There is an industry-wide awareness of the importance of carbonates in the commercial development and production of hydrocarbons. Yet there is a mystique about them that arises partially from the nature of their genesis and also from their higher velocity/density nature (even when geologically young), which has always made their seismic imaging a bit more challenging. Many advances address these issues, in particular, better and higher resolution processing and improved seismic data presentations. Most recently, replacing standard signal processing methods by holography has produced highest possible resolution (HPR) seismic imaging.

Ground rules for HPR

In conventional seismic imaging, four “ground rules” exist: There is a propagating waveform, all boundaries are sharp and well-defined, reflections are “scaled” copies of that waveform that broaden with reflection time as higher frequencies are lost, and so resolution decreases with reflection time. In holographic imaging, the counterparts are quite different. There is no propagating waveform; there is only propagating wave energy, and its frequency does not really matter —holographic images can be formed using a single frequency. Boundaries now are viewed as having realistic geologic properties. Low-energy boundaries such as shale/shale contrasts can have frequencies in the hundreds of Hz, while higher energy shale/sand boundaries top out at 60-160 Hz. Even higher energy reefal environments typically reach only 40-90 Hz. Modest as these limits seem, they are 3-5 times the values attained by standard signal processing approaches.

This image shows a conventional display of the South Texas holographic imaging. (Images courtesy of Norman Neidell)

An important corollary of HPR is that since any single frequency or range of frequencies can form the image, the loss of resolution with increased reflection time is far less pronounced. It diminishes somewhat as the imaging curves that need to be applied become less appropriate approximations to the real physics, and the requisite velocity information also is less precise. Bandwidths typically used for seismic acquisition offer quite robust holographic imaging. Color displays

The importance of data presentations cannot be overstated. Two issues should be raised: why inversions, and why such strange color combinations? Inversion displays are always preferable to reflectivity presentations, which today remain the dominant format. A single key advantage favors inversion, especially for carbonate reservoir regimes. In many instances, hydrocarbon-filled porosity is signaled by an interval velocity drop. This can usually be identified with ease for most reservoirs, consolidated or unconsolidated, carbonate, clastic, or even unconventional. Also HPR inversion is a “composite” display . It is a blending of a simple trace integration and a coarse interval velocity model developed from very detailed moveout velocity information No information from well logs is used in producing these displays. Hence, these inversion displays treat hydrocarbon indications in both high and low acoustic impedance circumstances without bias. With an effective use of highly contrasting colors, visual dynamic range is extended significantly, 20-fold or more for conventional black and white presentations, and fivefold or more as compared to typical color data displays. Such displays look more like geology.

The Austin Chalk and Eagle Ford

In the 1980s, the Austin Chalk was a common exploration play over a fairly large swath of Texas. Production from this objective was typically quite prolific but short-lived, often depleting in just a few months but producing good economic returns nevertheless. Horizontal drilling technology was evolving and growing in its use and sophistication.

A line in the conventional display (bottom) is compared to a composite inversion (top).

Wavelet processing with dense, high-resolution velocity work indicated three Chalk members. The lower member has the highest velocities, and the middle member has the lowest values. The velocity drops in each member could be readily recognized individually. Velocity drops in the high velocity and very consistent Buda could also be noted. Using information like this, dozens of Austin Chalk wells were successfully drilled with vastly improved economics as the vertical position within the section and lateral extent of the porosity developments were visible and known in advance.

Holographic imaging and inversion was performed in 2011. As before, velocity variations within the Chalk are visible. The Buda, Del Rio, and Georgetown below also are readily recognized. It’s important to note that synthetic seismograms cannot be used to make effective ties because of their restricted frequency content and inability to track the imaged detail.

In these images, the Austin Chalk is quite different and shows what appear to be several cycles of reefing. Two separate reefs are positioned on a discernible slope change on the Eagle Ford. Chalk velocities are at about 6,710 m/sec (22,000 ft/sec) in the lower members but only 4,730 m/sec (15,500 ft/sec) in the upper cycles, which possibly may be detrital material. These are still significantly above the velocities characteristic of the shales at this depth.

The two reefs are related by an atoll feature with a very distinct geometry. Such an environment can explain the cycles of possible detrital material seen above the harder Chalk. Within the body of the reef itself, velocities as low as 5,185 m/sec (17,000 ft/sec) can be seen.

A close-up of the previous figure shows the Eagle Ford interpretation.

A standard seismic data presentation of the parent data does not readily identify the reefs – a composite inversion is required. The two reefs would certainly not be readily identified even in the conventional presentation of the HPR data.

Another view of the same composite inversion cube focuses on the Eagle Ford formation. The trends can be mapped and are conformable with the regional dips. Fracture imaging has not been attempted, but the HPR image data would characterize fractures more effectively than conventional presentations. Most typical attribute calculations may be applied to this improved data.