Study of biodegradation processes along the Carnaubais trend, Potiguar basin (Brazil) – Part 2

The aim of this study is to estimate biodegradation rates of petroleum in natural reservoirs. For that purpose, a cross-section along the Carnaubais trend (ENE-WSW) in the Potiguar basin (Brazil) was modeled using Temis 2D. Most biodegraded reservoir oils are located in the southwest (onshore), within the Cretaceous sandstones of the Açu units that occur at shallow depth (<1000 m). Deeper reservoirs containing non-biodegraded mature oils are located in the northeast area of the section. Basin modeling predicts that fluids generated from the two main source-rock levels of the Alagamar Fm. mixed before the filling phase of the reservoirs. Therefore, a single fluid is representative of the oil filling the reservoirs before biodegradation and its composition predicted by compositional modeling is comparable to non-biodegraded natural oil samples. Compositional modeling enables the description of fluids in terms of light and heavy fraction (C14−/C14+) and the chemical proportion of saturates/aromatics and NSOs. Based on calculations of hydrocarbon losses in different reservoirs obtained in Part 1 of this study, biodegradation rates at geological time scales were calculated as a function of residence time in the reservoir, temperature history, and intensity of hydrodynamics.