WARNER, J. L.¹, D. K. BASKIN¹, R. J. HWANG¹, and M. E. CLARK^2
1Chevron Petroleum Technology Company, La Habra, CA
²Tengizchevroil, Kazakhstan

Abstract: Geochemical Evidence for Two Stages of Hydrocarbon Generation in Tengiz and the Origin of Solid Bitumen

Solid bitumen-free pores and sparry calcite cement after, or simultaneous with, solid bitumen suggest a complex history for the Tengiz field. There must be a period of brine saturation after a first-stage of oil migration, reservoir filling, and solid bitumen formation; followed by a second-stage of oil migration and reservoir filling of present-day oil.

Chemical and isotopic composition of (1) solid bitumen (2) present-day oil, and (3) H₂S dissolved in oil supports a two- hydrocarbon filling model for Tengiz, and demands that each generation have a different source. The +4 to 9 ^o/oo d³⁴S_CDT difference measured between solid bitumen (+5 to 7.5 ^o/oo) and present-day oil (+11 to 14 ^o/oo) suggest independent sources; if solid bitumen were genetically related to present-day oil, d³⁴S fractionation is expected to be, at most, a few ^o/oo. Sulfur in H₂S (+14 to +1 5 ^o/oo) is heavier than either bitumen or oil which suggests that it is has a third source, different from either the bitumen or oil source, because H₂S in equilibrium with organic sulfur is fractionated by about -15^o/oo.

We use fluid inclusion and solid bitumen elemental and reflectivity data to explore alternate two-stage models: (1) first generation oil deasphaltened by late migrating gas to form solid bitumen; reservoir is breached by faulting the salt seal; brine replaces first generation oil causing observed calcite dissolution and precipitation; reservoir is resealed; and filled with second generation oil, or (2) first generation oil fills the reservoir; reservoir is breached allowing hydrothermal solutions to pass through the reservoir transforming residual oil to solid bitumen and causing calcite dissolution and precipitation; reservoir is resealed; and filled with second generation oil.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas