Abstract
The mechanism of large-scale hydrocarbon accumulation in the Tethys domain has long been of interest to petroleum geologists. Located in the northern margin of the Tethys, the Amu Darya Basin (ADB) is well known for its large, indeed giant, gas fields. However, there has been surprisingly little research on the mechanisms of hydrocarbon accumulation in the basin. This paper investigates the accumulation history of the Samandepe Gas Field, a large condensate gas field in the Chardzhou step-fault zone in the northeastern ADB. Trap evolution in the basin is analyzed, the oil and gas sources and hydrocarbon charging periods are determined, and the process of hydrocarbon migration and accumulation explored to establish the relationship between hydrocarbon accumulation and the tectonic evolution of the Tethys. Geophysical logging, organic geochemistry, fluid inclusion analysis, basin modeling are used for analysis. The results show that hydrocarbon accumulation in the Samandepe Gas Field occurred in two stages. First, a lithological ‘large trap’ formed in a barrier reef belt in the Late Cretaceous Santonian, before the present anticline structure of the gas field formed. This trap was filled with oil generated in Upper Jurassic marine source rocks, forming a lithological paleo-oil reservoir. In the second stage, at the end of the Paleogene, the large trap was reformed as structural traps with reef-shoal reservoirs. Natural gas generated from coal-measure source rocks in Middle and Lower Jurassic strata migrated along basement faults, filling the traps. Excess natural gas displaced the oil, some of which dissolved in the gas, forming ‘gas washing’-type secondary condensate oil and creating a structural condensate oil-bearing field. The inherited paleo-uplift in the step-fault zone is crucial to large-scale hydrocarbon accumulation in the ADB.
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