Controls on the Formation of Favorable Reservoirs in Tight Gas Sands: Examples from Two Intracratonic Basins—Sichuan and Ordos Basin, Western China

Yilong, Li; Dongbo, He; Guoting, Wang; Qunming, Liu; Tiejun, Wei; Taotao, Chen; Lidan, Ji
Ordos Branch Institute, Reserch Institute of Petroleum Exploration and Development, Beijing, China.

Among pervasive low-permeability and low-porosity tight gas sand reservoirs, searching favorable reservoirs with relatively higher reservoir qualities are important in tight gas exploration. The origin of those favorable reservoirs have been widely discussed from depositional environments and diagenesis to natural fractures. Tight gas reservoirs from two intracratonic basins (Sichuan and Ordos Basin) are comparatively studied to reveal the main factors controlling the formation of tight gas favorable reservoirs.

Xujiahe Formation of Sichuan Basin and Xiashihezi Formation of Ordos Basin are both tight gas reservoirs with air permeability mostly less than 0.1 mD and porosity mostly less than 8%. The two formations share some commonalities: (1) Both of them were deposited in braided stream-deltaic-lacustrine environment which developed on stable intracratonic basin slopes with inclinations less than 1°; (2) both have been influenced by acid diagenesis fluid derived from coal seams; (3) natural fractures are rare or less important for reservoir quality.

In the Xiashihezi Formation, reservoir quality shows a strong positive correlation with sandstone grain size and composition and the latter are strongly controlled by sediment sources and sedimentation process—the coarse quartzose sandstones which subsequently turned into favorable reservoirs mostly formed in high-energy streams from quartz-dominated provenance. This kind of sandstones may be more resistant to compaction and preserve more initial intergranular porosity which can be pathways for later diagenetic fluids. Favorable reservoir development of Xujiahe Formation shows some similarity but appears to be slightly different: (1) quartz lithic sandstones with highest quartz amount have undergone intense quartz cementation, thus have the worst quality; (2) authigenic clay coats have played a significant role in porosity preservation;(3) reservoir quality is positively correlated with feldspar content, which means dissolution of feldspar may has enhanced reservoir quality.

Despite all the differences of diagenesis between the two Formations, initial homogeneity of sediments caused by sedimentation process is the common origin of favorable reservoirs. Rock texture, composition, and grain size, which were determined by the provenance and stream energy during deposition are the most significant controlling factors of favorable reservoir formation.

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