ABSTRACT

A set of 3-D rock-property models were generated for a Lower Miocene shorezone system at Powderhorn Field, Calhoun County, Texas. These models were based on detailed mapping of facies and rock-properties that involved 250 m of stratigraphic section over an area of 9 by 13 km. Examination of logs from 115 wells showed that 14 sandy depositional units averaging 3-30 m were encased in 15 shaly units. Effective porosity, P-wave velocity and bulk density were calculated from suites of logs from five recent wells, and then expanded to older wells through regression with SP and resistivity logs.

Idealized 3-D seismic models were generated by convolving a Ricker wavelet with the primary reflection coefficient series from stacked 2-D P-wave velocity and impedance models. The analysis of these models revealed that the commonly used approach of peak picking on a workstation may not generate correct stratal slices because seismic events do not necessarily follow geological time lines. In some cases, seismic events from sand bodies in different units may interfere, forming erroneous images. An event tying as many as six depositional units over 50 m has been observed. Phantom mapping between reference events produces truer stratal slices, and makes it possible to map, in detail, depositional systems from 3-D seismic data.

Stratal slices from real 3-D seismic volumes in Gulf Coast Tertiary demonstrate that sequential, fairly high-resolution amplitude images of various depositional facies can be obtained from lower frequency seismic data. Depositional facies patterns on those stratal slices provide much desired high-frequency information that may not otherwise be recognized on vertical seismic sections.