INSTALACIONES ELECTRICAS

The Tarma/Copacabana Group is by far the most widely distributed pre-Cretaceous unit in the sub-Andean basins, including the Ucayali and Ene Basins. Generally it is difficult to place an exact upper contact for the Tarma Group and the two units together are consequently often referred to together, as the Tarma/Copacabana Group. A separation of the Tarma and Copacabana groups can be established locally where the Tarma Group includes more clastic interbeds as in the Mainique Gorge area of the southern Ucayali Basin. The lower unit of the Tarma Group is a clastic unit that includes green sandstones, red siltstones, silty mudstones and anhydrite beds reaching 80 m. in thickness. The basal clastic unit of this interval is called the Green Sandstone member, which typically has good porosity and good reservoir potential. It is a green to brown, fine to very coarse cross-bedded, moderately sorted, glauconitic

Figure 10: An example of a 50 to 60 meter anhydrite unit within the upper Copacabana section that has been repeated by a thrust fault. The log on the right is the hanging wall section and the one on

the right, the footwall section. Note: The repeated section has been removed in the Huaya 3X well

in the stratigraphic stratigraphic cross-sections 1 and 2.

Figure 11: West to East seismic line through the Panguana well showing a) how the Copacabana has been erosionally reduced beneath the Base Cretaceous unconformity and b) The anomalously thick section of pre-Ambo sediments intersected in the Panguana well. The Basement pick is very interpretive and base largely on the results of the Panguana 1X well.

HUAYA 3X 2100 2200 2300 2400 2500 2600 2700 HUAYA 3X 1600 1700 1800 1900 2000 2100 2200 THRUST FAULT I THRUST FAULT I ENE COPACABANA MITU? ANH Chonta Tarma Ambo Devonian Basement Base Cretaceous/CopacabanaChonta

Tarma Ambo

Devonian

Basement Base Cretaceous/Copacabana

and chloritic sandstone. There is a sharp contact between the Green Sandstone and the underlying Ambo Group.

The green colored clastics diminish upwards and the upper part of the Tarma Group comprises micritic wackestones and dark gray mudstones establishing a gradational contact with the overlying carbonates of the Copacabana Group. The carbonates become a sequence of thick units of dark gray micritic and sparite carbonates, white to light brown crystalline dolomites, cross-bedded oolites, wackestones and cherts with distinctive fusulinid rich horizons in the upper part (Mainique Gorge, Agua Caliente and San Alejandro 1X wells). The group also include some clean 1 to 3 meter-thick anhydrite beds, occasionally 5 m thick, as in the upper Tarma Group in La Colpa 1X well and in the bottom 2/3 in the San Martin 1X well. In the 1950-2050m interval of the Huaya 3X well a 50-60m thick anhydrite unit was intersected within the Copacabana section and is repeated between 2430-2500m by a thrust fault at 2200m (Figure 10).

Thickness varies from 640 - 960m in the northern portion of the Ucayali Basin (see wells Huaya 3X, La Colpa 1X, Runuya 1X and Agua Caliente 1X in stratigraphic cross-sections 2 and 4) to 860 - 940m in outcrop in the Mainique Gorge and Atalaya areas (stratigraphic cross-section 9) and 990m in the Camisea San Martin 1X well (stratigraphic cross-sections 4 and 9), in the southern Ucayali Basin. Locally, the unit is partially reduced by erosion along the crests of Paleozoic aged structures such as in the Coninca 2X well were the Tarma/Copacabana has a thickness of 333m (stratigraphic cross-section 3) and in the Panguana 1X well where it has been reduced to 166m as demonstrated in stratigraphic cross-section 9 and seismically in Figure 10, or it has been completely stripped by erosion as in the Cashiboya area, (stratigraphic cross-section 1).

In the area over the Contaya arch where there is no Copacabana, it is presently unknown whether the Contaya Arch was a positive feature during Copacabana deposition, (the result of an earlier tectonic uplift and the unit was not deposited as suggested by Mathalone (1994)) or whether it is simple a result of uplift and complete erosion as referred to in the examples above. If the latter is true, the Contaya Arch became a positive feature in Late Permian time.

The Copacabana contains organic-rich dark gray to black mudstones deposited under flooding or anoxic conditions with source rock characteristics. Dolomitic wackestones interbedded with brown sandstones at various levels in the whole unit produce strong to faint oil smell in fresh broken surfaces. These intervals have TOC of 2.0 wt% and are mature for oil and gas generation in the Mainique Gorge, Shell (1997). Near the top, the carbonates are bioturbated and burrowed and are found underlying the basal Ene Formation mudstone, with no evidence of karsts or breccias. In the Huaya 3X well there is a common presence of dolomites observed near the anhydrite beds. These dolomites are brown gray and dark gray, locally vugular, micritic, oolitic and pelletoidal, which are remnants of the original limestones prior to dolomitization. The anhydrite/porous dolomite/organic rich carbonate association may constitute a potential petroleum system in this part of the Basin.

Figure 12: Distribution of the Ene Formation as mapped seismically in the Ucayali Basin. The seismic line shown in Figure 14 is located on this map

Figure 13: NW/SE stratigraphic cross-section flattened in the Upper Permian unconformity shows the late Permian post Tarma/Copacabana Group stratigraphy. Orellana 1X is in the SE Marañon Basin.