GALICIA'S ATLANTIC COAST BASIN -A NEW OIL PROVINCE?

Marcello Rebora
Taurus Petroleum AB
London Martin L. Keeley
Consulting petroleum geophysicist
London

The Galicia basin in the Atlantic Ocean off northwestern Iberia (Spain and Portugal) is a passive margin basin covering about 1.5 million acres (Fig. 1).

It has a similar history and geometry to other passive margins further south (Morocco) and north (Porcupine, Rockall), while lying opposite the Jeanne d'Arc basin with Hibernia oil field in pre-drift reconstructions.

Galicia basin exploration to data has been hampered by a number of problems:

Poor seismic data due to multiples caused by hard chalk on the sea bottom,
Steeply dipping seismic noise locally masking the main objectives,
Poor understanding of Upper Jurassic-Lower Cretaceous facies patterns, and
The political divide mid-way across the basin.

Only six wells have been drilled, with several finding reservoir absent on bald structures or because of poor seismic, being positioned off-structure. Yet the stratigraphic successions penetrated in these wells can now be integrated to establish a sedimentologic model of Upper Jurassic-Lower Cretaceous source-reservoir-seal rock associations of the type commonly found in giant oil fields.

CONCESSIONS, DRILLING

Concessions covering both Portuguese and Spanish sectors of the Galicia basin are held by Taurus Petroleum AB, a small Swedish independent. Contract terms are very attractive, being limited to 40% corporation tax in both countries and a partial 30% government back-in in Spain.

Taurus will be drilling the very large structural-stratigraphic Touro structure in mid-1994, with a 2,700 m well (Fig. 2). Lying across the political divide, explorers had earlier failed to recognize the size (potential reserves of 3.8 billion bbl of oil equivalent on an area of 37,000 acres) and integrity of this structure.

The most important nearby well, 1 Lula, was drilled in 1985 by Pecten/Salen Energy on a prominent basement ridge at the basin's seaward margin (Figs. 3, 4).

The primary Tithonian, Kimmeridgian, and Oxfordian reservoir targets were missing from this bald high, but oolitic grainstones were found in the overlying thinned Tithonian.

Seismic character and relief, as well as the subsurface facies, are now interpreted in terms of stacked Kimmeridgian and Tithonian high-energy fringing coralgal bio-herm complexes with sands and oolitic grainstones (Figs. 3, 4).

These bioherms lie on the landward side of the marginal basement Lula ridge and are known to pass further east into shales and sands.

Clear syndepositional and post-depositional relief, probably modified by karst phenomena, are apparent in the Kimmeridgian and Tithonian biohermal complexes on both seismic profiles and stratigraphic sections hung on the top Neocomian un-conformity (Fig. 5).

On the seaward side of the Lula ridge, there is seismic evidence for similar reef structures (Fig. 4). The reefs west of Lula and generally beyond the continental shelf are usually difficult to identify because, being on the hanging wall of large listric faults, they have been dropped to extreme depths.

Similar reefs have been found on seismic offshore Ireland in the Porcupine basin and, of course, those fringing the North American continent from the Gulf of Mexico to Canada extending in age from Upper Jurassic to Lower Cretaceous are well documented.

Throughout the Neocomian-Triassic section, although tight and in an off-reef facies, 1 Lula encountered good light oil and gas shows, with oil recovered on repeat formation test tool from the Berriasian-Portlandian. The source of this oil is believed to be reef-equivalent Kimmeridgian shales, found in the Galicia basin depocenter east of 1 Lula.

Being restricted to the axis of the basin, this primary source interval has not been penetrated by the bit but is known from the adjacent Lusitania basin. Indeed in Portugal bitumen from Upper-middle Jurassic sediments is locally used as road asphalt.

The Kimmeridgian oil-prone organic-rich shale is the main source rock in Northwest Europe and was deposited in a series of basins from eastern Greenland and Canada to the North Sea and off Ireland and Iberia.

The 1 Lula and other wells have shown that the Neocomian section is rich in shales and siltstones totaling nearly 1,000 in. Indeed the consistent presence of a seismic anomaly above the entire length of the Touro prospect is interpreted as a "gas chimney" (Figs. 3, 4). This anomaly is similar to the "gas chimney" on Ekofisk and other producing fields in the North Sea.

Seismic anomalies of this nature often have not been recognized for what they mean by the oil industry. A well known example is Ekofisk, nearly overlooked and finally drilled as priority No. 8 in the Norwegian sector in the late 1960s.

Sparker surveys along the Galicia basin (Fig. 6) have revealed numerous gas seeps at the sea bed, supporting the interpretation of a partially effective Neocomian top seal to gas. The seal is expected to be effective for oil.

Taurus and its predecessor Salen Energy have been active in the Galicia basin since the early 1980s. The company has now been able to resolve these problems of prospectivity by repeatedly experimenting with new seismic processing and acquisition techniques.

In fact, some lines that were seriously affected by multiples and noise have been reprocessed up to half a dozen times between 1975 and 1993 with continuous improvements.

Reprocessing has revealed that most of the noise was generated from the base of chalk where it is hit by the rising gas column. An appropriately tailored processing sequence is effective in nearly eliminating this noise.

Recently Taurus has shot 700 km of new data and reprocessed more than 2,500 km oF old data with remarkable improvements in terms of data resolution at reservoir level. A large number of Upper Jurassic reefs have been identified, some of them with tell-tale "gas chimneys." All these reefs were formed on tilted fault blocks formed during the initial stages of Atlantic rifting.

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