Three genetic types and prediction of sandstone densification in the Benxi Formation

The Benxi Formation of the Upper Carboniferous in the Yanchang field of the Ordos Basin is one of the important natural gas production layers, with reservoirs mainly composed of barrier, tidal channel, and shoal microfacies sandstones. Most of the drilled sandstones produce gas, but some of the drilled Benxi Formation sandstones have become dry layers due to densification. Identifying the origin and distribution of tight sandstone and effectively avoiding dry layers has become one of the important issues in the deployment of production wells. This article combines sandstone cores, sedimentary microfacies, microscopic characteristics, and logging responses to study the genesis and prediction methods of tight dry sandstone layers. The results indicate that there are three types of dry sandstone: tuffaceous basal cemented sandstone, carbonate basal cemented sandstone, and tightly compacted sandstone. The first type is concentrated and limited along the ancient volcanic crater near the ancient tidal channel of southern Jingbian; The second type is distributed in the relatively thickened area of the overlying Taiyuan Formation limestone; The third type is distributed in isolated small shoal sandstones, which may result in the lack of dissolution pores during the reservoir formation period which needs acidic fluids and oil and gas to enter the reservoir. This understanding is of great significance for the formation mechanism of effective reservoirs and the prediction of "sweet spot" distribution.