New, regional 2D seismic data can create fresh insights into basin architecture and regional play concepts, but low oil prices have forced everyone to look hard at every investment, particularly in frontier areas with lengthy payback periods. One low-risk approach to keeping the exploration prospect pipeline flowing, is to reprocess and re-evaluate old seismic data. There is an old rule of thumb that if your data are five years old, they are worth reprocessing, but if they’re 10 years old, they’re worth reshooting. Recent experience in Mexico has shown the potential to push these limits much further.

A NEW FRONTIER

Fig. 1. This original UT stack section shows a number of vertical discontinuities that could be interpreted as faults.

For many years, Mexico has been a major oil producing nation, with all E&P operated by state-owned oil company, PEMEX. The recent decision by the government to allow domestic and foreign operators to explore Mexico created a unique situation, in which PEMEX owned vast amounts of seismic, well-log and production data in the region, but other operators considering an entry into the country had access to little or no data. To these operators, the region was a largely unknown, but potentially prolific frontier.

COMPARING OLD DATA WITH NEW

Anticipating the dramatic regulatory changes in Mexico, in 2012, ION made an agreement with the University of Texas (UT) to reprocess and market 2D seismic data acquired in the southern Gulf of Mexico between 1974 and 1984. The UT data were acquired with state-of-the-art parameters for the time, but the instruments that were used lacked the dynamic range to record really deep data, and the long offsets required for deep imaging were not recorded. The reprocessed UT data (marketed as YucatanSPAN) delivered valuable new information about the region, but lacked data about the deep basin architecture required to fully understand the regional petroleum system.

Fig. 2. The UT stack section shows the final image produced through reprocessing.

In 2015, as soon as legislation allowed new data to be acquired, ION mobilized three vessels to the area to shoot new data, with specifications designed to image much deeper, and to deliver a more complete understanding of the regional geology. The older YucatanSPAN program provided critical insights into designing the new MexicoSPAN survey. Together, the two data sets provide a unique opportunity to validate the information contributed by the old data, but also to highlight the benefits of the new data.

REPROCESSING OLD DATA

Fig. 3. Modern acquisition and processing techniques can image fine-detailed shallow structures.

Data processing algorithms, and the computer systems that enable geophysical data processing have come a long way in the last 30 years, so the potential benefits of reprocessing old data are huge. However, reprocessing data that are 30-40 years old presents some unique challenges. Unless the data have been diligently stored and maintained, they may exist only on obsolete or physically degraded media, and some media may be unreadable or lost.

Additionally, observer’s logs and precise instrument configurations can be hard to find. For data recorded in an era before satellite-based global positioning systems (GPS), putting the data in the right physical location also can be challenging. Sometimes piecing together the fragments of a 40-year-old project requires considerable forensic skills to get every last scrap of available information out of the data. Once the data have been recovered and placed in a suitable format to enable modern processing, the real imaging work can begin.

Fig. 4. The techniques can also image critical deep structures.

The original UT stack section illustrated in Fig. 1 shows a number of vertical discontinuities that could be interpreted as faults. However, careful examination of the field data, and application of static corrections, showed that most of these vertical anomalies were, in fact, artifacts. The section shown in Fig. 2 shows the final image produced through reprocessing. In processing over 12,000 km of 30-40-year-old data, many significant improvements like this were accomplished through painstaking work by a dedicated team, with access to the right tools to overcome these challenges.

ACQUIRING NEW DATA

Fig. 5. A spectral comparison, showing new data extending the bandwidth at both ends of the spectrum.

Using a combination of modern acquisition and processing technologies, we are able to produce higher-resolution images that not only show great local detail, but also allow us to map deep reflectors, such as the Moho. The benefits of being able to map the Moho are two-fold. First, it allows us to get a better understanding of the basin framework by helping define the boundary between the continental and oceanic crust. Second, it can be used to help determine heat flow in basin modeling. Figures 3 and 4 illustrate how modern acquisition and processing techniques can image both fine-detailed shallow structures and critical deep structures.

DATA COMPARISON

From a geophysical perspective, it is interesting to look at the spectral comparison shown in Fig. 5. The new data extend the bandwidth at both ends of the spectrum, resulting in almost double the usable spectral content, enabling higher resolution, deeper imaging and the application of advanced techniques, such as inversion.

Fig. 6. Comparison of old YucatanSPAN data (left) and a fast-track time section from the newer MexicoSPAN data (right).

For the geologist, Fig. 6 is more interesting, as it shows how the improved geophysical performance leads to greater understanding of the basin. It shows a composite line, with the modern MexicoSPAN data on the right, and the older UT data on the left. The most profound benefits of the new data become apparent at approximately 7.5 sec (~8 km), where the often prolific Wilcox Trend can be seen clearly on the new data, but there are only hints of it on the old. The Cretaceous, which is not visible at all on the YucatanSPAN data, can be seen clearly on the new MexicoSPAN data at 9 sec (~10 km). The oceanic crust can be seen at approximately 11 sec (~13 km), and an additional deeper, strong reflector at 14 sec (~25 km).

Geophysical technology continues to evolve, and new data will always deliver a higher-resolution product that allows a more detailed interpretation. It also delivers deeper data that allows us to better understand the basin architecture. However, by using modern technology to carefully reprocess, some of the ambiguities seen in older data can be resolved, and useful information can still be obtained. 

Oil-free design for seismic airgun compressors

DR. (FH) Joachim Winter, sales and product manager, Seismic Compressors, Leobersdorfer Maschinenfabrik GmbH

Leobersdorfer Maschinenfabrik GmbH (LMF), known as the leading Austrian manufacturer of high-pressure compressors and a new piston compressor technology trendsetter, has successfully developed the first seismic airgun compressors to use completely oil-free technology.

If we look back to the standard compressor solution, an oil-injected rotary screw compressor is used to feed the oil-lubricated, high-pressure piston compressor. Both compressors are driven directly by a centrally located, electric motor or diesel engine. This concept is called the LMF compound compressor concept, which has become the number-one choice for marine seismic survey operations.

LMF has successfully developed the first seismic compressors to use completely oil-free technology.

The oil-free design is not new for LMF, as this technology has been used already for piston compressors feeding the PET-bottle blow moulding machines, producing up to 25,000 pieces of PET bottles/hr. Considering the experience of more than 15 years, it was only a matter of time before this clean and environmentally friendly technology was demanded by the marine seismic research companies.

The advantages of this oil-free design make all the difference.

The high-pressure air, up to 3,000 psi, is not contaminated with any oil content and, from an ecological standpoint, completely harmless to feed the airguns exposed to the sea ambient. This fact can be proven easily when taking a test probe out of the condensate. Due to the humidity in the ambient, liquid falls out after each stage of compression. Even this condensate is clean, clear and theoretically has drinking quality.

Beside the environmentally friendly, zero-pollution philosophy, this design pays off during the product lifecycle. The running costs are reduced significantly, as the costs for supplying the cylinder lubrication oil, and the costs for disposal of the oily condensate (collected in the oily water tank), are nearly disappearing. Even if the service demand for these kinds of compressors is increased slightly, the total costs of ownership (TCO) are downsized. 

The new LMF oil-free compound compressor design can be used as an energy source for appropriate airgun systems, whenever highest capacities with smallest space requirements and weights are demanded.

The basic design was kept unchanged, and it still combines the advantages of both rotating and reciprocating systems. A rotary screw block requires far less space than that taken by other low-pressure systems. The piston compressor completes compression after the screw has reached the pressure limit imposed by its geometrical design.

Its core components are an oil-free, double-stage screw compressor and an oil-free, balanced, opposed piston compressor in a three-stage design, made by LMF. Both compressors are coupled directly on both shaft ends to the main driver, mounted on one common base frame to result in a compact, rigid package.

The units are available with an output of 8 m³/min., up to 78 m³/min., 2,755 cfm free air delivery, and a pressure outlet of 2,000 psi or 3,000 psi. They also come along with the following product highlights:

• No lube oil requirement for screw compressor
• No cylinder lube oil requirement for piston compressor
• No logistic costs and efforts for screw compressor and cylinder lubrication oil
• No disposal costs and efforts for screw compressor and cylinder lubrication oil
• No oil content in condensate (100% clean condensate)
• No oil-contaminated, high-pressure air toward high-pressure distribution manifold and airguns

Thus far, two projects with completely oil-free compressor technology have been realized and installed successfully on seismic research vessels. Nobody knows how long the downturn in the seismic research business will last, or when marine research surveys will reach almost stable demand again. However, LMF is prepared for upcoming tasks, to supply both lubricated and totally oil-free compressor systems for marine seismic surveys.