Concentration of hydrogen sulfide in free gas of Dauletabad-Donmez field is uneven: It is practically absent in the northwest, whereas in the southeast its content reaches 0.948 percent. The gas dissolved in formation waters contains 2.2 percent hydrogen sulfide and is still present in these waters at more than 10 km from the field.
The isotopic composition was determined for the sulfur of pyrite of the gas-bearing sandstones, sulfates of the rocks and formation waters, and hydrogen sulfide dissolved in the formation waters.
Within Dauletabad-Donmez field the productive Lower Cretaceous Hauteriv sandstones are red on the west due to iron hydroxides. Hardly any organic matter is present. Small amounts of authigenic pyrite (generation I) occur as small globules. Then to the east where hydrogen sulfide is present these same sandstones are gray; the iron hydroxides have disappeared, and three generations of pyrite make their appearance. This color change extends to the eastern edge of the reservoir, and there the color goes back to red.
The generation I pyrite formed by alteration of sulfates, whereas the generation II, III, and IV pyrite resulted from action of hydrogen sulfide on iron hydroxides. The larger part of generation II pyrite consists of cubes up to 1 mm in size. It is clearly related to free hydrogen sulfide in the gas and has a heavy sulfur isotopic composition. Generation III pyrite consists of large poikilitic cement having incomplete crystal form, indicating relatively rapid growth with low hydrogen sulfide content of the gas. Generation IV pyrite consists of aggregates that formed in pores and cement by conversion of relict iron hydroxide, where hydrogen sulfide-bearing gases migrated from Upper Jurassic reservoirs into the Hauterivian reservoirs.
The sulfur isotopes in the study area are relatively heavy and are similar in both oxidized and reduced forms, indicating their genetic relationship. These fall into groups thereby reflecting variations in time and space of formation and conversion of the hydrogen sulfide in the Hauterivian sediments. Recognition of these groups in turn reflects the different structural features of the field. On the south is the semi-closed Dauletabad arch, and to the north is the broad Birleshik monocline with the Donmez structural nose. These structural features differ in thickness and lithology of the productive Shatlyk Horizon, temperature gradient, degree of fracturing, composition of the formation water, and other properties.
The heavy isotopic composition of the sulfur of the hydrogen sulfide of this field is a characteristic indicator of its origin in a high temperature zone at great depth as a result of reduction of sulfates in solution with organic matter. The scale of this process can be judged by the change in color of the rocks.
Taken from Pankina and others, 1986; digested in Petroleum
Geology, vol. 23, no. 9/10, one map, one diagram of isotope data.
Copyright 2002 James Clarke. You are encouraged to print out
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