This report contains an exploration review of Colorado Basin, located in southeastern Argentina, it has onshore and most extensive offshore sections with more than 125000 Km2 of total extension. The basin has more than 70 years of exploration history, with a few wells drilled, one of them recovered 39º API Oil from Early Cretaceous Syn-rift sandstone. Three main plays were identified, with high exploration potential: Rift Margin Play, Continental Slope Play, and Thrust belt play.
- Exploration History
- Regional Geology
- Petroleum System
- Exploration Potential
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Located in southeastern part of Argentina, it has more than 125.000 km2 extension, with an onshore and offshore (115.000Km2) portions (Fig. 1 a & b). Three major depositional centers were identified within the Colorado Basin, of which the deepest is supposed to hold up to 50000 ft. of sediment (Bushnell et al. 1999).
The basin has more than 70 years of exploration history, where 27 wells were drilled, 9 in onshore and 18 in the offshore section (Table 1 and Fig. 2). Several 2D seismic lines were acquired in different campaigns, and more than 2000 Km2 of 3D seismic (Fig. 2). All wells were dry, but oil samples have been recovered from the Cruz del Sur x1 well, proved an active petroleum system in the Upper Jurassic and Lower Cretaceous rift sequence (Fig. 2)
The Colorado Basin exhibits a complex history that makes it difficult to examine an onset of rifting that resulted in the final formation of the basin (Tankard et al. 1995). Following the Palaeozoic dominantly compressional setting, rifting began in the Early Jurassic along the southern margin of Africa and progressed southwestwards in Patagonia (Urien et al. 1981).Within this first phase of extension the San Jorge Basin was formed in a backarc environment and the basin became restricted in the mid-Jurassic (Urien et al.1981; Dalziel et al. 1987). Probably the Colorado Basin region was affected by this extension phase as well.
In Late Jurassic–Cretaceous, immediately before and during the break-up of the South Atlantic, extension propagated inboard and was widely distributed (Uliana et al. 1989).A linked system of NW–SE orientated basins, associated with a northwest-trending dextral shear system (Tankard et al. 1995) developed along the shelf, including the Colorado Basin (Kelley & Light 1993; Urien et al. 1995). Whether this stage was a renewed period of rifting or a continuation of Triassic/Jurassic extension is still a matter of debate (Light et al. 1993). The final opening of the South Atlantic took place in Early Cretaceous time (e.g. Rabinowitz & La Brecque 1979; Unternehr et al. 1988; Nürnberg, D. & Müller, R.D., 1991; Lawver et al. 1998) with inferred opening ages for Argentina/South Africa ranging from 127.7 to 135.5 Ma. Most plate tectonic reconstructions describe the opening as a combination of complex rift and strike-slip faults and a stepwise, northward-propagating rift for the South Atlantic.
The pre-rift substrate at the basin limits is formed by intrusive and metamorphic rocks from Precambrian and lower Paleozoic times (Fig. 3). In the rest of the basin, the basement for the Jurassic-Cretaceous sediments is formed by Permian continental black shales (Fig. 3 and 4). Beneath these, were deposited Permo- Carboniferous diamictite.
The black shales were drilled by Puelche well (YPF S.A.),with a partial thickness of 1500 m. After a well-defined angular unconformity, a Mesozoic pre-Maastrichtian sequence was deposited, that can reach more than 6000 m thickness in axial zones (Sin-Rift and Sag stages, Fig. 5)
The basal section called Fotín formation is composed by alluvial conglomerates, mostly restricted to NW-SE half grabens, with 2000 m thickness according with the seismic information. The upper part of the sequence, called Colorado formation, was divided in two sections, the lower is composed by continental deposits (in drilled positions)and the upper with better reservoir conditions, is composed by coastal deposits. The Albian Cenomanian deposits are expected marine facies, from deltaic to turbiditic in the slope part of the basin. The upper part of the sedimentary column (Maastrichtian-Cenozoic) is composed by marine sediments (shallow platform), up to 2500 m thickness.
The Cruz del Sur-1 well recovered a small amount of good quality (39° API) moveable oil from an Early Cretaceous syn-rift sandstone reservoir (Fig 2). The oil is believed to be sourced from Permian shale, and Paleozoic strata are imaged on the seismic data as a parallel bedded pre-rift sequence in the offshore Colorado and Salado Basins. The geochemical analysis performed in Puelches well indicates marginal conditions for dry gas generation in this section of the basin for the Permian lacustrine shales. The upper Permian section In the Estrella well, indicates oil potential generator with TOC values around 3.4%. A Neocomian source rock with some hydrocarbon potential was also encountered in the Cruz del Sur-1 well, but was immature at the well location. A 25 m-thick source interval was reported to have an average TOC value of 2.4%. These source rocks are more deeply buried in the center of the Colorado Rift, and could source hydrocarbons which migrated into rotated fault terrace plays, which are present along the basin margins. The Colorado Basin is separated from the Atlantic margin by an outer high.This high also access a syn-rift source rock kitchen (Fig. 6). The “oil seeps” detecting by radar images and the presence of thermogenic gas in samples of the bottom sea, are a strong indicator of an active petroleum/s system/s.
The older rocks with reservoir conditions detected by several wells are the Permian conglomerate sandstones, with low porosities due to the high compaction. In some places with strong deformation, could constitute natural fractured gas reservoirs.The Syn rift section presents sandstones deposits with porosities from 10 to 15%, but with high argillaceous matrix. The best reservoir conditions were registered in the Colorado formation, with porosities from 22 to 30%. In the western part of the basin, these sandstones were deposited in deltaic and fluvial environments (Enarsa, 2008).
The intercalated shales at the top of the Syn rift section are potential local seals. The regional seal for the Colorado formation (main reservoir), is the Pedro Luro formation (bathyal shales, clayey to marly sediments)
There are two main styles. In the continental platform, the basement highs and 3-way closure against fault of the Syn rift section are considered potential traps (Fig. 6).Rotated fault blocks with northwest oriented lineaments is the main structural feature in this section. In the slope zone, big structures (hundreds of km2) could generate variated type and size of traps (Fig. 7).
Three main plays were identified: Rift Margin, continental slope
Rift Margin Play
The Colorado Basin is separated from the Atlantic margin by an outer high (Fig. 6 and 8)
This high could also access a syn-rifts ource rock kitchen. Subtle closures have been imaged above the syn-rift faults at Early Cretaceous level over this high and later small reactivations caused faulting extending up to the Cretaceous level to allow migration into potential shallow water reservoirs. Bright, structurally conformable amplitude anomalies have been identified by YPF over at least one of these large Cretaceous level closures in their previously-operated Colorado blocks. The prospects lie in shallow water, and have good access to Neocomian source rocks sitting in the oil window (Argentina Energética, 2009).
The continental Slope Play
The seismic interpretation allowed the identification of big structural 4-way closure and 3-way closure against fault potential traps, conformed by basement blocks, with closures of hundreds of km2 (Fig. 9).The reservoirs (Cretaceous) are platform and turbidity clastics. The potential source rocks (Cretaceous and Jurassic), were originated in lacustrine and marine environments and the petroleum system would have adequate timing. The potential resource for each structure is 2000-3000 MBOE (Argentina Energética, 2009).
Thrust belt play
The thrust bel play is related to a structural closure that can reach hundreds of km2 (Fig 10).The reservoirs are marine clastics from the Tertiary, deposited in deep basin environments (turbidities). The potential source rocks (Cretaceous and Jurassic), are originated in marine environment and all the petroleum system would have adequate timing. The expected fluid is gas, according toDHI (gas chimneys?). The estimated resources are 800 to 1500 MBOE for each structure (Argentina Energética, 2009).
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