Interpreting Unconformities In Seismic Records And Outcrop

Abstract

1. ABSTRACT Understanding the nature and formation of unconformities allows for better interpretations of the sediment architecture within a basin and, hence, a more accurate play and reservoir evaluation. Sequence stratigraphy assumes that 1) relative sea-level controls the generation of sequences and unconformities to the exclusion of all other mechanisms, and 2) all unconformities on seismic profiles are unconformities in the rock record. Studies of the geometry of carbonate and siliciclastic shelves from seismic profiles showed that unconformities can form as the result/t of changes in the geometry of sediment dispersal systems and may be independent of sea-level. In addition, unconformities on seismic records may represent rapid transitions when studied in outcrop. Seismic modeling of carbonate platforms showed that, when using typical/ industrial/ frequencies, there are apparent truncations of the margin and significant basin ward shifts in the point of on/sp. The low resolution of the seismic tool also results in apparent pinch outs and pseudo-unconformities. Unconformities on seismic profiles need not correspond to unconformities in the Iithological record. The seismic modeling of outcrops ultimately provides a means for evacuating the accuracy of unconformity interpretations based solely on seismic response. New constraints in the interpretation of unconformity geometry will/ lead not only to more accurate p/ay evacuations but possibly also to revision of the "Vail" sea-level curve which is based largely on the interpretation of unconformity geometries from industrial seismic profiles. 2. INTRODUCTION 2.1 Unconformities and Sequence Stratigraphy Unconformities have long been regarded as the primary elements of physical stratigraphy. It was Hutton (1) who first recognized unconformities as boundaries between cycles of uplift, erosion and deposition. Unconformities in sedimentary sequences are boundaries that mark changes in the depositional regime within a basin and have long been regarded as the primary elements of physical stratigraphy. Sedimentary packages, representing depositional cycles, bounded by these unconformities are called sequences. In the 1960's and 70's the advent of digital seismic data led to the development of seismic sequence stratigraphy (2). Traditionally, unconformities and sequence geometries have been studied both in the field and from seismic profiles. Sequence stratigraphy is the study of these unconformity-bounded units with a view to determining both depositional chronology and selling. With the advent of improved digital seismic data these patterns came to be recognized on seismic profiles leading to the development of seismic sequence stratigraphy (2), These techniques have subsequently been applied to well logs and outcrops with little or no modification. 2.2 Stratigraphic Resolution of The Seismic Tool Not all seismic unconformities may represent unconformities in the rock record. Various authors (2 &3) argue that all reflecting horizons correspond to stratal surfaces or unconformities. From a theoretical viewpoint, this assumption is an oversimplification. The seismic tool responds to contrasts in acoustic impedance within the rock and acoustic impedance contrasts do not necessarily correspond to chrono-stratigraphic surfaces. In addition, seismic wavelets are usually in the order of tens of metres broad whereas most detailed sequence stratigraphy is carried out in the field with resolutions of centimetres or less.