Quasi-continuous Lithologic Accumulation System: A New Model for Tight Gas Occurrence in the Ordos Basin, China

Zhao, Jingzhou ¹; Fu, Jinhua ²; Wei, Xinshan ²; Liu, Xinshe ²; Wang, Xiaomei ¹; Cao, Qing ¹; Ma, Yanping ¹; Fan, Yuanfang ¹
(1)School of Petroleum Resources, Xi'an Shiyou University, Xi'an, China. (2) Research Institute of Exploration and Development, Changqing Oilfield Company, Petrochina, Xi'an, China.

The Ordos Basin has China’s largest deposits of tight gas resources in its 250000 square kilometers area. However, gas accumulation process and trapping mechanisms have long been in controversy and several models, such as lithologic or stratigraphic trap, deep-basin gas trap, and tight-gas model, were proposed. In this article, we demonstrate that the Upper Paleozoic gas accumulation is neither basin-centered nor lithologic, but is what we refer to as continuous lithologic accumulation system. The new model has following significant characteristics.

1. Every giant gas field comprises numerous smaller-sized lithologic traps that are superimposed vertically and combined or even connected laterally like strings of beads over vast area. Besides, most of lithologic traps are gas-bearing and no well-defined boundaries are diagnosed.

2. Water occurrence is complex. No updip water zone is observed. Gas-water contact is absent for most reservoirs. The complex occurrence of water is mainly controlled by factors of source, structure, reservoir and preservation, of which source condition is most critical.

3. The accumulations are predominantly underpressured and possess multiple pressure systems showing poor connectivity among the numerous smaller-sized traps. But It has been widely proved that the reservoirs were overpressured in geologic history.

4. The sandstones are mostly tight and extend vastly but exhibit a strong heterogeneity laterally, showing no trend of updip improvement for reservoir quality. Moreover, because the accumulation occurred within the lithologic traps, lateral seal condition is extraordinary, with no need of other seal factors like water locking.

5. Source rocks are nearly basin-wide in distribution and mostly interbedded with reservoirs so that gas generation is extensive and gas charge is pervasive. But gas charge is not strong enough for gas to thoroughly displace water in the reservoirs.

6. Migration is mainly short-distanced and dominated by primary migration. Its drive comes from two sources: one is overpressure caused largely by gas generation and another from diffusing of gas molecules. The migrating style is thus convinced to be non-Darcy movement.

7. The Upper Paleozoic gas accumulation system has experienced 3 stages of development: formation of tight sandstones; peak gas generation, extensive accumulation and development of overpressure; and recharging and adjusting of gas reservoirs and formation of underpressure.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.