Geochemical Evidence for Two Stages of Hydrocarbon Emplacement and the Origin of Solid Bitumen in the Giant Tengiz Field, Kazakhstan
J. L. Warner, D. K. Baskin, R. J. Hwang, R. M. K. Carlson, M. E. Clark, 2007. "Geochemical Evidence for Two Stages of Hydrocarbon Emplacement and the Origin of Solid Bitumen in the Giant Tengiz Field, Kazakhstan", Oil and Gas of the Greater Caspian Area, Pinar O. Yilmaz, Gary H. Isaksen
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Tengiz is a giant oil field on the northeastern coast of the Caspian Sea in Kazakhstan that produces a high-gravity, hydrogen sulfate (H2S)-rich oil from a reservoir containing abundant solid bitumen. Several lines of geochemical and petrographic evidence suggest there were at least two stages of petroleum migration into the Tengiz reservoir, both generated off structure from a marine source rock. The initial charge gave rise to solid bitumen, perhaps by a de-asphaltening process. Bitumen formation was followed by a period of hydrothermal activity, which thermally matured the bitumen to an insoluble pyrobitumen, produced bitumen-freepores, precipitated calcite on the bitumen, and mineralized parts of the Tengiz flank. Finally, a second petroleum charge, most likely from the same source at higher maturity, accompanied by a significant in-flux of H2S arising from thermochemical sulfate reduction (TSR) deep in the basin, filled Tengiz with its present-day oil.
The Tengiz reservoir consists of Carboniferous and Devonian limestones with mostly grainstone and packstone textures that define an isolated mound with a central platform and surrounding flank. Do-lomitization and silicification are sparse; cements are sparry calcite. The reservoir is divided into unit 1 (∼3950-4500 m; ∼12,959-14,763 ft), unit 2 (∼4500-5100 m; ∼14,763-16,732 ft), and unit 3 (∼5100 to >5600 m; ∼16,732 to >18,372 ft). Porosity average is 7% bivolume (BV) (range 0-20%). Where solid bitumen is abundant, it typically occupies 3% BV (range 0-15%), but bitumen occupies less than 1% BV in large parts of the reservoir. Bitumen is commonly encapsulated with calcite cement. Typically production preferentially enters boreholes from a few meter-thick intervals. Top reservoir temperature is 105°C, and initial pressure is 11,500 psi (79.2 MPa).
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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.