Architecture Characterization of Sandy Braided Fluvial Reservoir: A Case Study of P Oilfield (Neogene), Bohai Offshore, China

Abstract The architecture of sandy braided rivers, especially the mid-channel bars and the internal intercalations, plays a key role in the enhancement of remaining oil. Based on ancient outcrops, modern deposits and geological models, taken P oilfield as an example, this paper studies the different hierarchies of the internal architectural elements to describe the heterogeneity of sandy braided fluvial reservoir qualitatively and quantitatively, and proposes techniques to characterize the architecture of underground braided fluvial reservoirs. Based on the proposed distribution pattern and the related characterization techniques, the different controls of architecture elements on the distribution of remaining oil are analyzed: (1) sandy braided rivers have three filling patterns, dominated by sandy fillings. The mid-channel bars and braided channels are distributed in the pattern of wide bars and narrow channels, while the silting layers within mid-channel bars have dome-like and horizon-like distribution patterns; (2) the architectures are characterized from 5th to 3rd-order hierarchies successively under the principle of thickness control, scale constraint and multidimensional interaction. Single braided belts can be classified by three identification marks (the average width/thickness ratio is about 178, the width ratio of braided channels and mid-channel bars is about 1:3.8, and the width/length ratio of mid-channel bars is about 1:2.2); (3)Single-period silting layers are nearly horizontally distributed within mid-channel bars and multi-period ones are parallel with each other, generally 2-4 silting layers; (4)Reservoir architectural elements of different hierarchies exert different controls on the distribution of remaining oil. The intercalations control the remaining oil directly in the hierarchy of braided belts. In the hierarchy of mid-channel bars, semi-muddy filling channels have enriched in remaining oil due to relatively poor watered out degree as a result of disconnected sand bodies in the upper part with connected sand bodies in the lower part. Influenced by silting layers and petrophysical boundaries, the remaining oil within mid-channel bars is mainly distributed as “segment-like” enrichment at the top of vertical aggraded bodies under the silting layers and at the margin of poor water flooding. Overall, the research not only has good applications in the integral adjustment schemes of mature wells and development designs of news wells in oilfields, but also provides geological basis for well deployment of adjusting wells, especially for producing remaining oil in horizontal wells.