PETROLEUM OCCURRENCE

Location of Fields and Seeps

Seeps occur in the sub-Andean trend, where Devonian through Tertiary rocks crop out, and along the northeastern border of the province within Cambrian and older limestones. Most discovered fields are in sub-Andean reservoirs (Figure 2a) ranging from Silurian to Tertiary in age. All fields and shows – possibly except for the Cambrian seeps – are where Devonian source rocks are present.

The western sub-Andean is highly oil prone, with fields typically in oldest reservoir rocks. In contrast, the eastern sub-Andean and foreland producers are more gas-and-condensate prone and have reservoirs of all geologic ages.

Hydrocarbon Geochemistry

On average, older reservoirs generally are more oil-prone and have lower oil gravities, although both Tertiary and Carboniferous reservoirs do record nearly the full range (27-71°API) observed in the province (Petroconsultants, 1996). Gas-oil ratios (GOR) also have significant variability (100-73000 cfg/bo), but higher values typically characterize older reservoirs. Diasterane (C₂₇-C₂₉) biomarkers distinguish most produced and seep hydrocarbons as having been sourced by the same middle Paleozoic shales (Dunn and others, 1995), and they are further distinguished from Cretaceous sources south of this province by a low gammacerane index (Paleozoic 2 versus Cretaceous 30; ten Haven, 1996).

Migration-fractionation is proposed (Illich and others, 1981) to explain the gravity trend with reservoir age. Younger-reservoired, higher gravity hydrocarbons have significantly lower concentrations of normal paraffins, slightly lower concentrations of aromatics, and greater concentrations of isoparaffins and cycloparaffins. Such differences cannot be explained satisfactorily by differing thermal, burial and expulsion histories; regional source character variability; shallow bacterial degradation due to fresher water; or a segregation-migration process.³  Migration-fractionation results from differing solubilities of major hydrocarbon groups in water; i.e., cross-stratigraphic movement of fluids with imperfect exsolution.

Others propose that some gravity differences associated with reservoir age can be explained by different generation and entrapment histories, particularly along the Boomerang Hills depositional paleohinge, which underwent periodic episodes of extensional faulting nearly parallel in trend with later Andean compressional folding and faulting (Welsink and others, 1995). The Silurian source rock, geochemically similar to its Devonian counterpart, locally entered the oil window as early as Carboniferous time. Low gravity oils (35-40 °API) in Boomerang Devonian reservoirs could be relict accumulations of earlier phases of oil generation from deeper parts of the old Paleozoic basin. Baby and others (1995) believe the marine oil source rock has such low initial potential that generated oil does not expel easily, but the lighter compounds do expel.

Petroleum System Characteristics

Prominent Silurian and Devonian shale detachment horizons probably create lateral migration routes, in addition to unconformity surfaces prominent around the margins of the province. Belotti and others (1995) report the existence of two Los Monos stratigraphic zones of high fluid pressure that likely enable expulsion in both upward and downward directions, explaining charge in older reservoirs when no lateral juxtaposition occurs. The presence of both mature and immature source rocks on uplifted thrust segments suggests that both vertical and limited lateral migration are required to charge the folded and faulted structures.

Ultimately, 4.1 BBOE are predicted to be recovered from known fields (Petroconsultants, 1996). Table 1 illustrates the gas-prone nature of the known reserves, their distribution in structural traps, and the early production emphasis on oil (% of each commodity produced). Also shown is the dominance of Carboniferous reservoirs. More than half the known reserves are difficult to allocate to reservoirs because multiple horizons produce and inadequate data are published to make those distinctions.

SOURCE ROCK

Devonian and Silurian shale source rocks (Figure 7) were deposited in a semi-restricted, marine extensional basin covering most of the province. Both horizons are preserved throughout the area with thicknesses ranging from many hundred to several thousand meters each, but Devonian source rocks are typically thicker. Maximum Siluro-Devonian composite thickness is 4 km. Source rock quality possibly deteriorates to some degree because of lithologic variability near depositional basin margins (e.g., Brazilian shield, Asuncion arch, westward in the thrust belt) and perhaps over the central Chaco high, which was prominent and exposed by the Carboniferous period.

Silurian and Devonian shales are geochemically similar and locally have generated oil since as early as the early Carboniferous period (Baby and others, 1995; Dunn and others, 1995; Franca and others, 1995; Starck, 1995; Welsink and others, 1995; Wiens, 1995). Both intervals are type II to type III, mixed oil- and gas-prone source rocks. Although best TOCs are just 2 wt %, the shales have significant thicknesses. Los Monos hydrogen indices cluster in the 100-300 range but reach 500 mgHC/gTOC (Dunn and others, 1995). Corresponding oxygen indices cluster in the 10-50 range but can approach 300 mgCO2/gTOC.

Devonian Los Monos shales are the thickest identifiable and correlatable known source rocks in the Santa Cruz – Tarija Province. Overall, they have had the most favorable burial histories, with hydrocarbon generation windows coinciding with or post-dating the Tertiary time of major compressional trap formation.

OVERBURDEN ROCK

Regionally variable thermal gradients, post-Devonian isopachs, and Andean loading history determine when Siluro-Devonian source rocks matured – ranging from early Carboniferous to the present in different parts of the province (Figure 7, Figure 8a, Figure 8b, Figure 8c). Paleozoic and younger basin axes, where thickest depositional overburden exists, were near the eastern sub-Andean foothills or the present foreland (Dunn and others, 1995), but tectonic burial increased westward through the fold and thrust belt.

Carboniferous overburden varies laterally along the eastern sub-Andean from <1300 meters in the northern area, to 1600-2000 meters in the central region, to 1300-1600 meters adjacent to Argentina (Beer and Lopez P., 1993). It also depositionally thins into the western sub-Andean, to the north against the ancestral Brazilian shield, to the east against the Asuncion arch, and around the foreland’s central Chaco high (Carboniferous Izozog arch) in Paraguay. Carboniferous glacially supplied siliciclastics were deposited in alluvial to marine settings.

Shallow marine to continental Permo-Triassic strata are thin (mostly <1 km) in the sub-Andean and terminate in the foreland west of the central Chaco high. Jurassic rocks are mostly absent from the province. Cretaceous marine to non-marine rocks are also typically < 1 km thick with depositional margins resembling those of the Carboniferous. Cenozoic continental foreland fill ranges from 1-4.5 km, thinner to the west and onto the present forebulge axis in the eastern part of the province (Horton and DeCelles, 1997).

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³ fractionation by retrograde hydrocarbon condensation from a moving gas phase