Gravity Modeling and Its Implications to the Tectonics of the South Caspian Basin
James W. Granath, K. A. Soofi, O. W. Baganz, E. Bagirov, 2007. "Gravity Modeling and Its Implications to the Tectonics of the South Caspian Basin", Oil and Gas of the Greater Caspian Area, Pinar O. Yilmaz, Gary H. Isaksen
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The large isostatic anomaly dominating the northwestern part of the South Caspian Basin is best modeled as a root of South Caspian crust that has displaced the normal lithospheric mantle. In the eastern South Caspian, gravity modeling suggests that continental crust does not extend seaward as far as previously presumed, and that the Turkmen platform is composed of a thickened sedimentary section overlying Caspian Sea-type crust.
Satellite-derived free air gravity data (Sandwell and Smith, 1997) for the South Caspian Basin, combined with low-order onshore free air gravity, show a steep 120-mgal decrease from the Alborz Mountains along the south coast of the Caspian Sea (Figure 1). The negative gravity gradient continues to —150 mgal at the Apsheron sill, on the northern margin of the basin. This negative anomaly is not surprising in light of the 900-m (2952-ft) water depth for most of the southern Caspian Sea, the 20-km (12-mi)-thick sedimentary section, and the anomalous oceanic character of the crust. However, the negative gravity anomaly persists under the Apsheron and northward into the southwestern part of the central Caspian Sea, in an area of low topography and shallow sea bottom. It adjoinsthe negative Bouguer anomaly of the Greater Caucasus Mountains, but in the Caspian Basin, it constitutes a pronounced isostatic anomaly.
In contrast to the western Apsheron area, the eastern South Caspian Basin contains a 40-mgal gravity high that extends 200 km (124 mi) seaward from the coast of Turkmenistan. It is separated fromthe onshore by a small gravity low. Seismic data show no significant changes in thickness of the Tertiary section between onshore and offshore Turkmenistan.
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Throughout time, the greater Caspian area has maintained its position as one of the major petroleum provinces in the world. Its early history as a prolific producer of oil is well known. Caspian region exploration dates to the seventh century B.C., during the time of the Median Kingdom in today's southern Azerbaijan. Oil played an important role in the everyday lives of these ancient tribes of the region, and it is still a very important commodity today. The past two decades have seen many important advances in our knowledge of the geological evolution of hydrocarbon-bearing sedimentary basins. The success of modern exploration is, to a large extent, based on new advances in both deep and 3-D seismic imaging, as well as improved pressure-prediction and pre-drill oil and gas quality predictive methodologies, to mention just a few. Nevertheless, large areas of the greater Caspian region have remained unexplored due to a variety of factors such as deep-water conditions and zones with high pore-pressures in the South Caspian Sea and The Black Sea, and vast shallow-water regions with harsh winter ice conditions in the North Caspian Sea. This publication contains 12 extended abstracts and 6 full-length papers that discuss technology development, challenges in estimating proven and potential reserves, outcrop-based studies of potential reservoirs, regional tectonics and geodynamic evolution, and source rock and stratigraphic analyses of the greater Caspian area.