Thursday, January 4, 2018

Searching for Sand - A case Study

Sequence stratigraphy was applied in 1992 in the East Breaks area, offshore Texas. Data included a two-dimensional (2D) seismic line and logs from seven wells. The seismic line was processed for structural imaging and the structural interpretation used for other lines to view the basin as whole. In this case, the big picture shows a basin controlled by normal faulting to the north, which was the direction of the sediment source. Layers dip and thicken to the south. 

Initially ,seismic data and logs were interpreted indepedently to identify sequences and their bounding unconformities. Log-derived boundaries were compared with those from seismic data and the interpretation refined iteratively. Detailed seismic interpretation began with the most easily interpreted reflection patterns, and was pieced together - working upward, downward and back toward the wells - respecting the stratigraphy suggested by the sequence model.

Logs from wells on the seismic lines were converted from depth to time using nearest check shot - here, 3 miles (4.8 km) away. Sands interpreted on spontaneous potential, gamma ray and resistivity logs were associated with seismic reflections at the well and tracked along the seismic section. Shales indicated by logs were noted for correlation with fossil data from cuttings.

Next was integration of biostratigraphy. Fossils from cuttings help identify and date boundaries of each rock sequence. Fossil diversity and abundance are measured versus depth, which is converted to seismic travel time for easy comparison with seismic section. Fossils of planktonic (floating) organisms are more widespread than those of benthic (bottom-dwelling) organisms and are therefor more useful in establishing regional time correlations. However, in shallow-water environments, bethic fossils are used because nearshore conditions may be too variable for planktonic fossils.  

Fossils are also indicators of relative sea level. High fossil counts, or peaks, are associated with shales deposited during low sedimentation. Such conditions occur in the basin during time of high relative sea level, but also in deep water between fan deposits and outbuilding delta deposits. Two shale sections are expected within each sequence, one at the top of the slope fan and the other at the maximum flooding surface, associated with the furthest landward position of the shoreline. Biostratigraphy also holds the key to paleobathymetry - a measure of topography of the ancient ocean floor - needed to interpret the depositional environment.

Paleodepth is derived from benthic fossils with known depth habitats. Knowing water depth helps to interpret deep or shallow water rock types and expected layer thicknesses.

Once seismic, log and biostratigraphic data are combined, a final, color-coded interpreted section is made. Very high amplitude reflections may be highlighted with hatcing. These so-called bright spots are analyzed for anomalies in amplitude variation with offset associated with hydrocarbons. In the East Breaks example, the most promising prospect is a large, sandy basin floor fan. Shales interpreted above and below could provide seal and source rock, respectively. 

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