Contrastive Research of Fan Deltas and Braided River Deltas in Half-graben Rift Lake Basin in East China

Key words: 
• half-graben rift lake basin /
• fan delta /
• braided river delta

Abstract: Fan deltas and braided river deltas are widely developed along the direction of minor axis in half-graben rift lake basin of East China. Both of them can form good oil and gas reservoir, but the identifications of them in well core and geophysical data are easy to confuse. Through the comparison and analysis of sedimentary features, geophysical data, oil-gas-bearing and controlling factors, we conclude that fan deltas are formed by alluvial fans running into lake along steep-slope zone and its seismic reflection characters is wedge foreset. The sediment composition maturity and structural maturity of fan deltas are lower than that of braided river deltas. The sedimentary features of fan deltas reflect the gravity current and tractive current sediments combined effection. Braided river deltas are formed by braided river flow into lake far away along gentle slope zone and its seismic reflection characters is imbricate foreset. The sedimentary characters of braided river deltas reflect the tractive current sediments effection. Fan deltas can form fault-lithologic reservoir and lithologic reservoir. Its favorable reservoir zone includes underwater distributary channel, sheet-like sand of fan delta front and slump turbid fan formed in pre-fan delta. Braided river deltas can form strata-lithologic reservoir and lithologic reservoir of which favorable reservoir includes braid-river channel of braid river delta plain and underwater distributary channel, mouth bar, sheet-like sand of braid river delta front. Besides above, turbidity sandstone is also favorable reservoir in fairly deep lake. The sedimentary features of fan deltas and braid river deltas are controlled by palaeoclimate, tectonic-palaeogeomorphology and lake level fluctuations; Tectonic-palaeogeomorphology is the critical factor among them. The depositional model of fan deltas and braided river deltas in half-graben rift lake basin of East China is established by combining the sedimentary features difference between them so as to provide guidance to analysis and research for them in similar geological background.