This paper analyzes the sequence stratigraphy feature, lithology facies distribution, reservoir features and the distribution and control factors of the 3rd member of Xujiahe formation in Yuanba area, on the basis of the theory of high resolution sequence stratigraphy and core observation results. The target zone has been divided into one long sequence and five short sequences, corresponding to five sand groups. The experimental analysis of cores reveals that the reservoir belongs to low porosity and low permeability tight sandstone reservoir due to its poor physical properties, and this type of tight sandstone reservoir belongs to secondary tight reservoir because the efficient pores were formed during diagenesis procedure. Well test data confirm that the gas wells' productivities are very different and the upper three sand groups of the 3rd member of Xujiahe formation are relatively rich areas for natural gas. However, the distribution of gas in these three sand groups is still not uniform, which indicates that even in the same sand group, the gas performance varies dramatically from well to well. In order to find the reasonable explanation for this phenomenon, we conducted careful calibration between wells and logging curves to identify the reservoir features such as fissures and dissolution pores as well as analysis of influences of microfacies, lithologic belt, and sand body thickness. We draw a conclusion that favorable lithologic belt and sand body thickness are basic conditions for natural gas accumulation, and fissures combined with dissolution pores are key factors.

Yuanba area  /  the 3rd member of Xujiahe formation  /  natural gas distribution  /  secondary tight sandstone reservoir