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Accumulation Conditions for Ordovician Gas Enrichment in the West Margin of Ordos Basin, China

Xu, Wanglin; Guo, Yanru; Liu, Junbang; Jianrong, Gao
Ordos Department, Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China.

The Ordos Basin, which is situated in the north part of China, is one of the largest basins in terms of natural gas resources in China. Most oil and gas fields were found in clastic rock of Carboniferous, Permian and Triassic horizons. It was demonstrated that gas found in the top of Ordovician carbonate weathering crust in mideast Ordos Basin was from the overlying coal-bearing strata. However, natural gas accumulation from marine source rocks in Ordovician was found recently in the west margin of Ordos basin. The purpose of this paper is to describe the geological conditions of gas accumulation in the Ordovician in the west margin of Ordos basin and the identification of the Petroleum System.

The marine source rocks, occurred in the Kelimoli and Wulalike formations of Ordovician which consist of shale, mudstone and marlstone, are mainly distributed in the west margin of Ordos basin where sedimentary environments are deep ramp faces. The thickness of strata ranges from 20m to 200m, with the thickest deposition occurred in the west part and the thinnest in the east. Deposited in marine ramp settings, the shales are rich in organic matter and the TOC ranges from 0.19% to 4.55%. The kerogens are dominated by type I or II and the biological type is mainly the original marine invertebrates and algae. Thermal maturity is mostly in the range of 1.4 to 2.8% Ro.

Limestone bedding karst cavity reservoir in Kelimoli formation and beach face dolomite reservoir in Zhuozishan formation constituted the main space for the gas accumulation. The bedding karst cavity reservoir, with the average porosity between 4% to 6% and the permeability between 0.1×10-3μm² and 2.0×10-3μm², is the main gas reservoir while the dolomite reservoir mainly produce water. The limestone bedding karst cavity reservoir mainly distributed in the west with the thickness between 60m and 170m and is interbedded with the source rock while the dolomite reservoir mainly distributed in the east.

The overlying coal-bearing strata which is wide continuous distributed prepared the good cap rock with high quality. The vitrinite reflectance values and the homogenization temperature of fluid inclusions suggest that the onset of the oil generation started in the late Jurassic and gas in the early Cretaceous. There is a good exploration potential in the future.


AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012