Summer, 1994

About SCA
and geoLOGIC

SCA is a worldwide petroleum industry leader in professional consultancy and advanced training services. From major synergistic field studies to sequence stratigraphy, from property evaluations to prospect reviews, our staff of geologists, geophysicists, and engineers have the expertise and experience to provide you with the very best service and training available. Since 1988, we have helped our clients discover billions of barrels of oil and train for the challenges of the new millennium. We are proud to serve you and hope you enjoy reading geoLOGIC. For more information on SCA, please contact us today.

Subsurfave Consultants & Associates, LLC
10255 Richmond Ave., Suite 300W
Houston, Texas 77042
Phone: +1.713.789.2444
Fax: +1.713.789.4449
info@scacompanies.com

ESTABLISHING THE SEISMIC/SUBSURFACE CORRELATION

Establishing the correct seismic/subsurface correlation is a critical element in the interpretation process. It is especially important when dealing with stratigraphic or structural/stratigraphic traps delineated by seismic amplitude anomalies. A vertical seismic profile (VSP) is the most effective, method for determining the relationship between the seismic reflection and the subsurface data. However, a VSP is usually unavailable.

Synthetic seismograms are the traditional method of determining the seismic/subsurface correlation. Unfortunately, synthetics are rarely definitive. Most synthetic matches are equally good using reverse polarity and a half-cycle shift. Therefore, a synthetic correlation is ? 180 degrees. This is rarely sufficient for development work, and results in higher risk for exploratory prospects.

Subsurface Consultants & Associates, Inc. prefers to use fault surfaces to determine the seismic/subsurface correlation. Fault surfaces derived from 3-D seismic data are definitive when a fault cut in a well can be identified with certainty, the fault resolution of the data is adequate, and the well directional survey is accurate. Under thesecircumstarces, fault surfaces can provide a reliable seismic/subsurface tie.

This example is from the offshore Gulf of Mexico. The seismic display in Fig. 1 is an arbitrary line between a dry hole (#1) and a proposed well (#2). The geophysicist processing the data assured the client that the seismic data were zero phase: a positive reflection coefficient results in a peak on the seismic section. The primary objective is the Q Sand. The top of the Q Sand, a low impedance interval, should generate a trough on the seismic data, Fig 1. Updip of Well #1, the trough amplitude increases significantly and then decreases in amplitude until the trough terminates altogether. The high amplitude area was interpreted as a gas filled sand. Therefore, a location was proposed to test the Q Sand within the amplitude anomaly. Since a structural trap could not be demonstrated, an updip pinchout of the sand between two ENE trending faults formed the postulated trap.

The intersections of these two faults, striking parallel to the seismic line, and the seismic line are shown as the dashed lines on Fig. 1.

The #2 Well was drilled and found the Q Sand shaled out. A high velocity shale section was noted just below the Q Sand interval. Post mortuum analysis of the sonic and density log data from the #2 Well indicated the amplitude anomaly resulted from the high velocity shale interval. The seismic data are in fact reverse polarity. The revised seismic subsurface correlation is shown in Fig. 2. The Q Sand event is now a weak peak that terminates updip a short distance from the #1 Well. Note the excellent fit of the two fault cuts in the #1 Well and the single fault cut in the #2 Well to the fault surfaces using the revised seismic/subsurface correlation. The original seismic/subsurface correlation consistently placed the fault cuts deeper than the fault surface. This information was ignored because of the belief that the data were zero phase. Utilizing this information would have led to a more accurate assessment of the risk associated with this prospect.

It is important to use, more than one well to determine the seismic/subsurface correlation. Minor errors in the directional survey or small deviations in assumed straight holes will result in large differences in the intersection of the wellbore and the steeply dipping fault surface.


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Subsurface Consultants & Associates, LLC

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