An Evaluation of Permian Terrestrial Deposits as Potential Hydrocarbon Source Rocks, Southern Mongolia

Jerome Y. Amory, Alysa M. Keller

Hydrocarbon source rock evaluation in Mongolia is difficult due to a lack of well-documented stratigraphic studies. This is particularly true for the Paleozoic erathem, which has received less attention because of its complicated tectonic history. However, extensive potential hydrocarbon source rocks are reported in the South Gobi Coal Basin, which is part of a well-preserved belt of Permian terrestrial deposits in southern Mongolia. The stratigraphy, depositional facies, and associated source rock characteristics of several important localities along this epicontinental belt help evaluate the regional importance of the gas-prone Permian deposits.

The most promising Permian accumulation occurs in the Tavan Tolgoy basin, with an estimated 1.2 billion tons of high-grade coal. Volumetrically, this deposit represents an important potential source; however, it is limited areally to a small (90 sq. km) basin. Various units were examined and sampled to characterize the basin, including a 14 m-thick coal seam occuring in a fluvio-deltaic sequence. Preliminary pyrolysis results confirm the gas-prone character of these lithologies. Other thick (up to 62 m) coal strata are reported to occur in the basin to a depth of 950 m. younger basaltic intrusions probably had a locally favorable effect on maturity.

At other Permian localities, extensive meandering fluvial deposits occur. The regional dominance of fluvial facies suggests a prevailing humid climate, making the Permian a favorable period for the deposition of organic-rich sediments in other confined basins. The areal confinement of those intermontane basins, however, limits their hydrocarbon potential.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995