India Seismic Gets New View

By LOUISE S. DURHAM, EXPLORER Correspondent

The Andaman Islands, located in a basin known for its complex tectonic history, is frontier territory for explorationists – and a recent target for a seismic reprocessing effort. Graphic courtesy of Google Maps

When you reprocess a line of seismic data, the second time around can provide a passel of new info.

This was the case when Spectrum ASA reprocessed a 10,600-kilometer line of 2-D multi-client data in the geologically complex area offshore the Andaman Islands off India’s east coast.

“The seismic data are from five different surveys ranging from 1982 to 2001 vintage,” said Gary Scaife, geological adviser at Spectrum. “The seismic dataset extends over the fore-arc basin, volcanic arc and back-arc basin areas of the Andaman Sea Basin east of the emergent islands.

“The earlier processing was good for the time,” Scaife said. “We reprocessed using both industry-wide algorithms and our own in-house algorithms; the project included both pre-stack time and depth migration, or PSTM and PSDM, respectively.

“The main objective of the reprocessing effort was to provide a good quality regional survey,” he said, “and to improve the interpretability of the complex structures found within the dataset by improving the imaging of the entire seismic section, both shallow and deep.”

Spectrum was awarded the project via the Directorate General of Hydrocarbons in India.

The Andaman Sea Basin has evolved through a complex tectonic history beginning in the Cretaceous associated with the oblique convergence between the Indian and west Burmese tectonic plates.

Scaife noted the main tectonic elements that can be observed going west to east:

• Andaman Trench/Inner Slope.
• Outer High/Trench slope break.
• Fore-Arc Basin.
• Volcanic Arc.
• Back-Arc Basin.
• Mergui Terrace.

“The Indian sector of the Andaman Sea Basin is regarded as frontier territory, with only 13 wells drilled in the project area east of the Andaman Islands,” Scaife said. “The first of these wells hit gas in Miocene limestone.”

This discovery was determined to prove the existence of active hydrocarbon systems that contain generating hydrocarbon source, reservoir and seal features along with migration and trapping mechanisms.

“Even so,” Scaife noted, “the majority of the basin, particularly deepwater, has yet to be explored.”

Pleasing Potential

The Andaman Sea Basin lies between and on trend with the mature hydrocarbon-producing provinces of Myanmar to the north and Indonesia to the south.

Both these areas contain world class producing fields.

An example of limestone build-up – the basin’s cap rock. The section length is 28 kilometers.

Scaife emphasized that features observed on the Spectrum reprocessed seismic appear to be analogous to these fields and indicate these successful play fairways can be extrapolated into the Andaman Sea Basin frontier area.

Late Cretaceous and Eocene age sediments are reported to be the primary source rocks in the basin. They possibly are the source of the gas in the Miocene in the aforementioned discovery well.

“Biogenic gas is reported to be sourced from Neogene sediments as evidenced from the present day active mud volcanoes,” Scaife said. “Miocene carbonates and turbidites of the Mio-Pliocene are reported to be the reservoir rocks with intra-formational shales and tight limestone of Neogene age acting as cap rocks within the basin.”

Interpretation of the reprocessed data has shown that the Andaman Sea Basin harbors all of the necessary components for successful hydrocarbon exploration.

The interpretation identified potential source, reservoir (carbonates and clastics) and seal intervals and also structural and stratigraphic trapping geometries.

Scaife noted that direct hydrocarbon indicators (DHI) are observed, which include:

• Gas clouds.
• Bright and flat spots.
• Seabed pock marks.
• Vent mounds.

He added that bottom-simulating reflectors indicate the presence of gas hydrates.

No Slam-Dunk

Granted, the reprocessing program yielded significant results over the earlier processing.

But getting from there to here was no slam-dunk.

Scaif summarized the considerable challenges to the reprocessing effort:

• Data spread over a large area incorporating very varied geological terrains.
• Differences in ages and parameters of acquisition and the very sparse grid.
• Big bathymetric range from very shallow water to 3.5-plus kilometer water depth.
• Often dealing with high relief and abrupt changes in water depth.
• Structural/tectonic complexity, e.g., numerous steep dips with the angle and direction of dip varying dramatically within short distances.
• Complicated ray paths and considerable amount of data from out of the plane of the section complicating the velocity analyses and contaminating the multiples.
• Shallow reefs, rugose small-scale structures and very variable velocities scatter energy and complicate deeper imaging.

When all was said and done, the imaging was improved via:

• Reducing multiple contamination.
• Enhancing the signal-to-noise ratio by attenuating energy scattered by shallow geology.
• Improving the continuity of events.
• Enhancing the temporal and spatial resolution and, therefore the seismic character.
• Improved imaging of the deeper section.