Abstract

Mapping in the East Gobi basin, supplemented by seismic data, reveals a structural and burial history for basins adjacent to the Zuunbayan and Tsagaan Els oil fields. The tectonic framework was combined with available well and outcrop data to model the timing and magnitude of hydrocarbon generation.

Five structural episodes are recognized: (1) pre-Jurassic northeast-directed shortening that formed the tectonic fabric; (2) Middle Jurassic to Early Cretaceous rifting along northeast trends that formed the subbasins of the East Gobi basin; (3) late Early Cretaceous north-south shortening and inversion on existing normal faults; shortening caused left-lateral and reverse displacements on northeast-trending faults; (4) middle Cretaceous uplift and erosion, followed by (5) east-west shortening and right-lateral movement on northeast faults. Folds formed by inversion over Middle Jurassic–Early Cretaceous normal faults.

Modeling suggests that the bituminous member of the Zuunbayan Formation should be mature over large parts of the Unegt and Zuunbayan subbasins. Oil migrated from mature source areas toward several traps, including the Zuunbayan and Tsagaan Els fields. Modeling suggests that early oil (104–110 Ma) was generated in the Zuunbayan and Tsagaan Els area because of deep burial during the Cretaceous. Although generation began in the Early Cretaceous, peak generation in the Unegt subbasin occurred between 100 and 90 Ma. Generation continued at a decreasing rate up to the present day. Kerogen maturity (and oil field production) suggests that oil is the most likely product. Scoping calculations of hydrocarbon volumes generated indicate that the Unegt basin may have generated as much as 86 billion BOE.

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