Sedimentary Facies of Multi-Crosswell Profile Analysis

According to the analysis of the sedimentary facies of individual well, fluvial channel deposit of meandering stream developed in the research area during period of E1+2. The sedimentation types varied when came to period of E31, and they were mainly lacustrine deposit, delta facies of anastomosing stream and alluvial fan facies. Lacustrine sedimentation extensively distributed in period of E32. The clastic reservoir condition in E31 period was better than E32 and E1+2[1]. 6 Multi-crosswell profiles were chosen in the North-West Qaidam Basin, basically covered most research areas (Fig. 2). The following discussion will analyze the sedimentary facies distribution of each reservoir according to multi-crosswell profile.

The multi-crosswell profiles I-I’ is nearly NE-SW trending, crossing You18, Liang3 and Jianxi1 wells. The upper section of this multi-crosswell is shallow lacus facies, gradually changed to braided-river delta front and plain subfacies, presenting obvious retrograding sequence. It illustrates that the lake area has been expanding. The lake depth of Xiaoliangshan area is the bottommost.

The multi-crosswell profile II-II’ is nearly NE-SW trending, crossing You6, Feng3, Jian6 and Jian3 wells. It was bank lake sedimentary facies in the upper E31 section of You6 well and Jian6 well. And the top E31 section of Jian3 well was braided-river and floodplain deposits inserted into braided-river delta plain deposit. The sedimentary sequence was braided-river delta plain subfacies, delta front subfacies and bank lake facies from top to bottom. We could draw a conclusion that the provenance was in the northeast from sedimentary facies distribution.

The multi-crosswell profile III-III’ is nearly NEE-SWW trending, crossing Kai2, Kai3, Dun5 and Feng2 wells. Lacustrine sedimentation has been distributed to the southwest of Dun5 well, and the northeast part mainly spreaded braided-river delta facies sedimentation. When reached Feng2 well, the braided channel and floodplain facies could be seen. The profile illustrated that it displayed retrograding sequence which transited from braided-river delta plain subfacies to delta front subfacies, bank lake facies, and coastal-shallow lake subfacies in the vertical. It has been clarified that the lake was expanding to the northeast.

Multi-crosswell V--V' is nearly NW-SE trending crossing Lian3, Nan10, Feng3 and Dun 5 four wells. Coastal shallow and semi-deep lake facies were near Liang3 well. There were mainly bank lake facies around Dun5 well while the central part was braided-river delta facies, which made a deposition sequence not that obvious.

Crossing You18, You6 and Kai2 wells, Multi-crosswell VI--VI' is NW –SE trending. It distributed braided-river delta plain subfacies, delta front facies, bank lake subfacies and coastal-shallow lake subfacies from bottom to top, presenting a retrogradation sequence.