Abstract Tengiz is a giant oil field on the northeastern coast of the Caspian Sea in Kazakhstan that produces a high-gravity, hydrogen sulfate (H 2 S)-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 H 2 S 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).