Anatomy of Distributary Channel Sand, the No.68 Sandbody of Youshashan, Western Qinghai

Key words: 
• architectural element /
• sedimentary surface /
• geological model sandbody anatomy /
• distributary channel /
• Qinghai /
• Youshashan

Abstract: The bottom of the lower Youshashan Formation (Neogene) was deposited by a braided deltaic system. The No.68 sandbody was a distributary channel cut into the exposed lake bed during the lower stand stage. Sediments of the sandbody were transpoted by the longitudinal braided deltaic system located between the Qimantage and Argin mountains. The No.68 sandbody has the similar upward thinning structure and grain size sequence as most of fluvial sandbodies have. 3D sampling net work controls 158 meters long of the sandbody. The sandbody is 42.5 meters wide and 5.33 meters thick, it widens and thins toward the downstream, which is also one of the common characters of all deltaic front distributary channels. We recognized 12 lithofacies within the sandbody, they are: Gpr, Glt, Ge, Slt, Smt, Spr,Sla,Sbm, Sst, SIcr, SIh, Mr1. 6 lithoracies assemblages are composed of the 12 lithofacies, they are: Ge-Gpr-Glt; Glt-Slt; Slt-Smt; Glt-Sbm(Spr); Sst-SIt, SIcr(Gpr,Spr,Sst)-Mrl. There are 6 kinds of architectural elements in the sandbody, they are: lateral accretion(LA), longitudinal bar(LB), transverse bar(TB), simple gravel filling(GF), simple sand filling(SF), levee and sandflat(LF). LA is 8.8 14.4 m wide and has the ratio of width/thichness(W/T) 5.5 7.2, LB is 10 12.4 m wide and its W/T is 4.17 5.17, TB is 6.4 m wide and its W/T is 2.67, GF is 30 m wide and its W/T is 40, SF is 12 24 m wide and W/T is 15 30, LF is 1.76 m thick. LA,LB and TB are situated at the lower part of the sandbody, GF and SF develop at the upper part of the sandbody, LF occurs at the two channel banks. A 7 order surface hierarchy in the sandbody has been found in accordance with the sedimentary events. First order is the top and bottom surface of the sandbody, second order is sedimentary phase surface, third order is the surface of sub-phase, fourth order shows the surface of died channel bars, fifth order surface is the accretion surface within bars, sixth order represents the cross bedding set surface and the seventh order is the surface of laminar. By the surface hierarchy, the depositional process of the sandbody was reconstructed by 3 sedimentary phases and 6 sub-phases. Compared the architectural element reservoir model with the thin grid proto-type reservoir model, it could be concluded that the two models are very similar in permeability distribution. This tells us that we can build the predictable reservoir model using the method of architectural element analysis, with the predictable model we can predict remaining oil distribution and provide more useful information for EOR.