Gulf of Mexico Shelf:

ST-26 O Sand

A partially depleted 90+ MMBOE EUR oil and gas reservoir, from 11,000 to 13,000 feet

Movie #1: Shows a VoxelGeo-animated D3D*-impedance representation of a buried Common-Impedance Object (CIO) at South Timbalier OCS Block 26. It is interpreted to be the original high-GOR-oil-filled "O" sand reservoir at a depth of 12,000 feet.  The animation, color, and opacity control in this movie is provided by the use of a volume-visualization program, VoxelGeo (licensed from Paradigm Geophysical).  Our interpretation of this CIO, shown spinning and changing transparency to highlight various aspects of its stratigraphic complexity, is that of the Upper Miocene (CRIS K) "O" delta sand reservoir, discovered by Shell Oil in November 1968. It was subsequently produced by Shell (including a 1970 B-Platform blow out fire, extinguished by Red Adair) and is currently producing for EPL and Apache.  To date, it has produced over 90 million barrels of oil, and equivalent gas.

In this first movie, the yellow, red, and pink colors are formed by tens of thousands of tiny, opaque and translucent, high-resolution "cubes" (or voxels = seismic samples), with the lowest D3D*-impedance samples assigned the opaque yellow color.  They can therefore be seen inside the more translucent, red and pink, higher-D3D*-impedance voxels that surround them. Based on many years of working with D3D* volumes, a key assumption is that compared to the higher impedance values of the shales and water-filled rock surrounding the reservoir, the lower the impedance value, the more recoverable (slow and low density) hydrocarbons the local rock volume (voxel) is likely to contain.

Thus, some of the opaque yellow voxels are interpreted to represent small bypassed compartments (sweet-spots) of producible hydrocarbons, at the time the ocean-bottom cable 3-D seismic data were acquired by WesternGeco (December 1996). The mostly translucent, but sometimes transparent, voxels are interpreted to reflect uncommercial residual oil, left behind by the water-flooded production activity. Note the opaque yellow voxels down-dip in the reservoir.  Many of these fall between producing and water-injecting wells (not shown) and these are what we interpreted to be bypassed hydrocarbon zones, in this stratigraphically compartmentalized delta-front sand.  A conventional 3D image of the totally opaque seismic volume is shown at the end of the movie. The faces of the cube are Slices (horizontal time, vertical in-lines and cross-lines) of this display product and are useful for horizon-surface and fault-surface (structural and amplitude) mapping.

Technically, the "D3D*-impedance" of a small quantity of fluid-filled rock, is a name given to a calculated seismic attribute, assigned to each processed seismic sample on each migrated and integrated seismic trace. It is an "acoustic" seismic parameter, calculated by using all the non-seismic information available (including recording parameters, well velocity surveys, well-log-based synthetic seismograms, preliminary time structure maps, etc.), and by assuming that the earth's sedimentary crust is composed of complex buried 3D objects with "common-impedance" characteristics. This contrasts with the conventional processing and interpretation assumption of simple stacked and possibly faulted layers, whose thickness and lateral acoustic properties vary gradually and continuously. The D3D*-impedance is a relative quantity, not an absolute quantity, and is computed without measuring "amplitude vs. offset" (AVO) variations, and without depending on the accuracy of well log curves to exactly represent buried rock properties.  In fact, the D3D*-impedance can be thought of as an OVA (offset variations absent!) attribute, whose relative value (from -128 to +127, using an 8-bit digital word) is proportional to:

The natural logarithm of the actual Acoustic Impedance(AI) of a small volume of buried, fluid-filled-rock.

The Acoustic Impedance of a fluid-filled rock is an important measurable quantity, defined as the bulk density (in grams per cubic centimeter), multiplied by the near vertical speed-of-sound (of a compressional acoustic wave, in feet or meters per second) through the rock volume. Using a volume-analysis system (like VoxelGeo), each D3D*-processed seismic sample can be rendered as a volumetric-picture-element (or volume-PIXEL, or VOXEL), positioned in a virtual 3-D TWT-space, viewed on a desktop computer monitor. Each VOXEL has the dimensions of the seismic trace spacing, multiplied by the (seismic echo two-way-time) digital sampling interval. The volume of each individual voxel in this animated cyber-approximation of the ST-26 "O" sand reservoir, is close to 3.1 acre-feet (82 feet x 82 feet x 20 feet, or 4 milliseconds). At this depth and location beneath the shallow-water Gulf of Mexico OCS (Outer Continental Shelf), this high-GOR-oil-filled  "O" sand has a surprisingly uniform, measured (by acoustic well logs) velocity of about 10,000 feet per second.  The viewing direction was manipulated to render the D3D*-impedance CIO at various inclination and azimuths viewing angles, and snapshots were captured and organized into a series of (apparently rotating) color and transparency varying, animated movie frames. Finally, these frames were collected into a web-friendly, Shockwave Flash animation file format.

Click on the movie icon , to start and then control the playing of the movie.  On machines using Microsoft's Internet Explorer browser, a right mouse-click in the movie image window will display a movie-control menu, with which the movie can be stopped, zoomed-in and -out, moved around, restarted, and reversed.

Acknowledgement

VTV, Incorporated, gratefully acknowledges the written permission to publish these images, granted by Energy Partners, Ltd. [EPL], Apache Corporation, and WesternGeco (the owner of these multi-client seismic data).

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