Based on drilling, seismic and gas testing data, the structure, paleogeomorphology, sedimentary facies, trap, gas and water distribution are analyzed for Benxi Formation in Ganquan Fuxian area. The results show that reservoirs and stones deposited in tidal-flat-lagoon barrier environment. The sandstones mainly consist of two barrier bars parallel to the NW orientation shorelines, located on both sides of the ancient karst high belt. The underlying stratum in the middle east of the study area is the 6th Member of Ordovician Maiagou Formation. Due to the lack of Majiagou reservoir and downward migration channel, oil and gas are mainly accumulated in the east barrier of Benxi Formation. In the southwest of the study area, the ancient karst groove exited under the Benxi Formation, which make the coal measure hydrocarbon source rock to connect laterally with the karst limestone reservoir of 5th Member of Miajiagou Formation. Therefore, the 5th Member becomes the main gas reservoir while the western barrier of the Benxi Formation is relatively poorly charged with gas. The trap type is dominated by lenticular sandstone bodies, with a small amount of sandstone pinching out traps. The water zones consist mainly of thin sand layers with a single layer thickness of less than 2 m, which is tidal channel or barrier edge sandstone. Some thicker water layer sandstones are located between gas zone laterally, which may be related to the activity and destruction of local large offset faults during the reservoir forming period. The dry zones consist of tight sandstones with porosity less than 3.2% or permeability less than 0.07×10^-3μm², mainly composed of dark gray tuffaceous sandstone and light gray lithic quartz. The logging porosity of dry zones is less than 3.2%, or 0~3.2% under the microscope. The tuffaceous sandstone has dark color (dark gray, referred to as black sandstone for short), fine grain size (fine sandstone or silty fine sandstone), thin single layer thickness of about 0.5~2 m, and is randomly sandwiched in the light gray barrier or tidal channel sandstone. The upper and lower Member of Benxi Formatioin may contain one layer of tuffaceous sandstone, which is characterized by high volcanic dust matrix, high content of chert and rock debris, basal cementation, poor particle sorting and roundness. Light grey tight lithic quartz sandstone is characterized by clayey and dolomitic cementation, grain linear contact or stylolite contact, strong compaction and pore uneven distribution. According to the control of sedimentary facies and physical properties on gas accumulation, favorable reservoir distribution are predicted using seismic attributes and gas detection, it is believed that there are two barriers oriented towards the northwest that have developed in each of the first and second sections of the study area. The interior of the sandbar barrier is a highly productive area, while the physical attributes of the barrier’s edge deteriorate, leading to a decrease in productivity. The thick sandstone, which has a high water content, is mainly influenced by the activity of the Yanshanian North Northeast fault.

Based on core and thin section observations, as well as statistical analyzes of sandstone thickness and percentage of sandstone content from 1446 wells, we analyzed the coastal changes and control factors of the microfacies type, planar morphology and distribution of the Benxi Fm sandbodies in the Yanchang gas field. The results indicate that the Benxi Fm of the Yanchang gas field is generally composed of terrigenous coastal clastic barrier deposits in a medium to small tidal range environment, but the wave action in the southern region is weak, forming typical barrier sandbars and backbarrier deposits. The wave actions in the north change more strongly, forming three major microfacies sandbodies: reworked barriers, shoals and incised valley filled tidal channels. The barrier sandbodies are arranged in a belt or bead-like pattern parallel to the shoreline and are distributed longshore near the provenance. The oval-shaped shoal sandbodies are dispersed further from the shoreline, and the filling of tidal channel sandbodies is controlled by ancient geomorphic grooves. This research has important guiding significance for the identification and evaluation of reservoirs in the Yanchang gas field and similar geological settings.