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Basin Modeling Uncertainties Related to the Hybrid Devonian Petroleum System (Conventional Plus Atypical) of the Solimões Basin (Brazil)

G. Garcia¹, L. M. Araújo¹, and J. R. Wanderley Filho²
¹Petrobras, Rio de Janeiro, Brazil
²Petrobras, Manaus, Brazil

The Solimões Basin is a Paleozoic intractonic basin comprising up to 4,300 m of sediments covering 440,000 km² within the Amazon rain forest in northern Brazil. The stratigraphic framework comprises five depositional sequences limited by regional unconformities: Ordovician, Upper Silurian-Lower Devonian, Middle Devonian–Lower Carboniferous, Upper Carboniferous–Permian and Upper Cretaceous–Tertiary (Wanderley et alii, 2007).

Sills and dykes were intruded stratigraphically above the Devonian source rocks during the Penatecaua magmatic event in the Triassic, around 201Ma (Wanderley et al., 2007). In the modeled area, three sets of diabase sills were seismically discriminated. The lower set of sills (up to 168m thick) was intruded around 500 m above the source rock. The middle and upper sets (up to 500m and 449m thick, respectively) were intruded about 1,560 m and 1,140m above the source rocks. The dating methods do not have enough accuracy to support different ages of emplacement for the different sets of sills, despite the several surveys undertaken. Regardless of this uncertainty, according to the different modeled scenarios, the Solimões Basin can be characterized as a hybrid petroleum system, with up to 40% of the source-rock transformation ratio due to overburden and the remaining 60% related to the high heat flow during the magmatic event of sill insertion (conventional plus atypical petroleum system; Magoon and Dow, 1994).

The reservoir-source rock contact within the Juruá Formation is direct, with the Carboniferous reservoir overlying the Devonian source rocks. An effective seal is provided by Upper-Carboniferous evaporites (Carauari Formation). The main trap was formed in the Permian, and later amplified during the Juruá tectonic event at 125 Ma. The petroleum system efficiency was proved by discoveries of 11 gas accumulations and three oil and gas fields encompassing reserves over 500 MM BOE, with oil API gravity ranging from 37 to 65º and gas-oil ratio (GOR) up to 9000 m³/m³.

In this work the investigation was focused on the effect of the igneous intrusions on the thermal evolution of the Devonian source rocks. To test the uncertainty related to the emplacement age of various igneous bodies and their thermal effect on kerogen cracking, the PetroMod 3D software (IES-Schlumberger) was used. The investigation and modeled area covered three condensate and gas fields, which constitute the main petroleum province in the Solimões Basin. Usually, when a number of stacked sill sets occur, the upper set is the thickest and oldest (Linsser, 1973). In this case history, this would imply that the upper set of sills was emplaced before the others. At least in terms of sill thickness distribution, this pattern was observed in the Solimões Basin (Conceição et alii, 1993). Following this approach, four different scenarios of diabase sills emplacement were simulated to test the coherence between modeled and observed source-rock maturity and the physicochemical properties of the generated oil and gas. The results of the calculated thermal history were used in the GOR-Isotopes software (GeoIsoChem Co.) to compare the carbon isotopic signatures of gases sampled in condensate fields, and the modeled theoretical curves based on fractionation models for stable carbon isotopes.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012