Gas and water distribution in the tight gas sands of the northwestern daniudi gas field, ordos basin, China: Impact of the shale barrier

The production of gas from the Upper Paleozoic tight sands in the Daniudi gas field, Ordos Basin, China, is hindered by a lack of knowledge of the factors controlling water distribution. This study aims to clarify the origin of the water, reduce unwanted water production, and detect probable locations of "sweet spots". A geological study during field development was conducted in the northwestern region of the gas field by integrating production data with other core, well log, mud log, and geological data. The results showed that the gas and water distribution was apparently related to gas charge, with a close negative correlation identified between the thickness of shale barrier and the cumulative gas production on a per well basis. Therefore, the shale barrier has the significant influence on the fluid distribution in the study area. In addition, the shale barrier was found to play a dual role in hindering gas charge toward the upper reservoir and in sealing trapped gas in the lower reservoir, given the configuration of "gas generation below and gas storage above". This implies the wax and wane relationship. Insights on the shale barrier's effect contribute to an alternative strategy of "sweet spot" prediction for resource development in unconventional gas reservoirs, i.e., if improved technologies are currently lacking to discriminate between gas-bearing and water-bearing zones in tight gas sands, the spatial distribution of continuous shale can be predicted with 3D seismic data as an initial step, which is thought to be less difficult. The attempts to detect "sweet spots" should subsequently focus on the local area where the thinner shale barrier exists below the producing zone. This work not only can enrich the theory of hydrocarbon accumulation in the Ordos Basin, but also presents an analogue to tight sand gas for worldwide sedimentary basins.