A sequence stratigraphic model for reservoir sand-body distribution in the Lower Permian Shanxi Formation in the Ordos Basin, northern China

Abstract

The Lower Permian Shanxi Formation is the main gas-bearing stratigraphic unit in the northeastern Ordos Basin, China. Based on an integrated investigation of well logs, seismic data, cores and outcrops, the lower member of the Shanxi Formation has been subdivided into three third-order sequences from base to top, namely SQ1, SQ2, and SQ3. The three sequences consist primarily of stacked lowstand systems tracts with little deposition of transgressive systems tracts and/or highstand systems tracts. Detailed sedimentological and stratigraphic analyses indicated that the entire sequence is characterized by a regional transgression with incised valleys and channel fills marking the bases of each retro-gradational sequence, and swamp or inter-channel bay mudstone and coal deposits capping each sequence. The three sequences retracted source-ward sequentially from the sandstone-dominated SQ1 to the mudstone-dominated SQ3. The spatial and temporal distributions of the reservoir sandstones was primarily controlled by retro-gradational stacking patterns. Channel sand-bodies formed during the lowstand systems tracts of SQ1, SQ2 and SQ3 are excellent reservoirs, which form excellent reservoir–cap-rock assemblages with the overlying fine-grained sediments. The unique sequence stacking patterns control the accumulation of hydrocarbons. The reservoir quality of the SQ1, SQ2, and SQ3 sequences become worse gradually from base to top. Within each sequence, channel or incised valley fill of the lowstand systems tracts remain potential good expiration targets.