Application Of High-Resolution Seismic Reflection Data In Assessing The Size, Shape, And Lithology Of Near-Surface Sand Bodies

Abstract

ABSTRACT Several thousand kilometers of high resolution seismic reflection data were acquired from the east Texas continental shelf to map the major depositional systems formed since the last glacial -ecstatic lowstand. This work was followed by more detailed surveys of selected depositional systems using a variety of seismic sources and aimed at characterizing seismic facies of sandy systems. Several hundred sediment cores and oil company platform borings provided ground truth of seismic facies interpretations. Several types of sand bodies were investigated, including incised fluvial valleys, fluvial deltas, transgressive sand bodies, tidal inlet/tidal delta deposits, and shoreface deposits. Seismic reflection character, and therefore eismic facies, are controlled mainly by the scale of sedimentary structures within the deposit, the nature of bounding surfaces, and by lateral and stratigraphic variability in lithology. Distributary channels of shelf margin deltas and tidal inlet/tidal deltas complexes have large-scale cross stratification that is imaged with most high resolution sound sources. The main exception to this is fluvial sands that are buried beneath estuarine and marine muds within incised valleys. Delta mouth bars are characterized by a chaotic reflector pattern. Shelf sand bodies and sho reface sands have small-scale sedimentary structures that are imaged only with high frequency (3.5 kHz) sound sources. Attempts to estimate the size, distribution, and volume of sand bodies based on seismic data alone met with poor success. However, when good quality seismic data and reasonable core coverage are coupled with sedimentation odels, good results are obtained. INTRODUCTION Clastic sand and gravel deposits on low latitude (non-glaciated) continental shelves vary greatly in size, shape, thickness and internal stratification. One of the key problems in exploiting these sand resources is locating them, predicting their subsurface expression, and estimating their volume. High resolution seismic reflection profiling provides an invaluable tool for accomplishing these tasks. Good quality seismic data can be used to identify seismic facies which, when combined with the proper sedimentation models and sediment cores, provide a powerful tool for predicting the size and shape, and therefore the volume, of a sand body. In any seismic investigation the objective is to get the best possible resolution while penetrating deep enough to image features of interest. But there is always a trade-off between improved resolution and depth of penetration. It is important to know what is being imaged in terms of depth of burial, thickness, and the resolution required to image diagnostic features so that the right tools and acquisition parameters can be selected. The seismic expression of a sand body is controlled largely by internal stratification, which occurs at a variety of scales. The type of stratification is largely determined by the sedimentation processes that form the sand body, and therefore its depositional environment. There is a wealth of outcrop information on internal stratification for virtually every type of sand body and this information can be used to refine seismic facies models.