Abstract:

Fan deltas are widely developed in Xin’ancun and Dalianhe Formation of Tangyuan fault depression. According to core observation, well logging, seismic, lithology identification and grain size analysis, three subfacies including fan-delta plain, fan-delta front and pro-fan delta and eight microfacies are identified. From fan-delta plain to fan-delta front, the sedimentary characteristics of different microfacies changes regularly. The sediment textures such as the supporting type change from matrix-support to grain-support, the contact relationships between grains change from points to surfaces, grain separation from bad to good, the maturity from low to high, and the size of sediments from coarse to fine. The sedimentary bedding structures are massive bedding being miscellaneous in gravels that developed in fan-delta plain, and small-scale cross bedding with low angle and wave bedding developed in fan-delta front. The cumulative relative frequency curves are one-step and two-step type with low angle in fan-delta plain, but are two-step with steep slope and three-step type in fan-delta front. In the C-M diagram, the point group of fan-delta plain mostly concentrates in PQ section with large C and M value, and fan-delta front mainly concentrates in QR and RS section. Based on these studies, and combined with core characteristics and grain size analysis, it can be concluded that hydrodynamic mechanism of fan delta in this area is mainly traction current deposit, but gravity flow also plays some action.

Key words: Tangyuan fault depression, Xin’ancun Formation, Dalianhe Formation, fan delta, grain size analysis, sedimentary characteristics；sediments