Углеводородное изучение глубинных континентальных окраин, использующее несейсмические методы (стр. 3 )

В экваториальных областях осадочные бассейны часто проявляют себя как простые гладкие положительные магнитные аномалии и, на самом простом уровне, нужно только найти такие аномалии для предсказания местоположения бассейна. Malay бассейн в Малайзии - хороший пример (рисунок 11) где магнитная аномалия отражает основную геометрию бассейна. Хотя этот бассейн имеет более 14 км отложений, едва имеется какое-либо гравитационное выражение низкоплотностных отложений! Это из-за изостатического отклика поднятого уровня Моха ниже бассейна и демонстрирует важность использования как гравиметрических, так и магнитных данных скорее вместе, чем изолированно. Не всюду пространственное покрытие магнитных данных столь же хорошо, как показано на рисунках 10 и 11. В таких областях отдельные магнитные профили через бассейны могут обеспечивать важную информацию.

Рисунок 10. Карибские сравнения повторно обработанных опубликованных региональных магнитных данных и спутниковые гравиметрические данные в свободном воздухе. Отметьте подобия и различия (2° graticule для масштаба)

Рисунок 11. Полная интенсивность магнитного поля Malay бассейна оффшорной зоны полуострова Малайзия показывает гладкую большой амплитуды положительную аномалию по бассейну. (1° graticule для масштаба)

Заключение

Ценность исследования высокого решения (до 10 км в 2-3 mGal) спутниковых гравиметрических данных теперь оценивается многими международными нефтяными компаниями. Они используются для картирования геологических региональных структур до масштабов концессии для идентификации структур, которые являются достойными дальнейшего исследования. Вместе с магнитными данными, есть ли это отдельный профиль или хорошее пространственное покрытие, и данными о просачивании нефти, несейсмические данные станут ценным и эффективным по стоимомти инструментом исследования. Эффективность по стоимости в этом случае означает, что данные могут использоваться целенаправленно в тех областях, где закупка спекулятивных сейсмических данных будет оправдана. Это не может быть сообщением, которое сейсмические компании хотят слышать!

Ссылки

Fairhead, J. D., Green, C. M. and Dickson, W. G. [2001a] Oil exploration from space: fewer places to hide. [Разведка нефти из космоса: меньше мест, чтобы скрыться] First Break, 19.9, 514-519.

Fairhead, J. D., Green, C. M. and Odegard, M. E. [2001b] Satellite derived gravity having an impact on marine exploration. [Полученная спутником гравиметрия воздействует на морское исследование] The Leading Edge (August), 873-876.

Haxby, W. F., [1987] Gravity field of the world oceans. [Гравитационное поле мировых океанов] National Geophysical Data Center, NOAA, Boulder, CO.

Maus, S., Green, C. M. and Fairhead, J. D. [1998] Improved ocean-geoid resolution from retracked ERS1 satellite altimeter waveforms. [Улучшение океанически-геоидного решения от повторно прослеженных ERS1 спутниковых волн высотомера] Geophysical Journal International, 134,243-253.

Olgiati, A., Balmino, G., Sarrailh, M., and Green, C. M. [1995] Gravity anomalies from satellite altimetry: comparison between computation via geoid heights and via deflections of the vertical [Гравитационные аномалии от спутниковых высотомеров: сравнение вычисления через высоты геоида и через отклонения вертикальных] : Bulletin Geodesique 69, 252-260.

Sandwell, D. T. and Smith, W. H. F. [1997] Marine gravity anomaly from Geosat and ERS1 satellite altimetry. [Морские гравитационные аномалии от Geosat и ERS1 спутниковых высотомеров] Journal of Geophysical Research, 102, 10,039-10,054.

First Break. Volume 22, November 2004, pages 59-63

Special Topic * Non-Seismic Methods / Airborne

Hydrocarbon screening of the deep continental margins using non-seismic methods

J. Derek Fairhead, Chris M. Green and Kirsten M. U. Fletcher

Geophysical Exploration (GETECH), University of Leeds, LS2 9JT, UK

E-mail:*****@***com

Authors of UK-based company GETECH claim new refinements in resolution are providing increasing value for satellite-derived gravity data in hydrocarbon exploration.

A major focus of marine oil exploration is in the deep water parts of the continental margins where various non-seismic methods are playing an increasing role in helping to high grade target areas prior to seismic surveys or drilling. Methods currently used include gravity and magnetic as well as satellite-based Synthetic Aperture Radar (SAR) and/or its airborne equivalent to detect natural oil seeps that may be present on the sea surface. Once the subsurface structure has been seismically imaged, active source electromagnetic sounding is now being increasingly used to determine whether the target horizon (reservoir) contains a resistive layer to warrant drilling. Application of this latter technology by ExxonMobil in offshore Angola probably in part accounts for its incredible string of exploration drilling successes!

This article concentrates on how satellite-derived gravity is able to play a significant role in screening continental margins, often in conjunction with seeps analysis and other available geophysical data such as marine and airborne magnetic data. Previously the authors reported in First Break (Fairhead et al. 2001a) the ability of satellite gravity to reliably image anomalies down to 10 km (full wavelength) and showed examples from West Africa, Gulf of Thailand, Gulf of Mexico and offshore Yucatan Peninsula, Mexico.

Since that time GETECH has undertaken a major two year consortium study (2002-2004) to develop further its satellite processing technology and to map the gravity of all ice-free continental margins on Earth (Figure 1). This dataset extends 2 to 5 km from the coast out to ~ 500 km into the deep ocean basins. It can be used to recognise and map subtle features related to oceanic plate tectonic fracture zones that influence and segment the continental margins. The difference between this data and public domain solutions is that ERS1 satellite waveforms, that represent about half the data volume of the solution, were not well picked and introduced significant noise contamination for wavelengths below 100 km with little reliability of signals below 30 to 40 km wavelength. Filtering and reprocessing the original agency picked (tracked) data using 'geoid to gravity' transformation methods only slightly improved the resolution (after Fairhead et al 2001b). It has only been by repicking (or retracking) the altimeter waveforms that 'ultimate resolution' has been achieved, thereby bringing the data resolution fully within the exploration window.

Converting sea-surface height variations, derived from satellite altimetry, to free air gravity is not new. What is new is the ability to use existing satellite data to resolve anomalies down to 10 km wavelength and to within 2 to 5 km of the coast globally. This article discusses the methodology used and presents examples of how the solution is able to handle difficult data areas such as the mouths of major river systems, regions containing significant ocean currents, and areas close to the shoreline (Figure 1).

Figure 1. Coverage of the Global Continental Margin Gravity Study showing locations of examples used in this study.

In the mid-1980s William Haxby (1987) produced the first global marine gravity map from SeaSat satellite altimeter data with orbital track spacing of about 150 km at the Equator. Haxby's map had a significant impact on plate tectonic theory because marine free air gravity data were able for the first time to image uniformly the tectonic fabric of the Earth's oceanic crust. However, the track spacing was the limiting factor on resolution.

Since that time, much effort has been applied to improving satellite-derived gravity resolution. A major advance occurred in 1995, when the altimeter data from the Geodetic Missions (GM) of Geosat and ERS1 satellites were combining their orbital tracks, a track density of ~3 km was achieved at the Equator, with increasing track density toward the Poles. This resulted in a spectacular global marine gravity model developed by Sandwell and Smith (1997), based on the sea-surface height data provided by NASA (Geosat) and Eurimage (ERS1). Despite the dense spatial coverage of the orbital tracks (~3 km), it was surprising that the overall resolution of this new data set was no better than 30 to 40 km.

In 1993, GETECH started working with the International Gravity Bureau (Toulouse, France) and showed that by improving and refining the processing procedures, the resolution could be improved to about 20 km using the same publicly available data (Olgiati et at, 1995)

Visual inspection of the ERS1 repeat track data indicated a significant noise envelope, which suggested the major resolution problem was the way the return radar waveform onsets had originally been picked. GETECH thus set out to investigate this, by repicking (or retracking) the sea-surface heights from the 'raw' radar waveform data (Maus et al, 1998). Reprocessing the entire GM datasets of Geosat and ERS1 had not previously been done. This was due in part to the large data volumes involved, i. e., in 1997 it took a total of six months to download the entire ERS1 GM data onto 130 exabytes and three months to load them into the computer system. GETECH raised funds from UK Government agencies, followed by an oil industry consortium study, to develop the technology and to test it out over a selected area in the Gulf of Mexico. The study proved highly successful such that a further oil industry consortium was formed in 2002 to map the ice-free global continental margins (Figure 1).

Satellite processing method

The easiest way to understand what satellite-derived gravity is and how it is determined is to think of it as a mega seismic experiment to map the sea surface from 800 km above the Earth. The shape of the sea surface, if there were no disturbing effects such as wind, currents, tides, temperature variations etc., is an equipotential surface of the Earth's gravity field. The 'vertical gradient' of this equipotential surface (sea surface) gives the free air gravity anomaly which is measured at the sea surface, not at the satellite height of 800 km. The satellite seismic source is radar, with pulses of 13.5 GHz emitted at a rate of 1020 times a second. After 50- and 100-fold stacking of the data on board the ERS1 and Geosat satellites, the data were then transmitted back to Earth as 1/20 s and 1/10 s waveform datasets respectively. The stacked waveforms are spaced at 350 m (ERS1) and 700 m (Geosat) along track with sampling of each waveform at 3 ns (equivalent to 45 cm in sea-surface height). This sampling was increased to 1.5 ns over the leading edge of Geosat data.