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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Overpressures in the Sediments of the South Caspian Basin: Nature and Prediction
(1) Geology Institute of Azerbaijan, Azerbaijan National Academy of Science, Baku, Azerbaijan.
(2) Geoscience, Baker Hughes Inc., Houston, TX.
(3) Geology & Geophysics Department, State Oil Company of the Azerbaijan Republic, Baku, Azerbaijan.
On the basis of a systematization and summarization of studies of overpressures in different basins of the world, the authors identify two basic factors that contribute to overpressures in sedimentary basins: tectonic stress and the progressive rise in subsoil temperature with depth. A summarization of the results of all past studies of the patterns of distribution of fluid pressures in the South Caspian Basin (SCB), from well logs and actual downhole measurements to depths of approximately 7 km, makes it possible to identify two basic overpressure zones in this interval which are most distinctive in the Baku Archipelago. Depending on the lithofacies of the section, the top of the first overpressure zones lies at a depth of 600 to 1200 m. Farther down, to a depth of approximately 4 km, pressure gradients, while still high, are quite stable. We can observe the start of a new intense overpressure zone at a depth of 5 km. The authors demonstrate that overpressures in the upper zone are caused by uneven consolidation (underconsolidation) of rocks, while overpressures in the lower zone are caused by hydrocarbon generation. The lower overpressure zone is the most intense and results from the thickness of the shales, the concentration of organic matter in the shales, the type of organic matter, and the temperature conditions of its transformation into hydrocarbons. In this zone the greatest risk is associated with the start of gas formation at depths greater than 9 km resulting from more intense thermal breakdown of kerogen and the cracking of liquid hydrocarbons formed earlier. The results of the first attempt in the South Caspian Basin (SCB) to predict overpressures directly on the basis of seismic data are quite consistent with theoretical developments and traditional diagnostic methods.