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Abstract: Sour Gas Distribution in the Amudaria Basin, Central Asia

Marina Dakhnova, A. Ivlev, E. Shkutnik

The Amudaria basin is the main sour-gas bearing region in Central Asia. In this region, sour gases occur in Upper Jurassic carbonate-reservoir rocks as well as in terrigenous rocks of Cretaceous age, but the Upper Jurassic sulfate-carbonate complex is the main sour-gas bearing and producing complex. The chemical and isotopic composition of fluids in Upper Jurassic rocks show that sulfate reduction is the main process responsible for sour gas formation in the central part of the basin, where Kimmeridgian-Tithonian evaporites occur. The H2S content of gases varies widely (0 to 10 percent by volume), even within similar carbonate traps located close to one another in the same temperature zone.

Analyses of sour-gas distribution and composition in fluids in these areas indicate the main factors which control the variation of H2S content in Upper Jurassic hydrocarbon pools in the same temperature zones. These factors include 1) the carbonate sediment facies type (shelf, barrier reef, deep water facies), and 2) within the same facies, the characteristics of traps and pools (tight, gentle, structural, phase-type, etc).

The most favorable conditions for H2S accumulation occur in hydrocarbon pools confined to the barrier reef flat and the parts of the shelf closest to it. The least favorable conditions are in pools confined to local reefs or carbonate build-ups located within the deep-water facies zone.

These results are important for the prediction of H2S in hydrocarbon pools. In most cases, H2S in the Cretaceous complex is epigenetic. With the exception of Central Karakum zone, H2S distribution in this complex depends on the distribution and composition of Upper Jurassic evaporites.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France