Analysis of Sandbodies in the Braided Delta Front,WM Oi lfield, Turpan-Hami Basin

Key words: 
• braided deltas /
• underwater distributary channels /
• river mouth bars /
• depositional models /
• Turpan-Hami Basin

Abstract: The braided delta front is developed in WM oilfield, Turpan-Hami Basin, which is composed of the underwater distributary channels, mouth bars, distal sands, sh eet-like sands, and interchannel muds microfacies. The deposition features of t he braided deltas are low sedimentary maturation and abundant current structures caused by the traction current. The underwater distributary channels have an er osion surface in the channel bottom, and are fining upward from conglomerates in the base to fine-sandstones in the top of the channels. They are 2.5m in rhythm, and 4～10 m in complicate rhythms, and are 200～400 meters in width. The para llel bedding are well developed in the sandstones. While the mouth bars are cour sing upward from siltstones to sandstones, and are large than 4 m in thickness. The sheet-like sands are mainly sandstones, fine to medium grained, well sorted and rounded, and are mostly coursing upward. They are 3-4m in thickness. The rivers in the braided delta are strong, so that the distributary channels s till erode downwards after entering lake,and forms underwater distributary cha nnels as a result of water flow in the bottom of the lake, The underwater distri butary channels can frequently migrate laterally so as to form sheet-like sandb odies (complicated sandbodies of the distributary channels). Some uneroded inter channel muds are scatterd in the inner of complicated sandbodies of the distrib utay channels, and will be flowing barriers. The volumes of the mouth bars depend directly on the amounts of sediments carrie d by underwater distributary channels,while the shapes are partly influenced by the waves of the lake. The mouth bars may be ellipsoid parallel to underwater d istributary channels (i.e. perpendicular to the shore of lake),but may become fa n-like and even ellipsoid parallel to the shore of the lake by the modification of the waves. The nine kinds of relationship between the underwater distributary channels and the mouth bars are proposed according to the strength of the curr ents in the distributary channels and the strength of the waves in the lake. It has concluded the shapes of the mouth bars in the Sanjianfang Formation, WM Oilf ield, Turpan-Hami Basin. The underwater distributary channels of next stage may flow by the franks of the former mouth bars and develop forward; as a result,th e mouth bars are presented along with the franks of the underwater distributary channels. The underwater distributary channels of next stage, however, may flow on the top of the former mouth bars and erode them. They may cut part of the mou th bar deposits and may form composed sand bodies of the mouth bar (upper) and u nderwater distributary channel (lower) if erosion is weak. They way cut all the mouth bar as the latter is never presented if the erosion is strong and the form er mouth bar is too thin. A kind of thin-beded, fine-grained, lenticular sand bodies, which is called dis tal sands microfacies in the paper, is deposited in inter bars, shallow lakes in the weak wave region by the reworking of deposits of the mouth bars and underwa ter distributary channels by the weak waves or other currents in the lake. Howev e r, in the region of strong waves, the sediments of the underwater distributary c hannels and the mouth bars are by strong waves and migrated laterally to form co ntinuos, large scaled sheet-like sands. When the accomodation space of the lake basin is smaller, the braided deltas are well developed, and inter channel mud deposits are eroded by the underwater dis tributary channels in braided deltas, so as to form sheet-like sandbodies (compl icated sandbodies of the distributary channels). When the accomodation space of the lake basin is larger, the braided deltas are less developed, and the underwa ter distributary channels can not stretch too far away and channels sandbodies a re much more isolated, while the mouth bars are relatively more developed.