Static connectivity of fluvial reservoirs and their temporal evolution: An example from densely drilled subsurface data in the Sanzhao Sag, Songliao Basin

Abstract

Research on the static connectivity of reservoirs is of great significance for predicting the lateral continuity of sandstone bodies and determining the field development strategy. The main impacts of static connectivity are geometry, stacking pattern and distribution of fluvial sandstone bodies, which are controlled by changes in base level. This study addresses the static connectivity of fluvial sandstones of the Yangdachegnzy reservoir in the Yushulin oilfield with a dense network of wells. The Yangdachengzy reservoir develops a long-term depositional cycle (a third-level sequence) consisting of 3 mid-term depositional cycles (MDC1-MDC3) and 21 short-term depositional cycles (SDC1-21). Sedimentary facies maps of each stratigraphic intervals in the Yangdachengzy reservoir that are constrained to high well density from the Yushulin Oilfield exhibit: (i) as base-level falls, the planform style of sandstone bodies changes from isolated type to amalgamated type; (ii) how static connectivity evolves through base-level fluctuations and show how static connectivity is sensitive to the net-to-gross ratio and well spacing. The analyses show that the relationship between the net-to-gross ratio and static connectivity exhibits a sigmoid curve. Such a curve is sensitive to the well spacing and the planform style of sandstone bodies. When well spacing increases, the S-curve shifts to the right, with a narrow “cascade zone” and a relatively higher percolation threshold of net-to-gross ratio (e.g., 0.2 at 150 m well spacing). When well spacing decreases, the S-curve tends to be linear, with a large range of “cascade zones” and a relatively lower percolation threshold of the net-to-gross ratio (e.g., 0.43 at 450 m well spacing). At a given net-to-gross ratio, amalgamated-type sandstone bodies that developed at the lowest base-level stage are very prone to reach the fully connected status (reservoir connected over 0.8) with a relatively low well density.