The Analysis On Depositional Characteristic And Play For Petroleum And Gas In Initial Stage Of Exploration In Offshore Faulted Basin

Aiming at scarce information, inadequate acquaintance and simple method of faulted basins, this paper takes the evolvement of seimic facies and depositional facies as primary research contents, choosing the play for petroleum and gas as research emphases. Meanwhile, based on dynamic principles of depositional system and dynamic processes of the accumulation of hydrocarbon and supported by the analysis and simulation for basins, the paper selects six structures as significant evaluating targets: an example from Weixi survey in Beibuwan basin.Based on establishment of third-order sequences, in term of external geometric shape, internal reflection configuration, contact relationships of interface and characteristic properties of geophysics of such seismic impedance, there are five seimic facies and thirteen seimic sub-facies in the survey. Though seimic facies is not always accordant with depositional facies, Foreset facies usually indicates delta, fan delta and submarine fan, Parallel and Filling facies often involves in lacustrine hydrodynamic condition, Disorder facies reflects the distortion and disconnection of continuous deposits frequently by fault reconstruction.According to the viepoint of depositional evolvement, the whole basin experienced initial faulted phase in Palaeocene Epoch. Alluvial fans which were near provenance and showed disorder reflection in seismic profiles and flood plains with red layers developed well at that time. During Eocene Epoch, Beibuwan basin began expanding. Delta and fan delta could be found at higher position of the lacustrine area. In Oligocene Epoch, Beibuwan basin came into the evening of faulted phase. The tectonic evolutive phase of this basin transformed from faulted phase to depression phase gradually. Some stuctures in Haizhong depression even reversed at the end of Oligocene Epoch. Combined many depositional parameters(thickness of sandstone, thickness of mudstone, sandstone percent content), the depositional pattern of Liushagang formation of Palaeocene System is established. Eventually, the paper researches the dynamic principle on formation and development of depositional systems, which is co-effected by tectonic power, filling power and climatic power.In the light of five elements of play for petroleum and gas(source rock, reservoir, caprock, migration, trap), the distrubution of residual stratic pressure and the trend of fluid potenial, three plays can be identified in the survey which are complete play($MRTS), migrating play(MRTS) and incomplete play(RTS, TS).In the end, the separate geological risk assessments of six structures are evaluated by the analysis and simulation for basins and reservoir inversion. These structures nearly form in the same period, which begin in Eocene and Oligocene Epoch with Xishan Movement of Episodeâ…¡and finish in the end of Oligocene Epoch. Among these traps of structures, the trap of H2 belongs to typeâ… with fine conditions of reservoir better than others. However, that the summit of such structure deviates much hazards the exploration. The second fine structures are H1, W9s and W10-10 respectively. A great number of traps of those structures belong to typeâ…¡with restrained factors of source rocks and the capibility on sealing. The traps of W10-5 and W10-4 are not beneficial to reservoirs and many of them belong to typeâ…¢. They risk much on exploration and development with structures broken in part.