Reservoir characteristics of Dengying Formation in Hupingshan-Zouma area of western Hubei-Hunan region

Sinian carbonate rocks are widely developed in the study area, and these rocks constitute one of the most hopeful oil-gas exploration target strata where a breakthrough is likely to be made in the lower marine combination. Through oil-gas geological survey along the profile and the stratigraphic section measurement of the outcrops, in combination with the previous achievements, the authors restudied the strata and sedimentary facies of Dengying Formation in Hupingshan-Zouma area. According to systematic identification and analysis of the rock slices, casting body slices and reservoir property samples of the outcrops, the authors hold that the Dengying Formation is mainly composed of carbonate rocks, which can be divided into three lithologic sections in upward succession. The strata are in conformable contact with the underlying Doushantuo Formation, and in parallel unconformable contact with the overlying Niutitang Formation, with the total thickness being 242～339m, belonging to the sedimentary environment of tidal flat of the restricted platform-open platform-slope of the platform edge. In the strata, the reservoir thickness is 210.4m, which possesses 72.7% of the total stratigraphic thickness; the reservoir rocks are composed mainly of granular dolomite, micrite dolomite, psammitic dolomite and oolitic limestone; the reservoir space comprises mainly intercrystalline pores, intergranular dissolved pores and fissures; the porosities are mainly 0.69%～2.66%, with the maximum value being 3.52% and the average value being 1.52%; the maximum value of the permeability is 6.2812×10-3μm2. and the average value is 0.3000×10-3μm2. the main pore types are small throats and micro throats. On the whole, the reservoir property of the Dengying Formation is rather poor, and it is comprehensively evaluated as a Ⅲ-grade reservoir; nevertheless, structural fractures of the Dengying Formation are well developed, which can improve the reservoir property, and it is also hopeful for the reservoir to become a good reservoir. If there exist good caps and traps above the Dengying Formation, it is possible to find commercial gas reservoirs of certain sizes in the Dengying Formation.