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Minggu, 03 Januari 2010

SEEP AND OLI EXPLORATION

What is a seep? Why are seeps important? What relevance do they have to petroleum exploration? If seeps are present in a basin, will all the oil have leaked out? These are the most often asked questions regarding seeps.
Fig 1. Seeps from Oilfields schematic
Contents
Seeps and Oilfields
Seeps and discoveries
Offshore seepage
Follow-up studies
References
Further Information
Seeps and Oilfields
A good definition of a seep is ''the surface expression of a migration pathway, along which petroleum is currently flowing, driven by buoyancy from a sub-surface origin'' (Clarke & Cleverly,1990). Knowledge of where oil and gas seeps are emerging is therefore a key piece of evidence in reducing the risk on source presence in a new basin as seeps originate by leakage from buried oil and gas accumulations (fig 1). At the most basic level, this demonstrates that the basin contains a generating source rock and hence a viable petroleum system.
Work published by BP and others in the early 90's (Clarke & Cleverly,1990; Clayton et al.,1991) demonstrated that over 75% of the world's petroliferous basins contain surface seeps (Fig 2), the exceptions being those with unbroken regional evaporitic seals (such as the Hith Anhydrite in the Arabian-Iranian Foreland basin). Most seeps represent tiny but detectable volumes of oil and gas which are not significantly depleting the reservoir. Exceptions would be in some recent onshore fold and thrust belts where accumulations have either been breached or redistributed to tertiary traps and where the link between surface seeps and the leaking traps is more complex. Such geology, however, is rarely encountered in offshore basins so that problem does not arise. Confirmation of the presence of seeps, therefore especially in offshore basins, is positive and in the vast majority of cases is not indicative of breached or depleted traps.


Seeps and Discoveries
The knowledge that surface seepage has a direct link to subsurface oil and gas accumulations is not new and has been the stimulus for many of the world's early major oil and gas discoveries by the pioneers of our industry, as far back as the 1860’s in both Pennsylvania and Azerbaijan (figs, 3 & 4).

Fig 2. Typical large-scale onshore oil seep, Qaiyarah, Iraq
Fig 3. Early hand-dug well sited on surface seeps, Baku, Azerbaijan.
The following decades spanning the late 19th century and the first half of the 20th century heralded the first discoveries. Surface seepage was associated with Sumatra (1885), Texas (Spindletop, 1901), Oklahoma (1905), Persia (Majid-I-Sulaiman, 1908), Mexico (Golden Lane, 1910), Venezuela (Los Barroso, 1922), Iraq (Kirkuk, 1927), East Texas (1930), Bahrain (1932) and Kuwait (Burgan, 1938). (Yergin, 1991).
Fig 4. Baku, Azerbaijan; Discoveries based on onshore seeps. Production in 1898 (left) and 1998 (right)
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Offshore Seepage
Fig 5. (left) Gas (and oil-coated) bubble plume in water column. Fig 6. (right) Surfacing oil pancakes (gas bubbles burst and lost to atmosphere) - movie of surfacing oil pancakes - MPEG (4.2mb)
In the offshore, seeping oil and gas are often easier to detect due to the fact that oil is normally transported from the sea-bed vent to the surface as oil-coated gas bubbles (fig 5). At the surface, the gas bubble bursts and the oil remains on the surface as a thin oil film (fig 6). In calm sea conditions, these can often be viewed as beautiful, iridescent concentric shapes, typically 0.5 to 1 metre in diameter, known as 'oil pancakes'.
As seepage continues over time, these coalesce to form larger slicks that are detectable from aircraft (fig 7) and space (fig 8).

Fig 7. (left) Coalescing oil pancakes forming� large oil seepage slick (aerial view from c.100m asl). Fig 8. (right) Repeating oil seepage slicks on Radarsat data, Central Gulf of Mexico.
For offshore exploration, satellite radar (fig 9) or SAR (Synthetic Aperture Radar) can now offer the oil industry an effective, low-cost technique for reducing source risk in high-cost exploration environments such as the new deep and ultra-deep frontier basins. This is due to their ability to image surface oil seeps remotely with wide swath coverage (typically 100 x 100km scenes for ERS and 165 x 165kms for Radarsat Wide 1) and at low cost. Moreover, satellite data is free skies and is being continuously acquired, thus providing multi-temporal satellite data over any area of the globe. Such repeat seeps provides the location for follow-up surface sampling from which key geochemical information on the reservoired oil can be obtained ahead of the drill.
Fig 9. Typical Earth Observing satellite - movie of a satellite in a fixed polar orbit - MPEG (3mb)
SAR satellites scan the oceans continuously on fixed polar orbits. They have advantages over optical satellite systems, such as Landsat TM (e.g. Berry, 1995) and airborne systems (e.g. Williams, 1995) in that they observe night and day and penetrate cloud cover. SAR creates images of the sea surface detailing its morphology. Radar images map slicks (flat patches of the surface) that can be related by analysis to petroleum seepage.
For more information view our movie introducing slick detection using ENVISAT SAR; MPEG (7.8mb) .
Moreover, as the seepage rates direct from source-rocks are at least several orders of magnitude less than that from leaking traps (Clayton et al., 1991), it is highly unlikely that the size of slicks resolvable by satellite radar (minimum length of c. 100-150m) could originate directly from maturing source rocks. The size and/or permanence of a seep have no relationship to the size of the leaking traps.
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Follow-up Studies
The value of accurate seep locations on the ocean surface can be significantly increased when correlated with other data. On a routine basis, NPA do this with both Free Air (fig 10) and Bouguer Gravity but the link to seismic data is the most revealing. It is likely that genuine seepage-slicks will coincide with direct hydrocarbon indicators (e.g. phase reversals, flat spots, amplitude anomalies, gas blanking, etc.) on the seismic record. NPA supply a digital latitude-longitude database of slicks to facilitate direct comparison of seismic and sub-surface maps.
Fig 10. Seepage slicks (left) overlain to Free Air Gravity, Congo Fan, Angola. Fig 11. (right) Repeat seeps overlying undrilled structures, South Caspian Basin.
If positive comparisons to potential migration pathways, such as shallow-cutting faults, salt diapirs, mud volcanoes (fig 11), regional seal pinch-outs, etc. are confirmed, slicks and/or their associated gas bubble plumes could be located at sea (by means of fish finding sonar or shallow seismic reflection). Any slicks located can then be sampled at the surface (using specially treated nylon cloths, fig 12) from which crucial geochemical information on oil type, maturity and for oil-seep correlation can be derived (fig 13).
Fig 12. (left) Geochem sampling of a seepage slick, Gulf of Mexico. Fig 13. (right) Geochem Analysis of seepage and diesel slicks.

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