sonic and/or density logs, the reflection geometry of the seismic section can often give the interpreter a correlation style or concept that will permit proper correlations. INTRODUCTION The concept that primary seismic reflectors parallel bedding planes (time lines) and unconformities forms the basic principle of seismic stratigraphy and may be used to answer some fundamental stratigraphic questions critical to exploration. This paper will demonstrate that applying this principle in conjunction with the Law of Superposition permits 1) the recognition of unconformity surfaces and the subdivision of the stratigraphic sections into genetically meaningful units, 2) the reconstruction of paleogeometry and therefore paleogeography and paleoenvironments, 3) true chronostratigraphic correlations, and 4) the recognition of a stratigraphic trap. FUNDAMENTALS Primary seismic reflectors are generated at physical surfaces having a velocity and/or density contrast. In a sedimentary section only two types of reflection-generating interfaces are present at the time of deposition - stratal surfaces and unconformities. Each of these has chronostratigraphic ignificance. Stratal surfaces are the depositional bedding planes which separate the episodes of sedimentation. They represent ancient depositional surfaces and are therefore geologically synchronous over the area of their occurrence (Vail et al. 1977). Although unconformities are not time synchronous, and in fact are usually quite variable in terms of the hiatus they represent, they also have chronostratigraphic significance in that strata above the unconformity are younger than strata below, i.e. an unconformity separates older rocks from younger rocks.