Quantitative estimation of expelled fluids from Oligocene rocks, Histria Basin, Western Black Sea

Knowledge of changes in rock porosity and rock thickness within a sedimentary basin is important for identification and modeling the evolution history of petroleum systems. Because water is a phase that accompanies petroleum in its migration through a sedimentary basin, it is important to determine the volume of such mobile water masses as a means of understanding the development of petroleum systems. stria Basin is located in the Romanian sector of the Black Sea shelf where Oligocene pelitic rocks are widespread both horizontally and vertically. A petroleum system was defined in the Histria Basin in which the Oligocene sequence plays a dual role: source rock and seal for subjacent Albian-Eocene reservoir rocks. ork proposes a mathematical model to calculate water volumes expelled from the Oligocene rocks from the time of burial to the present day, as well as the timing of water expulsion. The model combines Magaraʹs model for calculation of total volume of expelled water from the beginning of burial with Athyʹs model for porosity evolution with depth. The basic assumptions are that porosity decreases exponentially with depth and that the volume of rock grains within the unit never changes. A “backstripping” method, utilizing a computer program written in Turbo BASIC language, was applied to determine the model parameters that were later employed to describe the timing of water expulsion. Sonic velocities derived from the acoustic log recorded from a well catalogued as 811 W Lebăda were used as inputs. ng results show that the Oligocene sequence is under compacted and that the relative variation of the expulsion factor, Δε, outlines three temporal peaks in water expulsion: Badenian-Sarmatian, Pliocene, and Quaternary-Present. The main peaks, Pliocene and Quaternary-Present, are superimposed on petroleum generation and expelling stages of Oligocene source rock evolution. These results support the view that the Oligocene sequence has a dual role within Histria petroleum system: (1) petroleum source rock, and (2) seal for older reservoir rocks.