About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Recent Exploration
Drilling Results Highlight
Significant Potential for
Portugal's Lusitanian Basin
By
Mohave Oil and Gas Corporation,
Houston, Texas
The Lusitanian basin is located on-and offshore, west-central
Portugal, extending from Lisbon to Porto, west of the
Iberian Meseta. Abundant oil seeps in the basin attest to
working petroleum systems, and have attracted industry interest
in the past. Despite intermittent drilling activity since the 1940s,
the basin is still relatively unexplored, with significant exploratory
tests being approximately 1 well
per 150,000 acres.
The Lusitanian Basin originated as a Late Triassic rift system
related to opening of the Tethyan seaway. This Mesozoic rift
system was superimposed upon an older, Hercynian orogenic
terrain of folded lower Paleozoic rocks and taphrogenic, Permo-
Carboniferous coal basins. Folds within the Hercynian terrain
trend NW-SE, oblique to the primarily NNE-SSW trend of
Lusitanian rifting, creating pre-rift, cross-basinal arches that
were episodically reactivated during subsequent tectonic phases.
Similarly, many significant Hercynian faults were utilized as
major rift half-graben boundaries, and hence can define syn-rift,
fluvial clastic depocenters. Such faults generally exhibited later
episodes of wrench movement as well
, particularly at the end of
the Middle Jurassic and during Tertiary Alpine orogenesis.
Regional Upper Triassic/Lower Jurassic evaporites divide the Lusitanian Basin stratigraphic column into subsalt and suprasalt hydrocarbon systems with associated exploration plays. The subsalt play targets gas reserves in syn-rift, Triassic continental sandstone reservoirs sealed by salt or anhydrite and sourced from underlying Silurian graptolitic shales and Permo- Carboniferous coals. This system is analogous to the Algerian Ghadames Bash, where prolific production has been established from Triassic continental sandstones, capped by Liassic salt, and charged by a second phase of expulsion from Silurian source rocks. The system is also analogous to the slightly older Southern North Sea gas production from Permian Rotliegendes continental sandstones, capped by Zechstein salt, and charged by Carboniferous coals.
Mohave and partner's first well
in the basin, Aljubarrota #1,
drilled a large, west-verging overturned fold, probably related to
partial inversion of an adjacent Triassic half-graben to the east.
Oil and gas shows were encountered in transition zone dolomites
and oolitic limestones at the base of the Dagorda evaporite
sequence
, indicating an approximately 500-m paleo-oil and gas
column.
Analysis
of oil extracts from the shows documented a
Lower Paleozoic oil source for these hydrocarbons. The Triassic
section encountered was a thin fault sliver. Mohave's second
well
, Aljubarrota #2, was drilled in the adjacent half-graben and
penetrated over 580m of syn-rift Triassic siliciclastics at a structural
position 470m low to the Aljubarrota #l. The Silves section
in the #2
well
had significant mud gas shows throughout, as
well
as substantial gas in the overlying post-salt section.
Suprasalt hydrocarbon plays targeting post-salt Jurassic carbonate
and clastic reservoirs have been the focus of previous
exploration in the basin. This petroleum system contains well
-documented
Jurassic source rocks and has produced
sub-commercial amounts of oil. The Mohave Aljubarrota #2
well
established the presence of an -700-m thick gas column in
fractured, vuggy carbonates of Lias/Dogger age (see inset photo
on cover). Mohave's Aljubarrota #3 and #2 Sidetrack appraisal
wells found water-free gas saturation in the target carbonates,
Unnumbered Figure. Steeply dipping beds of Lower Jurassic, organic-rich shaley carbonate source rocks at Sao Pedro do Muel, Porgutal.
End_Page 13---------------
but did not find the same density and openness of fractures which produced gas in the Aljubarrota #2.
Transitional dolomites and anhydrites of the upper Dagorda
evaporite sequence
have exhibited oil and gas shows when
drilled, and though past operators reported
log
pay, none of
these zones were adequately tested. Additionally, recent wells
drilled as part of a gas storage project intersected numerous,
porous, hydrocarbon-saturated dolomites in the Dagorda.
Seismic expression of this alternating dolomite/anhydrite facies
is mapable, outlining potential Dagorda dolomite reservoir
fairways. Porous oolitic limestones of the Late Dogger-age
Candieirros Formation have been penetrated in several wells and
offer significant potential where the oolite fairways overlap properly
timed structures which are sealed by overlying marly Montejunto Formation carbonates.
In summary, two petroleum systems are now documented in the Lusitanian basin, a subsalt petroleum system and a suprasalt petroleum system. The primary subsalt reservoirs are sandstones of the syn-rift Silves Formation, capped by a regional evaporite seal in the Dagorda Formation. Trap types are fault block closures and wrench-related anticlines. Multiple reservoirs are present in the suprasalt system, including Lower and Middle Jurassic porous dolomites and fractured vuggy carbonates, Middle and Upper Jurassic oolitic and reef limestones, and Upper Jurassic sandstones. Shaley and marly carbonates of the Brenha and Montejunto Formations offer regional seals. Traps are fault block closures, wrench-related anticlines, stratigraphic pinchouts, truncation traps, and reefs.
End_of_Record - Last_Page 15---------------