Time to Pick? No Need to Fear ‘Seismophobia’

Contributors: Supratik Sarkar

While seismic processing shops usually are the “professionals” when it comes to velocity analysis, the interpreter may have greater insight into the cause of the poor data quality.

In such cases, re-picking the velocities on a tight grid with a better knowledge of the geology can provide significant improvement.

The following example (figure 1) is from a tight turbidite formation in Mexico’s Chicontepec Basin, where the data are often handicapped by interbed multiples from overlying shallow volcanic sills.

While the deeper, non-targeted Cretaceous horizons were well resolved – appearing as coherent, broad-band reflectors – the shallower Eocene/Paleocene Chicontepec Formation reflectors of interest were “wormy” and narrow-band.

The original data were prestack time-migrated using a Kirchhoff algorithm, resulting in 50-meter offset bins ranging between 50 and 3,000 meters. The preliminary interpretation of the stacked data showed significant reduction of data quality below the volcanics.

Maps of the volcanics were made to highlight the problem areas.

The original migration velocities were then removed (figure 1b) using simple reverse normal moveout, followed by a velocity analysis on a dense 375-meter x 375-meter grid.

Seismic processors will recognize this workflow as comprising the key steps within a typical processing sequence – using simple software tools to generate a residual velocity analysis of the previously migrated data.

The key step was to take care beneath the volcanics to pick the slower Chicontepec reflectors rather than the strong, slightly faster interbed multiples (figure 1a), resulting in the flattened gathers like the one shown in figure 1c.

After the residual velocity analysis, the frequency content and the signal-to-noise ratio of the dataset improved in many places within the Chicontepec interval compared to the original prestack time-migrated data.

This residual velocity analysis resulted in better vertical and lateral definition of stratal units as seen by seismic amplitude as well as computed attributes (figure 2).

This velocity analysis also helped enormously to delineate turbidite channels, internal stratal geometries and the distribution of potential reservoir element.

Several volcanic activities in east-central Mexico from the Late Cretaceous to Miocene have been published. The burial history chart from the adjacent Veracruz Basin – where petroleum generation occurred from Upper Jurassic source rocks similar to those in the Chicontepec play – shows that oil generation and migration started around 20 Ma and continued through the Miocene.

Since most of the oil migrated after the major volcanic activities, improving the imaging below the volcanics also provided us ideas how the volcanic bodies enhanced fracture porosities in reservoirs in some areas – specially through enhanced seismic attribute images.

I wish to thank my adviser, AAPG member Kurt J. Marfurt from the University of Oklahoma, for his constant persistence and encouragement (to a geologist) to complete the work, part of which is reflected in this article.

I also would like to thank Pemex, as well as Sergio Chavez Perez from IMP, who provided the data for working this project. I am thankful to AASPI consortium at OU for all the support.

Comments (0)


Geophysical Corner

Geophysical Corner - Satinder Chopra
Satinder Chopra, award-winning chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer began serving as the editor of the Geophysical Corner column in 2012.

AAPG member Supratik Sarkar is an exploration geologist in the Deepwater/Frontier New Ventures Business Development group with Shell in Houston.

Geophysical Corner

The Geophysical Corner is a regular column in the EXPLORER that features geophysical case studies, techniques and application to the petroleum industry.


Image Gallery

See Also: Book

Desktop /Portals/0/PackFlashItemImages/WebReady/book-s61-Sediment-Transfer-from-Shelf-to-Deep.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3988 Book

See Also: CD DVD

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4464 CD-DVD

See Also: DL Abstract

Over the last two decades, numerical and physical experiments have repeatedly generated insights that contradict the sequence stratigraphic model that is near-universally used to interpret ancient strata in terms of relative changes in sea-level. This presentation will re-examine Upper Cretaceous strata (Blackhawk Formation, Castlegate Sandstone, Mancos Shale) exposed in the Book Cliffs, east-central Utah, USA, which are widely used as an archtype for the sequence stratigraphy of marginal-marine and shallow-marine strata. Stratigraphic architectures in these strata are classically interpreted to reflect forcing by relative sea level, but key aspects can instead be attributed to autogenic behaviors and variations in sediment flux.

Desktop /Portals/0/PackFlashItemImages/WebReady/reinterpretation-of-sea-level-driven-stratigraphic-architectures-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 11401 DL Abstract

See Also: Learn! Blog

Despite intensive research, the origin of dolomite remains subject to considerable controversy. This is partly because some of the chemical and hydrological conditions of dolomite formations are poorly understood. Here's a great chance to see an overview of dolomite research and highlight its major advances and controversies in recent years. This will be done by revisiting specific case studies and implementing newly developed concepts and models.

Desktop /Portals/0/PackFlashItemImages/WebReady/ec-fec-concepts-models-and-case-studies-of-dolomitization.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 11749 Learn! Blog

See Also: Online Certificate Course

Wind Energy Basics is an online course that enables participants to review, analyze, and evaluate opportunities in the rapidly expanding market for wind energy.

Desktop /Portals/0/PackFlashItemImages/WebReady/oc-cc-wind-energy-basics.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 1447 Online Certificate Course