Abstract:

Genetic analysis on different braided deltas is the basis of remaining oil prediction and reasonable developing policy making. Lithogeochemical and genetic stratigraphic analysis indicates that Sanjianfang Formation in Wenmi oilfield is deposited by regressive and progressive shoal braided deltas during long term base level cycles under the aridic climate in a fresh lacustrine basin. Genetic stratigraphic frameworks of individual sand units of Sanjianfang Formation are established with the method of detailed genetic stratigraphic correlation. Regressive braided deltas are smaller in scale, within which distributary channels range from 30 to 58 in width to depth ratios, and evolve from branched shape to fingered shape in areal geometry, and consist of finer lithofacies association of CSm→MSp-l→FSh→FSp-h→FStr→FSp-h→Fr. On the contrary, progressive braided deltas are larger in scale, within which distributary channels range from 53 to 108 in width to depth ratios, and evolve from fingered shape to branched and sheeted shapes in areal geometry, and include coarser lithofacies association of SCm→GCSm→CStr→FSp-h→FSp-l. Small scaled mouth bars are formed around the slope break areas of lacustrine bottom. Water alternating gas (WAG) injection can enhance the recovery efficiency further.

Key words: Wenmi oilfield, shoal braided delta, individual sand unit, sedimentary microfacies, stacked patterns of sandstone, depositional model