Abstract:

The 3P reserves of the Xujiahe Formation are large in scale in Yuanba area.However, due to the strong heterogeneity of tight sandstones, and less understanding on the distribution of relatively high-quality reservoirs, the proportion of proven reserves is low in the 3P reserves and the production tests are poor in effect.It is necessary to accurately predict the distribution of coarse siliciclastic rocks, from which relatively high-quality reservoirs are to be discovered.On the basis of sedimentary microfacies division by using well data, precise lithologic interpretation is achieved by gradual lithologic identification.The geological significance of peaks and troughs on seismic sections is clarified by analyzing the lithological associations on the top and bottom surfaces for various sand groups in the 3rd member of Xujiahe Formation (hereinafter referred to as Xu 3 Member); meanwhile, multiple methods are applied to select proper time window for extracting seismic attributes given the gap in seismic-well layering, and several seismic attributes that are not influenced by the overlying and underlying formations are extracted within the time window.In regard of the drilling data, we select the seismic attributes that are the most sensitive to coarse siliciclastic rocks and use them to map the distribution of the coarse siliciclastic rocks via seismic-well tie.On the other hand, to identify the relatively high-quality reservoirs within coarse siliciclastic rocks, we sub-divide the coarse siliciclastic rocks in lithology with core data, confirming that there are two categories of high-quality reservoirs well developed in the Xu 3 Member, that is, the medium-to-coarse-grained calcarenaceous sandstone reservoir and fine-grained arenaceous conglomerate reservoir.Furthermore, the distribution of the two categories of high-quality reservoirs is finely depicted under the constraint of the coarse siliciclastic rock distribution while establishing well logging interpretation criteria.The results serve to lay a sound basis for the large-scale development of the Xu 3 Member gas pools in high efficiency.

Key words: seismic attributes, lithofacies, sedimentary microfacies, high-quality reservoir, tight sandstone, reservoir prediction, Xujiahe Formation, Yuanba area in northeastern Sichuan Basin