Reprocessed seismic, including a line acquired more than 40 years ago, is providing new insight into an underexplored area of Colombia.
This reinterpretation in the Sinu and San Jacinto basins has upended previous thinking and could lead to a major new exploration effort in the area.
Searcher Seismic undertook the project to revisit a producing but mostly overlooked region of the country.
“The prospective Colombia onshore Sinu, San Jacinto and Lower Magdalena Valley basins have a large amount of legacy seismic data which has not been reprocessed using modern available algorithms,” said Karyna Rodriguez, vice president of global new ventures for Searcher Seismic in Guildford, England.
“This provided a perfect opportunity for a seismic reprocessing project, as poor imaging of legacy seismic data has led to unchallenged geologic understanding of the prospectivity in these basins,” she said.
Rodriguez and other team members will discuss the reprocessing project and the prospective exploration area in the Tuesday, Aug. 29, presentation “Breaking geological paradigms with reprocessed seismic date in the Sinu and San Jacinto basins onshore Colombia,” at the AAPG-SEG-SEPM International Meeting for Applied Geoscience and Energy in Houston.
Past Exploration Efforts
“In the Sinu and San Jacinto basins, exploration history dates to the 1900s. (A series of) small discoveries between the ‘20s and ‘50s showed that there was potential for oil and gas,” Rodriguez noted.
“However, commercial discoveries during the ‘40s and ‘50s moved attention to the Lower and Middle Magdalena basins, leaving (the) Sinu and San Jacinto basins largely underexplored,” she said.
Despite the positive well indications, “due to its poor quality and limitations, legacy seismic data (processing) cannot be used to understand the implications of the discoveries or to properly de-risk the elements of the petroleum system,” Rodriguez observed.
“Based on our experience of reprocessing data in structurally complex regions, Searcher believed this could only be achieved by reprocessing the seismic data,” she said.
This area of northwestern Colombia has long been considered prospective for both oil and natural gas. In 2022, NG Energy International of Vancouver reported a significant gas discovery in the Ciénaga de Oro sandstone with its Magico-1X well in the onshore Sinu 9 block, in Colombia’s department of Cordoba.
Colombia has been going through a debate on whether or not to ban future hydrocarbon exploration, a move proposed by leftist President Gustavo Petro, who took office in August last year.
Making Old Data New Again
Searcher works around the globe identifying prospective basins with legacy data that could benefit from being reprocessed through modern processing algorithms, in order to enhance subsurface imaging, Rodriguez said.
The Colombian basins fit the bill, with available seismic data that had never been processed with modern techniques, plus complex conditions that made subsurface imaging especially difficult.
“The complexities of structure and geology have meant that the subsurface seismic images acquired to date have various imaging challenges, including rough topography resulting in statics both as near-surface refractors and subsurface events, as well as making signal alignment difficult,” Rodriguez said.
“The strongly thrusted environment results in rapidly changing velocity due to highly structured geology. Legacy seismic suffers from poor (signal-to-noise ratio) due to penetration of the source and dissipation of source energy,” she noted.
Also, legacy acquisition methods from the 1970s to the 2000s are often associated with limited and/or poor offset distribution, Rodriguez observed.
“Older data acquisition methods for recording relied mainly on manual data entries with many errors in observer’s and survey notes. Capturing and fixing these errors can be a very manually intensive job, as well as building geometries and quality control by hand,” she said.
“This latter task is best undertaken by those senior geophysicists who worked on these problems 30-plus years ago, as that generation of geoscientist are familiar with the old methods and understanding observers logs,” she added.
Searcher sifted through available seismic data in the basins to find lines meeting its best reprocessing criteria, plus supporting information.
“A number of test lines were selected based on various geological considerations such as structural complexity affecting imaging, and other factors such as nearby well control, year of acquisition and available seismic information,” Rodriguez said.
“Two of the selected lines were acquired in 2006 and 2008, respectively, while a third line was acquired in 1980,” she said.
According to Searcher, the reprocessing included data loading and logistically challenging geometry quality control, followed by refraction tomography with integration of near-surface velocity into the initial velocity model and iterations of the isotropic depth model.
Velocity scanning and merging the velocity scan and tomography fields into a final velocity model were critical steps, the company reported.
‘Breaking Paradigms’
While lack of signal below surface mud volcanoes had been interpreted as mud diapirs, “velocities are much higher than expected for possible mud diapirs, implying that a different geological model can be proposed for these complex structures,” Searcher noted.
“Additionally, unexplored clear and thick Cretaceous sedimentary sequences have been revealed along the San Jacinto Fold Belt, reducing the longstanding uncertainty in the presence of a Mesozoic section in this basin and opening up the possibility for Sinu.
“A new, onlapping Oligocene sand play has been imaged which can be correlated to outcropping Oligocene sand sequences. Finally, a series of unexpected normal faults have been imaged for the first time in the San Jacinto Basin which could finally explain some of the structures drilled in the past in this area,” it noted.
Rodriguez said the company’s state-of-the-art technology “focused on critical aspects, including rigorous, near-surface velocity modeling – refraction tomography – and depth imaging with careful velocity model building. Time migration was not considered to be adequate.”
She referred to the resulting reinterpretation of the basin realities as “breaking paradigms.”
“The Sinu and San Jacinto basins have geological models built on legacy seismic data which have been widely accepted and not contested for many years. As the reprocessed data does not support many aspects of the old models, there is some resistance to change the long-established ideas,” she said.
“The application of advanced algorithms to reprocess legacy seismic data are challenging the established hydrocarbon systems and their inherent geological models directly. In the Sinu Basin, the trap systems previously modelled to be related to mud diapirs could in fact be related to the presence of salt,” she noted.
And along the San Jacinto Fold Belt, “the geometry of structures and the presence of a thick, unexplored Cretaceous sedimentary sequence are some of the key insights that will allow explorers to understand these prospective areas in a new and innovative way,” she said.
Going forward, a regional reprocessing project could allow a full, updated evaluation using enhanced seismic imaging at a large scale, Rodriguez said.
“Three test lines have already indicated that the geological models need to be revised and modified. A regional, reprocessed dataset should allow a better evaluation, and thereby encourage a new era of exploration activity in the very near future,” she said.